Differential flexure or field rotation

[Parallax]

I'm having a problem with egg shaped stars and not sure which of the above two are the cause.

SETUP - Imaging through the Explorer (fl 1000mm) + Atik 16, guiding with the ST80 (fl 400) + webcam + PHD + EQmod. The PHD settings are pretty much default. The 2 scopes are mounted on a DTM dual mount bar and seem rigid enough. All atop a HEQ5 Pro. Polar alignment seems spot on (I can guide in RA only for a good half hour). As each 300s sub comes in it is shifted by approx 2 pixels each time. That is, the image progressively creeps across the capture window.

I'm probably going to answer my own question here - I did notice last night that the image was creeping diagonally wrt RA & DEC. Is this indicitive of field rotation? Should I increase the fl of the guidescope (with a barlow)? Could it be that I have surpassed the limit of the mount?

Any help will be much appreciated.

Steve

[RobH]

Hello Steve.

Field rotation will manifest as stars rotating around the centre of the FOV.

If your guidestar is staying in the crosshairs with PHD, but the image is gradually creeping across the screen over a number of subs, this is differential flexure.

I was getting this but have since done away with using guidescope rings, and now just firmly clamp the guidescope in normal tuberings, and everything is done up tight with molegrips.

I still get egg shaped stars though, although I don't get the image creeping anymore, but am still trying to get to the bottom of this problem.

Cheers

Rob

[Merlin66]

Is the "creep" dependent on the time between subs? It may be just the difference between the HEQ RA drive rate and the guide rate???

[Parallax]

Both good points.

Rob - Maybe my idea of tight is not tight enough. I can't clamp the guidescope firm as my webcam doesn't give 100% reliable results in long exposure mode (and I can't afford to buy a QHY5).

Merlin - I am using the Pulse Guide method. Therefore, does your point remain valid?

Thanks so far.

Steve

[barkis]

Could your focuser be a suspect too perhaps?.

Ron.

[Parallax]

Ron - I did think that. Up until now I have not been locking the focuser screws. I have now done so on the ST80 and will try it out tonight. I have got the electric focuser on the Explorer and didn't think that that would move on its own but I will lock that as well.

Thanks

[blinky]

Watching this thread with interest as I have exactly the same problem. PHD Seems to track with no problems, the graphs are OK but I get pixel drift on each sub...

[steppenwolf]

I too sometimes have 'pixel drift' (easily seen by watching a line of 'hot pixels' on my stacked images). This drives me nuts as I have everything solidly bolted down but to no avail. I'd love to know where the flexure is although a weakpoint in my system is probably the Synta dovetail bar BUT as my guidescope is mounted to my imaging 'scope, they should both share the same root weakness!

[blinky]

I am the same Steve, I cannot figure out where it comes from. I was even debating getting an off axis guider and trying that!

[Parallax]

I have just given up for the night as I was getting nowhere. Before I came in I went round the whole setup checking for any movement where there shouldn't be. The webcam was loose in the eyepiece holder. It could have become loose due to the extremes of temperature in the obsy. I'll let you know if that was a contributing factor next time out.

At least I now know that I am not alone with problem.

Thanks again for your suggestions.

Steve

[PaulB]

I am also watching this thread with interest. I have I think a very similar problem, although I am still not sure it's this, or poor collimation.

In one image taken recently I had perfect guiding and I managed to image M5 in 2min subs with know movement. I am using Guidemaster to guide my G11.

On the left side of the image the stars are all nice and round, but if you look at the stars on the right, there elongated.

I also use a Baader MPCC attached to a BA-Astro filter wheel and this is all attached to my Atik 16ic. I use an Orion (UK) 2" Crayford focuser on my SPX 200-800 f4. I did wonder if it was the weight of the camera pulling on the focuser.

So I replaced all the thumb screws with bigger ones, and I also use three cap head screws at the base of this crayford and this has made the camera and focuser more firm. I also use a ZS 66SD mounted in rings with an Atik 2-HS attached. Again I have made sure that this is tight and firm.

If it was flexure wouldn't the whole image show elongated stars? And if it is poor collimation could this account for stars on the left showing up as round dots whilst stars on the right are egg shaped?

I can submit the image of M5 if you wish me to.

[blinky]

Paul I would say your problem is that the camera/MPCC/something in the focus path is not square on, this would account for elongated stars at one side.

[PaulB]

Thanks Blinky,

I will have a look at this.

[PaulB]

Can you test this and see whether or not this is the cause.

And is there a quick and simple fix?

[Parallax]

OK, this is starting to annoy me now.

I have locked the focusers and can't detect any movement by hand, but the problem persists.

I was monitoring the RA & Dec values, as shown by EQmod, whilst tracking (in both axes). During a 30 min period the RA fluctuated by +/- 1 sec but the Dec was +/- 9 sec. All the time PHD was kept the guide star where it should be. Obviously something is amiss but I'd be blowed if I know what.

At present I have the dual mount bar offset in the saddle. Should this bar be mounted centrally in the saddle so that the 2 scopes are equidistant from the mounts' centreline?

Steve (now with even less hair).

[Merlin66]

Steve,

The dual bar should be positioned to allow the two telescopes to "balance" each other.

So, if you have a heavy reflector on one side an a lightweight ST80 on the other, I'd assume the mount point to the HEQ would be almost under the reflector.

If you set the tubes up vertically, and you undo the Dec clamp they should stay in position. Does this make sense??

[Ant]

Can't really help with the flexure issue, but the odd shaped stars on one side I had earlier.

http://stargazerslounge.com/imaging-deep-sky/78651-m51-160minutes-some-rather-odd-stars.html

On the left were (almost) round stars, but on the right they were blumming strange.

It turned out to be a collimation issue. I bought a cheshire EP and the problem went away

Hopefully that will help on at least part of the problem.

Cheers

Ant

[Parallax]

Merlin - Yes that does make sense, and it is how I have mounted them. So that part is correct.

Ant - I remember that thread. Fortunately(?) my stars are consistantly oval.

Steve

[barkis]

Does Concentration seems to dwell mostly on the Imaging instrument when trying to figure out flexure issues, or does the guiding instrument get it's fair share of attention. I don't want to appear like I am butting in, as I am not doing any imaging at present.

The reason I am interested, is I noticed purely by accident, that the lens cell on my refracter , does not hold the doublet very securely. If I wring the neck of the scope, gently that is, I can feel the lens moving back and forth. I appreciate that they must not be held too tight, but this movement seemed a bit excessive to me.

It got me thinking that if some guide instruments were having a similar anomaly, then the lens may creep round in the cell slowly, and thereby cause guide corrections that were in fact false. Tiny corrections perhaps, but sufficient to affect the image. I would imagine that correction over a period of time, may cause stars to have a slight curvature.

Now I could be talking total B******s, but it's just my thoughts on it.

Ron.

[Parallax]

Ron - At this stage anything is possible. I will check it out.

Steve

[Milamber]

Old wisdom would say you have your scopes the wrong way around... guide with the longer FL is what you should do. Do you know the relative pixel sizes of the cameras? And have you done the sums for the pixel size/focal length/correction aggression to make sure your guide system is not over compensating because the difference between the two imaging systems is too big?

Arthur

[blinky]

Arthur,

I think you are now going to have to supply said sums! I never knew there was maths involved in this!

[Milamber]

Not sure they exist, but basically, if your guider has big pixels and your imager has little ones then you imaging pixel is going to rattle around inside the guiding one, creating this problem. If you see what I mean?

[Psychobilly]

Craig Stark - He of PHD Guide fame ... has written an interesting article...

http://www.stark-labs.com/craig/articles/assets/AutoGuiding%20Craig%20Stark.pdf

In which he dicusses amongst other things the guide scope requirements...

[blinky]

I just read that article and realised that I was doing something wrong! In the article it says the mount should be unbalanced slightly so it is against gravity, I always set mine so it is with gravity!