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Astrobits

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  1. Galvoptics http://www.galvoptics.co.uk/ VCSM ( Vacuum Coatings Scientific Mirrors ) http://www.scientificmirrors.co.uk and as mentioned above Orion Optics Nigel
  2. Ebony Star alternatives

    Find a Kitchen worktop manufacturer. They will have offcuts, probably for free. Nigel
  3. Comin' along - JWT

    I am sure we will get some interesting pics but not the same type as Hubble. The camera on the JWST is Infrared, 0.6 microns is the shortest wavelength it will record. It won't be able to record the blue-green portion of the spectrum from our own galaxy and those very nearby. Once the galaxies are red shifted enough then those wavelengths will fall within it's grasp. Nigel
  4. Where is the Delaware Diamond?

    They, or someone, bought it from the unofficial " Star Registry", hence it's faintness. Nigel
  5. Ed: Polythene ( polyethylene) is a naturally flexible polymer so no plasticisers are needed. However, as the B-BQ cover is made for outside use it almost certainly has other ingredients such as UV stabilizers and, presumably, pigments that do have the potential to bleed to the surface. They are not as volatile as the plasticisers used in PVC so as long as the cover is not touching any delicate optical bits it should be O.K. PVC in it's 'natural' form is very rigid, think rainwater goods like guttering, and needs plasticisers to make it flexible. Dave: Yes we also have the triangular identifiers but these only appear on containers, things like polythene sheeting and cling film don't get the mark. The marks are primarily to aid recycling as mixing plastics is not a good idea if you want a usable product out of the end. For example, Polystyrene and Perspex (aka Plexiglass ) are both clear rigid plastics in their natural state but they do not blend together, they are, like oil and water, not miscible. By the way, Ammonium Fluoride is used to etch glass as well as HF. It is at least less volatile although still pretty dangerous if not handled properly. When I had an old enamel bath re-coated with a polymer the operators used it to etch the enamel ( =glass) coating so that the bonding was as good as possible. Nigel
  6. There has been a lot of concern for many tears about the plasticisers from cling film entering the food chain. Steak and Phthalates anyone? Nigel
  7. If your plastic food wrap is based on PVC then it is full of plasticisers which will leach out. These make the plastic flexible and tacky. Just Google "PVC plasticisers" to get some of them, mainly long chain phthalates. If that is the case then water will not clean your mirror and you will have to try other, more vigorous, materials. I would start with soap solutions and if that doesn't do it then some commercial lens cleaner or organic solvent such as Isopropyl Alcohol. ( nearly used the abbreviation IPA there but here in the U.K. that is a beer and while it may wash a mirror it is much better being drunk) Nigel---hic!
  8. The 8.75" f/6.7 Mirror Grind

    Yes, pits can seem to be very persistent, but they do, eventually, disappear. I like to think of the grinding action in this way: As a large particle of grit rolls over the glass it strikes the glass with one of it's points something like hitting a big lump of concrete with a pickaxe. This causes a crack down into the glass. At a later time another particle creates a crack that intercepts the first crack and a lump of glass is freed and a pit is formed. This second crack does not necessarily meet the first crack at the bottom, the first crack going deeper. At the end of using a coarser grit the surface is covered with pits and there are sub-surface cracks left over. On starting a finer grit these create finer/smaller cracks, some of which meet the existing cracks creating a somewhat larger pit at the location of the deeper cracks. Again, the smaller cracks do not meet the remnant cracks at the bottom so the pits tend to be repeated at a particular location until they do, at last remove the traces of the coarser cracks. This action is repeated with each grade of grit. Have fun Nigel
  9. Go back to the dealer. If they can't fix it then a replacement OT should be offered. It should not be up to you to sort out a faulty instrument within the first few months from new. Nigel
  10. Moonlite focuser screws

    1/4-20 is good old BSW as used in cameras and tripods. Don't know where you would get nylon tipped versions. I would make my own, Nylon or acetal tipped brass as I have that size taps and dies. #10-32 is UNF. I guess that if you need replacements then Moonlight would supply. Nigel
  11. And a Happy birthday from me as well Peter. One day I might manage to visit your place. Nigel
  12. The 8.75" f/6.7 Mirror Grind

    What I do is to find the largest pit that I can and mark the edge of the mirror radially from the centre through that pit. I then measure and remember the distance in from the edge. After one wet re-check for that pit, ignoring the rest of the mirror. If it has gone then that grit is finished. If not, carry on. Nigel
  13. I am surprised that with situations like Alan has that there is no mention of the --STAT type telescopes where an adjustable mirror is used to relay light into a fixed telescope. There are a number of options but I am not aware of any manufacturer offering these, so all need to be made to order ( = expensive ). Binoculars can also be used this way by pointing them down at a mirror. This one at the 1900 World Exhibition in Paris might be a little on the large size but would cure aperture fever Nigel
  14. 16 " Open Truss Dobsonian Build

    I can see two potential problems with the mounting of your secondary mirror. The gap between the mirror and holder is very small and will make removing it difficult when you need to do so to re-coat it. In the picture it appears that the bonding agent is up to the edge of the mirror. This is bad practice as it can cause deformation of the mirror giving astigmatic views. The secondary should be mounted in a similar fashion to the main mirror. You can use one central blob of silicone no more than 25% the diameter of the mirror. If you prefer you can use three blobs but much smaller than a single blob positioned at about the 50% ( between 40-70%) zone . There should be about 3mm gap to allow the safe and easy removal of the mirror from the holder. Otherwise the build is looking good. Nigel
  15. Buying lens and mirrors

    You could try contacting Ian Poyser: http://irpoyser.co.uk/lenses/ He might have something suitable not listed on his web site. An alternative source for the achromatic doublet is to buy an old refractor telescope such as the Tasco type and take the lens from that. Flat mirrors can be obtained from old photocopiers but you would have to cut them down to size. Nigel
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