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About Astrobits

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  1. Astrobits

    Imaging opportunity - Comet Iwamoto

    It has already got to mag 6.5-6.7 according to four observers last night. Another opportunity for imaging. It will be passing M95 by less than 1 degree on the night of the 10/11th with M96 just over 1 degree away. Nigel
  2. Astrobits

    DIY star magnitude measuring device

    In addition to the problems noted by Vlaiv, photoresistors are both wavelength and temperature dependent. You will therefore need to control both to get consistent results. Also they are non-linear so you will need very closely matched comparison stars ( both in brightness and proximity ) to your variable to get good results. To be useful to the variable star records you will need to be comparable to the use of DSLR's where measured brightnesses are now quoted to the 3rd decimal place. I am like Vlaiv, not sure this will work other than as an exercise in electronics. Nigel
  3. Astrobits

    Possible mirror blanks

    As you don't have any 6" glass you might as well get a complete 6" kit. The 8" x 0.5" mirror you have would be a suitable tool for making an 8" f/8 mirror. On the Cloudynights forum there is some discussion on this topic and one small mirror there is claimed to be accurate to 0.2 wave. However, I have my doubts that the 17" is anywhere near that level of accuracy, I do not see that the design of an optical comparator would warrant the high level of accuracy needed in an optical reference flat, and consequent cost. Perhaps your local astronomical society can help you to get the mirrors tested. Nigel
  4. Astrobits

    Possible mirror blanks

    Andrew is quite right. If they are optically flat they will be of more value than same size mirror blanks. You just need to get them tested. If they are coated then it's tricky to do the tests. If they are uncoated then carefuly put the smaller one onto the larger and allow them to thermally stabilise for a few hours. You should be able to see fairly straight interference bands if both are optically flat. This won't work if only one of them is flat. Nigel
  5. Astrobits

    Possible mirror blanks

    The 17" is fine at 1.25". But the 8" is a bit thin if you have no experience. My 16" is 1" thick and performs fine. Go to Mel Bartells site and see how to make a 25" which is 0.5" thick. Mel's web site: www.bbastrodesigns.com Nigel
  6. Astrobits

    Lets hear it for moths

    Want to catch moths? just turn on a light. We know this but so do other denizens of the dark. Bats, for instance, know that there is a veritable smorgasbord of food around lights. And not just bats; if you live in the tropics you will get geko's clustering around lights just waiting for their food fly into their open mouths; I know, I have watched them. Another interesting phenomenon is that certain moths can detect the bat's sonic emissions and head for cover if they detect bats nearby. Strangely this defence mechanism is rendered U/S when the moths are near bright lights enabling the bats to catch them when normally they cannot. All of this changes the balance of nature and usually in unforeseen ways. Nigel
  7. Discussion on Venus Nigel
  8. 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
  9. 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
  10. 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
  11. 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
  12. 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
  13. 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
  14. 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
  15. 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

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