I have noticed a definite, slow drift whilst guiding with PHD. I am guiding through an ED80 with a DMK FW cam and GPUSB, imaging through another refractor. Both are mounted side/side on a G-11. All PHD default settings used, 2s exposures. G-11 set to photo mode.
PHD calibrates without a problem.
Drift appears to be in RA.
I don't think it is flexure, as both scopes seem very solidly mounted, and the drift seems sustained in one direction.
Though I've only used it on a handful of occassions, I haven't changed any default settings, and it appeared to guide fine as long as the star was bright enough (exposure length correct). I let it do the auto-calibration and didn't touch it.
It's annoying... The other night I did ten minute exposures on the horse head and it was fine.
Should the guide star be in the centre of the guide scope fov or does that not matter?
I've guided on stars in both the centre and towards the edge, but not at the very edge. I don't suspect it should make much difference as long as it's still visible.
I have the same issues Lee, with a side-by-side set up. When I switch to my ST7 with the on board guide chip, the problem does not manifest, so it points to something in my side-by-side set up.
I've yet to investigate this any further due to lack of an opportunity.
One has to be heavier then the other, unless both are the same size and have the same equipment attached to it.
Imagine a circle, place the side by side instruments, with the centre of the side by side conciding with the centre of the circle, in the circle. Now, do you think if you would rotate the circle that it would stay in one postion if you stop the rotation or would it continue to rotate and find it own balance point, heaviest side of the side by side.
I have found the same problem, does not matter how accuratly you are aligned with the pole, with Newtonians, SMC, etc. if you are not balanced aroung the circumference you will get drift in RA. In one position the heavier side will pull the scope, the drive will be constantly fast, in another position it will be the oposite.
Found that from my own experience, and from others, in 40 years of being in this hobby, a lot of it in deep space photography, the old way, that radial balance is just as important as alignment. Doesn't matter how good your mount is.
For a Newtonian I normally make two end plates with a short stubby shaft, bolt, in the centre. Sit the tube with the endplates attached in a couple of vees. Attach the focuser to the tube and balance around the circumference untill the tube stays, anywhere I rotate it, in place. I then attach the camera, or other equipment, one at a time, and rebalance, etc. The result is, that I now know exactly where and how much weight I have to attach to keep everything in balance. Sliding counterweight bar in the right position, with various weight works wonders, especially in the dark.
SMC and Refractors I used to balance, around the circumference, by having a board with two ball bearings, arranged in a vee, at both ends of the board. Sit the instrument on the ball bearings and watch it rotate of its own accord. the counter balance.
Two side by side instruments are harder to balance but the above might give you some ideas.
One has to be heavier then the other, unless both are the same size and have the same equipment attached to it.
Imagine a circle, place the side by side instruments, with the centre of the side by side conciding with the centre of the circle, in the circle. Now, do you think if you would rotate the circle that it would stay in one postion if you stop the rotation or would it continue to rotate and find it own balance point, heaviest side of the side by side.
I have found the same problem, does not matter how accuratly you are aligned with the pole, with Newtonians, SMC, etc. if you are not balanced aroung the circumference you will get drift in RA. In one position the heavier side will pull the scope, the drive will be constantly fast, in another position it will be the oposite.
Found that from my own experience, and from others, in 40 years of being in this hobby, a lot of it in deep space photography, the old way, that radial balance is just as important as alignment. Doesn't matter how good your mount is.
For a Newtonian I normally make two end plates with a short stubby shaft, bolt, in the centre. Sit the tube with the endplates attached in a couple of vees. Attach the focuser to the tube and balance around the circumference untill the tube stays, anywhere I rotate it, in place. I then attach the camera, or other equipment, one at a time, and rebalance, etc. The result is, that I now know exactly where and how much weight I have to attach to keep everything in balance. Sliding counterweight bar in the right position, with various weight works wonders, especially in the dark.
SMC and Refractors I used to balance, around the circumference, by having a board with two ball bearings, arranged in a vee, at both ends of the board. Sit the instrument on the ball bearings and watch it rotate of its own accord. the counter balance.
Two side by side instruments are harder to balance but the above might give you some ideas.
You might have hit the nail on the head Alex!
I have a similar prob with my newtonian in a side by side setup. The guidestar does not shift in PHD guiding but between every image there is a slight image shift always in RA just like what Lee is getting and I have contributed that to flexure in the OTA. But that flexure might originate due to the OTA not being balanced correctly?
Having a heavy focuser and a DSLR attached to one side of the tube would make it heavy on one side of the OTA. Maybe I should attach a counterweight opposite the focuser?
Do you have a pic of your side by side setup Lee? may help?
cheers Gary
Hi All,
Lee, when I played with side by side setup, I had very similar problems with PHD.
Fix: I placed the dovetail rail on the ground (with cameras & scopes attached) & placed a dowel or pen in the center point, where I thought the balance should be. I worked out by pressing with a finger , where I had to adjust the weight to to make it balance. Crude, but it worked for me.
PS, do not forget to have your scopes almost focused, as the weight moves again.
CS
Hmmmmm, very interesting, I have a similar set up on the G11 but mine is a triple side by side, i feel it is balanced pretty well, and the alignment is good, cant say that i have had a problem with it to date.
Could that be due to the fact that the G11 has the Maxxon motors, and not the normal stepper motors.
That's a lot of info!
Will post a pic soon - I balanced the two scopes as per this site - and they do balance well. I was thinking about this at work - the Horsehead and Tarantula were both east of the meridian, whilst the helix last night was west - maybe a balance issue.
Does PHD make adjustments in RA and DEC - I thought it did. If it's a balance issue, shouldn't the guide star shift with the image? - and thus guiding should correct for the imbalance?
Vexing.....
.
Does PHD make adjustments in RA and DEC - I thought it did. If it's a balance issue, shouldn't the guide star shift with the image? - and thus guiding should correct for the imbalance?
Vexing.....
One should imagin so, otherwise what good would be automatic guiding. Not that I know anything about PHD as yet.
As for correction for imbalance, hmmmm, maybe the motor is running at its maximum output or you have some clutch slippage.
This is a stack of 3x 10min exposures at the helix, complete with CCD fog
I stacked these without aligning to demonstrate the drift over 30 minutes....
I'm going back east of the meridian tonight, I need some success!
Lee, can you post your PHD settings?
Also, is there a way to alter the guide rate on your mount?
On the EQ6 & using a 1200mm refractor, I can use .50 or .75
With a smaller scope, I had to increase the rate to 1.
Seems something has changed, cause it was working a treat...
This is a stack of 3x 10min exposures at the helix, complete with CCD fog
I stacked these without aligning to demonstrate the drift over 30 minutes....
I'm going back east of the meridian tonight, I need some success!
This reply implies you get better results when tracking targets in the East if so then I am pretty sure it is a balance issue. I have to deliberatly unbalance my scope to be heavy in the East to prevent this behaviour, this means a rebalance after a meridian flip too, not to drastic - I have to move the CW up the shaft to the stop, quick and easy.
I thought I might have to do that...... In the East she's perfect - this shot is 10 minute subs....
Question - why isn't PHD screaming that the star is wandering off while this happens??
You’ve given me a bit to think over here too – cause my Vixen Atlux is about 1 arc second per minute off – but I wonder if its consistently always too fast or too slow?
If East of the meridian its too fast and West its too slow – I’d guess that weight balance would have to be the culprit – my counterbalance weight is to far down. If the opposite occurs my weights are too far up. Perhaps my near enough to the balance point isn’t good enough!
I love your idea of balance things around two ball bearings tapped into a sturdy wooden block! I will definitely do this to confirm if I have set my side by side dovetail plates at the correct point of suspension to really achieve the best balance possible.
The other balance factor is how I’ve set my counterbalance weights. I’ve held things with the counter weight bar level and just with my finger pressed lightly up and down until the pressure seems the same. Maybe I should use scales to confirm this pressure is as fine as I can determine it!
I have a lot of backlash in DEC and none I can see in RA – in my hand controller I set DEC backlash to 000 and leave RA at 100. I am wondering if this might cause PHD to chase the seeing (if it can't account for backlash properly - and possibly see-saw as a result) – as I see no noticeable DEC drift – but this very slow RA drift. Can I suggest try setting RA backlash to 0 if that's possible and just let PHD control things and see if that alters things - I will too!
I noticed with the scope facing South East at 30 degree elevation PEMPro said sideral rate was 0.994977 (I expected 1.00000) and I think it was slow on RA, facing West from memory it was a smidgeon too fast – so it hints I have the weights too far down!
This has been a terrific find for me if that’s the issue with my RA slight speed problem. I’ll keep you posted and play with PHD some more and see if we can find the best way to tune PHD!
PHD Guiding drift
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