I have found that my AstroDon E G2 Blue filter seems to require a -1 step focus offset compared to the other filters in my LRGB set. I use an Atik EFW2 filter wheel, and the driver supports a focus offset feature. This allows me to configure a focus adjustment per filter in focuser steps. I set it to -1, thinking that would do the trick, however every time I get to my blue filter, the focus is slightly off.
This wouldn’t be a huge issue except that I have some very difficult to resolve tilt issues which result in the slightly defocused blue stars to elongate diagonally in a couple of the corners. Looks terrible. I am using a Canon EF 600mm f/4 L II telephoto camera lens, big aperture, very fast, refractor…and all of the EF adapters on the market that fit within the 44mm of backfocus never have enough tension to handle the weight of a filter wheel and CMOS camera. I’ve tried to shim up the tilt, however it needs to be fairly precise, and as I continue to track throughout the night, the orientation of the FW to the pull of gravity changes as does the tilt.
The only solution has been to maintain dead-on focus, which is actually not all that hard with SGP’s AF feature. However, it seems to ignore the ASCOM focus offset I have configured in my EFW2 ASCOM driver settings. Is this just not a supported ASCOM feature? If it is, is this a known bug (I couldn’t find anything with a quick search of the forums.)
If this is not a supported ASCOM feature…is there any chance support could be added? I have found it is basically essential for my blue filter to work. I cycle through events, rather than complete each event one at a time, as this evenly distributes atmospheric conditions throughout all filters, both seeing and LP gradients. This helps a lot when combining the data as it avoids any real issues with atmospheric bloat across the channels (and my lens is very well corrected across the spectrum).
I can’t speak for ASCOM based offsets as the spec does not support them. That said, it seems like your filter wheel driver might just be trying to connect to your focuser driver to adjust position outside of SGPro. While this is fine and should not present any problems, I recommend using SGPro’s built in system:
Not sure exactly what’s being thought of here but ASCOM has filter offsets in the filterwheel specification. What they are is a way to store the filter focus offsets - a number associated with each filter. It’s up to the application(s) to use this if they wish.
Does SGP use it’s own independent filter offsets? No reason why not.
This is what I see in the Atik EFW ASCOM driver. A filter offset setting that allows me to set a relative offset from the current focus point.
I’ve had problems getting SGP’s system to work…however, maybe I’ve misunderstood it. So, based on the docs, I have to set ABSOLUTE filter positions for each filter? Then, SGP will calculate RELATIVE focus shifts for each filter during runtime based on those ABSOLUTE positions? Is that correct? I tried setting relative offsets in there before, and that definitely did not work.
The absolute positions is a bit confusing, because I don’t really want to force my filters to focus to a fixed position on the focuser…and I’m still not entirely sure I’m fully aware of how it’s going to work in actual practice. So, if I set a focus position of 5000 steps for my L filter, and adjust R (which apparently also needs an offset) to 5001 steps and B to 4999 steps. Then during the night, as my focus shifts away from 5000, down towards say 4950 steps (I build my own focuser, and am only able to half step my stepper motor at the moment, so 1 half step of the motor is the width of my CFZ…it works, but it’s not ideal), then every time I switch to the R filter it will go to 4951 steps, NOT 5001 steps, and for the B filter it will go to 4949 steps, NOT 4999 steps.
Yes, that is correct. I also use an Atik EFW2 and I was not even aware of the offset in the Ascom driver.
For example, here’s where I have my current filter offsets for Astrodon filters and my QHY163, set up in the “Define filter list”:
Lum 30,000
Red 30,007
Green 29,987
Blue 30,003
Ha 29,971
OIII 29,959
SII 30,012
These are relative numbers. It doesn’t really matter what number you start on the LUM with as long as you adjust the others accordingly.
Ok. thanks. I’ll try using absolute focuser steps for each filter (relative to my L filter, which is what I usually focus with) next time and see how it goes. Heh, I hope “next time” comes before the end of summer…I get about 1 night every 2 months or so!
It describes pretty well how the “Define filter list” or “Focus Pt” works.
I believe you must start AF run before starting SGP sequence run otherwise the SGP sequence run may move the focuser to the same default position as in “Focus Pt” in “Define filter list”.
