Quote:
Originally Posted by Bassnut
mmm, on a read of the manual, it only has a start and end point, not multiple points. It appears a line between 2 points might indeed be pretty straight.
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The OPTEC temperature compensated focusers are based to a strictly linear relationship. I am not aware of any solution which can handle non-linear dependency between focus and temperature, or other factors, for amateur scopes.
Classical scopes, such SCT with aluminum OTA, exhibit focus shifts from mainly the tube expansion/shrinkage, which is very linear with temperature (tube thermal inertia is much smaller than the mirror glasses). Other effects are less important, therefore a linear temperature compensation scheme does work well most of the time/nights.
Carbon fiber scopes and opened tubes, such as more advanced designs like RCT, do not have much tube length variation with temperature anymore (at least with well made carbon designs and related focusers), however mirror (lenses) radii changes are one of the main factor of focus shift.
For instance departure form spherical mirrors, such as hyperbolic mirror based scope designs, can be extremely sensitive to shape and alignment. Unless made of Zerodur glass, Pirex/quartz have enough left over volumetric thermal expansion coefficients to create a significant focus shift.
Some refactors are also very nonlinear in focus versus temperature dependency. When an nonlinear focus behavior is experienced this usually means a more complex process is at work here, and current temperature is likely not the only parameter, inertia plays also some role.
It should be remembered that for an RCT (or some other more advanced optical layouts) the best focus zone is usually tighter than the classical CFZ. RCT designs for instance trade coma for astigmatism, stars are round only in a very tiny focal plane located between the sagittal and tangential focal planes where a star would be elongated (due to astigmatism).
The CFZ is geometric in nature and does not account for the scope layout (SCT, RCT, ...).
When focus shift is due to the optics (radii, shapes, ...) the process may be highly nonlinear and unpredictable. For a reflector usually the primary mirror is much bigger than the secondary and have a significant larger thermal inertia leading to mirror figures evolving at different rates.
The resulting effect is not only a matter of temperature but also its rate of change, making any focus shift prediction solely based on external temperature challenging.
Using the temperature to infer the focus shift is a proxy which works well in some situations (such for SCT closed aluminum OTAs), but it does not provide any direct measurement of the actual focus.
As an example in this education page you can see the evaluation of the focus shift with temperature for a good popular carbon fiber 10" RCT:
http://www.innovationsforesight.com/...ime-autofocus/
This typical non-linear behavior usually does not repeat form night to night, unless the temperature profile (such as rate of change/derivative) over time is very similar.
In such situations periodic refocusing or continuous real time auto-focus may be the only effective solutions.
