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PHD guiding assistant says "HPF-RMS = 0.5 arc-sec" but actually guiding is 2.89 arc-sec (and some other guiding questions)

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#1 shitijb

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Posted 29 October 2020 - 12:42 AM

Need help in improving my guiding and making sense of some results, but first things first, my equipment:

  1. iOptron Cem70
  2. Orion 8" f/3.9 astrograph (focal length: 800mm)
  3. ZWO OAG with SSAG pro mono (so guiding image scale: 0.97"/px)
  4. PHD2 & APT

I see a few issues in the phd guide log that dont make sense or need help in improving:

  1. Guiding assistant rms = 0.5, but actual rms immediately after/before is much much worse at 2.89" (attached guide log)
    Tried guiding assistant twice, at the beginning and at the end and got similar result. Am I misinterpreting something?
  2. Backlash is enormous at >4000ms both times with guiding assistant
    I did not do any "east/west heavy" unbalancing, but balanced as best as possible, maybe this is a consequence of that? This is the screenshot I took after guiding assistant
    backlash
  3. Dither settle kept failing to settle
    My settings in APT are dithering stability: 1.4, Dithering settle time: 20, Dithering timeout: 60, Dithering start delay: 0. I guess my "stability" was too low for my terrible guiding rms? Is there a rule of thumb for this, like should this have been 3.0 since my rms was 2.89"?
  4. Any other patterns you see in the guide log that might suggest some improvement?
    I see some elongation in the scatter plot so I guess it could result in stars being more oval

Attached Files



#2 imtl

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Posted 29 October 2020 - 12:55 AM

With the Ioptron CEM mounts you need to make sure you have engaged the magnetic clutches properly. They have very little to non existing backlash since its a belt drive. Unless something's off. Make you the Dec clutch is engage properly.

 

Your calibration was not great you have an orthogonality error of 10deg.

 

A few more things I see. Your aggressiveness is way too high in my view and exposure time is too short at 1.5s. Most chances you're just chasing seeing like that.

 

Turn off dithering for now and fix your basic problems. Then put dithering back on.


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#3 michael8554

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Posted 29 October 2020 - 10:08 AM

The Guide Assistant measures RA with guiding OFF, so any RMS values shown are not a sign of Guiding quality. 

 

The GA graph shows the RA PE, and the Dec drift, a measure of PA error.

 

I can't tell if you accepted the GA RA MinMo suggestion, the setting used for the guiding was about 0.20 arcsecs, which I think is too low.

 

Also RA guiding was yoyoing about +/- 1.5 arcsecs around the zero axis, suggesting Aggr is too high, and exposure too low,  as Eyal says.

 

Your PA at 1.7 arcmins was good, so Dec should be plodding along with very few corrections, but Dec was also wobbling like RA.

 

Also Dec MinMo seemed too low as well.

 

Your good balance, together with the out-of-spec belt-drive backlash, means that Dec was free to wobble from one end to the other of the backlash.

 

So sort your belt tensions and worm mesh, or balance Dec heavy in one direction so that Dec backlash is always taken up.



#4 shitijb

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Posted 29 October 2020 - 11:54 AM

With the Ioptron CEM mounts you need to make sure you have engaged the magnetic clutches properly. They have very little to non existing backlash since its a belt drive. Unless something's off. Make you the Dec clutch is engage properly.

 

Your calibration was not great you have an orthogonality error of 10deg.

 

A few more things I see. Your aggressiveness is way too high in my view and exposure time is too short at 1.5s. Most chances you're just chasing seeing like that.

 

Turn off dithering for now and fix your basic problems. Then put dithering back on.

Thank you for the response, I am sure I had both clutches tightened properly, but will double check next time. I had exposure at 1.5s at the beginning and increased it to 2s and I think it improved things a little bit, will also try 3s next time. Will do a guiding only test to sort these issues, and play with aggressiveness as well like you suggest.

 

How do I fix the orthogonality error?

 

The Guide Assistant measures RA with guiding OFF, so any RMS values shown are not a sign of Guiding quality. 

 

The GA graph shows the RA PE, and the Dec drift, a measure of PA error.

 

I can't tell if you accepted the GA RA MinMo suggestion, the setting used for the guiding was about 0.20 arcsecs, which I think is too low.

 

Also RA guiding was yoyoing about +/- 1.5 arcsecs around the zero axis, suggesting Aggr is too high, and exposure too low,  as Eyal says.

 

Your PA at 1.7 arcmins was good, so Dec should be plodding along with very few corrections, but Dec was also wobbling like RA.

 

Also Dec MinMo seemed too low as well.

 

Your good balance, together with the out-of-spec belt-drive backlash, means that Dec was free to wobble from one end to the other of the backlash.

 

So sort your belt tensions and worm mesh, or balance Dec heavy in one direction so that Dec backlash is always taken up.

But doesnt that mean that without guiding I got 0.5 RMS and with guiding 2.89, so guiding made things worse (so indeed chasing seeing like you mention)? :O I'll try with higher exposure times, higher dec minmo, lower RA aggression & dec imbalance next time.

 

One thing I forgot to point out was that the temperature when I was imaging was -7 degree C (19 degree F), is it possible that things in the mount contracting lead to more play and hence more backlash? I dont have a reference to compare with (this was my first time with OAG without constant winds) but I can try on a warmer night to see the effect as well.
 



#5 rgsalinger

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Posted 29 October 2020 - 02:02 PM

First, just to be clear, the CEM70 does NOT have magnetic clutches. It has switches the are either on or off, just like my CEM120 or any Paramount that has ever been made. 

 

Second, the guiding cadence seems awfully fast for my taste. I'd try guiding at more like 3 or 4 seconds with this mount. 

 

Third, DEC backlash is almost impossible with this mount - it's belt driver so if the mesh is even roughly right you won't have any. Something else is going on. My guess is that there's a dragging cable of some kind. Can the OP post a picture of the system? 

 

Fourth, the minimum moves are way to low. These should be set roughly twice as high. These mounts do not react well to constant tiny corrections. 

 

Fifth, it might help to see if you can get better focus on the guide camera. Not sure that this can be done but it seems a bit high to me. 

 

So, if this were my system I'd be guiding at 3 seconds with min move set to .6 arc seconds and see how that works. 

 

I'm also wondering just how good the seeing was, how well balanced the system was. Newts are not trivial to balance properly.

 

Rather than make any mechanical changes, if you really think that this is a mesh issue, return the mount and get a new one would be my advice. 

Once you open it up, then you kind of own any problems. It's fun to do that if you are confident of your mechanical ability, otherwise don't do it. 

 

Rgrds-Ross



#6 imtl

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Posted 29 October 2020 - 02:24 PM

For some reason I read CEM60. I need to get my head checked. My mistake about the magnetic clutches! Sorry.



#7 shitijb

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Posted 29 October 2020 - 03:29 PM

First, just to be clear, the CEM70 does NOT have magnetic clutches. It has switches the are either on or off, just like my CEM120 or any Paramount that has ever been made. 

 

Second, the guiding cadence seems awfully fast for my taste. I'd try guiding at more like 3 or 4 seconds with this mount. 

 

Third, DEC backlash is almost impossible with this mount - it's belt driver so if the mesh is even roughly right you won't have any. Something else is going on. My guess is that there's a dragging cable of some kind. Can the OP post a picture of the system? 

 

Fourth, the minimum moves are way to low. These should be set roughly twice as high. These mounts do not react well to constant tiny corrections. 

 

Fifth, it might help to see if you can get better focus on the guide camera. Not sure that this can be done but it seems a bit high to me. 

 

So, if this were my system I'd be guiding at 3 seconds with min move set to .6 arc seconds and see how that works. 

 

I'm also wondering just how good the seeing was, how well balanced the system was. Newts are not trivial to balance properly.

 

Rather than make any mechanical changes, if you really think that this is a mesh issue, return the mount and get a new one would be my advice. 

Once you open it up, then you kind of own any problems. It's fun to do that if you are confident of your mechanical ability, otherwise don't do it. 

 

Rgrds-Ross

Here are a few pictures from the end of the imaging session (from just after the second guiding assistant run). I am not using through the mount cabling yet (I assumed it was not that important since Im using long cables, but maybe not)

 

PXL 20201025 140317113

 

The magnets & washers(?) + guidescope on the top is purely for 3rd axis balancing to offset the asi 183mm + EFW + OAG + guidecam weight on the focuser. With these it seems to be balanced nicely in all 3 axis. In zero position the camera + focuser points down as people recommend (actually offset an inch from exactly down, again for 3rd axis balance had to twist the tube inside the rings a little bit). I had spent a lot time on balancing at home and marked positions with tape.

 

PXL 20201025 140334969

 

Im not sure how to measure seeing in arc-sec correctly (always wondered this) but this was in central/eastern washington so it is unlikely to be better than 1" (from what I've read, since the jet stream is generally above). There were <10mph winds barring rare gusts and I dont remember any in the morning (so at least the second GA result should not be affected by winds)

 

On guide cam focus I tried my best to get it focused with main camera during daytime on a house roof, and this is where I stopped and tightened down everything:

 

111

 

Not perfect but I couldn't make it better. I did notice that since the roof is not vertical, the depth of field is still too shallow and some parts of the roof were in focus while not others even for main camera, so maybe main & guidecam were focused on different planes. But I tried trees and had the same problems (but worse since they were moving), and I couldnt find anything else to focus on. I couldnt bring the main camera to focus inside on anything in my house (maybe I need to use mirrors to increase the distance indoors!). Also I did NOT refocus as temperature dipped (a lot) and can see the stars are blurrier towards the end (because it was too cold and I had given up on imaging by that point looking at my RMS and mostly wanted to collect data to improve guiding)

 

For some reason I read CEM60. I need to get my head checked. My mistake about the magnetic clutches! Sorry.

CEM70 is a new mount so that confusion is expected, no reason to be sorry. I appreciate all your help! :)



#8 rgsalinger

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Posted 29 October 2020 - 04:30 PM

I see a bunch of long dangling cables. Why are you not using the through the mount usb and power ports? In cold weather the stiffness of those cables can cause all sorts of issues. I've been using the USB 2 ports on my CEM120 with my new USB 3 cameras for over a year now. That let's me have very short cables with no possibility of adding drag to the system. The difference in download speeds is trivial. 

 

If you really cannot just use the ports then at least get a USB3 hub and put it on the OTA or the saddle and eliminate the cabling that you've got. Now I can't prove that's one of the problems but I can say that it very well could be. 

 

Looks COLD.

 

Rgrds-Ross



#9 ryanha

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Posted 29 October 2020 - 07:01 PM

FWIW, I had 4000ms backlash on my CEM-40 (and saw a bunch of other threads like that).  I tried the gear mesh and other things.  Finally tensioning the DEC belt made the backlash go away (went from 4000+ms to 350ms).

 

http://www.ioptron.u...Tension_Adj.pdf

 

 

Thanks,

--Ryan


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#10 michael8554

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Posted 30 October 2020 - 12:47 PM

Focusing the guidescope on a distant object will only get you close to infinity focus.

 

Autoselect a star and get the HFD reading on the PHD2 Star Profile window as low as you can.



#11 shitijb

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Posted 30 October 2020 - 01:38 PM

I see a bunch of long dangling cables. Why are you not using the through the mount usb and power ports? In cold weather the stiffness of those cables can cause all sorts of issues. I've been using the USB 2 ports on my CEM120 with my new USB 3 cameras for over a year now. That let's me have very short cables with no possibility of adding drag to the system. The difference in download speeds is trivial. 

 

If you really cannot just use the ports then at least get a USB3 hub and put it on the OTA or the saddle and eliminate the cabling that you've got. Now I can't prove that's one of the problems but I can say that it very well could be. 

 

Looks COLD.

 

Rgrds-Ross

I see, didnt realize that long dangling cables would mean cable weight would be a factor as well...I will get my cable management in order and use the usb hub etc. on the mount. Yes, it was very cold haha, 19 F. Good experience though, I will be better prepared next time to deal with it.

 

FWIW, I had 4000ms backlash on my CEM-40 (and saw a bunch of other threads like that).  I tried the gear mesh and other things.  Finally tensioning the DEC belt made the backlash go away (went from 4000+ms to 350ms).

 

http://www.ioptron.u...Tension_Adj.pdf

 

 

Thanks,

--Ryan

Oh cool, I will check the tension on my belt. I was wondering that maybe low temperature caused more slack somehow? That would be a problem since I image in any of the dark sites in 4 directions wherever it is clear (and temps can be rather different in all of those places at this time, from 5 degree C to -10 degree C). Not sure if I can adjust this every time in the field.

 

Focusing the guidescope on a distant object will only get you close to infinity focus.

 

Autoselect a star and get the HFD reading on the PHD2 Star Profile window as low as you can.

Hmm, I will pay attention to and record HFD next time. I dont use a guidescope, but OAG. So I think infinity focus is not important during daytime calibration, just that main and guide cam are in focus at the same time. I then use bahtinov in the field to bring main cam into focus (and assume guidecam is still in focus). Im hoping there is no flaw in this process?

 

Thank you for all your responses and help, I plan to try your suggestions tonight or tomorrow night if weather cooperates.
 


Edited by shitijb, 30 October 2020 - 01:43 PM.



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