PHD Guiding

[whzzz28 (Nathan)]

I only recently started using PoleAlignMax, i used to do it all via PHD drift alignment.

Instead my procedure is now:
Setup mount as perfect as i can get it manually, this normally is within 5 degrees.
Run a PoleAlignMax and follow the procedures (if you are a long way out, you will probably run out of screw space to change azimuth settings...).
This normally gets me VERY close. I set my CCD to take 1sec exposures continuesly and then turn on cross hairs on my image, move the azimuth/alt a bit and wait for the next image.
Takes about 10mins to do the PoleAlignMax run.
Then start up PHD and do a drift align to fix the last little bit of error.

First time i did this it took me under 20mins to get a good polar alignment. When i was using PHD to drift align it often took me 30+mins.

[widow18 (Pete)]

Thanks everyone, lots of good information there.
Still would like to know why the numbers turned red, I will try and contact the designer.
My oval stars were not field rotation, I was only doing 180 and 120 second exposures anyhow. My finder FL is 400mm and imaging scope is 1000mm, don't know what that does to the equation.
Anyhow, thanks again.
Peter

[Garbz (Chris)]

Originally Posted by alistairsam

When you say flatlined, do you mean they're flat along the centre line or beyond the upper and lower limits?
I recently had one of my best guiding sessions where ra and dec were very close along the centre line throughout, but the images showed up elongated stars in the same direction, sort of star trail. I'm attributing it to a focuser tilt so will test for that by rotating my camera, but now am not too sure.
just wanted to check if ra and dec lines along the centre line was bad. I didn't think so.

Along the centre light. Really low OSC index. From the graph you'd think it's perfect, but the results showed quite clear lack of proper guiding with the stars bouncing around and trailing a bit.

I'm lead to believe that if you've got a very flat line then it's not actually good.

Originally Posted by whzzz28

No matter how good your guiding is, if your polar alignment is far off, your stars will be terrible.

I believe terrible is a very strong word reserved for people who are at the level of doing many minute exposures. Certainly the only time I've ever had an issue with guiding was when I pointed my telescope the wrong side of magnetic south so I was 22deg off. PHD noticed it quite quickly when it failed the calibration steps. Some of my best sharpest and roundest stars have been the result of plonking the telescope down and going for it. Guiding was definitely poor as the stars were moving around quite a bit unguided, and yet despite the PHD graph looking like someone was playing drum and bass next to the scope I got some 3min exposures with perfectly round stars.

From my limited experience field rotation is small enough that if the guiding can keep up with a star using a 1second maximum step (this is dependant on mount, guiding method and scope) then field rotation is not relevant within 1-2 mins.

[widow18 (Pete)]

This just in from Mr Stark.

Regarding red Osc numbers:

"The red there just says "Hey, this is probably not ideal..." The Osc-index shows the odds that two adjacent guide commands (in RA) are in different directions. On a perfect worm gear with no periodic error, this would be 0.5 -- the last motion not telling you anything about the direction of the next one -- pure noise. As we have PE, the ideal value goes down. Let's say it's an ultra-smooth worm but has some real PE (just not a jagged curve). Well, as you go up the curve, the next one is in the same direction as the last. Same deal as you go down it. The only time you change direction is at the very top and bottom of the wave. The ideal Osc-index would be very low.

For most folks, the ideal is somewhere around 0.3-ish-or-so-kinda. I color code it to say "Hey, you may want to check this out." If your guiding was great - great! If not, it may be that you've done something like reduced the aggressiveness too much.


Craig"


thats the answer.
Regards
Peter

[mithrandir (Andrew)]

The other trick to improve guiding GEMs is "east heavy". You'll track better if the mount is slightly off balance to the east side, no matter if it is OTA or counterweights. This means the RA drive is always pushing the scope/weights.
Perfectly balanced means RA pushes, coasts and pulls. This makes guiding the hardest.
West heavy and the scope/weight pulls the RA drive.