Originally posted by dakight: I *FINALLY* got autoguiding set up last night. I didn't do any imaging, I just let the telescope and guide cam run for a half hour or so and I was shocked at what I saw.
Congrats! Welcome to the wonderful (and sometimes frustrating) world of guiding.
Originally posted by dakight: In a nutshell the declination tracking ran from about -.5" to +.5" in a pretty regular pattern It could be tightened up a little probably but that's not my main concern. The RA was tracking from about -2" to +1". The rms error was on the order of 2.1" with large excursions from time to time.
Don’t worry too much about the numbers, especially at this early stage. Try to get your tracking line as flat as possible, and you do this by getting your polar alignment as spot on as you can. The best way to get your alignment accurate is to use the drift align function in PHD.
Take your time drift aligning, the more you put in to get it right, the better your tracking (and at the same time your scope is cooling down, so it isn’t wasting time you could spend imaging).
The more comprehensive answer is the error needs to be considered in the context of the arc-seconds per pixel of your imaging camera and your effective seeing. If the error translates into <1 pixel on your imaging camera, it doesn’t matter, if the translates into a few pixels, it probably doesn’t matter (1 pixel of error is more than acceptable, and pretty much immaterial for anything using a bayer filter). Lots of pixels is a problem, and that’s where you need to improve tracking.
This is why it is recommended you start out with a shorter focal length, as it’ll translate into a larger arc-seconds per pixel, and is more forgiving of tracking errors.
For effective seeing, there is little you can do - if seeing is terrible, it isn’t worth trying, and besides, you should be building you imaging setup to be within effective seeing - if you do so, that will mean any tracking issues with seeing are taken care of (by again having appropriate arc-seconds per pixel for your camera).
Originally posted by dakight: Would tracking error be reduced by using a shorter exposure time with the camera? I was using 2 seconds as suggested in the PHD software. I didn't try to play with that.
Use exposure time to get the exposure of your guide star right - you don’t want it bloated, nor feint. Don’t try to use it to compensate for something else, but be aware of he consequences of short and long exposure times.
Short exposures risk the guide star having too much variance is luminosity, but worse, doesn’t even out atmospheric effects, so the star may appear to move when it isn’t (thus the mount will be correcting for non existent errors, then try to correct its previous correction). Making corrections too often is also not good, as it keeps the scope constantly jittering about - better a periodic larger correction then lots of small ones.
Long exposures obviously risk the correction coming too late. For a poor alignment, it also may mean very large movements, possibly more than the limits you have set (though you can increase them if needed).
Typically though you want the exposure to be as long as you need, not as short as you can.
Originally posted by dakight: Another thing is that at the time I didn't know the focal length of the guide scope but I thought it was 230mm. Turns out is is actually 190mm so I'm wondering how much difference that will make.
Basically none, PHD cares about the ‘mass’ of the star being tracked (and uses this to track at a subpixel level). Between this and doing a proper calibration, it doesn’t really need to know the focal length for its calculations - I think I went months before inputting the focal length.
When it comes it handy is translating everything into arc-seconds (as when PHD knows the pixel size and focal length, arc-second calculations can be easily made). This is good because knowing arc-seconds means you can assess your tracking better and compare tracking to imaging.
Originally posted by dakight: I had some trouble with the PHD2 software. It didn't seem very responsive and it took a long time to connect to the camera and start doing anything but when it did it seemed to run OK but I could shut it down normally. I had to just close the window then when I started it again I was warned that another instance was already running.
Check your camera drivers, but it might just be the way that camera is. Mine only likes to be used on one particular USB port - plug it in the other and it just won’t work (and this is an expensive Starlight Xpress Lodestar).
Originally posted by dakight: I'm wondering it it would be worthwhile to use ASCOM instead. I think I have to make or buy an adapter cable. I have a serial DB9 to usb cable but my mount doesn't have a serial DB9 port. Instead it has the RJ-12 type "phone" port.
ASCOM is great. I highly recommend it if your mount supports it, but it does take some setting up. The RJ-12 port is the (analogue) ST-4 guide port. Old fashioned, but pretty universal. Very different from a serial port, which ASCOM needs to be able to drive your mount.
ASCOM isn’t needed for guiding. In theory it might give you better guiding (depending on mount), but it would be edge case stuff. The real benefit is ditching the hand controller and running everything from the laptop - using something like Stellarium or (my preference) Cartes du Ciel to plan your session, find targets and slew the scope, it also makes setting up the mount a breeze - no need to enter the date and time, and you can save your location details for later sessions.
My mount has a handy USB port, but if you do some googling you should be able to find out how you can set up your’s for driving with ASCOM.
