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Author Topic: Annoying problems with PHD2  (Read 5444 times)

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Offline mikejenningz

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Annoying problems with PHD2
« on: 10:16:14, 12 March, 2015 »
Hi Guys

 Is has been a little while since I last posted, but happily I’m getting back into my DSO astro imaging again.

My setup is a Celestron C8 SCT , guided by a Skywatcher ST80, using a QHY5 guide cam.  I use an unmodified DSLR for my main imaging.

The guide scope has 400mm focal length and the QHY5 works well with it.   Last night the sky was clear but seeing conditions maybe a bit questionable. I planned to do an imaging session targeting M81. I decided on 3 minute subs, and setup PHD2 guiding. Although I did manage to get quite a few successful shots, a problem kept happening whilst guiding.

I have my mount setup to slightly ‘north’ drift in DEC so that I just need to have PHD correct the DEC axis with ‘North’ setting on PHD. This is a tip I read somewhere and usually works pretty well. On the history graph I see the red DEC line slowly drift north and then be corrected with a few pulses as the mount moves the red line slightly above centre. However, last night I seemed to randomly get large pulses sending the red line on the graph southward, enough to leave the graph.  It didn't happen constantly and I got quite a few good 3min subs.  But on average maybe every 5 mins, and very annoying.

 I ‘m puzzled about what could be causing this?

 The guide star I had to use was not perfect and the SNR often was between 12 and 18, I adjusted the gamma setting to try to improve this. Later on in the session I found a much brighter star to guide on but the same problem persisted even with a SNR of 30. Also I tried slightly de-focussing the guide star, a technique I had previously read, maybe to combat poor seeing, and even out star movement?

 Also in PHD2 the ‘star profile’ spike would, whilst maintaining the spike shape, dance to the left or right briefly sometimes before settling back in the middle. Could this be the effects of ‘poor seeing’?  But this did not always match up with the excessive DEC pulses that destroyed the guiding. Also on occasion, the ‘star profile’ spike, would lose its shape but only for a few seconds, before returning to a good shape again. I also wondered if PHD calibration may need a tweak and could be responsible for the problem. Often my setup can take up to 30 or more steps on each of the axis’s calibration.

 On PHD2s advanced setup I have calibration step set to 950.
 Max duration is set to 2000ms in RA and DEC.

I've wondered if it could be flexure between the guide scope and main scope, but I've rarely been troubled by this in the past.
I think I need to do some more experiements, and unfortunetlly I don't have a screen dump of the graph to show you.  I never have any problem with the RA guiding.

Thanks,

Mick.   ;)

Offline Roger B

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Re: Annoying problems with PHD2
« Reply #1 on: 15:09:27, 12 March, 2015 »
Is this a first time with Phd2 ?   

Have to set up the calibration steps calculator to help,   It could be set up wrong for these steps

Roger
Scopes - TS65q, W/O 110 Triplet
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Cameras - SX694, QSI583WSG,Lodestar Guidecam, GP-Cam, QHY5II.
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Offline mikejenningz

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Re: Annoying problems with PHD2
« Reply #2 on: 10:32:21, 13 March, 2015 »
yes i'll have to have a check that the calib steps are set right.

I'm possibly thinking the DEC guide problems could be down to flexure, or just really poor seeing?

thanks,

Offline Roger B

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Re: Annoying problems with PHD2
« Reply #3 on: 11:02:15, 13 March, 2015 »
That's going to be trial and error I'm afraid.       Could be one could be both,  could be something else

Roger
Scopes - TS65q, W/O 110 Triplet
Mount - EQ6 PRO via  Eqmod,  Rowan belt modded
Cameras - SX694, QSI583WSG,Lodestar Guidecam, GP-Cam, QHY5II.
TS 9mm OAG
Quantum 7x1.25 EFW, Baader LRGB,Ha,OIII,SII Filters 
Quark chromo

SGPro, Pixinsight.

************AND NO TIME TO USE IT************

Offline mechanoid

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Re: Annoying problems with PHD2
« Reply #4 on: 18:36:39, 16 March, 2015 »
I'm fairly new to PHD2 as well, and I too have a QHY5-II on an ST80 guidescope controlling a Vixen GD-DX mount.  I've spent many moonlit nights understanding how to set the guiding, particularly the dec. axis.

I occasionally got the odd inexplicable spike in dec, which I think I've fixed.  Very close inspection of the final drive spur gears revealed a few tiny, hard particles in the mesh (otherwise known as muck !)  I removed the gears and thoroughly cleaned them in white spirit, then re-greased them and replaced wearing latex gloves to keep my grubby mitts off !

I've finally managed to get the tracking within +/- 2 arc secs RMS over a 20 minute period. I used a North only, low pass algorithm, with a slope weight of only 1, and a min move of 0.5 pixels. And an accurately aligned polar axis - PHD is a really great tool for doing this.

Do you have any log files you could post ?

On another issue, (hopefully unrelated), does your QHY5 occasionally fail to connect to the USB when first plugged in ?

Dave
Skywatcher Quattro 200mm C/F, Vixen GPDX, QHY10
Skywatcher ST80, QHY5II, PHD2
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Offline mikejenningz

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Re: Annoying problems with PHD2
« Reply #5 on: 13:39:58, 19 March, 2015 »
Hi Dave, thanks for the post.  I'm pretty nervous about taking the drive spur gears apart and hope it doesn't come to that.  I guess if it was any such problem with the gears it could show up in periodic error though? And therefore be predictable.

Congrats on getting your tracking so good.  I don't have any log files to post just yet, but hope to post some later.
Regarding the QHY5, I've never had problems with the USB connection, it seems very stable.  I always connect the QHY5 to the mount via ST4 cable first before plugging the USB to the laptop.  Failure on this sometimes gives me connection problems.

Mick

Offline mechanoid

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Re: Annoying problems with PHD2
« Reply #6 on: 17:16:10, 19 March, 2015 »
Hello Mick,

On the GP-DX mount the motor gearbox is connected to the slow motion worm via a pair of equal size brass spur gears that are easily accessed under a plastic cover. These are what I cleaned. I certainly wouldn't go delving into the reduction gearbox though.

I guess dirt on the RA axis would show up in the periodic error.  If its on the dec. axis most of the time it won't show at all. But if you're operating just as the dirt is in the mesh then the control will be very erratic and difficult to stabilise. The worm and wheel are probably far more tolerant of dirt though. I've never really wanted to clean all that grease off.

Can you move your axes by manually turning the worm directly ?  I found this invaluable in being able to "feel" how smooth the drive was when adjusting the mesh and balance.  Personally, I found it much better to balance both axes as accurately as possible.  When slightly out of balance (agreed, this may suit some mounts) the slip-stick and average torque increased significantly. Manufacturers of high end  mounts always stress that drive smoothness is the most important thing,  and I totally agree.

My intermittent connection problems were caused by a faulty QHY5 - a new one is on its way.

Keep on tracking !

Dave
Skywatcher Quattro 200mm C/F, Vixen GPDX, QHY10
Skywatcher ST80, QHY5II, PHD2
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Offline mikejenningz

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Re: Annoying problems with PHD2
« Reply #7 on: 11:49:32, 14 April, 2015 »
Here's an update on this, after my latest imaging session the other night.

Guiding starts of fine, but suddenly the dec guiding dives off the scale.  See examples, you’ll notice the red line take a drive.  Quickly though I stop guiding and restart and it continues as If nothing has happened.  Although it does ruining the current imaging exposure.   I was doing 2 min subs, and I’d usually get 1 sub ok, and it would drop sometime in the next sub.  It wasn’t always predictable, sometimes I’d get 2 or 3 subs.
My imaging session lasted just over an hour and I calculated the intervals in seconds between these events. 
From the start of the session the first event occurred after 359seconds
Then at: 245seconds and then:
314
114
148
137
130
383
269
217
finally 227 seconds 

This did allow my to get at least 17 good 2min subs of M51, but these intervals seem pretty random.  I wondered if the intervals could be related to the periodic error cycle of my mount but I think that is in the region of 600 seconds, and the events are occurring more frequently than that.

M51 was very high in the sky so a wouldn’t have thought scope balance would play a big part in this problem, but it is something I’ll need to check.

I was using a PHD calibration step size of 200 but running the calculate for the guide scope, the value  comes out at 400 so maybe should have change the value to 400.
I’m pretty puzzled at the moment…
Mick.

Offline mikejenningz

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Re: Annoying problems with PHD2
« Reply #8 on: 11:52:40, 14 April, 2015 »
Sorry, Not sure why the pics have appear quite so big , but if you move the scroll bar at the bottom you can see them all.

Offline Einari

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Re: Annoying problems with PHD2
« Reply #9 on: 13:33:19, 14 April, 2015 »
Don't know what the case could be.
Does this happen wherever the scope is pointed ?
What is your guiding exposure ?
Maybe could try if nose heavy scope would help ?

You could have resized your images before attaching.
Was it so that linked images are automatically resized but not attached ?
Tapio

Celestron C8, C9.25" & Hyperstar, iOptron CEM60-EC, TS 80 triplet,  Coronado PST, SXV-694,  ASI120MM/ASI174MC Cool

Offline psjshep

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Re: Annoying problems with PHD2
« Reply #10 on: 14:33:02, 14 April, 2015 »
I resized your photos to 800x600... can see them better

Phil
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Offline mikejenningz

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Re: Annoying problems with PHD2
« Reply #11 on: 14:48:23, 14 April, 2015 »
Thanks Phil for resizing  ;)

Does this happen wherever the scope is pointed?  yes it seems to have the same problem weather pointing east, west or straight up.

What is your guiding exposure?   In this case I started with 1sec and then went to .5sec.  Same issue with both.  I usually stick to .5sec

Maybe could try if nose heavy scope would help?   Yes , I will give it a try.  Also wondering about PHD settings.  In this session the red line seems to spend quite a long time above the centre line.  With Dec guiding set to 'north' I would have expected it to send a stronger pulse to slightly drop below the line and allow a slow drift back up to centre.  Previously this seemed to work, but will have to experiment more to find what works best.
 

Offline dciobota

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Re: Annoying problems with PHD2
« Reply #12 on: 17:06:57, 14 April, 2015 »
 Mick, I'm not familiar with phd2, just the original phd, but this looks to me like a classic case of backlash with a balanced dec setup.  It's likely that the spur gear is "bouncing" off the worm gear, and since the "bounce" may be to the south, there is no correction in that direction (since you've set to north only) until the spur gear settles back on the worm. 

 I see in your first post that you do have the dec offset, but I'm thinking as Tapio mentioned that a more nose heavy balances might help.  Another thing to try and fiddle with the calibration steps... maybe this is not as relevant with phd2, but with the old one I found there is a sweet spot in the number of steps to calibrate.  If it took less than ten steps to calibrate or more than twenty, the guide corrections would tend to overshoot or undershoot.  Not sure if that's still the case with the new version.

 Hope I didn't muddy the waters more.
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Offline mechanoid

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Re: Annoying problems with PHD2
« Reply #13 on: 17:34:03, 14 April, 2015 »
I've had the same sort of problem. Mine was caused by the worm/wheel mesh being slightly too tight. Driving the worm by hand made this obvious.  I suggest you try the effect of unbalancing on the "feel" of the drive. I always aim for perfect balance.  Any imbalance results in a horrible slip-stick feel in one or even both directions.  It mostly worked OK last night, so it seems to have solved it for me.

Dave
Skywatcher Quattro 200mm C/F, Vixen GPDX, QHY10
Skywatcher ST80, QHY5II, PHD2
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Offline mikejenningz

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Re: Annoying problems with PHD2
« Reply #14 on: 10:45:47, 15 April, 2015 »
yes, thanks.  No its not muddying the waters, its really helpful advice.

I do hope the problems are down to either backlash or scope balancing and are not anything more serious.  I need to change the calibration steps setup, or I think it took more than 40 steps to start guiding.

I would like to get more hands on with the mechanics of the mount , but do not have the confidence to go under the bonnet.  I want to learn more about the workings of my mount , or even mounts in general.  Are you aware of any good online resources?

thanks,
Mick.

 

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