FOV vs exit pupil question.

[parallaxerr]

Hi all,

I recently bought a Celestron 32mm plossl to go in my travel scope (66mm frac) for widefield observing and have been thoroughly impressed with its performance. So much so I'm considering the 40mm for my ED80.

Having crunched the numbers I'll get almost the same exit pupil with the 40mm in my ED80 as the 32mm in my ZS66, 5.3 & 5.4mm respectively. However on FLO it states "due to the 1.25" barrel size the 32 and 40mm actually show the same area of sky".

Does this barrel restriction affect the exit pupil? I can live with the smaller FOV (relatively) as it will still frame most DSO's just fine at such low mag but want to make sure I retain the calculated exit pupil.

Cheers, Jon

[parallaxerr]

Hmm, actually just read some pretty ropey opinions on 40mm 1.25" EP's in general, with respect to the feeling of "looking down a tube". Anyone experienced this? May have to look for a 2" alternative.

[John]

As far as I understand things, the 1.25" barrel does restrict the size of the field stop that can be fitted to the eyepiece, which in turn is what defines the apparent field of view. With a 32mm eyepiece the AFoV can be as much as 52 degrees but with a 40mm, having more or less to use the same max size field stop, the AFoV reduces to 43 degrees. This can seem a bit constricted compared with a 52 degrees AFoV although it has it's uses at times as well.

With the 2" barrel fitting the above AFoV's can jump to 80 degrees for the 32mm and 72 degrees for the 40mm because a larger field stop can be accommodated. Neither of these AFoV's will seem constrained.

The exit pupil is not affected by the AFoV though because it is derived from a combination of the eyepiece focal length divided by the scope focal ratio. AFoV does not come into the calculation.

See what others come forward with as well though - I seem to be loosing the plot lately on astro stuff

 

 

[Louis D]

You got it right, John.  Another way to think about exit pupil is that it is the de-magnified image of the entrance pupil (your primary mirror or objective).  To see this, put a low power eyepiece in the focuser and pull your head back from the eyepiece and then wave your hand in front of the OTA.  You should be able to see a tiny version of your hand waving in the eyepiece.

[Stu]

53 minutes ago, parallaxerr said:

Hmm, actually just read some pretty ropey opinions on 40mm 1.25" EP's in general, with respect to the feeling of "looking down a tube". Anyone experienced this? May have to look for a 2" alternative.

I think the 40mm can have some useful applications but mainly for longer focal length Maks for instance where you are trying to get a big enough exit pupil, but there is no point going to 2" eyepieces because of limitations on the baffle tube or you only have a 1.25" diagonal.

A 2" 30 or 31mm 82 degree afov would give you a wider field of view, keeping the exit pupil the same and would be worthwhile. You are in the realm of diminishing returns after this as you run up against the same limitations with the 2" barrel. A 41mm Panoptic for instance is 68 degree afov and has a 46mm field stop vs a 42mm for the 31mm Nagler. The 41mm is about the max though, because a 55mm TV Plossl has the same 46mm field stop so shows no more sky, but at a larger exit pupil; the same situation as the 32mm vs 40mm 1.25" Plossls.

One thing which is very important to consider with these eyepieces is that they are big and heavy! In a small scope like the 66mm it can give you issues with balance so it's worth doing plenty of research before committing.

[parallaxerr]

36 minutes ago, John said:

The exit pupil is not affected by the AFoV though because it is derived from a combination of the eyepiece focal length divided by the scope focal ratio. AFoV does not come into the calculation.

That was my gut feeling John. Thanks for clarifying.

[parallaxerr]

Stu - I noticed the size of the 40mm in the pics, looks big. It'll be going in the ED80 in place of my Baader Zoom so hopefully no weight/balance issues.

I think I'll try the 32mm in the ED80 and if I'm satisfied with the FOV, take a punt on the 40mm. 

The other EP option I came up with was this from Altair...

http://www.altairastro.com/Altair-45mm-KITAKARU-RPL-Ortho-Eyepiece.html

This gives me a bigger exit pupil at 6mm and the full 62 degree FOV as I do have a 2" diagonal in the ED80. It so happens to be a red Altair diagonal too, in a red moonlite so at least it would look good! 

Price point is the only factor to consider.

[Alien 13]

I do like big exit pupils, my 32mm is the only eyepiece I use would love one 80mm or more but perhaps a shorter FL scope would be a better option.

Alan

[parallaxerr]

20 minutes ago, Alien 13 said:

I do like big exit pupils, my 32mm is the only eyepiece I use would love one 80mm or more but perhaps a shorter FL scope would be a better option.

Alan

Lol I am in the opposite position, I need a little more FL to get higher mags with good exit pupil.