Can an exit pupil be too large?

[hunterknox]

I mean, I get the basic idea: if your exit pupil is larger than your actual pupil then you're spreading the light out to a greater extent than you can take in all at once and extended objects will appear slightly dimmer than with an optimised exit pupil but...

...isn't the plus side that a larger exit pupil is more like a window than a peep hole? Move your eye a bit and you're still taking in the vista. There's something to be said for that, surely?

[crashtestdummy]

the eyepiece becomes uncomfortable to use and isn't any fun.you have to look left and right inside the barell to see bits of image rather than a nice full field of view.try and loan an eyepiece at a meet up rather than spending your money on something you probably wont like

[rowan46]

at a dark site I don't find exit pupil a problem, but I only get to a really dark site about once a year. in my normal ight polluted urban skies a big exit pupil gives me grey splodges against a grey sky. It may be a porthole but it robs you of so much contrast especially when you are fighting for every bit you can get. So I would say under very dark skies its not much of an issue but under anything less than very dark you can have too large an exit pupil

[Uplooker]

Hi Tom,

I get what your saying but unfortunate that is not the case. If the exit pupil is too large you lose some of the instrument's incoming light i.e. you are not seeing all of the picture in its true brightness. this is especially important for most of the astronomical objects that we look at as they are generally quite dim (moon being the exception).  Think of it this way, why would you go for the largest aperture that you can attain / afford (for the purpose of grasping the maximum amount of light) only to reduce the amount of light that you are able to see due to some of the light not making it through your pupil to your retina.

Ian

[John]

I agree with Ian's assessment 

[nicks90]

hmmmm. i can see your point - but as the owner of an ep with possibly too large an exit pupil - it isnt that much of an issue tbh.

in my sw 12" dob I use a 38mm SW 2" panaview. Exit pupil is 7.7mm which is probably 1mm wider than my eye can go when fully dark adapted, But at such low powers and wide FoV it is not really an observing ep, its my finder ep and it does that job fantastically.

I have used it for an extended time on M45 and M31 to try and get 'everything in' and it was still a bloomin lovely experience. However i do understand what Crash says about it being tiring. Not eye strain type thing, but you do move your eyeball about a fair bit when using it. But its not often you need something with such a mahooosive FoV and most times its in there for 60 seconds whilst i find the object and then swap in a more appropriate ep.

So yes, for a proper observing ep, i would say you can have too large an exit pupil. But it shouldnt put you off getting a low power ep for finding stuff!

[bingevader]

I agree with John's assessment.

A bit like stopping down your telescope ( I did try it on the moon the other night because my daughter said it was bright, we weren't impressed).

The only proviso being that if the conditions are good enough, your site dark enough and the object bright enough, then it's horses for courses I suppose.

Cheers

[bambuko]

One thing none of you chaps mention is the type of telescope - depending on whether it is reflector or refractor the answer will be substantially different.

"...With refractors larger pupils do waste aperture. But the magnification is so low that the wasted aperture is of little concern: both image brightness and
resolution are as great as possible at that magnification. With reflectors, however, larger pupils do waste light, but primarily because the black spot in the
pupil caused by the secondary obstruction becomes larger. Both light loss and field shadowing occur with reflectors, but as with refractors there is no
resolution loss because of the low power.

...there is no practical limit to the low magnification that can be used with a refractor. But the secondary obstruction found on most reflectors does set limits,
because the shadow spot it forms in the exit pupil grows as the magnification is reduced..."

Quoted from: TeleVue.com: Advice Article List > Telescope Formulas, Common Telescope Myths

[swamp thing]

Think of it this way, why would you go for the largest aperture that you can attain / afford (for the purpose of grasping the maximum amount of light) only to reduce the amount of light that you are able to see due to some of the light not making it through your pupil to your retina.

Ian

Good point Ian, but as we get to bigger apertures it's often a case of we need a finder eyepiece. Yes we lose some light, but it's no real hardship. Eg I'd be quite happy to use a 41mm Panoptic in my f/4 Dob if hunting very faint galaxies. Using the 20" even at reduced aperture is still a country mile better than any finder scope. Admittedly once the target is found one would get up to a sensible exit pupil size before attempting any real observing though.

So my take is: for a finder eyepiece in a large scope there is no harm in an oversized exit pupil, but for observing, no I'd stick to the accepted boundaries. In smaller scopes it's more of a case of does one really need a finder eyepiece, the FOV of smaller scopes is much bigger.

[scarp15]

Good point Ian, but as we get to bigger apertures it's often a case of we need a finder eyepiece. Yes we lose some light, but it's no real hardship. Eg I'd be quite happy to use a 41mm Panoptic in my f/4 Dob if hunting very faint galaxies. Using the 20" even at reduced aperture is still a country mile better than any finder scope. Admittedly once the target is found one would get up to a sensible exit pupil size before attempting any real observing though.

So my take is: for a finder eyepiece in a large scope there is no harm in an oversized exit pupil, but for observing, no I'd stick to the accepted boundaries. In smaller scopes it's more of a case of does one really need a finder eyepiece, the FOV of smaller scopes is much bigger.

Until recently I owned a 35mm panoptic. At a dark location its primary function was as a finder eyepiece. With such a large exit pupil on my then 12" and now 14" dobsonian, it did leak a bit of light. However it was also very satisfying on larger brighter objects and also large objects such as the Veil, when used with an OIII filter. For mostly anything else, whilst it functioned very well as a finder, I would step up to a 20mm or 16mm eyepiece.

[Stu]

I tend to be of the opinion that if something works, then don't worry to much about the theory behind it. As a example, if I wanted to fit a large object into the field of view, and that required me to have an oversized exit pupil, I would be more bothered about the framing of the object than a bit of lost light.

I believe this to be a valid point when under dark skies, however under light polluted skies, exit pupil does make a big difference to sky background, and therefore the object contrast. I used to have a 41mm Panoptic, which gave a 6.3mm exit pupil in my 106mm frac. Under dark skies it was lovely, but from my normal observing sites it was horribly washed out and did not get enough use so I sold it. 31mm is now my lowest f/l ep and with its smaller exit pupil it is far more useable, with bit that much smaller a field of view.

I think the earlier comments about the different effects on refractors vs reflectors are well made and worth considering, particularly the part about secondary shadows.

Cheers,

Stu

[hunterknox]

OK then, follow-up question: have any of our older members ever sold a favourite wide field eyepiece because they felt they couldn't take full advantage any more?

[Scooot]

I can't remember

[scarp15]

Taking into account the comments made, if, at least some of your observing sessions take place at dark sky locations, a wide field low power (with large exit pupil) eyepiece has numerous advantages in framing large or displaced (i.e. M81, M82) objects, works well with an OIII filter such as on the Veil, Orion Nebula etc and is effective as a finder.

In my circumstances, I initially purchased the 35mm to work with my F10 scope in which it performs well (at dark locations). However when I started to use dobsonians, it received  much more use as a low power finder ep. Personally, I sold mine as I moved from an F5 to an F4.5 scope in which I felt there would be better option eyepieces at low power to take full advantage of the light grasp, even though I only use the scope at dark locations. However I feel that this is not (in my case) necessarily a compelling case for change, more of a curiosity to try something else (yet untried), as I was quite happy with what I had.

[Pig]

I don't think this guy has a problem  But his ears do get cold.