For celestial objects the accuracy of your mount’s pointing is primarily governed by the quality of what’s called the sky alignment matrix. For Celestron’s 6/8SE mount and similar mounts like the 8i and i-SE, this is usually about 5-10 arc-minutes (300 – 600 arc-seconds), well within the mount’s specification of 20 or so arc-minutes.
Flexure of the mount and optics can cause the pointing accuracy to be off by arc-minutes over the entire sky. Additional error is caused by backlash.
Bigger mounts will usually be more accurate due to their need to move scopes that are heavier and larger with substantially longer focal lengths and narrower fields of view.
The internal accuracy of the firmware programming and encoders is much higher. Celestron firmware uses 24-bit precision. The encoders on the mount motors usually have less precision and will round off commands sent by the firmware. For example, the 6/8SE has 4.8 million counts per rotation or about 22.2 bits precision, so will execute commands at this level. The firmware itself introduces some errors in a compromise In order to enhance the speed of GOTO operations.
If you want to achieve a greater level of accuracy with a mount like the SE mount, here are suggestions for RS-232 programmers from the engineers at teamcelestron.com (Please visit the website for more details on similar topics.)
First, execute a GOTO and wait for the mount to completely stop. Read out the mount’s position with a 24-bit read. Then use the AuxBus pulse guide commands to eliminate the error. The pulse guide commands basically allow you to add a small DC offset to the current position with sub arc-second precision and accuracy.
Of current production mounts (as of late 2012), only the original CPC (not the CPC DX) and the 6/8SE mounts lack pulse guide commands. For those mounts, use one of the track commands and then after the appropriate interval a stop command. A track command allows you to send a velocity to the mount. The velocity is given as a 16-bit number with a precision of about 0.015 arc-seconds/second. A “stop” command is a slew command with a rate of 0 to turn off the tracking.
Example: Suppose that after the GOTO was complete, it was 20 arc-seconds off. (NOTE: Errors on the order of 20 arc-seconds are always small compared to the accuracy of your sky alignment matrix, as described above.) Correct the error by sending a tracking command with a rate of 20.00 arc-seconds per second. After a period of 1 second, send the stop command, and the mount (should) have moved exactly 20 arc-seconds.
If all that matters is relative accuracy (to point within a few arc-seconds of a position today and then again to point to the same position as indicated by the hand control tomorrow), then this technique will be sufficient.
However, if you want point to within a few arc-seconds of a position over a large arc on the sky (up to 180 degrees), then a bigger and better mount is your best option.
