PhD2 pulse guiding and RA control with EQMOD

[practical....@gmail.com]

Does anyone know what happens to the RA slider in EQMOD when PhD2 is used -  e.g. does any setting of the slider get ignored; is it used to modify PhD2 behaviour  - or does PhD2 modify the EQMOD command?  

I've established from my PhD2 Guiding Assistant session (once Andy G kindly explained what I needed to do!) that I have a very slow but continual drift West in RA but DEC is fine.   The West drift means my EQ8 mount is running slightly fast.  It wouldn't matter much if I was imaging through my 80mm but as I'm mainly using a f/10 14inch SCT for imaging and PhD through my 80mm to guide, it makes quite a difference and prevents me imaging anything for more than 4-5 minutes per image, even when I have PhD2 doing the guiding.   EQMOD has a RA slider that can be set to slow the mount down but would PhD2 ignore this or be modified by it?

Peter

[Andy Galasso]

Hi Peter,

RA drift is usually just caused by ordinary polar alignment offset, though it could be caused by other things such as a non-sidereal tracking setting (lunar/solar) in the mount configuration, or even by flexure of the guiding assembly.  Here's another explanation of RA drift:  https://groups.google.com/d/msg/open-phd-guiding/ufB82xwalas/17mjyaoUCgAJ

When using EQMOD, make sure you have the settings described here: https://github.com/OpenPHDGuiding/phd2/wiki/EQASCOM-Settings .  If you end up changing the guide rate setting then you'll need to do a fresh calibration.

RA drift is ordinarily guided-out quite effectively by phd2 and should not affect your images.  If you have star trailing in your image subs it is unlikely to be related to RA drift, but more likely a symptom of differential flexure between the guide camera and the imaging camera.

Some recommended resources:

  PHD2 Best Practices

  Tutorial: analyzing phd2 guiding results

 

Andy

[Anun Earthly-Child]

Hello Andy

Links much appreciated - but on EQMOD and PhD2 settings I applied the recommended settings when I moved over to EQMOD two months ago (and after watching Chris Shillitoe's excelllent instructive videos on Youtube).   I have been using PhD for years with ST4 cable guiding although PhD2 only about a year and pulse guiding with EQMOD only since November.  On RA and polar alignment I would agree if we were talking about RA drift in one direction and DEC drift in another, but I have no significant DEC drift in any direction and PhD2 estimates my polar alignment error as no more than 0.5 arc minutes on a 40minute unguided test using Guiding Assistant following your helpful advice.   

I took a lot of time to drift align manually my permanent set-up when I put the 14inch SCT on the EQ8. In drift alignment as I am sure you know it is an iterative process and each time I point East and correct any DEC drift I then point South and correct any DEC drift and back to East again.  I have now reached the point where pointing East or South there is no discernible DEC drift looking at a bullseye target through the 14inch SCT at f/10 for fifteen minutes.  But in both directions there is the same RA drift towards the left of my field of view in the on screen target through the ZWO ASI 174MM.  That can only be down either to imbalance or the RA drive.  If there was any significant polar alignment error it would show up in DEC drift either when pointing South for East-West alignment error or pointing East for North-South alignment error.  If I declutch both axes nothing pulls mount or scope in any direction other than to go level or just stay where it is when I have the kit pointing up, on its side or with the scope parallel to the counterweights - I believe that is a balanced set-up so I am inclined to discount imbalance as the cause.

Having seen the result of the Guiding Assistant test it has confirmed my belief that whichever way I am pointing RA is running fractionally too fast.  If I used a small shorter focal length
such as my 80mm I can run for nearly 10minutes with the stars generally still looking round but the f/10 14inch with the 3,556mm focal length shows slightly elongated stars if I run past 4-5 minutes.  I think guiding through the 80mm isn't sensitive enough to spot the slight drift in RA seen in the f/10 scope and therefore doesn't correct it.  Hence I'd like to try the RA slider in EQMOD but I need to understand what effect if any that has on PhD2 pulse guiding.

Regards,

Peter

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[Andy Galasso]

Peter,

We can probably say more if we see your Guide Log. Do you want to post your guide log from a session where you had some guiding and a GA run?  Instructions here: https://openphdguiding.org/getting-help/

The likely suspect for elongated stars with an SCT + external guide scope configuration would be differential flexure. There's a good description of the problem and how to diagnose it in the "analyzing PHD2 guiding results" doc starting on page 13 (link to doc in prior post).   We can help you diagnose it too if you want to post a raw image sub (FITS file) along with the corresponding guide log.

Andy

[Anun Earthly-Child]

Hello Andy

I've uploaded the guiding log but please note it only starts properly from 17:58 as before tnat I had forgotten to change the settings t match the 14inch SCT I was using Guiding Assistant with raher than the 80mm scope that normally guides.

You should then have abot 40mins with GA momitoring and no guiding followed by a short period of guiding before clouds ended the session.  Using your tool to look from 17:58 I get a nice flat DEC line and a constantly rising RA line until guiding starts.

Peter

Peter AKA Practical Astronomer guide log for Andy to see

[Ken Self]

Hi Peter

You need to get a valid calibration before you try using the Guiding Assistant. Calibration is critical to the operation of PHD2

Yours has an issue with too few steps and no dec leg.

Use the calculator in the brain to get the right calibration step size then try again. You also need Dec guiding to be enabled for the calibration to work on both axes.

[Andy Galasso]

Right, there's no dec calibration or guiding because Dec guide mode is set to "Off" ... it should be set to "Auto" (the default).

You may also want to increase your exposure duration to 1.0s or more -- see PHD2 Best Practices, link in prior post.

It looks like you have exquisitely tuned your polar alignment to minimize dec drift. For most positions in the sky you can minimize dec drift or RA drift, but not both (reference: Hook, R.N., Polar Axis Alignment Requirements for Astronomical Photography). PHD2's drift align tool solely focuses on minimizing dec drift because RA drift is quite easy to guide out and we essentially do not really care about it for guiding purposes. Your RA drift measured by the Guiding assistant is 1.75 arc-seconds per minute, consistent with an ordinary polar alignment offset, not anything I would be concerned about. With RA guiding enabled there is no drift in RA (PHD2 is fully correcting it), and your RA guide error is about 0.5" RMS due to seeing and the mount's inherent periodic error.  That's actually pretty good -- you should not be seeing any star trailing in the imaging camera in the RA direction. If you do see star trailing in the image camera it is most likely due to differential flexure.  You can confirm that or rule it out by going through the procedure in the section on differential flexure in "analyzing PHD2 guiding results" (link in prior post).

Hope that helps, please let us know what you find.

Andy

[ross0g...@gmail.com]

Hello Peter,

Does anyone know what happens to the RA slider in EQMOD when PhD2 is used

Which slider are you referring to? The RA guide rate slider at the upper left of EQMOD, or the "Drift compensation" slider at the lower right?

If I understand your problem correctly you are seeing star trails when guiding with your 80mm guidescope mounted on your SCT, and you are seeing a significant RA drift when using the guiding assistant (thus unguided), despite having a PA error close to zero.  I believe these to be two separate issues and will comment mostly on the unguided RA drift.

... it has confirmed my belief that whichever way I am pointing RA is running fractionally too fast.

Like you, I have an EQ8 (recently purchased new) and was troubled by what I believe to be the same problem. I can fortunately report the problem was easily resolved. Please take a look at my posts to the EQ8 forum:

https://groups.yahoo.com/neo/groups/SkywatcherEQ8/conversations/topics/4103?guccounter=1

Despite being a rather pricey mid-range mount, the EQ8 appears to have some sidereal tracking rate inaccuracy. My mount's RA drift rate is approximately 2 arc-sec/minute, and I see this same RA drift rate no matter where I point in the sky, so it is unrelated to atmospheric dispersion, etc. My PA is also very good like yours, but it is not perfect, so some of the RA drift rate can be apportioned to PA error, and most of the remainder to my EQ8's sidereal tracking error (Andy/Ken/Bruce will hopefully correct me if I am wrong). Your mount's RA tracking error appears to be of a similar quantum to my EQ8, and I've been told that this amount of error is typical for the EQ8. You will also have periodic error to minimise too.

If you read my post at the above link you will see that the solution is easy: by adjusting EQMOD's "Drift compensation" slider (lower right side of the expanded EQMOD window) you can change your mount's RA sidereal tracking rate in increments of about 2 arc-sec/minute. No finer adjustment is possible unfortunately, but given your RA drift is close to 2 arc-sec/minute, you can at least minimise it to a large degree. Note that, after you make the slider adjustment in EQMOD, I think you have to close and then reopen EQMOD for the new setting to take effect.

Here is what my RA drift looked like before adjusting the slider in EQMOD:

And after adjusting the slider by 1 notch:

So my RA drift decreased from 2.8 arc-sec/min to about 0.2 arc-sec/min; quite an improvement. But note that my PA error increased slightly too. I think that is proof my PA was not perfect to begin with, and as such, variations in a mount's RA tracking rate will influence the measured PA error (please correct me if I am wrong).

PhD2 can easily guide out your EQ8's RA drift error, so I believe that is not the cause of the star trails you are seeing in your images. I'm confident you'll solve that problem by using an OAG rather than a separate guide scope.

RA drift is usually just caused by ordinary polar alignment offset

Andy and crew, while I have your attention, is there a formula that quantifies (approximately) how much RA drift is induced by PA error? For example a PA error of x arc-min produces an RA drift (at meridian/CE) of y arc-sec/min? 

Cheers,

Ross

[ross0g...@gmail.com]

Oops, those images are too small to view. Try these:

[Andy Galasso]

Thanks for the excellent info Ross.

is there a formula that quantifies (approximately) how much RA drift is induced by PA error?

 

There's a formula and a good description of it in the Hook article referenced a couple posts back in this thread.

[bw_msgboard]

Hi guys.  I’d like to add a note of caution here.  What you are measuring is the apparent drift *through the guide scope*, not necessarily the native tracking rate of the mount.  Unless you are using an OAG, you have to do some extra work to confirm the problem is really with errors in the sidereal tracking rate of the mount.  Maybe it is, I don’t know.  Frankly, I think anyone expecting to guide a 14-inch f/10 scope with a separate guide scope is smoking hemp – differential flexure is going to be a problem, especially with a scope that has a moveable mirror.  So fine-tuning the apparent tracking rate through the guide scope may do nothing to improve the images through the main scope and may actually make things worse.

 

Just my 2-cents worth,

Bruce

 

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[ross0g...@gmail.com]

Hello Bruce,

I should have made it clearer in my original post, I guide via an OAG on my Edge Hd 1100. I agree that adjusting the Drift compensation slider in EQMOD may not improve Peter's RA drift, but it will only take him 5 minutes on a clear night to find out.

 Frankly, I think anyone expecting to guide a 14-inch f/10 scope with a separate guide scope is smoking hemp 

Never a truer word spoken :-)  Sage advice like this no doubt saved me a lot of frustration, time, and expense by allowing me to not even attempt to guide my SCT via a guide scope.

Cheers,

Ross

[Anun Earthly-Child]

Andy, Ross and others

Following everyone's comments I had another look last night after a couple of occultation events. The sky finally cleared around midnight so after testing various options I redid PhD2 calibration and re-ran for half an hour on Guiding Assistant.  Found a star at +1 degree in DEC almost due South.  I had a bit of guiding at the start after my changes and would have run guided for a while at the end but as you will see from the uploaded log I ran into cloud just as I was about to finish with unguided.   I discovered one big error I'd made with PhD2, instead of a sensible number of guiding steps (it recommends 12 but I need a few more) I had it set for 60.   Moved it to 15 instead before I started.

During my test calibration after I turned on PhD2 I had real problems moving the mount in DEC so I stopped and tried in EQMOD and the mount moved effortlessly, so I tweaked the DEC settings in PhD2 and tried again.  I got some odd stuttering noises but it did manage to complete and start guiding. I then had to increase the percentage response in DEC from the previous guide command from default 10% to 50% to stop the mount grunting at me and being too jerky in DEC. That seemed to do the trick for the rest of the short guiding sequence before I switched to Guiding Assistant.

Please remember although I was having problems guiding through the 80mm I have been deliberately using the camera with PhD2  through my 14inch f/10 SCT for these tests as it is the 14inch that I need to know I can guide, whether I have to revert to off-axis or whether I can get it to work with the 80mm.   What my final file shows is the same low 0.5" polar alignment error (good!) and after adjusting the EQMOD slider to -1 as the Guiding Assistant had recorded I was going too fast in RA at +2 approx it now has a guiding error of -0.02 per minute.  . But it now shows a small negative drift in DEC of -0.13 per minute.

I suspect when I revert to using the 80mm I need to apply some changes to the calibration there as well if I am to match these results.

Thank you everyone for your help.

Peter

PS link in next post as am on wrong computer!

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[Anun Earthly-Child]

Am having to do the link manually as my laptop is not playing ball tonight.  The file is on:

https://openphdguiding.org/logsdl/PHD2_logs_7mvA.zip

Peter

[Anun Earthly-Child]

Sorry - link failed - this one should work: https://openphdguiding.org/logs/dl/PHD2_logs_aGIx.zip

[Ken Self]

Sorry to tell you that is the same log you posted a few days back

[Anun Earthly-Child]

I believe the second link is from 23 January - this is a screenshot.

Peter

[Andy Galasso]

Hi Peter,

The screenshot corresponds to this log: https://openphdguiding.org/logsdl/PHD2_logs_7mvA.zip

It's pretty clear that whatever you did greatly reduced the RA drift.

I have some general unrelated suggestions:

1. your image scale (0.35"/px) is too low and it is resulting in poor calibrations.  You should update to the newer version of PHD2 [https://openphdguiding.org/development-snapshots/], then run the new equipment profile wizard (Connect Equipment > Manage Profiles > New using Wizard) and follow the recommendation it will give you to use binning 2.  After creating the profile you'll want to run the guiding assistant again and apply the recommendations.

2. Use 1.0s-1.5s exposures as 500ms is too short and causes guiding to chase seeing (more info in PHD2 Best Practices)

3. Enable sub-frames (brain, Camera tab) to reduce the camera frame download latency and better track the 11s high frequency RA PE component.

Otherwise, the mount appears to behaving quite well and I expect you will be getting some great guiding results and images.

Andy

[Anun Earthly-Child]

Hello Andy G and Andrew J

I just wanted to thank you for all your help with PhD2.  Following the good results using a camera guiding directly through my f/10 14inch I dug out my old Off Axis Guider and Lodestar Mk1 guide camera for the Starlight wheel.  I have run everything on two successive nights now both in an arc from SE to SW and also in an area where I was having guiding trouble between NE and E.  With the new arrangement and the settings suggested (with one exception) I am running 7m subs comfortably at f/10 through the 14inch and every image is spot on top of the previous one. Result!

Peter