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Old 27-03-2012, 11:27 AM
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field rotation, focuser slop, coma issues

Hi,
I'm using an 8" F4 newt on a HEQ5Pro with a QHY8CCD, baader mpcc with the qhy8 spacer from peter tan, orion St80 guidescope with a qhy5 via a diagonal for guiding and st4 autoguiding.

I seem to have field rotation, curvature, coma and I don't know what else.

The attached images are single 3 minute subs, not stacked just colour corrected. As you can see, stars seem to have rotated at the corners in both images, and with an animation of both images that were taken a minute apart, the image seems to have rotated. I can't upload an animated gif here, but if you click the images consecutively you can see the shift. the OTA wasn't touched between exposures so framing is identical.
can someone pls tell me what's happening?

I initially had collimation issues with the F4, but I collimated with a cheshire just before this imaging run and it looked ok, managed to get the cross hairs aligned.

The ST80 guidscope is fitted to to the guide rings and not the OTA and is fitted very securely, but I didn't align the guidescope perfectly to the OTA, just roughly. will using a diagonal with the qhy5 cause issues?

I'm using a bintel 10:1 focuser and have checked there's hardly any slop. But I haven't checked the qhy8 ccd sensor itself to see if that's perfectly square.

The phd guiding graph was very smooth to well within 1 segment either side of the central line so just above and below. I used default phd settings and guiding in both ra and dec.
The figures were around 0.22 and 0.15 I believe, will attach that shortly.

my polar alignment was also quite good, I did drift align with the meridian star and the east star and didn't notice any n/s drift. I didn't spend too much time on it though.

I also ran the fits files through ccd inspector and the outputs attached which shows severe curvature or tilt or both, I'm not sure.

what normally causes field rotation in an eq mount? what other issues are apparent? should I increase the mpcc spacing?

I will be getting the cats eye kit for the 8" F4 and my 10" F4 but I don't think these are just collimation issues.
where do I start?

Thanks
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  #2  
Old 27-03-2012, 12:36 PM
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Links to GIF animations showing image shift for sequential images with no adjustment to scope or framing, 3 min subs taken a minute apart.

lagoon nebula M8 - Field Rotation
Trifid Field Rotation Animation
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  #3  
Old 27-03-2012, 04:43 PM
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Do I need to adjust the phd setting min motion depending on the image scale of the main scope and guide scope to reduce field rotation.

here are the calculations I've come up with using CCD Calculator

Imaging Scope & Imaging Camera

8" F4/QHY8
Pixel size - 7.8um x 7.8um
Array Size - 3032 x 2016
Chip Size - 15.7 x 23.6mm
Image Scale - 2.01 arcsec/pix
FOV - 67.5 x 101.5 arcmin

Guider Image scale

80mm F5 ST80 / QHY5
Pixel size - 5.2um x 5.2um
Array Size - 1280 x 1024
Chip Size - 5.3 x 6.7mm
Image Scale - 2.68 arcsec/pix
FOV - 45.7 x 57.1 arcmin

So would this mean a movement of 0.75 pixels of the imaging scope causes a 1 pixel movement of the guidescope or vice versa?
so would a min motion setting of 0.75 pixels in phd (2.01/2.68) be correct?
I know there's only one way to test it, but its clouded up as usual.
This is driving me nuts to say the least.
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  #4  
Old 27-03-2012, 05:20 PM
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tilbrook@rbe.ne (Justin Tilbrook)
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Hi Alistair,

Are you using a laser for final collimation?

I know I had similar results to yours, if the laser wasn't used.

Cheers,

Justin.
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Old 27-03-2012, 06:35 PM
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Hi Justin,

I did try a laser, but thought the cheshire was a bit more accurate in terms of secondary placement.
But if it was collimation, then the image wouldn't shift between exposures if the mount is continually guiding with phd? that's what got me to think of the image scales and guidecam/imaging ota colinearity.
I do have collimation to take care of though. also the ccd tilt if any.
I might also try increasing the mpcc spacing. I never really measured the qhy8/mpcc spacing as I assumed the adaptor would take care of that as it was custom built. But looking at the results in the ATM section with the custom spacer, he got best results at 59mm.
if only these clouds would clear.
here is the phd graph during those subs, I changed the scale to 50 frames for more granularity.
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Old 27-03-2012, 06:48 PM
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Alistair
I love the title of this thread - all those problems sound way to familiar. In reality you probably have a combination of those issue.
You're taking the right approach blinking the images and thinking of ways to first identify the problem before trying to fix it. Here is a brain dumpage:

For field rotation:
- I would say try align master to polar align. I'm a recent convert.
- select a guide star in the center of the image if possible.

For differential flex between guider and camera:
- review all mechanics, including primary mirror movement and movement in the focuser on the guide scope
- reduce weight wherever possible
- look into OAG or dual chip camera (both probably not possible)

For 'coma'
- review collimation again, you can borrow my catseye stuff if you want
- improve focus
- try tweaking the primary while imaging to see if you can find the sweet spot.
- try rotating the camera 90 or 180 degrees (sensor might not be square)

James
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Old 27-03-2012, 07:07 PM
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Looking at your triffid images and the CCDinspector plot it looks like the light cone is not intersecting the centre of the CCD. I'd check the focuser is square and the secondary is positioned correctly as a first step. The catseye tools will be the best thing to get the collimation right but you need to have the focuser right and the secondary in the right position to get the best out of the catseye gear.
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Old 27-03-2012, 07:48 PM
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Thanks james, Peter,
I do suspect the secondary is not centred properly under the focuser. while using the cheshire, everything lined up correctly, but the reflection of the end of the focuser tube was not perfectly centred on the secondary. I didn't expect it to cause field rotation though.
James, would be great if I could try out the collimation of the OTA the way it is at the moment with the cats eye tools and see just how far it is. I could bring the ota across. will send you a PM.
how about the imaging scales of the ota and guide scope. with the 2.01 vs 2.68 arcsec/pixel relation, would it help if I used a focal reducer on the guidescope? will using a diagonal with the guidescope cause issues?
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Old 27-03-2012, 07:56 PM
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Quote:
will using a diagonal with the guidescope cause issues?
See if you can avoid that if possible. You want it rigid, but not too heavy.
James
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Old 27-03-2012, 08:06 PM
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it does look like you have some rotation - I've not encountered this yet but I think I've read it can happen when your guide star is not close to the object you are imaging when polar alignment is out. How long did you drift align for?
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Old 27-03-2012, 08:16 PM
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Hi,
I let the star drift for about 5 minutes for each star, the meridian and the east.
But I used the guidescope for drift aligning, not the main OTA. I watched the North drift in phd, adjusted azimuth till it started drifting south, then adjusted back till there was no N/S drift. same with the East Star.
But I suspect, if the guidescope was not aligned with the OTA, I might have been a bit off.

Also, the guidescope is not that far off from the main ota, atleast visually looking at the two scopes, but I haven't really verified it. will do that.
James, just had a look at alignmaster, will give that a shot next time.

But how does the actual field rotation happen? i don't really understand the mechanics of it yet.

I'm going to open the IR filter on the qhy8 to measure height from each corner of the ccd to verify if the ccd is square in the camera. i've read that this can be a bit off because while collimating, we adjust with visual aids, but not really with the camera. will need that dessicant heater though.
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Old 27-03-2012, 09:50 PM
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I just checked the qhy8 sensor height to the thread at each corner and it was tilted by around 0.7mm at two corners.
will have to sort that out first.
also, the spacing to the mpcc doesn't look quite right.
from my measurements, its about 59mm at the moment.
The baader site and other sites mention the use of the mpcc stop ring for photographic use. is that necessary? I normally remove it and slide the mpcc/qhy8 into the focuser as otherwise, I don't reach focus with a standard profile focuser.
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Old 28-03-2012, 10:04 PM
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Thanks to a tip from theo, I managed to correct the qhy8 ccd tilt with some Al foil.
now to fix the other issues.
as for the baader mpcc, is it an issue if the stop collar is removed and the mpcc with the qhy8 spacer is inserted into the focuser rather than just the mpcc body?
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Old 28-03-2012, 10:23 PM
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I made my spacer so the stop collar could be removed and the spacer and MPCC slide into the focuser draw tube. The MPCC is meant to have 55 mm from the surface of the CCD to the telescope side of the stop collar. The stop collar is 2.7 mm thick so the spacing to the CCD side of the stop collar (or where the M42 thread ends) would be 55-2.7=52.3mm. Through testing I found the best spacing for my 10" f5 GSO newt is 16.5mm (CCD surface to front of camera t-thread)+43mm (spacer)=59.5mm from front of CCD to telescope side of the stop collar.
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Old 29-03-2012, 09:22 AM
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Thanks Peter.
many talk about inserting the camera, spacer and mpcc with just the mpcc body at the focuser brass ring stopped by the collar, but that would mean the focal plane would be around 110mm from the ota surface, so mirror to be moved up, bigger secondary, bigger focuser.
I don't see why it can't be removed.
Anyway, I think coma is not too bad in my case, it was more field rotation, but I'm pretty sure that's because I was polar aligning with the guidescope not the primary scope, and both were not aligned, plus the ccd tilt which I just fixed, as well as the diagonal on the guidescope and finally my collimation.
fixing them one at a time.
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Old 29-03-2012, 09:28 AM
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Quote:
it was more field rotation, but I'm pretty sure that's because I was polar aligning with the guidescope not the primary scope, and both were not aligned,
To be more precise - this won't actually cause polar misalignment, but it will accentuate trailing in your images if the guide star is a long way from the centre of the image. More likely is there is some movement between the guide scope and main scope over time (aka flex).
James
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Old 29-03-2012, 09:48 AM
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Hi James,
I'm pretty sure the guidescope wasn't too far away from the main scope.
I made sure there was no flex, but I didn't tighten the guidescope screws too tight. so that could have contributed.
If polar misaligment is unlikely to be the cause for field rotation, I'm back to square one.
can anyone explain how field rotation actually happens with an eq aligned mount? I can understand star trails with tracking rate errors, but I don't get how stars rotate within the 2 degree FOV.
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Old 29-03-2012, 09:51 AM
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Quote:
Originally Posted by alistairsam View Post
Thanks to a tip from theo, I managed to correct the qhy8 ccd tilt with some Al foil.
now to fix the other issues.
as for the baader mpcc, is it an issue if the stop collar is removed and the mpcc with the qhy8 spacer is inserted into the focuser rather than just the mpcc body?
I'd like to know about the alfoil fix!

Back on track though, I'm putting money on the problem being differential flex between the guidescope and main scope. The fact that the internal guide chip makes the problem go away certainly points to that.
Have you got any pictures of the setup?
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Old 29-03-2012, 10:14 AM
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Quote:
I'm pretty sure the guide scope wasn't too far away from the main scope.

It's not the distance between them, it's where they are pointing that’s important. And even then, where the selected guide star is in the FOV.
Quote:
I made sure there was no flex, but I didn't tighten the guide scope screws too tight. So that could have contributed.
There is always some flex, even if you can't see it. Thermal expansion is enough, for example. Everything can bend to some degree. You just might not be able to measure it.
Quote:
If polar misalignment is unlikely to be the cause for field rotation, I'm back to square one.
Polar misalignment is the cause of field rotation. The images will look worse the further away the guide star is from the centre of your image.
Quote:
can anyone explain how field rotation actually happens with an eq aligned mount?
Your mount is never 100% perfectly polar aligned, you want the alignment error small enough that it doesn't impact your images for the exposure duration you are using.


Hope this helps a bit...

James
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Old 29-03-2012, 11:51 AM
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Hi James,
what I meant was they weren't pointing too far away from each other, but then I never really checked, so quite possible.
I was thinking of getting the TS-9 OAG to avoid flexure but till then I'll have to stick with the St80 for a while. so will recheck everything again and try and align its FOV to the ota as much as possible.
I've downloaded alignmaster and will try my polar alignment with that. it looks pretty easy and is quite nifty. But does it account for the movement of the star whilst you're centering the object?

Peter, regarding the qhy8 Alfoil fix, I've attached a pic of the qhy8.
For me, the side with the single plastic stud was higher than the other side.

In this version of the qhy8, the plate under the ccd is sort of fixed to the ccd and the ccd sits in a socket on the pcb. so there was no way I could move just the ccd. only way was to adjust the pcb's tilt, but that would prevent it from sitting in the recess around the edges and the body won't close.
the Al plate under the CCD sits on the Al block of the peltier with thermal compound.
I just folded some Al foil to about 0.5 mm thick by 5mm x 15mm, and placed it on the peltier Al block corner where my CCD was lower.
so that basically tilts the whole CCD/PCB assembly a bit and because the PCB flexes slightly, it sits back in the recess and you don't have an air leak.
you do need to fill the 0.5mm gap between the side of the Al foil and the Al block with thermal compound else the ccd will only contact the Al Foil.

Other option is to use the newer UV/IR filter holder with the conical interface and the M48 connector with screws to adjust the tilt of the whole camera.
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