Larger Exit Pupil

[Littleguy80]

Thanks @Charic! Lots of great info there. I bit of experimentation is required I think. My current plan is to alternate between my 24mm and 18mm on the same target and pay close attention to the difference in brightness in the object and the background sky. Should give an indication of how much difference I'll see with a 32mm eyepiece. The general feeling from the comments so far is that the 32mm is just going to pick up more light pollution and give little benefit. 

[Stu]

1 hour ago, Littleguy80 said:

The general feeling from the comments so far is that the 32mm is just going to pick up more light pollution and give little benefit.

That is true, unless you are using filters. With a UHC or OIII filter I think the 32mm will give you benefits, as would a 40mm despite the limited afov.

[Littleguy80]

5 minutes ago, Stu said:

That is true, unless you are using filters. With a UHC or OIII filter I think the 32mm will give you benefits, as would a 40mm despite the limited afov.

Interesting. It's definitely going to be used with my UHC/OIII filters. I'd probably stick with with ES68 24mm for anything that didn't require a filter. 

[Louis D]

I don't know about the UK, but the 32mm GSO plossl is only $36 shipped within the US from Agena Astro.  At that price, it's a pretty low risk purchase.  That's like dinner for two at a mid-quality restaurant here in the states.

[Littleguy80]

7 hours ago, Louis D said:

I don't know about the UK, but the 32mm GSO plossl is only $36 shipped within the US from Agena Astro.  At that price, it's a pretty low risk purchase.  That's like dinner for two at a mid-quality restaurant here in the states.

@FLO currently have the Celestron Omni Plossls on offer at £35 plus shipping. This is what started me thinking about getting a 32mm eyepiece. I'm sure Mrs Littleguy won't notice once less dinner out  

[Charic]

....and of the two, 40 and 32mm. the 32mm is the better choice ( their words too ).

[Littleguy80]

After much deliberation, I've eventually decided to take a chance on the 32mm Celestron Omni Plossl. Over a number of sessions, I've compared the views of this versus my 24mm ES68 24mm. They both give the same TFOV with the Celestron giving lower magnification and the desired larger exit pupil. A lot of my testing focused on the Veil. I found a great article that breaks down it's various features.

 

The original article can be found here: http://www.astronomy-mall.com/Adventures.In.Deep.Space/Dissecting the Veil Nebula.html

The main negative of the 32mm plossl is the long eye relief. It's not something I've experience with an eyepiece before and even after a few uses I'm still adjusting to it. The 24mm is much nicer to use from this respect. 

Onto the big question, does the larger exit pupil make much of a difference? Under the right conditions, yes! There have been a few times where I've used the 32mm with my OIII filter and only really felt that I was getting a brighter image. Last night, when viewing the Eastern Veil, the image actually felt clearer. I got a much better sense of the shape and the finer strands of the nebula. It just looked more wispy and was recognisable as the nebula from the pictures, minus the colour of course. 

The increased brightness was also a help with the North American Nebula which was more easily seen with the 32mm. I had a couple of attempts at the Crescent Nebula with it too. On one occasion I had a sense of "something" being there but that's the best I could I say about it. 

Overall, it's been a worthwhile experiment and I'll be holding onto the 32mm eyepiece. For use without an OIII/UHC filter I would always go for the ES68 24mm though. It's a better quality eyepiece and the eye relief is more comfortable. The takeaway though is that a larger exit pupil for OIII/UHC filters through a small aperture scope does make a difference...for me at least!

[cloudsweeper]

Good move, Neil.  30 or 32mm is a very useful FL, giving more exit pupil, but generally not too much.  This is good for fainter objects, especially with low light pollution.  (Otherwise, the view can be a bit washed out.)

I don't find the eye relief problematic until I use 40 or 50mm EPs.  It's the other end of the scale that is awkward for specs wearers - anything under say 15mm.

Doug.

[Louis D]

1 hour ago, Littleguy80 said:

I've eventually decided to take a chance on the 32mm Celestron Ultima Plossl.

Where did you find such a rare beast as a 32mm Celestron Ultima Plossl?  I was only aware of the 30mm and 35mm versions as shown in this eyepiece family lineup.  I'd be happy to locate a 30mm version, let alone a 32mm version.

[Littleguy80]

1 minute ago, Louis D said:

Where did you find such a rare beast as a 32mm Celestron Ultima Plossl?  I was only aware of the 30mm and 35mm versions as shown in this eyepiece family lineup.  I'd be happy to locate a 30mm version, let alone a 32mm version.

ermmm... When I said Ultima, I meant Omni! Silly me! Sorry for the confusion!

[Littleguy80]

30 minutes ago, cloudsweeper said:

Good move, Neil.  30 or 32mm is a very useful FL, giving more exit pupil, but generally not too much.  This is good for fainter objects, especially with low light pollution.  (Otherwise, the view can be a bit washed out.)

I don't find the eye relief problematic until I use 40 or 50mm EPs.  It's the other end of the scale that is awkward for specs wearers - anything under say 15mm.

Doug.

Thanks Doug. I've only tried under some moderate light pollution with filters so far. Will be interesting to try it under proper dark skies unfiltered.

I'm fortunate enough to not wear glasses. The eye relief is pretty tight on my 5mm Ortho but I soon got used to that. No doubt, I'll soon adjust to the longer eye relief of the 32mm

[cloudsweeper]

7 minutes ago, Littleguy80 said:

ermmm... When I said Ultima, I meant Omni! Silly me! Sorry for the confusion!

Neil - the Omnis are very nice.  I had the 40mm, but now all my long FL EPs are 2" for the extra FOV!

Doug.

[Louis D]

6 minutes ago, cloudsweeper said:

Neil - the Omnis are very nice.  I had the 40mm, but now all my long FL EPs are 2" for the extra FOV!

Doug.

I have found I like 32mm plossls in my binoviewer to maximize exit pupil (within reason) despite the obvious vignetting due to the 23mm clear aperture.  In cyclops mode, I use only 2" eyepieces at 17mm and above.  Replacing my 17mm ES-92 with a 32mm plossl is almost a shock to the system.  It's hard to fathom anyone living with such a narrow apparent field of view in monovision.

[Littleguy80]

Sadly I don't have the option of 2" eyepieces. I've never used a 2" eyepiece either so I do have the advantage of not knowing what I'm missing out on

[jetstream]

3 hours ago, Littleguy80 said:

Onto the big question, does the larger exit pupil make much of a difference? Under the right conditions, yes!

Congrats Neil!

Like you, I prefer larger exit pupils for some things and the Veil is one of them. Once you get a scope that will take 2" eyepieces you might see another gain with a larger apparent field and the larger true field. I do see a difference between them, a noticeable difference.

A note on exit pupils, a nit pickers note really...central obstruction technically alters the exit pupil a bit- basically we observe with a scope of higher f ratio and smaller exit pupils a squeak (reduced aperture).

Are your skies in the 20.5-21 mag range where you observe as in the report?

[Littleguy80]

4 minutes ago, jetstream said:

Congrats Neil!

Like you, I prefer larger exit pupils for some things and the Veil is one of them. Once you get a scope that will take 2" eyepieces you might see another gain with a larger apparent field and the larger true field. I do see a difference between them, a noticeable difference.

A note on exit pupils, a nit pickers note really...central obstruction technically alters the exit pupil a bit- basically we observe with a scope of higher f ratio and smaller exit pupils a squeak (reduced aperture).

Are your skies in the 20.5-21 mag range where you observe as in the report?

Thanks Gerry. 

I was a bit worried as to whether it would prove a waste of money but I figured there's only one way to find out!

Interesting point on central obstruction. It did take me a minute or two to work that one through in my mind before I understood! 

I've estimated my skies at home to be somewhere between NELM 5 and 5.5 on a  good night. I think that puts them in the 18-19 region. 

[jetstream]

If you are like me the NELM can be hard to nail down accurately and I find I underestimate a bit. I have observed in mag 20 skies and the Crescent wasn't seen by me so the fact you are getting hints of it suggest a VG eye, well matched equipment and a bit darker sky than first thought possibly. That object is a super test of conditions. Again congrats for the fine observing.

Once you get to 21+mag skies you might need to sit down after viewing the Veil... all this work you are doing will pay off big time Neil.

If anyones interested the Crescent nebula is easily visible in a Heritage 130mm f5 scope.

[Littleguy80]

33 minutes ago, jetstream said:

If you are like me the NELM can be hard to nail down accurately and I find I underestimate a bit. I have observed in mag 20 skies and the Crescent wasn't seen by me so the fact you are getting hints of it suggest a VG eye, well matched equipment and a bit darker sky than first thought possibly. That object is a super test of conditions. Again congrats for the fine observing.

Once you get to 21+mag skies you might need to sit down after viewing the Veil... all this work you are doing will pay off big time Neil.

If anyones interested the Crescent nebula is easily visible in a Heritage 130mm f5 scope.

Thanks Gerry. I really appreciate that. It's always tempting to think that the only answer is more aperture but I really feel like I've overcome that. Not to say I wouldn't like to upgrade at some point but my little SkyWatcher Explorer 130M is serving me well right now!

My local astronomy club has an open night on a Friday so I'm just waiting for a clear Friday night so I can out to their dark site for some observing. As you say, Gerry, I might need to sit down and have a drink after that  

[Special K]

Congratulations on your purchase and the strategy of boosting the surface brightness with a bigger exit EP, and then filtering that down is a good approach!  

Surface Brightness = pupil exit squared, multiplied by 2. The human eye at 7mm is pulling nearly 100%, but as mentioned, age will decrease this and if light pollution is strong you won't get near 7 so aperture is being nullified. Therefore an EP providing 5mm and thus 50% is a good compromise. But even then we crave contrast so is helpful to go even lower to suit conditions.  To your original question, how much is the extra 1mm giving, it is quite a bit!  

[25585]

On 16/08/2017 at 15:43, 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

What size objectives did the scopes have?

[Alien 13]

55 minutes ago, 25585 said:

What size objectives did the scopes have?

Had a couple, objectives about 55 and 75 mm, I did have a rifle scope that had an 80 mm objective and X 5 mag.

Alan

[jetstream]

On 9/16/2017 at 15:50, Special K said:

so aperture is being nullified

The question is: what does aperture really do?

Some say it only plays a role in the needed or desired object size at a certain illumination?

 

[cloudsweeper]

1 hour ago, jetstream said:

The question is: what does aperture really do?

Some say it only plays a role in the needed or desired object size at a certain illumination?

 

Yes - amongst other things, greater aperture gives more mag for a given exit pupil, so an extended object of a given surface brightness will be larger in the EP.  Some say this makes it appear brighter or, at least, more noticeable.  (Others have other ideas.....an interesting topic!)

Doug.

[Alien 13]

It would be interesting to find out the role of a big objective when the exit pupil might be 20 mm or so, my thoughts are that all of the objective is still used for example you could have a big hole in any part or the middle like a Mak and still produce a complete image.

Alan

[Stu]

9 minutes ago, Alien 13 said:

It would be interesting to find out the role of a big objective when the exit pupil might be 20 mm or so, my thoughts are that all of the objective is still used for example you could have a big hole in any part or the middle like a Mak and still produce a complete image.

Alan

Along similar lines, is the exit pupil is larger than your pupil, do you just lose brightness or resolution too? Are you still seeing all the mirror or not?