ACP supports extended filter controls, including focus offset support and selection of filters for use in auto-focus and pointing updates. These optional settings for filter usage are contained in a file called FilterInfo.txt. This file must be placed into the ACP configuration file folder Public Documents\ACP Config. ACP includes a tool that can automatically create this file for you.
Before doing anything, make sure that the filter names configured in MaxIm's Filter Setup do not have any blank rows between the real filters. The best solution is to arrange your filters so they are contiguous in the wheel. If you must have an empty slot between others, give that slot a name like "Do Not Use". This will be a live filter though and will be measured in the process described below. Clear out the unused filter slot names at the end as well. If told to autofocus for a target (#autofocus), ACP (without the presence of FilterInfo.txt) will refocus after every filter change (very inefficient!), and it uses the configured "Clear" filter for pointing exposures.
Whether or not you have parfocal filters, we strongly recommend that you set up your focus offsets. It's so easy, see Setting Up Focus Offsets the Easy Way below. It will keep you from having to create the FilterInfo.txt file yourself, and give you the opportunity of adjusting the focus star mags for each filter (if you are not using FocusMax AcquireStar).Before doing anything else, go into MaxIm's filter setup and remove the 'Filter n" names from the unused slots. ACP depends on the fact that when there is a name present, it corresponds to a real selectable filter!ACP comes with a standard script FocusOffsets.vbs that will automatically determine focus offsets for your filter set and create the FilterInfo.txt file for you. If you have FocusMax installed, and AutoFocus enabled in ACP, simply run this script in ACP's console. It will make 5 focus position measurements through each of your filters then create the FilterInfo.txt file automatically. Load it into ACP's console and run it, then take a break! It may run for an hour or more.
If you have problems with your narrow band filters while running this script, you may need to create an initial FilterInfo.txt with 0 offsets but including brighter focus star magnitudes for the narrowband filters. See the next section for details. The 3 below is the "filter to be used for pointing updates and as the focusing reference filter" as set in ACP Preferences, Imaging tab. Normally this is the slot with your Luminance/Clear filter. The filters are in order they are in your wheel. The ';' starts comments which are ignored as well as the text thereafter. Here's an example that forces ACP to pick mag 4-6 stars for the narrowband filters. If you start with this, the FocusOffsets.vbs script will give brighter stars to the focusing software (FocusMax or PWI).
;
; Sample ACP extended filter info file
; Fields are offset, relative-to, use for pointing[, min-mag, max-mag]
;
0, 3, 3, ; #0 Red, relative to Clear, Pointing w/Clear
0, 3, 3, ; #1 Green, relative to Clear, Pointing w/Clear
0, 3, 3, ; #2 Blue, relative to Clear, Pointing w/Clear
0, 3, 3, ; #3 Clear, relative to Clear, Pointing w/Clear
0, 3, 3, 4, 6 ; #4 HAlpha, relative to Clear, Pointing w/Clear, Mag 4-6 focus star
0, 3, 3, 4, 6 ; #5 OIII, relative to Clear, Pointing w/Clear, Mag 4-6 focus starIf you are not using FocusMax's AcquireStar feature, ACP chooses the focus star and then calls FocusMax. If you have narrowband filters, or if your scope has a large aperture and/or a sensitive imager, the default range of focus star brightness (mag 6-9) can be a problem. You can override ACP's choice of focus stars by specifying a magnitude range for any of the filters listed in FilterInfo.txt. For large scopes and/or sensitive imagers and wideband filters, you may want to go fainter to prevent focus star saturation and/or really short exposure times. For smaller scopes and narrowband filters you may want to choose brighter stars to allow focusing and even shorten focusing time by reducing the needed exposure time.
The idea is to choose magnitudes that prevent saturation of focus stars, and result in focus exposures above a few seconds (to average seeing) and below tens of seconds (so focusing doesn't take forever!).
After using FocusOffsets.vbs to create your initial FilterInfo.txt file, open it up in Notepad (ASCII mode!). On each data line, the last two values are the magnitude range for the focus star to be used with that filter (see Format of FilterInfo.txt below). You'll see that they all are listed 6,9 (unlike the example below, which has the default values omitted). If you have narrowband filters (Hydrogen Alpha, Oxygen III, etc.) consider specifying brighter (lower magnitude) values. If you had to do this when running the initial measurement script (see above) the values you chose will be preserved. Once you have customized the focus star magnitude values, subsequent runs of FocusOffsets.vbs will use your mag values (improving its reliability!) and also preserve your changes.
Once you have FilterInfo.txt, ACP can avoid refocusing when it changes filters. If you have special requirements, or if you don't trust this approach (for example, if you have a focuser with poor repeatability), you can force ACP to refocus on each filter change. In ACP Preferences, AutoFocus tab, enable the option Force per-filter focusing even when FilterInfo is present.
If you want to measure your focus offsets manually:
You should probably focus several times with each filter and use the average of the positions.
Now create a file called FilterInfo.txt, using Notepad or other text editor, and place it into the ACP configuration file folder Public Documents\ACP Config. This file contains, for each filter on your system, the focus offset for the filter, the filter to which that focus offset is relative, and the filter to use for pointing exposures, and optionally the values of the magnitude range for the focus star.
FilterInfo.txt is an ASCII (not Unicode!) text file. It may contain spaces and tabs anywhere. Comments may be included, and they are indicated by a semicolon (';'). A line beginning with a semicolon will be ignored, and all text on any line that follows a semicolon will be ignored.
Filter info itself consists of one line per filter, in filter number order. Each line consists of three comma-delimited fields:
In ACP, filter numbers start with 0. In MaxIm, they start with 1. So, for example, filter number 4 in MaxIm is filter number 3 in ACP.;
; Sample ACP extended filter info file
; Fields are offset, relative-to, use for pointing[, min-mag, max-mag]
;
-10, 3, 3, ; #0 Red, relative to Clear, Pointing w/Clear
-5, 3, 3, ; #1 Green, relative to Clear, Pointing w/Clear
+15, 3, 3, ; #2 Blue, relative to Clear, Pointing w/Clear
0, 3, 3, ; #3 Clear, relative to Clear, Pointing w/Clear
-180, 3, 3, 4, 6 ; #4 HAlpha, relative to Clear, Pointing w/Clear, Mag 4-6 focus star;
; Sample ACP extended focus info file
; Fields are offset, relative-to, use for pointing[, min-mag, max-mag]
;
; LRGB Group, relative to Clear
;
-10, 3, 3 ; #0 Red, relative to Clear, Pointing w/Clear
-5, 3, 3 ; #1 Green, relative to Clear, Pointing w/Clear
+15, 3, 3 ; #2 Blue, relative to Clear, Pointing w/Clear
0, 3, 3 ; #3 Clear, relative to Clear, Pointing w/Clear
-180, 3, 3, 4, 6 ; #4 HAlpha, relative to Clear, Pointing w/Clear, Mag 4-6 focus star
;
; UBVRI Group, relative to V
;
+40, 7, 7, 4, 6 ; #5 U, relative to V, Pointing w/V, Mag 4-6 focus star
+10, 7, 7 ; #6 B, relative to V, Pointing w/V
0, 7, 7 ; #7 V, relative to V, Pointing w/V
-12, 7, 8 ; #8 R, relative to V, Pointing w/R <== NOTE (avoid filter changes)
-35, 7, 7, 5, 7 ; #9 I, relative to V, Pointing w/V, Mag 5-7 focus star