Astrobits

Or if you are ever considering visiting the South African Astronomical Observatory near Sutherland then you might be able to pick up some wine made on a local estate. I got some of these from a cafe in Sutherland. Nigel

You should have mentioned this to me on Friday. 🙂 I have a screw cutting lathe. The expensive bit is choosing Brass. Ally will be cheaper but that is, of course, your decision. I only have Brass rod to 35mm dia so would not be large enough. I do have larger dia. Ally so no problem/cost there. I am also hoping to do some anodising sometime so you might be able to choose your colour ( from those that I have available)😀 See you next time at BAS or pm me. Nigel

When I was making mirrors 20+ years ago I purchased my borosilicate blanks direct from Schott U.K. Ltd. The largest piece that they did as a single cast was 500mm x500mm square x 40mm thick. These I has water jet cut to a circle by a company whose name I cannot remember. If memory serves the 16"dia discs were 75mm thick. I suggest that you contact Schott U.K. and they will advise on who can supply their materials. ( Suprax and Duran are their trade names for borosilicate) Nigel

If the original mirror has not been replaced then I expect that it is round. If you feel the need for extra help then bring it down to me. I'm in Highbridge, about 7-8 miles from you. PM me for my contact details. Nigel

The Virtual Telescope Project is live streaming the Betelgeuse occultation by asteroid Leona starting at 01:00 12th Dec. Go to their web page and follow the link. Nigel

I don't know if Beacon Hill Telescopes are still operating. Barry was always very helpful and can/could supply any bits and pieces needed. His price list is dated 2010!!! but there are comments of ongoing business in 2020. Worth a phone call. Nigel

Black paints have a matte surface to help reduce reflections. However, at very low angles the light "sees" a much flatter surface than is seen at high angles of incidence. and therefore the surface tends to reflect more of the incoming light than we might have hoped. It's all in the physics of optical reflection at a surface. The only way to reduce these reflections is a physical barrier to the reflected light which is achieved through a variety of ways from adding a larger particulate ( e.g. sand ) to the paint or putting flocking or baffles on the surface. There is no way to reduce reflections to zero as there will always be small areas at the top of the roughness or baffle that will reflect a small amount of light. Nigel

For years a lot of collimation was done with a simple pinhole eyepiece. I have done this with my own 16" (f/4.5 ish ) reflector and had satisfactory views. I suggest that the OP uses this method first and then uses the telescope for a while before deciding to go into cheshire's, catseye's lasers etc. Nigel

I did reply to you pm's. The last one yesterday. Unfortunately I am not always able to reply to messages immediately, I do have other things to do and it might be day's before I can find time to respond. Nigel

I can also help. I have made mirrors up to 20" and whole scopes up to 16". I live about 45 mins drive south of Bristol at Highbridge. PM me for more info Nigel

Don't put grease anywhere near mirrors. Nigel

Cracks are very difficult to fill to the very point of propagation. So they are likely to continue after you think that you have dealt with them. The best way to stop a crack is to drill a hole, centered at the furthest point of the crack, and then fill and finish as you want. Nigel

Just seen this wallpaper in WILCO, called "COSMOS CHARCOAL". Looks like an image of Neptune there, amongst others. Should suit an observatory. Nigel

It is probably easier to go for a flat surface as the radius of curvature would make the normal testing for a sphere with pinhole and knife edge very difficult. For a 9.25" corrector, with full correction on one side, the ROC would be about 163ft. If half of the correction is to be on each side then the ROC would be double that. I don't think that I would like to try doing a Foucault test at those distances. This is just one of the ways to produce that complex corrector plate. In ATM book 2 they used several polishers of various sizes to figure the plate without the deformation technique. Testing was much more difficult though as they had to assemble the telescope for each test run. BTW the definition of a flat surface is that it is "the surface of a sphere of infinite radius". Nigel

Using this technique thin plates of glass are converted into corrector plates for Schmidt-Cassegrains. The suction pulls the material into a parabolic shape and the concave surface is then ground and polished to spherical. When the vacuum is released the figure is what is required for the corrector plate. Nigel