Instruction manual

Before you can effectively autofocus, you must have set up the RoboFocus properly. These are some good

starting points to use. The user will usually establish the internal RoboFocus settings for efficient operation

on a particular telescope-focuser combination whether focusing manually or with automatic focusing

software. Once the RoboFocus is set, the user will enter additional settings as necessary in the chosen auto-

focusing program. Note that the recipes we give below are derived from my own experience: they work for

me. Other users may find alternative methods of operation that are better for them.

The RoboFocus stepping motor operates with a configurable internal RoboFocus StepSize (RFSS) of 1-64

microsteps. This setting should be set low enough to give the resolution needed for a particular focus

system, and high enough to give the range of total movement needed. A directed move of 10 counts at

RFSS=4 is the same actual movement as 20 counts at RFSS=2: Both moves have 40 microsteps total

movement.

LX200 and other S-Cs. The RoboFocus applied to the LX200 focus knob generally requires RFSS=4 or

more to give the full range of focus motion without re-calibration. Total focus range will be about 35,000

counts. RoboFocus backlash setting should be at about 1000 or more microsteps, or about 250 counts for

RFSS=4. Direction of backlash correction should be OUT or CCW on the knob, which moves the focus

point OUT as a result of moving the mirror IN against gravity.

R&P. RoboFocus applied to any Rack and Pinion type focuser (including Crayford style) requires fine

steps to give fine control. With an RFSS=2, total focus range will be about 12,000 counts for a five inch

travel R&P, and about 2,000 counts for a Crayford focuser (usually only and inch or so travel). RoboFocus

backlash setting depends on the focuser, but about 60 microsteps (30 counts at RFSS=2) is typical.

Direction of backlash correction is normally IN so that the focuser will move against gravity.

We now discuss several different autofocusing programs. These are NOT detailed instructions, but will get

you started. As the programs are evolving, be sure to read the instructions on the version of program that

you have.

Using CCDSoft @Focus

@Focus instructions refer to the Critical Focus Zone (CFZ) which is the focus range over which an image

is “perfectly” focused. The CFZ is proportional to the f-number squared and is primarily based on the

resolution limit of the scope. In practice, the effective CFZ is often much larger due to atmospheric seeing,

under-sampling on the CCD, inaccurate guiding, or any other effect that smears the image. For an f/10

scope, the CFZ is approximately 0.01inch. For “perfect” focusing, you need to be able to make focus

movements at least as small as the CFZ. With RoboFocus on the focus knob of an LX200 or on an R&P

focuser, the least move is approximately 0.0001-.0002 inch, or about 15x smaller than the CFZ. Thus, one

could use an RFSS as large as 10-15 steps (in practice, we usually use smaller step size). For effective use

of automatic focusing, it is not necessary to know the particular CFZ value for the system, only that the step

size is smaller than the CFZ.

The recipe we give here (manual focus, step outward to out of focus, then run @Focus) is a bit different

from what is given in the @Focus instructions. We think our process is easier to use; also, we expect

customers will develop their own recipes for operation.

In @Focus, you will need initially to set the step sizes (position counts) that @Focus will use. As a start,

set the “large step” to about 10 counts (20-40 microsteps) and the small step to 1 count. Be sure and set the

backlash in @Focus to zero—RoboFocus is already handling backlash!

The first time you use @Focus, you should spend a moment and find the approximate best focus point by

hand using the maximum pixel value as a guide. In this process you will take an image, read the peak pixel

value, move the focuser then repeat. You are seeking the focuser position for the highest brightness value.

You can use the RoboFocus control program (RFCP) to do the movement, or you can use the stepping

available within @Focus where you can make single steps with your mouse. If you have far to go, the