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Crazy PHD graph


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Polar aligned the very best I could, kit set up and started an imaging run. Phd gave out this bizarre graph, what could make Dec act like this? After the initial spike upwards, the Dec levelled off and I was able to track well for 300sec subs.

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PA being out and DEC backlash can cause this. Guiding starts but the star moves so PHD tells the mount to move. It has to take out all the backlash though before the mount actually moves and by that time the star has move quite a bit. Once the backlash has been taken out, the guide commands actually start to move the mount so the star moves back to the correct position. Or maybe you accidently kicked/moved the mount in the darkness.

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PHD takes a little time to 'settle' after you first start it up, particularly for the Dec axis. If you were able to track well once it had levelled off then I don't think you have any problems at all.

It can be worth switching the Dec control from Auto to either North only or South only as appropriate, depending on which way off your polar alignment is. I've found that this has improved my DEC guiding. I think it helps if your PA is very close and it stops the switching of the DEC commands from north to south and vice versa, which is where the backlash will become evident.

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Yes I agree with the Backlash theory. Is your balance set up RA - east heavy, Dec camera heavy?

I too have guide settling issues with MaximDL. Nothing to worry about, particularly is your finally imaging with 300s subs.

I have read on the stark labs forum about moving the dec axis prior to guiding to take up the backlash.

cheers

Steve

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Yes, mine has several minutes to settle. My usual routine is to slew to the target using AT then run PHD, calibrate and start guiding on the best star. Then I check focus on one of the stars in the imaging camera using FHWM in Artemis. I find a star with binning to get a good enough image and double click it to enable FWHM then set binning to 1x1 to adjust the focus (this is still manual ATM). By the time I'm happy with the focus, PHD has had plenty of time to settle down. I then switch to the Sequencer and start imaging proper.

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The Dec does normally take a couple of minutes to settle down, usually goes half way up the vertical scale then flattens out with the RA. It's never shot straight up off the top of the graph and then back down again lol Must've been a heck of a lot of backlash to overcome? There's a couple of clear nights forecast this week, so I'll see how it is then :)

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