How do I fix this issue?

[Paul Haese]

Imaging with my RC12 via MaximDL for guiding and in OAG configuration.

Dec axis is perfect for guiding. Hardly moves off the centre line. RA goes up and down in an apparent sine wave.

This I have looked at:

1. Balance :near perfect and what it should be for a PMX.
2. PEC: on and PEC off, neither impact.
2a. I have protrack (skyX) on and that helps smooth out the errors.
3. Guide rate in SKYX: only seems to smooth out a lot when I have that set to 1.0-1.2 of sidereal. Weird. Anything near 0.5% of sidereal and I get really erratic behaviour.
4. In Maxim I have min move set to 0.04 for Dec and 0.05 RA as per calculator I used.
5. In Maxim I have max move set to 1.0 for both axis but have tried settings as low as 0.13.
6. aggression does not changing this at all, just slows or speeds up the changes. Currently set to 8 for Dec and 7 for RA.
7. Guide exposures set to 4 but have used as much as 10 seconds.

After a lot of reading I thought this might be mount oscillations caused form incorrect Maxim guide setting, though now I cannot think what the problem could be since playing around this these settings and finding these were about as close as I could get.

There does not seem to be any looseness in the mount at all. I had considered I may have installed the new worm incorrectly.

Effectively I get slightly elongated stars along RA in nearly all my exposures when pointing toward east but better results when pointed west (about 50% of images pointed west have round stars).

I have included my log file. As you can see there is little movement in the X axis being made to adjust guiding. Yet movement in the X axis is moving around. Seeing is generally good. Perhaps my settings are still wrong.

Any ideas?

[atalas]

Hi Paul

I don't use Maxim but a 0.04 minimum move seems very low to me?and better results to west but poor results to east smells like balance doesn't It?I mean to say that maybe the OTA is better balanced to the west?

[RickS]

Roland from Astro-Physics recommends a min move of no less than 1/4 of guider image scale. I don't know the parameters of Paul's system but for my 12" scope, OAG and a 2x2 binned Lodestar that works out at 0.02 sec (1/4 of 1.22 arcsec / 15).

Paul: sorry, no brilliant ideas from me.

Cheers,
Rick.

[Bassnut]

I have min move at 0.01 and max 2.00, not that I fiddled with it, didnt need too. Agressiveness sounds high. When I had perfect PA, I had set 3 and 2, now Ra and Dec at 4 and 4 since PA has drifted a bit. It would help a lot if you loaded the guide log into excell, made a graph and post that?.

[Joshua Bunn]

Maybe PEC isnt working at all.

Have you tried guiding in TSX Pro, is this with direct guide or relays?

Our equipment and conditions are relatively similar Paul, here are my settings that give great results for me: guiding through TSX, protrack on with a large model, and proper polar alignment. Min move of 0.2, max move of 2, aggresivness of 8, no delays after correction and guide exposures anywhere between 6-12 sec depending on filter and star brightness. 50arcsec callibration distance (maybe you could try to recalibrate), guide rate of 0.5 of siderial.

Josh

[Paul Haese]

Balance generally is pretty good but with a huge filter wheel it can be a bit hard at times. That said I have checked a balance a lot as SB mounts need to balanced really well generally to get best performance.




0.02 seems like an idea to me. I might have to give that a shot and see how that pans out. Going higher up to 0.07 gave pour results so I will try lower.



0.01-02 seems like a reasonable place to start. I will trial a bit with the aggression. My PA is at 0 nothing to move in either axis.

See the new log attached loaded into excel, but in winzip as excel files are not allowed here.

[Paul Haese]

Josh, I am not using TSX for capture or guiding. Though I am guiding via relays. Using a large model myself but that might need some more work since changing the back plate on my scope to the new GSO design. Did a re-calibrate last night; I was thinking something similar myself about calibration being wrong. I don't have the filter problem for guide exposure as I am guiding in front of filters.

0.2 on min move would create problems. I tried higher min and the guiding went all over the place.

Stuff to think about.

[Joshua Bunn]

Maybe there was confusion here, I was thinking of guider calibration, not tpoint. But if you changed your backplate without doing a new tpoint model, you definetly need to do a new full tpoint model. Protrack could be working against you.

Josh

[Paul Haese]

Yeah was thinking that myself, though not really a big problem but I will test again tonight.

[RickS]

Josh: if those parameters are in seconds (as they are usually specified) that's a min move of 3 arcsec and a max of 30 arcsec. Both quite excessive for a long FL scope.

[Joshua Bunn]

Hey Rick,
These are arcsec.

[Paul Haese]

Just checked and I turned off Pro track last night.

[RickS]

Cool... Maxim specifies them in seconds (time). One second = 15 arc seconds of sidereal movement.

[Shiraz]

there is a repeating RA pattern at the 230 sec worm fundamental, which suggests to me that the guiding is too slow to correct for rapid slope transitions in the PE. With 4 second exposures, mount response time and aggressiveness/hysteresis, your tracking loop is trying to (partially) correct for errors that happened maybe 8 or more seconds ago, which will mean that it could even be correcting in the wrong direction if the PE slope changes quickly. Have you tried guiding with short exposures - maybe 1 second - to speed up the guide loop? Also, do you have any raw PE data?

[Paul Haese]

Hi Ray,

I had not considered that as I generally don't try to chase the seeing but I might just give that a try. I have been doing sub exposures pretty long between 4-10 seconds and that is generally selected by CCDAP as a setting I have to ensure good guide stars. I have PEC data on the dome computer I will chase this up and post it here. Several PMX users in the southern hemisphere say that the correction does not see to work properly even when you reverse the side of the meridian and I a have found that too. Perhaps you will see a corresponding similarity.

[Paul Haese]

WinZip of log file for PE. Let me know if you cannot open it. :)

[Shiraz]

could be worth a try - expect a lot noisier error graph, but if the stars end up rounder you may be able to find an optimum compromise between seeing noise and guide loop speed....

I guess that even a slight imbalance in RA could mean that the worm and the 4 bearings that locate it would be loaded differently on east and west sides. Could that possibly explain why you seem to get no advantage from PEC - the PE is maybe not consistent east and west?

[Shiraz]

thanks Paul. that looks nice and smooth and almost shoots the exposure time theory out of the water :P. 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?

[Joshua Bunn]

Hi Paul, I noticed exposure times for your PE log were 3 sec, whats your reasoning behind this? This (http://www.bisque.com/sc/forums/p/21928/98558.aspx#98558) 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.

[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.

[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.

[Joshua Bunn]

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

[PRejto]

I'll just 2nd a few things already posted.

TSX never has worked for my former MX or MEII. PEMpro has always worked. Short exposures are good and Ray Garalak indicated that one shouldn't do more that 5 cycles or so as it makes the curve harder to extract.

Peter

[Paul Haese]

Thanks guys, plenty to consider here.



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?



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.



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.




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.



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.

[PRejto]

Paul,

I don't know what your RC12 weighs compared to a 14" under discussion here but reading through this post at SB, and in particular, the very last post on page 4, the issue sounds quite similar. The upshot is leading towards a conclusion that the MX might not take this much weight.

Peter

http://www.bisque.com/sc/forums/t/22660.aspx?PageIndex=4

[Paul Haese]

Yes possibly a problem there. I have calculated the weight of the equipment package. Total weight without including the 4 counter weights (which are down the bottom of the shaft) is 34kg all up. That is 7 kilo from capacity but well within spec.

My mount does not jump like that though. I am seeing a slight movement up and down over a period of time so the guiding looks like a slow sine wave over several minutes. Maybe it is too heavy but I had spoken to Daniel about the weight before and he did not say it was a problem.

Maybe I will move the counter weights up to the top of the mount and put on another weight to see if that makes a difference.

[gregbradley]

I bought PEMPro a while ago for this reason as I was not happy with PEC on my dark site PMX. But I redid the PEC one last time using Sky X and it worked. So not sure what was different. I may have loaded a later version of Sky X.

So perhaps try a later Sky X version if you dare as it may have been fixed in a later version as there was some internet traffic about this issue about 9 months ago.

Personally I would use Pempro and forget about Sky X and PEC. It certainly sounds like some hidden software issue that could go on for a while trying to find out what it is and a later version is just as likely to make something else not work!

Greg.

[Paul Haese]

Ok a bit of an update.

I have changed the min move in X to 0.02 and max to 2.0. I took off the PE and installed a flat line. Then I ran guide exposures for 1 second but found that did not have bright enough stars so I made it 2 seconds. The guide graph is a lot smoother now generally in RA. So I am getting somewhere. Stars are round at 10 minutes too. I have a bit of wind down the observatory tonight so I cannot go much further than I have now. I will report back once the camera comes back from the States.

Thanks so far for the help guys.:thumbsup:

[SkyViking]

Hi Paul, although our mounts are different I'll second shorter guide exposures. I use 0.6s most of the time and only go up to 1s on the odd occasion where no guide star is bright enough. And they don't have to be very bright at all in my experience.

Cheers,
Rolf

[Paul Haese]

The different f ratios impact on guide star selection Rolf. f8 has a big impact on how bright stars are even at 2x2 binning. I take it you are using f4 at 2x2? :)

[SkyViking]

Good point Paul, but that then makes me think your guide stars are oversampled. Would it not be better to use higher binning when guiding at f/8? Or a different guide camera with larger pixels?

[Joshua Bunn]

Good points Paul and Rolf! I must add, I bin my guide camera pixels 2x2 when guiding, so 0.73"/pix for imaging but 1.46"/pix for guiding. works a treat for me, and being highly sampled when guiding puts a lot of demand on the guiding and mount corrections - depending on seeing conditions and exposure time.

[Paul Haese]

I have been guiding at 2x2 so my image scale without the 0.73 reduction is 1.25 arcsecs per pixel. With the reduction it is 1.72 arcsecs per pixel. I wonder if binning at 3x3 will have a positive impact. Is this what you are thinking Rolf? I would be at 2.58. Not sure if that would work.

[Paul Haese]

I thought I would add an untouched sub with the new guiding parameters. Ignoring the stars are the edge of the field (the flattener is still not in the correct position), the stars in the center look tight and fairly round. This is a 10 minute sub guided at 2 second exposures with min set to 0.02 and max at 2.0.

[SkyViking]

Yes that's what I mean. From memory I might actually be guiding binned 3x3 already at f/4, but I'll have to check that next time I'm imaging. If that's the case I think you can safely bin at least 3x3 or even more when you are at f/8. Guiding is using sub-pixel accuracy anyway so should be fine. Worth a try - and if it doesn't make your guiding worse at least you'll have brighter guide stars.

[Paul Haese]

Yeah true. Some thing else to try. :thumbsup: