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CEM40G Frustration

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#1 ispybadguys

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Posted 27 February 2021 - 12:01 AM

I have had my first ever Mount about 4 weeks and now that I am learning a few things I feel like sending it back. I tried guiding over the last 3 days. Using Astroberry, the internal guiding algorithm complains, so I switched to PHD2. It had a complaint about the RA and DEC differing bo 144% but I got it to guide and stop complaining but it had over 9 arcsec p-p oscillation in RA. DEC was well behaved.  Also the HFD is over 3 pixels. Is that typical? My camera has 1.5 arcsec pixels so 9 is not encouraging. The guide focus adjustment mechanism is crap. You loosen a thumb screw and then try to position it to a couple of mils with your finger by sliding it. Is there some trick here? Couldn’t they afford a screw to adjust it?

 

I asked tech support about the guiding and they asked if I was using a PC and ASCOM so I dug out a PC to see if it was any different. PHD2 goes thru a calibration and complains about poor quality and asks if I just did a meridian flip and forgot to set the reversed DEC. It then starts tracking and drives the star out of the field. Repeat he the whole sequence with the same results.

 

Has anyone had problems like this? Oh I also bought iStarFi. What junk. If I move 15 feet away there isn’t enough signal to work and standing next to it I still had strange problems. I was going to buy a AZ EQ mount the EQ5 or 6 but some test data showed that the CEM40 had better open loop PEC errors. I have see star trails with 70mm focalength and 60 second exposures. I got to say the iPolar is really neat.

 

Thanks

 

Kurt

 

 



#2 Cfreerksen

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Posted 27 February 2021 - 12:12 AM

So sorry you are having problems. I have just the CEM40 plain w iPolar. It has been an excellent mount for the money. Good guiding and is very portable.

 

Your is the first I have heard of the CEM40G. I can't for the life of me understand why Ioptron made the CEM70G. I am sure it would be fine to guide with a 120mm fl scope on small refractors and the like. But with all that capacity you would think that there would be way more long fl scopes riding on that type of mount. Since you probably wouldn't get good enough guiding from the short fl guider for anything over 500mm fl scope it seems like a waste/gimmick.

 

Chris


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#3 bobzeq25

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Posted 27 February 2021 - 01:05 AM

HFD of 3 pixels is common, there's nothing wrong with it.  The algorithms used to determine the guide star centroid are very powerful.

 

9 arc sec guiding is bizarre.  Something is _really_ wrong.  That's an unguided number.


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#4 petercoxphoto

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Posted 27 February 2021 - 05:27 AM

Hi Kurt -

The CEM40 is a good mount. There are many things that could account for your problems and you need to make sure you're setting everything up correctly. Is your scope balanced properly? Are you certain you're polar aligned correctly? Do you have cables dangling all over the place that are pulling tight or catching on things? Is your mount level? Are you calibrating near the celestial equator (i.e. near declination 0)?

 

As far as focusing the guidescope goes, that's pretty standard. You don't need pin-point focus for that, getting close is good enough (i.e. you can see well-defined stars). It doesn't take long, then lock it in place and you'll rarely if ever have to refocus it.

 

If you remove the guide camera or change its orientation, you'll need to do a new calibration every time, so bear that in mind as well.

 

Did you enter correct details in PHD's new equipment wizard for guide camera pixel size and focal length? Those need to be 100% correct or you'll have problems. Bottom line, if PHD is consistently complaining that the calibration is poor, there's something not right with the setup. 

 

Cheers,
Peter



#5 michael8554

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Posted 27 February 2021 - 05:49 AM

No experience of Astroberry, so I'll stick to PHD2 comments:

 

Whichever way you have the Meridian Flip set, when Calibrating, PHD2 will move the mount what it thinks is West/East, then North/South, and will work out how the mount is set.

 

Calibration is key, errors here must be corrected before guiding.

 

144% difference is most likely due to large Dec Backlash, you should nudge the mount North until you see the guidestar move before Calibrating.

 

Post your PHD2GuideLog for more info.

 

Read these Good Practices from the PHD2 guys:

 

https://openphdguidi...best-practices/

 

How good is your Polar Alignment ? Needs to be 5arcmins or better.

 

Use the HFD reading in the PHD2 Star Profile window to fine-tune focus, HFD = 3 to 4 is good., HFD = 9 isn't.

 

15 feet range for a WiFi dongle doesn't sound bad, they're not for connecting from inside your house, from an adjacent warmroom would be the max.


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#6 ispybadguys

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Posted 27 February 2021 - 07:10 AM

Thanks to all of you for taking some time to help.
 
Prior to trying to guide I used iPolar to realign the mount so it at least showed it was right on. I have a Sony A7R4 with 200-600 mm zoom on the mount and it is pretty well balanced and only one USB cable coming off the mount which wasn't snagging. The mount bubble is in the center so it look pretty level.
 
I spend a hour trying to get the focus as good as possible. There seems to be some lag between the adjustment and PHD2 reporting with is why it took an hour and I wasn't very happy with the results.
 
I found the log file and I could attache it if you like but it is 8MB. I grep'ed for Alert and got the following:
 
20:43:30.415 00.003 5672 Alert: Advisory: Calibration completed but RA/Dec axis angles are questionable and guiding may be impaired
20:43:31.182 00.002 5672 Alert: Do you want to automatically restore this calibration whenever the profile is used?
20:44:11.554 00.003 5672 Alert: PHD2 is not able to make sufficient corrections in RA.  Check for cable snags, try re-doing your calibration, and check for problems with the mount mechanics.
20:44:43.972 00.003 5672 Alert: PHD2 is not able to make sufficient corrections in RA.  Check for cable snags, try re-doing your calibration, and check for problems with the mount mechanics.
20:45:13.171 00.001 5672 Alert: PHD2 is not able to make sufficient corrections in Dec.  If you have just done a meridian flip, check to see if the 'Reverse Dec output option' on the Advanced Dialog guiding tab is wrong.  Otherwise, check for cable snags, try re-doing your calibration, and check for problems with the mount mechanics.
20:45:46.202 00.001 5672 Alert: PHD2 is not able to make sufficient corrections in Dec.  If you have just done a meridian flip, check to see if the 'Reverse Dec output option' on the Advanced Dialog guiding tab is wrong.  Otherwise, check for cable snags, try re-doing your calibration, and check for problems with the mount mechanics.
20:46:16.955 00.002 5672 Alert: PHD2 is not able to make sufficient corrections in RA.  Check for cable snags, try re-doing your calibration, and check for problems with the mount mechanics.
20:50:23.982 00.003 5672 Alert: Advisory: Calibration completed but RA/Dec axis angles are questionable and guiding may be impaired
20:51:07.829 00.000 5672 Alert: PHD2 is not able to make sufficient corrections in Dec.  If you have just done a meridian flip, check to see if the 'Reverse Dec output option' on the Advanced Dialog guiding tab is wrong.  Otherwise, check for cable snags, try re-doing your calibration, and check for problems with the mount mechanics.
20:53:01.727 00.003 5672 Alert: Advisory: Calibration completed but RA/Dec axis angles are questionable and guiding may be impaired
20:53:40.944 00.001 5672 Alert: PHD2 is not able to make sufficient corrections in Dec.  If you have just done a meridian flip, check to see if the 'Reverse Dec output option' on the Advanced Dialog guiding tab is wrong.  Otherwise, check for cable snags, try re-doing your calibration, and check for problems with the mount mechanics.
20:54:51.669 00.001 5672 Alert: PHD2 is not able to make sufficient corrections in Dec.  If you have just done a meridian flip, check to see if the 'Reverse Dec output option' on the Advanced Dialog guiding tab is wrong.  Otherwise, check for cable snags, try re-doing your calibration, and check for problems with the mount mechanics.
20:55:21.870 00.002 5672 Alert: PHD2 is not able to make sufficient corrections in RA.  Check for cable snags, try re-doing your calibration, and check for problems with the mount mechanics.
 
I am wondering if there is some problem with the mount.
 
Astroberry is a Raspberry PI implementation of Kstars with Ekos so very similar to StellarMate. I set it up to use PHD2 to guide some the drivers to the mount are INDI drivers instead of ASCOM. It seems to work well but it is a lot slower to read and update the camera that running on the PC with ASCOM. Here are my only guiding results from that. The ASCOM would not guide it seemed to just run off the camera. I think the mount was being driven in the wrong direction. The new focus I spend an hour doing is worse that the focus I hand before readjusted it.

 

 Ok how do I attach an screen shot?



#7 starbuckin

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Posted 27 February 2021 - 08:17 AM

Where did you perform your PHD2 calibration? The scope should be pointed generally south and near the horizon.


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#8 ispybadguys

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Posted 27 February 2021 - 08:30 AM

Tha calibration was performed just before guiding was started. I pointed to Betelguese for focus. Because of a tree in my backyard I can't see below 30 degrees. So it was in the SE about 36 degrees elevation.



#9 ispybadguys

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Posted 12 March 2021 - 11:17 PM

Just to bring everyone up to date. My CEM40G first trip out to a dark site was a disaster in terms to getting any photos. After a careful iPolar alignment the GOTO performance was erratic. Pointing to the Orion Nebula was off by around a degree. Repointing to Sirius ended up off over 20 degrees. One other repoint was successful but several others way off. After about 5 hours out the RA axis just quit. I sent the mount back to iOptron. With so many problems should I just get my money back or let iOptron try and repair the mount? If I return it I was thinking about getting an EC version of the mount. Maybe a CEM70 or something else entirely.

 

Apart from the RA failure should I expect a mount to have good guiding and tracking performance out of the box or does it require a lot of adjustments to make one work. PHD2 complained of excessive backlash, RA DEC rate errors, and non-orthogonal axis. Guiding was 10 arcsec p-p using multiple stars.



#10 rgsalinger

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Posted 13 March 2021 - 01:00 AM

You should see if there's a club around your area that can provide you with a mentor. You should contact your dealer and let them know that the mount may be bad so that you can return it without losing any money. I doubt that it's bad, though from what  you have posted so far. 

 

Please post a photo of how you have the Sony zoom mounted on the system. Unless you go to considerable lengths to secure the lens to the mount you will get exactly what you are reporting. We've seen lots of people try to do this without properly supporting the lens. That may or may not be the problem here, of course. 

 

You really should focus on getting a good calibration. If you don't get one then figure out what's wrong. You will never get decent guiding without a good calibration.

 

Belt drive mounts do not have DEC backlash to any extent unless the belt is slipping. Use a star test to determine if that's actually the problem - not the guiding assistant because if you don't have everything just right the GA is not going to be able to differentiate between actual backlash and the star just not moving.

 

Please post a guide log from PHD. It won't be 8mb long at all.  From that we can probably determine if the mount is crap or if there's some workflow issue.

 

You really aren't providing much information about exactly what's going wrong with it - no log, no picture, etc. . 

 

 

 

Rgrds-Ross


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#11 petercoxphoto

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Posted 13 March 2021 - 05:56 AM

Just to bring everyone up to date. My CEM40G first trip out to a dark site was a disaster in terms to getting any photos. After a careful iPolar alignment the GOTO performance was erratic. Pointing to the Orion Nebula was off by around a degree. Repointing to Sirius ended up off over 20 degrees. One other repoint was successful but several others way off. After about 5 hours out the RA axis just quit. I sent the mount back to iOptron. With so many problems should I just get my money back or let iOptron try and repair the mount? If I return it I was thinking about getting an EC version of the mount. Maybe a CEM70 or something else entirely.

 

Apart from the RA failure should I expect a mount to have good guiding and tracking performance out of the box or does it require a lot of adjustments to make one work. PHD2 complained of excessive backlash, RA DEC rate errors, and non-orthogonal axis. Guiding was 10 arcsec p-p using multiple stars.

Sorry to hear you're having such a hard time of it. A good mount (which the CEM40 is) will track well out of the box - but everything needs to be set up correctly as detailed above. While I don't rule out that you may have gotten unlucky and have a broken mount, everything in your posts does suggest to me that this might be user error.

 

I'm not trying to be insulting here. I can also see from your posts that you're a very technical guy with a background in programming or systems administration - which reflects my own background. When I was getting started, I had various problems that I was certain were due to faulty equipment, but turned out to be something else entirely. I thought I had a great handle on how everything was supposed to work, and I just didn't.

 

Chief among these problems I had were bad guiding and tracking, and it was always something I was doing wrong.

 

As Ross says above, posting photos of the setup and a guide log would go a long way to helping us help you. As you've already sent the mount back to iOptron, I'd allow them to check it out. If they find fault with it, let them repair or replace (probably replacement is best seeing as you just bought it, repairs can always leave question marks about whether it truly fixed the problem). If they say there's nothing wrong with it, they're probably right.

 

I will make one note about iPolar. The camera is nice, but their software is just about the worst I've ever come across. The UI is terrible, and I've had several occasions where the software directed me to a totally incorrect polar alignment. I have zero trust in it. I have used the iPolar camera successfully with Sharpcap Pro's polar alignment routine, which is my go-to polar alignment tool. Some of your problems would be explained by very poor polar alignment, so you may be getting bit by this problem as well.

 

Another option for polar alignment is to use your guide scope and camera with Sharpcap Pro's routine - this is my preferred method. I only use the iPolar camera on my long focal length rig where I don't use an external guidescope.

 

Cheers,
Peter



#12 ispybadguys

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Posted 13 March 2021 - 03:05 PM

Peter, Ross

No offense taken at all. I appreciate you all taking the time to help me. Peter, you made an excellent guess I am an electrical engineer. I didn't know it showed that much. So there is is definite problem with the CEM40G and that is that the RA axis has fits and will not drive even with the manual slew. Re-seating the circuit board everyone has made it work but that isn't acceptable.

 

PHD2 Calibration reports three types of errors:
a) "very large Dec backlash "
b) "Non-orthogonality = 88.089”
c) "Expected ratio at dec=28.0 is 0.883, actual is 2.87"

I have not gotten a good calibration in many attempts. I followed the PHD guidance on setup and tried some guiding sessions with nothing attached to the mount, i.e. no camera or payload

 

I attached a log here. I chose a star near the zenith for my later attempts

 

Attached File  PHD2_GuideLog_2021-02-23_211217.txt   61.95KB   8 downloads

 

I don't have the mount here so I can't really take a photo. The Lens I have is a Sony 200-600 mm F6.3 lens with a tripod foot attached. It is well balanced fore and aft of the foot and nearly perfect balance in the RA axis. I ordered a new Vixen mount for it so I can make the DEC balance perfect. It is just pretty good with maybe an 1/4 of an inch-ounce  of force required worst case to hold the DEC axis.

 

I agree with you about the UI it is very primitive. The polar alignment itself seems easy but you can't really tell what it is doing. I have been using Astroberry and I like it a lot but I used PHD2 on the PC for my guiding tests so I could say that my tracking was using the recommended software.

 

Thanks

 

Kurt



#13 robbieg147

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Posted 13 March 2021 - 03:53 PM

Many will disagree but I believe it's best to start unguided, and perfect your technique until you find the limit going unguided? Then start guiding once everything is dialed in.

 

Your comment "Sony 200-600 mm F6.3 lens with a tripod foot attached" I don't like the sound of this? a lens like this needs proper mounting. Even my much smaller 200mm F2.8 I will have a supporting ring at the front.

 

I hope you sort your problems out soon.



#14 rgsalinger

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Posted 13 March 2021 - 05:07 PM

It looks as if you are trying to use the INDI driver and I have no idea how that works so take the following with a healthy dollop of skepticism. 

 

 

You are trying to guide at .5 seconds. That's almost impossible and I think it's causing the mount to oscillate as it tries to catch up with each command. Slow the cadence down to (say) 3 seconds. The rate is set way too low at .2 sidereal. You need to be at .5 sidereal (7.5 arc seconds/second) Minimum move is set to over 1 arc second. Cut that in half - .09 rather than .18.  Make those changes and see what happens.  

 

However, none of your calibrations worked well, as far as I can tell, but that may be due to the low guide speed you are using.  The calibration logs all say "RA Guide Speed = Unknown, Dec Guide Speed = Unknown". I'm not sure what that means but it may be an issue. They don't look good to me either.

 

 

Are you using pulse guiding? Are you calibrating near the meridian?  Until you get a good calibration you won't get really good guiding.

 

Rgrds-Ross


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#15 michael8554

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Posted 14 March 2021 - 05:14 AM

During Calibration PHD2 has calculated the guide rates to be "Norm rates RA = 8.0"/s @ dec 0, Dec = 7.0"/s",  which if correct is not unusual.

 

2 to 3 sec exposures would also give you a better camera SNR.

 

 

Here's what Ross is saying:

 

 

14thMar.JPG

 

PA is good and Dec (red trace) is chugging along at 0.61 arcsecs

 

RA (blue trace) is taking 8secs or more to correct those unusual 7.5arcsec oscillations.

 

Stiction and Backlash problems usually have a much flatter peak, with an abrupt final slope as the problem is finally overcome.

 

With corrections that get bigger and bigger until finally the mount moves, leading to a large overshoot and repetition.

 

Here it's much more gradual, the corrections are controlled, but taking too long - as Ross suggests, the guide rates may be too small ?

 

So see what slower exposures and offset RA balance achieves.

.


Edited by michael8554, 14 March 2021 - 05:16 AM.


#16 rgsalinger

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Posted 14 March 2021 - 08:17 AM

Ah I forgot that the cal rates are pixels/second and misread the guide speed in the calibration. Lesson learned.

 

"West calibration complete. Angle = -4.7 deg, Rate = 1.660 px/sec, Parity = Even
North calibration complete. Angle = -94.3 deg, Rate = 1.147 px/sec, Parity = Odd"

 

I still think that the cadence is the problem.  

 

Rgrds-Ross



#17 ispybadguys

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Posted 14 March 2021 - 09:08 AM

Thanks! This is most helpful. A comment on INDI. I have used PHD2 under Windows using ASCOM and using Kstars/EKOS using Astroberry on RPI. I prefer Kstars because INDI has a driver for my Sony A7R4. I am open to other tools but this is all I have found with the Sony Driver.

 

Knowing nothing about PHD2 I just used the default settings. All of my guiding attempts, be they INDI or ASCOM, pretty much look the same with bad RA and DEC being better behaved. Early calibrations we pretty high but later ones I am pointing at the Zenith. Guiding is over the USB connection the ST port i unconnected.  iGuide can do exposures up to 3.5 seconds so I will try that. I see some of the setting in the Advanced Settings where do I find the guide rate? I saw that somewhere when I was configuring it and I thought I used the default at 0.5 but apparently that got changed somewhere.

 

Thanks

 

Kurt



#18 petercoxphoto

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Posted 14 March 2021 - 09:14 AM

Early calibrations we pretty high but later ones I am pointing at the Zenith. 

 

Hi Kurt -

You don't want to calibrate at the Zenith. You should be calibrating close to the celestial equator (at or near 0 degrees declination) and close to the meridian. So more or less pointing south. I always pick something that's already past the meridian so the mount doesn't try to do a flip in mid-calibration!

 

If you haven't already, you should read and follow the instructions in this document: https://openphdguidi...ces_2019-12.pdf

 

Cheers,
Peter


Edited by petercoxphoto, 14 March 2021 - 09:20 AM.


#19 rgsalinger

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Posted 14 March 2021 - 07:03 PM

I pointed out the poor choice of calibration locations earlier in the thread. At the same time, my view is that the bigger problem is the guiding cadence, but the poor calibrations are not helping, that's for sure. 

 

Rgrds-Ross



#20 ispybadguys

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Posted 14 March 2021 - 10:40 PM

Peter, Ross

 

I did read that after you first posted it but I guess I did so without comprehension it but it does say equator. When I get the mount back I will do it right. Thanks for the help. I don't know if it will help with the non-orthogonality and rate errors during caibration but it is equally hard to believe the axis are a degree off from 90.

 

Kurt



#21 michael8554

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Posted 15 March 2021 - 04:55 AM

The Guide Rate does appear to be at 0.5X / 7.5arcsecs/sec  "Norm rates RA = 8.0"/s @ dec 0, Dec = 7.0"/s",

 

Here's a spectrum of the RA Errors:

 

PE.JPG

 

That peak is at 400 seconds, which is probably exactly the worm period of your RA drive.

 

In other words, common or garden Periodic Error.

 

Your mount may have Permanent PE correction built in, if so you could train it to minimise the PE.

 

The only confusion is why your guiding isn't coping with this PE   ?! :-<

 

Time to upload your GuideLog and DebugLog to the PHD2 gurus.

 

Instructions in the Help menu of PHD2.

 

Be sure to check you are uploading the correct files, date is in the filenames.



#22 ispybadguys

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Posted 15 March 2021 - 05:13 PM

That’s interesting. The PEC plot that came with the mount showed about a 1 arcsec peak error.

 

Kurt



#23 bobzeq25

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Posted 15 March 2021 - 09:54 PM

That’s interesting. The PEC plot that came with the mount showed about a 1 arcsec peak error.

 

Kurt

What!!!  That would be the greatest CEM40G the world has ever seen.  <smile>  The spec is +/-7.

 

Or do you have the EC version?  But that would be the worst EC the world has ever seen, the spec is +/- 0.25.


Edited by bobzeq25, 15 March 2021 - 09:58 PM.


#24 rgsalinger

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Posted 16 March 2021 - 01:04 AM

My understanding of the meaning of the frequency plot is different. It does not show the periodic error of the worm. It is used to analyze the "RA drive harmonics" - recurring errors at particular frequencies. It is correct that the worm frequency is 400 seconds. 

 

To whit, if you look at the first guiding run, you'll see a peak at a similar frequency and of only 8 arc seconds. So, to me that mount is somehow "jumping" 40 arc seconds every 405 seconds in the long run. I think that the graph excludes (or tries to) settles from dithers so that's not part of the problem. 

 

I ran PECPrep (from the EQMOD project) which has been around for a long time. It gave me about 23 arc seconds of PE for the OP's mount. However to measure PE you need an unguided run and this one is guided. So, the results are probably somewhat off. Needless to say the frequency analysis shows a peak but at 478 seconds which is not the worm period. 

 

When I look at that log I see a mount that is chasing the seeing with a rapid guiding sequence and is poorly calibrated. I'd like to see the OP fix those problems and then run the guiding assistant at this point. I'd also love to see an unguided PHD run that could be pushed into PECPrep. 

 

I'd love to see some better documentation about that frequency graph though. It would be truly fabulous is all that you have to do to figure out your mount's periodic error is just use the frequency analysis graph. 

 

Rgrds-Ross



#25 michael8554

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Posted 16 March 2021 - 05:36 AM

Hi Ross,

 

I'm afraid you understanding, and my understanding, of what the Frequency Analysis does, is quite different.

 

In my opinion it is showing the "RA Drive Harmonics", which are worst at 400secs, there's another small harmonic at 200secs.

 

But let's see what the PHD2 gurus say......




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