David's Astronomy Pages Notes - Session 304 (2008-09-22) Notes (S301)   Notes (Main)   Home Page   Notes (S305)

Goto Images from 2008-09-22

Temperature Coefficient - LX200 8"

Based on focus measurements from sessions S267 to S304,  a revised temperature coefficient of -160.6 steps per deg C has been estimated for my LX200 telescope normal setup (LX200 8", f/10 - operating at f7.4 , focal length 1498mm) .  This is equivalent to a focus change of 0.01365"/degC or 0.35mm/degC  (based on TCF-S step size of 0.000085").

The main dataset shows variation around the best fit line of +/- 150 steps (Standard Deviation : 86 steps) approximately equivalent to +/- 1 degC which are probably associated with uneven temperature variation along the length of the telescope tube or other temperature stabilization issue. 

4 sessions (S287, 289, 296, 304) appear be offset from the main dataset line by around 375 to 400 steps, but taken together these 4 points seem to show a temperature coefficient (gradient) that is similar to the main dataset.  The anomalous points are offset from the main dataset line by around +375 to +400 steps. This is equivalent to a temperature offset of +2.3 deg C which is considered outside of the normal range.    This level of offset has a noticeable and significant impact on star focus and FWHM, and requires identification and correction if remaining images are not to be compromised. The impact of a +375 focuser offset is shown by an analysis/simulation of data from session S304 (see Focus Profiles below)

The reason for these sessions being anomalous is not known, but they appears to indicate something real.   Initially (after sessions S287/289)  it was thought that the anomaly represented some non-linear expansion effect associated with the equipment at warmer temperatures, but more recent data taken from sessions of 10-13 degC which lie on the main trend line seem to dismiss this theory.  The anomalous sessions are intermixed with normal sessions and this seems to rule out any configuration change or shift with the telescope/imaging setup.    Focus offset retained itself throughout the anomalous session indicating that the anomalies are not due to non stabilised / warm scope, The current thought is that the anomaly is associated with some start-up 'step zeroing'  issue with the TCF-S or step position translation issue associated with CCDSoft or the TCF-S driver. Whatever the reason the erroneous offset always seems to be around +375 to +400 steps.

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