Help me choose an eyepiece?

[DanielC]

Hi everyone.

I'm trying to pick a wide field eyepiece and I'm looking at these two:

• 32mm Meade Series 4000 Super Plossl 1.25" Eyepiece, AFOV = 52.0°
• 40mm Meade Series 4000 Super Plossl 1.25" Eyepiece, AFOV = 44.0°

I plugged both of these in the Field of View Calculator, paired with a Celestron NexStar SE6, and they seem to have a near identical field of view:

 

So... what's the difference between a 32mm with a 52° AFOV and a 40mm with a 44° AFOV?  If they cover the same real field of view on the sky, wouldn't the 32mm with a 52° AFOV appear larger when you look through the eyepiece? I thought that a 40mm would always look more zoomed in than a 32mm but maybe it's not that simple. Can someone clarify this for me? And also, in this example why would anyone choose the 40mm over the 32mm? It looks worse in every way.

Thanks for the help!

[Kyle Allen]

A 32mm Plossl with a 52 degree AFOV will show the maximum area of sky possible in a 1.25 inch format. Longer focal lengths will yield less magnification but won’t show any more sky, unless you move to the bigger 2 inch format.

The larger AFOV will be more comfortable to look through and feel less restricted even though you are seeing the same area of sky.

The only reason I can think of for going for the 40mm would be to get a bigger exit pupil in a very slow scope like a Maksutov that is limited to 1.25 inch eyepieces. 

[DanielC]

14 minutes ago, Kyle Allen said:

A 32mm Plossl with a 52 degree AFOV will show the maximum area of sky possible in a 1.25 inch format. Longer focal lengths will yield less magnification but won’t show any more sky, unless you move to the bigger 2 inch format.

The larger AFOV will be more comfortable to look through and feel less restricted even though you are seeing the same area of sky.

The only reason I can think of for going for the 40mm would be to get a bigger exit pupil in a very slow scope like a Maksutov that is limited to 1.25 inch eyepieces. 

Thanks!

[Alfian]

 I appreciate you have narrowed your selection to the 32 or 40mm plossl but an alternative would be a 24mm 68 degree eyepiece such as the Explore Scientific 24mm as linked or if you can find one a second hand ES Maxvision 24mm which is essentially very similar but in a different casing. (I think the Meade SWA 5000 24mm is similar).  The 24mm will obviously give higher magnification a smaller exit pupil but as near as matters the same field of view which is as mentioned the largest you will get in the 1.25" format. 

I have both a decent 32mm Plossl (Revelation/GSO) and the ES Maxvision 24mm and I find the latter better to use and it is optically very nice. Its also reasonably well corrected in terms of pairing it with a fast-ish scope such as an F5. Definitely more money than a 32mm plossl but a good investment - especially if you can find one second hand.

[Louis D]

If you pair the 40mm with a nebula filter such as an OIII, the larger exit pupil may make the nebula appear brighter than it would with the 32mm.  This is especially important with longer focal ratio telescope such as CATs that produce small exit pupils with most eyepieces.  For example, to get a 6mm exit pupil with an f/15 Mak would require a 90mm eyepiece. 😲  Good luck finding one of those.

[DanielC]

12 hours ago, Louis D said:

If you pair the 40mm with a nebula filter such as an OIII, the larger exit pupil may make the nebula appear brighter than it would with the 32mm.  This is especially important with longer focal ratio telescope such as CATs that produce small exit pupils with most eyepieces.  For example, to get a 6mm exit pupil with an f/15 Mak would require a 90mm eyepiece. 😲  Good luck finding one of those.

Thanks. Can you help me understand this? For the SCT that I'm planning to buy, the focal ratio is F/10 so the exit pupil is 

pupil = (focal length of eyepiece) / 10

So the 40mm eyepiece would give me a 4mm exit pupil and the 32mm eyepiece would give me a 3.2mm exit pupil. So I googled for "exit pupil" and I saw that apparently a larger exit pupil means that there's more light going into your eye. But I don't understand that. If both eyepieces are already at the maximum field of view that the telescope can produce, aren't they both giving me every last photon that the telescope can collect?

Thanks!

[Louis D]

1 hour ago, DanielC said:

Thanks. Can you help me understand this? For the SCT that I'm planning to buy, the focal ratio is F/10 so the exit pupil is 

pupil = (focal length of eyepiece) / 10

So the 40mm eyepiece would give me a 4mm exit pupil and the 32mm eyepiece would give me a 3.2mm exit pupil. So I googled for "exit pupil" and I saw that apparently a larger exit pupil means that there's more light going into your eye. But I don't understand that. If both eyepieces are already at the maximum field of view that the telescope can produce, aren't they both giving me every last photon that the telescope can collect?

Thanks!

Correct about the total light flux, but the lower powered eyepiece is putting that same number of photons into a smaller area leading to a brighter image per unit area, albeit at a lower magnification.  Besides a smaller, bright object image, the background skyglow will also be brighter, washing out low surface brightness objects unless that skyglow can be reduced via a selective filter.

[MarsG76]

Which ever eyepiece focal length you decide on, have a look at the televue eyepieces... the 31mm Nagler Type-5 is a nice wide EP and the 11mm Nagler Type 6 is more close up but are 82° FOV and the Ethos 17mm is 100° FOV, so eventhough it magnifies more, you still have the wide view, just that the object is bigger.. actually you can't see the edges on that EP unless you really try and look for it... these eyepiece along with a 2" Powermate are what I use 99% of the time and nothing else (in my arsenal o'oculars) comes close to the views delivered by my Televue kit.... They're a bit more expensive but IMO they're worth it.