Eyepiece question

[gooseholla]

Hi,

I have a 32mm 2 inch eyepiece for my scope. On my 12 inch this gives a magnification of 50x. On my old scopes, a 32mm would have given 30 - 35x magnification. Now, do I need any focal length below a 32mm? Would a 38mm be too close to the magnification of a 32? A 40 - 50 mm out of the question due to exit pupil? what do people think? Will 50x be a good magnification to do the functions that were originally done at 32x magnification e.g. finding objects, nebulas, viewing large chunks of the night sky, milky way viewing, etc.?

[SwampStar]

Wish I could help, but it's a bit technical for me at the moment.

Would anything on this page help?

http://www.rocketmime.com/astronomy/Telescope/telescope_eqn.html

[John]

Your scope must be an F/5.3 I reckon. As a low power finder eyepiece a 38mm / 40mm might be OK but I would not go as far as a 50mm - the exit pupil would be way oversized.

When you go to a larger aperture you tend to get additional focal length too and therefore your eyepieces give higher magnifications and show smaller true fields of view.

With the relatively fast focal ratios of newtonians using long focal length eyepieces does give this oversize exit pupil. The solution is to use a 32mm eyepiece with the largest field of view possible - which is what Tele Vue were aiming to provide when they produced the 31mm Nagler. The downside is that such eyepieces have high price tags attached to them.

The trick is to find the right compromise that suits you !

[gooseholla]

I have a 32mm 2" which I reckon will give 1.6 degree view, and 50x magnification. I suspect a 38mm might be too close and a 42mm would be of a benefit in terms of sky area shown and TFOV in a perfect world. But was just checking. Think I might try a c. 40mm at some point just to see what it is like before splashing out.

[Capricorn]

Basically the new FoV is 1 degree now, the old one would have been 1.5 degrees.

You are down to 2/3 of what it was.

If you are using it on the Dobsonian then finding will be a little more difficult assuming you are not using a finder scope initially.

Your decision.

[gooseholla]

It only has a Telrad on it!

[coulthamst]

I find ( with a 200mm f6 dob) that going to lower magnification than a 32mm eyepiece gives becomes compromised by a less-black background sky, giving reducd contrast and a less pleasurable viewing experience. This may well be due to the limiting magnitude of my inner-city observing site (mag 4 on a good night) but I would be interested in the findings of others.