Pitch

[dark star]

Does anyone have any idea how much pitch I would need for polishing a 20 inch mirror? I am planning to start with a 14 inch pitch lap (as I aready have the base  for the lap that I used for my 14 inch mirror) for polishing and then a 10 inch lap for figuring. If I get the amount I would need for a 20 inch pitch lap this would allow for any loss of pitch. I will reuse the pitch from the 14 inch lap for the 10 inch, but some is always lost, as I throw away any that may have been contaminated by e.g. the cement base. I haven't got the mirror blank yet!

Galvoptics sell Special Pitch for £16.74 for 450 grams which is not cheap. I am checking the price from Stathis First Light in Germany.

I am tempted to see if I just might have enough to reuse what I already have, given the price so far!  However, I have already reused it 4-5 times and with each resuse I think the risk of contamination probably increases. I read somewhere that you can put the hot pitch through cheese cloth to get riid of any contamination?

David

[JRM]

Hi David,

  I purchaced 4.4 lbs or 2 kgs to make my 20 inch lap and had a little to spare after pouring but not much, hope it helps.

Rick M

[Glasspusher]

David,

If you are prepared to put in the considerable commitment in terms of time and effort to make a 20 inch mirror I don’t think it is worth compromising when it comes to materials. Spending a little extra on a good pitch of know quality is worthwhile, I would recommend Gugolz 55. It can be used as it comes without the need for additives to change the viscosity. 2 Kgs should be enough, but you will need at least two laps so a little more would be advisable.

I would recommend a lap size of at least 16 inches for a 20 inch mirror, the slightly larger size than you suggest  will prove more effective in dealing with problems that might arise during polishing/figuring.

John

[dark star]

Thanks for the replies. I think I will buy 3kg of pitch to be on the safe side. I think the advice from Glasspusher to get the best materials is correct. Especailly as I would like to try a differrent kind of pitch. The pitch from Germany is described as 28 degrees of hardness. Apart from Galvoptics I do not know another source for pitch in this country.

It would be interesting to see how a different type performs. From Galvoptics the pitch tends to be too hard. I added Boiled Linseed oil to soften it. This worked fine until the last time when I added to much and now the pitch is rather too soft. I am remaking it in to a 6 inch pitch lap from the 10 inch pitch lap. This is because I am getting the required parobala towards the outer areas of the mirror but the central area is not getting deeper. According to Terry, our resident expert at the Camden telescope Making Class, this size pitch lap should help to deepen the centre. I will be testing often to make sure there are no disasters!

I think using a full sized pitch lap worked well for polishing and getting a sphere for the 14 inch, so I think going with a 16 or 17 inch pitch lap for the 20 inch is a good idea. I will then most probably figure with a half size, 10 inch pitch lap.

I have sent an e-mail to Stahis First Light in Germany asking about prices.

David