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.
NGC 90 (center), part of a busy field in Andromeda. Exposed on 20130107, 20 minutes total exposure.
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.
In Windows, you can you can force an immediate synchronization by clicking the Update Now button on the Internet Time tab in Date and Time in Control Panel.
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.