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Old 05-10-2012, 12:14 AM
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CCD tilt

I've made a start with my gear and this is a first test of my qhy8 after adding an Al shim to reduce the CCD tilt.
Can someone confirm if this is indeed CCD plane tilt? That is the stars at the bottom right, centre and top left. Guiding was the best I've ever gotten after some very extensive PA.
I tried rotating the camera and the direction of the trailing did change.

The initial difference in distance between one side of the CCD to the top and other was over 3mm.
I reduced this down to about 1mm after some shims.

scope is an 8" F4, cam Qhy8 one shot with a baader mpcc using a spacer on an heq5pro,
this was taken 2 nights ago with the moon up high and from the city with no flats, darks, so is missing a lot of stars, but it was more of a test.
I finally got the drift alignment with Phd sorted. couldn't believe how easy it was.
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Last edited by alistairsam; 05-10-2012 at 12:45 AM.
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Old 05-10-2012, 06:50 AM
stevous67 (Steve M)
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From my experience, your image is indicating a collimation error. If your polar alignment is accurate, rotation is out. Flexure is out, as the error is not across the field (assuming you are using a guide scope with the F4). A tilt in the ccd will translate to an effect across the field (according to CCDInspector).

I have had this exact issue but with a different scope, and many claimed it to be rotation, but with excellent polar alignment, that's not likely. If the star elongation doesn't extend with longer exposures, it's not rotation.

What I found to cause this in my case was the optical axis failed to centre to the secondary mirror. Attempting to collimate under this circumstance caused this unusual elongation to one side of the field. Yours is a Newt, so I'm not sure the same can be said (mine is a CDK).

Good luck with it.

Steve.
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Old 05-10-2012, 10:00 AM
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Hi Steve,

Its possible the collimation was off, I took this defocused star image during the run. shows the secondary is not exactly centred.
I did collimate with a cheshire and a cats eye 2 pupil AC when the ota was not on the mount.
I then checked with just the laser when it was on the mount.
But I noticed very eggy stars while focusing, so thought the locking bolts for the cell maybe too tight, I loosened them, not too much of a difference.

if its collimation, will the elongation follow the camera when rotated, cause mine did. I rotated and the elongation rotated as well.
I then loosened the focuser lock screw as that was tilting the camera quite a bit and used rubber bands to keep the camera flush against the lip of the drawtube and then took the 47tuc images.

I'll centre the secondary again and do some more tests cause I'm not getting round stars in the centre even for 1s exposures.

Thanks
Edit: added an overlay image with the error. wonder if this is sufficient to show elongated stars in one side.
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Last edited by alistairsam; 05-10-2012 at 10:52 AM.
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Old 05-10-2012, 12:57 PM
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If its rotating with the camera then it is probably an optical train mis-alignment (tilt).
But it can also be collimation. Your collimation looks good so my suggestion would be to check your optical train.

Make sure the CCD is sitting flush with the MPCC - not 1mm tilted because it was screwed in wrongly (etc). Once that is all confirmed good, check your mounting on the focuser. It must be flush.

Another test - as you said you have a laser - put the laser in place on the focuser and then loosen the compression ring and rotate the laser in the focuser. Does the beam move a bit (rotate etc)? If so and you are sure you were not accidentially bumping hte laser when moving it then check your secondary collimation. If that is fine, then check your actual focuser. It could be out as well.

I have a sort of similar issue to you with my SCT. I think it is tilt, but i am not ruling out collimation.
Going back out tonight to check.
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Old 05-10-2012, 01:05 PM
stevous67 (Steve M)
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Hi Alistar,

If the fault follows the rotation of the camera, it's normally associated to the camera.But first ensure your laser hits the secondary dead middle.

Imagine if you hit to one side of the secondary instead of it's centre, you will need to adjust it off to one side to hit the middle of the primary. You then need to adjust the primary by tilting it to reflect the laser back to the source. Part of your collimation test should see if you can return the laser light back to the laser source, not just that you hit the middle of the primary. Thats right isnt it for a Newt? Please correct me if that's wrong.

So under the case where you have the bent light path, the star light coming in will need to follow this bent course to get to the CCD.

Your sample defocused star images are very good to demonstrate the issue.

Steve
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Old 05-10-2012, 03:33 PM
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Thanks
I'll try rotating the laser as well, I guess I need to mark the secondary centre using the correct offset so I can check if the laser is at the centre of the secondary and check with a cheshire as well.

One other test I could do is test with a DSLR. if I have the same elongation with the CCD and DSLR, then its either collimation or focuser tilt, but if the DSLR is ok then I guess I have to open the QHY8 again.

Steve, with the laser collimator that's what is normally checked, whether the laser hits the primary in the centre by adjusting the secondary and if the return hits the 45deg laser frosted glass in the centre.

But I'll use the cats eye as I can understand how the laser can be misleading.

what is strange though is that I was once intently adjusting using the catseye, I got the triangles overlapped correctly via both pupils and then had a look at the secondary, it was actually tilted sideways by more than 10 degrees, yet the cats eye showed good collimation.

how do you actually do a star test? do you defocus the star and collimate or adjust? I might buy those artificial star light sources to test while indoors.
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Old 05-10-2012, 04:23 PM
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Its tilt.

Something in your imaging train is not square. Stars are nice and round bottom left and whacked out on the right.

I am not an expert in collimation but I would have though that would give comet shaped stars near the edges rather than round in the L corner and elongated on the right side.

You should be using screw together type adapters. If you are using a eyepiece holder then that may be your problem. Screw together adapters ensure squareness.

If the scope is fast all this gets exaggerated and needs to be more precise.

Put your camera doing continuous focus images and try reseating it in the setup to see if it improves.

I used spark plug spacers as packers to get my setup square at times. It worked well. It only took 10 minutes as above.

Greg.
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Old 05-10-2012, 06:09 PM
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Thanks Greg,

I'll do a test with the dslr and compare with the qhy8 and that'll give me an idea of how far the qhy8 ccd might be tilted but if both are similar then its definitely at the focuser. I did try spacers but maybe not enough.

Its a normal bintel focuser, but I've got a moonlite on the 10" serrurier I'm building so might be better. I didn't get the threaded drawtube though. I got the one with the 3 screws.
now to wait for some clear nights to test.
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Old 05-10-2012, 10:13 PM
stevous67 (Steve M)
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Alistair,

Just do the checks. Ive had numerous optical errors, and one extremely similar to your case. I was also advised firmly the same that it was tilt - and it was not tilt!

Good luck,

Steve
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Old 05-10-2012, 10:19 PM
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Thanks Steve, will check everything suggested.
Cheers
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Old 06-10-2012, 10:55 AM
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Quote:
Originally Posted by alistairsam View Post
The initial difference in distance between one side of the CCD to the top and other was over 3mm.
I reduced this down to about 1mm after some shims.
I have the same sort of system and it was real PITA to align - some of these scopes work fine first up, but they can get to a state where collimation is possible, but something is still obviously way out of whack. Mine turned out to be the secondary position/alignment - suggest that Steve is on the money with his diagnosis - plus camera image plane tilt. A tip here is that all of the collimation guides I have read suggest that the secondary should be positioned using a sight hole, but they miss the point that the sight hole must be in the image plane - if it isn't, the mirror will be positioned wrongly. I was winding my focuser in so that I could easily see the whole secondary and got the secondary in-out-sideways position quite wrong as a result. This doesn't matter much at f5, but it does at f4.

Not sure what you mean by "distance" in the above quote. The surface of the CCD needs to be within better than about 30 microns of parallel to the front face of the camera at f4. Mine was 300 microns out, which I thought was lot, but your 3mm seems excessive - that is if we are talking about the same thing. If we are, 1mm is still far to much.

Your star test is defocused by too much to really show what the problem might be - suggest that you redo with maybe 1/10 as much defocus, which should start to show what is going on.

I used an additional shadow test to check overall mechanical alignment and found that to be extremely helpful, but am a bit reluctant to suggest it since others have found it to be a bit difficult to interpret what it shows. anyway, if you want to try it http://www.iceinspace.com.au/forum/s...ad.php?t=82230

Might also be a problem with alignment of the MPCC. As a first step, do some tests without it and make sure that you start out with good central stars and coma on the edges. Only add the MPCC when you have the rest of the system squared away.

I have used both Moonlite and GSO focusers on mine and found that the GSO is just strong enough to hold the QHY8 with acceptable flex - the Moonlite is rock solid. Needed to reinforce the OTA tube though, to stop that from bending in the focuser vicinity. Would suggest that your current problems are much more significant than focuser/tube flex.

Also, am assuming that you do the collimation with the scope pointing at the bit of sky you will be imaging from. These scopes move around internally - quite enough to mess things up if you have aligned at some other attitude (eg on the bench)

all the best Ray

Last edited by Shiraz; 06-10-2012 at 11:41 AM.
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Old 06-10-2012, 06:05 PM
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Hi Ray

I meant the depth from the top of the qhy8 body with the nose piece removed to the ccd surface. one side was between 2 and 3 mm deeper than the other when measured with a vernier. I might as well open and check again.
as for collimation, i was doing it on the bench with the cheshire and cats eye and then laser to reconfirm, and then just the laser when on the mount. But i might try the cats eye when on the mount at different positions indoors.

I've got enough info to test various things so will do that when the skies are clear. unfortunately that's a week away as its wet in Melbourne this whole week.
are the artificial star lights you get on ebay any good?
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Old 06-10-2012, 07:33 PM
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well I guess that is the main problem then - the measure from the front face of the camera case to the chip surface should be the same at all points on the chip (within about 30microns or so).

I can't think how the chip could be as much as 3mm out of alignment - that is an extraordinary offset - mine was 0.3mm and even that was pretty bad. It's a while since I did mine, but from memory the aluminium chip carrier+plate is in close (thermal paste) contact with the TEC, the TEC mates up against the heat sink and the heatsink bolts onto the camera body - can't think how anything could be 3mm out, unless the chip carrier is not fully in contact with the TEC or the TEC has come away from the heatsink?? If that is the case though, putting the camera back together should force them all back into contact. Definitely worth another look to see why it is that far out - you may even be able to see what is going on without dis-assembly if its that far out. Shimming will help, but will decrease cooling effectiveness. Maybe do a trawl on the QHY forums and also ask if anyone there knows what could be going on - Theo in particular knows these cameras well. there are also some threads that discuss the structure of the camera, but if you have already had it in bits, you have probably seen some of them.

haven't tried the artificial stars, but they should be OK if you have a long enough free path (probably need about 50m or so with that scope)

all the best Ray.

Last edited by Shiraz; 06-10-2012 at 09:44 PM.
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Old 06-10-2012, 11:43 PM
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Quote:
Originally Posted by Shiraz View Post

I can't think how the chip could be as much as 3mm out of alignment - that is an extraordinary offset - mine was 0.3mm and even that was pretty bad
Hi, sorry, my memory is not what it used to be.
It was about 0.7mm off the first time I opened it, not 3mm. I then brought it down to around 0.3 or so with the shim.
I just measured and the distance from each ccd corner to the outer body surface is 8.25, 8.3, 7.9 and 8mm

I 'll try adjusting the shim again to bring it within 0.2 or less
I had a closer look, and the reason why it's off is because the chip is not seated in the socket precisely. If only they had to pay a bit more attention before adding the sealant
To avoid cooling loss, I added more thermal paste in the gap caused by the shim. I just hope the central part of the pcb holding the sensor doesn't break off as its being raised by the shim.
I'll fix this first and check my collimation, focuser and the rest again.
Thanks
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Old 07-10-2012, 10:04 AM
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thats a pity - any chance of returning the camera or getting factory repairs done, since it was not assembled correctly?

Suggest you try for quite a bit better than 0.2mm - the depth of focus of the optics is only about 50 microns, so you probably want to get the chip parallel to the body to within about 30microns or so. Otherwise the defocus plus incorrect coma correction with the MPCC will produce very odd shaped stars.
regards ray

Last edited by Shiraz; 07-10-2012 at 10:26 AM.
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Old 07-10-2012, 11:01 AM
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Issue is I got it second hand here, so no warranty. I'll ask theo if he can have a look at repairing it if I can't get it any closer.
I have thought of reseating the sensor but I don't think I can remove the silicone.
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