Guiding Error Tests

See the 2nd set of tests here.

The following are some tests I ran using my Atlas EQ/G mount to see if using the EQMOD new Periodic error correction while also using an autoguider would produce better results than using the autoguider alone.

The following were the conditions of the test:
Scope: Astro-Physics 160 EDF (1200mm focal length) Camera: Test camera.
Guide Scope:  Orion 80mm Guidescope (910 mm focal length)  Camera:  Autoguider.
Software: PHD guide, EQMOD, K3CCD Tools.
Test camera:  Philips SPC900NC Webcam using K3CCD Tools 3.0 to log the data.  PEAS was used to analyze the raw data and graph it here.
Autoguider: Meade DSI II with 0.2 second exposures.
Test Star: SAO 147630 / 31 Eta Ceti.  3.5 magnitude.

The same star was used for each test, as was the same settings for each camera.  31 Eta Ceti has a declination of -10 degrees 8 minutes.  For the first two test the mount was almost perfectly balanced.  For the third test the mount was made significantly heavier on the counterweight side while tracking in the Eastern sky.  There was no wind and the sky conditions and seeing were fair with some haziness. 

Test 1: EQMOD PEC + Autoguider
This is the result of having the PEC turned on during the autoguiding session.  It appears as if the PEC and the autoguider would occasionally counteract causing a 3 or 4 arc second shift at about 400 seconds and again at 900 seconds.  

Test 2:  Autoguider Only
This is the result of not using PEC and just letting the autoguider work with the mounts sidereal tracking.  This still shows occasional shifts of about 3 arc seconds.  I have seen similar errors before while imaging which were solved by unbalancing the mount.  This lead to the third test.

Test 3:  Autoguider only + Mount off balance
With this test I unbalanced the mount making it counterweight side heavy so that the tracking motors would be lifting against gravity.  In the past I have found this to help with tracking accuracy.  This was done without PEC.

The best tracking was with PEC turned off and the mount off balance.  Average PE about .3 arc seconds with a maximum plus or minus of about 1.1 arc seconds.  This is quite good tracking for this mount and shows the importance of having the mount off balance while trying to do careful tracking..  

PEC + guiding and guiding alone seem to have about the same amount of error with almost 3 arc seconds plus or minus and an average error of almost 1 arc second.  Shifting the weight on the mount so that it is not balanced, but lifting against gravity improves tracking performance by a factor of 3!  This I found surprising and in the future in the field I will always adjust the balance accordingly. 

Based on these three tests it is more important to have the mount out of balance than to have PEC while autoguiding.  Further testing would be required to see if exposures over 1 second of the autoguider would be better with PEC than without. With a longer time between autoguider corrections PEC may become more significant in improving tracking accuracy.  For autoguider exposures 0.2 seconds PEC provides no advantage at all over autoguiding alone.


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