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Coma correction - WHAT am I doing wrong?


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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 :D 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!

coma_test.jpg

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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

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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?

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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.)

Olly

PS Get this working and you could do some collaborations with Mike (Yfronto.)

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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.

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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?? :D

Olly

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What are you eating in your new avatar?? :D

Olly

That 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.

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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:

M29_20s_test.jpg

Closeup:

M29_20s_test_closeup.jpg

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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

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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.

MX717test%20copy.jpg

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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.

post-16549-133877634977_thumb.jpg

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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

Cool. Thanks. It may be that that view is perfectly correct, and that my problem is caused by something else.

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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.

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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:

fast%20collimation.jpg

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.

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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?

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.... 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.

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