Focal length of guidescope for accurate drift polar alignment?

[Stephen Wong]

Hi

I've reviewed my images under PHD2 guiding and found that I've made a mistake on the polar alignment.

I first do a rough polar alignment and using the drift tools in PHD2. It appears that the trendline is good for one to two minutes.

My imaging scope is C8 with 0.67 reducer, imaging camera Atik 490ex. Resolution is 0.56" per pixel

My guidescope is a 120mm (focal length), zwo 178mc. Resolution is 4.13" per pixel

Is it that I need a longer focal length for a better drift alignment (or the guiding also)?

[bw_msgboard]

If you’re imaging with such a fine image scale on an SCT, you will probably need to switch to an OAG.  Otherwise, the differential flexure will likely be a big limitation.  You haven’t given us any numbers, but obsessing about polar alignment is usually a waste of time unless the alignment error is huge, like 10 arc-min or more.  But to answer your question, the guider focal length or image scale shouldn’t have any significant on polar alignment results.

 

Bruce

 

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[Dominique DANIEL]

Hi Sywon..

I'd experienced such difficulties with comparable setup.. I've tested an OAG. The problem is that the FOV is too limited, so it's difficult to find stars. Finally, I choose a low quality F400 scope as guiding scope, and I change the Evolution mount with a CGX one. 2 advantages: the 11kg payload was to heavy for the Evolution, and is not a problem for the CGX (limit is 25 kg), and there is a PEC (periodic Error Correction). The first session was dedicated to getting an accurate plane solve model, and a good polar alignment. Now it's done, and I spend my time in getting fine pictures! (I've also build a fixed pier).

Good luck

Dominique

[bw_msgboard]

I think your characterization of an OAG may not be accurate.  It was certainly true in the past but with the arrival of much larger and more sensitive guide camera sensors, finding stars is rarely a problem.  For example, I’ve been imaging remotely for 2 years now with an OAG on a 2450mm focal length scope.  The guider field of view is about 8 arc-min and I’ve never seen PHD2 fail to find at least one usable guide star – often more now with multi-star guiding.  These are usually 11^th or 12^th magnitude stars that are nearly invisible on the display, but that doesn’t matter.  Since I like to image small targets and especially galaxies, I am often shooting in star-poor regions of the sky.  Granted, you need to invest in a high quality guide camera but I don’t think there’s any reason to be afraid of OAG solutions.  I think Andy has had the same experience, in fact he was the one who convinced me I didn’t need to invest in a rotator. J

 

Regards,

Bruce

 

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[steve]

On 15/2/21 21:33, Dominique DANIEL wrote:

C8 with 0.67 reducer, imaging camera Atik 490ex. Resolution is 0.56" per pixel

Hi

How about getting a shorter focal length telescope for nights when the atmosphere will not allow guiding the c8? If then you do get a rare night where you can guide near 0.5", just swap over. That's what we do between or 6" 1200mm f8 and our shorter telescopes. It works well.

Cheers and HTH 

[Stephen Wong]

Guys

Thanks for your advices... allow me to upload the images and guide/debug log and explain a little bit... I took total 7 images. 4 on IC342 then 3 on M1. I made two animated gif so it's easier to see what happened.

On first 3 of the IC342 you can see the stars didn't shift much - showing the PA is good.... after these 3 subs I've found dew built up - it was so wet and I have to wipe the droplets from the front window - this must have moved the mount. You can see a significant image shift on the 4th frame - I've made a mistake that I thought wiping off the droplets gently would not move the alignment... I didn't notice it until I analyze these images later when I head home

Then I continue to took the 4th sub - I realized at that moment my site would never be good for IC342.... and then I changed the target to M1... at the time I saw the elongated stars, I didn't think it was PA - I was just wondering if my mount (Takahashi EM-11) cannot handle the weight.... until I head home and analyze these images and the guidelog and found it's in fact the PA...  

Thanks for your inputs - I just want to make sure I have a better way to ensure the mount is polar aligned with minimal guessing to be done.

19:53:04 performed drift calibration - result is ok

19:55:16 started guiding

19:55:52 ic_342_Light_001 start (8 mins)

20:09:38 ic_342_Light_002 start (10 mins)

20:19:55 ic_342_Light_003 start (10 mins)

20:33:00 slew mount (keep guiding)

20:33:37 guide stop

... found dew in front window... clean dew

20:55:18 ic_342_Light_004 start (8 mins)

21:42:40 M1_005 start (8 mins)

22:00:21 M1_006 start (8 mins)

22:08:37 M1_007 start (8 mins)

... then no images taken, only calibration, guide, test

22:36:30 re-calibration

[Stephen Wong]

I just review the log again and I have more questions:

1) it seemed that before the image shift the polar alignment error (7.9') was more than the one afterwards (4.0')? So does the RMS... that's seemed to be reverse to the fact?

2) you can see before the PA is off, the RA error tops at 2". After the mount was moved, the RA error is at 3".. the increase in error was not noticeable. The only noticeable difference was the RA correction is much frequent after the PA is off with a side by side guide graph comparison. Any hints on how I can detect the PA was off from the guide graph on the field? Or I'm being wrong that the guide graph wasn't suppose to tell the PA is off? (I may try the guiding assistance)

3) I did some drift alignment after all these images but I cannot tell if the result is accurate - the drift trendline was good but then when I started guiding again the RMS never went lower than 1". That's why I'm confused - it seemed to me I've performed the drift alignment, the trendlines are good and yet I have no way to tell if the mount achieve an accurate PA. That's also why I asked if I should get a guidescope with a longer focal length for a more accurate drift alignment.

[Ken Self]

Looking at your log I think PA error is the least of your worries. I don't see a single decent calibration in there and there are signs of significant backlash.

You need to work on getting a decent calibration. And because you are using the On Camera driver you need to get it right on every target.

The most important thing here is to clear the backlash on both axes before you start calibrating.

When you get a good calibration, and can do it reliably, then run the Guiding Assistant.

And take some time to read the best practices: https://openphdguiding.org/phd2-best-practices/

[Stephen Wong]

Thanks... If you take a look at the GIF images it's clearly the PA is off.

The calibration is mostly bad - I also noticed it.... however once or twice it was good.... I have no idea why it happened. You're right that it may be backlash - I'll try to clear it before I calibrate next time.