28 replies to this topic

[PEterW]

Glenn, are you offering to grind me a 12" F2 with an angle grinder and some sandpaper? Amateurs unless experienced are likely to be very bad grinding job and professional opticians hate to be told to do a "bad job"..... my optician friend at work always tries to get crazy smooth and flat finish when all I want is a "vaguely shiny" finish....

Cheers

Peter

[GlennLeDrew]

Peter,

Hardy har!

 

Actually, the grinding part is child's play. It's the figuring after polishing out where the optician earns his pay. But a system dedicated to *prime focus* work need not be finished to the degree of spherical correction required for small exit pupil visual work. As long as the departure from nominal on the surface has no locally strong slope, i.e., has a more gradual deformation with no pronounced zones, the p-v wavefront error could approach a full wave. The most important region to keep under better control is the outermost edge, because even a fairly narrow annulus here occupies a surprising fraction of the working area.

[Jeff Morgan]

The Pronto was the obvious winner, showing much higher brightness on M31.  At first look it appeared that the star fields were identical.  However, without much trouble, I was able to locate a nearby star that was barely (but clearly) visible in the Pronto that could not be seen at all in the dob.

 

This would be a good post for Refractors Forum. Or Reflectors. Or Eyepieces.

[t_image]

Since this thread never got off the ground with star magnitude tests,

maybe all psyched out from thought experiments,

and naysaying......

I decided to try it with the Sony a7s and a target easy to determine star magnitudes....

 

FWIW, A link for any future readers:

video demonstration

http://www.cloudynig...magnitude-test/

Cheers!