My imaging nightmare - time to throw in the towel?

[lukebl]

I think I work it out it the chip on the ccd board is not flat.

So I am just looking to see that evey thing is right up to the ccd.

Ah, I see where you're coming from now. I'm certain that the sensor is flat in the camera, as images taken without filters or coma corrector show nice round stars. By a process of elimination, it has to be the alignment of the coma corrector.

[FraserClarke]

Hi Luke,

Have you tried taking images with the coma corrector and WITHOUT a filter in the beam? It could be an abberation introduced by the filters. Do you have an "open" slot in the filter wheel that you could use to test this? Remember also that the filter will change the effective path length between the coma corrector and the chip -- but only by a mm or two, and it seems you've already covered that with your spread of distances.

Also, what is the relative size of the DSLR and CCD pixels? Are you seeing something with the CCD you never saw with the DSLR, simply because you're sampling the image better?

I think the images you take are great. I'd be pretty happy with the image quality you achieve (understand it is frustrating not to get it across the whole field). Certainly wouldn't suggest you need to consider throwing in the towel though

Fraser

[lukebl]

Cheers, Fraser! I don't think I've tried the 'no filter' option so I'll give that a go, although I'm not too hopeful!. As for the pixel size, the CCD actually has larger pixels than the DSLR, so in theory should be a bit more forgiving.

I don't think there's a real danger of me throwing the the towel. It's all too much in my blood to give it up!

[lukebl]

Oh dear. The saga continues. Sorry to bore you folk with this. There've been so many replies to my original thread that I've lost all sense of where I was!

Anyway, I think I've made a rather expensive mistake.

To recap, I have been using a SXVF-H9 with a Skywatcher coma corrector and getting oval-shaped stars for some reason when using the corrector, as shown here:

Well, thinking that it was the coma corrector's fault, I splashed out on a new Baader one, plus a few spacers to get the spacing right. The Baader one has a standard T-thread which fits onto the filter wheel without requiring an adaptor (unlike the Skywatcher one), so I thought it would fit squarer onto the filter wheel. Lo and behold, I get the same oval-shaped stars with that one too. AAAARRRRHGH! What a rash waste of money.

I thought that maybe it was the filter wheel not being square (It's a Scopeteknix which is a budget model), so I tried it without the filter wheel. Same problem.

The stars are round (albeit showing coma at the edges) when not using the corrector, so there is presumably nothing wrong with the camera. Help me. What on earth could be going wrong here? I really think I'm losing my marbles over this.

[Earl]

Luke after a discussion I had with Bern today about a flattener issue i was having, try rotating the camera 90 degrees, nothing else, does the orientation of the oval change?

Possibly if the ovals are orientated the same its an optical issue, if they rotate with the ccd rotating it could be a ccd issue.

If you look at the HA image I posted the stars are stretched on the right, ok in the center and the left does not look right, this was down to the flattener not been 100% true in the scope. (I thnk, I need another night to confrim)

[lukebl]

...try rotating the camera 90 degrees, nothing else, does the orientation of the oval change?

I think it does. I tried various alternative spacers and each time it was fitted back on the scope the camera was oriented slightly differently, with the result that the orientation of the oval changes a bit.

But what would that indicate?

[Earl]

If the oval moves when the camera moves it could hint to it been the ccd. is it possible that the coma corrector exaggerates the ccd chip not been level to the corrector

[lukebl]

If the oval moves when the camera moves it could hint to it been the ccd. is it possible that the coma corrector exaggerates the ccd chip not been level to the corrector

Oh dear. That doesn't sound good. Sounds expensive, too. How can I find out if the CCD's not level? Thanks for the tip, anyway.

[Earl]

I would think the best thing to do first would be take some images with the minimum amount of equipment in the way, just what is needed to achieve focus, the introduce the coma corrector in to that and compare the images, rotate the camera through 90 degrees and compare them, I think something is not flat, it could be a thread not sitting quiet right and just offsetting the angle of things.

[Coco]

My stars were eggy shaped 3 imaging sessions ago... I re-polar aligned and bingo

[lukebl]

My stars were eggy shaped 3 imaging sessions ago... I re-polar aligned and bingo

No, it's not my alignment. The stars are oval-shaped even with short exposures, and I have ensured that my alignment is spot on.

Definitely something wrong in the optical chain somewhere.

[ollypenrice]

Oh dear. That doesn't sound good. Sounds expensive, too. How can I find out if the CCD's not level? Thanks for the tip, anyway.

Simply try it in a different scope. There must be someone near you who can supply a scope for a night's testing. I think having a non-orthogonal chip is looking like a serious candidate at this stage. It isn't unkown and some cameras have adjustment built in.

What about the graph paper test? Photograph a piece of squared paper in the daytime. It may show deviations from square. Rotate the scope/camera and repeat. Rotate only the camera and repeat.

Or yet another test would be to try a different camera, again by getting in contact with anther local imager. Do you maybe have a DSLR, or could you borrow one for a test?

Have you contacted Starlight Xpress? They are very helpful.

Olly

[fatwoul]

Terminal newbie that I am, I've got nothing helpful to add, but I just wanted to say I really enjoyed seeing Garradd shimmy along in your video. I haven't processed my images yet, but if I have anything even half as good, I'll be really pleased.

[lukebl]

Many thanks fatwoul!

....Simply try it in a different scope...

Olly, you're a bloomin' genius! I dusted off my old 8" Newt and tried the whole coma corrector/filter wheel/CCD setup in that and, bingo, the stars are nice and round.

So what do I do with the 10" Newt now? What in the optical setup of the 10" would cause the stars to be oval when using the corrector, when it's fine in the 8"? I've had issues with the alignment of the secondary mirror (after I messed up its position), but my Cheshire collimator and a star test says that the collimation is OK. Is it possible to have good collimation, and still have the secondary in the wrong position, resulting in errors which are accentuated by the coma corrector?

Anyway, I know good results are possible with it, otherwise I wouldn't have unexpectedly ..er....cough...cough... won Picture of the Month in this month's Sky at Night magazine with this image of M5 (shame they cropped the image in the magazine):

[foundaplanet]

Re; Sky at Night photo of the month.

I thought that was probably you. Congrats..

Pleased you have got the frustrations kinda sorted. hope you can get the 10 fixed back up..

[ollypenrice]

Well done and, phew, light at the end of the tunnel...

I am a Newtonian numpty so cannot help regarding this problem but I'm sure someone out there will try to help. A solution will be possible. If the optical tests you are running suggest that collimation is OK then non orthogonality in the focuser comes back into the story, perhaps? Is the coma corrector very long? I tried an 8 inch Newt and found that with the coma corrector in the line the camera was just too far out from the tube and not sufficiently orthogonal for decent results. Optician's droop!

Olly

[Macavity]

"He's got an 'ology', and he says he's a failure"? (c.f. "Beattie" (BT) commercials!)

- Or at least a Picture in S@N! I did wonder... Congrats.

[SlipperySquid]

Well done on the picture of the month..it's lovely feeling isn't it

Could the problem be focuser sag? Maybe the comma corrector just pushes the weight up or the added length adds more leverage to the tube? If that makes sense? Just an idea? Plus if you want to sell one of your comma correctors PM me

Matt.

[lukebl]

Thanks a lot gents!

Although I think I'm nearly there with the focussing issue, I don't think that it's simply focuser sag. The focuser on the 8" Newt is much 'floppier' that the 10" and shows a distinct sag, but the stars are still round. Perhaps the focuser isn't aligned correctly. I'm loathe to fiddle with that, as that would mean adjusting the secondary, which caused me a huge amount of grief recently.

Incidentally, is there a 'scientific' way of checking the alignment of the secondary. Aligning the primary seems failry scientific with a collimator or checking the airey disk, but all the on-line guides regarding the seconday talk about looking through the focus tube till it looks 'about right'? Seems a bit vague to me.

[ollypenrice]

I think you need to get this to a Newt buff at an astrosoc. I've seen Ralf Ottow, a professional, collimate my 20 inch after a total strip down (really total) and he put no tool into the scope at all till he said, There you are, try the laser in that. I did and it was about 2mm off the centre dot on the return beam. If you know what you are doing, you know what you are doing. I know that I don't!

Olly

[lukebl]

The saga continues. I have now collimated my 250mm Newt perfectly and, guess what, OVAL stars again. Dammit.

I just wondered if the specified spacing of 55mm between the coma corrector and the CCD sensor is the same for all focal lengths? My 200mm has a FL of 1000mm and the stars are round, whereas the 250mm has a FL of 1200mm and the stars aren't round. Surely, the different focal lengths would require different coma corrector spacings?

I am experimenting further with different spacers, as the degree of distortion does seem to change depending on the spacing. Unfortunately, inclement weather means I haven't had sufficient clear skies to progress this further.

[Ransu]

i dont know have you check allready is your focuser in same line with optical path, or is there angle? I think you can easily check it using laser collimator. Just turn your focuser "in" and "out" laser on. If spot is not in the same place in both positions, then your focusers central line and scopes optical path are in angle.

[MartinB]

Your first animated top image looks pretty normal to me. To my aging eyes the stars look round. Every time an image downloads the guide camera stops tracking. When the image is downloaded the camera locks onto the star again. If the seeing is wobbly you can get this sort of movement.

[lukebl]

i dont know have you check allready is your focuser in same line with optical path, or is there angle? I think you can easily check it using laser collimator. Just turn your focuser "in" and "out" laser on. If spot is not in the same place in both positions, then your focusers central line and scopes optical path are in angle.

Thanks, but I've done all that! As my tedious and lengthy threads have explained, my collimation is now absolutely perfect, focuser is aligned perfectly, secondary is aligned perfectly, no focuser sag, yet the stars when using the coma corrector are oval.

I'm thinking it has to be the coma corrector spacing, as that seems to be the only solution left. The optimum 55mm spacing just doesn't seem to work with the 250mm Newt. (Having said that, I can't find any other spacing which works either!)

[lukebl]

Your first animated top image looks pretty normal to me. To my aging eyes the stars look round. Every time an image downloads the guide camera stops tracking. When the image is downloaded the camera locks onto the star again. If the seeing is wobbly you can get this sort of movement.

Thanks, martin. Actually, my rant has morphed from the issue over the wobbly guiding to the current issue of the coma corrector problems. I think I've sorted the wobbly animation issue. The basic problem is funny-shaped stars when using the corrector, as here: