DEC and RA PHD guiding graphs are mirror images. Normal?

[russ_gibbons]

My DEC and RA guiding graphs are often mirror images of each other, or nearly identical copies. (shown in PHD guiding pictures below).    The DEC vs RA graph in the lower right shows diagonal lines (indicating the two axis are nearly the same).   Usually the DEC & RA the graphs are very different (which I expect), but sometimes they are almost identical (which I don't expect).   Is there something going on wrong, when the graphs are so similar?

 

 

 

I'm using PHD2 S/W, with Orion SSAG auto guider camera, with Orion 80mm guide scope, on a Meade LXD55 mount.  Guiding in RA (hysteresis, or resist_switch) and guiding turned off for DEC (just polar aligned).  From California USA, looking toward the belt of Orion when it is high in the sky. Using DSLR with 300mm lens for imaging.

Thanks,

Russ

[Owmuchonomy]

That is very strange. I can't diagnose hopefully someone else will but it looks like it hasn't calibrated on a star or it has lost the target star judging by how far your crosshairs are away from your target box. The diagonal trend is symptomatic of something but not sure what yet.

[Zakalwe]

Why would you turn the guiding off in DEC? I would  switch it back on and then see what graph you are getting.

[DaveS]

Have you done a drift-alignment?

My guiding target shows hits all around the centre, with no overall trend, exept a couple of nights ago when I had a north-south trend briefly near the meridian.

[happylimpet]

I would definitely suggest increasing the calibration step size, as it looks like it isnt moving the scope enough to properly distinguish the RA and DEC axis directions. You can also 'force' RA and Dec to be perpendicular (which clearly isnt the case for your calibration)  which is probably a good idea but probably wont in itself fix the problem without a proper calibration.

[happylimpet]

and for heaven sake turn your dec guiding back on!

[ollypenrice]

Interesting graph.

FIrstly I'd set the guide camera to be aligned along RA and Dec whatever it says in the manual about the software coping. I just consider it good practice to remove as many sources of confusion (for me and the computer) as possible.

I'd guide in Dec for sure but not necessarily in both directions. When you have a little polar misalignment you can reduce spurious input and resultant backlash by guiding only in the direction needed to correct the polar error. We have one mount in particular which really appreciates this trick.

Olly

[r3i]

I'd guide in Dec for sure but not necessarily in both directions. When you have a little polar misalignment you can reduce spurious input and resultant backlash by guiding only in the direction needed to correct the polar error. We have one mount in particular which really appreciates this trick.

I often set the Dec guiding to one direction only , it certainly helps to smooths the guide graph particularly when guiding with an OAG at a long focal length.

[happylimpet]

Aha I didn't know you could guide in one direction in Dec! That'll help me for sure. Cheers.

[russ_gibbons]

Thank you for the replies.

I do plan to return to guiding in DEC, when I save up the funds to afford a proper mount.  It doesn't work well with my current low-cost mount.

History of this mount, if you are curious, otherwise skip this indented section in italics: When I had tried to guide in DEC, I would get a slow drift moving more than 40 arcseconds, then a quick sudden jerk with overshoot,  looked like a saw tooth waveform; the DEC movement appeared to suffer from Stick-tion, and would store up energy for a quick release like an earthquake fault-line.  My Meade LXD-55 mount is rather low end, and doesn't have ball bearings,  it has plastic ring washers between the rough cast iron mount surfaces.  So I spent many weekends polishing the surfaces of the mount and gears, changed to Teflon washers between metal parts, replace sticky grease with super-lube, excanged worms and gears between 3 mounts to find best combination (I bought more used mounts for spare parts, since the material aren't very sturdy and have broken several times);  in the end I was able to get an average  RMS error in DEC  about 4 times what I usually get in RA).   I found I get better results by turning off DEC guiding, and instead spend 20+ minutes doing polar drift alignment.  This allows me to take short 3-5 minute pictures for stacking, using my DSLR and 300mm camera lens, for the larger brighter objects (Andromeda galaxy, Orion Nebula....).  When I get a better mount (later this year I hope), I'll turn DEC back on, and use more magnification from my refractor and SCT scopes.  But I fear that the mirror image graphs of DEC and RA is a setup error that might follow me to whatever mount I use.

I've tried different calibration step sizes from 300 to 1000,  (the Value PHD calculates for my setup is under 500), changes in step size haven't seemed to make a large difference (although very small steps take longer to calibrate, and I hear that very large steps  decrease accuracy).  But I do notice that DEC movement usually isn't as big as RA movement during the calibration, so this may be a problem.

Doing DEC guiding in one direction did help, (given the sticktion I have in DEC movment, I only got overshoot in one direction instead of two).

I'll try rotating the camera, to see if this helps decouple DEC and RA movement. (Clouds are forecast for the the next week, and it is the rainy season here, so it may be a while before I have results).

Thank you,

Russ Gibbons

P.S.  below is graph with DEC guiding in one dirction, after working to make the mechanical gear movement as smooth as I could; taken when RA guiding was working better than usual on a good night;  (DEC guiding was not as smooth as doing a drift alignment and turning DEC off).

[happylimpet]

I use 2000 at 1500mm so I would deffo try more....but just as an experiment! But i remain hopeful.