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Help me fix my guiding...

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#1 MDWingsFan

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Posted 16 July 2019 - 03:45 PM

Attached is a link to the logs from this past Thursday and Saturday.  Polar alignment was as good as I could get it, and according to SharpCap was Good/Excellent.  No appreciable wind, etc.  

From the last thread way back when, I got the directions for mount adjustment from iOptron and performed an alignment based on their procedures.  Dec appears to be pretty good, which I would think speaks to a decent polar alignment, but RA is garbage!    The weather has improved and will likely be decent enough to troubleshoot this issue in the next few days.  Other than the suboptimal camera placement, I tried for 6 o'clock but couldn't get the camera to clear the mount head and balance.  My thinking is that it has to be balance, cable drag, and settings related...  Thanks in advance smile.gif

 

Edit Top pic is as guided last week.  Lower pic is as configured now.  I realigned the autofocus closer to the center line to minimize its effect on balance.

 

Not seen is the QHY mini guide scope on the opposite side.

 

https://drive.google...wfh?usp=sharing

Attached Thumbnails

  • IMG_1173.jpg
  • IMG_1175.jpg

Edited by MDWingsFan, 16 July 2019 - 03:47 PM.


#2 mewmartigan

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Posted 16 July 2019 - 04:43 PM

How much weight do you have on there? Also, it looks like you have your tripod pretty high. On my CEM25P I don't have the tripod legs extended at all as they can make things a bit wobbly.

 

What is the focal length you are imaging at? When I used a mini guidescope I was imaging at 360mm. Anything much higher than that and you may need to step up your guidescope.


Edited by mewmartigan, 16 July 2019 - 04:45 PM.


#3 MDWingsFan

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Posted 16 July 2019 - 04:56 PM

Total weight is < 16lb, or about 63% of the rated weight, last I weighed it out. The focal length of the scope is 600mm. I’ll try shortening the legs next session, but I’m my office I put it all the way up to keep it clear of snags and rug rats.

#4 mewmartigan

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Posted 16 July 2019 - 05:07 PM

At 600mm I would definitely suggest stepping up from the QHY miniguidescope. Maybe a 50mm guidescope at 200mm focal length.

 

But I am sure those who know a lot more than me about pixel scale ratios will chime in.



#5 MDWingsFan

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Posted 16 July 2019 - 05:09 PM

Been thinking about moving to an OAG to reduce the weight and solve scale issues...

#6 Stelios

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Posted 16 July 2019 - 07:57 PM

The guide logs would be MUCH more helpful than the debug logs

 

Out of your setup, one things that jumps at me is the counterweights at the low position. You should have the majority of the CW's flush against the top, with one small CW for fine balancing. OTOH, the magnitude of this effect is unknown. 

 

Your configuration will have moment arm problems. The moment arm of the scope + camera is quite high, and I would recommend a bigger mount. It's not simply a weight problem--a short, equally heavy refractor with camera in back would guide *much* better.

 

Still, send the guide logs and we can see if there's a setting that you can improve before hardware changes. Did you run guide assistant and follow recommendations? 



#7 MDWingsFan

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Posted 16 July 2019 - 08:23 PM

Updated the share with the logs.  I thought I grabbed the guide logs, but lo and behold I did grab the debug ones... Rectified!

 

I did run the assistant, an recalibrated before the Thursday session.  For the moment a bigger mount is out of the budget...  The ASI 294 ate that budget for the year ;)



#8 StephenW

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Posted 16 July 2019 - 08:28 PM

Looks like the guide logs are now available at the original link.

 

Looking at the log (and eliminating the data after your guide star was lost), your DEC RMS is really good at 0.56" but your RA is 2x worse at 1.19".

 

Unfortunately your guide log does not include a calibration or GA run - it would be really good to upload a log with both of those, plus another long guiding session.

 

PHD2's freq analysis indicates you have 0.6" PE at 200s and 0.4" at 300s - nothing huge - and not sure if either of those matches the worm period of your mount.  You are using the ppec algorithm, so that could account for the relatively low PE

 

I don't see any sudden or unexpected movements in RA, so I don't think this is cable drag.  Not sure if you are already running East heavy, but that might be worth a try.



#9 Stelios

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Posted 16 July 2019 - 08:31 PM

To understand the moment arm issues, please follow this link and scroll down to the discussion of Karen holding her bowling ball, er, moment arm considerations :) :)  This is something that is extremely important but little appreciated.

 

I'll take a look at your guide logs soon.



#10 Stelios

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Posted 16 July 2019 - 08:50 PM

OK, I removed all the star lost and dither settings, and I found the same things Stephen did-- 0.56" DEC, 1.19" RA, 1.31" total. Not great, but hardly terrible given the setup. 

 

What I would suggest: Try reducing the exposure time from 3" to 2" or even 1" (testing, of course). Your mount may need correction more often. Try NOT using PPEC as your RA algorithm, hysteresis might work better if your errors are caused more because of mount shakes than PE. I am not saying these will fix anything, but I would try them.

 

Finally, look at your stars. Are they round? If so, this can wait till you can afford a bigger mount. 

Note that cable drag might cause differential flexure (good guiding but bad stars) but won't cause bad *guiding*. 



#11 MDWingsFan

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Posted 16 July 2019 - 08:54 PM

No idea why the calibration/ga runs aren't there, it was done on 7/9...  Here's a shot of the calibration review data.

Attached Thumbnails

  • cal.png


#12 Stelios

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Posted 16 July 2019 - 09:05 PM

Hmmm--I know nothing about CEM25, is 0.75x the correct guide rate for that mount? A wrong guide rate will cause extra fluctuations.



#13 MDWingsFan

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Posted 16 July 2019 - 09:13 PM

0.5x is the default.  I made the change to 0.75 after a recommendation in Mar/Apr...   The weather stunk for much of the spring and early summer, so now that I've got decent weather again I'm hoping to try to dial things in while the weather appears to have provided me opportunities to trial and error w/o 3 month gaps...  Ah DC weather the last two years has been reminiscent of London or Seattle.



#14 MDWingsFan

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Posted 16 July 2019 - 09:23 PM

Here's the astrobin links for this weekend's images each is roughly 180 60s subs with darks, flats, dark flats:

 

get.jpg?insecure

 

get.jpg?insecure



#15 StephenW

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Posted 16 July 2019 - 09:48 PM

The result look very nice! :)

 

What is your measured eccentricity?  They don't look too far out of round to me.



#16 OldManSky

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Posted 17 July 2019 - 08:19 AM

I'm running essentially the same setup (pic below).

Stelios' point about the counterweight position (and moment arm) are important ones, especially on the CEM25P.  When I first saw your picture, I was having a hard time believing you could achieve RA balance with your weights and their positions -- without a motorized focuser, I needed more weight than you have to balance!  I used barbell weights on my first few runs (the original CEM25P counterweight plus 15 pounds of barbells), but I've since added another original counterweight and a 4.4-lb. smaller one.  That lets me get up higher on the counterweight bar, and that smaller moment arm does indeed make a difference in RA tracking.

 

It looks like your results are pretty good, but of course we always want to improve...good luck.

 

Oh, and if I may ask:  how long is your bottom dovetail?  I think I like the idea of having the dovetail long enough so the front part of the rings go *in front* of the secondary screws...

Attached Thumbnails

  • 6inch_ready.jpg

Edited by OldManSky, 17 July 2019 - 08:24 AM.


#17 MDWingsFan

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Posted 17 July 2019 - 08:53 AM

 

Oh, and if I may ask:  how long is your bottom dovetail?  I think I like the idea of having the dovetail long enough so the front part of the rings go *in front* of the secondary screws...

The dovetail is 13", I believe it was am Amazon purchase. I drilled out the last hole in the back to get clearance for the ring screw. The supplied dovetail was way too short to balance in dec...



#18 OldManSky

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Posted 17 July 2019 - 09:06 AM

The dovetail is 13", I believe it was am Amazon purchase. I drilled out the last hole in the back to get clearance for the ring screw. The supplied dovetail was way too short to balance in dec...

Yeah, I know :(  Thanks for the info.



#19 MDWingsFan

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Posted 17 July 2019 - 08:19 PM

Purchased a 2kg counterweight to go with the 2 4.5kg weights already on the mount.  That should bring the heavy ones up the shaft a bit. 

 

Also moved the guide rate back to the default 0.5 to get me back to original settings there.

 

Will try in order:

 

-moving back to hysteresis from ppec and re-running calibration then guide assistant.

-dropping exposure times from 3 to 2 seconds.

 

-Hopefully some combo of this will get my RA guide error closer to my dec error.  

 

-Longer term debate, moving to OAG, as well as moving from a newt to a 102mm refractor to make the balance easier.  Part of the reason, besides budget,  for CEM25P is a couple of herniated lumbar disks.  Lower weight = less chance of irritating them.


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#20 mewmartigan

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Posted 17 July 2019 - 09:06 PM

Hope it works out for you.

I have a William Optics z103 refractor on my cem25p and love it. Total weight with guidescope and camera is 13.2lbs.

#21 MDWingsFan

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Posted 29 July 2019 - 01:23 PM

Finally had a night where the weather cooperated after the new weight arrived.  I "think" the guiding was much better overall, the next post will have the zipped log from Friday night.  Here's a processing of the target for that night...  IC1396

 

get.jpg?insecure

Attached Thumbnails

  • IMG_1205.jpg


#22 MDWingsFan

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Posted 29 July 2019 - 01:25 PM

And the guide log...

 

Switched from PPEC to Hysteresis and let PHD2 guiding assistant suggest timings.  Also decreased the sampling period to ~2/2.5s.  

Attached Files



#23 mewmartigan

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Posted 30 July 2019 - 08:18 AM

I had some guiding errors recently also with my CEM25p. I made a few changes as suggested on various topics here and things actually got worse. I probably made too many changes at once. I also used outrigger washers to get the balance darn near perfect. The near perfect balance seemed to accentuate the backlash the system had.

 

So, I ended up making the mount slightly east heavy after achieving good balance. I also reset PHD2 to all the default settings. I have had it out two nights now and my RMS has been around 0.8" pretty consistently whereas before it was anywhere from 0.8"-1.5" and would jump. My imaging scale is 1.63"

 

The first night after resetting PHD2, the only change I made was to bump the DEC aggressiveness down from 100 to 70. Otherwise I let it calibrate and go with 2.5s exposures. I didn't calibrate the second night.

 

You have heavier gear with a more off-axis weight but I guess my suggestion would be to make it slightly east heavy and reset PHD2 to start from scratch.

 

The guiding in that first image looks pretty good.....how long were the subs?


Edited by mewmartigan, 30 July 2019 - 08:18 AM.



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