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Problem with elongated stars


konstantinos75
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Hi


Last night I started a astrophotography session with my equipment.

I enabled autoguiding and let the system taking L subframes for a period of 1 hour.

After checking the generated fits, I see in some frames the stars look ok - pretty round - while in some other frames the stars look elongated.


Fortunately the good frames are more than the bad frames.


Do you have any idea what is the reason that causes the stars to be elongated in some frames?


Thank you


My equipment: Vixen VC200L, Skywatcher HEQ5, SBIG ST2000xm

ppt.ppt

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offset the balance slightly to keep the gears meshed, otherwise there can be "bounce" which will give elongated stars.

It could be many other things though....a gust of wind for example.  I wouldn't get too concerned unless the ratio of bad subs to good was high.

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[PS - I've hidden the duplicated thread in the other section :smiley:]

I'd agree with Zak - Setting the mount so that it's east heavy should help to ensure that the gears are always being "pushing" as opposed to being allowed to "drop" by guiding adjustments... or it could also indeed be the result of a breeze...

As the drift is consistent across the entire frame, I'd normally offer up differential flexure as a suspect (even if using an OAG!), but the fact that your CCD is self guiding (with integral guide chip in the camera), that may mean having to rule that out here

So I'd also start looking at guiding errors...  I've sometimes had to dump occasional frames when high clouds move over(?), or, if the error's periodic(?) that could be down to a tracking issue which the guide camera guiding settings struggle to compensate for(?)... or even someone walking around the mount (if it wasn't on a firm footing?)

3 questions -

1.  What exposures were you using and how many?

2.  Was there any noticeable image drift between first and last frames? [if you load them into something like DSS and register/compute offsets, the dx and dy columns show this]

3.  What guide software do you use? 

The reason for asking the latter is that if you can get it to record a log file, I've frequently found that very useful in tracking down errors.    I personally use Maxim and have set up a spreadsheet which I load the log file into to display not only RA/DEC adjustments over the session but also cross reference them against guide star brightness - It's then very easy to spot periodic worm errors or see the guide star wanders if the transparency drops off.   However, if you're using PHD (or PHD2), there's something called PHDLab which can also be very useful when trying to analyse why certain frames may have gone astray...

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Hi

The subexposures were 1 minute. There's no noticeable drift between first and last frame (check attached image comp.jpg - first and last frame set as two layers in Photoshop with difference blending mode).

Also I have noticed some frames are affected by double-star effect. Check attached image double.jpg. This is an evidence that my problem with elongated stars is caused by wind.

post-25327-0-44468700-1417206227.jpg

post-25327-0-26706500-1417206244.jpg

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  • 5 months later...

Hi

The problem with the elongated stars still continues.

The images of M13 are captured with Vixen VC200L operating at 1280 mm and SBIG ST2000xm.

The autoguiding is done through the ST-237 guide chip of SBIG ST2000XM.

The above setup is loaded on Skywatcher HEQ5 mount.

During the astrophotography session, the telescope was pointing to the East and there's was no problem noticed with PHD guiding. The PHD guiding graph was smooth almost perfect. No wind blowing.

This is what I get after 1 minute exposure, elongated stars again...

Here you can see a sequence of M13 images during this session. As you can see the stars do not move between the frames. So this excludes in my opinion any problem of differential flexure or bad polar alignment.

http://www.albireo.gr/tmp/%CE%9C13.rar

The problem must be backlash in the motor gears of HEQ5 mount. But there's a possibility I am wrong with this assumption...

Before opening the mount and adjusting the backlash, do you have any ideas what could be the reason of elongated stars?

Thank you

post-25327-0-63428500-1432356731_thumb.j

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First thing... dont open the mount up (unless its already been tampered with). Difficult to tell if its PA, we would need to see a whole, uncropped image.

Standard checks:

If the elongation is consistent, and in the same direction across the whole frame - its flex, unlikely in your case since its on-camera guiding.

Fix: Tighten up your whole guiding arrangement... stiff as possible.

If the elongation appears to be radial (curved) across the whole frame - its PA.

Fix: use polarfiner http://myastroimages.com/Polar_FinderScope_by_Jason_Dale/

If the elongation only occurs say, from halfway across the frame and out towards the corners - its a tipped field (this will show in short, and long subs)

Fix: Go with threaded connections only, or ensure your camera is (very) securely mounted (compression rings are bad). Tipping is more difficult to solve since it can be caused by a number of issues.

Also, it wont be backlash - because in PHD that will manifest itself as a sawtooth pattern in the guiding graph (and 1min is too short a time for backlash to build). Something tells me it might be a mixture of perhaps two of the above. Try popping your DSLR on and see if you still get the same star distortion (better to simplify when hunting for issues).

One more thing to note is that when the setup is guiding and taking subs - back away from the mount! :police:  (stomping about isnt good for AP, especially if its on a concrete patio!)

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Hi

Thank you for your analysis about my problem.

Please notice that the elongated stars issue appears more when the telecope is pointing to the East.

With same setup described above I slewed the telescope to Zenith and captured a sequence of 10 minutes exposures of M51.

http://www.albireo.gr/tmp/M51.rar

As you can I still have elongated stars in the full frame of the image, the elongation appears radial this time. So it must be a combination of not accurate polar alignment including the differential flexure. Again this time the autoguiding was perfect. So this might be the reason the stars do not change position among the frames.

Any comment on these M51 images will be welcomed.

Thank you again

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  • 4 weeks later...

Hi

I have found some time to experiment more with my equipment and my problem with the elongated stars.
To remind you this is my equipment
- Vixen VC200L operating at f/6.4 
- SBIG ST2000xm dual chip camera
- Skywatcher HEQ5 synscan pro mount
- Skywatcher 9x50 finderscope with focal length 180mm
I have performed the following tests
test1)
point the telescope to the East - target star Vega
- take 1 sec exposure without guiding; as you can see the stars are round so this excludes problem with collimation of the telescope optics.
- start autoguiding, guiding scope is 9x50 finderscope, PHD guiding software is used 
take 1 min and 2 min exposures of Vega; as you can see the stars are elongated in both images.
Please find attached the PHD logs
Also have a look at image1.JPG, this is the PHD lab output after inserting the PHD logs
test2)
point the telescope to Zenith - target star Mizar
- take 1 sec exposure without guiding; as you can see the stars are round so this excludes problem with collimation of the telescope optics.
- start autoguiding, guiding scope is 9x50 finderscope, PHD guiding software is used 
take 1 min, 5 min, 10 min exposure of Mizar; as you can see the stars are almost round in all images.
Please find attached the PHD logs
Also have a look at image2.JPG, this is the PHD lab output after inserting the PHD logs
test3)
point the telescope to the East - target star Vega
- start autoguiding, guiding scope is VC200L, guiding camera is SBIG ST2000XM with dual chip ST237, MaximDL is used as a guiding software 
take 2 min, 5 min, 15 min exposure of Vega; as you can see the stars are elongated in all images.
Please find attached the MaximDL logs
Also have a look at image3.JPG, this is the MaximDL graph output
test4)
point the telescope to Zenith - target star Alkaid
- start autoguiding, guiding scope is VC200L, guiding camera is SBIG ST2000XM with dual chip ST237, MaximDL is used as a guiding software 
take 2 min, 15 min exposure of Alkaid; as you can see the stars are almost round in all images.
Please find attached the MaximDL logs
Also have a look at image4.JPG, this is the MaximDL graph output
You can download all the files here
So judging from these test results I have come to the conclusion that my problem can be caused by
(a) flexure by the VC200L focuser since a heavy camera SBIG ST2000xm is connected to this focuser
(B) HEQ5 mount has problem with RA motor; backlash present in RA; mount needs adjustment
I would like to hear your opinion also about my conclusion.
Thank you
Konstantinos
Greece
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Hi

I have found some time to experiment more with my equipment and my problem with the elongated stars.
To remind you this is my equipment
- Vixen VC200L operating at f/6.4 
- SBIG ST2000xm dual chip camera
- Skywatcher HEQ5 synscan pro mount
- Skywatcher 9x50 finderscope with focal length 180mm
I have performed the following tests
test1)
point the telescope to the East - target star Vega
- take 1 sec exposure without guiding; as you can see the stars are round so this excludes problem with collimation of the telescope optics.
- start autoguiding, guiding scope is 9x50 finderscope, PHD guiding software is used 
take 1 min and 2 min exposures of Vega; as you can see the stars are elongated in both images.
Please find attached the PHD logs
Also have a look at image1.JPG, this is the PHD lab output after inserting the PHD logs
test2)
point the telescope to Zenith - target star Mizar
- take 1 sec exposure without guiding; as you can see the stars are round so this excludes problem with collimation of the telescope optics.
- start autoguiding, guiding scope is 9x50 finderscope, PHD guiding software is used 
take 1 min, 5 min, 10 min exposure of Mizar; as you can see the stars are almost round in all images.
Please find attached the PHD logs
Also have a look at image2.JPG, this is the PHD lab output after inserting the PHD logs
test3)
point the telescope to the East - target star Vega
- start autoguiding, guiding scope is VC200L, guiding camera is SBIG ST2000XM with dual chip ST237, MaximDL is used as a guiding software 
take 2 min, 5 min, 15 min exposure of Vega; as you can see the stars are elongated in all images.
Please find attached the MaximDL logs
Also have a look at image3.JPG, this is the MaximDL graph output
test4)
point the telescope to Zenith - target star Alkaid
- start autoguiding, guiding scope is VC200L, guiding camera is SBIG ST2000XM with dual chip ST237, MaximDL is used as a guiding software 
take 2 min, 15 min exposure of Alkaid; as you can see the stars are almost round in all images.
Please find attached the MaximDL logs
Also have a look at image4.JPG, this is the MaximDL graph output
You can download all the files here
So judging from these test results I have come to the conclusion that my problem can be caused by
(a) flexure by the VC200L focuser since a heavy camera SBIG ST2000xm is connected to this focuser
( B) HEQ5 mount has problem with RA motor; backlash present in RA; mount needs adjustment
I would like to hear your opinion also about my conclusion.
Thank you
Konstantinos
Greece

I have recently been having a similar problem to yours. If the target is due south, west or south west I get elongated stars, I do 15 minute subs but the problem is present even on 5 minute test sub. If the target is due east or near the zenith the stars are round. I have offset the mount even more than usual but the problem persists. I have tightened everything up with no effect. Now I am suspecting either the focuser or the mirror housing flexing under its own weight. The PA is usually very good and the PHD graphs are as flat as a pancake. I have not tried an OAG as yet.

A.G

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Hi


Thank you for the reply.

In order to find out if my mount has a mechanical problem I performed another test last night. 


My setup last night: 


- Vixen ED81s f/5.2 

- SBIG ST2000xm dual chip camera 

- Skywatcher HEQ5 synscan pro mount 


Autoguiding was performed via ST4 with second chip of SBIG ST2000xm like previous time. 


As you can see from previous MaximDL guiding graph for test 3 with VC200L I have these values 


RA RMS error 0.786, DEC RMS error 0.934 


After performing star drift method for polar alignment, I have managed to reduce these values to 


RA RMS error 0.042, DEC RMS error 0.191 


So as you can see the guiding error is reduced in RA by 94.65% and in DEC by 79.55% !!!!!!!!!!!!!!!!!! 


After that, I pointed the telescope low to the East and took two exposures of 15 and 25 minutes of Deneb.

In both images the stars were pretty round. 


This proves that my mount has no mechanical problem. 


I will need to repeat this test with VC200L loaded on the mount, after performing the star drift method for polar alignment.

This last test will provide the answer if mispolar alignment was the reason for getting elongated stars... 


You can find attached my results :)

post-25327-0-35553800-1434774293_thumb.j

post-25327-0-94756700-1434774364_thumb.j

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Hi
Thank you for the reply.
In order to find out if my mount has a mechanical problem I performed another test last night. 
My setup last night: 
- Vixen ED81s f/5.2 
- SBIG ST2000xm dual chip camera 
- Skywatcher HEQ5 synscan pro mount 
Autoguiding was performed via ST4 with second chip of SBIG ST2000xm like previous time. 
As you can see from previous MaximDL guiding graph for test 3 with VC200L I have these values 
RA RMS error 0.786, DEC RMS error 0.934 
After performing star drift method for polar alignment, I have managed to reduce these values to 
RA RMS error 0.042, DEC RMS error 0.191 
So as you can see the guiding error is reduced in RA by 94.65% and in DEC by 79.55% !!!!!!!!!!!!!!!!!! 
After that, I pointed the telescope low to the East and took two exposures of 15 and 25 minutes of Deneb.
In both images the stars were pretty round. 
This proves that my mount has no mechanical problem. 
I will need to repeat this test with VC200L loaded on the mount, after performing the star drift method for polar alignment.
This last test will provide the answer if mispolar alignment was the reason for getting elongated stars... 
You can find attached my results :)

This is quite a detailed way of problem solving, however even the original PA errors are pretty small and the guiding software should be more than capable of guiding and keeping the stars round. My guess is that the problem is probably with the scope.

A.G

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