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

Not a new idea at all. Just Google "Mylar mirror for astronomy" lots of hits and plenty of ideas there. Nigel

It strikes me that you are using far too much weight on the lap. I never use any extra weight, just the weight of the lap/mirror and any connections needed to move it. With my mirrors up to 10" that was just my hands and I didn't push down on the lap/mirror any more than needed to move it. It might work more slowly but you seem to be going back and forth without getting where you want to be. A 6" mirror doesn't need a lap smaller than 6" until you need to attack very specific zones during the final figuring process. I was a member of an astro soc back it the 80's who had a stall at an annual event. The members would start and finish a 6" mirror within the one day of the event. Starting with two slabs of glass in the morning they would be able to observe, and show the visitors, the moon that evening with the ( unsilvered) mirror. It was said to me when I was learning to make mirrors that engineers do not make good mirrors because they tended to do repetitive operations as consistently as possible, without deviation. Using machines tends to do the same thing. This , more often than not, led to a very zonal surface. To make mirrors one needs to randomise every stroke which machines cannot do by themselves. Having nice loose joints in the machine helps to smooth out the regularities somewhat and horrifies some engineers when they first encounter such equipment. Nigel

I am having trouble relating your Ke values to the images. The Ke values are clearly way over corrected but the images look to be near spherical? Nigel

In order to get the unsuspecting customer to purchase cheap bins the manufacturers/suppliers often quote AREA magnification not the usual linear to make them appear super powerful. So technically they are not exactly dishonest. 10 x 50 bins have an area mag of 100 so could be quoted as 100 x 50. For those quoted as 90 x 60 just take the square root of 90 for the normal mag,= 9.5 approx, not abnormal for 60mm bins. The square root of 60 is 7.7 so those are probably similar to other 8 x 35's. Nigel

I don't know what proportion of Messier's list will be seen with 10 x 50's. However I can say two things. Firstly I have seen M1 with 10 x 30 binoculars. I have also drawn NGC288 ( mag 9.37 ) with 12 x 40 bins. By extrapolation to the light grasp of 50mm bins I would expect that most, if not all, could be seen in really dark skies. I am sure someone here will confirm that. Secondly, when starting astronomy the newbe will have to learn to see again. They will not be able to see everything the first night. Only with experience will they see more. Nigel

Putting close-up lenses onto camera lenses allow objects closer to the optical axis to be in focus. The field of view in this case can be much smaller than the diameter of the main lens itself. It follows that a close object in the view of one lens will NOT be in the view of an adjacent lens unless it is pointed at an angle towards the object. Just look at simple stereo microscopes, the two tubes are angled and pointing at one point on the specimen stage. Your binoculars are showing the early stages of the two fields are not fully merging as the two optical axes are parallel and if you could get to focus even closer the effect will just get worse. Your binocular manufacturers have decided that that is the nearest they are prepared to allow you to focus. If you really want closer focusing binoculars then you will need to purchase some that provide that facility. Nigel

That maybe so but as I was prepared to pay their "basic specs price" for just the lenses they would have made more profit than if I purchased the specs. Also they would have saved on cutting the lens to fit the frames, a further cost saving/increase profit. Nigel

The "filters" for close-up photography are simple low power lenses. For a diopter/dioptre ( D) of 0.25 you will need a lens of focal length 4 meters! For 0.125D the focal length is 8 meters! None of the scientific lens suppliers that I have looked at offer anything near your requirements. Try the online opticians for something with that low a D and choose the largest frames if they won't do the lenses by themselves. I have, some time ago now, tried to get one of the cheap online specs makers to sell me the lenses without the frames but they refused ( don't understand the logic of that). Have not pursued with any others yet but there are plenty more out there. Nigel

I hope that I am reading your post correctly. I think that Zambuto is referring to the "sweet spot" of a lap as being in the area either side of the fore/aft stroke direction encompassed by the central diagonal and either side within 80% of the diameter. The maximum effect of a lap occurs along the fore/aft diagonal, reducing effectiveness towards the sides. If you are offsetting the 80mm lap by 30mm then that puts the maximum effect at the edge of the central hill. This would do nothing to reduce the central hill and will probably make it worse by targeting the low zone around that central hill. You need to have the centre of the lap oscillating about the centre of the zone you wish to reduce. This is one area where simple machinery fails to give us the best results as it does not vary the position of the lap enough to prevent the lap effectively digging a hole for itself. As I said previously, I would concentrate on the outer zone first. Using the smaller lap by hand in a circular stroke, no more than about 1/2-1" in dia, over the high zone as the MOM rotates the mirror, making a flower petal pattern in the polishing medium. At present you are still trying to get to an optical sphere before going for parabolising. In parabolising you will be attacking the centre anyway and hopefully doing less and less work as you get towards the edge of the mirror. It is a fallacy that the surface after fine grinding is a sphere to optical standards. I once made a grinding tool that used rather large pieces of glass and it so happened that there were more gaps in a particular zone than elsewhere. Despite the mirror appearing to be a smooth surface, a sphere it was not. After polishing there was a large high zone at the location of the gaps in the grinding tool. Lesson learnt. Nigel

Perhaps you might like to think about making a Dall Null tester. You will need a plano-convex lens about 25mm dia with a focal length around 150-200mm and a bit of engineering to make up the tester. One of the drawbacks of the Foucault test on faster mirrors is that the light areas are very light and the dark areas are very dark making it difficult to see subtle zones. With a null tester the knife edge shadows appear as that of a sphere, i.e. the parabolic mirror will grey out much more gradually than with Foucault. Any high or low zones will stand out much more. I have used my Dall tester for my mirrors up to 500mm F/4. Personally, I would concentrate initially on the outer areas of the mirror as the central area will be partially covered by the shadow of the secondary and it contributes little to the image anyway. Nigel