Looking for guidance on guiding

Equipment Guiding Polar Alignment
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Andy Wray avatar
Andy Wray avatar
OK, so I have got my guiding down to about 0.5 arc secs RMS, yet my stars are still a bit oval, so looking for any tips you may have.

* DEC RMS is down at about 0.25 arc secs, so I guess I have my polar alignment pretty well sorted
* RA RMS is always about 1.5x as much as DEC, so hence the slightly oval star shape

I'm not doing the East-heavy thing, because I would like to be able to have an auto-meridian flip and continue imaging after the meridian ... or have I misunderstood that?

Anyone else had this RA issue and, if so, how did you fix it?

Here is a typical guiding graph:
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andrea tasselli avatar
andrea tasselli avatar
Try imaging at signficantly shorter intervals and see if it changes (becames rounder). If it does then you have an issue with the mount like high-frequency vibrations. If it doesn't then you have an issue with the optics.

P.S.: You can also check if the latter by imaging the NCP and switching off the tracking.
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Björn Arnold avatar
Björn Arnold avatar
Andy Wray:
OK, so I have got my guiding down to about 0.5 arc secs RMS, yet my stars are still a bit oval, so looking for any tips you may have.

* DEC RMS is down at about 0.25 arc secs, so I guess I have my polar alignment pretty well sorted
* RA RMS is always about 1.5x as much as DEC, so hence the slightly oval star shape

I'm not doing the East-heavy thing, because I would like to be able to have an auto-meridian flip and continue imaging after the meridian ... or have I misunderstood that?

Anyone else had this RA issue and, if so, how did you fix it?

Here is a typical guiding graph:

The MinMo on RA could be more aggressive, i.e. lower. Since RA doesn’t reverse, you don’t have to worry about backlash.

Did you run the guiding assistant? It should give you a good starting point

You may also try a bit longer exposures. 2 to 4 seconds.
Brian Dwyer avatar
Brian Dwyer avatar
Andy Wray:
OK, so I have got my guiding down to about 0.5 arc secs RMS, yet my stars are still a bit oval, so looking for any tips you may have.

* DEC RMS is down at about 0.25 arc secs, so I guess I have my polar alignment pretty well sorted
* RA RMS is always about 1.5x as much as DEC, so hence the slightly oval star shape

I'm not doing the East-heavy thing, because I would like to be able to have an auto-meridian flip and continue imaging after the meridian ... or have I misunderstood that?

Anyone else had this RA issue and, if so, how did you fix it?

Here is a typical guiding graph:

*Note: Looking at your RMS performance I would not expect you to have oval stars unless your focal length of your imaging scope is too large for your guiding setup. What is your main scope setup?

Your DEC looks smooth, notice the MinMo (minimum move) is 0.45. Your RA MinMo is pretty high incomparison at 0.65. I assume this is in 'pixels'. I usually use a MinMo for RA of 0.2 and sometimes slightly lower if I am trying to cure a gremlin. However, for your case I would dial it down to 0.4 and see how the guiding performs. RA Agression may be a bit high in your example, just keep that in mind. PHD usually recommends RA aggression of 70 (relative to DEC).

How is your PHD Calibration graph? You are calibrated, correct? What are your guidecam exposure times that you have tried? Do you have any guiding logs you can share?

Also: +1 to the comment suggesting to use the guiding-assistant. Make sure your guidescope focal length and camera pixel size are properly set for your configuration.

What are the exposure times you are using for your imaging camera? What is the focal length of your imaging scope? Did you investigate the 'oval' shape to make sure that the star is stretching in the RA axial direction?

Whew, lots of questions.
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Andy Wray avatar
Andy Wray avatar
Brian Dwyer:
Andy Wray:
OK, so I have got my guiding down to about 0.5 arc secs RMS, yet my stars are still a bit oval, so looking for any tips you may have.

* DEC RMS is down at about 0.25 arc secs, so I guess I have my polar alignment pretty well sorted
* RA RMS is always about 1.5x as much as DEC, so hence the slightly oval star shape

I'm not doing the East-heavy thing, because I would like to be able to have an auto-meridian flip and continue imaging after the meridian ... or have I misunderstood that?

Anyone else had this RA issue and, if so, how did you fix it?

Here is a typical guiding graph:

*Note: Looking at your RMS performance I would not expect you to have oval stars unless your focal length of your imaging scope is too large for your guiding setup. What is your main scope setup?

Your DEC looks smooth, notice the MinMo (minimum move) is 0.45. Your RA MinMo is pretty high incomparison at 0.65. I assume this is in 'pixels'. I usually use a MinMo for RA of 0.2 and sometimes slightly lower if I am trying to cure a gremlin. However, for your case I would dial it down to 0.4 and see how the guiding performs. RA Agression may be a bit high in your example, just keep that in mind. PHD usually recommends RA aggression of 70 (relative to DEC).

How is your PHD Calibration graph? You are calibrated, correct? What are your guidecam exposure times that you have tried? Do you have any guiding logs you can share?

Also: +1 to the comment suggesting to use the guiding-assistant. Make sure your guidescope focal length and camera pixel size are properly set for your configuration.

What are the exposure times you are using for your imaging camera? What is the focal length of your imaging scope? Did you investigate the 'oval' shape to make sure that the star is stretching in the RA axial direction?

Whew, lots of questions.

That was a lot of questions and some good advice:

The guiding assistant gave me these settings, although I did reduce from 1.5 secs to 1 sec exposure to try and smooth out the RA.
Calibration looked very good.
Focal length and pixel size are correct.
Exposure times in this case were between 30 secs (Lum) and 90 secs (RGB)

I think I will try reducing the RA MinMo and maybe reduce the aggresiveness.  Unfortunately I have clouds now for the next week.

I have tried guiding times from 3 secs to 1 sec.

I do need to learn how to identify the RA and DEC direction on my subs so that I can make sure it is a guiding issue.  Will do that next.
Brian Dwyer avatar
Brian Dwyer avatar
Andy Wray:
That was a lot of questions and some good advice:

The guiding assistant gave me these settings, although I did reduce from 1.5 secs to 1 sec exposure to try and smooth out the RA.
Calibration looked very good.
Focal length and pixel size are correct.
Exposure times in this case were between 30 secs (Lum) and 90 secs (RGB)

I think I will try reducing the RA MinMo and maybe reduce the aggresiveness.  Unfortunately I have clouds now for the next week.

I have tried guiding times from 3 secs to 1 sec.

I do need to learn how to identify the RA and DEC direction on my subs so that I can make sure it is a guiding issue.  Will do that next.

Can you share an image you believe has issue? I don't think a few minutes are enough to have oblong stars unless you have some extremely bad flexure that would not show up on the guiding graph (an untethered cable bouncing in the wind as an example). Also, I'm still missing the info on your imaging setup. Scope focal length + camera pixel size etc.
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Andy Wray avatar
Andy Wray avatar
Brian Dwyer:
Andy Wray:
That was a lot of questions and some good advice:

The guiding assistant gave me these settings, although I did reduce from 1.5 secs to 1 sec exposure to try and smooth out the RA.
Calibration looked very good.
Focal length and pixel size are correct.
Exposure times in this case were between 30 secs (Lum) and 90 secs (RGB)

I think I will try reducing the RA MinMo and maybe reduce the aggresiveness.  Unfortunately I have clouds now for the next week.

I have tried guiding times from 3 secs to 1 sec.

I do need to learn how to identify the RA and DEC direction on my subs so that I can make sure it is a guiding issue.  Will do that next.

Can you share an image you believe has issue? I don't think a few minutes are enough to have oblong stars unless you have some extremely bad flexure that would not show up on the guiding graph (an untethered cable bouncing in the wind as an example). Also, I'm still missing the info on your imaging setup. Scope focal length + camera pixel size etc.

906mm F4.5, 3.8u pixel size and here is a typical crop recently:
Brian Dwyer avatar
Brian Dwyer avatar
Andy Wray:
Brian Dwyer:
Andy Wray:
That was a lot of questions and some good advice:

The guiding assistant gave me these settings, although I did reduce from 1.5 secs to 1 sec exposure to try and smooth out the RA.
Calibration looked very good.
Focal length and pixel size are correct.
Exposure times in this case were between 30 secs (Lum) and 90 secs (RGB)

I think I will try reducing the RA MinMo and maybe reduce the aggresiveness.  Unfortunately I have clouds now for the next week.

I have tried guiding times from 3 secs to 1 sec.

I do need to learn how to identify the RA and DEC direction on my subs so that I can make sure it is a guiding issue.  Will do that next.

Can you share an image you believe has issue? I don't think a few minutes are enough to have oblong stars unless you have some extremely bad flexure that would not show up on the guiding graph (an untethered cable bouncing in the wind as an example). Also, I'm still missing the info on your imaging setup. Scope focal length + camera pixel size etc.

906mm F4.5, 3.8u pixel size and here is a typical crop recently:

Can you share a single Lum exposure (no stack)? And not cropped...
Andy Wray avatar
Andy Wray avatar
Brian Dwyer:
Can you share a single Lum exposure (no stack)? And not cropped...


I had to scale it down a bit due to file limit sizes, but here you go.  You can open the image in a new tab to get the full scale.  I did a histogram stretch on it to post on here and a quick denoise.
Brian Dwyer avatar
Brian Dwyer avatar
Thanks for the image!

Ok, I do see the slight elongation but I'm not sure I agree to call it oval (as in I wouldn't call it star smearing due to guiding error). There seems to be a good star shape and then a smaller smudge in the lower-right direction. I uploaded your image to astrometry.nova so I could see the directions for RA/DEC. The direction of the slight smudge I described seems to be 30-40 degrees out of phase with RA. It does not align with RA (in my opinion), which also leads me to believe this is not due to the RA graph you showed.

To me, I'm wondering if this is piched-optics or maybe even imperfect collimation?

I'll think more about. In the meantime, here is the astrometry job you are welcome to look at: https://nova.astrometry.net/user_images/6420237#grid

RA/DEC grid (RA goes nearly up and down): https://nova.astrometry.net/grid_full/6989854
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Lynn K avatar
Lynn K avatar
Andy, looking at the difference of the stars in the corners and center, I do not think the issue is guide error.  All four corners are slightly different, with the lower right the best.  It looks like a classic cast of slight tilt to me.  Could be the camera chip or the imaging train.  Rotate the camera and if the coner destortion is the same, then it is the camera chip.

Is it by chance worst in cooler/cold temps.  If so, there could also be some slight pinched optics. That can cause stars to be diamond shaped, which can appear oval.

Lynn K.
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Simon Pepper avatar
Simon Pepper avatar
I have had so many issues with tilt recently it could be that, but I am leaning towards one bad sub that may have got through into your stack? This looks like a prime example of when there are clouds and the guide looses its star and then you get one jump in RA causing almost a star tiny tiny line then a dot this it what it looks like to me. Go back and check each individual sub. if you cant see the culprit I reckon tilt. 0.5 RMS is good and at that FL will be fine I think.
Dale Penkala avatar
Dale Penkala avatar
Hello Andy,
Many good suggestions here so I’ll point out a couple that lead me to my suggestion.
@andrea tasselli made a very good suggestion. While I don’t think its your guiding (but possibly) this could help rule it out. I did this myself and its a great test.
Maybe I missed this but I didn’t see anywhere, where you stated what instrument your using?? However I’m right along with @Brian Dwyer and a couple others that are questioning possible pinched optics whether its in an refractor or mirror cell of a reflector. Certainly worth looking into.
Also, agree with the guiding assistant runs!
You may want to consider watching the episode on the astro imaging channel where the creator of PHD2 was interviewed on the software. When I get time this afternoon I’ll post the link.

Dale
Andy Wray avatar
Andy Wray avatar
Dale Penkala:
Hello Andy,
Many good suggestions here so I’ll point out a couple that lead me to my suggestion.
@andrea tasselli made a very good suggestion. While I don’t think its your guiding (but possibly) this could help rule it out. I did this myself and its a great test.
Maybe I missed this but I didn’t see anywhere, where you stated what instrument your using?? However I’m right along with @Brian Dwyer and a couple others that are questioning possible pinched optics whether its in an refractor or mirror cell of a reflector. Certainly worth looking into.
Also, agree with the guiding assistant runs!
You may want to consider watching the episode on the astro imaging channel where the creator of PHD2 was interviewed on the software. When I get time this afternoon I’ll post the link.

Dale

Thanks Dale!

Ref the pinched optics:  I'd love to check that out, but after 5 attempts I have still found it impossible to remove my primary mirror ... jammed solid in the tube.

I think the best I can do is focus on everything else for now and see how close to round I can get.

* I think I had some tilt in my imaging train for this session, so will attempt to eliminate that (N.B. my camera seems fine from a tilt point of view in general)
* I have noticed that my secondary mirror was not perfectly aligned to my focus tube (I could see only one primary mirror clip and not the other two using a Cheshire).  That's now fixed and I've recollimated, so just waiting for a clear night to test.
* Replacing the focusser with a low profile version which doesn't impede the OTA at all has made a big difference already.
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Dale Penkala avatar
Dale Penkala avatar
Andy Wray:
Dale Penkala:
Hello Andy,
Many good suggestions here so I’ll point out a couple that lead me to my suggestion.
@andrea tasselli made a very good suggestion. While I don’t think its your guiding (but possibly) this could help rule it out. I did this myself and its a great test.
Maybe I missed this but I didn’t see anywhere, where you stated what instrument your using?? However I’m right along with @Brian Dwyer and a couple others that are questioning possible pinched optics whether its in an refractor or mirror cell of a reflector. Certainly worth looking into.
Also, agree with the guiding assistant runs!
You may want to consider watching the episode on the astro imaging channel where the creator of PHD2 was interviewed on the software. When I get time this afternoon I’ll post the link.

Dale

Thanks Dale!

Ref the pinched optics:  I'd love to check that out, but after 5 attempts I have still found it impossible to remove my primary mirror ... jammed solid in the tube.

I think the best I can do is focus on everything else for now and see how close to round I can get.

* I think I had some tilt in my imaging train for this session, so will attempt to eliminate that (N.B. my camera seems fine from a tilt point of view in general)
* I have noticed that my secondary mirror was not perfectly aligned to my focus tube (I could see only one primary mirror clip and not the other two using a Cheshire).  That's now fixed and I've recollimated, so just waiting for a clear night to test.
* Replacing the focusser with a low profile version which doesn't impede the OTA at all has made a big difference already.

Hi Andy,
Boy thats odd that you can’t get the mirror out, which does seem to move in the direction of pinched optics, but I get it if you can’t get it out then work and adjust everything until you can’t get it any better.

Wishing you clear skies and best of luck!

Dale
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