Jump to content


40mm eyepiece with 12" Flextube?

Recommended Posts

Hi guys

I am wondering whether a 40mm focal length eyepiece would work with my 12" flextube dob.

Or will i get the kidney bean effect?

Has anyone any opinion as to the longest focal length eyepiece that could be used with this scope.

I currently have a 32mm Panaview eyepiece.


Link to post
Share on other sites

Hi Graham. I think the 12" Flextubes are about 1500mm focal length, so a 40mm EP

would give I nice low 38x. However that would produce an 8mm exit pupil, larger than

most peoples dilated pupil. In a light polluted area you would get a washed out

low contrast view. You may perhaps start to see the shadow of the secondary, but

I've found that I have to get to very low power to get that effect, a 40mm EP in my

6" F4 gives 15x and a 10mm exit pupil, and I can see the the secondary shadow.

It's hard to say what's best, maybe you already have it with the 32mm ?

But some use a very low power wide field to find objects rather than view them, so

the quality of that view is not so important to them.

HTH, Ed.

Edited by NGC 1502
Link to post
Share on other sites

I agree with Ed - 30-32mm is the longest focal length worth using with F/5 scopes if you want to get the benefits of all your aperture.

I guess that's why eyepieces like the 28mm UWAN and 31mm Nagler were designed.

Link to post
Share on other sites

I have a 12.5" Dob and an Antares 40mm that came in a set. It works fine, but I do find that it is a little pointless - I think your 32mm would probably be more useful.

Link to post
Share on other sites

agree with the general comments as above. in fact I sold my 35mm Panoptic as this gave a somewhat washed out view in my 12" f5.3 dob and put the funds towards a 26mm Nagler. I doubt I'd use anything lower power in an f5 scope or faster - I am leaning the other way if anything.

thankfully the paracorr I have committed to buy will bring the focal ratio of my f4 scope to f4.6 and therefore the exit pupil on the 26mm will be 5.6mm so not too bad.

Link to post
Share on other sites

I find a 40mm gives washed-out views in my F/6 APO. I still use it as a "finder-scope" with its 5.67 deg field of view :D, and of course in the C8, where it gives me 1.36 deg FOV at an exit pupil of 4.

Edited by michael.h.f.wilkinson
Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    No registered users viewing this page.

  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.