What's best 16" f4 masked to 6.7" f11 or 6" f11 Newt??

[angusb1]

Is the conclusion that an on-axis unobstructed scope would probably give the best performance then ?.

Aren't they called refractors ?

Sounds like we have found a great compromise. If you want to view DSOs leave your big reflector unmasked, if you want to view planets or the moon with it, use an aperture mask. Or buy a refractor but that won't give you the performance on DSOs that an unmasked reflector will. Versatility vs portability.

I still think I'm going to need at least 2 scopes

[michael.h.f.wilkinson]

I agree Michael that in principle, if you wait long enough in poorer conditions or have excellent seeing conditions then even a fast 10" scope of similar optical quality could probably beat my 'contraption'. That said the seeing is rarely good enough to make this the case in the UK at least and the difference really was astonishing. I am looking forward to further trials in due course.

Have you tried less dramatic masking? Using a 14" mask eliminates the edge of the mirror. If the figure is slightly off there, that would at least in part explain matters.

[Peter Drew]

Don't forget that an off-axis portion of a mirror is no longer a figure of revolution. Imagine an old 78 record, if you masked off a circular portion the grooves would not be circular, this is one of the small limitations of this pratice. Another is the shape of the parabola which falls off from the sphere as it reaches the outer areas. As for trepanning smaller off-axis mirrors from one large one, think of the stresses that would be caused as well as one edge being thicker than the other.

[Moonshane]

well, having just taken a punt on an f6 (I think) 6" newt, http://stargazerslounge.com/members-equipment-gallery/167602-you-cant-always-get-what-you-want.html I'll be able to make a pretty thorough comparison of all four types of set up shortly. I'll report back when I get a chance.