Focusing Techniques

Keeping your system in focus is, as might be expected, of critical importance in achieving high data quality.  Not only does excellent focus give smallest stars and highest resolution but it also gives higher signal.  So focusing and maintaining focus is very important.

Focusing Programs
There are three focusing methodologies supported in CCDAutoPilot - FocusMax, @Focus2 and Maxim's own focusing routine.  When properly initialized or calibrated, both will do an excellent job of achieving critical focus.

FocusMax is a free program put in the public domain by Steve Brady and Larry Weber.  It supports a wide range of focusers and is widely used and respected.  It works with both CCDsoft and Maxim.  It requires an initial calibration routine that might take 30 minutes or so but once done, will never need to be revisited unless you change cameras or the imaging telescope's focal length, by a focal reducer for example.  Consult the FocusMax help file for more details.

@Focus2 is a unique technology to Software Bisque and is incorporated in CCDSoft.  It supports most popular focusers.  Initial calibration requires determining the exposure time for a given magnitude star that results in a peak count of 25,000.  This must be done for each filter and can take 10-15 minutes or so for 5 filters.  Once done, it will not need changing unless the camera or imaging telescope's focal length is changed.  Consult the CCDSoft help file for more information.

Maxim Focuser  uses a similar approach as FocusMax by measuring the half-flux density of an out-of-focus star.  It does not require the calibration that FocusMax and, as a consequence, individual focus sessions will take a bit longer, perhaps 20-30% longer.  If focus offsets are required, then the Maxim Focuser must connect to an ASCOM hub so that both CCDAutoPilot and Maxim can access the focuser simultaneously.

The Problem
For each optical system, focusing must be maintained within the Critical Focus Zone (CFZ).  Classical optical theory as the best focus, limited to the wavelength of the incoming light and the size of the Airy disk, which is primarily a function of the imaging telescope's aperture.  Recent work suggests that with automated focusing programs, focusing should be targeted at being approximately one-third of the classical CFZ and this is what CCDAutoPilot's CFZ calculator gives.  Even assuming an accurate initial focus, the focus can change over the course of the imaging run due to a number of sources.
  • Filters:  Different filters may have different optical thicknesses, resulting in a change in focus as different filters are used.  Even parfocal filters, filters which have the same optical thickness, may not result in the same focus point if the imaging telescope has significant refractive elements.  The best corrected APO refractors will still show a focus difference between red and blue filters for example.  Reflective telescopes typically do not show this problem.
  • Temperature: Aluminum is a component in most telescopes and is subject to contraction as temperature decreases.  This usually results in a focus change, unless mechanically compensated.  And some lower cost telescopes might have optical elements made from non-zero temperature coefficient glass, leading to a potential change in focus.
  • Mirror Flop: This is an issue primarily with lower cost SCT's, where the mirror moves due to change in OTA attitude.  This issue may also be exacerbated by the meridian crossing "flip" with an equatorial mount.  In flipping from east to west, the OTA effectively rotates by 180 degrees!

    From the above it is clear that there is a need to focus during the course of the evening.  CCDAutoPilot provides a number of techniques to focus and has the ability to characterize your system.  You can characterize your system during moon time by a couple of techniques.  

    Filter Effect: CCDAutoPilot includes a Focus Offset Measurement Wizard that automatically determines the focus offset arising for each filter in concert with the characteristics of your OTA.  This requires a true absolute focuser whose position is completely repeatable.  If your focuser meets that requirement, you can use the wizard to determine the offsets for focusing. Simply center the scope on a suitable focus star, say magnitude 4 - 5, set your starting focus exposures appropriately on the Focusing page, select a reference filter(Green is suggested as a mid-band filter range) and the number of measurements/Filter (minimum 5 recommended, more is better), and hit the Measure button. The wizard will make the specified number of focus runs per filter, calculate the median for each filter, calculate the necessary offsets and enter them on the Focusing page.  If the entered offsets are within the CFZ (Use the calculator in the left pane of the focus page to determine CFZ), then you can use the same filter for focusing without worrying about the offsets Even if there are significant offsets, you can program in those offsets on the Focus page so that they will be added or subtracted as necessary to achieve excellent focus. When using focus offsets, be sure the system is in focus at the start of the session. A good way to do this is to use CCDAutoPilot's Focus Before Target Run Start as described here.

    Temperature Effect: This will probably take a full evening's run again during moon time.  You will need a method of recording temperature over the evening.  CCDAutoPilot can acquire temperature from a number of sources.  Set up a number of focus stars that will be within 20 - 30 degrees of the meridian over the course of the evening.  (This requires the Professional edition, since each focus star is a target.  For the basic edition, you will have to set up individual sessions.)  Unless you know you don't have a mirror flop issue, you should keep to one side of the meridian.  Set up SkyStar with Center Focus Star checked.  Do 5 series of a short exposure with the same filter.  At the conclusion of the evening's run, you can assess how much focus changes with temperature and how much temperature change causes you to exceed the CFZ.  You can then program CCDAutoPilot to focus on a temperature change that corresponds to 1/3 of the CFZ for example.

    Mirror Flop: Set up a number of series with the same filter and select focus at series start.  Choose a suitable value for the number of sets to insure the imaging time carries through a meridian flip.  Select Focus At Star Center with Center Focus Star checked.  For meridian flip settings, check Focus On Flip as an additional data point.  At the end of the session, examine the log to see how much the focus point changes across the meridian flip.  If you see a significant change, be sure to always use the focus after meridian flip option.

    Unknowns: The above covers what we know but other things can happen that we don't know about.  For example, a passing cloud can obscure the focus star and you may get a bad focus reference.  Periodic focusing, focus at series start even with focus offsets set up, can help catch and correct for those events.  See the Focusing topic in the Command Summary for details on the many focusing tools provided in CCDAutoPilot.