Guiding Error Tests Part 2

After my initial tests, I realized that in order to get a clearer picture I needed to do many more tests and have much better control to get any useful information.

Tonight I did the following tests:

  1. Baseline tracking without periodic error correction (PEC) or guiding.  This test establishes the mechanical error inherent in the mount.

  2. Tracking plus autoguider at 2 second rate of correction.  This test measures a realistic situation with a moderately faint guidestar.

  3. Tracking plus autoguider at 1 second rate of correction.  This is perhaps the most common situation one would see in the field.

  4. Tracking plus autoguider at 0.2 second rate of correction.  As discovered in my previous test, this provided the best tracking accuracy.

  5. Baseline tracking with periodic error correction (PEC).  This test shows how much a rudamentary PEC training session improves tracking performance.

  6. PEC plus autoguider at 1 second rate of correction.  This is the test to see if PEC helps at all to improve tracking performance over just using the autoguider.

The following were the conditions of the test:

Date:  November 6th 2007 - 8:30PM to 12:30AM
Slightly hazy, overall medium to poor sky quality.  I wouldn't bother taking the scope out on a night like this, but stars were visible and I was itching to do these tests.
Astro-Physics 160 EDF (1200mm focal length) Test Camera: Philips SPC900NC Webcam using K3CCD Tools 3.0 to log the data.
Mount: Orion Atlas EQ-G using EQMOD
Guide Scope:  Orion 80mm Guidescope (910 mm focal length)  Autoguide Camera:  Meade DSI II using PHD to autoguide.
Test conditions:  As learned from the previous test, the scope was left unbalanced exactly the same as the final test that yielded the best results on the previous test.  The balance was not changed for any of the tests tonight.  Each test was for approximately 1000 seconds (16min 40 seconds) to mimic about the longest exposure I would expect to use with my DSLR camera.  Both guide camera (Meade DSI II) and monitor camera (Philips SPC900NC) were carefully focused to optimize results.  Guiding was performed using the default settings in PHD Guide and the default settings in EQMOD pulse guide of x0.10 on both RA and DEC.  The first 4 tests were using the same 6th magnitude star high in the Eastern sky at about +5 degrees declination.  For the 5th and 6th tests I moved the scope to a different star about 10 degrees East due to the fact that by the time I got to the 5th and 6th test, the first star had moved to very near the meridian.  Moving past the meridian would have seriously impacted performance with the mount unbalanced on the counterweight side as the balanced shifted.  The second star was very close to the same magnitude and less than a degree different in declination. 
Polar alignment:  The mount was polar aligned simply using the polar alignment scope.  During the unguided tests I saw about 7 arc seconds of Declination drift in 1000 seconds of tracking.  I consider this good polar alignment.  However, see the Potential issues below.
PEC training:  I trained the mount for 1000 seconds using the same star as the 5th and 6th tests using PERecorder.  I saved the PEC training session using all the default settings in EQMOD.  I didn't change the Phase Shift or Gain in EQMOD.  
Potential issues:  During unguided tests the mount showed significant drift to the West. You can see these trends in the raw data gifs below.  Even though the average error was only 26 arc seconds, most of the RA drift was to the West with the PE curve stepping Westward.   I can't explain why this would occur.  It may be due to K3CCD seeing some Dec drift as RA drift with the camera not being perfectly square to the axis of the mount.  However I was pretty careful to align the axis of the camera to the movements of the mount by slewing back and forth with the crosshairs turned on with K3CCD Tools.

I ran out of time during these tests (it being a work night and all) to test PEC plus autoguider at 2 second rate of correction.  Before doing longer exposure tests, I would like to spend more time improving the PEC performance as I feel there could be a lot of improvement there which could substantially improve the PEC results.

Lets get right to the results.  All values are in arc seconds.

Data Summery
Baseline PE Test 1 2 Sec Guide
Test 2
1 Sec Guide
Test 3
0.2 Sec Guide
Test 4
Baseline PEC
Test 5
1 Sec Guide PEC
Test 6
Max + 24.65 2.01 1.17 1.09 13.12 1.44
Max - 26.17 2.91 1.61 1.18 18.08 2.1
Avg + 12.87 0.97 0.47 0.44 5.86 0.46
Avg - 13.01 0.96 0.68 0.44 5.86 0.45
Periodic Error Correction vs. No Periodic Error Correction
Baseline PE
Baseline PEC
Test 5
Max + 24.65 13.12 11.53
Max - 26.17 18.08 8.09
Avg + 12.87 5.86 7.01
Avg - 13.01 5.86 7.15
1 Second Autoguiding with and without Periodic Error Correction
1 Sec Guide
Test 3
1 Sec Guide PEC
Test 6
Max + 1.17 1.44 -0.27
Max - 1.61 2.1 -0.49
Avg + 0.47 0.46 0.01
Avg - 0.68 0.45 0.23

Test 1:

Test 2:

Test 3:

Test 4:

Test 5:

Test 6:

The main reason I wanted to do these tests was to determine if the new EQMOD PEC would work with guiding or if they would conflict.  Clearly the mount tracks fine with PEC and guiding.  As one would expect, the quicker the guiding, the less need there would be for PEC.  With 0.2 second guiding there is probably no need for PEC.  At 1 second guiding, the PEC didn't make any difference in the maximum error, but it did improve the average error significantly.  

At this time, I feel I need to work on improving the PEC as reports on the EQMOD Yahoo Group indicate that some users are getting +/- 3 arc second PE compared to my +/- 18 arc second PE.  Once I get significantly improved PEC I will redo these tests.

Testing with autoguider corrections longer than 2 seconds is probably not necessary, assuming one is guiding at all.  With 2 second exposures pointed at almost any random spot in the sky will show a suitable guidestar with a camera such as the Meade DSI II on an 80mm guidescope.  


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