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PEC on a CEM60 - Fail, advice needed.

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#1 cmassa

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Posted 05 February 2016 - 12:06 PM

I'm trying to smooth out guiding, I continue to have back and forth variation in guiding of +/- 2 arc-sec. I have tried many different variations in settings of PHD2. So I thought I would try PEC again. I'm using a trial version of PEMPro. Running multiple cycles of the worm I came up with a PE of about +/- 3.5 arc-sec:

 

CEM60 PEb.jpg

 

I recorded the generated PEC curve into the mount and then ran another series of worm cycles:

 

CEM60 PEC playback b.jpg

 

 

What I see is that I still have an oscillation of about +/- 3 arc-sec but it has a much shorter cycle length now, about once a minute. The PEC curve generated from that does not seem to correct for it. I did not add this curve to the previous one.

 

I started guiding and PEC playback in the mount (running the first curve). RA would never return to baseline, in fact, PHD was fighting to get it back down there, but I still have the +/- 2 arc-sec oscillation in guiding. When I turned off PEC playback, PHD succeeded in returning RA back to baseline.

 

Pec1 b.jpg

 

Any ideas on ways to interpret this or correct it? Has anyone else tried to use PEC with a CEM60?

 

Thanks,
Chris



#2 Raginar

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Posted 05 February 2016 - 05:13 PM

Well, either you have quite a bit of backlash (your mount spent the entire time trying to get back on centerline), or you weren't polar aligned and it was fighting the drift the whole time.  

 

It looked like it finally managed to get back to centerline when you quit.  Do you have a picture of your graph before you used PEC?

 

You might post this over at the PemPRO forums and see if Ray will take a look at them.

 

Chris



#3 turnerjs085

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Posted 05 February 2016 - 06:12 PM

What does it look like unguided with no pec? Your pempro capture looks really noisy, I suspect there may be a mechanical issue that needs to be resolved before doing pec... As far as what that might be, it beats me ;) I'd start checking the grease, cables, gear lash, balance etc. Just verify that it works the way it should.

Jeremy

#4 nxda

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Posted 05 February 2016 - 06:52 PM

Chris,

 

Not sure if this will help... take a look.

 

Will

Attached Files



#5 Tim C

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Posted 05 February 2016 - 08:43 PM

I've never had much luck with PEC improving anything on any mount and I've owned a bunch of mounts (I started calling it "fools gold" with me being the fool wasting so much time messing with it).  You shouldn't need it with a mount that has 7 arc sec of PE peak to peak unless imaging at very long focal lengths.  The noise in the curve could very easily be seeing...I saw jumps like that many times with bad seeing only to have it look incredibly smooth with the same mount on a night of good seeing.  How did your dec line look for that run... was it a little bumpy as well?  If so, that is likely just bad seeing.  I think your curve looks plenty good enough.

 

I had some so-so guiding at first as well with CEM60 but once I figured out what the mount likes it's been bullet proof after many hours of imaging.  The key for me was slight imbalance (camera heavy in dec and east heavy in RA... perfectly balanced didn't work so well especially near the zenith.  Definitely don't balance camera heavy near the zenith).  I also drift align in PHD which is usually very quick as I'm almost always within 5 arc minutes using the polar scope.  I shoot for 2 minutes or less to the pole.  After that, try very hard to minimize the number of guide corrections.  I use 3 second guide exposures (experiment with 4 or 5 sec too if needed) and I have my min move set so that it won't send a correction on every exposure.  I also limit my max move so that it doesn't overshoot (very important).

 

For what it's worth, here is what I do:

 

1. Drift align using phd2 and shoot for under 2 arc minutes of error
2. balance camera heavy in dec and east heavy on mount (my guiding was not nearly as good when perfectly balanced)
3. 3-4 sec guide exposures
4. Guide rate .5x
5. min move in both axis is .37 pixels which is .74 arc sec in my set up
6. Max duration is set to 1 arc sec (I recall this making a big difference in poor seeing as higher durations caused more oscillation).  You'll need to do some math here to convert milliseconds of guide pulse to arc seconds taking into account your guide rate.  Going from memory here and hopefully I did it right but at one point in time I worked out what each millisecond of duration equaled in terms of arc seconds of movement at 1X guide rate.  Not sure if I read it somewhere or was smart enough to figure it out myself.  Anyway, I worked out that one millisecond at 1x sidereal should equal .015 arc seconds.  So, I set the max duration to 1 arc second by multiplying 133ms x .015 arc sec x .5x guide rate = 1 arc sec.  I have it in excel so I can play with different parameters.  

7. RA: hysteresis 10 and aggressiveness 100
8. Dec: guide in both directions (auto) using the resist switch algorhythm.  My aggressiveness is 80

 

I hope that helps.  Remember, reduce the number of guide corrections and the minimize the possibility of over-correcting is the strategy (assuming everything else like cables, guide scope flexure, etc. isn't causing a problem).

 

It's easy to feel smart when your mount is working well as mine is but sharing in case this is helpful if you haven't tried this already.  Maybe there is something you haven't already tried.  I hope this helps.

 

p.s. Try backing off the gear engagement 1/8 turn after contact as well if you haven't played with that already. I think my backlash is minimized when doing that vs not backing off.

 

Tim

 

edit: I snapped this pic with my iPhone a few minutes ago.  It's what my guiding looks like on a night of typical not so good seeing.  You can see the frequency of corrections.

Attached Thumbnails

  • IMG_1538.JPG

Edited by Tim C, 05 February 2016 - 09:19 PM.


#6 cmassa

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Posted 06 February 2016 - 08:42 PM

Well, either you have quite a bit of backlash (your mount spent the entire time trying to get back on centerline), or you weren't polar aligned and it was fighting the drift the whole time.  

 

It looked like it finally managed to get back to centerline when you quit.  Do you have a picture of your graph before you used PEC?

 

You might post this over at the PemPRO forums and see if Ray will take a look at them.

 

Chris

 

 

 

My polar alignment was very good that night as the DEC corrections are minor. The deflection away from baseline at the beginning of the PHD window is when PEC playback was turned on. The PHD continued to fight the PEC playback and only made it back to baseline when I turned PEC playback off at the end of the graph. 

 

I have recorded my native PE in PHD2 using the guiding assistant:

 

RA Drift.jpg

 

This was back in November, and again my polar alignment was quite good as DEC remained in the middle. RA continued to trend one direction leading me to believe that my tracking rate was wrong, actually a little fast. What can also be seen in that PHD2 graph is the error which occurs about every minute. When I was doing that graph, I figured out that a setting of .9978 of sidereal improved the RA drift. 

 

Today, I took another look at that PEMPro data which was recorded at normal sidereal tracking rate. I removed the drift correction for RA and it shows this:

 

CEM60 PE expand a.jpg

 

Those 1 minute cyclic variations march forward. The first PE recordings are at the top, the last at the bottom. Maybe this is causing some of my problem?

 

Ideas?

Thanks,

Chris


Edited by cmassa, 06 February 2016 - 08:55 PM.


#7 Raginar

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Posted 06 February 2016 - 08:48 PM

Where is Orly when you need him?😂

#8 cmassa

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Posted 06 February 2016 - 09:02 PM

Tim, 

 

I tried changing some guiding settings last night to go along with your recommendations but just could not get guiding to calm down less than +/- 2 arc-sec. My seeing probably wasn't all that good but what I've found with my mount lately is to guide aggressively and constantly. I have RA aggressiveness at 100, hyst of 10. Max RA and DEC at 2000, guide rates in the mount at .90x. I use a min motion in RA of 0.05. This keeps the PHD2 graph at +/- 1 arc-sec. I usually use a 2-3 sec guide interval. 

 

When my mount was newer, I don't remember having this much variation, especially since I'm imaging/guiding at only 420mm FL. I wonder if there is something screwy with my RA drive. I took the cover off this afternoon and the RA axis components are all tight, no play at all to take out. 

 

I'm going to set my tracking rate to 0.9978 tonight and see if it makes a difference. 

 

All ideas and comments welcomed,

 

Chris



#9 cmassa

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Posted 06 February 2016 - 09:03 PM

Where is Orly when you need him?

 

Sorry, new to these forums. Who's that?

 

Chris



#10 Tim C

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Posted 06 February 2016 - 10:15 PM

Chris,

 

Those pempro curves look good to me with low overall PE and pretty smooth.  Just eyeballing it, looks like your RA PE equates to about a max of around .15 arc seconds of error for every 1 second of time or ~.5 arc seconds for a 3 second guide exposure.  That should be easy to guide out so I'm puzzled.  What does the min motion of .05 equate to in arc seconds in your setup?  That seems very low but not sure what your guiding image scale is.  

 

Do you have a screen shot showing the + / - 2 arc sec RMS when guiding and is that just in RA?  Each mount's sweet spot is likely a little different but mine definitely did not like really aggressive guiding... that cause a lot of unnecessary guide corrections and oscillations.  

 

 

Tim, 

 

I tried changing some guiding settings last night to go along with your recommendations but just could not get guiding to calm down less than +/- 2 arc-sec. My seeing probably wasn't all that good but what I've found with my mount lately is to guide aggressively and constantly. I have RA aggressiveness at 100, hyst of 10. Max RA and DEC at 2000, guide rates in the mount at .90x. I use a min motion in RA of 0.05. This keeps the PHD2 graph at +/- 1 arc-sec. I usually use a 2-3 sec guide interval. 

 

When my mount was newer, I don't remember having this much variation, especially since I'm imaging/guiding at only 420mm FL. I wonder if there is something screwy with my RA drive. I took the cover off this afternoon and the RA axis components are all tight, no play at all to take out. 

 

I'm going to set my tracking rate to 0.9978 tonight and see if it makes a difference. 

 

All ideas and comments welcomed,

 

Chris

 

 

 



#11 cmassa

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Posted 06 February 2016 - 11:16 PM

At my image scale with the guide camera (3.68 arc-sec/pixel), 0.05 pixel equates to .18 arc-sec.

 

Chris



#12 orlyandico

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Posted 07 February 2016 - 12:13 AM

I was gonna chime in earlier but got sidetracked.

 

The "marching 1 minute cycle" sounds like a non-integer term, probably from the gearbox.  If indeed present it's probably impossible to get rid of.

 

I noticed in the original post that the CEM60 owner used a rather fancy PEC curve. In my experience, any correction more fancy than the fundamental (uncheck all terms above the 1X term) doesn't buy anything. So I would re-record the correction but only using the fundamental (which is a pure sine wave).



#13 Tim C

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Posted 07 February 2016 - 08:02 AM

Hey Chris,

 

.18 arc sec just seems too low to me for min move. That would seem to assure that a guide command is sent with every exposure since you'll be chasing the seeing even when seeing is fairly good causing oscillation.  You have some 45 sec to 1 minute stretches in your worm period where the PE is pretty flat... there should be very few corrections being sent during those periods on your worm but the .18 will be causing corrections due to the seeing.  Your mount may require different settings than mine but I still think your strategy should be less aggressive to reduce the number of corrections.

 

Check out this excellent paper at the link below.  It really helped me understand what my approach should be and come up with a guiding strategy that worked for me.  Here are few quotes from it:

 

"This is the first principle: strive to make the setup so that it needs the minimum
number of guide corrections possible."

 

"...do everything that can be done to not chase the seeing."

 

 

 

http://acp.dc3.com/M...ding11-2005.pdf

 

 

At my image scale with the guide camera (3.68 arc-sec/pixel), 0.05 pixel equates to .18 arc-sec.

 

Chris


Edited by Tim C, 07 February 2016 - 08:03 AM.


#14 cmassa

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Posted 07 February 2016 - 10:02 AM

Thanks Tim,

 

I have tried to follow that but for some reason it's just not working for me. Here's some guiding last night with MinMo set at 0.2:

 

PHD min mo 2.jpg

 

And here it is with MinMo set to 0.05:

 

PHD min mo 05.jpg

 

What I have been seeing is that as I try to increase MinMo or guiding interval to lessen "chasing the seeing", the line for RA just gets worse but a guide command is still sent almost every time.  I have also tried a very calm approach to the guiding of this mount by putting the aggressiveness much lower but it gets worse as well. 

 

I know from looking at the RA line that it is guiding back and forth and overshooting a lot but I haven't been able to stop it. My RA RMS error is lower when guiding aggressively. 

 

My polar alignment appears a little off last night since DEC is always guiding in one direction. When I finished my drift alignment, PHD2 reported it at about 1 arc-min but when I was running the guiding assistant it said it was about 2 arc-min. 

 

Just some info if you need: scope is an AT65EDQ (420mm FL) piggybacked on an 8" Edge SCT and I'm using a QHY5L-II mono guide camera through an Orion thin OAG. I'm using the ASCOM chooser in PHD2 so I can set the binning to 2 in the camera driver. Then in the brain I change the camera pixel size to 7.5 from 3.75 since it doesn't seem to know I'm using the binning setting. 

 

Thanks for all the advice, I think we've moved from helping me use PEC to getting my guiding better and that's a good thing. If I don't need to use PEC then I really don't want to. 

 

Chris


Edited by cmassa, 07 February 2016 - 10:06 AM.


#15 Raginar

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Posted 07 February 2016 - 10:36 AM

Looks much better Chris.  Maybe PEC would help with Orly's advice?

 

Chris



#16 Tim C

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Posted 07 February 2016 - 11:49 AM

Yeah that .74 rms should be plenty good enough.  I seem to be around .55 to .85 most of the time depending on the seeing with ~.65 being the most common result.  Just goes to show that there is more than one way to skin a cat as they say!  Glad you're getting better results.




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