PHD2 Erratic graph

[willuk2010]

Hey,

so, last night i tried attempt 4.. at the Orion Nebula... couldnt get above 30s without having eggy stars..

This time i used AlignMaster to try and polar align, i thought i did ok, but once i started guiding PHD2 was doing strange things with the graph, i would have though if it was a polar alignment issue, would the peaks and troughs be more uniform/symetrical? rather than just random drops.. I hope that makes sense, i cant really explain it! lol

There was nothing near the mount, it was out on the patio while i was in the summer house on the PC... unless one of my cats jumped on it!

[Zakalwe]

Hard to know, but I would suggest not picking a star so close to other bright stars. The PHD algorithm might get confused and "jump" from the guide star to one of the other ones. I'd also go for a star that's not so bright and saturated, as a fully saturated guide star makes it harder for PHD to calculate the centroid.

[willuk2010]

Hey Zakalwe,

Makes sense, that was using the "auto find me a star" thingie.. and thats what PHD selected... Next time, ill try some different stars, see what happens.

[r3i]

It might be getting confused

Hard to know, but I would suggest not picking a star so close to other bright stars. The PHD algorithm might get confused and "jump" from the guide star to one of the other ones. I'd also go for a star that's not so bright and saturated, as a fully saturated guide star makes it harder for PHD to calculate the centroid.

You just beat me to it :smile - I  was thinking the same that PHD is possibly getting confused by the 2 bright stars either side of the guide star.   PHD is quite sensitive so try to pick a star on its own that not so bright.

[Russe]

Very odd indeed! +1 on the above mentioned

what exposure time did you use? 2s?

the picture seems a bit bright - bigger stars are harder to guide from - I think I read this in the PHD help file...

I usually guide from 1s exposures but was recently thinking, maybe if I went shorter it might just work better - just a thought.

[willuk2010]

Hey Russe, that was 1s exposures in PHD..

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[Russe]

Hey Russe, that was 1s exposures in PHD..

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mhm... at a loss then..

but I had all my equipment play up Saturday night - weather change in static???

[willuk2010]

Am I right in thinking if it was poor polar alignment it wouldn't do what I'm seeing on the graph?

also I'm having some balance issues, I'm waiting on an extra AVX counter weight.. Could poor balancing cause this?

Also lastly, there is an error number I see, that is visible in APT too, can be at a Max of 2.55.. (Total RA/DEC error is It?) What should this number range between? In order to know guiding is good, and eggy stars will be at a minimum?

Cheers!

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[glowingturnip]

+1 on not picking a double star - I had a session aligned on one of a close pair once, the graph was lovely and smooth, and the scope wasn't complaining, but on reflection, about 10 of the subs had trailing at exactly the same angle and separation as the guide star pair.

your erratic guiding error there seems to be in Dec - if it was just poor polar alignment, then i think you'd see a dec line that constantly drifts off in one direction until phd tries to pull it back.  Would make more sense with the star pair theory, since they will be at slightly different declinations to each other, and phd will be randomly flipping between the two.  I may be wrong, but i don't think balance would affect Dec tracking at all, just RA ? 

Regarding the error numbers, iirc the 'RA Osc' is an important number and has an optimal range for it (which I think is not too far off from 0.35 as you have it) - check in the PHD instructions. 

I didn't know there was a PHD2 out, will have to have a look...

[willuk2010]

Cheers guys, I will try selecting a dimmer isolated star next time rather than using the auto select option see how it goes.

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