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Old 24-08-2013, 09:37 PM
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camera loan request

Hi All,

Over the last 4 or so months ive been trying to chase down the source of my star elongation in my images. (my thread on refraction at the horizon was about trying to find the issue)

The gear is a cdk 12.5, PME, stl11000 with bolt up adapters.

ive tried every test i can think of to try and track down the source of any movement in the scope and so far i havent found the source. Ive tried many things and Steve Mohr has been assisting me over the phone and email but im still unable to eliminate this elongation.

What hapens is i would point to the zenith and take a picture and collimate there and all stars on the chip would be round. then i would slew to say 40 deg altitude in the south and take an exposure of 30 or 60 sec (same as zenith) and the star elongation would be to the top left of the chip - over the whole chip.

then i would slew to the north and take an image there at 40 deg altitude and the elongation would switch to the top right of the image and still over the whole chip (not just in one corner).

I dont think its guiding error as my guide graphs are indicating sub pixel guiding. the elongation is more than this. Im not using a guide scope, but the internal guide chip on the stl11000.

One other thing is the elongation of the stars (wherever i point in the sky) is parallel with a line from the horizon to the zenith.

So, i want to iliminate the camera from the picture. I would like to try another camera on the scope to see if the problem still exists or goes away. I have a 5d mkii but i cant be sure it wont move as its only using thumb screws to hold things in place.

Would anyone with an stl11000 lend me their camera to try this out? i could possibly use a stl 6303 but the mounting face plate needs to be the same as an stl11000 so my adapters fit.

Of course i would pay for insured postage etc to and from WA.

If someone would be kind enough to do this, please post here, PM me or phone me on 0428650851.

I'm all ears for sources of the elongation as well

thanks
Josh

Last edited by Joshua Bunn; 26-08-2013 at 02:20 PM.
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Old 25-08-2013, 12:52 AM
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Tandum,

No its not primary mirror movement as its fixed place. The scope is a Planewave CDK 12.5". Yeah the primary is fixed in place and it appears not to move from my bench tests using a micrometer.
Thats all ive got to go on - and the fact that collimation seam to hold (just by defocusing and getting concentric circles) at orientations where in focus stars are elongated.

Thanks
Josh
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Old 25-08-2013, 05:39 AM
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Can you post a pic of your imaging train?
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Old 25-08-2013, 11:19 AM
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Hi Bert,

Sure, I will get the pictures up soon.

Josh
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Old 25-08-2013, 02:01 PM
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Josh, it can't be the camera with the symptoms you describe...if it were you always see some sort of elongation, the problem is elsewhere

Cheers
Peter
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Old 25-08-2013, 03:19 PM
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Thanks Peter. Yeah, it doesnt look like it would be the camera judging by the elongation, but im at the point where i just wanted to eliminate what i could. Is there anything in the ccd compartment that could come loose to cause elongation? I see the chip has some springs pushing down om it, do they hold it firm? is there anything else that holds the chip in?

I will try some 5 sec exposure and see if the elongation persists. Previous exposures were 30 or 60 sec.

Heres some pictures of my imaging train. more in the next message.

thanks
Josh
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Old 25-08-2013, 03:21 PM
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and the rest.
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Old 25-08-2013, 04:17 PM
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Here are two 200% crops from some images. One is elongated to the top right, the other to the top left. the only difference between the images is a meridian flip, they were both taken around the south celestial pole. The same thing happens when shooting low in the north.

The elongation is all over the chip like this.

thanks.
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Old 26-08-2013, 05:56 AM
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Josh,

Sorry you are having so much trouble.

Have you tried rotating the camera 90 degrees and repeating the experiment?

Can you eliminate the secondary support? That is, have you slewed to a position where you get elongation and re-checked collimation? Could it be focuser shift?

Also, maybe you should turn off anything in software that might be contributiing such as PEC and Protrack, and then take even shorter exposures. Maybe your model isn't great at 40 degrees and Protrack is moving the scope??? Also, if you take much longer exposures in an area where you get distortion does the distortion increase at all over time?

PS You should have come to AAIC because Rick Hendrick was there and maybe he would have been able to help! There were more experts in attendance than you could shake a stick at!!!

Peter

Last edited by PRejto; 26-08-2013 at 06:28 AM.
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Old 26-08-2013, 06:23 AM
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Have you talked with Rick Hedrick about this from Planewave?

Something in the scope is shifting. Secondary, primary, focuser, possibly the corrector.

The Secondary mirror is the one I would suspect first. Focuser second, primary last.

Greg.
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Old 26-08-2013, 08:22 AM
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There has to be some differential movement between the guiding and imaging chips if your guiding graphs are correctly indicating sub-pixel guiding. What rate are the corrections being applied at? Are the guide graphs sooth?
Cheers,
Andrew.
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Old 26-08-2013, 09:04 AM
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Yes that is a possibility too. If your mount is unbalanced or it shifts in balance at different angles (do you have a piggybacked scope?) or you camera filter if sticking out at an odd angle. Dragging cables.

See if you can check the balance of your rig at those angles you know you get elongated stars. It may be way off even if balanced when horizontal.

I always balance my scope at a typical imaging angle not horizontal which of course you never image at.

Greg.
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Old 26-08-2013, 10:44 AM
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Hi, Sorry for your troubles.

I have not much experience at all (only dreaming of such a set up), but as a sanity check you should try (unless you already did) calibrate your guide error and elongation etc in arc-seconds.

Also, try to work out in what 'axis' the elongation is (sorry you mention the 'axis' in your post, would that be 'vertical'?).

cheers,
Bram

Last edited by torsion; 26-08-2013 at 10:47 AM. Reason: commented wrt the axis of elongation
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Old 26-08-2013, 10:47 AM
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thankyou to you all for your replies.

Quote:
Originally Posted by PRejto View Post
Josh,

Sorry you are having so much trouble.

Have you tried rotating the camera 90 degrees and repeating the experiment?

Can you eliminate the secondary support? That is, have you slewed to a position where you get elongation and re-checked collimation? Could it be focuser shift?

Also, maybe you should turn off anything in software that might be contributiing such as PEC and Protrack, and then take even shorter exposures. Maybe your model isn't great at 40 degrees and Protrack is moving the scope??? Also, if you take much longer exposures in an area where you get distortion does the distortion increase at all over time?

PS You should have come to AAIC because Rick Hendrick was there and maybe he would have been able to help! There were more experts in attendance than you could shake a stick at!!!

Peter
Hi Peter,

I think i tried rotating the camera some months ago, but i will definetly do it again and see what the results are.

I have slewed to different positions in the sky (where the problem areas are) and the collimation seems to hold by looking at the defocused star and looking for concentric circles. The focuser is on a lead screw which stops focuser travel, but i cant be sure its not shifting sideways with gravity, there was also something else that happened after moving the focuser that i need to retest.

Ill have to take a longer exposure and see the results. But, the pictures i posted here were 60 sec guided, and the guide graphs inducate guiding to 0.1 - 0.6 of a pixel at 2.5 m FL. for 1.2 arcsec / pix. image scale.

yeah, would have been good to be there, ive had to talk to Rick via phone about this issue.

Quote:
Originally Posted by gregbradley View Post

Have you talked with Rick Hedrick about this from Planewave?

Something in the scope is shifting. Secondary, primary, focuser, possibly the corrector.

Greg.
Thanks Greg,

Yes ive been on the phone and email to Rick and will be again when he gets back to the US.

I can rule out the corrector as i just checked that and its tight, no movement from months ago. My money would probably be on the focuser or secondary. What about tube flexure (twist from one part of the sky to the other?)

Quote:
Originally Posted by alocky View Post
There has to be some differential movement between the guiding and imaging chips if your guiding graphs are correctly indicating sub-pixel guiding. What rate are the corrections being applied at? Are the guide graphs sooth?
Cheers,
Andrew.
Hi Andrew,

Guiding graphs are good. there cant be any differential flexure between the chips as im using the internal guide chip on the stl (both in the camera head), not an external guide scope. Unless your of the opinion there can be differential flexure in there?

Quote:
Originally Posted by gregbradley View Post
Yes that is a possibility too. If your mount is unbalanced or it shifts in balance at different angles (do you have a piggybacked scope?) or you camera filter if sticking out at an odd angle. Dragging cables.

See if you can check the balance of your rig at those angles you know you get elongated stars. It may be way off even if balanced when horizontal.

I always balance my scope at a typical imaging angle not horizontal which of course you never image at.

Greg.
I will reballance at the low angles and do it again to check. my guide graph indicates there is no problem with guiding at these altitudes (40 deg) though. There are no cables to get snagged on, everything is through the mount and the mount is free to move where ever it wants.

thankyou all, I will work on these suggestions and see how it goes.

Josh

Last edited by Joshua Bunn; 26-08-2013 at 11:03 AM.
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Old 26-08-2013, 10:55 AM
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Quote:
Originally Posted by torsion View Post
Hi, Sorry for your troubles.

I have not much experience at all (only dreaming of such a set up), but as a sanity check you should try (unless you already did) calibrate your guide error and elongation etc in arc-seconds.

Also, try to work out in what 'axis' the elongation is (sorry you mention the 'axis' in your post, would that be 'vertical'?).

cheers,
Bram
Thanks Bram,

guiding error is between 0.1 - 0.6 of a pixel for 1.2 arcsec / pix. this is 0.12 - 0.72 arcsec of error. translate this to the imaging chip with an image scale of 0.73 arcsec / pix and thats an error of 0.16 - 1 pix.
These guiding errors have given me round stars before. so i dont think its the guiding. My seeing isnt as good as the guiding error either.

And the axis is a mix of 50 % RA and 50% Dec as the x axis on the chip is in line with Dec and the y axis RA.

thanks
Josh
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Old 26-08-2013, 02:53 PM
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Quote:
Originally Posted by Joshua Bunn View Post

Hi Andrew,

Guiding graphs are good. there cant be any differential flexure between the chips as im using the internal guide chip on the stl (both in the camera head), not an external guide scope. Unless your of the opinion there can be differential flexure in there?


Josh
Hi Josh - that's kind of what I'm getting at. If it is flexure in the optical train, the guide chip should be guiding it out. If the guide graphs are that good, then the guide chip isn't seeing the same movement as the imaging chip.
Only external-to-camera things I can think of are vibrations at a frequency higher than the guider will register, but they are unlikely to induce a linear error. Have you tried turning off the cooling fans?
Anyway - good luck!
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Old 26-08-2013, 03:29 PM
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Hi Josh,

Well, you have a lot of input here! Why not take very short unguided exposures and check this out? Wouldn't that just eliminate one complication? With a mount like yours and good PA you shouldn't see anything except round stars in 5 sec or so! Obviously then if you don't see the error it leads back to guiding, no?

Perhaps a statistical analysis using CCDInspector would help by comparing aspect ratios at different points in the sky (with short exposures). With everything secure you probably shouldn't see any change in aspect as long as focus is holding while you slew.

Peter
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Old 26-08-2013, 05:24 PM
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Quote:
Originally Posted by alocky View Post
Hi Josh - that's kind of what I'm getting at. If it is flexure in the optical train, the guide chip should be guiding it out. If the guide graphs are that good, then the guide chip isn't seeing the same movement as the imaging chip.
Only external-to-camera things I can think of are vibrations at a frequency higher than the guider will register, but they are unlikely to induce a linear error. Have you tried turning off the cooling fans?
Anyway - good luck!
Andrew, yeah i will try it without the cooling fans on. interesting thoughts re flexure there. thanks.
Quote:
Originally Posted by PRejto View Post
Hi Josh,

Well, you have a lot of input here! Why not take very short unguided exposures and check this out? Wouldn't that just eliminate one complication? With a mount like yours and good PA you shouldn't see anything except round stars in 5 sec or so! Obviously then if you don't see the error it leads back to guiding, no?

Perhaps a statistical analysis using CCDInspector would help by comparing aspect ratios at different points in the sky (with short exposures). With everything secure you probably shouldn't see any change in aspect as long as focus is holding while you slew.

Peter
hi Peter,
Yes, although my 30 sec unguided exposures showed exactly the same results as my guided 60 sec exposures, i will definetly do some 5 or so sec unguided images when the clouds clear.

thankyou all.

Josh
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Old 26-08-2013, 05:35 PM
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Good guiding and elongated stars seems to be contradictory statements.
The only time I have seen "good guiding" (of course its not good guiding if the result is elongated stars) was when I had differential flexure and I was using a guide scope. So the guide scope was seeing the guide star just jim dandy but the imaging chip was showing drift due to something not being rigid with the guide scope.

If in fact your guiding is good then that only leaves optical movement like the secondary is moving slightly or the primary is distorting slightly.

When you grab these parts are they all rigid? Mirrors, tube, focuser?
See if any move when you push/pull them.

Greg.
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Old 26-08-2013, 06:06 PM
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Quote:
Originally Posted by gregbradley View Post

If in fact your guiding is good then that only leaves optical movement like the secondary is moving slightly or the primary is distorting slightly.


Greg.
If the primary or secondary move slightly then the guider would register this movement also. It would also effect collimation I guess. If it is only in corners of images then you can rule out anything to do with the guider because elongation would be in the center of the frame too. My money is on the secondary or focuser flexing.
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