Miniumum exit pupil ?

[SAW]

Hi,

What's the minimum exit pupil I should roughly be looking to go down too ? I don't wear glasses but I doubt my eyes I like they were when I was 20 ! Looking to get a 6mm or 9mm Tak Abbe, depending on what scope I select in astronomy tools it's saying anything from 0.44 to 0.90mm exit pupil. Is 0.44 going to be usable ?

Thanks.

[cloudsweeper]

As I understand it, an exit pupil of 0.5mm corresponds to the Resolution Limit.  But since the RL is essentially a scientific convention, it is possible to go lower to say 0.4mm (approx.) for example when trying to split doubles.  Seeing and eyesight also become limiting factors, and "floaters" become more noticeable.

(The phenomenon is governed by aperture.)

Doug.

[Stu]

Is this in the Tak Simon?

I have an annoying floater in my observing eye, so tend to use binoviewers at high powers in smaller scopes these days. I do find that 0.5mm is about as low as I go though, and do quite regularly use a 3 to 6mm Nagler zoom in the Tak for high power. I tend not to go much below 4mm, or sometimes 3.5mm which is just around 0.5mm. For doubles it is fine at the highest powers but for planetary and lunar the floaters get in the way.

[JeremyS]

+1 for the NZ 3-6. I use in both my Tak FC76 and FS102. You can tune the mag (and exit pupil) to the object and conditions. I find it a bit easier to use than my 4, 5 and 6 mm Orthos (which are very nice eyepieces)

[SAW]

Thanks. 

So might get the 9mm Abbe and maybe add the 3-6mm Nagler later on ?

[Buzzard75]

As has already been discussed, smaller exit pupil means you're more likely to start seeing floaters. The light is being concentrated into a smaller and smaller area. It will get so small that at some point you'll start seeing these floaters actually blocking the light path. I start seeing them when the exit pupil is less than 1mm. Others can go down to 0.5mm. A lot of it depends on the age and condition of your eyes so it's all going to be different from person to person.

On the other end of the spectrum, if the exit pupil is too large, you're likely to notice a darkening in the center due to the obstruction of the secondary mirror when using a reflector telescope. Using a refractor and having a large exit pupil is essentially the same as reducing the effective aperture of the objective lens. What that exit pupil size is and how large it can be before you start to notice its effects is going to differ from person to person and scope to scope of course. General rule seems to be about 7mm being the maximum for most people. Some people's pupils can dilate larger.

[darkskyastronomy]

It depends on the objects you are looking at. Double stars can take a lot, 0.4mm exit pupil. Planetary nebula can also take really high magnifications. With bright objects, like Jupiter, you will be limited to smaller exit pupils around 0.75mm.

 

[Rob Sellent]

Just adding to what the others have already said, the smallest exit pupil depends on a number of factors, notably, seeing, the object being observed, the quality and stability of the telescope and the visual acuity of the observer.  As exit pupil decreases one reduces contrast, dims the view and magnifies the imperfections in the optics, the atmosphere and the eye. 

My own comfortable minimum for bright objects like Jupiter or the Moon is around 0.6mm/0.5mm. After that I have trouble blinking away floaters. However, there is no rule to this. Every eye is different. I have seen sketches from very experienced observers  viewing at around 0.2mm exit pupil . Bionic eyes is an understatement.

Double stars are the exception. You can really crank up the magnification because essentially at that stage you're just splitting pixels