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. I’m going to presume there’s 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 there’s a good reason for it.
Ok, so the “offset” isn’t a number you enter anywhere. it’s merely the difference between your starting focus points. If you start green at 15000, and start Ha at 15500, the offset is 500…and it will always be 500, regardless of where green is actually focused at.
So let’s 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 it’s at 14900 now. Still good. Green is focused, everyone’s happy.
Now, you change to Ha. The focuser will move to 15400 (NOT 15500) because the offset from green is +500. SGP knows you’re on green, you’re in focus, and Ha was green+500…so it moves +500. It neither knows nor cares (other than doing the basic math) that the filter settings say “15500”…it just knows “15500-15000 = 500”.
To SET your offsets then, you basically just autofocus EVERY filter in a single testing run, save those numbers, and bam…you have your offsets.
Once again…it doesn’t matter that the exact focus points won’t be the same tomorrow because it’s 5° warmer…it only matters that the difference between them won’t change. (And it shouldn’t)
So here’s 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, doesn’t matter, you just mentioned green so…green). Let’s just say, for example purposes, it winds up focused at 14000 tonight.
- In Control Panel -> Focus click “Set Focus” to set Green’s focus point to 14000. UNCHECK the box for “Adjust all other filter focus points”.
- Change filters to any other filter you wish. Let’s say, Red.
- Focus with red.
- Control Panel -> Focus -> Set Focus.
Repeat this process for each filter, being sure to not change the other filters’ focus points each time.
Ta da. You have offsets. If you go to Control Panel -> Filters -> Define Filter List, you will see the “Focus Pt” for each filter. The difference between any 2 is the “offset” 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 -> Focus you’ll find a checkbox for “Auto adjust focus per filter”. This will cause SGP to move by the offset between current and new filter any time the filter changes. All other autofocusing steps aside…when you change filters from X to Y, SGP will check the difference between X and Y, and move by that much.
Now…IF you’ve told it to use Auto Focus, it gets a bit more complicated :
- In the auto focus settings, there’s a box for “Auto Focus On Filter Change”. If this is checked, then any time you change filters, SGP will autofocus with the new filter. If you’ve told it to Auto Adjust per filter, then it will apply the offset first, before autofocusing.
- There’s another box for “Auto focus with filter (dropdown to select filter)”. If this is checked, SGP will always change to the selected filter to autofocus. It will apply that filter’s offset or not depending on what you’ve 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.
There’s several ways you can go about it, really…and much of it depends on how you like to image. I, for example, prefer to cycle through filter events when imaging…so autofocusing on filter change would be a nightmare for me, as I’d 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 I’m 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 “auto adjust per filter” and “auto focus on filter change” and NOT use “Auto focus with filter”. 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.