PHD2 Guiding Issues

[gnomus]

I had a lot of issues last night with my guiding.  Last time out I had managed to obtain OK-ish guiding.  This was off a tripod in my garden.  Last night was in my new observatory using a pier-mounted set up that I had carefully polar aligned the night before.  I was expecting better results.  

My target was the 'coathanger' asterism.  After calibration, I got the following error message:

My other settings are as follows:

Do these look OK?  I am using my ZWO ASI120-MM-S attached to a ST-80.  

I tried recalibrating several times, but always got the same result.  My guiding was not very good with frequent deflections into +/- 4" territory:

I also kept getting Star Lost low SNR error messages.  I find it quite awkward to select a suitable star.  If I select something of reasonable brightness, the Star Profile window will show it clipped off at the top.  My SNR ratio on the chosen star tends to hover around the 4 mark. Typically, I will use exposures of between 1.5 and 3 seconds.  

Any help would be much appreciated.

[gnomus]

I should add that when I was getting these 'star lost' messages, skies were clear.

[Owmuchonomy]

Hi. One thing that stands out for me in your commentary is the star SNR. When guiding with PhD2 my calibration star usually has a SNR around 30 and say a range of 20 to 60. I use a lodestar and an Altair 80mm guide scope. I would suggest that a better star selection, higher gain on your guidecam or some related parameter needs attention. Having said all that there are folk on here better qualified than me to comment.

[gnomus]

Hi. One thing that stands out for me in your commentary is the star SNR. When guiding with PhD2 my calibration star usually has a SNR around 30 and say a range of 20 to 60. I use a lodestar and an Altair 80mm guide scope. I would suggest that a better star selection, higher gain on your guidecam or some related parameter needs attention. Having said all that there are folk on here better qualified than me to comment.

Thank you.  I agree.  My SNR looks like this:

The trouble is that this gives me the following in the Star Profile window:

My camera gain is set to 95% (which I think is the default).  If I push the exposure length, or choose a brighter star then I get a flattened peak in this window.

[libraryman]

Last night I was getting perfect guiding using a star with a SNR of 14 iirc..with a fwhm of 1.5..ish carnt remember exactly but...your choice of star is quite important and it can be the difference between medioca guiding and good guiding.

The other thing I note is the deviation in RA on your calibration, looks odd to me , I expect to see a straight blue line not one with a biggish kink in it.

I run the qhy5L-ii which might be similar to yours, I run with a gain of 12..... Lower gain will allow you longer exposures without saturation...

My seeing was good last night in the early dors, it too can inflict pain if its very bad.

Ray

[gnomus]

Another thing occurred to me.  During my calibration, there were around 20+.  These were followed by only 3 East  and a bundle of "Clear Backlash" steps.  There were also a lot of North steps (a bit more than West) followed by 5 South steps and a couple of "Nudge South"s.

This is my "Review Calibration" window, which shows a bit more information than a previously posted image:

[gnomus]

..... Lower gain will allow you longer exposures without saturation...

Ray

Thanks Ray.  And of course, less gain will give less noise, meaning that I can have a higher SNR ratio.  (I think I was getting hung up on the 'S' element and forgetting about the 'N'.)

So, in summary, I should aim for a higher magnitude star, but reduce its "brightness" (saturation), by setting a lower gain value in PHD.  

I was also a bit disappointed with the FWHM score (and sometimes it was much worse (>5) than the one shown in the image above) given that I focussed the guide scope using a Bahtinov.  A better guide star would probably give me a better FWHM score though wouldn't it?

[libraryman]

This made me think about my gain, I set mine in the ascom chooser, I can't set it in PhD for some reason, must be drivers but....iirc, PhD reports a gain of 95....odd when I know its set at 12

Your calibration figures in RA suggest to me that you need to balance east heavy with your weight, you should not get a nudge in east....south nudge is common

Ray

[Davey-T]

Got exactly the same message last night, just changed to side by side setup from piggy backed. PHD1 wouldn't move the mount to calibrate but PHD2 seemed to calibrate OK.

Like yourself it all worked fine before so more faffing about, at least the Moon was out so didn't waste any imaging time.

Dave

[gnomus]

Got exactly the same message last night, just changed to side by side setup from piggy backed. PHD1 wouldn't move the mount to calibrate but PHD2 seemed to calibrate OK.

Like yourself it all worked fine before so more faffing about, at least the Moon was out so didn't waste any imaging time.

Dave

It is a little frustrating when you 'invest', in an attempt to make things easier, but then things get harder!!!     

I can see that I am going to have to get one of those expensive mounts that don't need no guiding....    

[graemlourens]

Hey Gnomus.

You do not need to get an expensive mount at all.

I was fighting with Guiding for a long time (around 2 months) until i got it right.

If you want to go unguided, you will be pushing lots of cash out of your account. Starting at Mount, going over OTA to focuser. Everything has to be perfect. With guiding it does not

Stay on it and try going back to basics.

- have you got loose cables hanging/dragging

- Is everything tight (really tight)

- Are you in focus with your ST80?

- SNR of 4 is really really really low. My gain is set to 12, i think this could really be your problm with such a high gain. But i don't have that set option in PHD2, but you can choose it in the ascom settings probably of your camera?

I have the same setup (i assume you have ST80 piggybacked on ED80 ?) and it runs very well with PHD2, never have  a problem finding a star.

Regards, Graem

[gnomus]

Hey Gnomus.

You do not need to get an expensive mount at all.

I was fighting with Guiding for a long time (around 2 months) until i got it right.

If you want to go unguided, you will be pushing lots of cash out of your account. Starting at Mount, going over OTA to focuser. Everything has to be perfect. With guiding it does not

Stay on it and try going back to basics.

- have you got loose cables hanging/dragging

- Is everything tight (really tight)

- Are you in focus with your ST80?

- SNR of 4 is really really really low. My gain is set to 12, i think this could really be your problm with such a high gain. But i don't have that set option in PHD2, but you can choose it in the ascom settings probably of your camera?

I have the same setup (i assume you have ST80 piggybacked on ED80 ?) and it runs very well with PHD2, never have  a problem finding a star.

Regards, Graem

Thanks Graeme

I was (sort of) joking about the expensive mount.  

I paid some attention to the cabling - I have power for my filter wheel and CCD along with USB cables for wheel, CCD and ZWO.  I have bound these together (with velcro for now) and have attached them to the scope ring (half way up the train) and then to the equipment.  I don't think these are dragging.

I think everything is tight.  The pier and the mount are rock solid.  When I hold the scope end, however, I can 'jiggle' it around a tiny amount.  I wondered if this might be due to a loose saddle.  I am going to see if I can tighten this up this afternoon.

I focussed the ST80 with a Bahtinov mask.

I am sure that the SNR is the issue.  I am going to connect everything up this afternoon to see if I can adjust gain downwards.  Clear Outside is telling me that I might get an hour or two tonight to have another go....

Yes ST80 is bolted to the ED80 with no play of any sort.  The issue is not finding stars (there are loads) - it is that most of them blow out with my high gain.

Steve

[Pompey Monkey]

It might not solve the issue of the guiding, but setting the correct position in ASCOM (EQMOD) would stop the orthogonality issue in the sanity check:

Assuming that you are in the UK as your profile suggests!

[gnomus]

Thank you.  As I am using a CGEM, I cannot use EQMOD (I believe).

[gnomus]

OK ... I've connected up again.  In PHD 2 using the "ZWO ASI Camera", I  do not have access to the "Camera settings" button (it is greyed out).  However, using the "Brain", I can set the gain in this dialogue:

This seems to remain at 12 when I come out of PHD2 and restart it.  Does this seem correct?  Looks like I will need to wait for some darkness now to test it.

[MattJenko]

Funnily enough I had significant guiding problems last night and had the same issues as you, with my RA/DEC rate differences quoted at over 260%. I was getting very low SNR values as well, and put it down to me trying to do imaging close to the full moon (long exposure Ha images) as well as tree skimming. I will also have a play around with gain settings tonight and see if I can rectify the issues.

[libraryman]

I can see that you calibrated at 19degs Dec, if you can get a good calibration at zero Dec, you do not need to recalibrate when you go higher.

E.g. I did a calibration at Dec zero just east of the meridian. with 14 steps W..then 14 steps N...the returns to South & East don't do anything other than return the star to the centre.

I just reload it each night, where ever I image.

Ray

[glowingturnip]

Another thing occurred to me.  During my calibration, there were around 20+.  These were followed by only 3 East  and a bundle of "Clear Backlash" steps.  There were also a lot of North steps (a bit more than West) followed by 5 South steps and a couple of "Nudge South"s.

This is my "Review Calibration" window, which shows a bit more information than a previously posted image:

phdh.jpg

that's not a problem, that's your 'fast recenter after calibration' option coming into play - I use that too and find it gives me better calibrations.

You could try guide-speeds on the mount of 1x rather than 0.51x - Craig Stark, the original PHD guy, recommends that.  Your other settings look pretty much like mine.

Agreed that star looks too low in SNR, mine usually look like the below.  I wouldn't worry about getting the fwhm/focus spot on, I believe PHD actually likes a small amount of soft focus as it helps it determine the centroid better, and actually, it would help with any overexposure problems you seem to be having too (though your guidestar doesn't look anywhere near over-exposed to me ?)

One thing that does occur to me though is that that kink in the blue line (the Dec line) in the calibration screen suggests that the mount may have wandered slightly in RA whilst it was calibrating in Dec.  Then in your guiding graph you've got a big long meander in RA with corrections all going the other way before it was eventually able to bring it back.  Just wondering if there might be any mechanical reasons for RA tracking to be sub-optimal ?

[gnomus]

that's not a problem, that's your 'fast recenter after calibration' option coming into play - I use that too and find it gives me better calibrations.

...

One thing that does occur to me though is that that kink in the blue line (the Dec line) in the calibration screen suggests that the mount may have wandered slightly in RA whilst it was calibrating in Dec. Then in your guiding graph you've got a big long meander in RA with corrections all going the other way before it was eventually able to bring it back. Just wondering if there might be any mechanical reasons for RA tracking to be sub-optimal ?

Thanks for explaining 'fast recenter' to me.

There is no obvious mechanical issue. Are you suggesting that this could be a dragging cable? I will check all these again before tonight. Am I correct in thinking that it is best to bring the cables up to the middle of the scope before taking them down the pier, vertically?

When doing fast slews to objects, my CGEM is a little noisy and from time to time there is a brief, slight change in pitch in the sound of the motor. It has always done this, however, and I have had significantly better guiding. I had heard CGEMs could be a little on the noisy side.

[glowingturnip]

i wouldn't worry about the noise change, i think the slewing algorithms change slew rate when they get close to the object.

Haven't had the luxury of cabling up a pier I'm afraid - I just have a rat's maze of dangly cables from my tripodded mount, any that can drag on a camera I clip to elsewhere, and i just periodically check that no cables are getting tight anywhere

[gnomus]

i wouldn't worry about the noise change, i think the slewing algorithms change slew rate when they get close to the object.

Haven't had the luxury of cabling up a pier I'm afraid - I just have a rat's maze of dangly cables from my tripodded mount, any that can drag on a camera I clip to elsewhere, and i just periodically check that no cables are getting tight anywhere

Thanks again. I think my cabling is probably all right then. I am hoping that this is all some glitch caused by choosing too low an SNR star. I am hoping to get a chance to try it out tonight. I will report back.

[graemlourens]

smth that just popped into my mind, but probably has no relevance to your problem:

Do not bundle USB cables with power cables. Or so i heared I don't know if it also affects low voltage cables, but with bad shielded USB cables i believe it can affect the signal slightly.

In my case i was fighting Camera timeouts, and i changed a lot of things, one of those was recabling, and it disappeared, but no idea what really solved the problem in the end.

2 cents.

Regards, Graem

[gnomus]

Thanks. I did try guiding again the other night, but it was a lost cause. So much high cloud that I couldn't get a reliable fix. I was, however, able to test out that the Gain control was working, and I was able to get some higher SNR stars, which is encouraging.

Another thing occurred to me. My last attempts at guiding were on stars whereby the scope was pointing quite high in the sky. I wonder if that might also have been a problem. I will try calibrating on a Dec zero (or thereabouts) star next time.

Finally, I am never sure how good my guiding needs to be (or can be) with my current set up. When I examined the subs I got from the session above, they were not actually all that bad (I only got Red data and one Geen sub). The stars were round and not too large. I am still new to this and really have no idea what accuracy of guiding I can expect with my CGEM.

[MattJenko]

https://www.youtube.com/watch?v=LXFGRta98rs

This talk by one of the PHD2 devs was very enlightening for me. Hope it does the same for you.

[gnomus]

https://www.youtube.com/watch?v=LXFGRta98rs

This talk by one of the PHD2 devs was very enlightening for me. Hope it does the same for you.

Matt

Only half an hour in so far, but thought I should break off to thank you for posting this link.  I think it will be very helpful.

Steve