Exit pupil in high/low magnification help?

[mbaddah (Mo)]

Hi there

Was wondering how much of a difference would there be if looking at say for example Saturn in a 25mm eyepiece F5 (exit pupil 5mm) compared to 10mm ep (2.5mm exit pupil)? Will you see much more detail with a exit pupil of 5mm compared to 2.5mm?

I'm 25 yrs old and assuming eyes have dilated to max in dark night, many thanks for your help.

[ngcles]

Hi Mbaddah & All,

In viewing Saturn which is a very small high surface-brightness object (as are all the planets) the question of exit pupil plays a minor role. Neither do you really have to dilate much --as you will see.

The telescope's ability to show detail on a planet (assuming diffraction limited or better optics) is governed by the diameter of the aperture but very much limited by the seeing at the time of observing.

The Dawes formula provides that the size of the airy disc a telescope produces in arc-seconds (R) is R=4.56/D where D is the diameter of the aperture in inches.

So for simplicity's sake, assuming a 10" 'scope, the smallest detail visible on a planet (or the moon for that matter) is about 0.46 arc-seconds across, but only in _perfect_ seeing.

The problem is that the seeing is rarely perfect, infrequently very good and commonly average or ... worse.

If you view at low power with a wide exit-pupil, all the detail that the telescope/seeing will permit to be seen is there in the image, but it is usually much too tiny to be seen comfortably. So to see it comfortably, we apply magnification. More magnification doesn't increase the resolution of the 'scope, it just makes what is there easier to see. Unfortunately, applying magnification also amplifies any imperfections in the seeing. The seeing will limit what magnification can be practically used while retaining good image quality.

As I see it, the trick is to use a magnification that enlarges the image of the planet so that it is big enough to see what can be seen, but no so much that the seeing degrades the quality of the image. The amount of magnification you can "get away with" while retaining image quality will change from day-to-day, if not hour-to-hour.

Higher magnifications produce a smaller exit-pupil and this can be an advantage -- For example: a 10" used at x250 produces an exit-pupil about 1.0mm. That means that the light from the 'scope will pass trough only the very centre of the cornea where there are likely to be less aberrations in the eye itself -- hopefully making for the best image possible.

Hope this helps!

Les D
Contributing Editor
AS&T

[mbaddah (Mo)]

Originally Posted by ngcles

Hi Mbaddah & All,

In viewing Saturn which is a very small high surface-brightness object (as are all the planets) the question of exit pupil plays a minor role. Neither do you really have to dilate much --as you will see.

The telescope's ability to show detail on a planet (assuming diffraction limited or better optics) is governed by the diameter of the aperture but very much limited by the seeing at the time of observing.

The Dawes formula provides that the size of the airy disc a telescope produces in arc-seconds (R) is R=4.56/D where D is the diameter of the aperture in inches.

So for simplicity's sake, assuming a 10" 'scope, the smallest detail visible on a planet (or the moon for that matter) is about 0.46 arc-seconds across, but only in _perfect_ seeing.

The problem is that the seeing is rarely perfect, infrequently very good and commonly average or ... worse.

If you view at low power with a wide exit-pupil, all the detail that the telescope/seeing will permit to be seen is there in the image, but it is usually much too tiny to be seen comfortably. So to see it comfortably, we apply magnification. More magnification doesn't increase the resolution of the 'scope, it just makes what is there easier to see. Unfortunately, applying magnification also amplifies any imperfections in the seeing. The seeing will limit what magnification can be practically used while retaining good image quality.

As I see it, the trick is to use a magnification that enlarges the image of the planet so that it is big enough to see what can be seen, but no so much that the seeing degrades the quality of the image. The amount of magnification you can "get away with" while retaining image quality will change from day-to-day, if not hour-to-hour.

Higher magnifications produce a smaller exit-pupil and this can be an advantage -- For example: a 10" used at x250 produces an exit-pupil about 1.0mm. That means that the light from the 'scope will pass trough only the very centre of the cornea where there are likely to be less aberrations in the eye itself -- hopefully making for the best image possible.

Hope this helps!

Les D
Contributing Editor
AS&T

Thanks for that was very informative. Will the above apply for DSO as well such as globular clusters?

[ngcles]

Hi Mbaddah & All,

Mbaddah wrote:

"Will the above apply for DSO as well such as globular clusters?"


While this is not a really "vexed" question the answer is complex and something like the answer to that age old conundrum "How long is a piece of string?"

I said above that Saturn is a high-surface brightness object -- it is, but I'm now going to re-define that. Saturn is an extremely, extremely, extremely high-surface brightness object -- compared to even the highest surface brightness DSOs -- they just ain't in the same ball-park!

Perhaps the highest SB of all non-stellar DSOs are small, bright planetary nebulae (PNe) -- for example the Blue Planetary in Centaurus (NGC 3918), or the Homunculus Nebula in Eta Carinae, and they descend from there all the way to tiny (ie < 2 arc-mins) faint galaxies, faint PNe and the really difficult, ultra-faint, small, unresolved globulars etc.

This is what _I think_.

First, for DSOs that can be mostly resolved into stars (open clusters and bright globs), your first concern is to use a magnification that resolves as many stars as can be seen in your aperture, while nicely "framing" the object. If say a cluster is 30 arc-minutes diameter and assuming you want to see all of it in one go, you use a magnification that produces a field large enough to take the whole thing in, plus a frame around it (if possible) so it can be seen in context. Using the above example, I'd be thinking something like about a 45 arc-mins field or perhaps a bit bigger. My 20mm Nagler in the 18" is perfect, producing x111 and a 44 arc-min field If it is a globular you could also use a bit more magnification (ie smaller pupil) to bring out the faintest stars (for reasons below).

Second, for very large (ie super-huge) extended objects like big nebulae (Eta Carinae -- NGC 3372 or M42 or the Veil nebulae complex), the biggest galaxies (like NGC 253 or NGC 55) etc you would use all the field you can that will effectively frame it, _but_ I wouldn't exceed an exit pupil that approximately matches your fully dilated pupil. If there are bits in that extended object that you'd like to scrutinise, lift the magnification for a higher contrast view of smaller areas.

But, the bulk of Deep Sky observing (well, my Deep sky observing and I think most deep sky observer's observing anyway) are either small, very low SB objects like small faint galaxies, small faint globs or small to tiny (relatively) high surface brightness objects like many PNe.

Third, in the case of small, unresolved (ie not stellar) LSB objects, there is I feel, that there is a "magic" exit pupil that maximises the contrast between object and background sky. I believe it is a bit either side of 2.0mm. For a 10" ''scope, say _about_ x130, 12" x150 and for my 18", a bit over x200. This exit pupil range seems to darken the background while not stretching-out the little patch of fuzz so much it disappears entirely. In short it means maximum contrast. Magnification can be your friend!

If you happen to glance at some of the observing notes I post here occasionally, you will see with my 18" observing small galaxies I almost exclusively use either x185 or x247 -- which bracket that 2mm exit pupil range. That approx 2mm exit pupil is enough magnification to significantly darken the background sky, while rendering somewhat fainter stars visible, while enlarging it enough to be comfortable to see easily, while not stretching it too much to "dissolve" into the background sky. A dob of butter will only cover so much bread!

Fourth, if the object is small _and_ has good SB, like a bright planetary, you can use somewhat more magnification I regularly use x247 and x317 and sometimes x371 and x465 on PNe like NGC 1535, NGC 2392 and IC 418 with the 18" which is getting down near the 1mm size -- if the seeing allows. In say a 10" for example, this would translate to somewhere between x180 and about x260

Fifth, in the case of things like your "average" globular (lets say 6 arc-mins diameter and 8th magnitude) you are aiming for a field big enough to see it and frame it, while also using an exit pupil small enough that the faintest stars your aperture can show, can be seen. This usually translates to about 1.5mm exit-pupil or even a bit less. If you want to maximise the number of faint stars visible (or perhaps just see _some_ stars in a faint or really faint glob) go a fraction higher again.

Lastly, if you are only feeling your way, the best way to go about it is to start at low magnification and increase slowly in increments through the eyepieces you have. As you are observing and as part of this process, you will reach a point where you will say something like "I think it looked better in the last eyepiece I tried a moment ago" and you go back to that one -- and that is the best view!

Sorry the answer (well, my answer) was so long!

Best,

Les D
Contributing Editor
AS&T