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The 200PDS has a focusser tube, with an adapter ring at the camera end that is larger in diameter than the rest of the tube. This is also true of the compression ring replacement I have bought which also has 2 screws 120 degrees apart to secure the coma corrector. I added a 3rd screw so now I have 3 screws 120 degrees apart. I also have an Explorer Scientific long bodied coma corrector attached to either ZWO ASI 2600MM or 2600MC cameras with APS-C sensors. The larger sensors made corner star problems worse. Because of the different diameters inside the focusser tube there is considerable slack with the coma corrector at the focussers end where it meets the tube. I thought the compression adaptor would help centralize the optical train but it doesn't. I inserted 3 pieces of Gorilla gaffer tape at the scope end inside the focusser tube to help take up the slack and centralize the optical train but I am still getting variable tilt issues and probably some back focus errors. It seems to depend on which coma corrector and camera I use but with variable results I don't know where I am. Also the ZWO cameras are known to have sensor tilt issues so that complicates things. It occurred to me that changing the optical path by 90 degrees in a Newtonian has unfortunate consequences, apart from collimation difficulties which I think I am OK on. Is this why professionals use Ritchey Chretien Cassegrains, which have the optical path all along 1 axis? I don't want to spend years sorting out tilt issues. I have used CCD inspector in ASTAP to quantify the problem but how to do it is something else. Does anyone have the answer to the sloppy coma corrector in the tube problem? What it needs is for it to be a proper sliding fit and if not, to be locked in place without changing its orientation. Would a better focuser help? Also ASTAP image inspector always gives the smallest stars in the centre and bigger at the corners. This must be field curvature? Is this something one has to accept as a design limitation? I can't find any information that the Explorer Scientific reduces field curvature. I tried focus pocus to help sort out tilt and backfocus issues but it failed before it finished as results were too variable. Any help would be appreciated. |
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Replacement with a focuser better designed to handle these issues is always an answer although the price would probably on par with the tube itself if not more. The Moonlite focuser I have has none of those issues with the long CC (SW but they are pretty much all the same) . A possible way around with the current one, if the CC can be secured to a position whatever the tilt might be, is to buy an external tilt corrector, such as the following (disclosure, I have one): https://www.gerdneumann.net/english/astrofotografie-parts-astrophotography/ctu-camera-tilting-unit.html This unit is also sold by at least one UK retailer. As an additional option for those with both the inclination, the skill and the tools needed, is to manufacture a new draw tube with a better match or a threaded fit if the CC supports one. |
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| Thanks Andrea. At the moment the possibilities are that there are camera and focuser tilt issues. I also think I have to sort out a backfocus problem. Until 1 is eliminated then it will be very difficult to rectify either. A new focuser and tilt corrector will cost much more than the scope. Am I right in thinking this is why pros go for the Ritchey Chretien Cassegrains? I read a recent post from someone who had bought many scopes said that his was the simplest to operate and best to own that he had had. Mind you they cost a lot. I can understand that it must be difficult to attach a CCD chip to a board at the exactly the right angle so I wonder how many camera have tilt issues? |
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David Moore: There are many telescope designs out there but the cheapest and the simplest to operate is still the newton configuration. RC design does not guranteed orthogonality per se and has a number of strings attached yet it is compact and aplanatic by design (and less sensitive to collimation issues), which means they can grow in size to an extent that cannot be matched by the newton configuration (which isn't aplanatic but has a rather gentle field curvature and very little astigmatism). This said, with the current crop of CMOS cameras tilt issues are bound to affect quite a number of newtonians owner, more so if in the f/4 (or faster) variety, myself included. In my case I solved the issue with use of shims in one case and the aformentioned tilting unit in the latter (once I get other issues sorted). |
