One of the reasons most refractors tunes are still made of aluminum rather than carbon fiber-even high end telescopes-is that the aluminum tube also contracts, somewhat offsetting the change in focal length of the lens. Hence, the need to refocus inwards with a large enough delta T. The net result is that the focal length of the lens shortens as temps fall.
Also, the glass contracts, changing the radius of curvature. Generally speaking, as the temperature drops the refractive index of the glass changes. I'd be curious to here from other refractor owners about this. I do know that the glass has an opposing effect, but for my stuff, the metal contraction seems to dominate. Each requires that the focuser moves OUT and not in as the temperature drops. At some point I will implement this on my current setup. Changing focus based upon temp change and the expansion coefficient of your OTA is the best way to go, IMHO albeit a bit more expensive in terms of hardware. This works acceptably well, but in the past I have used the same focuser with a 5 micron accuracy DRO, and manually adjusted focus based upon temperature change, during exposures of up to 30min duration, whist autoguiding with an OAG for each sub.
Dslr focus stacker software#
I have a USB controlled crayford focuser and my control software (Astroart7) is programmed to autofocus between subs. I usually use 4 - 5 minute subs with my 10in ~F7 SCTs. The differences between good and bad should be very pronounced in the software's eye. My questions is, is it sufficient to take multiple shots with no focus interventions an depend on subtle focus differences to be processed, or intentionally refocus each time to generate a broader statistical distribution of focuses from which to choose. In reading about stacking, the articles seem to say take lots of exposures and the software will select the intricate differences and combine the best. If focus is killing your frames it's normally operator error. Many other factors come into play to ruin your frames but rarely focus. I set my focus as best as I can get it, don't fiddle with it and let the exposures rip.
Sometimes it seems to reject on the basis of sky background, other times on the FWHM of the star image but commonly on the "score", however that is derived! Like most others I use DSS and I have yet to figure out what criteria it uses to select "good" from "bad" frames. My workhorse scope, a 80 mm f 7.5 is set at the beginning of the session and although I might re-look at the focus after a few hours I never see any change even if the temperature has shifted by 10 C. In a short fl scope (600 mm?) focus change will probably go unnoticed and continual adjustment is really not necessary. A longer focal length scope will be more susceptible to changes in focus due to temperature changes and other variables.