Help! New CGEM II having issues with PHD2...

[Steve Rifkin]

Hi, I'm in desperate need of help!  I just purchase and received a brand Celestron CGEM II mount this weekend, put it together, balanced it in RA and DEC, and added my scope (C8), guidescope (WO 50mm, yes, I know, a little small, but it worked with Phd2 and another oldmount of mine with the same scope) , and guide camera (ZWO ASI120MM-S).

(First I should add that I reset my PHD2 configuration from what it had been, as previously,  it had been used with the same scope equipment above, except, it was on my old, and backlashy --in DEC-- Advanced GT mount.  So, I cleared out PHD2's config in prep for the new CGEM II mount.)

I leveled the CGEM II mount and polar aligned it with my Pole Master.  No issues.  I did a 2+3 star alignment.  No issues.  In fact, this is the first time I've had Go-Tos as accurate and centered compared to my older mounts.

I then connected the guide camera and mount to the laptop and ran the PHD2 new profile wizard.  I set the camera pixel size to 3.75 and the guide scope focal length to 200.  I then set the mount guide rate by having it grab what it needed from the scope.  (I'm assuming it ended up as the default .5 value, as that's what the number remained as..)

Then, I proceeded to calibrate.   I'm pointing the main scope to Castor off to the west.  (I didn't check the clear sky clock to see what it thought "seeing" was for the evening, but at least, for most of the evening, it was at least clear.)

What I was dreading happened...a problem.  Specifically, an advisory:  Calibration indicated that there was little south movement probably due to a very large dec backlash.  And this is where my problems began.  It's a brand new mount.  Visually slewing and using the hand controller, I had no visible backlash with the hand controller "rate speeds"  down to 5 (9 is the fastest, 1 is the slowest, I think... and I don't recall how those hand controller numbers actually translate to the tracking rates on these CGEM IIs) in either RA or DEC.

I was really disappointed to see PHD2 complain about this with my new mount.

Anyway, I trudged on and let guiding commence.  The graph was all over the place.   (Don't worry, I include the logs as well.)

So, I tried Guiding Asst.  And when it ran, I had it set to measure backlash.  And it claims to have failed to measure DEC backlash.  Again, I just don't understand what's going on.

Regardless, I tried several guiding attempts.  I took advice from Guiding Asst in some of the attempts to change the minmo numbers, but they made no difference. I even tried a few attempts with more than the default 1 second exposure time, trying with even up to 2.5 seconds.   In all cases, the RA and DEC lines were not huddled close to the center point line.  I was able to get *some* 30 second exposures without elongated stars, but most 60 sec. exposure attempts failed with elongated stars.  And I could certainly visually see the graph was just not looking good like my old DEC-backlashy mount's when attempting guiding.  (A quick word about my old mount... it was an Advanced GT mount with horrible backlash easily evident in DEC just visually using it with the hand controller.  And for that mount, of course, PHD2 complained about its backlash, but it had infinitely better-looking graphs than what I'm getting for my new CGEM II.)

Back to the CGEM II... With the CGEM II, I went through several guiding attempts (not long attempts, because they looked horrible right away and never really settled down.)

Then I moved to another star, Alkaid (last star in the dipper's handle.)  This time, I'm pointing Northeast and pretty high.  In this case, guiding was no better, but my RMS numbers for DEC were half as small as they were for RA.  And again, a bad graph.  (Tried adjusting aggressiveness and some minmo --guiding asst again-- to no avail.  Dec was always half of RA in this instance.)

I gave up, and before I packed it in, I checked to see if the mount's Go-To capability was still working.  I told it to slew to Arcturus (high in the east), and it was spot on, dead-center.

I'm not convinced my mount is bad.  I'm not yet convinced my mount is having DEC backlash issues.  But, I am concerned.  And of course, terribly disappointed at the graphs.  How can my guiding be worse with the new mount compared to that old mount of mine with visible backlash issues?  I'm very confused and am thinking I'm just missing something.  Or there's something particular I have to set to PHD2 to work with a CGEM II.  (Longer mount guide rate?, etc?)

(Also, I just mentioned "longer mount guide rate".  If I need to up it from the .5 to possibly 1 --which I read about in some other postings--, I read that Phd2 doesn't actually change that guide rate, and instead, you have to change it in the mount itself?  I can't find anything in the docs about mount guide rate, only tracking rate... and I don't want to change tracking rate for when I'm doing visual work.  I was thinking that perhaps, at the time I run Profile Wizard, instead of letting it connect to my mount to grab the mount guide rate, I just enter a number there in the wizard replacing the default value of .5   Will that work?)

I am still very new with PHD2, so please be patient with this newbie.  A friend of mine, also new to PHD2, suggested I write to you all, the Open PHD Guiding group, in that you might be able to help me through this.  I can't imagine this is failing so badly for my new mount, when my new mount is working so well for non-guiding purposes.    And I feel really badly about the whole situation.  I spent a lot of money for this new mount and am really hoping it's not defective, but merely needs some PHD2 tuning.

Any advice to get me going would be great.  If you're a CGEM II user and use a similar scope setup, I would especially like to know your PHD2 configuration.

Logs (zipped) are attached for last evening.  I also took some snaps with my phone of the PHD2 screen throughout the evening while guiding (zipped.)  Sorry, they're not the best quality, but I thought they might come in useful?

Thanks so much in advance!

Steve

[Brian Valente]

Hi Steve

welcome to PHD. 

you are in the right place

It sounds like you have some good understandings of what to do, but there are a couple gaps in your approach. For example, you really don't want to be guiding on big bright stars. stay away from them

May i suggest you start with a baseline guiding as outlined in this document and get back to us with the guidelog there:

https://www.dropbox.com/s/os1thorvswmzaul/How%20to%20create%20a%20baseline%20for%20guiding%20results%20using%20OpenPHD2.pdf?dl=0 

you may be repeating many of your previous steps while doing this, but please do this in order and follow the steps as written. that will give you and us a solid baseline for your guiding. There are too many missteps and short runs in your current guidelogs for it to give us a clear picture

Regarding your rather long message about your brand new mount, here's the thing: all mounts of this design have some amount of dec backlash. Visual performance is pretty much irrelevant when it comes to comparing these very small movements required for imaging. 

Backlash is not necessarily bad, but you need to be able to quantify it, and determine if it's a normal amount or there is something mechanical that needs to be addressed. PHD has beautiful tools for handling backlash, and hopefully it will be handled in a matter of a few tweaks

Also here is my personal advice regarding "things you've read or heard" about PHD. 

If it's not part of the official PHD documentation, help pages, or comes from Andy or Bruce, disreard it. There is too much bad or misinformed suggestions out there to go through it point by point. My advice is forget about what you've heard. start fresh now

 The good news is I don't think you are that far from good guiding. Your uploaded guiding sessions are far too short to really draw a conclusion, but looking at your 32nd run, your overall RMS was 0.92" which if you can even it out is very solid guiding. Your calibrations look reasonably good as well.

So don't panic - nothing so far has suggested there is something terribly amiss. your mount is not perfect - even if it's new - and i recommend you let go of thoughts like that.  It's okay, it will likely work great for what you need. 

Brian

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Brian Valente

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[Steve Rifkin]

Hi Brian, thanks so much for your comments; I will check out the link you provided and attempt to create a baseline and report back.  By the way, a note that in either case (Castor or Alkaid), I was not actually  guiding on those two  stars.  Those stars were the subject of my C8's frame, as I was trying to get starfield images with those big stars as the prime subject of those images.  However, in both cases,  I let PHD2 choose the guide stars, and in each case, it chose tiny stars other than the Castor and Alkaid, which was the plan.  Is that a problem for testing guiding?   Can I not have bright stars in the main scope's field of view, but have the guide scope guide on a PHD2-selected guide star?

Steve

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[Steve Rifkin]

Brian, I forgot to mention in my reply post to you earlier this morning, with preparing the baseline, when I run Guide Assistant, should I be checking off that it should "Measure Declination Backlash" as well?  (...as I had done the other evening, when it kept failing with the error,  "Measuring backlash:  Mount never established consistent south moves- test failed")   The baseline document does not state whether I should or should not check off that option when running Guiding Assistant.  Thanks!   --Steve

[Brian Valente]

Okay thanks for the clarification Steve, it's fine to have them in the frame, you just don't want to guide on them

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[Brian Valente]

Yes, leave it checked. you should at least be able to characterize the backlash amount. 

that test may fail, but don't feel like it's the end of the world, sometimes that happens and it ends up guiding fine. the point of the baseline is to get the 'big picture' for your mount

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[Steve Rifkin]

Sounds good.  I do plan to make sure I head a little north (with the HC or online controls) prior to the calibration, as suggested, to try to remove any DEC backlash before the calibration. (Something I had not done before.)  I'm confused still at how removing the DEC backlash prior to calibration will affect actual guiding when one does actually have some DEC backlash in the mount. In other words, if you mask the actual DEC backlash by eliminating it just before calibration (so that calibration doesn't see it), once you start guiding,  doesn't that cause a problem since there actually is some DEC backlash in the mount?  Or does it now work that way?   -Steve

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[Brian Valente]

There are two separate issues here

one is you want an accurate calibration of your DEC axis movement, and for that you don't want backlash to impact the movements. that's why you bump north first

second is once you have a successful calibration, you want to quantify your backlash and then if needed use the backlash compensation within PHD to manage it. 

So for calibration, there's a slightly different goal in mind

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[Steve Rifkin]

I understand now, thank you Brian!  Now, just waiting for some clear skies to do the baseline.  :-)

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[Steve Rifkin]

Hi Brian, we finally had a clear, windless evening.  I set up the CGEM II, balanced with the C8, guiding scope/guide cam and DSLR, same as before.  Polar aligned with Polemaster  I followed the docs you pointed me to to establish a baseline.  I chose to go to the vicinity of Procyon, which got the scope almost in the orientation your picture described.  However, I had to slew the RA a bit to raise the weight bar to be parallel.  At that point, the weight bar on the mount pointed west, parallel to the ground, telescope was on the east side pointed southwest at probably 70 deg (I could be wrong.)   See the attached zip file that includes Guide log and a pic of the orientation of the scope.

I started PHD2 with default values and a fresh profile.  Set for 3 second exposures. Ran calibration.  Got the usual error about little south movement.  As soon as it started guiding, I ran Guide Assistant (GA).  If you look at the logs (again, in the zipped file), you'll see that the Guide Assistant failed measuring backlash on its first attempt!   Ran GA again, and this time, it was able to measure backlash, but reported it to be over 8 seconds!  It also had me make  some suggested adjustments, which I did (minmo.)  As it started guiding again, I ran GA again to see if there were any other additional adjustments it would recommend.  Again, it was able to measure backlash, but reported over 8 seconds. And the only recommendations it made this time around were the ones it had already had me do after the 2nd GA attempt.  (Why does it ask me to do what's already been done?)

So, now that you have all the baseline log data,and  what I'm interested in finding out are:

1) your recommendations to improve the guiding, and

2) The backlash being reported in GA.. could it be an issue with my new CGEM II mount?  This is really important.  Testing the unit recently,  I was getting 15 second backlash reports by GA. that is, during the times it didn't fail the DEC measurement in GA (because it's failed before, just like it failed one time out of 3 tonight..)   Whether its  8 seconds (tonight), or 15 seconds (the other night), they both sound like very long backlash periods for a new mount.  I have a window to return the mount,  if this looks to be an issue that I'm going to have to keep dealing with.  If it it sounds like it's a mount issue to you, now's the time for me to act on a return.  If you do not think it's a mount issue, what would be your recommendation to shorten the backlash?  And what would be a reasonable backlash max value?

I also wanted to note that later on this evening, after I already wrapped the baseline activity, I played with Phd2 for a bit, trying out different settings (no logs attached for that), and what I kept finding were issues with RA being twice the error value as DEC, no matter what I did.  I even played with re-balancing a bit (making the mount east or west heavy(, and still, RA was generally 2x DEC, and so any imaging I tried tonight (after the baseline was already wrapped up), the stars were all egg shaped (and the stars egg shapes were always such that they were narrower at the 11 oclock and 5 o'clock ends of each star.)

Anyway, any advice for making changes is appreciated.  Of course, if you feel the mount is having bad backlash based on the data, and that I should return it, I guess there would be no changes to make on the mount until I got a replacement (which means, running a baseline again.)

Thanks in advance!

Steve

[Steve Rifkin]

Oh, I forgot to add to my post from a moment ago (the one with the baseline logs you wanted to see)... I did, fact, bump the mount north before I ran the first calibration for the baseline.  Yes, I saw the stars move when I did the north bump, so that *should have* released the backlash before the calibration.

Also, at some point when I was not guiding, I took some 10 second images of the starfield that I was pointed to to see if there was trailing... and there a little.    In my old broken Adv GT mount, I used to be able to get *at least* 15 seconds before any noticeable elongation. So, that makes me still concerned whether a lot of what is going on is related to the actual CGEM II mount.  But, I want to see what the data I provided tells you about the mount.

Steve

[peter wolsley]

Steve,
I use a old CGEM mount so I took a look at your latest guiding log.  Here are my observations.

-Looking at your calibration run I calculate a DEC backlash of roughly 88 arc-seconds.  I believe the guiding assistant reported DEC backlash values in the 60 arc-second range.  My old CGEM has 50 arc-seconds of backlash so your mount has a lot of backlash.  This might be reduced by mechanically adjusting the backlash.  I have done this with my mount and I feel that the best achievable with these mounts is going to be close to 50 arc-seconds solely because of the gearboxes and plastic gearing. These style gearboxes require backlash between each gearset so that there is no binding.  Because there are typically 4 or more gearsets in these gearboxes this backlash adds up.  There is backlash between the wormgear and it's associated gearsets.  These can be adjusted and I believe can improve your situation.

-Your RA guiding indicates that you have not enable the PEC logic and/or you have not attempted to train your mount's PEC.  Your RA guiding has a dominant 176 second oscillation which should be significantly reduced if you get this PEC logic up and running.

-Your RA guiding also suffers from two fast oscillations.  One has a period of 15 seconds and the other has a period of 16.3 seconds.  These are causing the fast oscillations in your RA guiding.  I don't believe the PEC logic in your mount is going to do anything for these fast oscillations.

-The PHD2 PPEC algorithm for RA has help some users with these fast oscillations.  You might want to look into using this algorithm.  I would suggest decreasing your exposure time to 1 second to give this algorithm more data to work with.

-You are also use autoguiding gains that are approx 50%.  You should consider increasing these values to 80% or higher.  Higher autoguiding gains allow PHD2 to correct for guiding errors quickly which is needed to correct these fast oscillations.  A higher autoguiding gain for DEC will also allow PHD2 to correct DEC backlash events more quickly which is always desired.

-Your pic shows a C8 so you are interested in high magnification photography so sub-arc-second guiding is important.  If I look at your long guiding session and ignore the initial unguided portion I see your guiding was roughly 1.35 arc-seconds total rms.  If you do some of the things I have mentioned here you should be able to achieve 1 arc-second rms which is respectable.  Something else to consider is whether your seeing is going to limit your results.  A great mount is not going to be able to improve poor seeing.

Overall, I think we are building a case for Celestron to get involved.  Your mount would be just fine for someone who only wants to perform visual observations or public outreach.  For astrophotography it is not as good as it should be.  I would suspect that a replacement CGEMII may perform better but I would first want Celestron to show concern before.

My two cents

Peter

P.S. The photo you provided does not show your DSLR attached to your scope and you said you balanced the mount with the DSLR attached.  If you switched out your DSLR for an eyepiece...I hope you rebalanced the mount.  The motors in these mounts are not powerful and should never be loaded significantly.  Always treat them as a precision clockworks that only performs when they are only slightly off-balanced.

On Sunday, April 12, 2020 at 7:36:03 PM UTC-4, Steve Rifkin wrote:

[Steve Rifkin]

Hi Peter!  Thanks so much for reviewing my logs.  I really appreciate it.  As it is, I've only had the new mount for less than a week, and it has been experiencing so much of this backlash when attempting to guide that I contacted Celestron today and am hoping for a replacement unit.  I'm guessing that out of the box, the CGEM II units should not have that bad a DEC backlash.  Guess I got a bad one.  Hearing your comments confirms that for me.

For the DEC backlash, GA says it's 8 seconds (I guess that's real-time) and you converted it to movement/distance?  How high is reasonable for backlash (in seconds or fractions thereof) for a mount like a CGEM?  I assume 8 seconds is way high.

In terms of RA, no I have not done anything with PEC, but it sounds like I do to start doing so.

That dominant 176 oscillation (that might be treatable with PEC) and the two fast oscillations you referenced, can they also be the sign of hardware issue in a mount, or do they have more external causes (cables hanging, etc.)?

Do you think having the RA errors being twice the value of the DEC errors, as I see in my case, as causing the egg-shaped stars I'm getting?

Peter, I'm going to have to play dumb here (as I'm still fairly new to a lot of this.)  When you referencing autoguiding gains, exactly what are you referring to by that term (and where can I find those settings in PHD?)  I was looking up the term "autoguiding gains".  I found a reference to "gain" in the Guide Algorithm docs comparing "gain" to "aggressiveness".  I also see "gain" discussed as predictive and reactive gain in the PPEC algorithm (not that I'm sure exactly what that means...)  When I look into my PHD2's PPEC algorithm for RA, I see that my predictive weight is 50.  Is that  autoguiding gain?    And that's what you suggest I should bump up to 80?  If that's the case, what would I do with the Reactive weight (currently at 60)?  Or do I have the gain thing all wrong and you're referring to something else? 

I will take your suggestions to heart.  I need to learn about PEC on the mount and exactly how to train the mount.  I also need to learn about what the PPEC algorithm does and what to set there.

Yes, your point about my mount being good for observation... I already have a Nexstar SE and a Meade LX-200 10" classic, and they are my "go to" scopes for observing and public outreach (well the Nexstar is for that... not so much the monster of a Meade!)  So, I don't need to have the CGEM II be that!  I have the astrophotography bug and have done a lot with 15 second non-guided images with both my Meade and my Celestron Advance GT mount (that has been having issues lately, and is for now, retired.) Getting the CGEM II was my hope that I could get longer exposures that just 15 seconds!   (I've used my old Adv GT mount with PHD2 at some point, and it did ok at times.  Got round stars.  Might be fatter round stars with that mount, but still, not egg-shaped!)   My hope is that the horrible backlash in this new CGEM II is really a problem with that mount and not indicative of an issue with the CGEM II line.  Otherwise, I chose the wrong mount to try to achieve some longer exposures.

Thanks!

Steve

[peter wolsley]

Steve,

If you have had it less than a week then I think it's worth talking to Celestron.  I get the impression there is a fair bit of variability in performance between identical Celestron mounts.  I had to work on my old CGEM before I was satisfied.  It's not perfect but it is consistent and I can deal with consistent.

The 8 seconds of backlash is where terminology gets into it.  This also pulls autoguiding gains into the picture.  Autoguiding gains are values that are set, using your handcontroller, in the mount's firmware. PHD2 can query the mount to discover what these gains are IF the ASCOM platform is installed on the user's computer and the drivers are install for their mount and the user is actively trying to get ASCOM up and running.  Autoguiding gains typically range from 0 to 100%.  100% equals sidereal rate which is roughly 15 arc-seconds/second.  I quoted a DEC backlash value of 50 arc-seconds for my CGEM mount.  I use an autoguiding gain of 50%(I should bump this up but I am stubborn).  My 50 arc-seconds would be equivalent to 50/15=3.3 seconds if my autoguiding gain was 100%.  Because it is only 50% the 3.3 seconds doubles 6.6 seconds.

 I quoted you that your DEC backlash is roughly 88 arc-seconds.  Because your autoguiding gain is also 50%, this 88 arc-seconds translated to 88/7.5 = 11.7 seconds  This is worse than the 8 seconds that the PHD2 guiding assistant calculated but this is because the guiding assistant uses an entirely different method for determining backlash.  I use calibration data which tends to yield pessimistic values.  I always calibrate each night and I calculate my backlash every night using my calibration data.  It's a easy check that lets me know that nothing is out of whack.

I think you are right to suspect that the unequal RA/DEC rms guiding will produce egg shaped stars.  Differential flexure could also be lurking in your system.  Combined with longer exposures the differential flexure is going to be quite visible.  I use an 8" EdgeHD SCT and it has a lot of differential flexure compared to my 80mm refractor.  I believe in the PHD2 documentation there is a description of how to test for differential flexure so you can look into this further.  Using an off-axis guider also corrects for differential flexure.  I have resisted going that route only because I like the convenience of using a widefield guidescope.  Astrophotographers are a quirky lot...at least I am!

Peter

The fast oscillations are typically flaws in the RA gearbox.  I believe these flaws are exaggerated by not ensuring the RA axis is well balanced.  Some off-balance can help guiding but don't assume that your RA axis drive motor is as strong as a stump puller!  I know of CGEM users who have stripped the gearing in their RA axis because some of these tiny gears are plastic.  Decreasing your exposure time and using higher autoguiding gains can help to reduce the magnitude of these fast oscillation.  I know of users who have used the PHD2 PPEC algorithm with some success in correcting fast oscillations.

I am interested in what Celestron has to say.  I suspect you are going to find that you typically have to pay for performance.

Peter

On Thursday, April 16, 2020 at 4:55:17 PM UTC-4, Steve Rifkin wrote:

Hi Peter!  Thanks so much for reviewing my logs.  I really appreciate it.  As it is, I've only had the new mount for less than a week, and it has been experiencing so much of this backlash when attempting to guide that I contacted Celestron today and am hoping for a replacement unit.  I'm guessing that out of the box, the CGEM II units should not have that bad a DEC backlash.  Guess I got a bad one.  Hearing your comments confirms that for me.

For the DEC backlash, GA says it's 8 seconds (I guess that's real-time) and you converted it to movement/distance?  How high is reasonable for backlash (in seconds or fractions thereof) for a mount like a CGEM?  I assume 8 seconds is way high.

In terms of RA, no I have not done anything with PEC, but it sounds like I do to start doing so.

That dominant 176 oscillation (that might be treatable with PEC) and the two fast oscillations you referenced, can they also be the sign of hardware issue in a mount, or do they have more external causes (cables hanging, etc.)?

Do you think having the RA errors being twice the value of the DEC errors, as I see in my case, as causing the egg-shaped stars I'm getting?

Peter, I'm going to have to play dumb here (as I'm still fairly new to a lot of this.)  When you referencing autoguiding gains, exactly what are you referring to by that term (and where can I find those settings in PHD?)  I was looking up the term "autoguiding gains".  I found a reference to "gain" in the Guide Algorithm docs comparing "gain" to "aggressiveness".  I also see "gain" discussed as predictive and reactive gain in the PPEC algorithm (not that I'm sure exactly what that means...)  When I look into my PHD2's PPEC algorithm for RA, I see that my predictive weight is 50.  Is that  autoguiding gain?    And that's what you suggest I should bump up to 80?  If that's the case, what would I do with the Reactive weight (currently at 60)?  Or do I have the gain thing all wrong and you're referring to something else? 

I will take your suggestions to heart.  I need to learn about PEC on the mount and exactly how to train the mount.  I also need to learn about what the PPEC algorithm does and what to set there.

Yes, your point about my mount being good for observation... I already have a Nexstar SE and a Meade LX-200 10" classic, and they are my "go to" scopes for observing and public outreach (well the Nexstar is for that... not so much the monster of a Meade!)  So, I don't need to have the CGEM II be that!  I have the astrophotography bug and have done a lot with 15 second non-guided images with both my Meade and my Celestron Advance GT mount (that has been having issues lately, and is for now, retired.) Getting the CGEM II was my hope that I could get longer exposures that just 15 seconds!   (I've used my old Adv GT mount with PHD2 at some point, and it did ok at times.  Got round stars.  Might be fatter round stars with that mount, but still, not egg-shaped!)   My hope is that the horrible backlash in this new CGEM II is really a problem with that mount and not indicative of an issue  secondswith the CGEM II line.  Otherwise, I chose the wrong mount to try to achieve some longer exposures.