Astrobits

Three options: 1. As above get a good GEM 2. Go for an equatorial platform 3. see this thread for a DIY option: (Or use a commercial add-on version as mentioned by Brantuk) If you want to do photography then the 3rd option will require a field derotator on the focusser. Nigel

Although the Russians got most of the stuff and some personnel from Jena, other members of the staff got to the west and started up the factory in Oberkochen. Products from there carry the 'Carl Zeiss' name while the Jena factory continued under the 'Zeiss Jena' name. Nigel

As far as I can discover, the first two numbers refer to the year of manufacture. In your case this will be 1983. I have a pair of 12 x 40's that I bought new in the early 70's and they have a serial number starting with 73. The Russians acquired the Zeiss factory ( in Jena ), all it's equipment, designs and workers during the last war. These were copied, and where possible improved, for much of the Russian optics manufacturing which is why they are so good. Up to the fall of the Berlin wall they exported only the best quality but afterwards we started to see some of the reject items that previously only went onto their local market. Nigel

Some 20+ years ago I queried the then importers of Russian stuff, TOE ( a Russian government owned company ), if the eyepieces for their telescopes were available separately. After all, owners of their telescopes could loose/break an eyepiece and the design of the telescopes ( at that time ) was such that many other eyepieces did not fit properly and would not focus. The response from the Russian factory was in essence " we sell all our telescopes with eyepieces, why would you need replacements. Eyepieces are not available separately." A member of TOE staff did comment to me that they could not order what they wanted. Instead, a container would arrive at TOE and they would find out what they were going to have to sell only when they opened it. Obviously it depended on what the factory was making at the time. Looks like not much has changed. A lot of Russian optical products are based on the techniques and patterns that they got when they took over the Zeiss optical works during the second world war. That is why they were/are so good. Nigel

If that's John's opinion then so be it. If, as John says, you have a deep zone around the central hill ( which is presumably below your final parabola ) then reverting to fine grinding is the best way to get back on course. I find that Ronchi tests are not my cup of tea, it is difficult to tell from your last image just what the profile is. I prefer something a little more positive. Using the fringes at the edge of the knife shadow in a normal Foucault test, for example, to give an approximate depth/height of any zones. That way I can pick up any wayward area before it gets too bad. For the 20" mirrors that I have made I have never used a grinding or polishing lap bigger than 13", but then I used a machine that I had built and a spherometer that reads to 0.001mm ( direct reading of 2 waves although I could estimate to 1 wave ). The spherometer kept the grinding on target and when polishing the disappearence of the pits was closely observed and the stroke changed so as to be even over the whole surface. Fine grinding and polishing will go much quicker now you have so much experience under your belt. Nigel

I too have had times when nothing appeared to be happening to my mirror. At those points I have decided to change something significantly and see what happens. The last images show a central hill. I would probably make up a very small lap, about 1/3rd the dia of the hill, certainly no more than 1/2, and work entirely within the central area and see if that made a difference. I wouldn't worry about creating zones there as you can easily switch back to a larger lap once some progress has been made. Don't forget that the central area will be obscured by the flat when in use so it doesn't really matter what happens under that flat. ( Cassegrains have a rather severe central hole ). I have used laps as small as 2" on a 20" mirror and on occasions even just my thumb. I also use a heavier tool base making mine with plaster and sealing with varnish. As stated previously, the biggest problem with figuring large fast mirrors is that significantly different amounts of glass need to be removed from adjacent areas of the surface and that cannot be done with large tools. So a bit of practise with small laps now will give you a feel for them when you actually get to the figuring. Here's the 200" Palomar mirror being worked with a relatively small lap: Probably equivalent to a 2" lap on your 22" mirror. Nigel

You're right, I have made flats ( but not for many years ). If I get round to making another one I might post about it. Nigel

Surely, once the primary is finished you will start on the flat? Nigel

Looks like a nice job, well done there. The eyepiece labelled "SYM.ACH 9mm" would indicate that it is probably a pair of symetrical achromatic doublets ---- which is essentially the Plössl design although manufacturers have played fast and loose with designs and labels. Nigel

Just my small portable 16" Nigel

The fact that you are generating a hill in the centre indicates that you are removing glass from the outer areas. You are going in the right direction. Keep it up. Nigel

I understand your desire to ge to the end asap but I would not recommend starting to parabolise before getting the edge correct. The reference point for the parabola is the edge and if you don't know where that is it is too easy to overdo the centre. Then you have more edge work to correct a hyperbola and you know how much effort it takes the change the edge of a large mirror. Looking at your lap there are only 6 facets spanning 10". I would expect to have facets about 1" in size so have 10 x 10 on that lap with sub-faceting with a mesh. Keep polishing................. Nigel

Coming along nicely. There is always more work needed at the edge than at the centre and big, fast mirrors accentuate this. It just goes to emphasise how easy it is to overdo the centre which then leads to an awfull lot of work. Nigel

Quite right. If the centre gets too deep ( hyperbola ) then the rest of the mirror has to be taken down and that represents a LOT more work than it took to overdo the centre in the first place. That's why the old books refer to it as the "fatal hyperbola". My stroke would have up to 2" overhang at the centre of the stroke, going to 3"-4" at the end. With the large, fast mirror you are making the parabola is very steep and significant amounts of glass need to be removed right up to the edge and there is only one way to remove that. Nigel

I would use the 10" lap and a W stroke centered about 3"-4" from the edge and see how that affected the "hill" near the edge. If you do a Foucault test try to see the faint interference bands near the knife shadow when the knife edge is away from the coc. They are 1 wavelength apart, ie. 1/2 wave on the glass. This might give you some idea of how much glass will need to be removed Nigel

Looks typical of a crater-on-top-of-a-hill. The problem in this situation is to know whether the bottom of the crater is/is not below the required point for the final paraboloid and it is too easy to go too far. Just remember that the centre of the tool does the most work so John is absolutely right - work off centre until the edge is correct, then you can take the centre down to finish. Nigel

Are you curving the surface of the wood lap base to match the mirror curve? If you are using it flat then the pitch coating will be of different thicknesses over the lap and will have variable flow at different points across the lap. This could cause problems in figuring. I always cast my lap bases against the finished ground surface using a simple wall from a sheet of plastic or card to give me different diameters of lap. Nigel

Shurely the need to offset the secondary applies only when you are using a minimum size secondary ( as is shown in all the above diagrams ). As most Newtonians are fitted with oversized secondaries to increase the size of the fully illuminated field, the extra size of the secondary prevents the problems mentioned. The only effect of having the oversized secondary centrally placed is a fully illuminated field that is not concentric with the optical axis. This is hardly noticeable in visual ( and only at low powers ) and will only affect photographic applications if the sensor size is so large that it's boundaries exceeds the boundaries of the fully illuminated image area. Nigel

The RAS thread is BSP ( British Standard Pipe ) so just get a couple of bits of pipe and find someone who could turn one bit down to 1 1/4" push fit. Nigel

I'd go for a smaller lap. 17" is a bit too close to a full dia lap for my liking. But whatever you use make sure that the first priority is a full polish and the figure is secondary until that full polish is reached. I never tested my mirrors until they were fully polished. That way I avoided the temptation to call a parabolic figure with pits "done". Nigel

There is no need to achieve a perfect sphere before starting figuring. If you have a raised centre then work on the centre only. After all, you have to work on the centre of a sphere to get a parabola. If the centre is depressed ( but not too much ) then some of the figuring work has already started! Finish the polishing first then work on the areas that your testing shows are not in the correct parabolic zone using smaller tools as necessary. It might be wise, before you finish polishing, to cast some smaller tool bases so that they will be ready for you when you need them. Nigel

It is possible that using a full diameter tool ( tool on top ) you are causing the centre of the tool to become depressed and hence lessening the polishing of the centre of the mirror. Try going to sub-diameter tools. I would never consider a full sized tool for a 20". All my 20" mirrors were made, tool on top, with 13" and smaller tools. Have you felt the warmth of the mirror and tool after a bit of polishing? Nigel