This section contains options that are related to the focuser and the auto-focus methods. They are described in detail in the Auto-Focus Documentation.
As a summary:
- Use FilterWheel Offsets: determines whether the focuser should move per the defined offset when the filter wheel changes filter
- Auto Focus Step Size: the number of focuser steps that the autofocus routine will move by between autofocus points
- Auto Focus Initial Offset Steps: the number of focus points that will be used on each side of perfect focus by the autofocus routine
- Default Auto Focus Exposure Time: the exposure time in seconds that will be used by autofocus, if filter times are not set
- AF Method: the autofocus method to be used
- AF Disable Guiding: determines whether autoguiding will be stopped during the autofocus routine
- AF Curve Fitting: the curve fitting method used for finding best focus based on autofocus points
- Focuser Settle Time: the amount of time, in seconds, that should be awaited after a focuser move before starting a new exposure
- AF Number of Attempts: the number of attempts the autofocus routine should be retried in case of unsuccessful focusing
- AF Number of Frames per Point: the number of frames whose HFR or contrast will be averaged per focus points
- Use Brightest n Stars: the number of top brightest stars that the autofocus routine will use - 0 means there is no limit
- AF Inner and Outer Crop Ratios: ratios (as numbers between 0.2 and 1) that will determine a centered region of interest for autofocus
- Backlash IN/OUT: the focuser backlash in the IN (decreasing position) and OUT (increasing position) directions, expressed in focuser steps. A tool described in the Focuser Backlash Measurement Section is available to measure it
- Binning: the binning to be used for Autofocus exposures
Filter Wheel Configuration#
The Filters defined in the Filter Wheel list are used in various places in N.I.N.A., especially in:
- The Sequence Tab: each sequence item can use its own filter for capture
- The Plate Solving routine: it can be set to use a particular filter, to have lower exposure times for plate solving (e.g. using L rather than HA for example)
- The Auto-Focus routine: like plate-solving, autofocus can be set to use a particular filter
For the above to work well, it is necessary to define the proper filters available, if necessary their filter offsets, and any filter to be used for Auto-Focus from this view.
The screen looks like the below:
Typically the first step for a user when first setting up the filter wheel is to click on the Import Filters from Filterwheel button. This will take the information about the filters from the filter wheel itself (if any is available), and automatically populate the list in N.I.N.A. based on that information.
If this doesn't work, it is possible for the user to use the + and - buttons to manually add or remove filters. Note that the Position order of the filters in this tab should match the order of the physical filters in the filter wheel.
For example, if a filter wheel has the following filters:
- Luminance filter at position 1
- Red filter at position 2
- Green filter at position 3
- Blue filter at position 4
- H-Alpha filter at position 5
then the screen should be configured as per the screenshot above, starting with the L filter, and going in order until the HA filter.
Changing filter information#
It is possible to double click within the table to change the name of the filter (used throughout N.I.N.A.), its focuser offset and its Auto Focus Exposure Time directly.
Most filters are not exactly par focal, meaning that when changing filters, the ideal focus distance changes slightly. This will cause an imaging system that was in perfect focus with one filter to be slightly out of focus with another filter. This can be a big problem for precise imaging, requiring an additional autofocus run each time the filter is changed.
To avoid this, it is possible to set filter offsets, which are the amount of focuser steps that the focuser should move by when switching from one filter to another.
For example, I could run the autofocus routine on each of my filters one after the other (with hopefully very little temperature change in between), with the following results:
- L filter achieves perfect focus at focuser position 5000
- R filter achieves perfect focus at focuser position 4990 (10 steps fewer than L filter)
- G filter achieves perfect focus at focuser position 5030 (30 steps more than L filter)
- B filter achieves perfect focus at focuser position 5045 (45 steps more than L filter)
- HA filter achieves perfect focus at focuser position 4988 (12 steps fewer than L filter)
If we take the L filter as the reference filter, we can set up all the filter offsets relative to the L filter, as below:
- L filter offset 0 (reference filter)
- R filter offset -10 (10 steps fewer than L)
- G filter offset 30 (30 steps more than L)
- B filter offset 45 (45 more steps than L)
- HA filter offset -12 (12 steps fewer than L)
This is what has been done in the above screenshot.
Note that for this to work, the Use FilterWheel Offsets parameter under the Focuser Options needs to be set to On.
Auto Focus Exposure Time#
The ideal auto-focus time can change per filter, particularly between broadband and narrowband filters (in the above example, the narrowband filter requires an exposure time 5 times longer than the broadband filters). This can easily be set up here.
Finding a good exposure time for autofocus is further explained in the Auto-Focus section
Auto Focus Filter#
From this screen, it is possible to set (or unset) an autofocus filter, which will be used by the autofocus routine (if the Use FilterWheel Offsets setting under Focuser Settings is set to On). This can be done by simply selecting a filter in the list, and clicking on the Set as Default AF Filter button. The same button can be used to unset the Auto-Focus Filter.