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davidc135

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Everything posted by davidc135

  1. I'd have a look at the threads and check to see if there is any damage preventing the reassembly. If so, a local machine shop would be able to repair it. Either way there must be some such businesses in Inverness who could help. If the threads are good and clean but difficult to engage I've found that slowly rotating one element counter clockwise against the cell until a slight click is felt sometimes does the trick, but you may have tried that. David
  2. There is a thread over on CN where one Askar 185 was so so in the indoor Ronchi test being a bit over-corrected with a wide central hill. Whilst the owner felt his 150 apo showed a bit more planetary contrast, he still thought the Askar was no slouch and was pleased with his purchase, especially given the price. But he bought it more for DSOs. I expect the 8.5'' achromat, in spite of CA, got the best out of its aperture by having an excellent figure and it may be that it's that consistency which suffers with lower priced large apo objectives. No doubt there will be more indoor tests/field reports on the 7'' Askar in the near future. David
  3. It isn't too difficult to disassemble and clean the objective if it's taken step by step. Remove dewshield. How is the cell fixed to the tube? I'm assuming that it isn't collimatable but screwed in place. Collimatable cells would be attached by three pairs of push/pull scews. Remove cell. As Louis says check for a retaining ring which should have two indents to aid removal. Usually it would be on the outside of the glass but it's possible that it screws in from the rear. Remove ring. To remove the objective first find a cylindrical object say 3'' diameter and of ample depth such as a small glass on which is placed a protective tissue or cloth. This supports the objective whilst the cell is lowered down carefully avoiding any jamming on the sides and possible resulting 'clamchips'. With the objective high and dry, inspect for any side pencil marks or scratches indicating best orientation. If there are none it's still an idea to mark a V or similar with a waterproof sharpie just in case it matters. A V can also be a quick check on the order of surfaces during reassembly. Check what they use to separate the two lenses. It could be a ring, or three foil or similar spacers. Retain as the spacer thickness is especially important in a fast doublet. I'd give them a soak in tepid soapy water and carefully rub the surfaces with fingers. The lenses can be rinsed and drained with the last drops removed with a tissue before being allowed to thoroughly dry. Ensure objective is reassembled in the correct order. Hope this helps David
  4. An update. I'd assumed that the surfaces of the B270 float glass were sufficiently parallel and hadn't checked. A disappointment to see Airy discs as little spectra! What can I say? Both corrector discs had around 125 microns of wedge where 25 should be the limit and so there was nothing for it except to regrind one side of each plate. It's likely that the nearly finished corrector will suffer some astigmatism due to irregular polishing. Avoiding pressing down too hard will help whilst interference fringes will show likely problems. The pattern around the neutral zone should be circular with any variation being improved by selective pressure. On this earlier 4'' c.p extra time spent pressing down at 10 and 4 O'clock evened the fringes out. The gram weights were to centre the fringes. There's still a little astigmatism in my DX-6 but it gave sharp images at x370 during spells of good seeing in the garden, on high contrast objects. David
  5. If the Orion Optics VX8L upgrade is truly 1/10 pv and smooth I don't see it being easily bettered visually on the moon and planets. David
  6. I'd contact Stathis Firstlight in Germany who many people have dealt with, unless you have done so already. If he doesn't have your thickness in stock he can order it. David
  7. My orange tube C-8 has an SN of 870505 and was made in the early 90s, I was told. I wouldn't offer more than you would want to resell it for, in good conscience, if it turns out to be disappointing. It may well be fine but I expect it's a bit of a lottery. David
  8. Very nice find. Highly respected manufacturers. Look forward to hearing how it performs. David
  9. Definitely spend a few pounds on a double layer of reflectix. I'm not sure how much seeing varies from place to place throughout the UK. I suppose it must be influenced by local microclimate but I'd be surprised if the 9.25 sct didn't live up to its potential where you live, at least reasonably often. David
  10. It's well worthwhile doing a star test and checking that all is well at focus. Out of focus there may well be colour effects. A 50 micron artificial star 20 m plus away will be more independant of the weather and seeing than a real star. There may be a touch of spherical under-correction. And it's fun. David
  11. Looking forward to following progress and you're welcome to pop over any time. David
  12. It would be quite a beast and probably needs a permanent observatory to be enjoyably easy to use. I have the necessary kit including a large optical flat and would be happy to test the objective for only postage cost. I'd be cautious about sending the lenses by post however well they are packaged although the two lenses of a 4'' Tak apo crossed the Atlantic twice without mishap. If the two lenses are taken out of their cell and individually wrapped before being double boxed I expect that they'd be OK. Orion Optics UK provide a testing service for mirrors and probably also for refractors. David
  13. Scts aren't very sensitive to changes in back focus. Telescope-Optics.net, page 10.2.2.4.2 says scts of f2/10, regardless of aperture only vary by 1/23 wave correction per inch change in back focus, on axis. Similarly Edges, but their off axis imagery suffers more. I should think CKs would be similar to non ACF scts due to their both having spherical secondaries. CCs are much more sensitive which is why they don't have mirror focusing. David
  14. Alternatively, I could Ronchi test a singlet in auto-collimation in rgb and see how much of a difference there is. David
  15. False colour effects in singlets and achromats would be mostly due to defocus but Vla Sacek in 'Telescope-Optics.net' gives lengthy equations to calculate the SA contributions of both the crown and flint in a doublet example. Varying the wavelength would give an idea of sphero-chromatism. I'll take a deep breath and see if I can get to grips. The form of a crown lens in a doublet would be far from optimum in a singlet. David
  16. How much more evident is sphero-chromatism in, say, an f8 singlet than in an equivalent achromat or apo? Disregarding defocus. David
  17. Edit. Another part of the optical train to check is the diagonal if Peter's suggestion doesn't help. Try the scope without. David
  18. Astroreflect seem to guarantee a minimum Strehl of 0.95 which, if true, would give me confidence. David
  19. You could try Grovers Optics in the UK, email: hello@grovers.biz. At one time they supplied Celestron made corrector plates to customers in the UK and might send them overseas. Prices were moderate. Corrector plates would be a very tough challenge for a beginner. 1. 'Making Schmidt Cassegrain Telescope Optics' by Robert Piekel. Uses petal shaped facets to fine grind the complex curve. 2. There should be stuff on the net about Bernhardt Schmidt's original method using a partially evacuated pan to support the edge of the plate as it's ground and polished. 3. My spinning method requires a machine to be made. The 9.25 has a slower primary which very much eases the amount of asphericity needed on the corrector. However... Unless the maker has some experience with producing mirrors from scratch I definitely wouldn't advise tackling a corrector plate, regardless of which method was chosen. David
  20. I didn't know about dark-star.it. Thanks for posting. David
  21. Thanks John. The key for me was using a vertical spindle. It was a stroke of good luck that a simple technique naturally wanted to produce a Schmidt curve or something close. In my case I am right handed and the spindle spun clockwise. The short stroke begins with the workpiece on top and centre over centre. It is pushed forwards and returned to the same centre over centre position all the while with most downward pressure being applied to the edge of the glass at 2-3 o'clock position. The left hand merely steadies. Sometimes the work is displaced slightly to the right.This I repeat a few times and then allow the glass to rotate a little in my hands before the process is repeated and so on. For mild aspherics as in say a Schmidt Newt just average hardness pitch would be fine from start to finish but deeper aspherics require a softer pitch that might need to be poured on a hard rubber mat which in turn is pitched to the base. I use tractor tyre inner tube. Surfaces with shallow aspherics should be polished out using ordinary technique before being aspherised. So straightforward enough. It takes a similar time to any other surface although repeatedly remaking the softer pitch lap towards the end of this job stretched it out. This corrector plate or one for a C-8 would be at the limit for this technique. The other limitation is the flexibility of glass in the thicknesses that commercial scts can accomodate which is about 6mm max. This can cause irregularity of polishing which shows as astigmatism although this could be fixable. To do a good job some figuring is needed towards the end to get a decent result but probably no more than with other surfaces. In the Foucault knife edge image above there is a slight raised zone towards the edge best seen at 1 and 7 O'clock positions. I'll cautiously use a ring lap to wear it down but it's easy to get local polishers wrong and make things worse. David
  22. Contact interference fringes form against an optical flat. Edit. The corrector surface is slightly concave which pushed the neutral zone further in from 0.71r Figuring in the later stages was helped by a petal lap. The corrector plate was propped up on a plywood jig during testing. David
  23. I've nearly finished figuring a replacement corrector plate for my Dynamax 6 sct. Two 156mm discs of 5.5mm B270 glass were fine ground and then polished on a vertical spindle running at 35 rpm. Polishing and figuring took place at the same time and needed around 6 hours per side. The glass was worked back and forth using short strokes and the desired Schmidt curve was gradually produced. The knife edge image shows a diagonal symmetry probably due to non alignment coma. Some slight zones can be seen. The large obstruction is from the 8'' sct used to produce collimated light. David
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