w7ay I wonder if that has more to do with the OTA itself instead of the filter thickness; i.e., how "APO" is the telescope. One day, perhaps manufacturers will design optics that are not full APO, but have perfect convergence at OIII and H-alpha wavelengths, ha ha.
I'm sorry, i confused the offsets. Ha and OIII are in a relative short range being about < 100 steps apart. SII is about 400 steps away from Ha. EAF is able to focus them all correctly when imaging in SHO. Side note: SII produces huge halos on bright stars where the others do not, so i guess its quality is worse than that of Ha and OIII and that may be the cause to having a bigger offset.
When i shoot LHaRGB and it comes to Ha, EAF is often not able to focus because the focus point of Ha is too many steps aways from the focus point of the other filters. I guess It is related to the V-curve ASIAir is producing. This is my perception: let's say the focus point for LRGB is around 5,000 absolute steps (just fictious values, i need to measure the actual values), at the beginning of creating the V-curve, ASIAir adds a fixed amount of steps to extend the range in which it might find the focus point for the current filter – let's say it adds 1,000 steps, so it begins at 6,000 absolute steps counting downwards in subsequent tries. If Ha's focus point is at 6,200 absolute steps, it would lie outside the range of the V-curve and the focus process eventually fails. Now that i think of it, i guess it would be better to start with Ha so ASIAir has a bigger chance to find focus for the LRGB filters, but this of course would just solve it for a single autorun session, not for plan mode, when imaging several targets successively.
Btw: all filters are from the same manufacturer and have the same thickness of 2 mm. And i activated the reverse option in the EAF settings in the ASIAir app.
w7ay I don't know what your EAF scale is. [snip] What is your EAF scale?
Unfortunately, me too 🙂 At least at the moment. It's on my todo list to measure it.
w7ay I assume you are using a Bahtinov mask right?
Yes, i do for the initial setup. But during a whole night of imaging, i let ASIAir do its autofocus routine multiple times (usually on filter and temperature change).
w7ay Perhaps you have the typical EAF-to-SCT focuser set up which have EAF scales in the hundreds of EAF step per µm?
I can tell, that on the SpaceCat, EAF has a maximum range of 10,000 steps and it won't let me change this using the ASIAir app. On my SCT i have a different EAF which had a maximum range of 60,000 steps by default.
w7ay Speaking of the EAF, I have special EAF mounting plates fabricated
I was thinking of that too when i was evaluating if i could add a Crayford Focuser to my SCT. For those i looked at, i had to find a special solution as the ZWO plates would not work out of the box. In the end i gave up because the backfocus of the SCT is so tight that it's hardly possible to achieve proper backfoucs with a Crayford in the image train. Today, i'm using the ZWO SCT adapter plate to attach the EAF:
And on the SpaceCat i use the DeepSkyDad solution:
Both are the 12V EAF models. I have one spare 5V EAF but until now i did not use it.
Olli
