After the initialization, typically high power tracking will show two residual errors.
The first is a slow steady drift, commonly a couple of arcseconds per minute of time. This is caused by small initialization errors such as not precisely centering the star in the crosshairs when initializating, and mount errors such as the azimuth and altitude axes not being exactly perpendicular.
The second is periodic error caused by the gearing, commonly the worm not being centered on its shaft or butting the gear properly. This shows as a slow oscillation of the star back and forth across the crosshairs.
The program has both periodic error and drift error cancellation features.
The PEC, or periodic error correction, file consists of two columns. The first is a counter, 0 to 199, that indicates the index. The index is the resolution or number of steps that the PEC is divided into. Commonly this is spread over a single motor rotation or 200 fullsteps, but may be spread over any number of fullsteps. For instance, spreading the PEC over 400 fullsteps would cover the PEC caused by a double turn worm. Set the number of fullsteps that a PEC cycle will cover with the variable FullstepsPerPECArray. Mark a synchronization point on the motor shaft or the worm shaft that indicates the starting position, corresponding to an index of 0. Next to the index numbers in the PEC.DAT file are numbers that that indicates the amount of PEC in tenths of an arcsecond to apply at each index. For instance, if a single turn worm causes 2.3 arcsecond of periodic error with the motor shaft at a 45 degree angle, then index 50 would have a value of 23 (assuming 200 fullsteps per motor revolution). See the PEC.TXT file to see how to enter these values. The altitude PEC comes first, with indexes of 0-199, then the azimuth motor comes with indexes of 0-199. Remember that you have 200 PEC values per axis, no matter how many fullsteps you spread the PEC over.
Here are more notes on PEC by Chuck Shaw: "I am Chuck Shaw. I
recently finished converting my 14.5"f/5 dob into an AltAz
system using Mel's plans and software. I concur, it is MUCH more of a
"system" than I ever imagined!!!
I also just went through coming to grips with nulling out PE and I have a few "lessons learned" that might be helpful....
1. Use LOTS of power when running in Guide mode to build Guide.dat I used two barlows stacked into each other (a 1.7x and a 2.8x and a 12mm illuminated reticle eyepiece and an ~1808mm scope gives about 720x...... use AT LEAST that much if you can.... (Slightly defocused blobs are easier to guide with than points anyway!!)... You ARE using an illuminated reticle EP aren't you??? Do not try to keep a focused image of the star in the center of the box. Either defocus till the star fills the box, or put the star in the center of an intersection of two lines, or put it in a "corner" where its just touching two lines. Try different techniques to see which works best for you. I found the biggest source of inconsistencies in my data sets was ME and my poor guiding!!!!!! My reaction times usually had the wrong direction and were late..... (duh......) :-) Play with the orientation of the handpaddle also to see if a "fly to" type of guiding is easier.....
2. Use the guide routine several times to nail the drift as well as possible BEFORE you try to write a file to capture PEC data.
3. For Azimuth data collection, be sure and aim the scope due south, near the celestial equator. This will minimize motion from Alt, and maximize the motion in AZ....
4. Shell out to DOS without stopping tracking after a Guide run, Do a "copy c:\guide.dat PE-AZ1.txt", then a copy "c:\PEC.txt guide.dat" (say YES to overwrite) , then type exit and SHAZAM the scope will slew back to the star you were guiding on and you are ready for another run!!!!! Name the next file PE-AZ2,txt, etc...... This allows you to capture several runs very quickly, and any system config changes between runs are minimized.
5. VERY CLOSELY inspect for any "wobble" in your worm bushing. I use 359:1 Byers gears. I had to make the bushings for the worms and discovered the bushing has a tiny bit more slop than it should (the clearance is about 0.001 and Byers tells me it should be about 0.00005). I decided to fabricate a "tailstock" out of a small aluminum angle and a brass machine screw. I ground a point on the end of the machine screw and it goes through the bracket and presses into the center ground dimple in the end of the worm. That made a HUGE difference in obtaining repeatable data. I think the problem was the worm's wobble was somewhat random. That, in turn changes the mesh slightly, and that induces PE.... If your worm is on a shaft with bearings/bushings at both ends, you still need to look for this!
6. Look at the couplings for the motor to worm. If they are flexible and you are using the motor bearings to stabilize the worm at all (if there is slop in the bushings you are in this category), you need to go to a precision coupling. I bought several commercial ones and found they were slightly out of round!!!! This in itself introduced PE!!! Thats OK though, since the Software will compensate for it.
7. Reinitialize the motor/worm positions in between runs to make VERY sure that you know what position corresponds to the 0 position in the guide.dat data I failed to do this a couple of times and got accurate but skewed curves..... You have to remember to turn off PEC and then back on after the shafts are positioned !!!!!!
8. Remember the data take will NOT start at zero unless you time it that way. Do not try to get cute and time it, just start it and use your spreadsheet program to cut the four runs of data into separate columns and then move them up/down in the spreadsheet to line up the steps in each run. Then delete the step number columns (all but the far left one). Graph the 4 curves and throw out any that are obvious guiding errors. Then average the rest and you are there!!!
9. Remember after the data file is copied into a file called PEC.dat to reinitialize the shafts to their zero position. I keep forgetting to do this!!!! The system also seems to get lost after a couple of hours of operating.... An occasional reinit of the PEC alignment takes less than 15 sec and helps "clean up" things. I should not have to do this but......
I have the sets of curves from my data takes I can send you. I was having the inconsistencies in taking data initially, and then the phase skewing till I stabilized the worms too.... perplexing, but a methodical working on each potential source of PE will find the culprit!!!
The performance of the system is simply astounding when all is completed..... I agree with Mel that software timing errors are the least likely culprit. Too many times I wanted to blame the software for problems but Mel has simply done too good of a job!!!!! It was ALWAYS something I did!!!! (don'tcha hate it when that happens!!??) :-)"
Here is Joe Garlitz's excellently explained analysis method for PEC.
Vincent Steinmetz periodic error analysis of belt and pulley drive http://xmcvs.free.fr/ep/asca_eng.htm
To null residual drift and optionally write GUIDE.DAT, follow the following procedure.
As a pattern to copy, a dummy PEC file is included with PEC values of 0, named PEC.TXT, found in the scope.zip file. Rename it to PEC.DAT if you wish to use it. Turn PEC on after synchronizing both motors' output shafts to a predetermined angle. Turn tracking on, set the handpad function to 'guide', set the speed button to microstepping, and when the guide star is centered on the crosshairs, flip the mode switch momentarily to the left, turning on guide. The scope will now move at GuideMs speed, a slow additive/subtractive speed from the tracking rate. Using the hand paddle direction buttons, keep the guide star centered in the crosshairs. Soon beeps will be heard, signifying the synch point of the PEC cycles, the alt beep being higher in pitch than the az beep. Flip the mode switch on the hand paddle momentarily to the right to end guide after hearing at least two beeps from each axis. A GUIDE.DAT file is written with PEC indexes and the resulting guide values as input from the hand paddle. This can be used to create a real PEC.DAT file. Import the GUIDE.DAT file into a spreadsheet, and graph the curve. You can throw several GUIDE.DAT files into the graph, to get an average PEC. Smooth the values and place them in the PEC.DAT file.
Regardless of whether the PEC.DAT file exists, and regardless of whether PEC is on, drift values are calculated on the spot, displayed on the screen, and adopted immediately by the tracking function.
If only drift calculation is desired, center the star in the crosshairs, initiate guide mode by a momentary flip of the handpaddle switch to the left, guide for a minute or two, then end guide mode by a momentary flip of the handpaddle switch to the right.
Drift nulling can be done again, on top of the last calculated
drift null values to further refine the nulling, if desired. Usually
once through the drift nulling process is good enough until the scope
is pointed at a very different location in the sky. After moving to
the new part of the sky, repeat the drift null process after first
zeroing the drift values via the menu.
Since the last section was written, much has been done to enhance the generation of periodic error correction on the fly. The two principal errors affecting guiding accuracy can be fixed at one time: the periodic error of the gear that the stepper connects to, and drift. The drift may need to be tweaked after pointing to another section of the sky.
There are two methods to choose from: fixup an already generated PEC or generate a PEC from scratch
The PEC in use by scope.exe can be displayed by using the hotkey 'c'. If you wish to save this PEC to PEC.DAT, answer yes.
To synchronize the PEC with the motors, use the handpaddle to move both motors to their respective synchronization angle, then select 'p' or pec on/off on the keyboard. PEC is now synchronized with the motors and will remain synchronized unless the motors stall.
To generate PEC from scratch, or to to fixup an already generated PEC, put the hand paddle into guide+save mode, and guide until at least two high beeps and two medium beeps are heard, preferably until a low beep is heard, indicating that the guide array has been filled. The higher pitched sound indicates each time the altitude motor crosses the synch point. It must cross twice in order to generate a full motor rotation of data. A medium pitched sound is used to indicate the azimuth motor's crossing of the synch point. Multiple rotations of the motor will yield multiple PEC analysis files. Better results may be obtained by focusing on only one axis, then repeating the entire procedure for the other axis.
If using an equatorial telescope, or using an altazimuth telescope where one axis is moving very slowly (altitude axis when on the meridian for instance), then you will have to either misalign the scope or find another section of the sky where both motors are moving at a nice rate.
Use the 'u' and 'v' hotkeys to inspect the just completed guiding corrections. If you wish the guiding corrections will be saved to PECALTxx.TXT or PECAZxx.TXT files, where 'xx' starts at '00' and is incremented for each file saved. This allows you to build several iterations of guiding corrections in order to average them for a more accurate PEC. In addition, the drift should shrink to zero as it is refined. Use the 'e' and 'f' hotkeys to read in all the PECALTxx.TXT or PECAZxx.TXT files, average them, then display the results graphically, including how the pec will look if the averaged files are added in.
Use the 'L' hotkey to reload PEC.DAT.
Set TestString to "PreloadGuidexx.dat" to preload guidealt.dat and guideaz.dat upon program startup. This gives an opportunity to analyze and pull out pec data from earlier guiding efforts.