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An interesting thread. Autoguiding is a black art.
Here are some tips that I have learnt. I have only used PHD briefly and it seems very user friendly and does some things automatically. I use CCDsoft.
Basics:
0. Balance:
If you have a guide scope on top and then balance with the scope horizontal it may still be unbalanced with the scope at an angle as the centre of gravity shifts. Make sure it is balanced at an angle which is where you actually guide anyway.
Under balance I would include cable drag and cable management. Poorly handled cables or transformers dragging on the mount can throw off autogudiing or result in 1 out of 3 images being no good.
2. Alignment:
Perhaps less important but still a basic. Have your mount level where you can - it all helps. If on a tripod make sure it won't sink in soft soil under the weight.
Have your guidescope square to the camera and it should be seeing what the scope is seeing. Take an image with your scope and one with your guider and adjust until square or at least make sure it is parallel with the OTA.
If you have a portable setup and image at the same location but lug your gear out each time then try to make the polar alignment repeatable. By this I mean put markers on the ground for the exact position of the tripod or tape marks on your mounts RA and Dec scales etc.
With my dark site mount I found I could take it down there, shove a torch on the polar alignment scope or a laser and it would hit the ceiling. I placed a small piece of electrical tape on the ceiling. I would simply adjust the mount until the laser or torch (laser was better) was on that mark. That would take about 1 minute and it was back to being aligned where I had spent a couple of hours getting it right and marking the ceiling for next time.
3. Flexure:
If you are using a guide scope then you are one step away from the actual image you wanted guided ie the main camera's image. So anything that could make the guide scope shift slightly form what the main camera sees will mean the autguider will be moving the scope away from what the main camera images. Screw fittings, loose focusers, weak mountings of the guide scope, heavy guide cameras, dragging guide camera cables and transformers, main OTA and camera weakly fitted or not totally secure or loose focusers will mean it will be very hard to get round stars at 10 min exposures. Get everything tight, strong and rigid. Threaded adapters are better than eyepiece slide in with tightening screw type adapters.
3. Polar alignment has to be really really accurate. You can buy a cheap screen overlay from Andy's Shotglass Astronomy site called Star Targ.
Using that it becomes easy to polar align using your CCD cam or DSLR if you download the images to a computer.
Otherwise use a reticulated eyepiece and drift align until you have it veyr close.
There is free software to assist with polar alignment like PoleAlign Max. There are others.
PemPro has a polar alignment wizard that is really hot and worth the cost of the program just for that. They have a free trial.
Software Bisque T-Point is also very very good and takes it to a high level of accuracy but that is more for a permanent mount.
I have found in several years of imaging that the most important thing in autoguiding was the polar alignment. You can fiddle with software all you like but if your Polar Alignment is off then you will go nowhere.
Time spent on improving polar alignment is the most productive thing here you can do.
How good do you want it?
Perfect.
4. Only now do software settings come into it. Guide exposure times are important. Work out by trial and error which give best results. I found with my NJP 1 second guide exposures gave best results. The Paramount seemed best around 4 seconds. In poor seeing it is better to use a bit longer as the guider can chase the seeing.
Aggressiveness - If polar alignment and balance are good and the correct guide exposures for your mount are being done then this should be perhaps medium like 6-8. Too high and the mount will overcorrect. Watch the guide errors. You see a - go to a heavy + after one correction it probably means you are overcorrecting. Back it off.
There is an article about how to set backlash setting. I think its at the CCDware site, or Maxim site.
5. Guide star selection:
The first thing I do if I see bad errors and my setup has been working fine is to select another guide star. Don't select double stars. You want one that is bright enough to survive a small cloud but not bloated and too bright. Sharp and tight and fairly bright.
6. Callibration.
The software needs to know how much of a correction moves the mount a known amount. Pick a star that is by itself, that is quite bright. Set the callibration time long enough so the movement between callibration exposures is 50 pixels at least. Sometimes it improves autoguiding to recallibrate at different parts of the sky or on the other side of the meridian. It depends on how well your mount is guiding.
7. Min/max move:
I usually set this to .1 and 1. I figure a correction needed more than 1 is an error or blip in the seeing or wind and not periodic error. Under .1 and I don't want to correct it as it probably has not moved to be worth it.
8. PEC:
If your mount supports it then PEC takes you that last little step is the PEC is uneven.
9. Offaxis guiders or self guiding is better than a guide scope. Offaxis guiders are the modern way to go as the guide camera is in front of the filters and you can do narrowband. Also you have higher quality guide cameras and its easy to pick up a guide star.
10. If you've done all the above then you need to reduce your mount's PE.
A better worm (some have upgrades available). There is an ad at Andy's Shotglass Astronomy to coat gears of mounts with a hi-tech low friction coating that is supposed to dramatically reduce PE. Perhaps that is worth a shot (I don't know if it works, just saw the ad).
Join Yahoo Groups for your mount to get tips specific for that mount.
I had a Vixen Sphinx once. The mount was very stiff in rotating in one axes. That had to be adjusted as it came out of the factory too stiff and that caused issues. I don't know about other Vixen mounts.
I hope that helps.
Greg.
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