40mm eyepiece with 12" Flextube?

[gfletcher]

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.

Graham

[NGC 1502]

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.

[John]

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.

[citaylor]

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.

[Moonshane]

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.

[michael.h.f.wilkinson]

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 , and of course in the C8, where it gives me 1.36 deg FOV at an exit pupil of 4.