David's Astronomy Pages Notes - Session 276 (2008-03-12) Notes (S266)   Notes (Main)   Home Page   Notes (S285)

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Optec TCF-S Focuser (2)

[ See also Previous notes TCF-S Focuser Notes 1 - 2008-02-19 ]

Initial Impressions / benefits at 2008-03-19:
The new focuser equipment arrived in mid-February and initial  impression after 3 sessions was that the TCF-S is a worthwhile investment, providing :

• firmer connection between CCD Camera and Scope.

• repeatability in focuser position, with digital readout of focus position

• remote focusing capability through network / observatory laptop 

• automated focus adjustment for changing ambient temperature  (correcting focus drift)

• realtime and recorded measurement of ambient temperature and temperature changes

The TCF-S is a far more heavy duty focuser, than my previous NGF-S focuser, which removes the previous collimation-limiting flexure between the scope and CCD camera. This should also mean that I will also get better/more reliable flat frames.  With the step motor in the TCF-S I can now move to / return to a specific focus position with confidence.  I don't have to wait at/revisit the observatory to make focusing adjustments.  I can also maintain better focus as the temperature increases/decreases during unattended or automated operation of the observatory.

Earlier Teething problems
Teething problems with TCF-S focuser 

• shut-down issue.

After shutting down CCDSoft and hibernating the Observatory Laptop, I find that I'm unable to turn-off the focuser at the hand set.  I have to restart the laptop and fiddle around with focuser PC connection to get to a position where I can switch off the focuser. (Simply turning off the Power to Focuser would of course mean that the current focus position/temperature are not stored on the EPROM ready for the next session)

FWHM Improvements

Although TCF-S has improved focus quality, sharpness of stars under my normal imaging conditions (5-180s exposure, unguided, near sea-level site)  is still significantly impacted by factors other than focus qualiity, these factors are general poor seeing, seeing variations/cloud, wind/telescope shake, residual periodic error, star drift due to residual polar misalignment, star drift due to drive/post slew errors for certain poorly balanced sky positions.

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Focus Profiles

Focus profile was measured on a couple of occasions during Session 267 (2008-02-22).  These are shown in the graph below.

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Temperature Coefficient

Based on focus measurements from several sessions (S267 to S275), including points measured in profiles above, a temperature coefficient of -158 steps per deg C has been estimated by my LX200 telescope normal setup (LX200 8", f/10 - operating at f7.4 , focal length 1498mm)