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James Vincent

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  • Content Count

    32
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About James Vincent

  • Rank
    Nebula

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  • Website URL
    astropictures.co.uk

Profile Information

  • Gender
    Male
  • Interests
    Telescope and mirror making, deep sky and planetary imaging.
  • Location
    Chelmsford Essex
  1. Hi These telescopes have a Grade A David hines Mirror in them and are well worth doing up. I restored a 8.25 inch a few years ago, How bad is the secondary mirror? Jim
  2. yes you are correct, its the 4 lens type like the Petzval corrects for field curvature . The third lens can be used to correct some off axis aberrations. The standard refactor or reflector the field curvature is related to the focal length of the telescope, the shorter the focal length the greater the curvature . :}
  3. Hi Mike Is it possible to draw out the ray diagram of the proposed set up i am a little confused. The Ronchi screen is usually positioned where the the knife edge would have been.
  4. An 8 inch dob is easy to use and will make a good first scope. To get the best out of it ensure it is collimated correctly http://astropictures.co.uk/culminationpg.html Also allow the telescope to cool down if it has been kept in the house before using it, then enjoy. Apparently after receiving a telescope it is usually cloudy, the number of cloudy nights is normally proportional to the aperture in inches. Lol
  5. I do Agree that Refractors give a higher contrast image than reflectors because there is no obstruction, reflectors also suffer from coma that inversely proportional to the F ratio. is the secondary mirror on your dob glued on to the mounting or is it in a cell? The reason i ask is that lots of dobs suffer from distortion of the secondary mirror causing astigmatism.
  6. Globular clusters start looking good in a 10 inch telescope, however omega Centauri or O my God Centauri we called it, looks great in binoculars when observed in Australia.
  7. Have you ever had clear sharp views through your 14 inch dob ?
  8. Absolutely, resolution is proportional to aperture. My 20 inch dob makes planets look fab A 8 inch TMB APO dos not come close. A 14 inch dob should easily out perform a 120 ed no question. Maybe there is a problem with the dob. i e check the collimation, make sure it is cooled down before using it. Are the optics good, the main mirror may be 10 th wave but what about the secondary mirror? is the secondary mirror distorted because it is glued on with silicon sealant.
  9. A big dob its the only way to Go. Aperture wins, a 10 inch dob will beat a 5 inch refractor and a 8 inch SCT on all objects. providing it has good optics
  10. The best telescope is the one that is easy to set up and use, as it will get used the most. if you belong to a club have a look through the members telescopes before deciding.
  11. look for a dob on UK astro buy and sell , you could pick up a 12 inch light bridge for less than £500 if you are lucky.
  12. Just a quick tip on laps, it is not necessary to strip an old lap, it is possible to pour a new one over the top of the old it saves time and reduces the mess.
  13. You can use the fixed post machine to parabolise however the turntable will need to be running very slowly, push the tool over the mirror as described in my last post by hand, you can of cause use a arm as Nigel suggested. The back and fourth motion of the tool should dominate . Don't forget to press the tool to get good contact prior to figuring, then work the mirror for 20mins then test it. Don't press to hard when figuring you could end up with a rough surface. The tool should feel like its heavily lubricated when figuring, when the tool is in good contact with the mirror and the water and polishing powder mix is correct.
  14. Not sure what you mean by top post machine, but a driven table rotating at a few rpm would be would make life easy. Using a 50% sub Dia tool tool with the tool on top pushing from one side of the mirror to the other with a 20% overhang both sides will deepen the center. This stroke should be off set so that it dos not go through the center of the mirror, reduce the amount of overhang if there is a turned down edge. 3 phase Motor speed controllers are great they will allow you to remove some of the pulleys but not all of them because the motors lose torque at very low speeds. The controller will also allow a three phase motor to be controlled from a domestic single phase supply, i use these on my machines.
  15. Hi Rick You do not require large powerful motors to make a grinding/polishing machine, 1/2Hp is plenty. lots of electric motors and car parts ( Mondeo rear stub axle very handy for turntable) on E bay. When testing the mirror use the Foucault test and a Couder mask and input the data in to a program like figure XP . You can not figure a mirror using the Ronchi test on its own. Hope the wrist gets better soon.
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