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Guiding Question, still some drift over hours?

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#1 stacaz

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Posted 09 April 2020 - 11:55 PM

I got into guiding and did very well with a typical quide scope/cam combo, with wider field main imaging scope and ASI1600 -type images, stacked, sharp, etc. I recently moved up to 1000mm main scope, and noticed that I get noticeable image drift in one direction over an hour or so, even WITH guiding, and the guide star stays in the middle the whole time, and I also get a decent PHD2 graph around and under one arc-sec. RMS... Anything thoughts on how the overall tracking over an hour or more, WITH guiding, can lead to a one-drectional drift over an hour?  My subs still have basically round stars, on most frames...   My mount is a Celestron AVX..   Time to upgrade to an EQ6-R Pro?  Specifically, I guess, why is my guide star still showing in the middle of the Star Profile screen in PHD2, while my main cam image subs are drifting in one direction over time, perhaps several arc seconds per minute...?  Thanks!


Edited by stacaz, 09 April 2020 - 11:59 PM.


#2 scadvice

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Posted 10 April 2020 - 12:07 AM

 Flexure? A picture of your setup would help us.


Edited by scadvice, 10 April 2020 - 11:28 AM.

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#3 bridgman

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Posted 10 April 2020 - 12:08 AM

I'm trying to reconcile two of your comments:

 

"My subs still have basically round stars, on most frames"

"main cam image subs are drifting in one direction, perhaps several arc seconds per minute"

 

How long are your exposures for each sub ?

 

I don't remember offhand if PHD2 has the ability to zoom the guiding image - wondering if the image scale on PHD2 is just too small to see the drift ?

 

The other possibility is that your guide scope and main scope are moving relative to each other, but that seems less likely.



#4 WadeH237

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Posted 10 April 2020 - 09:06 AM

If your guide star is still essentially centered, but the main image has drifted, this is flexure.  Guaranteed.

 

Upgrading your mount will not help.  Moving from a guide scope to an OAG (or ONAG) will eliminate it.  Until you are able to do that, your best bet is to keep your sub exposures short enough that the stars are round, and then repoint at the target periodically.


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#5 SMigol

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Posted 10 April 2020 - 10:30 AM

I have also chased this problem.

 

Things to consider:

 

Flexure is the quickest to diagnose - put your guide camera on the same optics as the sensor and your rig can drag, flop, droop, and wiggle all it wants but those errors will be guided out.  This is the main advantage of the off-axis or on-axis guiding setup.  

 

You'll need a sensitive guide camera to do this.  Time to upgrade to a 16 bit camera vs the 8 or 10 bit webcam.  Get a lodestar or something similar.  The star images from these cameras have significantly better signal and the guiding is also better.  Yes, the webcams are good enough, but once you start pushing exposures longer than 10 minutes, I've found that that there's not enough information to stay on target.  

 

If you want to put multiple sensors on one guide setup (dual cameras, etc), then you'll eventually need to resolve the flexure issue.  

 

Here's a decent link that covers star trailing issues: http://www.astropix....iled_Stars.html



#6 stacaz

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Posted 12 April 2020 - 09:24 PM

Here is a photo of my setup with guiding. I can't feel any slop or play. The drift is about the width of the whirlpool galaxy over about 2 to 3 hours. Not very much, but it's consistent, night after night. 

 

Also I ran the wizard on PHD2, and it had a weird (supposed to be perpendicular) graph on the backlash graph. It said don't use exposures longer than 1.5 to 3.5 seconds.  Huh?

 

https://ibb.co/yRp5jcv


Edited by stacaz, 12 April 2020 - 09:28 PM.


#7 Iver

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Posted 12 April 2020 - 10:01 PM

Could also be mirror shift.


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#8 scadvice

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Posted 12 April 2020 - 10:02 PM

Have you posted you're last question on the PHD2 forum?

 

https://groups.googl...pen-phd-guiding


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#9 Michael Covington

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Posted 12 April 2020 - 10:41 PM

Here is a photo of my setup with guiding. I can't feel any slop or play. The drift is about the width of the whirlpool galaxy over about 2 to 3 hours. Not very much, but it's consistent, night after night. 

 

Also I ran the wizard on PHD2, and it had a weird (supposed to be perpendicular) graph on the backlash graph. It said don't use exposures longer than 1.5 to 3.5 seconds.  Huh?

 

https://ibb.co/yRp5jcv

That's the guide camera exposure, not the astrophotography camera exposure.  Typically 2 seconds.



#10 stacaz

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Posted 12 April 2020 - 11:28 PM

Here is a stacked image from about 70 subs at 1 min each. See the stars are still basically round. I just switched to SGP with a color camera, and I am still learning SGP and processing of FITS files...  I have dithering on, and you can see it in the subs, but it still slowly drifts in the RA direction in a steady way. 

 

Very slight "footballing" of stars and centers of galaxy. The direction is downward, in the image, I assume that is East-West (RA).

 

https://ibb.co/4WTCB4K


Edited by stacaz, 12 April 2020 - 11:33 PM.


#11 stacaz

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Posted 13 April 2020 - 12:29 AM

Playing some more with GRBG, RGGB, etc. debayer settings in DSS for processing the FITS files from SGP. And then playing with Photoshop. Same set of subs from earlier. Better image here to see the footballing over 1 min subs (times) about 100 of them, about 70 used for stacking. So minor, but so frustrating to solve!  I really don't want to use an OAG to get in the way of the main imaging cam...

 

https://ibb.co/yPzPQDt



#12 stacaz

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Posted 13 April 2020 - 01:39 AM

So I'm getting the feeling that a regular Celestron 8SE SCT is just not quite good enough for multiple-minute Deep Space Astro Photos....even with the 6.3 reducer/flattener...  frown.gif  Time for an HD SCT, or 127mm refractor?

 

I know that when I focus the SCT, the imaging camera image moves quite a bit, even with the mount having an electrical motor focuser.. Crappy Mirror movement design... 


Edited by stacaz, 13 April 2020 - 03:56 AM.


#13 WadeH237

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Posted 13 April 2020 - 09:56 AM

So I'm getting the feeling that a regular Celestron 8SE SCT is just not quite good enough for multiple-minute Deep Space Astro Photos....even with the 6.3 reducer/flattener...  frown.gif  Time for an HD SCT, or 127mm refractor?

 

I know that when I focus the SCT, the imaging camera image moves quite a bit, even with the mount having an electrical motor focuser.. Crappy Mirror movement design... 

All of your posts are still indicative of flexure.

 

I would caution against trying to get around this by buying a new OTA.  Every system that I've ever used with a guide scope has some flexure.  Every one.  It does not matter if you are using a refractor or an EdgeHD scope (I image with an EdgeHD, and the mirror clutches are not sufficient to prevent it).  It's just a matter of the degree to which it exists.

 

The best answer is the very thing that you are trying to avoid:  an OAG.  Configured properly, an OAG does not "get in the way" of the main imaging camera.  The pick-off prism should be adjusted so that it is just out of the field of the main camera.  Yes, there is a bit of a learning curve in setting one up.  But once you get past the learning curve (probably a night or two), then it just fades into the background and works.  The other two possible solutions are an ONAG (more expensive than an OAG, plus uses lots of back focus) or a self-guiding camera (SBIG used to make these, but they've switched to having an OAG integrated into their filter wheel on the current crop of cameras.  The downside to the self-guiding cameras, is that the guide sensor is behind the filters, which is a pretty significant limitation.)


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#14 SimonMiller

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Posted 13 April 2020 - 08:58 PM

Flexure is the quickest to diagnose - put your guide camera on the same optics as the sensor and your rig can drag, flop, droop, and wiggle all it wants but those errors will be guided out.

“Easy” to understand/appreciate but not quick to demonstrate if you don’t own/have the equipment e.g. an OAG setup?

 

Could the problem be shown independently of PHD2 by showing the amount of flexure and/or mirror movement present (and also no need to upgrade the mount) by:

 

1. Switch off any guiding

2. Align the guide scope “temporarily” to point at a “target” in the main scope

3. Capture short exposure from main and guide camera and verify they show the same “target”

4. Move to a new “target” by rotating in RA about two hours on to a new target

5. Capture another pair of short exposures

6. Compare relative alignment from each pair of short exposures in step 3 and 5 looking for movement

 

In step 4 you are trying to replicate what happened over several hours, to show that there wasn’t flexure or mirror movement. If there is then have to consider if OAG would eliminate the problems.

 

Simon


Edited by SimonMiller, 13 April 2020 - 09:26 PM.


#15 SonnyE

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Posted 14 April 2020 - 01:18 AM

Every time I run across a thread like this I thank my lucky stars I became a refractor nut.

Coming down to the wire, I had an Astrograph in one wish list. And I had a Refractor in the other wish list.

 

I chose the path of the least chaffing.

Purely dumb luck. But almost all of my luck is that way. wink.gif

I'm a lot smarter now. But so is my dumb luck.



#16 schmeah

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Posted 14 April 2020 - 12:13 PM

Every time I run across a thread like this I thank my lucky stars I became a refractor nut.

Coming down to the wire, I had an Astrograph in one wish list. And I had a Refractor in the other wish list.

 

I chose the path of the least chaffing.

Purely dumb luck. But almost all of my luck is that way. wink.gif

I'm a lot smarter now. But so is my dumb luck.

But this is flexure. And flexure has nothing to with whether or not it’s a refractor or SCT. If you have a guide scope on it, you will likely see some flexure, perhaps less noticeable at shorter focal length. But all imperfections are less noticeable at short focal lengths.

 

Derek


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#17 stacaz

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Posted 14 April 2020 - 11:12 PM

OK, Thanks all!! I am doing some comet Atlas stuff tonight, not getting any appreciable movement over a hour, maybe one arc-minute. Likely mirror flexure with change in temp. Here in the desert the temp swings are quite wild from sunset to midnight. I will try OAG and be done with it.  Thanks again!



#18 scadvice

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Posted 15 April 2020 - 11:29 AM

There are some 'you tube' videos by James Lamb talking about setting up the Off-Axis Guider you might be interested. Here is one of them.

 

https://www.youtube....h?v=x_Od8hp7dg0



#19 SimonMiller

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Posted 25 April 2020 - 03:04 PM

James makes the point that the mount's performance (guiding) is perhaps impacted more by where the weight is distributed than the absolute amount.

 

It's commonly assumed that a "good" mount that's lightly stressed for AP will be a good solution e.g. 1/2 of it's rated capacity (that's quoted for visual astronomy).

 

With an OAG you put the weight closer to the centre of gravity so the moment of inertia (weight * distance squared) is greatly reduced vs having a big guide camera "hanging off" the main scope.

 

Adding weight can be beneficial e.g. take a counterweight and supplement it with another weight - can now achieve balance with it closer to the centre of gravity - reducing moment of inertia.

 

Simon



#20 scadvice

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Posted 25 April 2020 - 07:05 PM

I've recently moved to an OAG  from a 80mm guide scope and found though my guiding was pretty good before, it has improved quite a bit with the leaner system. Even with the bad seeing.. However, I've only been able to use it for the last three clear nights. High clouds have been a imager's nightmare here this month.




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