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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.
Latest - Temperature Coefficient Plot (2008-09-22) |
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Estimated FWHM impact associated
with |
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Earlier Plots (now superceded) : |
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This Web Page: | Notes - Session 304 (2008-09-22) |
Last Updated : | 2015-05-16 |
Site Owner : | David Richards |
Home Page : | David's Astronomy Web Site |