PEC was a bust and a note on focusing

After recording the periodic error in the AP1200, I was very excited to try it out.   Last night, I started my O3 imaging run of M16 and turned on PEC.   (Called "PEM" in the settings.)  It was a very odd result.   There were these periodic large excursions in the DEC that only happened above the "zero line" in the PHD2 graph.   The first two 20 min exposures were pretty decent -- except for vertically stretched stars!   :(

The other thing to note is that when the large excursions DIDN'T happen, the guiding was really good.   I'm guessing like .3 arc sec error in both DEC and RA.

So here's where my lack of methodical troubleshooting comes into play:  the other time I've seen
this kind of error was when I was shooting my 135mm lens and had no way to lock down the lens and when the lens was "sideways", I had these oblong stars.   My theory then was simply that there was "sideways" creep which corresponds to tiny slippage coincident with the DEC axis.

On a weird hunch, I tried moving the Dec axis by hand and it easily moved!   So I tightened
down the knobs that hold the DEC in place.   But to hedge my bet, I also turned off PEC.   Well,
it went back to the old predictable behavior and the stars were basically round again.   But my guiding was around .6 to .8 for both DEC and RA.   The stars were noticeably shimmering.

The thing to note is that I did TWO things to address the issue and I'm sure only one of them actually fixed it.   In my next session, I'd like to try PEC again but I need to make sure the DEC is tightly locked down.

Another thing I wanted to note was my focus behavior with the bahtinov mask.   So, I always
start the imaging run with a bahtinov focus run which can take like 10-15 minutes because I have to use 30 sec exposures due to dimness of the O3 data.   I put some tape down on the focuser tube so I can get pretty close.   Because the focuser slips easily (ARGGGGGHHHHHHHH!!!!!), I have go through many more iterations of unlocking/relocking the focuser than I would with a decent focuser.  About every 3 exposures (1 hour), I will focus the image.

So, my initial understanding is that as the temperature drops, the metal contracts, right?   Right?!?!?  This would imply that I would have to move the camera just a tiny bit further away, right?   Well, for some reason, I have to do the opposite -- I have to move the camera CLOSER in toward the objective.   This has been a curious mystery to me.

My only theory about this is that the spacing in the triplet lens does shrink which causes the focal point to move back.   That sounds crazy to me but that may be what's happening.

Regardless, I've wondered if I can improve my focusing routine.   So when I start an exposure after focusing, I'm "perfectly" in focus.   So after time, it can only get worse.   When using 20 min exposures and falling night temperatures, I'm probably slightly out of focus at the end of the 20 minutes.   By letting SGP Pro do a second exposure, I'm pretty much guaranteeing that it will be slightly out of focus.   So, I've thought about going slightly "closer in", then starting a run of 3 exposures so it "falls into" focus somewhere in the second exposure.   It's very time consuming to focus so I feel like I need something to keep me from spending half the night focusing.

[Later:  To do this, I have to look at the bahtinov pattern and figure out which "lobe" is smaller WHEN the focus tube is too far INSIDE -- meaning closer to the objective lens.   Then settle on this slightly INSIDE focus so that as temperatures fall, it will become ideally focused.   I don't know if this is a bad plan or what.   I also would need to check temps to see how much of temperature drop requires a focus adjustment.]