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.
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.
