Ad Astra Observatory

I saved off one of my larger tracking logs last night, and decided to analyze it for Periodic Error in the mount. To help the data, I downloaded PECPrep from SourceForge. After an initial problem, I realized that I had to get the 1.5.3 full package, not the 1.5.3 exe-only upgrade. 1.5.3 knows how to parse track logs from MaxIM DL v5.

I read in the tracking log, with the following settings:

focal length 400 (Pumpkin)
pixel size 13.75 x 16 um (ST-4)
everything else default (bin 1×1, 0, 0, no check)

I set the mount to “G11”.

The verdict:

Peak PE+ : 12.42 arcsec

Peak PE- : -12.89 arcsec
RMS PE : 1.88 arcsec (wow)
Avg PE+ : 1.60 arcsec
Avg PE- : -1.30 arcsec
Max rate: 7.82 arcsec/sec (ouch)
Max Delta + : 16.88 arcsec
Max Delta – : -10.66 arcsec

Avg. Noise : 0.02 arcsec

Trend Y = 0.0000X + 0.3

Those last two (if I’m reading them right) indicates that the data is relatively good; a linear trend in Y means (to me) that I should have turned on Dec compensation in the settings. The noise figure means (to me) that the data is relatively clean wrt wind, people tapping on the OTA, etc.

So the overall feeling I’m getting here is that the mount is generally good; an average of <+/-2” of PE = 4” to 5” peak-to-peak (from the RMS = 1.88” figure) is excellent. The mount is totally worthy of doing astrophotography, which we knew already; it shows flashes of greatness.

Quite troubling, however, is the Peak PE (25” peak to peak) coupled with the high max rate ~8”. This means that the PE is not smooth; that things will be going along fine and then all of a sudden, a 1-pixel jump can happen (ST-4 = ~8”/px). This is also something I’ve noticed.

Looking at the Frequency spectrum, I’m getting spikes in the graph exactly where I was expecting and hoping: at integer divisions of the worm period (and not, for instance, in the bearings, races, or motors). This means to me that I should be able to tune the PE down a little bit by adjusting the worm.

I need to spend more time analyzing the data.