How to automatically apply focus offsets for each filter relative to Lum focus
I want to focus at all times using my Lum filter and have SGP apply known focus offsets from Lum for each of my other filters. I know the offsets from FocusMax. How can I apply them in SGP?
I’m using Astrodon filters in a QSI 683 on an f/5.4 Tak FSQ-85. They are not parfocL at this focal length (454 mm).
Windows 10
SGP V 4.5.0.1527
You can set in Filter Setup the focus point by filter:
Select the filter for Auto focus:
And enable Auto adjust focus by filter:
SGP will then move the focuser during the filter change according to the offsets you’ve set.
Thank you very much for this response.
I had noted the “Focus Point” column in the manual, but found (and still find) the concept puzzling. The Focus Point for Lum changes constantly through the night with temperature. Therefore so do focus points for the other filters—but there will be a constant DIFFERENCE between Lum and the other filters in a non-parfocal situation.
Seems to me it should be possible to enter the known focal DIFFERENCES between Lum and the other filters, which remain constant.
So if at the outset of a session, if my Lum focal point is 7000, and I know the Red focal point will be 6996, then why can’t I enter a -4 for the Red filter?
That’s how FocusMax works, but FocusMax doesn’t play with SGP.
Best, Bob
SGP works almost the same: you do specify exact focus points, but in practice SGP only applies the difference in defined focus points when moving from one filter to another. There is the option to “Set initial position of the focuser at sequence start”. When you select that SGP will move the focuser to the defined focus point for the filter used at the start of the sequence. I prefer to disable this option and ensure rough focus prior to starting the sequence, as that point can indeed change a lot depending on ambient temperature and other factors. SGP will then start at that rough focus point when running an autofocus procedure. Consecutive filter changes will then only apply the difference in defined focus points, just as you would expect with FocusMax.
I’ve also found it confusing entering a focus position that’s only valid the moment I enter it. I understand SGP only looks at the difference.
Has anyone ever set LUM focus to, for example, 1000, then set the other filters plus or minus that? If SGP only looks at the difference, at least this way it’s easy to do the math - 1021 is 21 more than 1000.
Yes, SGP only looks at the differences when utilizing offsets but stores the absolute position for easily getting near focus when using the “Focus for ” option. For instance if you rack your focuser in at the end of your last imaging session and pack things up the “Focus for ” option allows you to get back to a mostly in focus area of your focuser travel, we can only do this by storing the absolute position.
Then when applying offsets SGP takes the difference between those two filters and applies that to your current in focus position. Say you have:
Lum 1000
Red 1026
You focus with LUM and the position is 987. When switching to Red SGP moves the focuser to 1013.
For this reason we recommend setting up the offsets once everything is thermally stable and setting the offsets for all filters in that session.
Hope that helps to understand why we have it setup that way.
Jared
