21 replies to this topic

[lRaziel1]

Hello everyone.

 

So i have this doubt because i want to acquire a new eyepiece, specifically for observing the brighter DSOs such as globular clusters and nearby nebulae.

 

We know that small exit pupil found on medium power eyepieces benefits observing DSOs and galaxies since it may reveal details that wont show on low power eyepieces.

 

Also, we know that the shorter the focal lenght of the eyepiece, the dimmer the image gets.

 

So, normally, the best eyepiece for DSOs would be the one with an exit pupil of 1.5-2mm, right?

 

 

My question comes here: Does the same apply for small apertures?

 

 

Given that i have a F/10 small aperture scope and prominent light pollution that makes DSOs such as Andromeda Galaxy already too dim and hard to spot, which would give the best experience given that i need all the light i can get?

 

A 40mm eyepiece (cant afford any longer) would give me a 4mm exit pupil, and a 15mm one would give me 1.5mm exit pupil.

 

 

In between these 2, which one would help me resolve better these objects or which one is overall a better pick? Considering i already have a highpower eyepiece and a 26mm eyepiece that allows me to fit the pleiades perfectly in my FOV.

 

Many thanks!

Edited by lRaziel1, 19 February 2020 - 05:19 AM.

[Redbetter]

If you only observe in bright sky, then low power eyepieces will result in bright images--not much dimming of the background.  In bright sky more magnification (smaller exit pupil) generally helps by darkening the background. In dark sky larger exit pupils have more utility than they do in bright skies.

 

Observing galaxies or nebulae in bright skies is not very rewarding.

[Jon Isaacs]

Hi:

 

With significant light pollution, the 15 mm will be much more useful. A 40 mm 1.25 inch eyepiece offers no wider field than a 32 mm Plossl and only about 25% wider than a 26mm Plossl.

 

Many extended objects benefit from magnification because they're larger even if they're dimmer because the sky is dimmer so no contrast is lost.

 

Clusters and stars benefit from magnification because the stars are points and do not dim with magnification but the sky dims so the contrast is dramatically improved. Just going from a 26 mm to a 15,mm dims the sky by a factor of 3, the contrast is increased by a factor of 3 and stars more than a magnitude dimmer will be visible.

 

Jon

[Miranda2525]

Need to know what size your telescope is, but dark skies are more rewarding regardless.

[sg6]

In exit pupil terms anything from 1mm to 6mm will be OK - it should all go in the eye. That is a fair range, and if you lose some with say a 7mm or 8mm exit pupil it isn't the end of the world. People use binoculars during the day and most of the light is lost as a binocular will have typically 7mm and the eye will be down at 3mm. Estimate some 80% is lost.

 

Probably betetr selecting on field, get something that is around 1 degree to frame the globulars easily - assume most are in the 0.5 degree area.

 

Bit unsure that equating exit pupil with brightness is good.

If you have a 6mm pupil and a 4mm exit pupil then all the light enters the eye.

If you have a 6mm pupil and a 1mm exit pupil then all the light enters the eye

The difference is that one image on the retina is 4x bigger then the other so 1/16 as bright.

The exit pupil is more a measure of how efficent the whole lot is in getting light to the retina, not necessarily how bright, or at least not directly.

 

The reason you will "not" see Andromeda is simply it is too big to get in the field of view of the scope. You need a magnification down towards 20x and preferably 15x to actually see it all.

 

Down there the exit pupil is I expect greater then the eye pupil so you lose light - it is therefore dimmer - but you get to see Andromeda. Which oddly you would not see fully with a more efficent, read smaller, exit pupil.

 

I think you could be better working out the resulting fields of view of eyepieces in your scope rather then exit pupil. Only get concerned if the exit pupil is too small and so hightens eye ball problems.

[CrazyPanda]

Hello everyone.

 

So i have this doubt because i want to acquire a new eyepiece, specifically for observing the brighter DSOs such as globular clusters and nearby nebulae.

 

We know that small exit pupil found on medium power eyepieces benefits observing DSOs and galaxies since it may reveal details that wont show on low power eyepieces.

 

Also, we know that the shorter the focal lenght of the eyepiece, the dimmer the image gets.

 

So, normally, the best eyepiece for DSOs would be the one with an exit pupil of 1.5-2mm, right?

 

 

My question comes here: Does the same apply for small apertures?

 

 

Given that i have a F/10 small aperture scope and prominent light pollution that makes DSOs such as Andromeda Galaxy already too dim and hard to spot, which would give the best experience given that i need all the light i can get?

 

A 40mm eyepiece (cant afford any longer) would give me a 4mm exit pupil, and a 15mm one would give me 1.5mm exit pupil.

 

 

In between these 2, which one would help me resolve better these objects or which one is overall a better pick? Considering i already have a highpower eyepiece and a 26mm eyepiece that allows me to fit the pleiades perfectly in my FOV.

 

Many thanks!

The best exit pupil for a target depends on several key factors:

 

1. The apparent size of the magnified object against your retina (e.g. how big it appears to be to your eyes)

2. The contrast of the object with respect to light pollution levels

3. Similar to #2, how well defined the edges of the object are. Are they well defined like a planetary nebula, or do they fade off into nothing like an elliptical galaxy?

4. Whether the object is an extended/diffuse object like a nebulae or galaxy, or a star cluster

5. What you want to do with the object (observe details in it, or just say that you saw it, or attempt to observe color in it)

6. Whether you want to use high quality, narrow bandwidth line filters (O-III, H-Beta etc).

 

Some objects look great at very high magnification in exit pupils as small as 0.3mm. Some objects require the most field of view you can give them (exit pupil becomes irrelevant here). Some objects never show detail at any magnification. Some objects are so diffuse that higher power makes it harder to distinguish where the edges are. Some objects look good at a mix of different magnifications/exit pupils.
 

If you want a good "general purpose" DSO eyepiece, then I think anything from 15mm to 18mm is a good fit. If you want to use a line filter like an O-III or H-Beta, then a 32mm or 40mm is a good choice.

 

A smaller exit pupil (0.75mm to 1mm) on globular clusters like M13 and M3 would be beneficial. It will darken light pollution and help resolve more stars. Though this exit pupil might not be great for detecting really small NGC globs which will essentially look like nebulae in your telescope. For those smaller, fainter globs, you'd want to use a brighter exit pupil to detect them.

 

You might want to consider getting a simple 8-24mm zoom eyepiece. This will let you find an optimum magnification & exit pupil for a variety of objects. You'll find that some objects have a tendency to "pop" once you hit that sweet spot of magnification and exit pupil that your eye/brain prefers.

[Jon Isaacs]

Bit unsure that equating exit pupil with brightness is good.

If you have a 6mm pupil and a 4mm exit pupil then all the light enters the eye.

If you have a 6mm pupil and a 1mm exit pupil then all the light enters the eye

The difference is that one image on the retina is 4x bigger then the other so 1/16 as bright.

The exit pupil is more a measure of how efficent the whole lot is in getting light to the retina, not necessarily how bright, or at least not directly.

 

 

The area of the exit pupil is directly related to the surface brightness of an extended object, galaxies, nebulae, the planet's and the sky. It's independent of aperture, the sky is equally bright in a 3 inch scope and a 15 inch scope of the exit pupils are identical.

 

The reason only the bright core of Andromeda is visible under light polluted skies is that it's brighter than the rest of the galaxy, the rest of the galaxy is washed out by the light pollution.

 

Jon

[Redbetter]

 

Bit unsure that equating exit pupil with brightness is good.

Exit pupil is a direct measurement of image brightness (only limited at the larger exit pupils by the eye's own pupil.)  It is a very basic thing that experienced observers and longtime CN readers/commenters should understand after a time.  Jon explained it well.  

[25585]

The area of the exit pupil is directly related to the surface brightness of an extended object, galaxies, nebulae, the planet's and the sky. It's independent of aperture, the sky is equally bright in a 3 inch scope and a 15 inch scope of the exit pupils are identical.

 

The reason only the bright core of Andromeda is visible under light polluted skies is that it's brighter than the rest of the galaxy, the rest of the galaxy is washed out by the light pollution.

 

Jon

Its got logic but still seems a riddle.

 

Generally as an eyepiece's focal length decreases, so does the diameter of its field lens, letting less light into the eyepiece. Depending on ranges used, I have found using a good Barlow 2x with a 10mm, gives a brighter view than a 5mm eyepiece, 7mm & 2× than 3.5mm for examples. 

Edited by 25585, 19 February 2020 - 04:43 PM.

[Asbytec]

If you only observe in bright sky, then low power eyepieces will result in bright images--not much dimming of the background.  In bright sky more magnification (smaller exit pupil) generally helps by darkening the background.

Red, as you know and just to be clear, of course the (sky + object)/(sky only) contrast remains unchanged at all magnifications. I don't observe from bright skies, so I have no real experience. But it does seem feasible darkening the scene should help with dark adaption which is needed for DSO viewing. And even though the object/sky contrast remains unchanged, it makes sense image scale on the retina still applies as it does under darker high contrast skies. So, I think I can agree with you even though object contrast remains unchanged. 

 

If so, I might be tempted to use the same 1 to 2mm exit pupils as I find useful on smaller and bright DSOs under darker skies (some NGC galaxies, for example) while still expecting to see less simply because the contrast is not high enough to be detected across much of the dimmer parts of the nebula. If I'm understanding correctly, effectively and since object/sky contrast is reduced, we actually have to magnify the (any given) dim object more so than under dark skies in order to at least detect it against the bright sky background. 

[Ihtegla Sar]

Its got logic but still seems a riddle.

And to further add to the riddle of the "exit pupil"  . . . there are some very experienced observers here who insist that anything below .5 is completely wasted/unusable/provides no more details than can be seen in a larger exit pupil (I hope I am paraphrasing all that correctly).  And yet, there are other very experienced users here who say that (with very high quality refractors and eyepieces) they can use magnifications high enough to produce exit pupils as small as .22 and see details they could not see at lower magnifications/higher exit pupils, and they have the sketches to back it up . . .  

Edited by Ihtegla Sar, 19 February 2020 - 05:09 PM.

[lRaziel1]

And to further add to the riddle of the "exit pupil" . . . there are some very experienced observers here who insist that anything below .5 is completely wasted/unusable/provides no more details than can be seen in a larger exit pupil (I hope I am paraphrasing all that correctly). And yet, there are other very experienced users here who say that (with very high quality refractors and eyepieces) they can use magnifications high enough to produce exit pupils as small as .22 and see details they could not see at lower magnifications/higher exit pupils, and they have the sketches to back it up . . .

I think that's when light pollution and aperture comes into play.
Because a small exit pupil can sometimes provide better views, it may cause the opposite effect if your aperture is already too tiny to begin with.

Under my very specific to experience, on a 70/700 (F/10) on bortle 8 sky, my 26mm (2.6 exit pupil) can barely make out Andromeda's core, you have to sort of look on your peripheral vision to really make it appear, while on my 9mm eyepiece (all stock cheapo kellners btw) with a 0.9 exit pupil, i couldn't locate Andromeda's core at all.
That's what arose my question. As in, matbe with low power eyepieces, making the objective as small as possible in the FOV,light wouldn't be so spread out in a way that allowed me to make it out.

Sort of confusing tbh, its not a very treated subject, but understandable since its a very specific situation.

[lRaziel1]

The best exit pupil for a target depends on several key factors:

1. The apparent size of the magnified object against your retina (e.g. how big it appears to be to your eyes)
2. The contrast of the object with respect to light pollution levels
3. Similar to #2, how well defined the edges of the object are. Are they well defined like a planetary nebula, or do they fade off into nothing like an elliptical galaxy?
4. Whether the object is an extended/diffuse object like a nebulae or galaxy, or a star cluster
5. What you want to do with the object (observe details in it, or just say that you saw it, or attempt to observe color in it)
6. Whether you want to use high quality, narrow bandwidth line filters (O-III, H-Beta etc).

Some objects look great at very high magnification in exit pupils as small as 0.3mm. Some objects require the most field of view you can give them (exit pupil becomes irrelevant here). Some objects never show detail at any magnification. Some objects are so diffuse that higher power makes it harder to distinguish where the edges are. Some objects look good at a mix of different magnifications/exit pupils.

If you want a good "general purpose" DSO eyepiece, then I think anything from 15mm to 18mm is a good fit. If you want to use a line filter like an O-III or H-Beta, then a 32mm or 40mm is a good choice.

A smaller exit pupil (0.75mm to 1mm) on globular clusters like M13 and M3 would be beneficial. It will darken light pollution and help resolve more stars. Though this exit pupil might not be great for detecting really small NGC globs which will essentially look like nebulae in your telescope. For those smaller, fainter globs, you'd want to use a brighter exit pupil to detect them.

You might want to consider getting a simple 8-24mm zoom eyepiece. This will let you find an optimum magnification & exit pupil for a variety of objects. You'll find that some objects have a tendency to "pop" once you hit that sweet spot of magnification and exit pupil that your eye/brain prefers.

I think im gonna go ahead and get one of those 8-24 eyepieces from AliExpress.

I heard that at the longer focal lenght its like looking through a straw, is it true?
If so, is it so bad that the stock 26mm would be a better choice on the smaller magnification range?

[Asbytec]

"And yet, there are other very experienced users here who say that (with very high quality refractors and eyepieces) they can use magnifications high enough to produce exit pupils as small as .22 and see details they could not see at lower magnifications/higher exit pupils..."

I've asked the question many times as to why that would be true, and it's usually related to observing planets. The usual answer is the quality of the optics can handle it, they can take magnification without image break down.

I've agreed with being able to take high magnification, but image break down probably has to occur on the eye as the bright low contrast image dims considerably at very small exit pupils. Not the fault of the optic, rather the fault of our failing high resolution visual acuity at low light levels.

At some point, Jupiter's image becomes too dim to see well. At least it's bright low contrast detail, I'm sure many of us have seen this effect. Some variance for personal acuity must be taken into account, and I've presumed a bit of efficient throughput might allow for larger image scales.

I am not sure how this applies to already low light and averted vision, needed for DSO, where the eye is sensitive to low light levels and suffers lower resolution.

Edited by Asbytec, 19 February 2020 - 08:34 PM.

[Redbetter]

Red, as you know and just to be clear, of course the (sky + object)/(sky only) contrast remains unchanged at all magnifications. I don't observe from bright skies, so I have no real experience. But it does seem feasible darkening the scene should help with dark adaption which is needed for DSO viewing. And even though the object/sky contrast remains unchanged, it makes sense image scale on the retina still applies as it does under darker high contrast skies. So, I think I can agree with you even though object contrast remains unchanged. 

 

If so, I might be tempted to use the same 1 to 2mm exit pupils as I find useful on smaller and bright DSOs under darker skies (some NGC galaxies, for example) while still expecting to see less simply because the contrast is not high enough to be detected across much of the dimmer parts of the nebula. If I'm understanding correctly, effectively and since object/sky contrast is reduced, we actually have to magnify the (any given) dim object more so than under dark skies in order to at least detect it against the bright sky background. 

 

It is not the contrast so much as the aesthetics that I am referring to when observing in more light polluted conditions.  The sky looks bright to the naked eye, and it looks bright in an eyepiece with large exit pupil.  More modest exit pupil tones that down.   Going from 7mm exit pupil to 4mm is worth 1.22 magnitude (or MPSAS in terms of surface brightness reduction.)  Of course one's pupil might be less than 7 so the impact might be somewhat more muted for that increment (e.g. 6 actual pupil to 4mm = +0.88 MPSAS darker).  

 

The other side of this I mentioned was that in bright sky larger galaxy and nebula targets are badly washed out.  So the large exit pupil eyepieces have less utility and one will likely spend more time observing targets that benefit more from magnification/small exit pupil rather than low power and large exit pupil.   In such conditions lower power eyepieces serve more as finder eyepieces or for framing several objects at once.  I don't tend to target large nebula with filters in suburban conditions because so much less is detectable compared to the same objects in dark sky with large exit pupil and filters.

 

Yes, the increase in apparent size does help with what are already lower contrast situations in brighter sky.  It isn't that the target is too dim, it is that the background is so bright that the contrast is low--even if it would normally be higher in dark sky conditions.  The most obvious example is the very large apparent size of the Milky Way naked eye.  In 19 MPSAS conditions the Milky Way is a very low contrast haze seen mainly in averted vision.

[Jon Isaacs]

It is not the contrast so much as the aesthetics that I am referring to when observing in more light polluted conditions.  The sky looks bright to the naked eye, and it looks bright in an eyepiece with large exit pupil.  More modest exit pupil tones that down.   Going from 7mm exit pupil to 4mm is worth 1.22 magnitude (or MPSAS in terms of surface brightness reduction.)  Of course one's pupil might be less than 7 so the impact might be somewhat more muted for that increment (e.g. 6 actual pupil to 4mm = +0.88 MPSAS darker).  

 

The other side of this I mentioned was that in bright sky larger galaxy and nebula targets are badly washed out.  So the large exit pupil eyepieces have less utility and one will likely spend more time observing targets that benefit more from magnification/small exit pupil rather than low power and large exit pupil.   In such conditions lower power eyepieces serve more as finder eyepieces or for framing several objects at once.  I don't tend to target large nebula with filters in suburban conditions because so much less is detectable compared to the same objects in dark sky with large exit pupil and filters.

 

Yes, the increase in apparent size does help with what are already lower contrast situations in brighter sky.  It isn't that the target is too dim, it is that the background is so bright that the contrast is low--even if it would normally be higher in dark sky conditions.  The most obvious example is the very large apparent size of the Milky Way naked eye.  In 19 MPSAS conditions the Milky Way is a very low contrast haze seen mainly in averted vision.

 

A few thoughts:

 

- Increasing the magnification helps with small objects but with larger objects that are past any detection threshold in terms of size, there's magnification is not much advantage.  Dimming the sky dims the object.  Stars and clusters are a different story since stars do not dim at increased magnifications.  

 

As far larger DSOs like the Veil, I do observe them from my urban backyard using filters.  It's fun and while they're better seen under dark skies, they represent an challenge, something to do under light polluted skies.  

 

One night I made some measurements using my O-III and Ultrablock filters from my backyard. That night it was about magnitude 18.4 and if I remember correctly, using my SQM-L, I measure the sky brightness through the O-III filter at about 21.3 and through the Ultrablock at a 20.6.  

 

These are pretty close to the numbers I calculate assuming 14nm and 28nm bandwidth filters and a visual bandwidth of 200nm or so.  

 

I agree that it's mostly aesthetic but the bright view of the sky and an object has a certain attractiveness all it's own.

 

Jon

 

 

 

[Redbetter]

And to further add to the riddle of the "exit pupil"  . . . there are some very experienced observers here who insist that anything below .5 is completely wasted/unusable/provides no more details than can be seen in a larger exit pupil (I hope I am paraphrasing all that correctly).  And yet, there are other very experienced users here who say that (with very high quality refractors and eyepieces) they can use magnifications high enough to produce exit pupils as small as .22 and see details they could not see at lower magnifications/higher exit pupils, and they have the sketches to back it up . . .  

I am not sure what the targets or sketches being referred to are since what is useful also depends on the target and the aim of the observation as well as the observer's visual acuity.  For example I don't know that anyone is claiming 0.5mm is the limit for resolving double star patterns or trying to resolve planetary moons or asteroids.  And with small refractors I tend to find in good seeing that the limiting magnitude of the scopes are in the 0.4 to 0.6mm range most of the time--the problem is that the spurious disk is becoming an extended object and blends into the surrounding sky, just not enough photon hits in a small enough patch of retina to localize it.

 

What I have seen of the refractor planetary sketches is that more extreme magnifications/exit pupil don't reveal more than what I see at more traditional ~0.5mm exit pupil limits with the same aperture.  I have asked what is being seen when the mag/inch becomes quite high and so far it hasn't been anything that I don't already see in the ~0.5mm range. I will use a bit more magnification to separate something at times...but it is something I am already seeing at 50x/inch and am trying to confirm. 

 

I can already begin seeing diffraction patterns in some stars at ~1.4mm exit pupil, about 18x/inch.   There is only so much detail/resolution available because of diffraction, and my eye seems to be taking that in by the time I reach the ~50x/inch range.  Past that I am actually looking more at details in the diffraction pattern itself.   

 

This is the heart of the problem to me:  some folks claim very high mag/inch without loss of contrast, etc., but they don't report more actual detail than I see with the same aperture at lower power.  Some will claim that 100x/inch or 120x/inch works well for them on planets or the Moon.  However, they aren't seeing/describing detail commensurate with the magnification level.   Therefore the difference seems to be with the scale that some need to employ to resolve the same features.    

[Hesiod]

I prefer to stay above 0.5mm, but when my eyes are especially tired sometimes resort to higher magnifications.

I found that, obviously, do this more often with smaller telescopes: I have pushed the fl55 to 200x a few times, but never observed at more than 400x with the C8.

I have never seen more details at 200x than at 100x in the FL55 but, when am tired, sometimes have had to increase the magnification at 150x-200x in order to see the detail scale I am used to observe at 100x

[Ernest_SPB]

So, normally, the best eyepiece for DSOs would be the one with an exit pupil of 1.5-2mm, right?

 

it depends...

there are a number of DSO types and a lot of observing conditions.

E.g. under good sky

some wide and fine nebulae (like some Sh2) sometime requires exit pupil 4-5 mm

many popular Messier "likes" exit pupil 3-4 mm

small galaxies better seen with exit pupil 1.5-2.5 mm

for some globular clusters and planetaries it is normal to set magnification that provides exit pupil with diameter 1 mm 

etc.

Under light polluted sky optimal magnification can become higher. 

 

Does the same apply for small apertures?

Small aperture reduces number of available objects and their visible details, but in most cases optimal diameters of exit pupil to observe the same objects are the same (more or less).

[Asbytec]

"Some will claim that 100x/inch or 120x/inch works well for them on planets or the Moon. However, they aren't seeing/describing detail commensurate with the magnification level. Therefore the difference seems to be with the scale that some need to employ to resolve the same features."

I'm sure many of us have taken our scopes to ludicrous magnification, just for a look see (try in on Jove), greater than 0.5mm exit pupil or somewhat more 50x per inch. Not much to see up that high too far above 50x per inch except stars. Diffraction limited or aberrant, whichever, in good seeing.

I can begin to see Airy discs, too, around 1.5mm+/- exit pupil, but generally hang around 0.4 to 0.6 for lunar and planetary. Though well above the standard resolution of the aperture for average acuity (like me), I believe we can see larger bright low contrast planetary detail a little easier.

The mechanics of image scale seem to play a similar role to the critical size for detection of DSOs allowing us to detect such features. The scope can take ludicrous magnifications, it's the same image we see at lower magnification, after all. The question is can we survive small exit pupils and low light levels. I agree, maybe some folks with less acuity may need high magnification and risk a dim planetary image trying to see detail.

Kind of drifting from DSO observing in light pollution, but it's an interesting and useful side note to exit pupils and acuity of the very human eye/brain system of the observer. As Jon has mentioned, the eye likes a bright image, and it likes a large image. Strike that balance. Generally, magnify an image until you can't see it well then drop down a notch. Or two.

Edit: It's been said smaller apertures tend to smaller exit pupils because the eye is starved for image scale, while larger apertures tend toward larger exit pupils because they can hit a higher magnification with a larger exit pupil (magnification is the ratio of aperture/exit pupil).

Edited by Asbytec, 20 February 2020 - 07:10 AM.

[BKSo]

My only scope is 4" F/10 and I have both 40mm, 26mm, and 15mm eyepieces. Under urban sky the 15mm is my main eyepiece, both for open clusters and finding smaller targets, along with a 8.3mm for planets, globular clusters and planetary nebulae. The 26, 40 and 64 are mainly reserved for dark sites.

[faackanders2]

Both.  The more the merrier.  Good to have options