Should I train my drive (PEC) for auto-guide accuracy

[Robert_T]

Hi All, I just can't seem to get consistent auto-guiding from my LXD75 mount using PhD. It seems fine for a while, then will wander off or start requireing large corrections that show on the image. I've tried tweaking up and down every setting in PhD I can find. I did fiddle with a setting or two in the autostar menue, but that seemed to make thinsg much worse and I could no longer calibrate ... don't know how I got that back, but it's scared me off touching any of the autostar mount settings.

I haven't gone to the trouble of "training" the drive for PEC and wondered whether that might be a cause. Is this generally considered necessary for good auto-guiding performance:shrug:

[g__day]

I'd say it would help - but more when your set up is fully operational anyway. Its common to train PEC using guiding software like PHD - so if PHD isn't working - well your PEC training may not be too fine!

PHD takes a bit of work, I find the Yahoo Groups stark-labs very helpful and Craig is a great guy who answers questions really promptly. I would check your settings on PHD and see that you have everything optimally set. What parameters do you use by the way - more information would be good, because I think you have to fix your guiding over-correcting before PEC will help.

[Robert_T]

Thanks G_Day, I wouldn't say PhD isn't working. Just my guiding seems fine sometimes and others not. Attached is a screen view of the seetings I'm currently using (or ended up) with in PhD, though I've really pushed these up and down a lot. e.g. I've had RA aggression up as high a 120%, pulse length as high as 1400 (I'm using a very short - 380mm FL - guidescope).

I've tried increasing and decreasing the search window and have ranged the min pixel movement from 0.25 to 0.1.

Everytime I think I'm onto something and it begins to work fine for minutes or even 10's of minutes at a time it then either very slowly loses sync drifting away from the cross-hairs, or starts to oscillate in big corrective jumps that eventually get it to lose sync ... I get the feeling that PEC might be involved with settings suiting one part of the drive train not being suitable for another:shrug:

[g__day]

So you're guiding on the 80mm or 66mm refractor and imaging on the C9.25?

I typically have the calibration step around 400ms or lower and the min pixel before pulse guides are sent around 0.75 - 1.1 pixels (seeing, pixel size and focal length dependent).

That last point causes folk alot of grief I believe. If your seeing is poor, and a star covers alot of very small pixels you don't want to in effect over sensitise PHD. Ron Wodaski (google) has an online pixel calculator that can be used to assess how much of the sky (arc seconds) each pixel of your guide camera is seeing and whether you should be binning pixels to form a super pixel. Its pointless over driving the sensitivity of the guide camera when seeing is poor.

Can you confirm which camera is on what scope - that they are firmly mounted to reduce differential flexure and mention the focal length of your guide scope and the pixel size of your guide cam?

[Robert_T]

Ah, sounds like I have over-sensitive settings that are being affected by seeing...that would explain the guiding OK sometimes and then not if it is seeing dependent.

As for which scope, I use every combination (or will do)... e.g. 66mm to guide camera on my 80mmED (most common), 66mm to guide the C9.25 with reducer, C9.25 to guide and camera through the 66mm, guiding with the 80mm and piggy-back lens on top (next most common).

I have rock solid guide rings and mounting. I'm confident that flexure is not an issue.

cheers,

Rob

[g__day]

Can I suggest the following settings.

Set the pulse length between 300 - 600 ms - whatever it takes so it calibration takes about 25 pulses to move the star initially far enough - then always use this setting.

Use a aggressiveness between 70 - 90% (I haven't mastered this one yet myself).

Try several runs with aggressiveness say at 80% with minimum pixel before a pulse is set to 0.7, then 0.75, then 0.8 etc all the way up to 1.3 pixels

Keep the image refresh rate up to around one update every 2-3 seconds if you can - this seems ideal for me using a Meade DSI (mostly with an 80mm refractor). If you are updating 60 times a second - you will being seeing a lot of atmospheric turbulence.

Try the latest version of PHD - 1.8.3, enable logging and see if drift looks both balanced in all axes directions (indicating good polar alignment) and smooth vs jaggy (indicating fewer seeing or over correcting errors).

Hope this helps!