#1  
Old 25-05-2015, 03:00 AM
konstantinos75
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Location: Athens,Greece
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Problem with elongated stars

Hi

Last night I started a astrophotography session with my equipment.

I faced a problem with elongated stars in all my images.
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...
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?


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.
Any comment on these M51 images star elongation will also be welcomed.

Thank you
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  #2  
Old 25-05-2015, 06:55 AM
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Shiraz (Ray)
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I can't see much difference between the two sets of images?

In any case, the distortion could be either optical or guiding.

Check if it is optical by taking a short (eg 1 second) exposure of a few bright stars and see how it looks - if the distortion is still there it's optical and you will need to re-align the scope and camera.

If the phd graph is smooth at that focal length and pixel size, you could possibly have set the guide exposure too long to correct for short term jitter. again, suggest that you find a bright star and try guiding on it with 1 second guide exposures - that will at least tell you how much short term tracking error there is.

Since it guides differently when east, suggest that you also try increasing the weight bias in RA - these mounts need a slight bit of imbalance in both RA and DEC to work properly.

as a general comment, that scope/camera is a fairly bulky load for the mount - it should be possible to get it to guide OK, but it will be very sensitive to how you set it up. You don't have to open the mount to adjust the worm backlash - it can be done easily with the mount in one piece.

Last edited by Shiraz; 25-05-2015 at 07:34 AM.
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  #3  
Old 03-06-2015, 07:32 AM
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MelD (Melvyn)
Mel Davis

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Location: Sydney Australia
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Elongated stars

"You don't have to open the mount to adjust the worm backlash - it can be done easily with the mount in one piece."

Hi Ray, would you care to explain this for us please?

Regards,
Mel
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  #4  
Old 19-06-2015, 12:46 PM
konstantinos75
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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
http://www.albireo.gr/tmp/all.rar


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
www.albireo.gr
Greece
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  #5  
Old 20-06-2015, 12:39 AM
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Shiraz (Ray)
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Quote:
Originally Posted by MelD View Post
"You don't have to open the mount to adjust the worm backlash - it can be done easily with the mount in one piece."

Hi Ray, would you care to explain this for us please?

Regards,
Mel
Hi Mel - sorry I missed your post. this shows how: http://www.astro-baby.com/heq5-rebuild/heq5-we1.htm

Quote:
Originally Posted by konstantinos75 View Post
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
http://www.albireo.gr/tmp/all.rar


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
www.albireo.gr
Greece
Hi Konstantinos.

You have established that there is not an optics problem - that's good.

Since your guiding chip is in the same body as the imager, I think that it is unlikely that flexure of the focuser is the problem (it would affect both chips and be guided out). The most likely problem with the mount is backlash - try adjusting both axes to get rid of as much as possible without binding. It is also worthwhile unbalancing the scope slightly in both axes so that the offset weight ensures that the worms and gears remain in close contact when the scope is buffeted by minor wind turbulence.
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  #6  
Old 20-06-2015, 01:34 PM
konstantinos75
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Location: Athens,Greece
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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

http://www.albireo.gr/tmp/logs.rar
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  #7  
Old 20-06-2015, 02:36 PM
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Shiraz (Ray)
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excellent - seems like you have fixed the problem.
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