Recent developments in mirror making

I did a little more grinding till 11th Jan 2008 by when I was able to completely eliminate the central area on the two surfaces which was not getting abraded. Now all parts of the mirror and the tool are getting abraded. This is good news. The bad news --- the tool is still concave although not much. I have been able to measure the concavity of the tool and it is approx 1.5mm. This is much better than the 4-5mm depression that I had when I started working the first day. The #36 grit has worked really good in helping to reduce the tool concavity and in bringing in the two surfaces in contact. Another thing I have been doing over the last few times is that I have used the chordal strokes. Rather than doing the 1/3rd normal center-over-center strokes, I have been using the chordal strokes with about 1.5" offsets between the centers of the tool and the mirror. Both these factors, the #36 grit and the chordal strokes have greatly reduced the tool concavity. One problem though is that the amount of #36 grit is only about a quarter pound or so. This is likely to last just one or two sessions and I really need to make sure that the tool is at least flat, if not convex, before this grit size ends.

More things have happened. Yesterday (23rd June 2008), I was out at the meeting of the ATM club of MAS (the local astronomy club). I was able to use the 12" f/7.5 scope to look at Mars and a few other objects. the seeing was not particularly good so we used a few filters. The important part was that my 6" f/5 mirror was coated with Aluminum oxide and then an overcoat of Silicon-Oxide (SiO). The whole process took about 45mins of which nearly 30mins was spent in bringing the coating-jar to a vacuum or rather as close to a vacuum. The actual pressure was less than 1/10000th of the normal atmospheric pressure. After the coating, we did some basic testing on the mirror and things are looking good. Now comes the hard (and somewhat boring) part of building the remainder of the scope. The most important part will be the mirror cell. Most of the experienced people have told me that it is far better to buy rather than make the mirror cell. I am also thinking of buying the secondary mirror, eyepiece focusser and the eyepieces. I already have 3 eyepieces, 30mm, 20mm and 10mm. Since this mirror is a wide-field scope, I might have to buy an eyepiece of 35-40mm. That will give me a wide field with a 6-7mm exit pupil.

Finally, yesterday I have purchased a 8" mirror blank yesterday. The blank is about 1.3" thick. I also saw the 8" tile-tool but it was also concave (another concave tool) and so I straightaway rejected it. I will now have to look out for a 8" tile blank that I can use to grind this mirror. My plan for this mirror is to build something around f/8 to f/10. This would be a good scope for planetary and also for work that requires a decent magnification. It is occurring to me that the 12" mirror is a big step-up from the 6" mirror that I have been working on. It is a better idea to complete a 8" (or 10") mirror before attempting a 12" mirror. Let us see how this goes.