PEC Has Changed On My MX

[PRejto]

OK, while I "endlessly" trouble shoot I noticed that my PEC wasn't working as well as it has since last May. I've not changed anything on the mount, or even removed it from the pier. I don't even recall removing the scope.

I used Pempro to redo the PEC curve. The first thing that stands out is that my peak to peak uncorrected error has increased from 2.6 arcsec in May (Red curve) to 4 arcsec (green curve). The two curves are both similar and different. Currently the new curve works better than the old curve, but perhaps not quite as well as the old curve worked back in May when I measure .4 arcsec after correction. Currently the best I can get is .9

I feel pretty confident about the data as they both used 7-9 worm cycles of data at 0 degrees on the meridian at .65 (green) -.75 (red) arcsec resolution.

Is it usual for PEC curves to just change over time? Could it be an indication that the mount needs lubrication? Something else, or should I forget about this and just accept that it's working now?

Thanks,

Peter

[Logieberra]

How's your belt tension? That may be subject to change over time. Nice & firm?

Also, an uncorrected raw reading of 2.4 to 4 is seeing dependant. I've seen different PEC results from night to night, based on seeing alone, but the underlying shape of the error curve remains roughly the same.

This would explain a .4 to .9 corrected curve on different nights. The .4 must have been superb seeing. .9 is as low as I can go, from the Canberra suburbs.

[Logieberra]

Also, did you collect data with the same equipment on each run? Same Autoguider binning, barlow - the same image scales etc?

Something I've just bumped into -- my Kendrick dew heater settings have a bearing on my FWHM focus results! Until recently, I only ever saw 4s and 5s. Bumping Mr Kendrick down to 40% power, I now see high 1s and 2s. This no doubt would affect my PEC collection readings. I'll only measure PEC with Kendrick OFF from now on...

[PRejto]

That is a most interesting observation! You'd need to assume the dew heater was setting up some sort of convection over the lens, no? Perhaps you've got it located in a bad spot. I will need to also experiment with this one.

As for your first post I would find it odd if seeing wasn't somehow factored out in the PEC calculation when the curve was fitted. I have actually seen the opposite of what you report; namely that the PE measure without correction is pretty stable from night to night. If it wasn't stable and you generated a PEC curve from that data then your correction amount would also be all over the place and dependant on the seeing being the same as when you measured it. On the other hand I do agree with you that measuring the correction is quite seeing limited, but again I think the seeing is somehow mathematically supposed to be removed from the calculated amount.

Anyway, I'm not too sure about this so it might be a factor. The thing about my curves are that they are partly the same but with 2 areas of large difference. I don't see how that can be seeing.

No, the two runs were not done even with the same camera, but the camera angle and resolution for both runs was carefully noted, and both image scales differed by only .1 arcsec. One ought to get very similar results...and in fact a comparison of the two curves 5 months apart show pretty much the same shape...and both curves have worked, one now and one back then. I guess what I'm saying is the new curve has got to be valid or it wouldn't work.

The belt tension seems good, though perhaps it is fractionally looser than it was. I could try tightening it up a tad. Maybe that's all it is!

Thanks,

Peter

[cfranks]

Another possibility, Peter. Your worms may have worn the odd high spot off the wheel teeth so refit the worm to wheel when you check the belt tension.

Charles

[Logieberra]

AP ships their mounts with PEC pre-flashed to them. I understand that the PEC curve is good for a year or so. Don't quote me. As a mount wears in, a PEC adjustment is in order.

Also, if you adjust your RA plunger settings (SB recommend 2.25 to 2.5 turns 'out') that would alter PEC, as this changes the gear mesh/friction between worm and main RA gear/cog. Have you adjusted them? That could explain these minor deviations in PEC.

[PRejto]

Hi Charles,

I did adjust up the belt tension on both axis yesterday and I also relubed the ring gears. I didn't redo the cam stop tension but will before taking new PEC measurements next week.

Thanks for the tip.

Peter

[PRejto]

Hi Logan,

I have never touched the RA plungers so it wouldn't be that. I did post the old/new curves over at the CCDWare forum. Ray said he thought my curves looked odd and will investigate. Odd or not, the old curve certainly worked well. I think some of the MX mounts show up the pinion gears and you see those quick almost sinusoidal waves up/down. Maybe that is what looks odd to him. We will see.

Thanks,

Peter

[gregbradley]

Your PEC Curve looks nothing like mine. Mine is much more gentle up and down with only a little bit above and a little bit below the midline on both my PMX and PME. As I recall there are different curve fitting algorithims you can use.

The first time I did PEC curves they looked like that and did not improve tracking on my PMX. I redid them on a night of good seeing and after I had improved PA and did a 20 minute run.

Was this done with Pempro or Sky X?

But the bottom line is do they work and if you are getting round stars in 10-15 minute exposures then you have arrived and I would stop fiddling with it.

Greg.

[PRejto]

Greg,

I used Pempro since I have never yet had a good result from TSX. Every single time I run PE measurement I get those little bumps. One person on the SB forum said those were pinion gear errors. I certainly have had terrific seeing on occasion and I still get the bumps.

Yes, I'm getting a decent correction result from the new curve. The purpose of my post was to 1. ask if changes in PE were common over time, and 2. to see if the amount of change I am now measuring is worth being concerned about.

I think I'm getting answers to both questions! I will be running the measurements again as I have tightened up the RA belt and I re-lubed the ring gear. I'm curious if this has changed anything.

Thanks for your input!

Peter

[gregbradley]

[Yes, I'm getting a decent correction result from the new curve. The purpose of my post was to 1. ask if changes in PE were common over time, and 2. to see if the amount of change I am now measuring is worth being concerned about.

I think I'm getting answers to both questions! I will be running the measurements again as I have tightened up the RA belt and I re-lubed the ring gear. I'm curious if this has changed anything.

Thanks for your input!

Peter[/QUOTE]

I have not done PEC that many times but I did get different results sometimes with different software. If you were getting bad results with TSX and now good guiding with Pempro I think you have answered your question. You would not want the PEC curve to be the same.

The belt is supposed to be finger tight and with no backlash when you pull on it. Chris Venter has a video he made of changing his belt and its very useful. I would not overtighten it either.

Greg.

[Logieberra]

"The belt is supposed to be finger tight and with no backlash when you pull on it. Chris Venter has a video he made of changing his belt and its very useful. I would not overtighten it either".

Good point. Another Aussie MX owner/user really overdid his belt tension and got mounthitsomething slew errors. Must have been super tight! Automotive tight even!

I press firmly on one side of the belt. If it introduces movement on the opposing side - it's not tight enough. After working with large electric RC helies for years, I do prefer working with belts and pulleys. Very straight forward to re tension.

[PRejto]

Greg,

Just to pedantic(!) I've never had a working result from TSX, but did have a previous very good result from Pempro back in May. For some reason it recently stopped working which seems odd. So, the curves I'm comparing are both from Pempro. I'm just wondering why the big changes?

Peter

[Logieberra]

Here's some advice I received from SB over private e-mail, re MX belt tension:

"Yes, belt tensioning is very tedious. We use a belt tensioner so they are all 15-20N. The happy medium is what we are looking for. You want it tight enough that there isn’t any backlash but not so tight that there is strain on the motor shaft."

[gregbradley]

Oh I see.

Yes the changes are strange. Something obviously has changed. With so many variables it may be hard to pin down.

If you are getting good results now then that is the main thing.

Greg.

[PRejto]

That's still fairly subjective isn't it?

I released the belt tension by unscrewing all the screws a bit. Then I increased tension by putting a fingernail on one of the bolts and pressing down to tighten the belt while tightening the screws. I'm really not sure if I may have tightened it too much. I ran some slews with no problems, and the mount homes. Do you think that is an indication that it's OK? I felt the belts "might" be a bit loose when I initially checked them, but perhaps I've gone to the other extreme.

Peter

[kimrichards]

I am more and more convinced that the fix that SB gave us earlier this year for SkyX and MX PE corrected the rotation error for the Southern Hemisphere but that an issue with the phase of the correction remains.

I can produce reasonably consistent uncorrected PE curves but after correction the residual error curves are far from consistent and mostly worse.
(This is not related to the Pointing West button and am using latest build).

I have tried on several nights with numerous runs and I always come to the same conclusion to leave PEC off.

Has anyone in the Southern Hemisphere produced a repeatable residual error with an MX?

1. Using SkyX
2. Using PEMPRO

That is, recorded the PE, applied PEC, run a corrected log and found the residual PE to be less than before and below 1 arc sec peak to peak.
And then were able to repeat the whole process with a similar result.

Thanks

Kim

[PRejto]

Hi Kim,

I just answered your similar post at SB.

Short answer, no. But I can with Pempro.

Now, I've only tried once (recently) using TSX (Win7) and failed, but on same night with same camera setup using CCDSoft's camera to run Pempro I had success on the first go. My PE went from 4 arcsec peak to peak to .9

A test of "1" is not definitive, but I suspect there is still a problem. Interestingly, I compared the TSX generated curve (both versions!) to the working Pempro curve and I could not even see any similarities no matter how I rotated, inverted, or translated the curves.

Have you tried Pempro? Why not try it. It has a very generous trial period and it really isn't difficult to use. Perhaps if you got a working curve saved to the mount you could compare it to non-working curves from TSX. Perhaps you might see a way to "translate" the curves and then understand what is not working in TSX. I will also try again when I feel more motivated. I'm kind of in a why bother mood about this since I have a solution. I'm pretty sure the new automatic version of PEC may also fail for us in the S.H. if the problem we still seem to have isn't identified.

Peter