Let me put in a plug (once more) for a fairly simple enhancement to the auto-focus routine that, IMHO, would make a DRAMATIC improvement in focus reliability for central obstruction scopes, of which the many focus challenges they experience are well documented above.
The focus routine needs a sanity check feature, such that if the focus position it is currently working in or the end result focus value do not pass this sanity check, the focus would return to a certain value that is known to be reasonably good.
The sanity check I propose is the least square fit focus line (LSF line) that has been derived over several nights or at least a fairly sizable number of focus runs over a wide range of temperatures. The relationship between focus position versus temperature is close to a straight line for most scopes and its specification only requires two numbers, a slope (which is the change in focus position for 1 degree change in temperature), and the intercept… This is easy to determine using any of dozens of free sites on the web if you feed them pairs of focus positions/temperatures.
I have determined these parameters for every scope I have ever owned, which have included several refractors, a 12" RC, a 14" Celestron Faststar, an 11" Celestron RASA. All of these scope had a consistent LSF line that I could use reasonably well for an entire night of imaging, only doing an initial focus run.
The proposed focus enhancements could work this way with SGP:
- focus routine would accept the slope, ie. focus change value per degree temperature change. It already does and uses this value to compensate for temperature changes between focus runs.
- A new input: the focus position at 0 degrees C. This is just the intercept of the least square fit line. No measurements are actually required at this temperature. A value of 0 would disable this feature.
- When the first focus of a sequence run is started, SGP would automatically move the focuser to the focus position on the line determined by the current temperature for its staring position. From there it would do the usual focus routine. This is something that should always be done with the current routine manually to give the best results.
- A second new input: MAXIMUM allowed deviation from the LSF line. This is the sanity check part of the proposal. If the new focus position deviates too much from the LSF line, this is probably a bad focus run, and you are much better off just using the LSF line value. A value of 0 would disable this feature.
- A third new input: Number of retries if the focus value exceeds the maximum deviation value. Could default to 0, which is what happens now.
- The normal center point of the focus process of moving the focus range progressively further from the original starting point would not exceed the maximum allowed deviation. This would keep the focus routine from running off into never never land.
Implementing these suggestions would not keep you from using the focus routine exactly the same as you do now if your scope does not have a consistent LSF line.
What I have experienced many times over the past several years using the SGP focus routine is the following:
- for refractors, it works spectacularly well, giving perfect V curves, most of the time. However, even with a refractor, at times a focus run will be flawed, perhaps a wind gust or light cloud at just the wrong time puts it out of whack.
- for the central obstruction scopes, most of my focus runs were quite good with very acceptable results. However, a fairly large percentage were not, and have given me an hours worth of imaging that was out of focus. For those times it would have been much better to just use the LSF line value. In fact, much of the time I just let SGP apply the set focus delta value for the entire night, after establishing a good focus at the start.