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Astrobits

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About Astrobits

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    Somerset
  1. Astrobits

    Source of 1m of 8" Ducting

    When I was making dobs some 20 years ago I found a local one man dust control business who made his own ducting. I had a number of tubes made by him for 8" and 10" mirrors. The maximum length he could make as a one piece was 4' ( 1.2M) as that was the length of the rollers in his machine. Nigel
  2. Astrobits

    Advice On Dob Mirror Cells Please...

    I am not familiar with the SW mirrors/cells but it appears from your pics that SW mounted this mirror from a central boss only. Certainly the molded ribs would not permit the conventional triangular mounting systems as there are eight ribs and it is not going to sit neatly on 3 x 3 supports. I would suggest that you keep the SW cell mounting system as is and arrange it in the mirror box of your choice. Nigel
  3. Astrobits

    Mirror grinding

    Davey, Peter, In a previous post Tonyhaz mentioned that he had a 10" mirror re-coated and the comment from the re-coaters was that it needed re-grinding. I am assuming that this post refers to the same mirror. Nigel
  4. Astrobits

    Mirror grinding

    Check the price of a new mirror and deduct the cost of a blank from that. For a 10" the I am guessing blank will be somewhere between £50 and £150 depending on your material. Galvoptics no longer list float glass blanks on their web site, Beacon Hill list a 10" x 1" float glass at £51. It is possible that with an unknown glass blank ( your mirror ) there might be a reluctance to undertake the re-working as any fault in the glass could cause problems with it in the future. As I assume that the mirror is your Dark Star one furture problems are unlikely. Nigel
  5. Astrobits

    Cure for Aperture Fever?

    In the pics it looks like they have four of these sets, Multi-mirror telescope anyone? November Sky & Telescope is a big telescope issue. Don't read if you suffer from Aperture Fever. Nigel
  6. The cell that failed for me didn't have any horizontal screw holes and it failed diagonally through the sheet from a surface screw hole. A very difficult way to break MDF. I expect that Aquarium Silicone will be O.K. I have just looked at my supply, it is Ever Build Everflex 225 and it particularly mentions glass to glass applications. Other products are available Nigel
  7. I'm don't know if the Evo Stick adhesive will do the job. Ordinary mirrors are silvered on the back and this is then overpainted so the Evo Stick is intended to stick to paint. You're welcome to try it but I have used the greenhouse silicone for over 20 years without problems and don't see any reason to try something different the next time I stick a mirror to MDF. I found priming the MDF was essential as without it the failure was always the silicone to MDF bond. In the 90's I sent an 8" F/6 Newtonian tube via couriers who proceeded to drop it with such force that the 19mm MDF in the cell broke sending the mirror down the tube, with the remains of the cell still siliconed to it, to hit the secondary That proved to me that the procedure that I have adopted was good ( apart from the use of couriers that is ). I have come across the felt option mentioned in Ed's post and I found it difficult to separate the mirror from the cell when needed as the felt blocks up the saw teeth and, being wool, doesn't surrender to wire very easily. It is also very important to use only three blobs of silicone and not too large, 1" dia for an 8-10" mirror is sufficient. . Use 4 or more, or go too large in area and you are immediately in astigmatism territory. My 16" mirror has been on three blobs of less than 2" dia for at least the last 10-12 years. Nigel
  8. NO. It will not give you two important properties. 1. space between the MDF and mirror to remove the mirror easily next time. 2. prevention of astigmatism by contacting the mirror at only 3 points ( small areas, about 1" dia ) only. If you have a raw MDF surface then prime it with a thin coating of epoxy and roughen it. Apply three blobs of Silicone ( I use the clear type that mentions use for glass applications ) , enough to give a blob about 1"dia and 3mm thick and place three spacers about 3mm thick on the cell before presenting the mirror to the cell and pressing down to the touch the spacers. Wait 24 hours before removing the spacers to let the Silicone cure. Job done. Nigel
  9. Astrobits

    Rebuilding 18" dobsonian

    There are different textures to laminate surfaces, some are better than others while smooth is very poor ( indicating that coarser might be better ). I have used coarse textured laminate from a variety of sources with perfectly acceptable results. You can usually get some free off-cuts from a worktop manufacturer ( if there's one nearby). My 16" is on such a sourced laminate. You will also get different movement characteristics by changing the size and type of plastic against which the Formica if rubbing, PTFE ( various brands available e.g. Teflon, Fluon etc. ) being the defacto norm for the type of plastic. Size you can vary to get the best performance. Nigel
  10. Astrobits

    Rebuilding 18" dobsonian

    "EBONY STAR" is an American trade name for Thermosetting resin ( eg phenol/formaldehyde ) sheet used primarily for worktops. If the books that mention "Ebony Star" had been written in England they would probably have used the trade name "Formica". So where "Ebony Star" is mentioned just use any textured worktop material for the base of your rocker box. Nigel
  11. Astrobits

    Rebuilding 18" dobsonian

    The split blocks on my 16" were made with MDF and have survived for over 10 years so far. With natural wood the orientation of the grain is important as all splits will be along the grain. Ply should be better but make sure that it is not bent too much when clamped up. Nigel
  12. Kodak's Technical Pan film no 2415. A fine grain film that responded to "Hypering" or Hypersensitising. There were several ways that were used. Simple baking in an oven ( water tends to desensitise film ), baking in nitrogen or even in Hydrogen ( v. dangerous but was the best result ). Nigel
  13. Yes, He used any suitable supplier, mainly David Hinds, but I still believe that he started making his own at some point. Therefore the early telescopes would be purchased mirror sets but the later ones will be a mixture of purchased and Dark Star made mirrors. Nigel
  14. Yes, I believe that Dark Star did start making their own mirrors later in the life of the business. Unless DS was for David Sinden. Might be an idea to check the figure on the mirror? Nigel
  15. As I understand it, Richest Field refers to the maximum number of stars you can see in the field of view. I am not sure that DSO's are considered. As aperture increases the number of stars visible increases, and to see the widest field the lowest magnification is needed. However, as the magnification decreases so the exit pupil increases and when the exit pupil exceeds your eye's pupil then the effective diameter of the objective decreases thus limiting the number of faint stars visible. Thus everything is interlinked and the result is, as far as I am aware, the Richest Field instrument has been determined to be at 9X63 for a 'normal' 7mm eye pupil. If you have an eye pupil of only 5mm then 9 x 63 will only be working at 9 x 45. Therefore your own Richest Field instrument depends on you and your own eye's pupil dia. Multiply that with the magnification you want and you get the maximum diameter objective that you can use. Nigel
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