# What range of exit pupils work for observing the full moon?

I'm observing the full moon, from a major city, with heavy light pollution and dust.

The objective size is fixed, for this comparison, something between 4 and 6 inches. The question is about the tradeoff between magnification and dimness of the image. I'd like to know what range of exit pupils works best. Again, the goal is to observe the full moon.

To put it differently:

• What's the smallest exit pupil at which the moon gets so dim that you can't take advantage of the increased magnification? ("Empty magnification").
• What's the largest exit pupil at which the moon is bright enough, so a larger exit pupil doesn't help?

The only reference I could find is this, which claims that 1-2mm works best for the moon. Is that correct?

Since the magnification is determined by focal length of the objective/focal length of eyepiece it is difficult to have an eye-piece with the same f ratio as the objective. Assuming pupil aprature 0.25" then the ideal eye-piece aperature will be 0.25" for most effective light transmission of the eye.A generally acceptable f8 would have a focal length of (0.25x8)=2". A 2" eye piece x 100 magnification would require an objective focal length of 200",for f8 this means 200/8 which is 25" diameter.This would give comparable brilliance at that magnification. Any deviation from the ideal means loss of resolution due to lens errors of manufacture,the combination of these errors result higher arc" resolution. You will find a lower magnification more useful as the lower mag system may effect finer arc",res and additional light to eye/optical system efficiency. There are no easy answers and sadley cost raises the barriers

Light pollution does not matter for the Moon. Even transparency doesn't matter that much. What does matter is seeing, a.k.a. air turbulence.

It is very rare that an exit pupil smaller than 0.5 mm is useful for anything - perhaps for some tight double stars, but that's about it. So take that as a hard lower limit.

In terms of a "soft" limit, it depends. If seeing is excellent, if the optics are very good, if the instrument is in perfect collimation and at perfect thermal equilibrium, then you can push e.p. below 1 mm, especially for a small aperture like the range you mention.

If seeing is less than perfect, if optics are so-so, if the instrument is miscollimated or too hot, then the minimum usable e.p. goes up. Each one of these factors influences the observation, so it's hard to pin a number.

Bottom line is, use what works for you.

claims that 1-2mm works best for the moon. Is that correct?

2 mm seems too big for a small 4… 6" aperture when observing the Moon. It might be okay if you want to capture the whole disk, but it seems large if you're interested in small details.

Again, it's all relative.

I have a 6" reflector with a self-made primary mirror of excellent quality. I always take care to collimate the scope, and to keep it at thermal equilibrium. I live in California, so seeing is often good. It's not uncommon for me to use less than 1 mm exit pupil when observing the Moon. I use 0.84 mm as a starting point, sometimes pushing it down as low as 0.6 mm.

If I use an e.p. of 1.1 mm for the Moon that means seeing is pretty bad - but in those cases I'd rather do something else, instead of being frustrated by bad seeing. But at that e.p. in a wide eyepiece (82 degrees apparent field) you see the whole Moon at once, which is always nice when you do a public demo (sidewalk astronomy).

For much larger apertures, the ideal exit pupils tend to be somewhat larger even when observing the Moon.

To predict seeing, go on the Clear Sky Chart site, find a location near your home, and look at the third row in the chart (labeled Seeing); when it's dark blue, seeing is good.

For the largest practical e.p., don't worry about it. The Moon is so bright, losing light is not something you should pay any attention to at all.

## FOV and Exit Pupil

Can someone explain to me AFOV vs TFOV and exit pupil and how you calculate them for a specific scopes and eyepieces.

I think I understand FOV but can't figure out how it is calculated or how to determine what the maximum FOV certain scopes will allow. Specifically trying to figure out for an AD8 as I have one ordered.

I understand what exit pupil is referring to but again don't know how it is calculated or what practical differences there are when observing. What are the pros and cons to having a larger or smaller exit pupil? Are certain ranges better for observing lunar/planets vs DSO or better for different levels of magnifications?

I did a quick search but couldn't find anything that specifically address the above questions. Please feel free to direct me if this has already been discussed. Thanks.

### #2 SeattleScott

Let’s use a 40mm 70 AFOV eyepiece as an example.

1200/40=30x magnification.
70 AFOV divided by 30 magnification equals 2.3 degrees TFOV. In reality more like 2.2 but the formula comes close enough for government work.
Exit pupil is 40/6, your scope’s F ratio. F ratio is focal length divided by aperture. So exit pupil about 6.7mm. Definitely on the large side but manageable under darker skies.

### #3 rhetfield

To further expand on exit pupil:

In theory, the human eye can see up to 7mm exit pupil. Anything larger is wasted and on a newt, a person might even see the secondary obstruction.

The reality is that many - especially older - people can't see more than 5-6mm.

On the other end, less than 1mm exit pupil starts to get to small - things look dim and grainy. People start to see their eye floaters. By 0.5mm things can be pretty bad. Bright things like the moon respond better than dim things like DSO's.

### #4 CassGuy47

There's a direct correlation between exit pupil and useful magnification. If you're young, and you're in a dark sky location, you can take advantage of the benefits of a 7mm exit pupil. In the case of a scope with a 200mm aperture, 28X would be your lowest effective magnification. 200mm/28x = 7.1mm EP. If you're a senior citizen, your pupil probably can't open larger than 5mm, even under a dark sky. In that case, the lowest effective magnification would be 40X. 200mm/40x = 5mm EP. If you use a magnification lower than that, it would be like trying to pour 9 ounces of water into an 8 ounce glass. You would lose the extra water due to spillage, just like the extra light would spill over.

On the opposite end of the spectrum, the theoretical magnification limit is 2 times the aperture, so on the high-end 200mm x2 = 400X. That will result in an exit pupil of .5mm. 200mm/400x = .5mm EP. It's generally accepted that beyond that magnification, there is no further increase in detail, and the image will become unacceptably dim. Of course, with poor seeing, you won't be able to use 400x. I hope this helps!

### #5 Jon Isaacs

To further expand on exit pupil:
<. snip. >

I like this study, it measured the dark adapted pupils of 263 people from about 18 years old to about 80..

18 to 19 years (n=6), the mean dark-adapted pupil diameter was 6.85 mm (range: 5.6 to 7.5 mm)

20 to 29 years (n=66), 7.33 mm (range: 5.7 to 8.8 mm)

30 to 39 years (n=50), 6.64 mm (range: 5.3 to 8.7 mm)

40 to 49 years (n=51), 6.15 mm (range: 4.5 to 8.2 mm)

50 to 59 years (n=50), 5.77 mm (range: 4.4 to 7.2 mm)

60 to 69 years (n=30), 5.58 mm (range: 3.5 to 7.5 mm)

70 to 79 years (n=6), 5.17 mm (range: 4.6 to 6.0 mm)

There are senior citizens with dark adapted pupils greater than 7mm.. One of them would be me.

### #6 AtlantaAstro

I like this study, it measured the dark adapted pupils of 263 people from about 18 years old to about 80..

<. snip. >

How does one find out what their personal dark adapted pupils Max out at? Is this something the optometrist would tell me during an eye exam? I haven’t had an actual in-depth eye exam since I was a kid probably.

### #7 TOMDEY

These questions repeat so often that it seems time to create a little cheat-sheet >>>

AFOV (Apparent Field of View) How Expansive the view is as seen through the eyepiece. Expressed in Degrees.

TFOV (True Field of View) The Field Diameter on the Sky. Expressed in degrees or arc-minutes, where 60 arc-min = 1 degree.

F (Focal Length of the Telescope) Objective Lens, Mirror, or combination. Expressed in millimeters.

D (Telescope Aperture) diameter. Expressed in millimeters (preferred) or sometimes inches, especially for larger telescopes.

F# (Focal Ratio of Telescope) A dimensionless number related to étendue.

f (Focal Length of the Eyepiece) Expressed in millimeters.

m (Magnification) How much bigger an object appears as seen through the telescope + eyepiece a dimensionless real number.

d (Exit Pupil Diameter) scope + eyepiece. Image of the Telescope Aperture that appears behind the eyepiece where you put your eye's pupil. Expressed in millimeters.

S (Field Stop Diameter) the hole inside the eyepiece that defines the field edge. Expressed in effective millimeters.

m = AFOV/TFOV

AFOV = 2 atan (S/2f)

and are nuanced by such things as distortion and other aberrations. But they are decent rules of thumb. There are literally hundreds of other useful relationships that the professionals immerse in. But 99% of end-user utility is captured in the handful above and a couple dozen others that address such things as resolution, visual discrimination expectation, Eye Relief, limiting magnitude, brightness, luminance, etc. Pocket the six above and you are already ahead of most avocationals. There are lots of popular books covering all this and far more. Tom

### #8 Dave Mitsky

How does one find out what their personal dark adapted pupils Max out at? Is this something the optometrist would tell me during an eye exam? I haven’t had an actual in-depth eye exam since I was a kid probably.

Methods of measuring pupil diameter are discussed at the end of the article posted at https://skyandtelesc. a-pupil-primer/

### #9 Dave Mitsky

Can someone explain to me AFOV vs TFOV and exit pupil and how you calculate them for a specific scopes and eyepieces.
<. snip. >

The answers to some of those questions can be found at https://skyandtelesc. -magnification/

I don't do the calculations myself anymore.

Edited by Adun, 28 March 2021 - 10:47 PM.

### #11 Virtus

Thank you all for the responses this was definitely helpful!

One more related question how does field stop of 1.25" vs 2" EPs/focusers apply to FOV?

What EPs would give the widest TFOV for an AD8?

### #12 rhetfield

I don't do the calculations myself anymore.

I use http://astronomy.too. /field_of_view/

That is one of my most favorite sites

### #13 AtlantaAstro

Thank you all for the responses this was definitely helpful!
<. snip. >

I believe these are the True Field of View Limits but please somebody correct me if I’m wrong:

a lot of folks like the Televue 24 Panoptic as it’s the widest for a 1.25”

my 2” Agena 38mm SWA70 is the widest I can get for my 2”

I made this chart a while ago. I hope it’s accurate:

Max TFOV for 1.25”: 1.79°
- 32mm 52°, 26mm 62°, 24mm 68°, 20mm 82°, 16mm 100°,
Max TFOV for 2”: 2.9°
- 55mm 50°, 41mm 62°, 37mm 68°, 32mm 82°, 25mm 100°, 23mm 110° (41mm panoptic or 55mm plossl, 38mm agena SWA 70°)

### #14 rhetfield

Thank you all for the responses this was definitely helpful!

<. snip. >

The 2" EP will have a much wider FOV. Somewhere around 1.75 times larger depending on the individual eyepiece. That is why it is very good to have a 2" focuser on a scope the size of yours.

I will let others advise on individual eyepiece recommendations. Go to the eyepiece forum for many recommendations - keeping in mind that many will be outside your budget and about half will be discontinued and rare on the classified listings . Generally, plugging the different Explore Scientific EP's into the astronomy tools website listed by others would be a good start. Probably start with 60-70 degree eyepieces unless you have a chance to look though a wider one before buying.

I believe these are the True Field of View Limits but please somebody correct me if I’m wrong:

<. snip. >

I believe those limits are specific to a particular telescope. Not applicable to the general population of 1.25" focusers.

My Z114 has 1.25" focuser and easily offers 4° TFoV.

A more accurate description would be that focuser size limits the maximum field stop of eyepieces that can be designed for that size. So for example you can't build a 25mm 82° eyepiece in 1.25" format.

### #16 GGK

True Field Of View in degrees = (eyepiece field stop diameter in mm / telescope focal length in mm) x 57.3.

The 57.3 is a constant to get the answer to degrees.

Max field stop is the eyepiece barrel diameter with no smaller field stop orifice inside. A 1-1/4 eyepiece has a barrel diameter of around 27mm. The 2 inch barrel diameter is around 46.5mm, so has potential to increase TFOV by around 1.75X

Decreasing focal length increases TFOV, so a focal reducer is also a way to see more of the sky without moving to 2” eyepieces if you have a slow scope like an SCT. Faster scopes have a wider view.

Note that other TFOV estimates in this thread using Eyepiece AFOV / magnification are also good rough estimates, but AFOV is not an exact representation of field stop diameter, so there will be a small error, which generally makes no difference at all, because the focal length is not exact either.

Edited by GGK, 29 March 2021 - 09:34 AM.

### #17 AtlantaAstro

I believe those limits are specific to a particular telescope. Not applicable to the general population of 1.25" focusers.

<. snip. >

Thank you for speaking up haha I’m pretty new so I’m not 100% sure. Well, if anybody wants to know the limits of a 90mm, f/10.1 scope, here you go! Ha

### #18 SeattleScott

With that scope you could hit 2.2 degrees wide with 2” but the exit pupil would be pretty large. Not great for light pollution. With 1.25” you could hit 1.3 degrees with a good exit pupil.

Most people with a F6, particularly if they live near the city, opt for something around 30mm, maybe a 30mm 82 deg for about 2.0 degrees wide with a better exit pupil than a 40mm 70 degree eyepiece. Or dropping down to 30-32mm with 70 AFOV will give an appreciable larger view than 1.25” without an excessive exit pupil or excessive weight (or cost).

Some step down to a 27-28mm 68 AFOV in order to manage cost, weight and exit pupil. But at that point you aren’t getting much wider view than a 1.25” 24mm 68 and now you have to mess with adapters and probably extension tubes.

Edited by SeattleScott, 29 March 2021 - 10:14 AM.

### #19 TOMDEY

How does one find out what their personal dark adapted pupils Max out at? Is this something the optometrist would tell me during an eye exam? I haven’t had an actual in-depth eye exam since I was a kid probably.

You can do that at home. I came up with this long ago (oft copied but never matched for its magnificence). You hold it up close to your eye and find which bar just blocks a brightish star. The nice thing is that it measures your actual pupil that night from that location, under those viewing conditions. People also use such things as drill bits. which are fine. although reflection off the polished edge can confuse the reading and the disclaimer "don't poke your eye out" creates concern.

Annual eye exam and corrective glasses are more than just prudent. The greatest cause of irreversible eye damage is --- neglect! The exam can detect glaucoma, cancer, ruptured vessels, scar tissue, blind spots, retinal detachment, cataracts, constricted field, sensitivity, puckered membrane, color discrimination, floaters, abnormal pressure, infection. great list of metrics and concerns. And of course whether eyewear will improve your vision. For some reason we astronomers are more likely to neglect our visual health than are regular blokes. We get stingy when it comes to test and eyeglass cost, yet happily plunk down $500+ for a single Premium Eyepiece --- makes no sense whatsoever. Tom #### Attached Thumbnails ### #20 Virtus With that scope you could hit 2.2 degrees wide with 2” but the exit pupil would be pretty large. <. snip. > Is this for the Orion Astroview 90 or AD8? ### #21 SeattleScott ### #22 Virtus With that scope you could hit 2.2 degrees wide with 2” but the exit pupil would be pretty large. <. snip. > I'm in a yellow/Bortle 4 zone would the 6.4mm exit pupil of the Agena SWA 38mm cause an issue. Ideally I would like to be able to frame the Pleiades which I believe is about 2 degrees. Not considering cost differences, would the extra 0.2 degrees of the above be worth it over say an ES 30mm 82? That would be the exact same 5.1mm exit pupil as the 30mm Superview that comes with the scope correct? As the AFOV of the EP doesn't affect exit pupil if I'm understanding correctly. ### #23 GGK Correct. APOV has no impact on exit pupil. Exit pupil = eyepiece focal length / telescope focal ratio (f#). 30mm is 30mm regardless of brand, barrel size, or APOV. The challenge with evaluating exit pupil is that it's heavily reliant on the person. Everyone replying will have a different opinion and everyone is right. In Bortle 4 skies with my eyesight, anything greater than 4mm exit pupil starts to look washed out. The stars don't get any brighter, but the background sky does. LPR filters don't work with new LED light pollution, so the only way to darken the background sky is to lessen the exit pupil. The best large star cluster view for me is an exit pupil in the 2.0 to 2.8 range. Since I have a SCT with an effective focal length somewhere around 2150 to 2200mm, my TFOV with a 30mm Pentax XW is just under 1 degree. If the object I'm viewing is a little smaller, I much prefer the view through the 22mm Nagler. Not because the Nagler is a better eyepiece (I rate image quality as equal) but because the background sky gets a little darker with a smaller exit pupil / higher magnification while the stars don't change much. This improves contrast and the view is richer. However, the 22mm Nagler drops me to about 0.8 degrees TFOV. As you can see, each object will have it's best eyepiece. When viewing Pleaides, I go for the widest view I can get, which in my scope is a 40mm Pentax XW at nearly 1.3 degrees. Viewing smaller objects with the 40mm is not as nice as the 30mm or 22mm for reasons stated above. However, Pleaides is bright enough to maintain contrast with the lighter background sky - plus I need all of that TFOV to get all 7 sisters in view. Although I do look at Pleaides occasionally with my C8 SCT, I get a much more spectacular view through my 16x50 Pentax binoculars. I see the 7 sisters surrounded by many outer stars, all framed nicely with open sky. It's why I use my binoculars so much when viewing very large bright DSOs. ## Exit pupils Exit pupils: A second way to choose eyepieces (other than power per inch of aperture) is to match the eyepiece exit pupil to the type of observing you want to do. To find the eyepiece exit pupil -the diameter of the beam of light coming out of the eyepiece - divide the eyepiece focal length by the telescope focal ratio. Thus, while a 10mm eyepiece has a 1mm exit pupil with an f/10 scope (10/10 = 1), the same eyepiece has a 2mm exit pupil with an f/5 scope (10/5 = 2). The higher the power, the smaller the exit pupil. The brightness of extended objects (galaxies and nebulas) is proportional to the square of the exit pupil. Therefore, a low power 4mm exit pupil (4 squared = 16) is four times as bright on galaxies and nebulas as a medium power 2mm exit pupil (2 squared = 4). To put it another way, twice the power results in one-fourth the brightness on the faint fuzzies outside our solar system. On the other hand, the brightness of a point of light (a star) is a function of the aperture of your scope - not the exit pupil.The bigger the aperture, the fainter the star you can see. Stars do not get dimmer as a scope's power increases and the exit pupil gets smaller. Extended objects do,however, and the sky (the most extended object you'll ever see through your scope)becomes progressively darker as the power goes up. The result is that faint stars are usually more visible at higher powers, as the contrast between the unchanging star brightness and the progressively darker sky background increases. From dark sky sites, a 5mm to 7mm exit pupil is best for observing Milky Way star clouds, open clusters, large nebulas such as the Veil, etc. From mildly light-polluted suburban sites, a 3mm to 4mm exit pupil improves the contrast of these large-scale objects by darkening the light-polluted skies somewhat without overly dimming the objects themselves. A 2mm exit pupil typically most closely matches the area of highest resolution in your eye and gives you good detail for planetary, lunar, and globular cluster observing. The sharpness of those details is likewise improved by a 2mm exit pupil, as a smaller exit pupil minimizes astigmatism at the edges of your dark-adapted eye. Also, the visibility of small galaxies and planetary nebulas is often enhanced by the darkening of the sky background with a 2mm exit pupil. A 1mm exit pupil gives you maximum planetary detail and is excellent for splitting binary stars. A 0.5mm exit pupil is useful for splitting close double stars, but only during very good seeing. ## What range of exit pupils work for observing the full moon? - Astronomy I've simplified and integrated my visual detection algorithm into my telescope designer with an object catalog and eyepieces selector. Go here. Enter the object total magnitude and object size as commonly given in catalogs. The program will calculate the surface brightness along with the surface brightness at magnification. Other useful values are calculated and displayed. These values are then used to search through the Blackwell data to determine object visibility for a range of apertures. Detection thresholds for various eye pupils are plotted as long as the object fits into the magnified field. Simple contrast is also calculated, which when combined with object apparent size, is an useful approach to predicting object visibility. For more, see the notes below. MPAS = Magnitude Per Arcsecond Squared Sky background brightness is 21.5 MPAS for a dark site, 18.5 MPAS for a city site. The chart plots the visibility of extended objects like nebulae and galaxies. The object is visible when the eye's perceived contrast is greater than zero. However, because of the impreciseness of object magnitudes and object sizes, it is better to divide the log contrast into zones. Log contrasts greater than 0.5 are easy, contrasts down to 0.25 are visible, contrasts between -.25 and 0.25 are difficult and log contrasts under -0.25 are not visible. For comparison, half the aperture and double the aperture are also plotted. The plot stops if the object is too big to fit into a 100 degree apparent field of view eyepiece. The visibility is plotted logarithmically to match the eye's performance. The contrast is the object's brightness + the sky background brightness divided by the sky background brightness, mapped to the eye's response at different sky background brightnesses, object brightness and apparent object size. To be clear, the object's contrast never changes. But the eye's ability to detect the object depends on three factors: the sky background brightness (light pollution and exit pupil), object brightness and apparent size of the object. The chart is plotted for a range of exit pupils. The exit pupil is calculated by dividing the eyepiece's focal length in mm by the telescope's focal ratio. For instance, a 30mm eyepiece with a f/5 telescope produces a (30/5) 6mm exit pupil. A 10mm eyepiece with the same telescope yields a 2mm exit pupil. A 6-7mm exit pupil is typically the widest field lowest practical magnification and a 1/2 to 1mm exit pupil the narrowest field highest practical magnification. The apparent sky background brightness at the eyepiece is solely mediated by the exit pupil. In other words, at a given location, the sky background brightness will be identical regardless of eyepiece and telescope when the exit pupils are the same. The contrast percentage is included. Experienced observers have no trouble viewing objects with 6% contrast and can observe objects with difficulty down to 3% or so as long as the object is at least of 3-5 degrees apparent size. Use an Allen wrench to match your instruments to your eyeball. Your pupils constrict in a bright environment to limit the amount of light that reaches your retinas. In the dark, your pupils dilate to admit as much light as possible as part of the dark adaptation process [Hack #11] .Astronomers refer to pupil diameter as entrance pupil size because it determines how much of the light from a binocular or telescope can enter your eyes. Measuring your entrance pupil size when your eyes are dark adapted gives you a key piece of information to help you select binoculars and eyepieces that are best suited to your own eyes. Maximum dilated pupil size varies with age and other factors. A child under 10 years of age may reach maximum dilation of 8mm or slightly more when fully dark adapted. A young adult’s entrance pupil may be as large as 7.5mm. As we age, our eyes may no longer dilate as fully as when we were young. By age 35 or 40, we may be limited to 6.5mm or less dilation, by 50 or 60to only 6mm or less, and by 80 to only 5mm. (This is not invariably true some 60-year-old eyes can still dilate to 7mm, and some younger eyes cannot dilate to a full 7mm.) For maximum light gathering, you want the exit pupils of your binocular and telescope to be no larger than the entrance pupils of your dark-adapted eyes. This delivers the maximum possible amount of light and allows you to see the brightest possible image. If the exit pupil of the instrument is larger than your entrance pupil, you waste light. To calculate the exit pupil of a binocular, divide the objective size by the magnification. For example, a 7X50 binocular delivers an exit pupil of 50/7=7.1mm, while a 10X50 binocular delivers an exit pupil of 50/10=5mm. To calculate the exit pupil of a telescope, divide the focal length of the eyepiece in millimeters by the focal ratio of the scope. For example, a 25mm eyepiece used in an f/5 scope delivers an exit pupil of 25/5=5mm, while a 35mm eyepiece in the same scope delivers an exit pupil of 35/5=7mm. If your fully dark-adapted entrance pupil is 5mm, for example, it’s pointless to use a 7X50 binocular because it delivers a 7.1mm exit pupil. In effect, your 5mm entrance pupil stops down your 50mm objective lenses to 35mm. You could instead use a 7X35 or 10X50 binocular, either of which delivers a 5mm exit pupil that matches your entrance pupil, and the images would be as bright as those you see with the 7X50 binocular. Similarly, looked at solely from a light gathering perspective, it makes little sense to buy an eye-piece that has a focal length longer than the focal ratio of your scope multiplied by your entrance pupil. If your entrance pupil is 5mm, for example, the longest eyepiece you should choose for use with an f/5 scope is 5 x 5mm =25mm. You may choose to buy a longer eyepiece despite the fact that it “wastes light” because that longer eyepiece provides a wider field of view. For example, Robert, whose entrance pupil is about 6.5mm, routinely uses a 40mm Pentax XL eyepiece in his 10” f/5 scope. That eyepiece provides a huge 8mm exit pupil, and in effect turns the 10” f/5 scope into an 8” f/6.3 scope. That doesn’t really matter, though, because Robert is seeing that larger field of view as brightly as it is possible for him to see it. Your eye doctor can measure your fully dark-adapted entrance pupil for you, but you can also determine it for yourself. To do so, you’ll need a set of metric Allen wrenches. Allow yourself to become fully dark adapted, which may take half an hour or more. Look directly at a bright star, and hold one of the smaller metric Allen wrenches along your cheek so that the long portion crosses your eye parallel to and near the eyeball, as shown in Figure 1-4. Move the wrench up and down until it is centered on your pupil. You’ll see the star split into two stars, one on each side of the wrench. Substitute larger Allen wrenches until you reach a point where the star no longer splits, but is visible only as a single star on one side or the other of the Allen wrench. The size of that Allen wrench is the size of your fully dark-adapted entrance pupil. If you observe frequently from a light-polluted site, repeat the experiment there. You may be surprised at the difference light pollution, particularly from nearby local sources, makes to your dark adaptation. For example, if your entrance pupil is a full 7mm at a truly dark site, it may be only 5mm at a brighter site. Your eyes operate on the same principles as any optical instrument. Light gathering ability varies with the square of the aperture. That means a 7mm entrance pupil admits nearly twice as much light (7 2 versus 5 2 ) as a 5mm entrance pupil, which in turn means that you can see nearly one full magnitude deeper from the darker site [Hack #13] . Get Astronomy Hacks now with O’Reilly online learning. O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers. ## What range of exit pupils work for observing the full moon? - Astronomy Be Proud of Who You Are - Be Proud of Your Heritage Our friend David L. Fisherowski is well versed in many applications of Astronomy and has offered his support in this field of study. A little bit about David he was Born 9/15/48 in Erie Pa. spending 27 years there before moving to Boyertown, Pa. where he now views the heavens. David went to Erie Cathedral Prep 1966 then onto higher education at Edinboro State College(now University) 1970 and he felt the need to go further and did his Graduate work at Edinboro, West Chester, Boston University and College of the Pacific. His first and last job found him teaching at Boyertown Area School District 8th and 9th grade Earth and Space Science. David spent 35 years teaching students the foundations of earth sciences and sharing his love of space.(retired) 1970-2005(35 years). David was also the JV Basketball coach 1975-78, Varsity Football line coach and defensive coordinator from 1986-2011(26 years) David’s Hobbies include: Geology(All aspects), Weather analysis and photography, Astronomy and astrophotography and Americas favorite sport Football. We encourage you to visit David’s Web site listed here. Here you can learn about the heavens, the weather, the DDS system ( now in developmental stages), astronomy and much more. http://daveshorizon.blogspot.com/ We also have our friends at AccuWeather.com community helping us out. Please feel free to visit their Astronomy blog daily. http://www.accuweather.com/en/weather-blogs/astronomy A Simple Guide to Backyard Astronomy The Earth’s Moon The wonderment of the night sky is a passion that must be shared. Tracking the phases of the Moon, if only to plan how much light it will put into the sky at night, and bookmarking the Clear Sky Clock, affectionately known as the Cloud Clock become as common as breathing. The best observing nights fall about a week after the Full Moon until a few days after the New Moon. However, don’t wait for ideal and see what you can see every night no matter where you are. I offer this simple guide to anyone who wants to look upward and behold the magnificence of the night sky. Light pollution complicates observing the night sky if you live near a city as the brightness of the sky will determine how much you will be able to see. Star gazing in Pennsylvania can be a wonderful experience in your own backyard or an incredibly fantastic experience if you are willing to drive 2 to 8 hours to find a dark sky. For more information about curbing light pollution and saving money at the same time, or to see a light pollution map, check out All it takes to get started is a lawn chair and a pair of binoculars Then you will want to know where to look for beautiful things in the sky, and how to know where you have pointed your optics Soon you will want something to hold the binoculars steady, bigger binoculars, a more comfortable lounge chair, warmer clothes and something to keep the dew off your optics Then well, Pandora’s Box has opened, and you realize that aperture is everything, the cost of mounting the optics and accessorizing them far exceeds the cost of the optics themselves, and you are starting to wonder how nice a gift you need to buy for your spouse so that you can acquire a new piece of astro stuff. Where and When to Go to look at the Stars Your backyard is a good place to start Most local astronomy clubs have public outreach programs and advertise public viewing nights. If you like camping, try attending a star party. Even if you don’t own a telescope, star parties are fun to attend. People with big and small, homemade or expensive telescopes are happy to show them off and let you take a look. They are great opportunities to see and try things before you buy. Some of the ones I am familiar with are listed below, but there are wonderful star parties all over the country: The Mason-Dixon Star Party near Lancaster, PA usually in June or July Stargaze in the Spring, and the No-Frills in the Fall at Tuckahoe State Park, MD sponsored by the Delmarva Stargazers club Almost Heaven Star party in August at the Mountain Institute on Spruce Knob, in West Virginia The Black Forest Star Party in Cherry Springs, PA in September Dress for the Occasion! Always carry a jacket, hat and boots. The cosmos is a very, very cold place, and it just sucks the warmth right out of you in the absence of sunlight. By day, the sun can be intense and burn your skin. A daytime, safari-style hat with a broad brim and covers the back of your neck, though no fashion statement, will help protect your face and neck from getting sunburned. Sun screen, lip balm, moisturizing lotion and insect repellent are four things you do not want to be without in the middle of nowhere! Even in summer, most astronomers battle cold temperatures at night! Changing the clothes next to your skin at dusk, wearing wool socks and a hat that covers your ears will go a long way to keeping you comfortable. Dew can be a problem and soaks through clothing, so water resistant outerwear is a good idea. A pair of gloves comes in handy when the dew freezes on your equipment Wearing a hat will help keep your feet warm as fifty percent of your body heat escapes from the top of your head. If you find that insulated boots are too bulky, try a pair of NEOS Overshoes. They fit over your regular shoes, are very light weight, pack flat, and keep your feet warm and dry in water, mud, frost and snow. Red Flashlight A flashlight that shines a red and not a white light is essential for navigating your way around in the dark on the observing field and reading the star charts. You can either cover a flashlight you have with rubylith, a red cellophane, or buy one with a red LED bulb. Many clubs that sponsor public outreach programs will be able to provide you with red cellophane for your flash light when you arrive. I have come to prefer the kind of light you wear around your head so your hands are free. WHY RED? Human eyes adapt to the dark by dilating the pupils and filling up with a chemical called visual purple. A white light will cause immediate pain and take your eyes another 30 minutes to readjust to the dark. How to Find Things in the Sky There are many maps of the sky available. The selection is overwhelming, so I will name my favorites. Photographs on the covers of astronomy books and maps are spectacular, but you will NOT see those objects in that beautifully illustrated way in a small telescope or with a pair of binoculars. You also will not be able to see the color shown in these photographs, so don’t be disappointed at the outset! I own two bookshelves full of atlases and star charts, yet I cannot read them in the dark with a red flashlight because the print is too small and/or they cram so many objects on the chart that you can’t tell where they are. Many of the charts included in this guide will be ones I created for myself and are specifically limited to large, bright objects you can see with a small telescope or binoculars. Most of the objects can be seen to some degree with the naked eye in a dark sky, and many of them can be found using optics in the horribly light polluted skies of any city. Most amateur astronomers use a combination of charts and gizmos to assist them in finding celestial objects. There is even software that can be used on a laptop computer or Pocket PC that will guide you or a telescope to targets you choose. Many new telescopes come with “Go-To” motors and software built into them or something like Sky Commander can be used with large Dobsonian telescopes to find and track objects in the sky. Planetarium Software Planetarium programs can be installed on a computer that will show you what is available for viewing at anytime – both past and future. Most of my favorites are available for free on the web. Sky View Finder Stellarium http://www.stellarium.org/ Red dot pointing devices mounted on binoculars , cameras or telescopes work very well. A Rigel finder stands up a little higher and gives our face a little more room to aim the scope. You can aim the red dots at a specific point in the sky, then look through your scope. If everything is lined up properly, the object you seek will be centered in the eye piece.. A stand-alone “star finder” is the new Celestron Sky Scout. Not only does it have a lighted display that is more or less possible to read in the dark, it can also talk to you if you use the ear buds that come along with it. It works by turning on its GPS and gets a lock on several satellites. Then you can point it to any area of the sky, day or night, and it will tell you what object you are looking at. You can even ask it to find something for you, and more of the lights will blink on the display the closer you get. Meade has a similar product made of plastic called the mySKY, however the picture is misleading because you will NOT see that beautiful galaxy in the window! DDS (Dave’s Dynamic System) For years mankind has had to battler the elements of mother nature. When it comes to capturing the night sky, well lets just say one word DEW. For the most part it’s rather easy to set up the camera, all you need is a long lasting power source, good tripod, good SLR camera, and a timer. However remember that element called dew? Well as the sun sets, the stars come out and it’s time for mother nature to create dew. What is dew? Dew is water in the form of droplets that appears on thin, exposed objects in the morning or evening due to condensation. As the exposed surface cools by radiating its heat, atmospheric moisture condenses at a rate greater than that at which it can evaporate, resulting in the formation of water droplets.When temperatures are low enough, dew takes the form of ice this form is called frost. Because dew is related to the temperature of surfaces, in late summer it forms most easily on surfaces that are not warmed by conducted heat from deep ground, such as grass, leaves, railings, car roofs, and bridges. Yes, here we have a common occurrence which will happen on many nights, so the question remains… your trying to capture the night sky and the dew will surely form on your camera and its lens. Before the DDS was invented astro folks would use a heat strap wrapped around the lens and a bag placed over the camera. The heat strap was powered by a large battery, which should last all night. David L. Fisherowski being a science teacher, understood the dynamics of dew and knew that if you apply a condensed project focal layer of air to affected area the dew in question will be unable to form. So in short after years of fine tuning and tweaking the first DDS he developed what we have today the DDS standard, the Mobil DDS, micro DDS, Suspended DDS and the AC/DC interchangeable DDS. For more information visit Dave’s Blog at http://daveshorizon.blogspot.com/ This video explains in detail the workings of DDS https://www.youtube.com/watch?v=m47qGLPiZNc&feature=youtu.be Binoculars for Astronomy Many large deep sky objects look better in binoculars than in a telescope due to their larger field of view. Although any pair will do, bigger binoculars gather more light than smaller ones. Larger binoculars (15×70 and larger) should be mounted because it is hard to keep the image steady whereas smaller models are easily hand held. There are also “image stabilizer”models that keep the target from moving. Because the pupils of the eyes dilate in the dark, binoculars for night use need larger exit pupils ratings than binoculars that will only be used in daylight.The exit pupil is simply the objective (front /larger) lens diameter divided by the magnification (the smaller lenses next to your eyes). A pair of 10×50 binoculars would have an exit pupil of 5 10x30s have a 3 8x56s have an exit pupil of 7. The larger the exit pupil, the brighter the target image will appear. In general, a larger field of view is better, but more magnification reduces field of view so the observer needs to balance the factors. A telescope may have a view of only 1 degree, but many objects in the night sky are much larger and can only be seen in their entirety with binoculars. It is like the difference between viewing something the size of your fingertip versus the size ofyour hand at arm’s length. Another very important consideration is whether the binoculars will be used with or without eyeglasses. For use with eyeglasses, more “eye relief” is needed. Long eye relief generally is at least 18 mm. There are two types of prisms used in binoculars: porro and roof. Porro prism binoculars cost less. “BaK-4” (barium crown) glass is better quality than “BK-7” although the latter is not necessarily bad. Coatings are used to focus the different wavelengths of light to a single point to minimize color aberrations and halo effects. “Fully multi-coated” is best. Roof prism binoculars are better if they are also “phase coated”. The older Celestron Giants 20x80s were made with Japanese optics are the very best large binoculars. Although they are no longer made pairs may become available. I fasten them to a parallelogram mount attached to a surveyor’s tripod. On a dark sky, I can see nebulas very well – especially using only one nebula filter fastened to one eyepiece. A favorite binocular mounts is the Couch Potato Telescope Chair. It is made to handle mounting of small and medium sized binoculars up to 15×70 lightweight models. The entire assembly folds flat and easily fits into a car. It offers a 360 degree rotation and good height adjustments. You can buy it already assembled, or buy it in kit form, or just buy the plans and make it yourself. The chair is made by Sim Picheloup in Houston, Texas. His website is: http://www.geocities.com/lwraif/SimP/ Telescopes for Back Yard Astronomy The best telescope is the one you use all the time. The telescope tube is the least expensive item of the set-up once you consider eye pieces and mounting the thing. Bigger and heavier is always better, but as you get older, weight will become more of a consideration than money. Depending on the construction of the telescope, the image you see is usually upside down, and/or reversed left to right. A roof prism, although not necessary, will correct the position of the image. A spotting scope gives you an image that is realistically oriented. There is an extensive selection of telescopes available in every conceivable price range – reflectors, refractors, Dobsonians, Schmidt-Cassegrains, and Maksutovs. They all do different things and it all comes down to personal preference and what you want to look at. You can even buy telescopes for only a few hundred dollars that have GOTO computers attached to them that will point themselves to whatever you want to look at in the sky. There are endless focal length specifications and eyepieces. Many decisions and trade-offs need to be balanced in selecting a telescope as no one model meets all needs. Some scopes to consider The Yukon 6x100x100 Spotting Scope is very light weight and versatile. It has built in zoom and the eyepiece can be easily rotated to different angles. Both the scope and the tripod are in the$350 price range. Don’t waste your money on anything less than a Carl Zeiss tripod. The website for the spotting scope is: http://www.yukonamericas.com/index.asp The Celestron NexStar 4SE has high-quality Maksutov-Cassegrain optics with Celestron’s premium StarBright XLT coatings. It weighs 21 lbs including the tripod, is ultra portable and features a precision optical system with 1,325 mm focal length (f/13) with a 105 mm (4-inch) mirror. This means it has a much narrower field of view than the above telescopes, but can see much fainter objects in light polluted skies. It gives very good detail on the planets. It is also completely computer-controlled (GoTo) and sets up in minutes. The database has 40,000 objects. The first time I set up mine, I simply leveled the included tripod, adjusted the included red dot finder, pointed it to one bright object in the sky

The Coronado Personal Solar Telescope (PST) is the specialized little telescope that has only one purpose – viewing the sun! NEVER, NEVER point any other optics directly at the sun without special filters. You will fry your eyes and your optics! You can see solar storms on the surface of the sun which appear as black sunspots when viewed head-on, or prominences, pictured below, when viewed at an angle. Sunspot activity is indicative of solar flare activity that causes the Northern

Lights (Aurora Borealis). You can see predictions of aurora activity at: www.spaceweather.com

LOMO 6″ Maksutov Telescope. This is a short tube telescope with Russian optics mounted to a fork mount on a heavy duty tripod. The wedge attached to the tripod enables the scope to “track”, follow the rotation of the earth, when a small, 9-volt battery operated servo motor is connected. This way the object in focus does not move out of view. Both the telescope and fork mount are made of steel. Because this set-up is very heavy I think twice before taking it anywhere. However, it has extremely sharp images and doesn’t move in a strong wind!

Star Charts

Most star charts I have seen are cluttered with too many objects in a small space for me to be able to read in the dark with a red flashlight, so I made my own charts using the Deep Space Astro Card software. The darker the sky, the more objects you will be able to see. You know when you are looking at a dark sky when the clouds appear black instead of white! Of course, the bigger the telescope the more detail you can see! The planets always travel the same path through the sky, called the Ecliptic. The constellations located along this path are known as the Zodiac. You can always check the almanac data in the newspaper to find which planets are visible and when. While stars focus to a point of light, planets focus to a solid object. Venus can only be seen in the morning and evening, and Jupiter and Saturn can be seen during the night as well as evening and morning depending upon where they are in their orbits around the sun. Often, these very bright objects are miscalled the Morning or Evening Stars. Mars is visible only every 2 years and looks red. You can tell if you are looking at Jupiter because you can usually see at least 3 little moons near it. Saturn, with its rings, is the most impressive object in the sky next to the moon! Only two double stars on are included on this list of objects, but the sky is full of them. Two easy ones to see with a small telescope are Mizar in the handle Big Dipper and Albireo in the constellation Cygnus. My charts contain Open Clusters, Globular Clusters, Nebulas and Galaxies. Because galaxies are so diffuse they are hard to see in light polluted skies. You need a larger telescope to see most of them, however, the closest one, the Andromeda Galaxy, is best viewed with binoculars. Galaxies can only be seen when looking out of the plane of our own Milky Way Galaxy which appears as a narrow, bright celestial cloud spread across the sky. The Open Clusters will also be best viewed in binoculars. Globular clusters and nebulas will appear as “faint fuzzies” in the sky.

List of 93 Treasured Objects in the Sky

This a list of 93 things you can see in the sky with a small telescope or pair of binoculars and 15 star charts to help you find them. The moon, planets, and comets are NOT included because they are in different places each night. The chart also includes a map key to assist you in finding these treasures. There is a column that lists each object’s magnitude – the lower the number the brighter the object. Under light -polluted skies, or nights when the moonlight is bright, you may only be able to see Mag 4 or less. The right ascension (RA ) is when an object rises in the East with smaller numbers rising earlier. The Declination shows the degrees the object is located North (+) to South (-) of the celestial equator. Some objects are best viewed in binoculars because they are so large.

Good Binocular Objects – These objects are included in my list of 93.

Nebulas Orion Nebula, M42 Nebulas in Sagittarius, The Lagoon (M8), the Trifid (M20), the Swan (M17), Eagle (M16) The North American Nebula, NGC 7000 in Cygnus The Dumbell Nebula (M27) in Vulpecula The Helix Nebula, NGC 7293 near Aquarius Open Star Clusters The Peiades, M45 The Hyades in Taurus The Alpha Persei Group in Perseus The Double Cluster in Perseus The Coma Cluster in Coma Berenices, The Beehive (Praesepe, M44) in Cancer, M6 and M7 above the tail of Scorpius, Globular Clusters (look like fuzzy balls in binoculars as do comets), M13 and M92 in Hercules, M22 in Sagittarius, Omega Centauri in Centaurus – only visible here in March and April, Galaxies, The Andromeda Galaxy, M31 – look between Cassiopeia and Pegasus, The Triangulum Galaxy, M33, Star Chains, Kemble’s Cascade in Camelopardus (between the North Star and the Double, Cluster in Perseus), Messier Charts, Charles Messier (1730-1817) was a French astronomer, and along with his friend, Pierre Mechain, compiled a list of 103 “faint fuzzies” they observed with small telescopes over the skies of Paris. Comet hunting was a very popular pastime back then, and since comets also appear as “faint fuzzies” in the sky, Messier compiled this list so they would not be mistaken for comets.

Astrophotography is a Pandora’s Box all unto its own. You will NOT be able to see these beautiful colors through a small telescope, however using a Nebula filter, you will be able to see some nebulosity as green.

HOW TO SHOOT THE NIGHT SKY

SO YOU WANT TO SHOOT THE NIGHT SKY (Including Lightning) ……..

This is just a short basic overview on how anyone can shoot the night sky.

Although sunset photos are attention-getters, you and your digital SLR can get great shots of the night sky after the sun goes down. Please note it is very difficult to shoot the night sky with a point and shoot camera, but it can be done. Now you first task, is to find a great spot with no ambient light from cities, also be aware of floodlights that turn off and on. And the weather plans a big role as well shoot on nights that have a lower-humidity, the fall and winter seasons makes for clearer pictures.

Another item that you will most likely need is a lens warmer. Now I have tested many items claiming to keep your lens warm all night, hand warmers, hand wraps, etc. None of these work all night long, however I did find a product which worked not only on cameras but telescopes as well. The firefly heater by Kendricks Astro Instruments http://www.kendrickastro.com/astro/dew_flyheaters.html has this powerful device that will supply heat to your camera through the roughest, dampest nights anyone can imagine. We have been shooting the night sky this past week looking for meteors but we have had a lot of low level fog, and the lens never gathered drops of water.

I also place a plastic bag over the entire camera keeping water away from battery and housing itself. I cut a small hole in one end to fit the lens over and use a rubber band to hold bag around the lens. Now back to taking your picture.

To capture a photograph of a scene complete with stars, you need to keep your aperture open a long time. You also need A tripod, this is a must! You need to use a remote trigger to operate the shutter. ( I found mine on amazon for \$8.00) My timer is a digital timer allowing me to walk away and let camera shoot all night long.

When you photograph a scene that includes starry skies, you want a huge depth of field to keep everything in focus, so use a small aperture that has an f/stop of f/16. If you rely on the camera to expose the scene, you don’t see any stars at all. Therefore, shoot this type of picture by using the B Bulb setting – shooting mode so that the shutter stays open until you decide to close it, which you do remotely.

The lowest ISO setting on some older cameras and Nikons is ISO 200. But if your camera has a lower setting, use it. A focal –Length range of 28mm to 50mm lets you either capture a wide expanse of landscape and stars, or zoom in for a tighter view.

I also shoot a high asa and shorter time frame. I then use a free program to stack my pictures. http://www.markus-enzweiler.de/software/software.html

A sample of stacking pictures – I used over 900 pictures to get this effect

Taking pictures of the night sky

Ok find yourself a dark location, backyard, field, flat roof top, but make sure your far enough away from any kid of lights. Also make note a full moon will often bleach out the night sky.

Mount your camera on a tripod and get it set up, including attaching the remote shutter trigger. Set the lens to manual focus and the lens focus to Infinity. This setting, combined with the small aperture, gives you a huge depth of field

You can attach a hood if you want to, it helps.

After you compose the picture, press and hold the remote release. Experiment with different exposure times. Start out with an exposure of about 30 seconds. Release the remote trigger and review the image:

## Should I wear eyeglasses while observing?

By: The Editors of Sky & Telescope July 19, 2006 0

### Get Articles like this sent to your inbox

Should I wear eyeglasses while observing?

If you are near-or farsighted, with no other eye problems, definitely take them off. You’ll have to tweak the focus, but you’ll see objects in the telescope just as clearly as if you had 20:20
vision. By taking off your glasses you’ll avoid the “tunnel vision” effect often caused when glasses keep your eyeball too far from the eyepiece.

If you have astigmatism, however, you aren’t so lucky. You should wear glasses for all low-power observing, but you can probably get away with taking them off when you are examining the Moon, planets, or anything else at high magnification.

Arizona optical designer Richard Buchroeder devised this handy rule. If the amount of “cylinder” (in diopters) in your eyeglass prescription is D, you can remove your eyeglasses whenever the telescope’s exit pupil (in millimeters) is 1/√ D or smaller. A larger exit pupil means your uncorrected eye will contribute more than 1/4 wave of aberration, and image quality will noticeably suffer. Let’s say your eye needs a cylinder correction of 0.7 diopter (which is not much). The maximum exit pupil is then 1/√0.7, or 1.2 mm. Since the diameter of the exit pupil equals the aperture divided by the magnification, you’ll be able to use a 4-inch (102-mm) telescope at 85× or higher without wearing
glasses.

## Does everybody have the same choices?

Using LTImage in a computer room

Not quite. One of the strengths of Go Observing is that it can adjust to the type and age of the user. A primary school child will obviously have different interests from an A Level physics student.

Also, a Teacher account gives access to a wider range of observing programmes than a Student account. Because of this, teachers can happily let their students work on their own without worrying about them getting lost or overloaded with too many choices.

Teachers can set up entire class sets of usernames using the Registration Management tool in the "My account" menu.

## What range of exit pupils work for observing the full moon? - Astronomy

To determine the aperture, look for a number such as 7x50. The last number, the "50" in this case, is the aperture in millimeters. The larger this number, in general, the brighter the images will appear. The tradeoff is that the bigger the aperture, the bigger, heavier, and more expensive the binocular.

### Exit pupil

The best binoculars for astronomy are models with exit pupils from 5mm to 7mm. The diameter of the light cone projected from these binoculars nicely matches the diameter of the pupil of the human eye, assuming the eyes are dark adapted to nighttime conditions. These binoculars are providing as much light as your eyes can accept and, as a result, are producing the brightest images for their aperture.

Most human pupils can open no wider than 7mm, so a binocular with an exit pupil wider than 7mm would waste light -- only the center of the light cone could enter the eyes. In fact, there are no binoculars with exit pupils greater than 7mm. Those with exit pupils equal to 7mm (7x50, 8x56, 9x63, and 11x80 models for example) are often called night glasses. These are some of the best models for astronomy.

As we get older, however, our pupils lose their ability to open wide at night. For those of us over 40, our pupils commonly open no wider than 5mm to 6mm, even when our eyes are fully dark adapted. Under these conditions, binoculars with a 5mm to 6mm exit pupil are best. This includes 7x42, 8x42, 10x50, 15x80, and 16x80 models.

### Actual field of view

Field of view is often given in "feet at 1,000 yards" or "meters at 1,000 meters." These numbers tell you how wide an object would completely fill the field if it were 1,000 yards or meters away. But in astronomy we are looking at objects across the universe. For us, a field of view measured in degrees makes more sense. A field of 5 degrees, for example, encompasses 10 Full Moon diameters or the distance between the Pointer Stars of the bowl of the Big Dipper.

To convert "feet @ 1,000 yards" into degrees, divide by 52.5. To convert from "meters @ 1,000 meters," divide by 17. A binocular said to give a field of 367 feet at 1,000 yards, a common number, will produce an angular field of 7 degrees. A binocular with a field of 120 meters at 1,000 meters will also yield a 7-degree field.

A field of 5 to 7 degrees is normal for most 7x binoculars. Wide-angle models may show 8 to 10 degrees of sky.

### Apparent field of view

Getting as wide a field as possible might seem best, but there are tradeoffs. Wide-angle binoculars usually exhibit distorted or out-of-focus star images at the edges of the field. And with wide-angle binoculars you usually have to sacrifice what is known as eye relief.

### Eye relief

An eye relief of 10mm to 15mm is normal for most eyepieces. Less than that and you have to press your eyes uncomfortably close to the eyepiece lenses.

If you wear glasses and wish to keep them on while viewing through your binoculars, you'll need binoculars with at least 15mm of eye relief. Models with eye relief of 20mm or more are ideal for eyeglass wearers. This requirement may rule out wide-angle binoculars (those with apparent fields of 60 to 70 degrees) for eyeglass wearers, as few of these models have eye relief greater than 15mm.

### Coatings

When inspecting a pair of binoculars, look down onto the main lenses. Do you see lots of white reflections from lights or windows? If so, it is a sign the optics are not multicoated or not coated at all. In a fully multicoated binocular the reflections will be dim and will look deep green, blue, or purple.

### Type of prisms

For astronomy, all but the finest roof prism binoculars are outperformed by porro-prism models. Roof prisms split the image into two and then recombine it, a process that can lead to spikes of light off bright stars on lower cost models. Porro prism binoculars of any price tag don't have this quirk.

However, not all porro prism binoculars are equal. Ones with prisms made of BK7 glass are economical but suffer from some dimming of the edge of the field. Better binoculars use BaK4 glass prisms for the brightest field of view.

### Similar Content

lets imagine I wasn't to see a nice DSO about 15' size and I think it should look good nicely framed with a 1 deg field of view in the EP..
Which would give the better (or higher probability of seeing anything at all ) view from a semi urban light polluted home site (e.g Bortle 6)?
a) an 100mm f/6 refractor (fl 600mm) and a 10mm EP (60 deg afov, gain 60x = fov pf 1 deg)
(and exit pupil of 100mm / 60 = 1.6mm)
or
b) a 200mm SCT with focal reducer to give f/6 (fl 1200mm) and a 20mm EP (60 deg afov, gain 60x = fov of 1 deg)
(and exit pupil of 200 / 60 = 3.3mm)
My gut feeling is that the SCT should give a better view just based upon its 2xaperture - but Im not sure I understand fully the maths why.
Is the larger exit pupil going to result in a better / brighter / more successful view?
Or will the view be 'roughly' the same ?
Or have I got it all wrong.

Had alot of fun observing galaxies last night, despite poor transparency.
Picking up dozens of galaxies in Virgo, Leo triplets, and M51, M63, M94.

I did however fail to detect the Galaxy cluster in Leos mane. (ngc 3185, 3189, 3193 etc)
I`ve observed them before (3 of them), but I cant remember the eyepiece focal length used.
Can you recommend the best optimal Power / exit pupil for these faint Objects?
I use 8"dob F6.