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Hi Arun,

PE evaluation is probably the most critical evaluation of a mount. It is like a "startest" for optical systems.? It's where the rubber hits the road.? It can be done in one evening...even hazy skies and poor seeing are fine for this.??

?If PE evaluation shows the error is mostly from the worm, PEC can eliminate most of it and save any autoguider corrections.?

?If PE evaluation shows the problem is not at the worm frequency, PEC may do nothing or make the mount perform worse. If the PEC curve has lost its phase it could double the mount tracking error. So the evaluation is the first thing to do....don't jump to PEC yet.??

You have to know a few "gotchas" which are:

PEC correction in the Gemini system plays back the correction curve in synch with the worm.

But, the worm has no "zero point" detector, so you must always park your mount, so the PEC curve you measured is matched in "phase" (really 360 degrees x time / worm_period of 239.4 sec on the G11).? (The latter for mounts having a phase or zero point detector is called Permanent PEC or PPEC by some mount makers.)

Even if your gemini power goes out, the Gemini is still supposed to know the last recorded worm rotation position. It is in SRAM.? If your Gemini loses its SRAM backup battery power, that synch to the worm is lost.? Then you replace the battery and redo the PEC data.

Also know that:

Gemini-1 has a built in PEC recording function.? And you can also average multiple PEC training runs.? I have used it and it works well.? I don't have any G2s but they are supposed to also have it built in.

The Gemini.net has a checkbox to tell the Gemini to employ the PEC and this must be done each time you boot up, as Gemini defaults to NOT using the PEC.? There is a Gemini.net checkbox to turn this PRC on after bootup.? That's a nice feature.

You can use the free program PECprep.exe found in the online EQMOD group software system.? That has all the settings to analyze G11, Titan, GM8 and many other brand mounts.? That program can create a PEC file for some mounts, but not the Gemini units.? It was vmcreated initially for the Synta EQ6 or Orion Atlas mount....hence the group named EQMOD.??

For taking the necessary data, these are the best steps:

1. Only use your main scope on the mount dovetail...not a piggyback scope.

2. Use PHD2 and your autoguide camera at prime focus.

3. Point to a star near intersection of Meridian and Equator.??

4. Let PHD2 calibrate (tracking is on, and the Gemini.net is set to G for Guide speed.

5. Drift align to get excellent polar alignment. PHD2 has an good tool to facilitate that.

6. Now shut off PHD2 autoguiding pulses (leave it just tracking)!? It's under the Brain icon.

7. Return to a star near meridian and equator intersection.? Center the star in the middle of your Autoguide video camera.

8.? Start tracking this star with PHD2 not autoguiding.

9. Let the star be tracked for about 40 minutes.??

Find the latest PHD2 log files, usually in a Windows system in the folder c:/Documents/PHD2

There will be 2 files...one is an Error file, of no interest.

The other file is the log file you do want.

Copy that file and name it like:

G11_10inSCTf10_ASI178MC_Jul-3-2020

That way you will recall the camera ("ASI178") and the scope "10inSCT"?

Now open the PECprep program, and choose the Mount as your mount RA drive, say G11, from the top menu.??

Then import the Tracking file as PHD2 and locate the file from the file menu.? The program will ask for the scope FL in mm, and the camera pixel size in X and Y in microns??

Then the program will show its analysis. Press the button "Auto filter" to get the best results.? An excellent G11 mount will have about 1 arcsec RMS.? You should be happy with 2 arcsec or better (was spec'd by Losmandy for the Titan mount!).? Above 3 arcsec to me suggests there are some adjustments to make.? Note the GN8 has half the size ring gear so I think it's PE would be double that of a G11 with the same worm.

Go to the last tab, and see the origin of the frequency peaks the program finds. You should expect the largest peak is the worm fundamental at 239.4 sec.? That means PEC could clean that up.??

If my PE is below about 1.5 arcsec, I consider that equal to my seeing fluctuations.? ?Then I only use autoguiding with like 3 sec exposure time.??

If your first panel view of the star tracking shows any abrupt jumps...then you have a puzzle to solve:

... either something bumped your scope, or the camera got snagged on a cable, or a clutch disk slipped, or mount leg is not locked down, or the ground shifted, or an earthquake, or mirror flop, or camera was loose and shook or....the Oldham coupler was loose, or a bearing glitched, or dirt on the ring gear, ....

...there you need to carefully look at every little thing.??

If my PE is not acceptable using it's existing RA worm, say over 1.5 arcsec, I will swap the DEC worm for the RA worm in hopes the DEC worm was by chance the better worm.? They are all made the same, but one will be better than the other...

Have fun!!!

Michael

On Thu, Jul 2, 2020, 12:12 PM Arun Hegde <arun.k.hegde@...> wrote:

Has anyone tried PEMPro and found implementing a PE correction to make a meaningful difference in tracking? It seems like it should, but it also seems like PHD2 should be able to correct a long period low amplitude PE all by itself.

Hi Michael,?

Thanks for the very detailed write up. There is one thing? that your post clarified, which is how the PEC curve is synched to the current worm position. My follow up question is this: if the mount's RA axis is accidentally bumped after it has been shut down - which is very possible when we manually set up and tear down- will Gemini lose track of the worm position? I realize that when I restart, I can level the RA and DEC axes to set the CWD position again, but will using a level be sufficiently accurate to reproduce the CWD position at which the PE curve was generated? A different way of saying this - does Gemini know the absolute position of the RA worm??

Arun

Has anyone tried PEMPro and found implementing a PE correction to make a meaningful difference in tracking? It seems like it should, but it also seems like PHD2 should be able to correct a long period low amplitude PE all by itself.

Has anyone tried PEMPro and found implementing a PE correction to make a meaningful difference in tracking? It seems like it should, but it also seems like PHD2 should be able to correct a long period low amplitude PE all by itself.

Hi Brian,

I've been considering purchasing PEMPro as well to analyze my PE. Does PEMPro have a way to load the PE curve into the Gemini ASCOM driver or into the Mount firmware itself? I guess I am asking how you implement a PE correction.

As the title says.? What's the best way to determine P.E.?