View Full Version : Autoguiding woes
3rd of November 2008 (Mon), 06:17
Hi all, been having a few Auto-Guiding problems lately.
Not long ago, rather than try any imaging, I spent a few hours trying to Polar Align my mount accurately. I used a method I read about whereby you turn the mount tracking off, set your camera up and locate a bright star in the south, near the meridian. You open the camera shutter, then manually slew the mount, at 1x speed, along the RA axis so that the star moves across the camera frame. After a minute or two you stop, and slew the other way, to move the star back. This makes a '<' or '>' shaped star-trail image, as the star moves north or south while you're slewing east and west. After several adjustments I got the image to be a '-' shape, meaning good alignment. Did a similar thing on a star in the east, near the horizon.
As a test I put an eyepiece on my 1200mm scope and left it tracking a star for about 10 minutes, and I was seeing hardly any drift. So I marked my patio where the scope was and all I need to do is place the mount in the same spot each time. (I always leave the legs extended.)
I'm still quite well aligned, but my problems are with Auto-Guiding. I use my Orion ST 80mm f/5 as the guide-scope, and have tried it with a 3x Barlow. It seems to lose track of the guide-star quite easily, and makes all sorts of adjustments to the mount, and the result tends to be a worse image than if I wasn't guiding at all. ???
My alignment isn't as perfect as it could be, and at 1 or 2 minutes at 1200mm I do get trailing. I'm in two minds as to whether to spend another few hours increasng my alignment accuracy, or persever with Auto-Guiding. The problem is my mount, which is quite good, isn't brilliant, and is probably showing errors.
3rd of November 2008 (Mon), 09:07
I've never tried autoguiding before but could the problem be either in the camera (not sensitive enough) or the software (not accurate enough) that does the guiding?
It seems like the importance of the alignment of the mount decreases since it's always being updated with where it should be aimed.
Just pitching out some thoughts.
3rd of November 2008 (Mon), 11:07
Dunno Jeff, it could be those things. The weird thing is, if I watch the star through the webcam for a bit, and it doesn't move much, then I click on it and tell PHD to start guiding, the star seems to mvoe around loads more. Perhaps the signals the software is sending is a bit much and it causes the star to move about in all directions...north, south, east and west!
Perhaps using the 3x Barlow isn't the best idea. However, my best guiding experience was when I shot M31 with my 400mm Orion ST80 and guided using my 1200mm...that seemed to work really nicely.
3rd of November 2008 (Mon), 11:37
Like with drift aligning, once you've made your southern adjustment and then made your eastern adjustment you need to repeat both. Each adjustment effects the other slightly so by repeating you increase accuracy of your alignment. One repeat should do it.
1200mm is tough even on a superior mount to yours and mine. So as you know auto guiding is definitely in order. In my experience using a 3x barlow might gain you more similar focal length to your imaging scope but it also requires almost perfect seeing conditions. Anything less will make your auto guiding software over correct. Magnifying poor seeing provides your guider with a vibrating strobe light to guide on. I would use a 1.5x or even try guiding without a barlow just to see if there's some improvement. Your mount likely does have a good bit of periodic error and it is contributing to the problem. Without correction built into your mount software you really can't do much about it. You can evaluate your PEC by turning your tracking off while taking a 30 sec. to 1 min. exposure. Periodic error will show up in the star trails as peaks and valleys. As I mentioned you won't be able to reduce PEC without physically changing your mounts gears(since your mount has no internal correction) and even if you wanted to I'm not sure high precision gears can be had for your mount or the cost that would incur. Autoguiding accuracy depends on many factors and gives the best results when you can ID the weaknesses and culprits causing the problems. Some can be fixed, some we have to live with and try to minimize. If it's any consolation, even if you had PEC correction in your mount you couldn't run the feature while auto guiding, it works against the auto guider too much.
Alignment and gear balance is job one. Without attention to these two, you might as well shoot unguided. When alignment is off the autoguider is stressed just as it is when seeing conditions are poor. So the goal is to do your best to make the autoguider's work easier and as a result yours too.
Flexure is another issue that can play into this. If your guide scope is mounted on less than solid hardware and attachment means flexure will occur. The weight of the guide scope will cause the hardware to flex. This is hard to imaging but it occurs in seemingly solid hardware. As the flex occurs the autoguider is stressed to hold onto the guide star. It's good to review all mounting hardware stability especially as you add optics and change configurations. I'm not saying this is part of your problem, it may not be at all but I thought it was worth mentioning.
What exposure time are you using with PHD? I stay around 1 to 1.5 seconds. Most mounts in our price range need shorter exposures for more frequent response.
Lastly, when you are balancing your scope/mount do you bias the weight slightly toward the east? In other words, a bit weighted to the east with your counterweight or weight added to the scope end. Personally I perfect balance and then add a 2 pound ankle weight to the rig on the east side when imaging, especially when the counterweight is very low to the ground. The reason for this is to not let the mount gears to mesh without some tension or resistance. If balance is absolutely perfect the gears will operate less evenly and smoothly, basically teetering in the slight tolerance spacing between the gears. So a bit of bias weight in the east means that some tension or resistance is being applied to the gears as the scope tracks in sidereal. Many times I'll shift the bias weight in or out from center and slight tracking error disappears. It takes a little testing.
5th of November 2008 (Wed), 06:34
Try this little investment... (it's in my TO BUY list).
edit: I plan to use it with my Celestron NexImage 1/4" CCD sensor 5.6 micron sq. (640x480)
5th of November 2008 (Wed), 06:47
Thanks ebann, that does look useful. I might get it for those rare times I am going to move my mount to an alternate site.
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