That is exactly what SGP will do, if told to do so. However, it seems counter to your early statement :
SGP can allow either. It can be told to focus with one filter, and use only offsets for the others, or it will use the offset, and THEN focus with the others. You have to decide which.
Ok, I hope this is a typo. Iām going to presume thereās not a 160,000 step difference between Ha and Green.
There is one. (More on that in a moment)
That is not what happens, and thereās a good reason for it.
Ok, so the āoffsetā isnāt a number you enter anywhere. itās merely the difference between your starting focus points. If you start green at 15000, and start Ha at 15500, the offset is 500ā¦and it will always be 500, regardless of where green is actually focused at.
So letās say you start a session, and you autofocus with green. Because of temp, humidity, whatever changes, green winds up autofocusing that night at 14950. Fine. You shoot a few frames, temp changes a couple degrees, you autofocus green again, and itās at 14900 now. Still good. Green is focused, everyoneās happy.
Now, you change to Ha. The focuser will move to 15400 (NOT 15500) because the offset from green is +500. SGP knows youāre on green, youāre in focus, and Ha was green+500ā¦so it moves +500. It neither knows nor cares (other than doing the basic math) that the filter settings say ā15500āā¦it just knows ā15500-15000 = 500ā.
To SET your offsets then, you basically just autofocus EVERY filter in a single testing run, save those numbers, and bamā¦you have your offsets.
Once againā¦it doesnāt matter that the exact focus points wonāt be the same tomorrow because itās 5Ā° warmerā¦it only matters that the difference between them wonāt change. (And it shouldnāt)
So hereās your process :
- Fire up the rig, slew somewhere with a good number of scattered stars for focusing.
- Focus with, say, green. (Start with any filter you want, doesnāt matter, you just mentioned green soā¦green). Letās just say, for example purposes, it winds up focused at 14000 tonight.
- In Control Panel ā Focus click āSet Focusā to set Greenās focus point to 14000. UNCHECK the box for āAdjust all other filter focus pointsā.
- Change filters to any other filter you wish. Letās say, Red.
- Focus with red.
- Control Panel ā Focus ā Set Focus.
Repeat this process for each filter, being sure to not change the other filtersā focus points each time.
Ta da. You have offsets. If you go to Control Panel ā Filters ā Define Filter List, you will see the āFocus Ptā for each filter. The difference between any 2 is the āoffsetā for that pair.
Now, what will SGP do with this info? Depends on what you tell it to do. There are a BUNCH of options.
In Control Panel ā Focus youāll find a checkbox for āAuto adjust focus per filterā. This will cause SGP to move by the offset between current and new filter any time the filter changes. All other autofocusing steps asideā¦when you change filters from X to Y, SGP will check the difference between X and Y, and move by that much.
Nowā¦IF youāve told it to use Auto Focus, it gets a bit more complicated :
- In the auto focus settings, thereās a box for āAuto Focus On Filter Changeā. If this is checked, then any time you change filters, SGP will autofocus with the new filter. If youāve told it to Auto Adjust per filter, then it will apply the offset first, before autofocusing.
- Thereās another box for āAuto focus with filter (dropdown to select filter)ā. If this is checked, SGP will always change to the selected filter to autofocus. It will apply that filterās offset or not depending on what youāve said above to Auto Adjust. Once that filter is focused, it will change back to the requested filter, again applying the offset or not as told to do so.
Thereās several ways you can go about it, reallyā¦and much of it depends on how you like to image. I, for example, prefer to cycle through filter events when imagingā¦so autofocusing on filter change would be a nightmare for me, as Iād be autofocusing each frame!
So instead, I autofocus with filter (Lum in my case), auto-adjust per filter, and do NOT autofocus on filter change, instead relying on the offsets to keep me in focus. Especially handy when Iām shooting NB, because NB focus runs can take a very long time, and can produce some questionable results on certain types of targets.
If you prefer to work through an entire event at a time, then you might instead want to enable āauto adjust per filterā and āauto focus on filter changeā and NOT use āAuto focus with filterā. This way, SGP will autofocus when needed (temp, number of frames, etc) with whatever the current filter is, apply a reasonable offset when changing to the next one, autofocus that one to tweak focus, and move on.
Works well for broadband, but can be problematic with NB for the reasons described above.