Problem with non-orthogonal RA-Dec

[Michael Boccabella]

I have been having a problem the last few times I have been out to image and was hoping someone might have some thing that could be tested. The log included is for an Orion SSAG camera on an ST80 short tube. The imaging scope is a 102 mm refractor. When doing calibration, often times I will get a non-orthogonality error. When I ask for details, the the plot is hsown and the number reported (most times 2 or 3 degrees, but occasionally as much as 6 or 7 degrees).

There are 3 nights upcoming that will be clear, but the moon will be out so it is a great time to figure out what the problem might be.

Doing some research online, one of the major causes that I found for this error is poor polar alignment, but using the drift align as part of PHD2, I had the alignment error pretty low, at least I thought.

Any help or suggestions would be appreciated.

Thanks

Mike

[bw_msgboard]

Hi Michael.  I think the trouble-shooting section in the Help guide can give you a push in the right direction:

 

 

 

 

And you have probably seen a few alerts about the computed guide rates being suspect.  That would take you here:

 

 

You can also use the Manual Guide tool to clear the Dec backlash, any technique you like that will show the stars in the guide field moving consistently north before you start calibration.  Have you measured the Dec backlash with the Guiding Assistant?  It looks like there’s quite a bit there, pretty typical for this mount.  All of that said, you seem to be getting very good guide results so maybe you don’t want to do much tinkering around here.

 

Hope this helps,

Bruce

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[Michael Boccabella]

Hi Bruce,

Thanks for the tips. I will follow these tips tonight, along with using the Guiding Assistant to measure the PA error, Dec backlash, and RA tracking error

I agree with your assessment that the guiding looks pretty good, but the results are not what I would like to see. Attached is a photo taken 16-Mar-2020 at 1:46. It is a 600 second exposure and there are stars show elongation.The log file for this image was uploaded in the original post.

This afternoon I am going to do a complete re-balance of the mount to be sure the Dec is balanced properly. Once the system is aligned I will do the GA to see what the results are.

Again, thanks for the suggestions. I will post tomorrow to let you know how things progress.

Regards,

Mike Boccabella

[Michael 8554]

Looks like Tilt and Coma to me ?

Michael
Wiltshire UK

[Michael Boccabella]

Michael,

I want to understand why you feel it is tilt and coma. This is a learning process for me and want to know what in the image makes you think that. What I see is the same streak through out the entire image, The length and direction of the streak in the upper left is the same as the lower right, so I thought it would be guiding. I do see the possible tilt that you mention.

Please explain so I can understand what would cause you to think it is coma and tilt.

Always looking to understand more.

Thanks

Mike

[Brian Valente]

Hi MIke

i only have time to dash off a quick response here, so I apologize if it's somewhat truncated

it looks like you have a number of things going on here.

Looking at your guide log, the last session shows DEC corrections in only one direction, which suggests your polar alignment is off. However in your earlier session of the same duration (segment 20) it doesn't show this. It makes me wonder if you have something slipping in your setup?

Your stars do show a fair bit of optical aberration across the image which to me looks more like coma. Regarding the star shape, usually with guiding problems i tend to see tube-shaped stars, where the star is moving but has the same shape. in your case they are tapered at both ends, like a football (american football that is :) ). so i'm not sure it is a guiding problem. 

you correctly pointed out that your orthagonality is not too much. More importantly looking at your guiding results, the overall RMS of your last session was 0.67" with RA being .40" and DEC being .54". that's really good guiding, and I don't think that would cause your unusual star shapes

what is your imaging scale? 

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Brian 

Brian Valente

portfolio brianvalentephotography.com 

[bw_msgboard]

Hi Michael.  In your earlier message, you said the image was taken at 01:46.  I took that to mean that’s when the 10-min exposure completed.  If so, we can see some guiding problems during the period when the image was being taken:

 

 

The problem is the big Dec movements in green in the second half of the exposure.  The Dec guiding RMS during this period was almost 2x larger than the RA RMS, which is often a problem.  This is how you can analyze the source of problems, by carefully matching the time periods between the guiding and the exposure.

 

So it’s credible to me that the elongated stars were caused by this.  Do you see these in the preceding and following frames?  If not, it isn’t an optical problem like tilt and it certainly doesn’t look like coma to my eye.  The quickest way to isolate optical problems is to take a series of short exposures on a fairly rich star field, with the exposures short enough to not really need guiding.  Then you can look at the full-frame, non-compressed raw images with whatever tool you want to use.  CCDInspector, for example, will give you an evaluation that can be very helpful.  Anyway, unless you have these particular problems on all your frames, I don’t think it’s an optical problem.

 

As for the big Dec movements, those look like something loose in the guiding assembly or something pulling on it at certain times.  As bad as they look, they correspond to linear movements of the camera sensor by about 5 microns.

 

Hope this helps,

Bruce

 

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From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Michael Boccabella

Sent: Wednesday, April 01, 2020 8:45 AM
To: Open PHD Guiding

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[Michael Boccabella]

To all who replied to my post, thanks for the suggestions. I went out yesterday evening and re-balanced the scope. The Dec axis was not correctly balanced.

After that, I reviewed both the guide scope and the imaging scope and found that the imaging scope was loose near the focuser. The screws that allow the focuser to rotate for changing camera angle were not as tight as they should be and it introduced some wobble. The thumbscrew was tight, but the 3 set screws that keep it straight in the tube had some play. Also, the guide scope focuser had a little give. I tightened everything up. After alignment and polar aligning, I was able to capture the attached file. The guiding log is also attached. 

These are things I should have thought to check on my own, but I did not. The image is a 600 sec. sub. With the moon out, it is pretty washed out, but I wanted to see what tracking was at 600 sec.

Thanks for all of the help.

Mike

On Tuesday, March 31, 2020 at 12:24:01 PM UTC-4, Michael Boccabella wrote:

[Brian Valente]

Hi Mike

the image looks pretty good for 10 mins. there are some optical aberrations which make it a little harder to analyze, and jpg always introduces compression which destroys the details. a link to a .fit file is always more helpful

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