PHD Guiding drift

[Lee]

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.

Should I have changed any settings???

[iceman]

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.

[h0ughy]

PHD should keep it centered, maybe your drift alignment is out, remember you had a guess after using the PAC to find SCP?

[Lee]

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?

[Lee]

Dave - I was under the impression that then i'd get field rotation. My alignment is within 5' of the SCP?

[iceman]

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.

[Dennis]

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.

Cheers

Dennis

[[1ponders]]

Lee is the drift observable on the screen over time or only in the results of your images?

[Aster]

Lee

How have you balanced the side by side scopes?

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.

[Garyh]

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

[DeanoNZL]

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

[leon]

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. :shrug:

Leon :thumbsup:

[Lee]

That's a lot of info!
Will post a pic soon - I balanced the two scopes as per this site (http://www.aozc64.dsl.pipex.com/tips/g11gemini.htm#balancing) - 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..... :shrug:

[Aster]

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.

[iceman]

My thoughts too, Lee. If it was unbalanced, surely it would just make more corrections?
How does it manifest itself in the final image?

[Lee]

This is a stack of 3x 10min exposures at the helix, complete with CCD fog :rolleyes:
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!

[DeanoNZL]

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...:)

[JohnH]

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.

[Lee]

I thought I might have to do that...... In the East she's perfect - this shot (http://www.iceinspace.com.au/forum/showthread.php?t=26062) is 10 minute subs....
Question - why isn't PHD screaming that the star is wandering off while this happens??

[g__day]

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!

[Aster]

Lee,

Have you read the Advanced Settings and played around with the RA Aggressiveness Setting.???

Just a thought

[Lee]

No - all settings are default so far.... will have a play soon though....

[g__day]

Ah, I adjust min movement before a guide pulse is sent, and RA aggressiveness down - yo yoing is my main enemy.

With that said I re-balanced the scope - two 5kg weights were a cm too far down I reckon - now even a 10 minute unguided shot at 2300mm focal lenght looks far better - given across is 30 arc minutes - I'd say under 20 arc seconds of drift in RA - huge yah!

My choice of autoguide camera - a DSI on a 1500mm focal length MAK is probably now limiting me. Have a look at a guided 10 minute versus unguided 10 minute at 5 second PHD adjust intervals on a real faint star. Only a botched levels and curves applied here - look for stair trails.

Surprising the unguided one is on the left! I'd say re-balancing the shot so far has created terrific results for me! Now if I could juist autoguide better :)

[Garyh]

I have been giving it some thought also to try to work out if its a balance or a flexure problem between the 2 OTA`s.?
My thoughts are setup guidescope with camera and PHD and have a illuminated reciprocal eyepiece in the other scope at high power with a star centred. Make sure you are well east or west of the meridian. Let PHD do its thing..Does the star stay centred in the other scope? Give the scope a very slight tap every now and then. does PHD pull it back to where it was or does its position change slightly every time in the same direction or is there drift in the same direction?
Now change the eyepiece and the camera around and try the same things again? does the star drift or move the same way or is it now moving the opposite way?
My thoughts are if the movement is reversed then there`s some sort of differential flexure betwen the 2 scopes but if it still keeps moving the same way it maybe some misbalance in the setup?
I`m not sure if this is right? someone correct me if my idea is mislead!
I will give this a go myself if this cloud goes away anytime soon!
cheers Gary

[bird]

Something I see with my own scope is that the primary tilts a bit in its cell as the scope moves across the sky, due to the very "soft" mounting system I use. This means that it slowly points away from my guide scope even though the two are firmly bolted together.

So this or some form of tube flex might be the answer :-) It has to be something that causes the two scopes to drift apart in relation to each other, no doubt the guide scope is still doing it's job and keeping the guide star where it ought to be.

You can check this, watch your collimation carefully and see if it changes as the scopes drift apart. If you started off in perfect collimation, then recollimating should bring the two scopes back into alignment again.

cheers, Bird

[Aster]

Gary

Had the same idea, but couldn't try it out as my shoestring guide system won't be ready until January next year.

On the other hand, looking at tech-guru test photos who needs automatic guiding :)

[Aster]

This has nothing to do with PHD Drift but might be interesting to some to overcome imbalance of side by side scopes.

Many years ago when I first started in astronomy and photography I had a 8" newt sitting on a very cheap japanese mount. Touch it and the object you were looking at just about moved out of the FOV. Couldn't put a guidescope on top of the newt as the whole lot would have collapsed. Also couldn't afford to buy or make anything else.

After a lot of reading, going to the library, and a lot of thinking, grey matter still worked in them days, I came up with what I thought was a brilliant idea. Why not put the guidescope where the counterweights are. No extra weight to destabilise the mount and no extra counterweights.

Found a nice piece of aluminium U channel, made a couple of adjustable tube rings ,mounted the lot on the end of the dec shaft and the problem was solved. Used that type of setup for a couple of years doing deep space photography guiding manually with excellent results.

Was going to do the same thing if I would have used the 304mm tube assembly on top of the EQ6 mount to save weight.

[Garyh]

Hi Alex,
Now thats a idea I havn`t seen in a long time!
In my very oldest book on Astrophotography they used to attach there wideangle plate cameras in a similar fashion on the end of the dec shaft.
:thumbsup: good idea!

[Lee]

A problem with that is the guide scope wouldn't clear the observatory walls for low objects.....