TCS Startup
When you first start TCS (typing TCS at the DOS prompt), you'll notice that the focus motor starts running and the focus seems to be in a runaway mode. This is normal. The telescope has two focus position encoders: an analog (potentiometer) encoder that reads absolute position and a digital incremental that counts steps in finer detail.The analog encoder is good to +/-1 focus unit (the display is in tenths). The digital encoder is good to better than the displayed tenth of a unit.
Both encoders are needed to start the telescope. At TCS initialization, the analog encoder is read and its value stored. The focus motor is then sent to the end of travel where a limit switch is engaged. At that point the digital encoder is zeroed and the focus is driven back to its original analog readout value.
One consequence of this operation is that the actual telescope focus shifts slightly across TCS restarts since the analog encoder is good only to +/-1 unit. In practice this shift is too small to notice but you might want to take a close look at images if you restart TCS during the night.
The telescope does not respond to commands while focus is initializing. Note that if this process does not finish (say, because of a broken wire to the focus motor) then the telescope will never reach an operational state.
Focus Sensitivity
With the wide field correctors installed and the 24 micron (1.15 arc second) pixels on the SDSS CCD, focus shifts of 0.5 units are noticable. With smaller pixels or if the correctors are removed, you will need to focus more carefully.For SDSS observing, one should adjust focus when it's one or more units off.
For visual use offsets of +/-1 unit are noticable. Your eye can take care of smaller changes.
Focus Readout Scale
One unit of focus movement shifts the secondary mirror by 0.0004 inches. Larger numbers mean the secondary is farther down so final focus should be set from larger numbers to smaller to prevent gravity induced backlash.On the handpaddle, FOCUS OUT means the focal plane of the telescope moves away from the telescope, towards the ground.
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