Larger Exit Pupil

[Littleguy80]

I keep pondering a longer focal length eyepiece to get a larger exit pupil for viewing large diffuse nebulae through a UHC/OIII filter. Currently I have an ES68 24mm giving an exit pupil of 3.4mm in my scope. Options are a 32mm Celestron Omin Plossl (4.6mm exit pupil) or a 30mm Vixen NPL (4.3mm exit pupil). So to manage my expectations, how much difference does an additional 1mm exit pupil make to the brightness of the image? 

[John]

My own experience is that shorter focal length eyepieces work better for me than longer ones. Last night I started out using the 31mm Nagler on DSO's and put an O-III filter on it to view the Veil and others. The exit pupil with my F/5.3 12" dob is 5.85mm. The views looked good but when I switched to the 21mm Ethos and an 3.96mm exit pupil I found the nebulae seemed better defiined and the background sky darker. I'll hesitate to say that the nebulae were "brighter" but they seemed more distinct and stood out agains the background sky better to my eye. This is not just my experience last night but is what I regularly encounter and why my 31 Nagler gets less used than my 21 Ethos.

Under darker skies than I get here I seem to recall that the difference was less defined between the 2 exit pupils.

I realise that does not really answer your question and it's only one persons experience so it will be interesting to see what views come forward from other observers

 

[Ricochet]

It isn't really about increasing the exit pupil diameter but the area of the exit pupil. I think a 32mm -24mm difference should be perceived as the same as the 24mm - 18mm difference. 

[Littleguy80]

3 minutes ago, John said:

Under darker skies than I get here I seem to recall that the difference was less defined between the 2 exit pupils.

Thanks John. Very interesting observations. I guess when light pollution is a factor then it's a trade off that everything gets brighter with the larger exit pupil, including the LP! Maybe there's little to be gained because you just get more of the "bad" light to filter as well as more light from the DSO?

[Littleguy80]

3 minutes ago, Ricochet said:

It isn't really about increasing the exit pupil diameter but the area of the exit pupil. I think a 32mm -24mm difference should be perceived as the same as the 24mm - 18mm difference. 

That's useful. I'll try a comparison of my 24mm and 18mm to see how much difference it makes

[John]

There was an interesting thread on here a while back on this topic:

 

[scarp15]

Engaging with a substantial large exit pupil is quite a specialist observational pursuit. For example I use a 31mm nagler with my 14" dob, 5.84mm / 6.7mm exit pupil with and without paracorr.  A target such as NGC 1435  Merope Nebula, a diffuse reflection nebula found in the Pleiades will benefit from such a low power, wide field, large exit pupil configuration. Another circumstance would be for filter use, OIII and H-beta filters upon large and faint diffuse nebulae. In each case a dark transparent sky and fully dark adapted pupils are a necessity. For moderately light polluted back gardens neither these targets or such a large exit pupil would necessarily become applicable.

[Alien 13]

In my early days of astronomy I was often under pitch black sky's (1960s) with ex military spotting scopes with exit pupils well into double figures and even as high as 25 mm and the views where marvelous, I do miss the large objectives with single figure magnifications, I assume they cant be made today for some reason.

Alan

[jetstream]

1 hour ago, Littleguy80 said:

an exit pupil of 3.4mm in my scope.

 This is my minimum exit pupil for objects like the Veil etc using 100 deg EP's. I also find that widefield (82 deg) and hyperwides (100 deg) seem to offer more "contrast" (yeah I know its the wrong use of the word) than 50 deg EP's -possibly through engaging more of the eyes receptors. In other words my 32mm plossl goes unused for this type of observing.

For many an exit pupil of around 5.5mm is very good- under truly dark skies.

Your own eye plays a big role here too... honestly I think that 3.4mm exit pupil is small for what you want to observe but I would go with 2" eyepieces to get larger ones. If your scope doesn't take 2" I'd buy one that does down the road. Just my thoughts,Gerry

[Littleguy80]

7 minutes ago, Alien 13 said:

In my early days of astronomy I was often under pitch black sky's (1960s) with ex military spotting scopes with exit pupils well into double figures and even as high as 25 mm and the views where marvelous, I do miss the large objectives with single figure magnifications, I assume they cant be made today for some reason.

Alan

Wow! My understanding was that there wasn't any benefit to an exit pupil larger than 7mm as that's bigger than your actual pupil? That's not based on any great knowledge of the subject though!

[Littleguy80]

1 minute ago, jetstream said:

 This is my minimum exit pupil for objects like the Veil etc using 100 deg EP's. I also find that widefield (82 deg) and hyperwides (100 deg) seem to offer more "contrast" (yeah I know its the wrong use of the word) than 50 deg EP's -possibly through engaging more of the eyes receptors. In other words my 32mm plossl goes unused for this type of observing.

For many an exit pupil of around 5.5mm is very good- under truly dark skies.

Your own eye plays a big role here too... honestly I think that 3.4mm exit pupil is small for what you want to observe but I would go with 2" eyepieces to get larger ones. If your scope doesn't take 2" I'd buy one that does down the road. Just my thoughts,Gerry

Thanks Gerry. I don't have the option of a 2" eyepiece with my scope. Maybe I'll just put those pennies in the "New Scope" piggy bank  

[Alien 13]

8 minutes ago, Littleguy80 said:

Wow! My understanding was that there wasn't any benefit to an exit pupil larger than 7mm as that's bigger than your actual pupil? That's not based on any great knowledge of the subject though!

It probably has more to do with easier eye placement, a lot of rifle scopes have magnifications around 2-6 and some can have objectives over 80mm.

Alan

[cloudsweeper]

8 minutes ago, Littleguy80 said:

Wow! My understanding was that there wasn't any benefit to an exit pupil larger than 7mm as that's bigger than your actual pupil? That's not based on any great knowledge of the subject though!

That's about it, Neil, although the figure depends on your eyes and your age.  If it's too large, you effectively waste light, but - I believe - you can still enjoy wider field views.  The other thing about large exit pupils is that the background has not dimmed much, so the view can be washed out, especially if you're looking at faint objects.

Doug.

[jetstream]

1 minute ago, cloudsweeper said:

If it's too large, you effectively waste light, but - I believe -

It's more complicated than this IMHO.

After hearing Alans (Alien13) experiences a while back I tried super large exit pupils with success on a few objects, but in general I stick to a 5.5mm maximum for general dark sky use. The goal here is to help members get into a reasonable "ball park" in terms of exit pupils and we can explore other options once experience is gained.

[cloudsweeper]

4 minutes ago, jetstream said:

After hearing Alans (Alien13) experiences a while back I tried super large exit pupils with success on a few objects, but in general I stick to a 5.5mm maximum for general dark sky use. The goal here is to help members get into a reasonable "ball park" in terms of exit pupils and we can explore other options once experience is gained.

You can indeed use large values.  I just wanted to point out the possible pitfalls of doing so.

Doug.

[Mark at Beaufort]

From experience I have found that an exit pupil between 4 and 5 seems to best meet my requirements which is why I start with my 20mm Myraid in my 12" Dob (exit pupil 4.01). I have used this website to determine what is best for me - http://www.stargazing.net/naa/scopemath.htm

I have a 30mm EP which is great for star hopping but for me it washes out the sky too much.

[jetstream]

7 minutes ago, cloudsweeper said:

You can indeed use large values.  I just wanted to point out the possible pitfalls of doing so.

Thanks for this Doug, great point.

In general I do not use super large exit pupils and have bought a scope (200mm f3.8) that brings along many benefits for specialized nebula observing, including the avoidance of the super large exit pupil (ie 42mm LVW/standard reflectors) to increase the TFOV ...

[alan potts]

I often use the 41mm Panoptic in scopes giving an EP of 8 plus, I know it wastes an amount of light, in effect making the scope a smaller aperture as the light cannot enter your eye, even though I am told mine are a young 61. The neighbourhood owl enjoys it though.

Alan 

[Alfian]

50 minutes ago, cloudsweeper said:

The other thing about large exit pupils is that the background has not dimmed much, so the view can be washed out, especially if you're looking at faint objects.

I think this is the main factor for me that generally makes me stick to exits of 5mm or less. As a binocular fan used to wider fields of view I quickly found years ago that when trying to emulate the low power/wide field with my 'scope,  the washed out grey views were not compelling.

[Alien 13]

This can be a complicated subject, I dont know the maths but an over sized EP does waste light but not necessarily the resolution advantage of a bigger objective lens on the scope etc.

Alan

[alan potts]

53 minutes ago, Alien 13 said:

This can be a complicated subject, I dont know the maths but an over sized EP does waste light but not necessarily the resolution advantage of a bigger objective lens on the scope etc.

Alan

Alan,

I believe the only reason light is wasted is because it cannot enter the eye. I am lead to believe a healthy 21 yo can dilate to about 7mm and sometimes a bit more, even though my friend who is a eye Doctor tells me mine are young for my age only needing a very weak reading glasses, she also tells me mine will dilate to 6mm at very best and probably less.

i know when using refractor you do not have an obstruction in the light path, this can become an issue on reflectors at 8mm ExPu and above. If I use my 35mm Pan on my 190mm F5.26 it is fine and that's giving close to 7.0mm but the 41mm at 8 plus shows a marked secondary mirror, you can still use it though it is a bit annoying. With the moon out the way I have dark skies and no doubt that helps alot. All that said I do enjoy putting the 41mm in the 805mm F7 refractor and using x20 and 4 degrees of field.

Alan 

[John]

I bought a 55mm 2" eyepiece a few years ago wistfully thinking that it would show me huge tracts of space, the whole of M31, etc, etc. Well the field was indeed as wide as the 2" barrel would allow but with the F/6 scopes I had back then any DSO's were totally washed out and the background sky a pale orange / grey colour

I think it was about then that I started to save up for a 31mm Nagler .....

My widest true field now is 3.83 degrees when I use the Nagler 31 with my F/6.5 Vixen ED 102. The exit pupil is a reasonably sensible 4.77mm.

 

[cloudsweeper]

7 minutes ago, John said:

I bought a 55mm 2" eyepiece a few years ago wistfully thinking that it would show me huge tracts of space, the whole of M31, etc, etc. Well the field was indeed as wide as the 2" barrel would allow but with the F/6 scopes I had back then any DSO's were totally washed out and the background sky a pale orange / grey colour

I think it was about then that I started to save up for a 31mm Nagler .....

My widest true field now is 3.83 degrees when I use the Nagler 31 with my F/6.5 Vixen ED 102. The exit pupil is a reasonably sensible 4.77mm.

 

How true John.  All this exit pupil stuff makes a strong case for a nice slow SCT!!

Doug.

[Louis D]

I do find I like the views of nebula through 32mm plossls in my binoviewer better than with 26mm and shorter eyepieces despite the severe vignetting caused by the 23mm clear aperture of the binos.  It condenses the light into a smaller patch making it stand out better.  In mono-mode, not so much.  I think it's a one eye vs. two eye thing.

[Charic]

On 16/08/2017 at 14:27, Littleguy80 said:

I keep pondering a longer focal length eyepiece to get a larger exit pupil for viewing large diffuse nebulae

Not sure its just about having a larger exit pupil, more to do with having the right exit pupil?

The real advantage with a longer  focal length eyepiece is the lower magnification, and wider field of view.
This view, the 'true field'  is controlled solely by the telescopes focal length and the field stop of the eyepiece in use.
The field stop is important, if you can determine what size it is? I have seen  two references where the field stop sizes are recorded for our information, Tele Vue being one company. Now at something like 27mm your at the physical limit for a 1.1/4" eyepiece, so no matter how long the focal length of the eyepiece, any eyepiece with a field stop this size, in 1 1/4 fitting will provide the widest true field. To go bigger, you could jump to 2" eyepieces with larger field stops.

From comments already supplied, you'll see that the shorter length eyepieces can for some folk  show an improvement. In fact two different  focal length eyepieces, with the same  common field stop should produce better contrast  using the shorter focal length, given the same field of view! Why? Magnification! Magnification has the effect of darkening the sky, making for a better contrast between the two, target & Sky.

So far, I have mentioned Field Stop's, Magnification and  focal lengths for telescope and eyepiece, but as yet, no exit pupils!

Its possible to have too large an exit pupil, so trying to understanding some of the reasons and theory beforehand is quite important, but there's huge amounts of info on the web! Some folk may already own telescopes that are  specially designed for wide field views, with their much shorter focal length scopes, and using for example an eyepiece of say  50mm could end up with exit pupils in excess of 10+ mm which exceeds the reference  of 7mm to shame!

Having an exit pupil that is larger than your dilated eye will give a nice bright provided by the system,  but your 5mm eye (for example ) can only use some of that exit, not the full amount, so effectively reducing the 'aperture' and reducing the performance of what your trying to achieve in the first place!

One way to choose the perfect fit exit pupil, without understanding all the theory is to select the focal ratio of the telescope in use and multiply that by the size of your dilated eye ( it can be measured with a rule, or ask an optician ) Its a good guideline, but can be exceeds by a few mm either side of the mark. My perfect fit would be 29.7mm? I chose and use a 32mm 70° to good effect. If I use something longer, especially on a reflector, then there are limits at which I'll starting seeing the secondary obscuring the image and my floaters!