[ATM] Intro and slitless tester question

[gfphoto1 at earthlink.net (Gary Fuchs)]

Hello all,

I've been browsing the archives (and read the FAQs) and haven't found any "intro" messages, so my apologies if this is inappropriate.

We got our first real telescope about 15 months ago - a 3" f/9.4 refractor. Then attended our first Stellafane (and first time camping - rain and mud were fun). We're about 10 hours into polishing an 8" f/5 mirror for what I plan to be an EQ mounted Newtonian reflector. And have started on a 5/16" thick, 6" f/25 mirror for the "folded Herschelian" described in the Sky & Tel article. Glass and tool cost $5, so not too big a risk. 

We're reasonably active in our club - though it would be nice if there were more ATMing going on. 

I've built a Stellafane type slitless tester which seems to work pretty well. Even made the fishing line grating, but have since upgraded to a chip of 88 lpi glass. I'm reading everything I can get my hands on about Foucault and Ronchi testing.

With the grating, it's easy to find inside and outside focus, but not very clear exactly where focus is. With the knife edge, it also seems unclear. The mirror darkens gradually seemingly regardless of position. Do I need to use a pinhole source to get to where the mirror illumination will suddenly blank out? If so, does it need to be fixed? I feel like I'm missing something very obvious here.

Any help appreciated!

Thanks,

Gary Fuchs