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About Astrobits

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  1. You will find that the dry grits are very heavy and do not readily get suspended in the air. Even the polishing Cerium Oxide doesn't become airborne very easily. However it is best to make up the polishing slurry well in advance to allow any agglomerates to break down before you use it. Nigel
  2. A few thoughts. Having made a number of mirrors I can confirm that keeping things wet will totally eliminate any dust to the extent that you do not need any eye or face protection. Nothing flies up from steady wet working. Don't try to make a slurry of the coarser grits in a bottle, it will just settle far too quickly and become more of a hindrance than a help. I only made slurrys with the two finest ( aluminium oxide ) grits that I used. The coarser grits ( silicon carbide) were applied to the work from a shaker jar, a coffee jar with holes drilled in the lid, or alternatively just using a spoon to ladle from the bulk container and sprinkled over the tool. I don't know how thick your tiles are but you better plan on making a second tile tool when the first one wears out. If the tiles are about 6mm thick they are unlikely to last to the finest grit. You will then need another to finish off. This second one can be made to the curve that was generated by the first but will still need bedding in. I used 10mm thick glass tiles and they will last for one mirror 8" mirror. When placing your tiles don't use a regular pattern, that will almost certainly lead to a mirror with zones, Also make sure that there are no very narrow spaces between the tiles where grit can become difficult to remove. I usually filled the spaces between the tiles with candle wax ( melted in a pan and spread around where necessary with a hot air gun ) which could be melted out at the end of a grit size ( using the hot air gun ) bringing any embedded grit with it, then fresh wax was poured on sealing in any wayward bits of grit. Just make sure that the level of the wax is below the level of the tiles if you do this. As regards plaster then any plaster will do, just make sure that your casting is allowed to thoroughly dry ( it takes days. However, beware-- all my faster drying attempts using heating in an oven resulted in the tool cracking) and then waterproof with varnish. Wet plaster is not as strong as dry plaster. When casting a plaster tool you can use surprisingly flimsy appearing material for the circular dam wall. I have used polythene bags ( the thicker ones like the large dog food bags) cut to size and held with sticky tape around the mirror blank. The advantage of using a flexible material is that when you pour the plaster into the mould the shape naturally becomes round whereas a rigid dam will not. For cleaning the tool and mirror during grinding I do not think that sponges are the best choice. Just dunk them in a bucket of water and use your hand to brush the water over the mirror surface ( a finely ground mirror surface feels wonderful). More care needs to be taken with the tool as there will be sharp edges. Here a nail brush is most suitable to get between the tiles. For the work area then copious water flow would be best. For examining the ground surface a 10X loupe is sufficient even at the finest grit sizes. The higher the power, the smaller the area in view and the longer it will take to examine the entire surface to find that one rogue pit. More important than magnification is method of illumination. I made a light box with a glass top. A piece of black card with holes in it ( approx 1" square chequerboard style) was on top and the mirror placed on that. It is easy to focus on the pits at the (out of focus ) boundary between dark and light. I am sure that you will have many questions once you get going. Have fun. Nigel
  3. I note that your location is Fishguard. Is the deposit on your equipment salt spray deposit? If so you will have to dissolve it off with water with little or no rubbing in the first instance. You can follow that with Isopropyl alcohol/Baader fluid and finally distilled water. Nigel
  4. It is almost certainly true 7x 50. A popular ruse by cheap manufacturers to make the bins seem very powerful is to quote the mag as area not linear. All 7x (linear) 50 bins give 49 x area mag. Often used to see ads in the magazines years ago for 50x mag bins. Nigel
  5. Are you sure that it's the telescope and not your eyes? Eyes change with time and Opticians cannot always fully correct for astigmatism. They can only correct for the major axis and there could be some minor astigmatism at a different angle remaining. I know because I have this problem and I can detect changes in my astigmatism over time-scales of less than 1 year. Just look at the Moon with and without any glasses you use. Is there a another, fainter, image nearby? Do any glasses you use eliminate all secondary images or is there still a faint secondary image remaining, probably in a slightly different position? Whatever, if you can detect a secondary image then you have uncorrected astigmatism. I would not have thought that at 10 years old your telescope would be deteriorating unless it has had a knock severe enough to put the lenses out of alignment. Nigel
  6. I too have a mirror ( 8 3/4" dia ) with very similar writing on it. I believe David Hinds used a 4 figure code which is present on your's and mine. Nigel
  7. While it is quite easy to test primary mirrors with a variety of tests, it is not so easy with secondaries. During manufacture, secondaries are tested against a reference flat when the interference fringes can be seen. Once the secondaries have been coated and in use for a while there are two problems that arise. Firstly, they will have some material ( dust/dirt ) that has settled on the surface and has stuck so that the usual gentle cleaning does not remove it. It does not need to be large enough to be visible, a few wavelengths in size is enough to prevent a glass reference surface getting properly parallel to get a good set of interference fringes. This can be overcome by using a liquid reference surface. Secondly the secondary is very reflective while the reference surface is not. The difference causes the fringes to be very faint because of the imbalance in the reflectance of the two surfaces. Using a liquid reference surface where the mirror is under the liquid, usually water, this can be partially overcome by adding some water soluble dye which attenuates the reflectance of the mirror. It needs rather a lot of dye as the layer of water over the mirror needs to be very thin in order to get stable fringes. I have tried this and it works to a limited extent with the fringes a bit easier to see. Nigel
  8. 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
  9. For doing occultations, IOTA ( International Occultation Timing Association) require analogue cameras coupled to a VTI ( Video Time Inserter) and recorder. Nigel
  10. 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
  11. 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
  12. 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
  13. 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
  14. 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
  15. 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
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