Vignetting with aperture mask

[Moonshane]

Hi all

I have just created a couple more holes in my aperture mask http://stargazerslounge.com/diy-astronomer/183426-aperture-mask-16-f4-dobsonian.html

I think that painting the thing black has highlighted an issue of which I was previously unaware but presumably has always been there. There is a slight vignetting of the edge of the field when using the mask and this gets worse the smaller the hole (I have 170mm, 130mm and 100mm holes). The holes get further away from the OTA walls as they get smaller.

It's not a major issue but would be nice to cure.

It only manifests itself with my TV plossls, BGOs and my Nagler zoom (although strangely in the case of the zoom it appears on the other side of the eyepiece than the others).

It does not appear with more complex eyepieces such as Ethos, Nagler 26mm and Radians.

If I loosen the eyepiece and jiggle it, the vignetting alll but disappears.

This last point suggests to me that I may have uncovered the reason - i.e. the focuser is not quite square to the OTA. Before I start fiddling, does this make sense to anyone as a likely reason?

Cheers!

[nytecam]

Sorry to ask but what is the purpose of the aperture masks on a 16"? Have I missed something?

[Moonshane]

it reduces aperture but increases focal ratio and therefore contrast and detail is improved and extreme brightness reduced. you do lose potential resolution though with less aperture. also, the atmosphere works in 'cells' which if unsettled can created issues with larger apertures with a constantly unsteady image. when the atmosphere is very steady the full aperture works well at higher mags but a smaller aperture matches the atmosphere a little better and the images stay steadier for longer periods.

[nytecam]

Ok - I only use an aperture mask in my 12"SCT for planetary viewing in daylight with an oval off-axis mask:hello2:

[Moonshane]

interesting, I always thought that anything but round created weird shapes? or is that only with stars?

[michael.h.f.wilkinson]

An oval mask creates an oval point-spread function. It does not distort the planet, it will produce an effect not dissimilar to slight (cylindrical) astigmatism. I never use an aperture mask. In my experience, in well collimated and well-figured optics, an aperture mask reduces sharpness more than does the central obstruction (which in my C8 is comparatively big). In bad seeing an aperture mask has a certain effect of reducing the blur (because you are looking through a single seeing cell (roughly), at the expense of more motion (apparent motion caused by multiple seeing cells causes is averaged out in big scopes). On such nights I just give up o planets and go DSO hunting .