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Astrobits

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About Astrobits

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  1. From a purely theoretical ( and not exhaustive ) point of view:- A mirror doubles the surface error so a 1/10th wave error becomes 2/10ths wave on the wavefront A prism also has a reflective surface so it also has the same effect as the mirror. However, the prism also has two plane transmission surfaces which will, individually, affect the wavefront only half as much as the surface error, so x2 1/20th is 1/10 error. Thus a prism could have a 3/10ths error on the wavefront if all surfaces are 1/10th wave. In addition, the two plane surfaces of the prism act as a plane parallel window and to a converging wavefront this will add some spherical aberration and the angled light will also undergo some dispersion in relation to the wavelengths of light, i.e. chromatic aberration. These last two effects will have very little effect on the final image as the distance over which they can operate is very small. Of course, not every diagonal will live up ( or down ) to these expectations. Nigel
  2. A repeat of the 2015 broadcast. On BBC Radio 4 now ( Friday, 2:15-3:00pm ) and presumably on catch-up. Nigel
  3. As I understand your description, the card disc will be underneath the baseboard. That's where the feet are placed and you might have to move those inward so they don't foul the disc. That might make the whole telescope less stable. Also, how do you plan to make sure that on rotating the disc you keep it concentric with the rotation axis of the telescope? Nigel
  4. Agreed Nigel
  5. I would think not. Paint is an organic polymer which is easily degraded while the coatings on optics are inorganic and not easily attacked. Nigel
  6. The main problem when going to ever larger apertures for solar is the extra energy collected by the larger objective. Your ERF has to handle more than 50% extra energy when going from 80mm to 102mm aperture. Hence the increased cost. Nigel
  7. Stopping down the aperture as you suggest is exactly the same as in camera lenses. Varying aperture with fixed focal length gives variable focal ratio although camera lenses do this internally it is perfectly acceptable to do it externally as you suggest. Nigel
  8. The mirror has to be held very loosely so that it can follow the curve. If you set the overhang greater than 50% there is every possibility of the mirror tipping over the edge of the tool so max about 45% if centre over centre. With an offset then the max stroke will be less. You should get there with about 30% ( 1/3 ) stroke with a small offset. More info would be useful. What is the size of your mirror, Have you made mirrors before? Nigel
  9. It is amazing how flimsy the barrier can be when casting the tool base. I have used polythene from a dog biscuit bag as the barrier without any reinforcing When the mortar is set make sure that it will sit firmly on it's base before finishing it off. It is v. annoying if it rocks as you try to push the mirror over the tool. Nigel
  10. The optical components might well be figured as a pair but the mechanical components will be common. I think that Peter's offer will be the best. Nigel
  11. "I do have a generally layout for the scope cobbled together. It is a Serrurier truss ~900mm f/6 150mm. I'm going to try to keep the trusses symmetrical front to back, but the COG may change depending on how things weigh up. " "It has a 1.2" /1.92° diagonal 100% illuminated area which will cover a cropped DSLR sensor using a 2" secondary. The central obstruction is 33%. " He is doing a 150mm diameter telescope. The 900mm refers to the focal length. Nigel
  12. Monica Grady on the search for life on Mars. Nigel
  13. As title. Nigel
  14. The music is "Alpha" by Vangelis Nigel
  15. As title. Radio 4 is having a number of programmes on the theme of Mars this week. A mix of factual, speculation and Sci-Fi. Starting today at 2:30 with a dramatisation of H.G.Wells 'War Of The Worlds'. Nigel