Exit pupils

[GavStar]

I've got an 85mm refractor and have been enjoying the my first views of Saturn recently.

However the maximum magnification I've been using is 133x and I'd like a bit more, say 170x.

Problem is at over 40 (just!) I am seeing some floaters with an exit pupil of 0.64mm. If I used 170x magnification I'd have an exit pupil of 0.5mm which I think may be too small. (When I tried 133x magnification during the day the view looked like a bit of a spiders web - I was quite shocked)

What minimum exit pupil do people normally use comfortably?

thanks

Gavin

[John]

I find I can use down to .53mm without seeing floaters. Thats a 4mm eyepiece with my ED120 refractor giving 225x. When I use the 3.5mm eyepiece in the same scope (EP .47mm) I do start to see them against a bright, extended background eg: the lunar surface. I've tried a 2.5mm eyepiece which gave an exit pupil of .38mm in my ED102 refractor but the floaters were much more pronounced to the point of being very distracting.

[Laurie61]

Hi gavin, floaters are not too bad for me at 1mm E.P and above. This is one of those areas where people have there own preferences and will depend on what they feel happy with but about 0.5mm is about the minimum. This might be a good excuse for you to get a bigger scope

[GavStar]

Yes, a new scope could be beckoning, currently looking at 100mm fracs to give me the extra magnification. I think more than that woud be too heavy for what I want.

[umadog]

I have a lot of floaters and so prefer to be above 1 mm to keep them at bay.

[Schorhr]

Interesting, does one have a link to floaters regarding exit pupil? I notice them frequently during the day, but never in the telescope (neither with 130/650 & 2.5mm eyepiece nor with the 102/1300 & 3-4mm eyepiece).

Does the type of eyepiece matter? I think I did notice when using a 4mm Plössl. The eyepieces I use now are HR Planetaries with comfortable eye relief.

[bingevader]

Hmmm, interesting.

My floaters are either in the way or they aren't!

I hadn't realised exit pupil made a difference.

Can some one enlighten me as to the effect please?

Cheers

[acey]

The exit pupil is effectively a small aperture that you're looking through, and if it's too small then diffraction disrupts the view. Try looking through a pinhole with a diameter of half a millimetre or less to see the effect. This places a limitation on the maximum magnification that can be used for a given aperture, since exit pupil = aperture divided by magnification. (Exit pupil has nothing to do with the particular type of eyepiece used, only the magnification and aperture, or more properly "entrance pupil").

The traditional rule of thumb (devised by 19th century telescope makers) was to have a maximum magnification of 50 times the aperture in inches, in other words an exit pupil of 1/50-th of an inch, or 0.5mm. (The metric equivalent is 2 times aperture in millimetres). But everyone's eyes are different - some people prefer not to go below a millimetre, others can go below a half. You've just got to see for yourself what your own limit is.

The other limiting factors are the atmosphere, target, telescope quality and tracking ability. With my 12" dob the highest power I use is a 4mm eyepiece giving an exit pupil of 0.8mm. That's a magnification of x375, and any higher would be hard to track, even if the atmosphere were steady enough to make it worth while.

[bingevader]

I get the exit pupil bit, sorry, maybe I didn't make myself clear.

What is the effect of exit pupil on floaters?

As I said, mine are either in the way or not on a particular night, what EP I use doesn't really matter.

Cheers

[acey]

Diffraction spreads light from a narrow beam into a disc. A disc of light can cast a shadow. Small exit pupil creates diffraction, which creates shadows of floaters on the retina.

[bingevader]

Thank you!

It always delights me that you can learn something new every day!

My floaters can be pretty bad at times, but I've yet to see shadows.

In the not too distant future, I'm hoping to get a 5mm EP that'll take me down to .8mm eye relief, it'll be interesting to see if this shadowing effect then appears.

Cheers

[Schorhr]

Strange that I never noted this before.

So eye relieve does not affect it at all?

I just tried 2.5 & 3.2mm HR Planetary and 4mm Plössl (as those are the closest eyepieces I have that have different eye relieve) on the small 76/300.

I can't say that it was worse in the 2.5mm HR or 3.2mm HR (0.6-0.8mm exit pupil), subjectively I would say in the 4mm (1mm exit pupil) it's just as bad if not more.

I tried against the cloudy evening sky and the brickwall next door. I will check the next time I'm actually observing with the actual telescope :-)

The only problem is to get a good view through the 4mm Plössl at all

I can't believe I regularly used that. Okey, it's available for €8/7gbp, while the erfle and planetary eyepieces cost 30-45€/25-38gbp (but offer more then 16mm eye relieve and almost 60deg afov)...

[acey]

Diffraction spreads light from a narrow beam into a disc. A disc of light can cast a shadow. Small exit pupil creates diffraction, which creates shadows of floaters on the retina.

Actually I'm wondering now if I've stated this correctly. As an experiment, I just tried looking at the daylight sky and distant trees through a pinhole (fraction of a millimetre across) in a piece of paper. This is analogous to the effect of using an eyepiece with a sub-millimetre exit pupil.

First noticeable thing is how dim the image looks, second is the presence of diffraction (i.e. dark marks rather than a completely clear field) which makes things look blurry. Third thing is noticeable floaters.

The first two are sufficient reason not to use too small an exit pupil. As to the third, I think it's simply the case that when you've got such a small spot of light landing on the eye to create an image, anything in the eye of a similar size scale (i.e. passing floaters) is going to be very noticeable, since it intrudes on a large proportion of the light being received.

Try the experiment and you'll see what I mean.

[CSM]

Floaters - hmm, another reason to move to imaging.

[LeeB]

my main eye is the left for detail and sods law says that's the one with the floaters, I notice them often in the day especially when its bright, for the same reasons as above they disappear if I wear sunglasses and the iris opens a bit. At night I never see them above a 1mm exit pupil and I try and limit myself to 250x with the 10 for that reason. if its gets necessary I swap eyes. but no matter how I train the right eye it never seems as easy to see detail as the left.

[bingevader]

Floaters - hmm, another reason to move to imaging.

Pah, We laugh in the face of our floaters!

Fascinating! Just spent a funny five mins staring out the window through a tiny hole in a piece of paper!

Yep darker, yep fuzzier, yep more definition to the floaters. I say more definition, because mine are there all the time, I just try not to focus on them! However, through the hole, they are brighter and better defined, crazy!

Maybe I'll try a 5mm EP before buying one. Although, there won't really be any difference to my observing, if they're in the way at the moment they spoil my observing, if not I just try to ignore them.

My reason for the EP would primary be luna observation, so my thinking is that the floaters would obscure a proportionately smaller area of the moon's surface at greater magnification!

Cheers

[umadog]

Diffraction is present in all pupil sizes, though. Diffraction only starts to do crazy stuff when when the aperture diameter approaches the wavelength of light. We're orders of magnitude away from that here. Diffraction can't be significantly worse at smaller pupil sizes, because if it was your vision would deteriorate in bright light.

I think the reason you see more floaters with a smaller exit pupil is simply because the floaters are a fixed size and as the pupil diameter becomes smaller the floaters can block proportionally more of the light. So when they stray into your field of view there is proportionally more darkening. When the pupil diameter becomes really small, you're starting to create a pinhole camera and get sufficient resolution to see shadows of the floaters on the retina.

When you look through the pinhole during the day (you can make one right away with your fingertips) the centre of the field becomes sharper but the field edges dim rapidly. Even though I have -4 myopia, I can see relatively clearly without glasses when looking through a pinhole. The problem is the edge darkening, which I don't think is due to diffraction. I think it's because off-axis light doesn't get through pinhole because it's at the wrong angle. I suspect the very gradual drop off in illumination is because the pinhole is out of focus (so you're seeing a blurry edge).