Focuser zero intercept

Sequence Generator Feature Requests
1

This is probably more in the “that would be cool to have” category and I know that there are more pressing features in the works, but eventually it would be nice to be able to specify a focuser zero intercept to go along with the temperature compensation coefficient. It would probably be pretty easy to do and might dovetail nicely with upcoming changes to autofocus. I’ll explain:

One of the commonly-accepted tenets of autofocus is that it works well when you start from fairly good focus. However, when a new session begins, the focuser is typically at the position of the last image from the previous session. For large temperature swings and/or scopes with a very narrow CFZ, the starting focuser position for the new session may be too far off for AF to work. This is almost always the case for me. When I start a session, I have to manually (not physically, but with the move buttons) move the focuser back out quite a bit to get focus close enough to start AF. Basically, I am unwinding all of the temperature-related focus moves from the previous session. This could be relatively easily automated if we know both the temperature coefficient and the zero-intercept (the focus point at zero degrees). These would be entered by the user. Upon startup - if enabled - the starting focuser position for the current temp could easily be calculated by multiplying the temp coefficient and the current temp. If temperature response of the OTA is fairly linear (it usually is) this would automatically put the focuser very close to good focus to start the session.

Since manually tweaking the focus at the beginning of a session in order to get it close enough for AF to work is the only manual step that I have to do when starting a session, this would nearly close the loop on complete automation.

Anyway, just a thought.

Tim

2

This used to happen to me on a regular basis, when using a C11. Significant change in temp since previous evening’s imaging (especially if many days or weeks have passed) would often require manual intervention before autofocus would work.

Also, even if it could manage to complete an autofocus run, it sometimes wouldn’t make it that far (during a fully automated imaging session) because plate solving comes before autofocusing, and platesolving would fail because stars too much like donuts. It still amazes me how tolerant PS2 can be when it comes to non-focused stars, but it does have limits.

DaveNL

3

I’ve been thinking focus position should be defined in this way for some time. I was looking at this from the ASCOM perspective and wanting to define the focus positon, F0 as the focus position at 0C. This would be related to the focus motor position, Fm as follows:

F0 = Fm + Tc * Temp
or
Fm = F0 - Tc * Temp

where Tc is the temperature coefficient and Temp is the temperature.
ASCOM would useT0 all the time. There’s no reason why SGP shouldn’t do this.

If the focus position is truly a function of temperature and the temperature coefficient is correct this should work. If this isn’t correct then it won’t but I don’t think that the situation will be significantly different to now.

4

OK, then please go for it!

Bernd