David's Astronomy Pages Notes - Session 285 (2008-05-02) Notes (S276)   Notes (Main)   Home Page   Notes (S286)

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Optec TCF-S Focuser (3)

[ See also Previous notes TCF-S Focuser Notes 1 - 2008-02-19 ]
[ See also Previous notes TCF-S Focuser Notes 2 - 2008-03-12 ]

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Temperature Coefficient - LX200 8"

Based on focus measurements from S285 combined with measurements made during earlier sessions (S267 to S275),  a revised temperature coefficient of -155.9 steps per deg C has been estimated by my LX200 telescope normal setup (LX200 8", f/10 - operating at f7.4 , focal length 1498mm) .  This is equivalent to a focus change of 0.013"/degC  or 0.34 mm/degC (based on TCF-S step size of 0.000085")

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Focusing - Star Analyser 100

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Focus Offset - Star Analyser 100

Focus profiles produced for a selected star confirmed a significant focus position offset between the Star Analyser 100 and normal C Filter (see Figure below).. The profiles established a focus offset of 1550 steps for the Star Analyser 100, equivalent to 0.13" or 3.35mm (based on TCF-S step size of 0.000085").

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Autofocusing

New software routines were used from Session 284 onwards to automatically adjust the TCF-S focus position, based on the established temperature coefficient , datum focus position and the filter being used.  Because the autofocusing calculation is performed external to the TCF-S box or TCF-S software, it is possible to capture telescope tube temperature as a near continuous graph throughout the session using a program thread.  The same thread also captures a near continuous reading of CCD temperature and cooling.

The following chart shows the telescope tube temperature for Session 284 (2008-04-24) and a provisional S filter offset. It shows the fall in ambient (tube) temperature from initial reading of +8 degC to a final temperature of +3.5 deg C.  Compensating changes in focuser position through the session are shown, along with larger changes in focus position related to autoselecting the S Filter (Star Analyser 100) with it specified focus offset.  

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Star Analyser 100 / Spectroscopy

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