Ah, OK re: the TOS. Thanks! It is kind of an ATM topic too, but most of the info I'm seeking relates specifically to making a scope safe for solar viewing, so I figured I'd put it here.
Original post was this:
A 4.5" f/8 Meade Newt OTA that I picked up at a nearby thrift store a while back had badly degraded coatings on its mirrors (pic here), so a few days ago I decided to strip the coatings completely and use the clear glass to make a little dedicated solar Dob. A couple of questions for those who know more than I do (so, pretty much everyone here):
Googled-up opinions seem to vary as to whether the flat back of the primary should be clear (a diverging beam is reflected back up the tube) or frosted (light is diffused off the back surface). Mine is clear. I'm kind of leaning toward roughing it up with sandpaper under running water, but I will not be able to reverse that if I change my mind after doing it.
What's the best option for a secondary mirror? I have both coated and uncoated secondaries. I plan to mount the required ND filter into the focuser. Would a coated secondary call for the same ND3 filter as a Herschel wedge? What density would be needed if the secondary were uncoated? The original secondary is now clear, but will give a double image off of its rear surface. I could use the long face of a right-angle prism from an old porro set, in the manner of a Herschel wedge variant, though I'd rather have something elliptical. An uncoated elliptical flat with a wedged profile to deflect the ghost image would be my preference, but I don't have the means to make something like that.
I figured an uncoated secondary would cut the primary's 5% reflection down by another 95%, but I'm guessing the resulting .0025 would still be too much without further filtration. I assume that I also need a UV/IR filter, as a Newt's image path doesn't pass through any glass as it does with a refractor.
I have both front-end AstroSolar filters and a Lunt wedge for my refractors; this project is just to scratch the gotta-make-something itch, and avoid having a decent piece of glass go to waste...