Still having guiding trouble

[Sparrow]

Anybody here good at interpreting the PHD graph?

I've not figured out how to save an image of the graph so I'll describe it.

The blue line (RA) is reasonably well behaved and stays close to the centre line with only small deviations.

The Red line (Dec) is the problem. For a few minutes when guiding starts it will behave and then very gadually drift downwards away from the centre line. It looks like it's trying to correct but just doesn't make big enough corrections.

I have tried various settings on the following:-

RA aggresiveness 100

RA hysterisis 10

Dec guide mode Auto

Dec algorithm normally "resist switching" but have also tried lowpass filter

Dec slope weight 5.00

Calibration step (ms) tried anywhere from 750 up to 1800

Min motion (pixels - tried from .15 up to .60

Noise reduction - none

Time lapse - 0

Camera gain 100%

I'm using the 8" newt on the CG5 mount with the 4" skywatcher as guide scope using the MEade DSI I with a 50% reducer. Getting good strong images of stars. Pulse guiding through the telescope handset. The rig is set slightly nose heavy as per recommendations.

Calibration is always successful - the number of steps depends on the various settings I have tried but on my usual settings it calibrates in about 20-25 steps. The only thing that puzzles me is that when watching the calibration, it works fine in the first 3 directions (right, left up) But on the last one where it should move the image down, it's counti8ng up calibration steps but the image doesn;t seem to move back down and ends up away from the original position when it starts guiding.

One last thing - This problem last night was while I was imaging the veil which is very high up from here in Chester - the scope was probably about 10 degrees from vertical. I wondered whether this might be a factor.

ps - I did check and nothing was snagging the mount such as cables etc.

Thanks in anticipation of help

John

[greenkat]

Just a thought, is the scope balanced ok ? Maybe a bit of slipage ?

[Glider]

To me, everything sounds OK except, Veil, big bright star in the middle!!

If the 'star' you choose is so bright that the star profile is flat(saturated) then you can have problems.

I even run slightly defocused.

Have a search around SGL threads for these issues.

[OzDave]

It sounds a bit odd that you don't see calibration movement in all four directions.

Try setting the calibration step size to 5000 so it tries to calibrate in really large steps. It should calibrate faster, but you should also notice the scope moving in each direction.

The calibration step size doesn't affect the guiding accuracy so using a really big step size is ok, and I use it sometimes to overcome backlash, especially in DEC.

The other thing it could be is DEC backlash. If you see the star moving one way in DEC calibration, but not back the other way, it could well be DEC backlash affecting you. If so, you'd maybe want to use the Resist Switching mode so that PHD will only try to make DEC corrections in one direction to avoid having to deal with the backlash.

Personally though, if you're getting big DEC deviations, maybe the polar alignment is out and you'd be better off doing a more accurate alignment. At the moment, I go for a better alignment and only guide in RA. I don't know if that is a good idea or not though, but has worked out ok so far for me.

Dave

[r3i]

Can't remember where I read it but the general consensus was to alter the calibration step size so that the calibration is achieved in 10-15 steps.

I had very similar problems in the past. As mentioned already, the main things you need to check are polar alignment and balance. PHD can go only go so far in compensating for errors.

Another issue may be mount capacity - is the equipment that you have on your mount overloading it?

[ollypenrice]

Dec backlash problems can be reduced by running camera end heavy, just as you aim to run slightly west heavy, to keep the gears meshing on the same side.

I don't know PHD but does it have a parameter for cancelling guiding once a certain level of error has been reached? In this case if your polar alignment was off then that limit might be reached quite quickly and you'd then not get any corrections sent through.

Olly