Masking

[bish]

Hello all,

I've read a few people on this forum who mask their dobs to make their scope slower for planetary viewing. My 10" dob has given me some excellent views of the planets but wondered whether it would be worth having a go at masking the aperture down. Would it be worth while using a 10" f4.7 (f1200mm) dob, or is it more for larger scopes. If I mask to a 6" scope then it would be f8. and slower and the aperture would be alot smaller than the 10".

Cheers

[Moonshane]

hi Bish

you really need to mask between the vanes avoiding the secondary; the point of masking is 1) and slower focal ratio but also 2) an unobstructed view.

I'd think with a 10" scope you'd get about a 100mm aperture at f12.

[Moonshane]

ps

try it with a piece of cardboard first no loss of cost and you'll see how it works.

[Mike Hawtin]

If you use a centrally placed mask then you will degrade the image quality due to the proportional increase in secondary obstruction as well as causing a reduction in resolution from using a smaller aperture. If you use an off axis mask you will gain in image contrast due to the absence of diffraction effects but will lose resolution again due to the smaller aperture and will not see a benefit from a longer focal ratio as the off axis angle of the primary's light cone remains unchanged . The apparent improvement in image quality using masks is just that, "apparent", no different than using the full aperture with a lower magnification that doesn't show the effects of the atmospheric "seeing".

Mike

[bish]

Thanks for your replies. I was also thinking about masking down and using a solar filter but am a bit cautious about DIY solar filters and also might be better off getting a small frac and solar filter for this purpose.

[ollypenrice]

If you use a centrally placed mask then you will degrade the image quality due to the proportional increase in secondary obstruction as well as causing a reduction in resolution from using a smaller aperture. If you use an off axis mask you will gain in image contrast due to the absence of diffraction effects but will lose resolution again due to the smaller aperture and will not see a benefit from a longer focal ratio as the off axis angle of the primary's light cone remains unchanged . The apparent improvement in image quality using masks is just that, "apparent", no different than using the full aperture with a lower magnification that doesn't show the effects of the atmospheric "seeing".

Mike

I agree with the first part of this but I'm not sure about the second. If the seeing is not that good might you not expect fewer distortions from a narrower incident beam? I stress that I don't know.

What about a mask with four holes placed to avoid the spider? Would you not expect similar resolution to the full beam without the diff spikes and without the excess of light? Wouldn't this be a humble interferometer?

Olly

[John]

I would have thought a mask with 4 small holes the equivalent of having a larger central obstruction and thicker secondary supports ?

[Mike Hawtin]

Olly, I have tried it out myself in the past and the results were pretty underwhelming. Four off axis masks did produce a cleaner looking image but it was markedly fainter and the resolution far less than the full aperture, no interferometer effect that I can recall. Not surprising really as the mask blocked about half the incoming light and image formed from the combined beams would have been subject to the same atmospheric distortion as the full aperture even without taking the loss of resolution due to the smaller apertures into account.

Mike