Exit pupil

[NessieHunter]

Reading a thread on here I see mention of exit pupil and theoretical limitations on eyepieces. I read the sticky at the top but didn't see any mention of this. How do you calculate the exit pupil and just as importantly, what should it ideally be. I have the Skymax pro 150 with an fl of 1800 and f12. Having only 2 eyepieces at the moment, the stock 28mm 2" and a meade 12mm 1.25" I am looking to expand this selection. Does the exit pupil change depending on whether it is 2" or not? I know the mak is not ideal for deep sky but would like to get something lower powered for a maximum FOV but would need to know the limitation.

Many thanks

Neil

[ollypenrice]

First calculate the magnification of the EP-telescope combination (focal length of 'scope divided by focal length of EP) and then divide the aperture of the 'scope by the magnification in order to calculate the exit pupil, which is the diameter of the ray of light leaving the EP. Since the best a young, dark adapted observer can hope for in terms of pupil aperture is about 7mm an exit puil larger than this will not fit pass without loss into his or her eye. A figure more like 5mm will apply to those of us less fortunate with our eyesight!

Olly

[haitch]

The easiest way to get round it is to go for wider angle FOV EPs rather than longer focal length - there is a trade off though as there is a direct correlation between FOV and depth of pockets.

In any case I've got a couple of EPs that go over my maximum in my dob but I still find the nice wide field views good even though I know I'm wasting light.

[perrin6]

I've read that older people's (50ish plus) iris's don't enlarge more than about 5mm. So for my scope that's 335/5 = lowest mag. of X67 (otherwise you lose light),

Alan

[Macavity]

With a MAK150, you will always be below ~2.5 - 3mm with the exit pupil, so under most (imaginable) conditions "safe". Exit pupil is mostly a limit for short focal length refractors or maybe reflectors with large secondary mirrors.

I don't know what the CLAIMED apparent field of the stock 28mm 2" is really. I do know that e.g. a 32mm 2" eyepiece with ~72 degree apparent field of view will work well visually in this scope. These are relatively common, and a "safe bet" (re. vignetting etc.) with your type of scope. Wider field or longer focal length will usually be more specialised, more expensive and MAY start to vignette.

Default eyepieces rarely push the limits. With a decent 32mm 72 deg eyepiece, you should get a field of around 1.2-1.3 deg TRUE field on the sky. This would be big enough for the majority of brighter DSOs - Moreover and usefully, just that BIT greater than the 1.0/1.1 deg limit, for the scope, imposed by 1.25" format etc.

[NessieHunter]

Thanks Olly, Haitch,

If I understand this correctly and not being as young as I used to be, I should probably be aiming at a 5mm exit pupil to maximise the available light. With an aperture of 150mm and a minimum mag of 150/5 = 30x, the limit of the eypiece would be 1800/30 = 60mm? In which case I could use pretty much anything, if my maths is correct.

Neil

[NessieHunter]

Thanks Alan and Chris, I didn't realise that it wasn't an issue so much with f12 scopes so probably not really worth worrying about in this case.

Many thanks

Neil

[haitch]

Oh yes, as a member of the dob brigade I forget it's not much of an issue with most folded scopes.

[Merlin66]

Guys,

The exit pupil has nothing to do with the apparent field of view or wide angle eyepieces etc etc other than the possible effect on eye relief ( the distance the exit pupil sits behind the eyepiece)

If you do use a wide angle eyepiece then you may experience "kidney bean" drop out of the FOV due to the steep cone of light to the exit pupil.

The size of the exit pupil is always the aperture/ magnification.

[haitch]

Guys,

The exit pupil has nothing to do with the apparent field of view or wide angle eyepieces etc etc other than the possible effect on eye relief ( the distance the exit pupil sits behind the eyepiece)

If you do use a wide angle eyepiece then you may experience "kidney bean" drop out of the FOV due to the steep cone of light to the exit pupil.

The size of the exit pupil is always the aperture/ magnification.

Hi Merlin,

I think you might have misunderstood my comment. I was just pointing out that if you don't want to go for a longer f/l because you reach your exit pupil limit you can see a larger piece of sky with a wider FOV EP of the same focal length.

[brianb]

If you do use a wide angle eyepiece then you may experience "kidney bean" drop out of the FOV due to the steep cone of light to the exit pupil.

The size of the exit pupil is always the aperture/ magnification.

Indeed - but an oversize exit pupil, as well as spilling light, also makes the central obstruction (in reflectors & catadioptric scopes) much more of an issue, this can cause an effect similar to "kidney beaning" as well. Personally I find 4mm exit pupil big enough with an SCT.

With refractors you can use a magnification so low that the exit pupil is bigger than the eye's pupil size, but you don't get any benefit from doing so - if you really need a wider field at lower magnification, a smaller scope with a shorter focal length is a better idea (yes, you do need more than one instrument!)

[Macavity]

I think the problem I mostly find with "exit pupils" is their small size RELATIVE to the vast expanse of eye lens. In short, I tend to loose track of the bloomin' thing in the dark. <G> Moving from (shorter) Hyperions to (TMB/HR) planetaries seems to be a positive start... Who knows where it will end - Orthoscopics?

[gustavo_sanchez]

Maybe this is a dumb question, but what exactly is exit pupil?

[Merlin66]

Exit pupil - Wikipedia, the free encyclopedia

This is a circle in space behind the eyepiece where all the rays from the objective, going through the eyepiece, converge.

The distance between the eyepiece and the exit pupil is called the eye relief.

To see as much of the image as possible the eye should be positioned at the exit pupil.

[Space Beagle]

Here's a useful site for calculating all this kind of stuff without having to use fingers and toes!

N.A.A. Telescope Math Calculator

Doc