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Minimum useful exit pupil


Paz

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When thinking about what eyepiece focal lengths to get, the longest worth getting is limited by the size of exit pupil you can fit through your own pupil.

But at the other end of the spectrum I understand that 0.5mm is considered to be the smallest useful exit pupil below which you get problems with image quality, seeing the floaters in your eyes, etc.

Is the 0.5mm minimum exit pupil rule fairly standard for everyone or does it vary quite a lot in practice like the maximum useful exit pupil size does?

What is the smallest exit pupil others use in practice - and on what kinds of targets?

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The smallest that I use is an exit pupil of .4mm. This is given by a 3mm eyepiece used with my F/7.5 120mm refractor. The magnification given is 300x and I use this mostly to split tight binary stars and occasionally on the Moon. I would also try it on Mars when it's at opposition later this year.

I do see some floaters when observing the Moon with this exit pupil.

 

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It depends at least on the telescope features, seeing and floaters. With my TV60 which is optically very good, but also very short, I reach 0.3mm exit pupil (180x). I also used ~0.2mm (206x) and the only real loss was resolution. Generally, I would say that it is can be worth using very short exit pupils to split very tight doubles (very tight for the telescope), particularly if these have different colours. In this case, despite the loss of optical resolution, the eye can still resolve the different colours, and so, you still have a gain in resolution somehow. 

Ah, forgot to say, I don't have serious problems with floaters. 

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On extended objects like Jupiter, half a millimetre is the limit for me. That's when I definitely see diffraction effects from the aperture. These tend to blur the details on the object so that higher magnifications do not help to see more.

On the Moon's terminator, I sometimes go a little higher in magnification. The strong contrast of the views helps to save the image quality down to exit pupils of about 0.4 mm.

To identify very close double stars I can imagine a benefit of even smaller exit pupils. The two diffraction patterns may overlap and the Airy disks may merge, but as long as the combined Airy disk is at least elongated a double star is identified as such.

In general, the most pleasing views I get with exit pupils of about 0.7 mm or larger.

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I am pretty much like the others in the 0.4 area, I do find floaters though vary from session to session, some nights I see them, other nights with the same equipment I won't. I have even seen them long before getting to EP of 0.5  sometimes. I use a cleaning fluid to clean my eyepieces on a regular basis but whether this actually changes anything I do don't really know, I have to say though the use of it is nothing to do with an attempt to remove them.

Alan

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I tend to stick to around 1mm as I find this most rewarding in brightness, detail (often determined by seeing anyway) and reducing adverse effects from floaters.

This isn't saying that I don't use and find useful exit pupils down to 0.3mm on the moon and doubles. I guess at the end of the day as with much in astronomy it is down to personal preference as although I can use 0.3mm it is not a preference of mine.

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If seeing allows .5mm with one of the smaller scopes, I put that one away, get a bigger one out and observe at a higher exit pupil/longer focal length ie 10" f4.8. In those conditions optimizing aperture and exit pupil can provide amazing results.

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About 2 mm is my limit I have never understood the upper limit though views through a Military spotting scope with an exit pupil of 14 mm looked brilliant without any of the hovering eye contortions trying to get the image in view.

Alan

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1 hour ago, Alien 13 said:

About 2 mm is my limit I have never understood the upper limit though views through a Military spotting scope with an exit pupil of 14 mm looked brilliant without any of the hovering eye contortions trying to get the image in view.

Alan

The upper limit is only a guideline related to the maximum dilated diameter of the pupil. If the exit pupil is larger than that not all the light will make it into your eye. 

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3 minutes ago, John said:

The upper limit is only a guideline related to the maximum dilated diameter of the pupil. If the exit pupil is larger than that not all the light will make it into your eye. 

Do you not also get problems with the shadow of the secondary above a certain level John? Obviously with refractors it is pretty irrelevant so long as the sky background remains dark enough.

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8 minutes ago, Stu said:

Do you not also get problems with the shadow of the secondary above a certain level John? Obviously with refractors it is pretty irrelevant so long as the sky background remains dark enough.

Yes you can start to see the secondary shadow with obstructed scopes.

I prefer to stick with reasonable size exit pupils even with refractors - I assume that my pupil does not dilate any more with one type of scope than it does with another ?

I've tried 50mm 2" plossls and erfles with scopes that I've owned and I've never been very impressed with the results even when the sky is dark.

 

 

 

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4 minutes ago, John said:

I prefer to stick with reasonable size exit pupils even with refractors - I assume that my pupil does not dilate any more with one type of scope than it does with another ?

 

 

No, of course there's no reason it should.  My point was just that if you want to frame an object in a refractor and it means the exit pupil is too big then there's no reason not to, whereas in a reflector the secondary shadow issue is there.

Having said that, I've never been under a sky dark enough to use an exit pupil that big without being totally washed out, so your point is probably the more valid of the two ?

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I nearly bought a 52mm Antares erfle that was on the Astroboot a few days back. It was priced at £25 !. But then I thought, though it's dead cheap, I'm going to get the same results that I did when I had a Vixen 50mm plossl a couple of years back - lots of sky but rather washed out DSO's. Someone else got that bargain.

 

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I figure if I enjoy the view what does it matter that most the light is being wasted as long as it didn't cost the earth that is. That said I get washed out skies at 28mm so I'd think the skies here would resemble more the surface of the sun in white light if I tried a 52mm in my f/5  :D:D

I've always said that what may work for one person doesn't necessarily work for another. Astronomy has so many variables to it not only in selection of equipment but also in peoples locations and personal preferences that it is difficult to get a straight answer when asking some questions. The best approach often seems to be try before you buy at star parties or local clubs. Failing that the next best thing is to buy used at a good price and if it doesn't work out sell it on, often for little or no loss.

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Thanks for the replies - it looks like there is no hard and fast rule. I've done the maths on my observing and I don't generally go much under about 0.7mm at the moment. I've got scope/eyepiece combinations that give the option to go to 0.4mm or even 0.2mm but both give very poor views.

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