[Paul Haese]

Thanks guys, plenty to consider here.

Originally Posted by Shiraz

thanks Paul. that looks nice and smooth and almost shoots the exposure time theory out of the water . However, there is a relatively abrupt transition from negative going to positive on a couple of cycles that could possibly explain your results. Shorter exposures are probably still worth a try. With such consistent PE, you should be seeing large benefit from PEC - wonder why it doesn't help?

Yes it is odd but even if there is no PEC guiding should take care of the problem.

Maybe the abrupt transitions are a warp in the new worm?

Originally Posted by Joshua Bunn

Hi Paul, I noticed exposure times for your PE log were 3 sec, whats your reasoning behind this? This post explains it a little. Its usually recommended to use 1 sec exposure times or the like, and that's what i use - or less.

Actually I am puzzled by this too as I knew that it should be a second or under for good sampling of the data and I am sure I set this correctly at the time. Might have to have another sampling of it and see if the results differ. Another log I have from the night before has 3 second exposures too.

Originally Posted by gregbradley

PEC on the PMX seems touchy. I did some PEC on my PMX and at first it did not seem to make any difference like you are experiencing. There is the possibility of course that the East/West is back to front on the data so its told to push when it should pull but that probably isn't it as that would most likely worsen. Although one time I did have that reversed it didn't seem to make a lot of difference.

3 second intervals for the PEC log may cause an issue as you want more frequent samples for an accurate PEC curve. Also keeping the other points in about which spot in the sky you use for your PEC guide star.
Another would be to do the PEC curve on a night of good seeing. I haven't read the instructions for a while but I do seem to recall shorter exposure times being recommended to get a more precise sample. Also about a 20minute run (requires good polar alignment otherwise the guide star drfits out of the imaging box). When you think about it 3 seconds is probably way too long as these corrections in the PEC curve whilst they tend to be smoothed out they still move a fair bit over 3 seconds so you don't want a PEC that is always coming in late or averaged over too long a time so the corrections are always coming in late either too little or too much.

Having said that I doubt that is the reason why you are having trouble as it would still make some difference as the curve is smoothed out a fair bit during the final steps of making one. It sounds like its way off, out of synch and your autoguider is having to correct it for bad PEC adjustments.

But apart from that it may be best to ditch Sky X PEC and use PemPro as no doubt you know the above. Especially being a remote system and you don't have many nights to experiment to find out it was some software bug in the SkyX after several nights of trying.

I used Precision PEC for my PME and that got a nice curve. It was a subtle sine wave which looked much like my PME's PEC curve.

My original PMX curve seemed too much for correction. On the PME its a pretty gentle almost sign wave type PEC.

There were in the past numerous threads about Sky X and PME PEC being out of phase so there is a question mark over it.

When I redid my PEC using Sky X on a night of good seeing with the Polar alignment already very very close (probably not as close as yours is now) I got a very good result.

Another factor of course is how stable are you optics in terms of flexure and mirror shift? Protrack corrects for flexures of various types, PEC of course for gear errors.

Another thing that threw my PMX off at one stage was a bit of dirt or something got on the worm so about every 3 or 4 minutes I would get a bad PE spike that threw the guiding out for a few seconds resulting in double stars in the final image. A replacement worm corrected that. Not sure what the spike was caused by in the end - dirt? Damaged worm from the slipping gear/cam issue? Either way it fixed and I do get a noticeable improvement in my PMX using PEC so it can work even if perhaps a bit touchy.

You are imaging at 3 metres though and I image at 1260mm on my PMX so everything is 3X exaggerated on your setup compared to mine.

On my PME I find a 300 point Tpoint model does improve roundness of stars using Protrack at the same time as PEC on at 3 metres focal length. So Protrack is correcting some minor flexes that are slow to show up in the image. It can be odd as when I watch the guiding some of the corrections are larger than what I would think is ideal yet the images show round stars. That must be the Protrack flex correction being added to the PEC curve correction plus the usual autoguiding corrections if they all coincide in the same direction.

My first action would be to redo the PEC using Sky X and make sure its exactly as per the manual and if still no go then try PemPro or Precision PEC and see how that goes.

There is no spike in your PE curve so PEC ultimately should be pretty easy.

Greg.

The East/ West issue is certainly a problem. I have switched between both and not seen any difference. I still get the oscillations but no change in the extent of those oscillations.

PA is good and the guide star did not drift after 20 minutes.

As you know my seeing is good at my location so even an average night of seeing is still pretty darn good.

The guide star has to be very close to ecliptic for best results and that is certainly what I did. It also must be no dimmer than mag 5 or else the guide results will be less than satisfactory.

I am certainly considered getting pempro to resolve the very real possibility that the SkyX has an issue. Certainly at that point.

I am pretty certain the optics are pretty reliable now as I have installed the new back plate and I cannot find any discernible flexure in my images now. ie stars being slightly different shapes in different parts of the sky before the back plate change over.

Focal length is not quite that long but still long enough and yes compared to the other system this is not a piece of cake. My FSQ system is a flat line most nights in both axis and hence why my refractor images are pretty sharp.

Originally Posted by Shiraz

Paul, hope you don't mind, but would like to confirm with Josh that this means that his approach is to use high time resolution PEC to remove most of the PE and then mop up any residual errors with slow (6 seconds or more) updates through the guiding.

No problem Ray. I am happy to hear all manner of ideas. In fact I did try some of the mods to Min and Max last night and I got it running pretty smooth near the Helix. When I moved over to NGC253 it went to pieces again. So I am starting to get some where.

I will be sorting PE with a higher sampling than at present.

Originally Posted by Joshua Bunn

That sounds about right Ray, high sampling of the PE. And then Protrack does some work to.

Protrack certainly has an impact. My model needs running again now with the change of the back plate but the previous model of 288 points really did help keep stars tighter.