Why is my guiding drifting?

[lukebl]

Hi folks,

I feel I should know the answer to this, but can't get my head around it.

What is causing this guiding issue? These are 2 minute exposures, each separated by about 20 minutes. Guiding was with an ST80 guidescope and PHD guiding, and the graph was nice and level. Polar alignment seems to be good.

As you see, the stars are drifting in a steady and even trajectory across the field, apparently solely in RA, even though PHD says guiding was good. I feel it's something very obvious which I'm missing! I thought flexure would be less smooth, and poor Polar Alignment would cause field rotation?

Incidentally, you can see 16th magnitude Asteroid 10811 Lau in the centre of the field moving in a different directon to the stars.

[Davey-T]

Can you post your settings, could just be RA aggression too high.

Dave

[lukebl]

3 hours ago, Davey-T said:

Can you post your settings, could just be RA aggression too high.

Well, the RA agression was set to 100%, so I've reduced it to the default 70. But that wouldn't explain why the stars were drifting continuously off the field of view.

I guess it might be flexure. I've started using my hefty old SXVF-H9 as a guide cam, and its weight might be making the guidescope sag. I have an off-axis guider which I'd use if I could, but I just can't get any stars to focus with it.

[alexbb]

It might also happen that you have a good PA and small periodic error and you guide on a hot pixel. How level is your guiding graph? Small chance though.

Edited April 7, 2017 by moise212
chance

[Jannis]

I have the same problem with my scope using a finderguider. Graph is relatively nice, but over the hours it drifts way off from where it started. Could be flexure, but i dont know. Hope someone Else know. If its indeed flex we are struggeling with, i think at least i will concider going for oag instead - i just dont know what i neef to buy...

[kens]

Try this:

Open two PHD2 sessions. You open the second one with "-i 2" on the command line

Set up the first session as usual with your guide cam

Set up the second session with your imaging cam and disable guide pulses

Start the two sessions guiding simultaneously

Session 1 will guide the scope whilst session 2 will just record the movement of the guide star in the imaging scope

Any deviation in session 2 is due to differential flexure. Working out the cause is still a challenge but the shape of the graph can help

[Thalestris24]

Hi

Try running the phd2 guiding assistant - it will give you lots of info. Maybe your balance is off? It looks like guiding is doing its job as you don't have star trails. What does your Phd2 target look like and what value of RA Osc are you getting? You want the latter to be around 0.5-ish and you want the target to be closely spaced around the centre - not to one side.

Louise

[lukebl]

10 hours ago, Thalestris24 said:

..... It looks like guiding is doing its job as you don't have star trails....

That's precisely the problem. The guiding isn't doing its job. The stars are trailing in the same direction as the drift, as you'd expect. I've decided it must be diff flex. Just need a clear night to check it agin.

[Thalestris24]

7 hours ago, lukebl said:

That's precisely the problem. The guiding isn't doing its job. The stars are trailing in the same direction as the drift, as you'd expect. I've decided it must be diff flex. Just need a clear night to check it agin.

Oh, ok - looked ok at a glance! As I said, check your balance in all 3 axes, and also check your pa.

ps I assume you're tracking at sidereal rate? Dithering off?

Edited January 28, 2017 by Thalestris24

[ollypenrice]

I have to say that it looks very like PA to me. On what basis do you say that PA seems good? The simplest test is to take a 'first' image and a 'last' and combine them, then look at the edges of the frames. Any disparity in the alignment of frames, assuming reasonably round stars, comes from PA. 

 

Olly

Edited January 28, 2017 by ollypenrice

[nucdoc]

I would also say it looks like PA.  I see this when I image planets, for planetary imaging I use a secondary mount in my back yard, it is set up with simple alignment with a polar scope, I do not spend time doing drift etc, as this is not needed when I use a video camera shooting hundreds of frames over only 60 seconds.  During the exposure the planet slowly drifts, and eventually I have to put it back in the centre after I have shot several 1 minute data sets. Looks just like what you are showing

Mark

[lukebl]

Sorry for reviving this thread, but it's driving me bonkers. To recap, the field of view of my camera drifts over time, even when guiding seems perfect. My polar alignment is within 1 arc-minute of the pole, according to both Sharpcap and PHD, which I would have thought was close enough. Yet, the frame drifts evenly and steadily by a few arc-minutes over the period of an evening's captures and not rotationally. The stars appear round, but only because I restrict myself to 4-5 minute exposure max.

I am using an ST80 guidescope, which is well clamped down to reduce flexure. I get identical effects when using a finder-guider. I would use my Off-axis guider, but have never managed to locate a guide star with it.

Edited April 7, 2017 by lukebl

[Thalestris24]

Hi

What happens if you run unguided? What does the PHD2 guiding assistant say? What does your guide log say? PHDLab?

It looks to me like you're mostly drifting in RA so could be a PHD settings issue or a balance issue.

Louise

[lukebl]

2 hours ago, Thalestris24 said:

....What happens if you run unguided? What does the PHD2 guiding assistant say? What does your guide log say? PHDLab?

It looks to me like you're mostly drifting in RA so could be a PHD settings issue or a balance issue....

I've had a look at the log with PHD2 Log Viewer, but have absolutely no idea how to interpret what I see!

I still don't understand how it could be balance. If PHD says it's locked-on and guiding, and the guidescope is firmly fixed with no flexure, I don't see how it could possibly drift like that. Surely it should stay put, or at least rotate around the guide star if it's PA?

EDIT: Here's the PHD2 Log Viewer. Is it good or bad?

Edited April 7, 2017 by lukebl

[Thalestris24]

Hi

It looks like you are dithering? When I use dithering I do it in RA and DEC and select the spiral pattern. You can use the Guiding Assistant to see how your mount is performing with guiding off. You should then just see the effects of periodic error and seeing and get feedback on PA (though it seems to return instantaneous values).

Louise

[lukebl]

No, I'm not dithering. By the way, the mount has had the Rowan Belt mod, if that makes any difference.

[Thalestris24]

1 hour ago, lukebl said:

No, I'm not dithering. By the way, the mount has had the Rowan Belt mod, if that makes any difference.

It might. I have the Rowan mod on my Heq5 and found the balance became quite critical. No East heavy, and accurately balanced in all three axes - RA, DEC and vertical. I had to adjust mine to in order to ensure the guiding corrections in PHD2 were centred around the bullseye. If they are to one side, you'll get the drift.

Louise

 

[StuartJPP]

I had a similar problem a while ago and it was caused by differential flexure. I was using a large and heavy Canon lens as an OTA and the supplied tripod foot is only attached at one point (the rear)...Once I braced it at both the front and the rear of the lens my problem went away. Now if I don't dither I can get well over 45 minute subs with no trailing (I only do 5 minute subs anyway).

In my case it was the actual lens/OTA that was flexing...In your case it is probably the guide scope itself if your problem is indeed flexure.

You could point the scope in the opposite side of the sky and see if the movement is still the same direction with gravity acting on your setup in a different way.

Just remember that the flexure can be totally imperceptible to the naked eye...We are talking tiny, tiny movements.

[Oddsocks]

I am not a PHD user and don't know the software that well but there is interesting information in the log window, statistics box shows RA RMS was 97.19 pixels against DEC RMS of 1.79 pixels, graph shows clearly periodic error but the scale is too large in the posted image to be able to see the guiding corrections and Luke reports the stars are trailed in the RA direction. To me that suggests that PHD is locked to the guide star but no guiding corrections are being issued to the mount. Luke's PA is good so no drift in DEC but no mount corrections would cause star deformation in RA direction due to PE and gradual drift over time in RA due to mount tolerances/ belt mod/wrong RA rate set, etc etc.

To fault find further and as already noted above I think you would need to set up normally take a series with guiding enabled and then immediately after another series with guiding disabled. If there is no difference between the two series that would indicate that for some reason PHD guiding corrections are not being issued to the mount even though PHD is locked to the guide star.

Other than that you could go into ASCOM diagnostics and enable logging then read the log after a guiding session to see if guiding corrections are being sent to the mount (assuming you are using ASCOM guiding of some kind).

The PHD guiding log can be read like a normal text file and it may show the guide star's centroid position throughout the run, if the x and y centroid position is the same at the end of the run as it was at the beginning but the main camera image is drifting then that would indicate either flexure, rotation, or the guide camera image is not being updated.

Depending which capture software you use in some software packages you can configure storage of the guide camera images along with the main camera images, a quick comparison of the start and end run images from both cameras would confirm flexure or rotation, if the guide camera images show no drift but the main camera does then = flexure or rotation, if drift is seen in both guide camera and main camera images then guide corrections are not being sent to the mount.

How far off axis is the guide telescope compared to the main telescope? is the guide star selected in the same frame as the main camera? A big differential and a little polar misalignment would create drift due to rotation.

Edited April 8, 2017 by Oddsocks
Added guide camera image comparison notes.

[ollypenrice]

What happens if you take a first and a last sub from a run and then align them on the stars and combine them? The stars wll align but what about the frames of the two images? If those don't align then that's because of your PA. I'm wary of the claimed PA measurements offered by the various software packages because I've seen good reason to doubt them, using the test I describe.

Olly

[kens]

1 hour ago, StuartJPP said:

I had a similar problem a while ago and it was caused by differential flexure. I was using a large and heavy Canon lens as an OTA and the supplied tripod foot is only attached at one point (the rear)...Once I braced it at both the front and the rear of the lens my problem went away. Now if I don't dither I can get well over 45 minute subs with no trailing (I only do 5 minute subs anyway).

In my case it was the actual lens/OTA that was flexing...In your case it is probably the guide scope itself if your problem is indeed flexure.

You could point the scope in the opposite side of the sky and see if the movement is still the same direction with gravity acting on your setup in a different way.

Just remember that the flexure can be totally imperceptible to the naked eye...We are talking tiny, tiny movements.

I've also been chasing down a differential flexure problem with similar symptoms. The guide graph shows that for half the time at least the guide star was centred, yet the stars in the image are moving. By the way, screen shots are pretty useless for analysis - you need to attach the guide log itself.

Attached file shows the images from the session stacked without alignment so the movement is clearly shown. I think one of those hockey sticks is the guide star but can't be sure. The guide star was kept centred in the guide scope the whole time.

To home in on the problem I pointed at the pole and took a series of images from both guide camera and imaging camera as I rotated the mount 90 degrees in RA East and West. I did this by running two PHD2 sessions as I described earlier and File | Save As at each point. The combined traces are attached. For a bit of fun I edited in lines showing the movement of BQOct.

The plots clearly showed a problem with the imaging scope as the trace should have been circular in both cases. So at least I could eliminate at least half the possibilities. Two possibe causes came to mind: the dovetail mount and the camera attachment. Some preliminary adjustments to the dovetail have been promising - an after shot had a circular trace but I'm not sure it will stay that way.

In short - there is no easy solution without gathering diagnostic data. Hope this technique may help you. Its work in the southern hemisphere as there are some nice bright stars near the pole

 

 

TEST-all-30s.tif

[lukebl]

Thanks, folks. A lot of things to think about there!

Looking at my EQMOD settings, I noticed that the PulseGuide Settings are at the minimum rate of 0.10. Is that too low, or would that not have any effect on the problems I'm having?

 

[kens]

It is unrelated to the drift problem. Assuming what we can see of your guide log is representative, PHD2 is keeping the guide star centred so there is no major problem there. 

So if the stars in the image are moving the most likely reason is differential flexure.

If you could attach your PHD2 guide log it would really help the analysis of your problem by removing a lot of guessing and assumptions.

[Thalestris24]

Hi

I think when calibrating in PHD2 the pulse guide settings need to be at max (0.90) - I leave mine on that anyway. RA aggression settings in PHD2, on the other hand, need to be adjusted to what works best. As I said before, the bullseye in PHD2 is a good guide for monitoring drift over a session.

Louise

[iapa]

On 07/04/2017 at 16:54, lukebl said:

I've had a look at the log with PHD2 Log Viewer, but have absolutely no idea how to interpret what I see!

I still don't understand how it could be balance. If PHD says it's locked-on and guiding, and the guidescope is firmly fixed with no flexure, I don't see how it could possibly drift like that. Surely it should stay put, or at least rotate around the guide star if it's PA?

EDIT: Here's the PHD2 Log Viewer. Is it good or bad?

Hi,

I notice your graph is pixels. 

I have often seen an various places be people who know these things that you are better using the arc-sex/pixel scale - for  variety of 'technical' reasons, but I think one of them is their better reflects how the star is moving in relation ration to your optics.

Will have a dig around incase I've kept some relevant links