Aperture vs Exit Pupil

[PeterC65]

I have a 127mm MAK and recently added a 72mm APO (432mm FL) , partly for the wider field of view (which it certainly provides), partly for the sharper images (which is noticeable), and partly to get bigger exit pupil size from any given eyepiece.

With my ES 68 degree 24mm eyepiece for example, I get a 2mm exit pupil with the MAK but 4mm with the APO, and in general the APO gives twice the exit pupil size of the MAK with the same eyepiece.

I was hoping that the increased exit pupil size might allow me to see fainter galaxies (I can only really see M31, M81 and M82 with the MAK) and make better use of my UHC filter to see fainter nebulae (I can only really see M42 and M27).

So far I've only used the APO a couple of times but if anything most objects appear less bright with this scope than with the MAK and so it seems that the smaller aperture of the APO is reducing brightness despite the bigger exit pupil size.

What is the experience of others?

Can I expect an improvement in object brightness from the APO due to the bigger exit pupil size or is the reduced magnification taking away any theoretical advantage so that in practice it is all about aperture?

 

[Nik271]

To observe faint DSO you need BOTH large exit pupil and some magnification, precisely how much magnification depending on the size of the object.

Your APO  with a 24mm EP will give you magnification of about 18 which is basically a binocular class magnification. WIth a wide field of view it is ideal for open clusters (Pleaides must look wonderful in it) and the large DSO: Andromeda and Triangulum galaxies, The Pinwheel galaxy M101,  M42, the Veil nebula, North America nebula and so on. 

The sky brightness is very important as well: large exit pupil makes everything brighter including the sky background. So if you have significant light pollution large exit pupil does not help on its own.

The same EP with the Mak on the other hand gives you about x60 magnification, much better for the smaller galaxies. 2mm exit pupil is actually sufficient to see them, personally I consider 2-3mm exit pupil the ideal for small DSO observing.  I have observed many galaxies with my 127Mak e.g. M81,M82, M51 in Bortle 5 sky as well as many planetary nebulae.  This is not to say that 4mm exit pupil is useless, not at all, one big advantage to it  is that you can put narrowband filters to improve the contrast of some nebulae and still have bright image. The Veil nebula is a prime example where an OIII filter does a very good job with a 4mm exit pupil. I recenlty observed it with binoculars (20x80 so again giving 4mm exit pupil like your scope) with a filter inserted in the eyecup.

I hope this helps. There is a lot of discussion of image brightness and exit pupil in some recent threads in the observing section.

 

 

Edited January 8, 2022 by Nik271

[drjolo]

Observing faint objects is a little bit complex process that is performed by eyes and brain in the same time. Ability to spot low surface brightness objects depends also on its angular size. The larger the object is with the same surface brightness, the easier to observe. 

In your case, enlarging the exit pupil does not make galaxies easier to observe, because they have now lower angular size. Plus 72mm APO has significantly smaller aperture than 127mm MAK, and basically you collect less light. Telescope aperture is the most important thing for observing faint targets. Once you enlarge aperture, you may play with eyepieces, powers and exit pupils. 

You may check this book for more details: Human Vision and The Night Sky: How to Improve Your Observing Skills,  Michael P. Borgia

Edited January 8, 2022 by drjolo

[NGC 1502]

For me as a visual only observer objects look brighter when using a larger aperture at the same magnification. I think that’s simply because a larger aperture gathers more light.

[John]

The 127 mak-cassegrain (which I think has a working aperture of 120mm) will be capturing nearly 2.8x as much light as the 72mm refractor does. That will make more of an impact on the views of DSO's than the larger exit pupil does I think.

 

[PeterC65]

12 minutes ago, Nik271 said:

Pleaides must look wonderful in it

M45 does indeed look magnificent through the APO, so much so that I may not bother observing it with the MAK in future, although the MAK does show more nebulosity around the fewer stars that fit in to its FoV.

15 minutes ago, Nik271 said:

This is not to say that 4mm exit pupil is useless, not at all, one big advantage to it  is that you can put narrowband filters to improve the contrast of some nebulae and still have bright image. The Veil nebula is a prime example where an OIII filter does a very good job with a 4mm exit pupil. I recenlty observed it with binoculars (20x80 so again giving 4mm exit pupil like your scope) with a filter inserted in the eyecup.

I have Bortle 4 skies here. The Veil nebula is one of the objects I am targeting with the APO but unfortunately it's behind trees at the moment. I do have a UHC filter which significantly improves nebulosity, so what you say about observing the Veil with binoculars makes me hopeful of seeing it when it comes back in to play.

[Nik271]

This is the (long) thread I remember which discusses the issue of exit pupil, aperture and magnification when observing DSOs:

 

[PeterC65]

2 hours ago, Nik271 said:

This is the (long) thread I remember which discusses the issue of exit pupil, aperture and magnification when observing DSOs:

Useful thread. I think this article - How Telescope Aperture Affects The View - linked in the thread, explains things very well.

It seems that magnification is the issue. You need higher magnification to see fainter objects (so as to make them bigger), and to get that, and maintain the same brightness (same exit pupil size), you need more aperture.

So the 72mm APO should be good for larger faint objects.

 

Edited January 8, 2022 by PeterC65

[Zermelo]

Yes, contrast with the background sky is important for extended, faint objects. I'm also B4 and I was surprised quite how much difference a small change in magnification made, especially when seen in a zoom EP.

[SuburbanMak]

Despite many enjoyable distractions, my main observing project is to track down all the Messier objects with a Mak 127 so although I don't know much about exit pupil vs aperture,  I can add something practical on what this OTA can do on galaxy targets in the field. 

So far I've seen 30 galaxies with the Mak of which 24 are Messiers.  It took me a number of sessions to get my eye in after the obvious M31.   M81 & 82 were next and then after multiple attempts I picked up the Leo Triplet.  From there I did a good deal of prep of star hops in Stellarium to identify the various smudges in Markarian's chain/Virgo  (observing notes below in case you find them useful).   I also did a couple of recce sessions with an ST80 to familiarise myself with "mini-asterisms" in the starfields local to my targets on nights when the moon was up. 

The biggest things I have learned are: 

That a darker site makes a HUGE difference to the Mak's capabilties (solid Bortle 4 vs. light Bortle 5).  

Getting your eye in to picking up "suspicious" smudges in the 9x50 finderscope improves session by session and is definitely a knack - one I am having to re-acquire after a spell of looking at brighter targets. 

Lots of these galaxies have been little more than fuzzy stars with the almost-five inch Mak, but I have seen good structure in a few particularly on the darkest, most transparent nights - notably M51, M99, M100 & M63 stand out in my notes.   

The biggest learning is that it takes patience at the eyepiece but the Mak is much more than just a planetary/lunar tool. 

One final thought, a brilliant resource for us Mak 127 users is Phillip Pugh's eBook in the Patrick Moore series "Observing the Messier Objects with a Small Telescope" - Pugh uses exactly the scope we have and his notes on how to find and what to expect are great, he doesn't beat about the bush on the difference between which objects can yield decent views vs. as he describes it "one to tick off before you die". 

Clear skies! 

 

 

 

 

Edited January 8, 2022 by SuburbanMak
Grammar!