lukebl Posted July 9, 2011 Share Posted July 9, 2011 Aaargh. Someone help me PLEASE before I do something stupid! I've just got an SXVF-H9 CCD, and I am trying desperately to get some decent coma-free images with my 250mm Newt and Skywatcher Coma Corrector. I've used the Coma Corrector very successfully with my DSLR, with nice round stars right to the edge. However, I just can't seem to lose the coma with my CCD imaging, even with the corrector in place. Here's a couple of images showing out-of focus stars in the corners and at the centre using a) no corrector, and with the corrector in place.The fairly round stars in the centre tells me that collimation is reasonably OK, and the elongation at the corners indicates the coma. However, the coma looks just as bad with the corrector in place. Could it be just that the Corrector isn't at the correct distance. It is roughly the same distance to the sensor as with the DSLR, but is that distance really critical? In which case I'll just have to play around with spacers till I get the distance right. Or is something else amiss?And please don't suggest I give up the Newt and try imaging with an APO refractor instead! Link to comment Share on other sites More sharing options...
gbealnz Posted July 9, 2011 Share Posted July 9, 2011 Hi Luke,he he, I did have an 8" Imaging newt, and went back to the refractor for that reason, but you can beat this I am sure.The H9 will be miles more forgiving anyway, the chip is way smaller.Spacing?With the MPCC (which I assume you use), the DSLR would have screwed straight on, and then the MPCC/DSLR inserted into the focuser, correct?The H9 is slightly different in that you need to have "about" 55mm from the rear shoulder of the MPCC to the chip. The H9 has a rough distance of about 17mm from the chip to the front face (metal face) of the camera, so you need to fill the space with a T extension or something similar, to the tune of about 38mm (55mm - 17mm =38mm). In my case I sometimes screw the MPCC into a Baader 2" nosepiece, plus a 7.5mm Baader extension tube and this gets me close.Is this what you are doing? If it is, then perhaps a little experimentation to try a few differing distances?Gary Link to comment Share on other sites More sharing options...
lukebl Posted July 10, 2011 Author Share Posted July 10, 2011 Just now measured the Corrector-to-sensor distance and it's 60mm, instead of the 55mm it should be. Does anyone know if that extra 5mm is sufficient to prevent the coma corrector doing its job, and is causing the results shown above? Link to comment Share on other sites More sharing options...
steppenwolf Posted July 10, 2011 Share Posted July 10, 2011 I would say that 5mm over-length would be more than enough to upset the optical correction. Baader recommend ± 1.0mm and you are well outside that. Link to comment Share on other sites More sharing options...
ollypenrice Posted July 10, 2011 Share Posted July 10, 2011 Steve's right, 5mm is way too much error and the faster the system the more it matters. If you go to the Telescope Service or Baader Planetarium sites (I find TS easier to navigate) you will find all sorts of gadgets for absorbing space!They also do a nice line in small apo refractors while you are there... (Exit stage left persued by a bear.)OllyPS Get this working and you could do some collaborations with Mike (Yfronto.) Link to comment Share on other sites More sharing options...
lukebl Posted July 10, 2011 Author Share Posted July 10, 2011 Thanks guys. Just the information I was needing. (I was kind of anticipating Olly's comment regarding refractors!) Link to comment Share on other sites More sharing options...
Merlin66 Posted July 10, 2011 Share Posted July 10, 2011 Hmmm, the replies are assuming a Baader CC (?) whereas you mention using a Skywatcher CC - is the Skywatcher designed for the Newt or just refractors????? Link to comment Share on other sites More sharing options...
lukebl Posted July 10, 2011 Author Share Posted July 10, 2011 Hmmm, the replies are assuming a Baader CC (?) whereas you mention using a Skywatcher CC - is the Skywatcher designed for the Newt or just refractors?????My one is definitely designed for the Newt, and I think it is virtually identical to the Baader one. I've measured all the distances when it's fixed to the DSLR with the proper attachments, and the corrector-to-sensor distance is 55mm, the same as the Baader one. Link to comment Share on other sites More sharing options...
ollypenrice Posted July 11, 2011 Share Posted July 11, 2011 Thanks guys. Just the information I was needing. (I was kind of anticipating Olly's comment regarding refractors!)If you get results anything like Mike's your revenge will be sweet indeed!!!What are you eating in your new avatar?? Olly Link to comment Share on other sites More sharing options...
lukebl Posted July 11, 2011 Author Share Posted July 11, 2011 What are you eating in your new avatar?? OllyThat be Patrick Troughton (Dr. Who number 2) playing his recorder! And if I don't get results as good as Mike, then I will be very cross. Link to comment Share on other sites More sharing options...
lukebl Posted July 15, 2011 Author Share Posted July 15, 2011 Well, I've now got the Coma Corrector to sensor distance correct, but I've now got egg-shaped stars when imaging with my SXVF-H9. Anyone got any suggestions why this is happening?This is a single 20sec exposure, with an enlargement of a small area. The elongation of the stars seems to be in a consistent direction across the frame, unlike normal coma, and isn't caused by tracking errors as the stars are the same shape with a very short exposure. I don't think the fault is with the corrector, as it works fine with my DSLR. Collimation seems to be OK, as does the alignment of the camera to the imaging plane.I know that imaging with a 10" isn't everyone's preference, but it's worked very well till now with my DSLR.Whole frame:Closeup: Link to comment Share on other sites More sharing options...
Starlight 1 Posted July 15, 2011 Share Posted July 15, 2011 Hi Luke I am watching all your posts as I am using just about the same set-up as you, hopefully all your answers will help me as well, p.s. just down the road in Norwich, maybe we could have a get together one day and try and help each other with the issues.Les Link to comment Share on other sites More sharing options...
lukebl Posted July 15, 2011 Author Share Posted July 15, 2011 Hi Les. Hope that my tribulations can help!Just done another quick test image of an out-of-focus star, this time without the coma corrector. Please ignore the dust bunnies, and this image was taken with the MX716, not the SXVF, but it still shows that the stars are rather egg-shaped or oval. So it's not the coma corrector then. Doing a bit of homework on the web shows that this is a sign of astigmatism. As I haven't had this problem before, could it be a sign that the secondary is out of collimation or, worse still, that the primary has become deformed? Any help would be appreciated. Link to comment Share on other sites More sharing options...
Starlight 1 Posted July 16, 2011 Share Posted July 16, 2011 Hi Luke it look like the secondary is out of collimation ,I think you find the secondry is off set to one side on the 250pds and not in the middle. Link to comment Share on other sites More sharing options...
lukebl Posted July 16, 2011 Author Share Posted July 16, 2011 Thanks, Les. I am rather tearing out what little remains of my hair right now. I've always been a bit smug about collimation, but I realise now that if the secondary's out, then you'll never get it right. I tinkered with the secondary a few days ago, and I think that's where the problem lies.One issue is that when the secondary is centred in the OTA (i.e. adjusting the vanes till the screw holding the secondary is in the dead centre), it isn't centred in the focuser. It's offset slightly. This implies that the focuser isn't pointing directly into the centre. Here's a view through my Cheshire collimator to show what I mean. You can just see the edge of the focuser on the right-hand side of the secondary reflection. So, do I adjust the secondary so that it's central to the focuser, or central to the tube? Or is it Ok and leave it alone. However, if I leave things as they are I get eggy stars.Does this all make sense? Someone please put me out of my misery. Link to comment Share on other sites More sharing options...
Starlight 1 Posted July 16, 2011 Share Posted July 16, 2011 Hi I am just about to go shoping so when I get back I set my tube up and have a look as i only use my 250pds 5 time and its still as it was. and I try to get you some photo to help you as well.les Link to comment Share on other sites More sharing options...
lukebl Posted July 16, 2011 Author Share Posted July 16, 2011 Hi I am just about to go shoping so when I get back I set my tube up and have a look as i only use my 250pds 5 time and its still as it was. and I try to get you some photo to help you as well.lesCool. Thanks. It may be that that view is perfectly correct, and that my problem is caused by something else. Link to comment Share on other sites More sharing options...
Starlight 1 Posted July 16, 2011 Share Posted July 16, 2011 You can see by the photos you put in its look like a egg. Link to comment Share on other sites More sharing options...
lukebl Posted July 16, 2011 Author Share Posted July 16, 2011 You can see by the photos you put in its look like a egg.Not sure what you mean! Can you re-phrase. Link to comment Share on other sites More sharing options...
Starlight 1 Posted July 16, 2011 Share Posted July 16, 2011 You see when I put some photos in .Look at your picture, you can see it is out left and right. I can see purple edging on the left hand side of the primary mirror, with black edging on the secondary. Just give me a couple of hours and I will sort it out. Link to comment Share on other sites More sharing options...
lukebl Posted July 16, 2011 Author Share Posted July 16, 2011 You see when I put some photos in .Look at your picture, you can see it is out left and right. I can see purple edging on the left hand side of the primary mirror, with black edging on the secondary. Just give me a couple of hours and I will sort it out.I think the image is misleading. The purple edging is just the light falling at the top of the OTA (reflecting a blue tarpaulin!). The offset of the reflection in the secondary is part of the design of fast Newts, as this image of a correctly cllimated scope shows:Perhaps the fact that the 250PDS has a larger secondary mirror results in part of the focuser appearing on the edge of the reflection. Could anyone else with one of these have a look with a Cheshire collimator and tell me if they see the same thing. Link to comment Share on other sites More sharing options...
Starlight 1 Posted July 16, 2011 Share Posted July 16, 2011 ok looking down the Cheshire collimator lining the wires up with the Cheshire collimator with the 4 veins on the spider, the top edge lines up witn the top of the primary mirror so the big black edge is showing most of the way round and the secondary mirror is off-set to the left. The edge of my mirror lines up with the cross in the Cheshire collimator. How can I take a picture to put in? Link to comment Share on other sites More sharing options...
lukebl Posted July 16, 2011 Author Share Posted July 16, 2011 .... How can I take a picture to put in?I just held my DSLR up to the collimator, flush with it, and took a picture through the pinhole. You need to be very close to the hole, though. Link to comment Share on other sites More sharing options...
Starlight 1 Posted July 16, 2011 Share Posted July 16, 2011 ok photos Link to comment Share on other sites More sharing options...
Starlight 1 Posted July 16, 2011 Share Posted July 16, 2011 With out Coma correction in place Link to comment Share on other sites More sharing options...
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