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Guiding Performance - why PE isn't everything

Here are three guiding graphs, all of Capella, all with a 50mm finder guider and Meade DSI.

1) 0.28 px RMS (@ 9.71"/px) = 2.72" RMS

2) 0.11 px RMS = 1.07" RMS

3) 0.05 px RMS = 0.49" RMS

So #3 is not leaps and bounds ahead of #2 - but the difference is, #3 guides like that practically forever. No deviations, no jumps, no backlash...

I think this is something that is overlooked when talking about mount performance... periodic error isn't everything. Having smooth and low PE will give you decent guiding performance, but...

1) is the PE the same on both sides of the meridian? (unless you move the CW to be always east heavy) i.e. how sensitive is the PE to balance

2) how is declination guiding performance? is there a large dead spot? (this information is useful because if the scope gets nudged by wind or cable drag, how long does it take to reverse direction and correct the DEC issue)

3) do you have to watch the mount like a hawk to ensure that guiding performance is always within acceptable limits?

my #1 above is the best case I could get with that mount. Worst case is much worse (huge DEC deviations that don't correct...)
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TAK FS-102, GSO 12" F4 Newt, EQ8, Atik 11000, QHY5L II and nothing but clouds. Gallery
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Did you happen to take long exposure photographs with each setting? I'd be interested in seeing the difference in star shape.

It's also odd that the OSC index isn't switching in the third test. Based on the graph, I'd thinking #3 is the best, but look at the OSC. It doesn't seem so good.

The most important thing is the pictures.

Lackawanna Astronomical Society
My Astrobin

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Adam, that was due to misalignment in altitude. So the RA runs away in one direction.
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This RA drift might invalidate your test. If there is significant drift then the difference in guiding performance with and without PEC is reduced. Also, by using Capella for your test, which is about 44 degrees declination cuts the effective periodic error down by about 0.7x.

A proper test of the effects of PEC on image quality should be done when RA/Dec drift is minimal, seeing is good, and on a star near Dec=0 near the meridian. That will separate the effects of periodic error from other tracking error sources.

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