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I think the answer is No, but just throwing this out there: Is it possible to have the following in the imaging train and still achieve focus? * Field rotator * Coma Corrector * OAG * Filter wheel * An imaging camera like the ASI1600MM which has a 6.5mm back distance to its sensor I have all but the field rotator and all is well, however I can't see that my Skywatcher 200PDS has enough back focus to accomodate it as well. |
5.87
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In the standard ZWO 55mm backfocus config (+1mm for filters) you squeeze an OAG and a camera and a filter wheel. The OAG has 16.5 mm optical length, and you have 13mm between OAG and filter wheel (20mm). Does that fit with the back focus requirements of your scope/corrector? Manual rotators have about 13mm optical length, so in principle it should work if you replace the 13mm m42 tube (usually a 2 male m42 adapter (2mm) and a 2 female m42 spacer (11 mm), so net 13m male/female) with a manual rotator with two m42 threads. If it gets too tight, there are OAGs from TS Optics at 11mm net, give you some extra space. For an motorized rotator (like a Pegasus Falcon) you need to find more like 22mm, which probably gets too tight. Matthias |
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11.01
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In the standard ZWO 55mm backfocus config (+1mm for filters) you squeeze an OAG and a camera and a filter wheel. The OAG has 16.5 mm optical length, and you have 13mm between OAG and filter wheel (20mm). Does that fit with the back focus requirements of your scope/corrector? Manual rotators have about 13mm optical length, so in principle it should work if you replace the 13mm m42 tube (usually a 2 male m42 adapter (2mm) and a 2 female m42 spacer (11 mm), so net 13m male/female) with a manual rotator with two m42 threads. If it gets too tight, there are OAGs from TS Optics at 11mm net, give you some extra space. For an motorized rotator (like a Pegasus Falcon) you need to find more like 22mm, which probably gets too tight. That's what I thought and so my answer is definitely no. Because the SW coma corrector has too long threads, the gap between the OAG and the filter wheel is reduced to an extent that not even a manual field rotator will fit. I was looking for a motorized field corrector and have definitely run out of back focus for that  My dreams of a setup that supports totally remote control will have to wait until I upgrade the OTA.
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5.87
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I feel with you, I try to find a combination that fits a external focuser, an ONAG, a rotator, and a FW into to 146.5mm of my Edge11 - all „affordable“ ones (whatever that means) miss, often just by a few mm. There is a solution, but it costs more than my scope … Matthias |
4.72
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Hi, I just purchased a GSO 6" F/6 Newtonian and recently did a lot of work figuring out the imaging train. Right off the bat, I'm unsure about the field rotator, and I don't recall ever seeing that in the newtonian forums on Astrobin or CloudyNights. The coma corrector is probably going to dictate your imaging train equipment. Different coma correctors have different backfocus spacing requirements. The most common is ~55mm like in the Baader MPCC, the dedicated Skywatcher CC, the Explore Scientific HR CC, etc. Other coma correctors like the Baader Rowe CC, or the GSO/Apertura/Orion/OPT/Astro-Tech generic CC has a broad requirement of 70-80mm, which allows you to use more equipment in your imaging train. In your case I would recommend using the dedicated Skywatcher CC because you'll know it'll work with your OTA and achieve focus. According to the ASI website, you should at least be able to fit both the Filter Wheel and OAG between your camera and CC. You'll be able to manually rotate the field simply by rotating the imaging train in the drawtube. |
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3.15
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Get a filterwheel with built-OAG. They exist as far as I know and should create enough additional backspace room for all the parts you want. CS Sean |
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5.87
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| Unfortunately that does not help, these combos need more space between FW and sensor (otherwise you don‘t get the guider in focus), so they end up again with ~54mm |
6.77
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Andy Wray: It's not the Skywatcher 200PDS that determines your back focus. It's the coma corrector that determines the back focus and how much space you have in between the coma corrector and the camera. I did find this coma corrector - it has a far larger back focus of 91.5mm (most others have 55mm). I have 0 experience with that coma corrector so I have no idea how it performs. - But in theory, that should give you enough space in between the coma corrector and the camera to put in a rotator. Before that though; what do you need a rotator for? I'm no expert. And others that have experience with rotators might be able to give better information. But I see it as just another element that can fail/move/vibrate/cause flex/backlash, etc into your imaging train. And here is the tricky one. Technically you would need flats for every single angle that you use, as dust particles and stuff on the mirrors won't rotate together with your imaging train, and you would need new flats for that. - same goes for collimation, that one needs to be absolutely spot on, errors there are less noticeable if you stick to one angle.. but rotating the imaging train in a Newtonian will be affected by imperfect collimation. Which might also affect vignetting slightly.. And again, would require flats for every angle that you use. Depending on your use, it might just be a lot easier to stay clear of it and stick to the rotation/crop tool in photoshop. Good uses are Alt/Az mounts, remote rigs, or big mosaic use.. but for single target use in the back yard.. I might stay clear. |
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| Image train rotators (to distinguish them from field rotators as used in Alt-Az configs) aren't really worth their money for what they do and much less worth the additional hassle of having to re-calibrate flats and everything that goes with it, including obviously PE curves. So much so that none of the remote service providers that I know of offer this as a add-on extra. |
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11.01
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andrea tasselli: OK, that's given me food for thought. I guess with the right camera and OTA combination I could just take care of image composition in post and with mosaics it would just be some careful planning. Good point and one less mechanical thing to worry about. |
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| I thought i might add that there are focusers with built in rotators, which won't mess up your backfocus. While these also perform better than the stock focuser of the PDS and often has an electronic focuser built in they are not cheap by any means, see nitecrawler for example. |
