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About Astrobits

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  1. I have made Acrylic sheet. I worked in Perspex research. The sheet is cast against ordinary glass sheets and will therefore AT BEST take up the surface morphology of the glass. If you test ordinary glass sheet you will find that it is not optically flat but covered with hills and valleys of at least a few wavelengths high and low. These undulations will be present on BOTH sides of an Acrylic sheet. As mentioned in above posts the bulk of the material may, or may not, be homogeneous. All these defects will degrade any image in addition to your spherical primary. Chriske mentions that float glass is used in some SCT's but that will have been selected for homogeneity and optically polished both sides to produce the complex curve SCT's require. Maks require more simple curves but of very short radius and the thickness of the corrector is important. Go check the formula for a Mak. Nigel
  2. For doing occultations, IOTA ( International Occultation Timing Association) require analogue cameras coupled to a VTI ( Video Time Inserter) and recorder. Nigel
  3. Quite. Which is why I qualified my remark with "Done properly it is very accurate". The spacing between mirror and lens is critical . Putting the lens on the end of the focussing tube is not going to give the required result. Nigel
  4. Actually Olly, that is the basis of the null testing set-up for parabolas. A simple positive lens is used to put in the same spherical aberration, but opposite sign, when testing a parabola at it's centre of curvature. Done properly it is very accurate. In the Jones-Bird design it is a negative lens that also has the effect of lengthening the effective focal length of the instrument. Nigel
  5. The fact that the round(ish) laser spot becomes a line on the primary mirror worries me. Looks like severe astigmatism in the lens or secondary mirror. Try inserting a piece of white paper before the laser hits the secondary. If it is a line instead of a spot then the lens is a bad one or is not square on to the optical axis. I would suspect that a previous owner fiddled with it or it has moved during transport. If it is still a spot then the secondary is questionable. Whichever is at fault you will never get good focus without fixing it. I suggest that you go to your local astronomy club ( there is one in Melbourne ) and seek their help. Nigel
  6. I don't see the lists when I view as guest on my tablet but they do appear when I sign in. There's even more SGL stuff if viewed as desktop though as noted by John above. Nigel
  7. I don't think I saw a mention of alignment shift with focus/backlash previously in this post. Sorry if I missed it. If that is the cause it still implies that there must be a limit to such movement while the telescope is in one position. You don't mention if you have an observatory but views from the house would indicate not. Pity, as you could leave the scope pointing in one direction for a day or two and then take some shots without moving the scope to another sky position. Any slow slop should have reached it's limit by then. Shots of the brick wall might not be representative due to the mirror being out of it's normal operating position. I was thinking along the same lines as Dave with the extra mass of the RASA causing extra friction in some components or slippage in a clutch ( would not have thought so with the claimed load capacity but you never know). Nigel
  8. While I agree that the movement in one individual exposure is only arc seconds, your post on the 18th: Quote: "To illustrate the issue better, these are four 300 second exposures guided. Guiding was helpful in this case, but up to a point after which it couldn't keep up with the drift. I would have to stop guiding and exposing and re-centre the image every five subs otherwise it would slowly drift quite a ways". end quote. clearly states that the movement continued for at least 5 consecutive subs at which time the total movement was sufficient to require re-centering the target. This amount of movement is much greater than a few arc seconds and should have caused an out of collimation situation if only one of the optical components was moving. As this does not seem to be the case then the whole of the optical system must remain in their correct positions. The fact that the guiding set-up works at some points indicates that there is nothing moving within the RASA when the guiding is on.at that time. I cannot see this trail going anywhere but back to the mount and drive system. Nigel
  9. If whatever it is that is moving is doing so consistently over a considerable period of time without stopping then logically whatever bit is moving will fall over or out!! I assume that that hasn't happened. Also if something in the optical train is moving to shift the star positions then the collimation will be out at some point and should show up in your longer star trails as a changing of the width of the tracks in at least one corner. I see no evidence of this. I think you should look again at the drive train of the mount. Nigel
  10. Looking at your images, it strikes me that where there are long trails they are far too regular to be caused by some sloppy component. Surely, a sloppy component would slop from one position to another fairly quickly and then stop. A sloppy component would have a limit to it's movement so the movement should become less with time in one telescope position. Do repeat subs show the same amount of movement over the same time periods? Nigel
  11. 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
  12. 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
  13. 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
  14. 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
  15. 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
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