Revised filter offset focusing


As new features for v3 are being considered, I would like to propose that the current filter offset focusing routine be redesigned to make it easier to implement and more reliable in use.

In the Auto Focus Options dialog box, I would like to see a check box that clearly states:
“Use filter offset focusing.” When checked, a drop list allows the selection of the filter to be used for initial focusing; this would normally be the LUM filter. It might be useful to have a focuser position value for the LUM filter but that position would have to be temperature compensated.

In the Filter Setup dialog box, I want to be able to make entries that clearly identify that relative filter positions are being specified. For example:

   LUM  filter offset:   0
   RED  filter offset:   +145
   GRN  filter offset:   +88
   BLU  filter offset:   -44

But filter offset focusing is a two part problem. You have to get accurate values for those filter offsets. Trying to run AF routines manually will typically result in values that may not be all that accurate. The better solution is to have a “Filter Offset Wizard” that would automatically run an entire series of AF runs for each filter and calculate an average value. This would be a fairly long process since you could be looking at 15+ second exposure times for NB filters. This also introduces the possibility of needing to take into account temperature compensation while the wizard is running.

All this relative movement also means using backlash control. If the focuser’s own ASCOM driver does not implement backlash adjustments, then SGP will need to do it. This might be part of the wizard.



I just went through this process manually, by setting “Auto focus on filter change” and running a series of focus runs for all my filters on different nights at different temperatures. The different temperatures allowed me to come up with a temperature compensation value in steps/deg, then I adjusted the values of the focus positions for temperature (since you are correct, they vary enough during a single run to require correction, and yes I did need to use backlash compensation). Once I had the corrected focus positions for a number of runs, I averaged the offsets for every filter and entered a base focus value (this would be for Lum in your case; I have a 5-position wheel and used Green) + offset into the Filter list. Not sure a wizard would have made this much easier since I had to run the focus tests multiple times over several nights at different temperatures to get the numbers I needed…


I agree with the OP and in fact have suggested something similar here before. Basically there are two “problems”.

  1. The filter offset setup and terminology are not at all obvious, especially to new users.
  2. Obtaining accurate offsets requires many offset runs since single runs are wildly inaccurate (and I have the data to prove this). I have done this manually and entered the results in a spreadsheet in the past. This works but could be automated for easier user friendly results.

This has a lot in common with the thread about the usage of the term “step size”. In other words, we long-time users understand it but we still do not agree that the use of non-standard meanings of terms is a good idea.


I agree and I see this as the biggest single improvement that can be made to SGP. I’ve tried using offsets on a few occasions and just couldn’t get them setup accurately enough. I now always focus per filter and this wastes a lot of imaging time.

If the log files weren’t auto deleted after two weeks, but after a user selectable period, then maybe these could be parsed and accurate offsets calculated?

Or indeed, a wizard that just cycles through auto focus routines for each filter over a set period of time and displays calculated offsets with a method of displaying accuracy confidence.


We’ve talked about doing something like this for a while. It’s definitely something we’d like to get to eventually…I just don’t know when at the moment.



I think it is a great idea. I am trying to use offset at the present time and being a new user I am totally lost. So anything to make this process easier has my vote.