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About Astrobits

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  1. 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
  2. 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
  3. 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
  4. And a Happy birthday from me as well Peter. One day I might manage to visit your place. Nigel
  5. 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
  6. 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
  7. 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
  8. You could try contacting Ian Poyser: 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
  9. Don't trust gluing to HDPE unless you have checked the bond strength with your chosen glue. PVC plumbing pipe is much easier to bond to, Polyolefins ( LDPE, LLDPE HDPE ) are notoriously difficult to bond with adhesives. Personally, I would never trust an adhesive bond to polythenes, and I have spent a number of years in Plastics research. With a wall thickness of 6mm you could screw an aluminium plate to the angled end of your HDPE pipe and glue to that. Nigel
  10. I don't know whereabouts in Gauteng you are but there is a group on the West Rand and one in Pretoria as well as the J'oburg lot. There might be more established since I left SA. Check out the main ASSA web site. Nigel
  11. It won't work. Firstly, as Chris says, you need the tool curved to your final mirror curve so you need to generate a suitable mould in the first place. Secondly, the normal abrasive grit, Silicon Carbide, breaks down to smaller bits so you would need to keep renewing the surface with fresh grit or the grinding action will very quickly grind to a halt ( pun intended ). The only grit to remain for long enough is Diamond. This would be expensive, as would be the cost of all the resin needed to make half a dozen tools for the different grits. Commercially, the curve is generated with diamond tools. A cup shaped tool with diamond grit edges will generate any curve you want. Look for Diamond concrete grinding discs in your local builders merchant. You will still need to use finer grits with a matching tool before you can polish the mirror. Have you contacted the mirror making group in the Johannesburg ASSA centre? They can give you help and advice. Nigel
  12. You can still use the lens formula to determine the three parameters given your space available. If your object to lens distance has to be 5M you can determine the image distance for various focal lengths of lens. eg. if you use a lens of 2500mm focal length then the image will be 5M from the lens and be the same size as the object. Is your observatory 10M long? Use the formula to find the optimum positioning and focal length of your lens. The classical object to show lens "rules" is a candle. Not suitable? Nigel
  13. Here for interest are two pages of spot diagrams for some common eyepieces for F/10 and F/5 telescopes taken from the book: Telescope Optics by Harrie Rutten and Martin van Venrooij The one labelled Abbe is the orthoscopic design that he ( Ernst Abbe ) developed. You can see the deterioration of all the eyepieces when going from an F/10 to an F/5 telescope. Nigel
  14. It might well be that the mirrors and eyepieces are all good. It's just that steeper light cone from the faster mirror that makes the difference. Many eyepieces work well with shallow ( slow mirror ) light cones but their performance deteriorates as the cone gets steeper ( faster mirrors ). Nigel
  15. It didn't take too long but this was over 20 years ago. An experienced mirror maker can make a finished 6" mirror within a day and that includes figuring to a parabola. Polishing the tool for use as a lens would be just as easy and without the figuring stage, somewhat quicker. What space have you to do this demo? That will define the limit on focal length of your lens. Can you not move the object nearer to the lens you have and produce a larger image at a greater distance? It doesn't change the principle you are demonstrating. With the object at twice the focal length from your lens the image will also be at twice the focal length away and at 1:1 size ratio. That will consume 2000mm of your available space ( with a lens of f= 500mm ). Move the object nearer the lens and the image appears even further away and larger ( with the object at the focal length the image will be at infinity ). Use the lens formula to work out the distances: 1/u +1/v = 1/f u= object distance v= image distance f= lens focal length Nigel