From: "Dan Chaffee" 
To: 
Reply-To: "Dan Chaffee" 


>I'm not sure how I'll get on star-testing with a full sized,
>but un-coated secondary, but will try this next. I still
>have a hankering to chase the figure round a bit more, to
>polish the sleeks out, and to try and improve the surface
>roughness a bit. (Anyone going to talk me out of it?)

Andy,

Firstly, don't bother with star testing an uncoated primary with an
uncoated diagonal. The image is too dim for any meaningful analysis
whatsoever.

Second, from the looks of the images you may have full correction in the
outer 12mm or so; maybe too much. The mirror really has too much dog
biscuit to hope  further paraboloizing will clean it up. I would return to
a sphere and get that surface creamy smooth. Don't worry about sleeks or a
few scratches; they are *nothing* compared to the overall figure. I don't
know what your lap's pitch temperment is, but it may be too hard to produce
a smooth surface due to not achieving full contact. Maybe press more often,
use slower strokes, ease off the pressure--all these tactics could be
beneficial given we don't see how one another works. Personally, I've
tended to advocate getting the best, ultra-smooth sphere one can before
forcing a paraboloid out of it. The few times I've tried to parabolize a
slightly rough surface in the hope of ironing it out in the figuring
process have yielded unacceptable results.

Dan Chaffee

--- BBBS/NT v4.00 MP