Instruction manual

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the focus out 0.001 inch. Remember that a move of 5 steps (using the RFCP) at a step size setting of 4
corresponds to a move of 20 microsteps.
Thermal Effects. Because of the magnification inherent in the S-C design, most S-Cs change focus with
temperature ( 5-7 ºF shift is sufficient to change focus). RoboFocus temperature compensation can
compensate most of this shift. The compensation will not be perfect because the scope does not always
react the same way, and because the temperature of all parts, including the RoboFocus sensor, may not be
tracking together. The major scope part contributor to thermal focus shift appears to be the tube itself. So
long as the temperature sensor and the critical parts do track external temperature changes fairly well (to
within 3-5º) then compensation will be good. An external (optional) sensor mounted on the telescope tube
may also be used.
Backlash. The manual focus knob has a large amount of backlash and may amount to as many as 500
microsteps. The RoboFocus backlash compensation will remove most of this but the backlash must be set
to a large enough value.
Mirror Shift. During focusing with the main focus knob, the main mirror will usually shift to one side,
moving the image as much as several arc minutes. This may move the image out of the field of view of a
CCD camera. Again, if the RoboFocus backlash is set to a sufficiently high value, most of this effect is
removed.
Mirror Flop. As the scope is moved from one orientation to another, the main mirror may move (“flop”)
in its cell. This may shift the image and/or change the focus. This is primarily a potential problem if one is
imaging while passing the meridian. If one is taking a long exposure using multiple shorter exposures (e.g.,
20 10 minute shots), flop may cause one image to be rejected, but the rest can still be combined using
software alignment techniques (assuming the focus is still acceptable).
To eliminate the effect of mirror flop, and to improve focus operation, some astronomers use the manual
focus knob on a S-C to set “coarse” focus then lock the mirror in place. Fine focusing is then done using
one of the zero backlash focusers equipped with RoboFocus. This is an effective strategy, but does require
more expenditure. It also increases the optical path length raising potential balance, optical alignment, and
focus range problems.
Automatic Focusing Strategies
Third party software can support automatic focusing using the RoboFocus. For example, the newest
version of CCDSOFT (Software Bisque) incorporates @Focus, which is an automatic focus routine.
CCDSOFT is a standalone program that operates the camera and processes images. The newest version of
MaxImDL (diffraction Limited) is also a standalone program that provides automatic focusing using a
somewhat different algorithm from @Focus. Another program called FocusMax (formerly known as HFR
Focus) is now available as Freeware. FocusMax uses scripting to drive the CCD camera program
MaxImDL, so it is not a stand-alone program. Each program has its advantages, which we will briefly
describe. All three programs are effective in operating RoboFocus, with the choice depending on personal
preference and the features desired.
CCDSoft and MaxImDL connect directly to the RoboFocus controller through the serial port. Because
RFCP cannot be run at the same time, you do lose the data and controls available in RFCP. FocusMax
controls via RFCP, so the screen shows RFCP and you can use RFCP at any time. A big advantage of
seeing RFCP is that you will have the temperature display available at all times.
You can connect the focus program, do your focusing, and then close that part of the program. However, if
you leave the program running, you can recheck the focus during the session. During the observing
session, if you decide to refocus you may be able to do so using a star in the image field, or you may have
to shift the scope to a brighter star. You may do this “manually”, or you may use automatic scripting for an
automated session. In general, the programs will refocus a scope in 1-3 minutes.