Periodically there’s a thread on the AstroPhysics forum on why the scope isn’t exactly on target, and sometimes a lot off target, after start up and un-parking. There are lots of mechanical reasons for this: if the optics of the scope are not parallel to the scope’s axis (“unorthogonality”); flexure of the scope in relation to the mount; and “mirror flop” (bane of the schmidt-cass); as well as atmospheric refraction (if the target is at low altitude).
If the error is mostly East-West, it’s probable that the mount’s time is a bit off. In permanent installations, the time is grabbed from the controlling PC. The PC, if it’s connected to the internet, syncs time to a time server via the Windows Time Service. As it turns out, the PC time syncs to a reference only once per week, usually on Sunday night. It’s typical for a PC to be off by several minutes. How does that translate into centering your target?
Each second of time is 15 arc-seconds on the sky; a minute of time is 15 arc-minutes on the sky.
With my setup, a Meade 12 inch schmidt-cass, with an SBIG ST-10XME, I get a field that is 17×11.5 arc-minutes. If I’m more than a minute off in my time settings, the target is off the chip.
How can you update your PC time more frequently than once a week? The NIST (National Institute of Standards and Technology) is probably the last word in time accuracy and is probably the source your PC is syncing to. There are a bunch of applications that can set your PC time; NIST has a list at http://www.nist.gov/pml/div688/grp40/softwarelist.cfm. Most amateur astronomers mention Dimension 4 (free) from Thinking Man Software. It makes it easy to setup time sync on a regular schedule.
Presumably this is calling the Windows Time Service application, w32tm.exe, located in c:\windows\system32. You can also run this from the command line as w32tm. Or, you can set it to run as a scheduled task using Task Scheduler in Administrative Tools.
If you use a workflow with Maxim or TheSky, you can use their plate-solving utility to sync the mount to its actual position, then slew to the target. Here’s my start-up procedure:
- Power up mount, dew heaters, camera, and focuser
- Start FocusMax; this connects to focuser, starts Maxim, connects Maxim to ST10 camera and guide camera.
- Start TheSky; link the mount (telescope); this launches the ASCOM V2 control panel. Unpark the scope from the ASCOM panel.
- In Maxim, connect to telescope (mount)
- In TheSky, slew to a target
- In Maxim take focus frame. Here’s where you think, “that doesn’t look like NGC 7007!”
- If the target is off center, you can right click it in Maxim and select “Point telescope here”.
- If the target is nowhere in sight, select Pinpoint astrometry from the Analyze menu. Click Process. The software will hum and purr for a while and hopefully stop with a solution. Assuming a solution,
- Open the Observatory panel in Maxim, select the Telescope tab, and click “Sync”. The mount now thinks it’s pointed where it is, in fact pointed.
- Go to TheSky and slew to your target again, and it will be, god willing, in the center of your frame. Any time-based errors have been resolved from the source — the position of the stars.