Quote:
Originally Posted by gregbradley
One thing we didn't talk about is your guider setup. Are you using an offaxis guider or a guidescope? Guidescopes are subject to differential flexure. Offaxis guiders are usually way superior.
Here are fairly common settings:
1. The better the mount and the better the polar alignment then you tend to use longer exposures on the guide camera and shorter exposures if all is not good. By longer I mean 6 seconds which is what I find works best on the better mounts. What works for yours is trial and error. I used to use 1 second with a Tak NJP mount.
2. Guide rate .5X. Some mounts have some backlash so you can put in say a 1 second delay between corrections to help with that. Some software lets you set the aggressiveness separately for the 2 axes, others don't.
3. Aggressiveness. Similar to the above. The better tracking the less aggressive it needs to be. Again a bit of trial and error and watch the guiding errors. But 6 is setting I use a lot. You can raise that if the seeing is good.
If the seeing is bad you don't want to be chasing the seeing so watch the corrections and if they go from - to + a lot then you are chasing the seeing and the mount is correcting some of the last correction instead of the seeing.
4. Min/Max: Min: something low like .1 Max 2 to 3. If your polar alignment is good and your tracking is good then you don't want a correction for an error of 3 - its probably PE or a wind gust so set it at 2 so larger "errors" are ignored.
5. I find with PEC the reported errors can get high for a little bit and then lower and that is the PEC kicking in and correcting the larger wave of errors the worm cycle is producing.
6. One of the first things I do if I get too large errors is select another star.
You don't want too bright a star. A nice semi bright star is ideal especially if some light cloud goes over.
7. Roland Christen from AP recommends calibrating your autoguider near where the celestial pole and the meridian intersect.
8. AP has some other informative things about tracking. Like don't have all your counterweights at the bottom of the shaft. Its better to have more weight and have it near the top of the shaft and one weight down near the end of the shaft to get balance as there is less resistance to movement.
Ideal balance is not perfectly balanced but slightly biased to mesh the gears in east/west, I think its to bias towards the west and similarly bias the weights towards the camera. RA and Dec autoguide differently.
There is an article about these points on the AP website I think its under "technical".
Pulse guiding through Ascom is supposed to be superior to ST 4 relay guiding.
I use both an SBIG STi guider and an ASI290 which is very clean and very very sensitive so getting a guide star is easier.
Greg.
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Thanks for the tips - greatly appreciated. Definitely some things for me to try next clear night! One point I did note, around the optimal location to calibrate the guider. I had heard similar and so tried doing this one day, but got awful results compared to just calibrating in the part of the sky I am imaging. Maybe I did something wrong? Will have to try again when the clouds part.