Huge error in DEC on polar-aligned SWSA

[Max Romanchenko]

https://openphdguiding.org/logs/dl/PHD2_logs_ceQg.zip

I encountered an unusual error, and couldn't resolve it on my own.
I set up everything as I successfully did dosens of times, polar-aligned, started calibration to further drift-align, and got an error that there is "little or no east movement". I checked ST4 cable, - all connections were tight without tension etc. I did several attempts to do polar alignment from scratch, with subsequent calibration pointing at different parts of the sky.

When imaging even without guiding, just tracking, there was just enormous drift in DEC, even though I polar-aligned pretty precisely via build in polar scope.

My best attempt at 2024-06-07 00:14:29 looked like two minutes of decent RA tracking with 400'' of DEC drift.

How is that kind of drift even possible? 

[Bruce Waddington]

You've specified that there will be no Dec guiding done at all so it's not surprising that your mount is drifting in Dec.  The drift could be caused by polar mis-alignment but it could also be caused by something moving or sagging in the guiding assembly or by cable dragging.  That's something you will need to track down.  That said, you're operating in a very degraded mode by using ST-4 guiding with no information about scope pointing position or many other things that should be known.  The failed calibrations have already told you everything we can know - when the guide commands are changed from west to east, the mount doesn't react at all.  Try using the Star-Cross tool (Tools menu) to get a clear picture of how the mount is reacting to pulse-guiding.  It's quite likely that the ST-4 cable is damaged which is one of many reasons we discourage this kind of configuration - you should really start using ASCOM guiding for your mount.

Good luck,

Bruce

[Max Romanchenko]

Ok, guys, I figured out the problem. Indeed, "something was moving".

The story developed the following way: 
1. I had several successful night sessions following the same alignment routine as usual, - no strange DEC drift.
2. This night the error reappeared: no matter how well my polar alignment was - stars drifted in DEC with a constant speed of several seconds a minute. Hmmmm...

The only difference between successful and failed sessions was geography: good tracking was happening while photographing in the Northern province of my country.
This led to the culprit of the problem: looks like the friction mechanism on the DEC bracket loses friction when the ambient temperature is above a certain level (we are having extremely hot summer nights), so a slightly misbalanced setup slowly drifts to the heavy side, no matter how hard I tightened the screws.

Good balance in DEC did the trick.

Issue solved!

середа, 12 червня 2024 р. о 05:17:30 UTC+3 bw_m...@earthlink.net пише:

[Bruce Waddington]

Glad you figured it out, thanks for letting us know.  Now maybe you can move beyond ST-4 guiding... :-)

Bruce