Rotating main camera after calibration

[Chuck Schreiner]

The other night, after I had calibrated PHD2, I took a couple of test shorts - the stars looked good. But I wanted a little different orientation, so I rotated the main camera maybe 20*, and did not re-calibrate PHD2. The rest of the night the stars were elongated. (I didn't recognize this connection until the next day). Tracking was good all night (under 0.9, typically 0.7)

Would that cause the elongation, or do I need to keep digging? 

[Bruce Waddington]

If you rotate the guide camera, you must re-calibrate unless you have an automated rotator.  That was undoubtedly the source of your elongated stars.

Bruce

[Chuck Schreiner]

No, I just rotated the main camera, which is independent of the guide camera.

[Brian Valente]

Chuck

We need to see your guidelogs by using the guidelog uploader

https://openphdguiding.org/getting-help/

Also can you take a picture or two of your setup from different angles

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Brian Valente

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[Chuck Schreiner]

Hi Brian,

Here is the guide log: 

https://openphdguiding.org/logs/dl/PHD2_logs_wdRw.zip

My setup is broken down (I travel to image) so I can't take a pic of it. It is a 152mm refractor with a 60mm guide scope/camera - the main ZWO camera is attached to a feathertouch focuser that can rotate the camera to frame the image as desired. This is what I rotated. 

After thinking about it, I don't see how this would impact guiding. 

Thanks, 

Chuck

[Chuck Schreiner]

I don't know how to read the log...I hope I uploaded it so others who do know how can see it. When I was imaging, another guy there said he thought, even though my overall error was low, that the fact that the error in my RA was 2x that of dec was the issue (and that I should use guiding assistant). Can this be seen in the log? Would this be the issue?

https://openphdguiding.org/logs/dl/PHD2_logs_wdRw.zip

[Brian Valente]

Hi Chuck

RA is a bit higher but far from 2x. RA is typically higher than Dec on most mounts since RA is always running.

Taking out a few blips in your guiding the overall and RA/Dec RMS for your two runs is:

 

There is a noticeable residual error around 32 second period. That is likely near the motor, you can try adjusting the motor mesh. Losmandy has a video here (the topic is noise, but same adjustment) https://www.youtube.com/shorts/Vqqjz40qAu4

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[Chuck Schreiner]

I thought  I had replied...guess not. Thanks, Brian. I will try that. I need to learn how to read, analyze the guide log....

[bw_m...@earthlink.net]

https://openphdguiding.org/tutorial-analyzing-phd2-guiding-results/

 

 

 

From: open-phd...@googlegroups.com <open-phd...@googlegroups.com> On Behalf Of Chuck Schreiner
Sent: Wednesday, April 17, 2024 7:26 PM
To: Open PHD Guiding <open-phd...@googlegroups.com>
Subject: Re: [open-phd-guiding] Re: Rotating main camera after calibration

 

I thought  I had replied...guess not. Thanks, Brian. I will try that. I need to learn how to read, analyze the guide log....

On Tuesday, April 16, 2024 at 1:00:01 PM UTC-7 bval...@gmail.com wrote:

Hi Chuck

 

 

RA is a bit higher but far from 2x. RA is typically higher than Dec on most mounts since RA is always running.

 

Taking out a few blips in your guiding the overall and RA/Dec RMS for your two runs is:

 

 

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[Chuck Schreiner]

Thanks, Bruce. Am reading it now. Very interesting. And well-written!

[bw_m...@earthlink.net]

Thanks, Chuck, let me know if you have questions about it. 

 

Bruce

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[Chuck Schreiner]

Hmm. After looking at that article and my log pretty carefully (and fiddling with the Losmandy motor as per Brian's suggestion) - it's not at all clear to me from that analysis what the issue might be. 

This is a newish telescope, pretty long focal length for me (@1200, f/8) and I really want to get the stars tight. How best to test? I am in Los Angeles but I go to the bluffs of Palos Verdes which are a little darker to test, and drive 2-4 hrs, camp out during new moon to image. The one thing I have learned is getting guide stars in tight focus really helps. As does good seeing (and darker skies!) 

I have not used PHD's Guiding Assistant for a while. I'm going to use that when I test. Losmandy has an upgrade to Gemini to Level 6 that evidently improves the mount's performance. I am trying to avoid off-axis guiding, as I understand that to be a pain to learn. Is a 240 guide scope/ASI 120MM camera insufficient for this size telescope?

Any other suggestions as to what to do to test? What else might I pay attention to?

[Brian Valente]

Hi Chuck

I'm not sure i'm following you here. What is the issue you are trying to troubleshoot? What has happened since the last feedback i sent you?

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[Chuck Schreiner]

The problem is elongated stars but a pretty good tracking error. (<0.7) This is for a Stellarvue 152 triplet, 1200 f/8 refractor and G11G mount. I have 2 data points from a week ago - one, the degree of elongation increases with exposure time (suggesting to me it's not something like camera tilt). My initial shot before I rotated the camera and then re-plate solved, had good stars, but after this they were all elongated. Was this just circumstantial or did that rotation/re-centering cause something to get out of whack?

I have watched the video you suggested and adjusted the motor (but could hear very little before and after difference).  I have not yet re-tested (clouds). You posted earlier after looking at my log  ' There is a noticeable residual error around 32 second period.' I am trying to learn how to analyze the log, but I could not see what you were referring to. 

My friend, who is much more experienced than I am, believes part of the problem might be the mounting rings might be too close together (my top plate for the guide scope is only 12" and thus this is the width of the rings) - I ordered a 2nd 19" plate to match the one that came with the telescope. This could reduce flexure, he thinks, if that's the issue.

What I am trying to figure out is how to go about debugging this. 

Thanks very much, Brian, for your help and interest.

[bw_m...@earthlink.net]

I’m sure Brian will get back to you on this.  But one of the reasons I directed you to the tutorial on log analysis is because it has a discussion of differential flexure (in the appendix).  This is a likely source of your problem as your friend is suggesting.  You can probably see if this is the problem even without being able to do more nighttime testing.  Here’s a possible recipe:

1. Find a series of main camera images where you are getting elongated stars.  Note their timestamps
2. Use the LogViewer to look at the guiding performance during those same periods.  You will be looking to see if there were big excursions or if the RA and Dec guiding rms values were very different – e.g. as much as 2x different.  If those things are always present, then you have a mechanical problem of some kind that needs to be fixed.  If they are not present, then you probably have differential flexure.  This is the hallmark of differential flexure – good guiding results but elongated stars with the main camera.
3. If the guiding in step 2 looks good, blink through the main camera images or stack them without first aligning them.  You are likely to see that successive images move in a consistent direction from one main exposure to the next.

 

Good luck,

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[Chuck Schreiner]

Bruce,

Thanks. After working thru the first kinds of errors described in the log analysis, and pretty much rejecting them, I ended up thinking it was flexure, but frankly got intimidated by what this implied. The section on it didn't sink in. Now I've taken a deep breath, and I'm ready to wade into it. It's just that I have read so many discouraging stories about off-axis guiding that ... ugh. 

So when I blink the images, they absolutely drift in the same direction. So, then I looked at images I took with the same telescope, but with the reducer/flattener (FL = 888) vs just the flattener (FL = 1200). There is no elongation, no drift in the blinking of the images. 

Could this be an issue with the flattener? Or is it just that with this additional FL, the flexure becomes apparent? 

Thanks again, 

Chuck

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[Brian Valente]

Hi Chuck

There are two possibilities when having low guiding RMS but elongated stars:

1. Differential flexure (when using a separate guidescope). Bruce pointed out this way to tell, but let me emphasize it: take all your images for a night and stack them without first registering. If you see the elongation goes along a path you are suffering from differential flexure. this is a good article that discusses this phenomenon https://astronomy.mdodd.com/flexure.html

2. One axis has higher RMS than the other: even if your total RMS is low, if one axis has a higher rms than the other, you can get star elongation. The best (and easiest) way to evaluate this is with the log viewer. Just look at each session and look at the contributions of RA and Dec. the greater the difference, the greater the elongation. (log viewer also removes dithers and settling, so it's more accurate than just watching the RMS go by)

here's an example (not yours) of low RMS and the axes being relatively close to each other

Here's an example of one of your runs, you can see RA is slightly higher, but not significantly

In your case I think you may have a little of each. 

Brian

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[Brian Valente]

>>>Could this be an issue with the flattener? Or is it just that with this additional FL, the flexure becomes apparent? 

I think it's unlikely the optics would create differential flexure. I think your latter suggestion is probably the case, it's more apparent. 

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[Chuck Schreiner]

Brian  - thanks again. I am learning a lot thru Bruce's and your input. I trace the graph on log viewer - I see the top data line above the graph that gives the data at any point, and assume the (RA,Dec)=(num1,num2) is the RMS. There are moments in most images where there are significant discrepancies (>.20), but these usually don't last more than 1 or 2 frames. 

If this is part of the cause... What improves this? (Is this likely a setting or a hardware issue?)

[mj.w...@gmail.com]

Hi Chuck

" I have read so many discouraging stories about off-axis guiding "

Most OAG difficulties are due to miss-matched cameras and OAG.

1. The guidecam sensor and the imaging camera sensor both have to be the same distance from the prism, to both be in focus.

Simple optics that many can't seem to grasp :-<

2.  With some OAGs that have a  long helical guidecam focuser, it's not possible to place the sensor close enough.

Often because there's a FF or FR with 55mm Backfocus on the imaging camera side, constraining the required prism to guidecam sensor distance.

3. Best to have a guidecam that's mono and has large pixels, to give sensitivity to find faint guide stars, and to give a sensible pixel scale for guiding.

Though small pixels Binned works too.

And a large OAG prism helps, there are some with a 12mm x 12mm prism, which doubles the area of the Field of View compared to the common 8mm x 8mm prism.

Again there's a common mistake of using a guidecam with a tiny 4.8mm x 3.6mm sensor with that, the guidecam sensor needs to be more like the 12mm x 12mm of the prism.

4. If you don't have a permanent setup, remove the OAG complete with cameras, to avoid having to refocus.

5. Remember to  recalibrate PHD2 if you rotate the OAG to find guide stars, of if you have dismounted it.

Michael

Wiltshire UK

[Brian Valente]

>>> I see the top data line above the graph that gives the data at any point, and assume the (RA,Dec)=(num1,num2) is the RMS. There are moments in most images where there are significant discrepancies (>.20), but these usually don't last more than 1 or 2 frames. 

can you send a screenshot of what you are looking at

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[Chuck Schreiner]

Michael, 

Thanks! I have copied your note for reference. I also watched this video ( (1) Setting Up An Off-Axis Guider - YouTube) till midnight last night and it and you are saying the same thing. I am a little less intimidated now! Of course, like everything astronomy, it means several 'astro dollars' (1 astro dollar = $100 USD)! 

Cheers, 

Chuck

[Chuck Schreiner]

Brian, here's a screen shot. As I understand it, the frame I have circled has an RMS delta between RA & Dec of 0.79. I see this level of difference 2-3-4 times in each exposure. I don't know a) if I am reading this correctly and b) what level of error is significant. I don't see what looks like a periodic error, but all this is new territory for me. 

Thanks,

Chuck

[Bryan]

Chuck

You can expand the Y-axis to view subtle changes better.  Use the various icons on the right side of the graph to zoom in, out, reset, pan.  Similar option for X-axis at the bottom.

You can do the same on the fly in PHD2 using the dropdowns on the left side of the History tab

Finally, you can fine-tune the RMS analysis by selecting certain portions of the graph in the Log Viewer. Right click for a brief menu.  You'll have to tinker to learn how to use it.  It does come in handy.

Bryan

[Bruce Waddington]

Quickly corrected displacements that small are pretty normal and they wouldn't account for your elongated stars.  I think the evidence is strong that you have differential flexure.

Bruce

[Chuck Schreiner]

Bryan, thanks! I had not right clicked at all, and was only vaguely aware of independently expanding x vs y axes (I just zoomed the whole thing). 

Question: How do you select frames to analyze (when you right click)? I couldn't figure out how to do that, nor understand exactly what I was seeing when I chose the analyze options. 

I really appreciate the input. Getting this sorted out is important to me. I invested a lot in a big scope/beefier mount and now want to get solid data. 

On Sunday, April 21, 2024 at 6:47:04 AM UTC-7 Bryan wrote:

[Chuck Schreiner]

Bruce, I agree. This is flexure, mainly. But while I am in the analysis phase, I am just trying to improve tracking in all ways it could be improved. But mainly, I have a bigger plate for the guide scope arriving Monday - but I doubt this will really improve much. We'll see. Meanwhile I am neck deep in looking at OAG/guide cameras. I assume this will be needed. 

Q re OAG. It seems to me that some ability to focus via the OAG (eg, Helix focuser) would be worth the extra $. I am trying to figure out what fits with this 152mm triplet, 1200mm fl refractor. I have enough astro stuff I am not using (and have been too lazy to sell until now) that I can afford the right solution. 

Any suggestions re OAG/camera? 

Thanks,

Chuck

[bw_m...@earthlink.net]

As with most things in this hobby, I would say you won’t get more than you pay for.  So you probably want a high-sensitivity, monochrome, binned guide camera with a large sensor and an OAG with a prism large enough to fully illuminate the camera sensor.  A helical focuser is a nice feature although focusing is usually a one-time (if tedious) operation.  You will also want to think about the form factor of the guide camera (e.g. ‘cigar’ shape or not) to be sure you can reach focus with the guide camera when the main camera is in-focus.  Just my 2-cents of course.

 

Bruce

 

From: open-phd...@googlegroups.com <open-phd...@googlegroups.com> On Behalf Of Chuck Schreiner
Sent: Sunday, April 21, 2024 11:13 AM
To: Open PHD Guiding <open-phd...@googlegroups.com>
Subject: Re: [open-phd-guiding] Re: Rotating main camera after calibration

 

Bruce, I agree. This is flexure, mainly. But while I am in the analysis phase, I am just trying to improve tracking in all ways it could be improved. But mainly, I have a bigger plate for the guide scope arriving Monday - but I doubt this will really improve much. We'll see. Meanwhile I am neck deep in looking at OAG/guide cameras. I assume this will be needed. 

 

Q re OAG. It seems to me that some ability to focus via the OAG (eg, Helix focuser) would be worth the extra $. I am trying to figure out what fits with this 152mm triplet, 1200mm fl refractor. I have enough astro stuff I am not using (and have been too lazy to sell until now) that I can afford the right solution. 

 

Any suggestions re OAG/camera? 

 

Thanks,

 

Chuck

On Sunday, April 21, 2024 at 9:51:55 AM UTC-7 bw_m...@earthlink.net wrote:

Quickly corrected displacements that small are pretty normal and they wouldn't account for your elongated stars.  I think the evidence is strong that you have differential flexure.

 

Bruce

 

On Sunday, April 21, 2024 at 6:47:04 AM UTC-7 Bryan wrote:

Chuck

 

You can expand the Y-axis to view subtle changes better.  Use the various icons on the right side of the graph to zoom in, out, reset, pan.  Similar option for X-axis at the bottom.

 

You can do the same on the fly in PHD2 using the dropdowns on the left side of the History tab

 

Finally, you can fine-tune the RMS analysis by selecting certain portions of the graph in the Log Viewer. Right click for a brief menu.  You'll have to tinker to learn how to use it.  It does come in handy.

 

Bryan

 

 

On Saturday, April 20, 2024 at 9:01:32 AM UTC-6 schrein...@gmail.com wrote:

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[Brian Valente]

Chuck did you post a few pics of your setups from different angles?  

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[Chuck Schreiner]

Brian, no, I did not. I travel to image and then break it down. I can, of course, reassemble it and take some images if that would provide relevant info. 

[Bryan]

Chuck

I certainly could have given a bit more detail!

With a log file open in Log Viewer, right click on the graph and select Exclude all frames.  The entire graph will become lightly shaded and the RMS stats will go to zero, because there is no included data.

With the SHIFT key held down, simultaneously use the mouse to select a portion of the graph in which you are interested by sliding left to right across that portion with the cursor.

That section will become unshaded and the stats table will indicate the data FOR THAT SECTION ONLY.  You can select multiple non-contiguous sections by repeating the SHIFT-cursor action.  The zoom in and out icons still work on the entire graph.

You can analyze the selected sections beyond the stats table by right clicking in the unshaded section and click 'Analyze selected frames'.  My experience is that you must have selected at least one minute of data to allow this analysis.

Bryan

[Chuck Schreiner]

Bryan, ah! There's the missing piece! Thanks again. Seriously. I really appreciate the help. Cheers, Chuck