Instruction manual

5. Focusing Strategies

Introduction

There are as many focusing strategies (almost) as there are astronomers. Regardless of the method you use,

the RoboFocus allows you to return to the same setting, and to make small, known changes to your focus.

Focusing may be performed “manually” (taking an image, measuring the focus, moving the RoboFocus,

then reimaging) or you may focus “automatically” by using one of the automated focus programs now

coming available. Either way, focusing should take no loner than a few minutes.

In general, if you are imaging planets or other extended objects, you will normally focus on a nearby star,

then move to the object to be imaged, knowing that the focus is correct. This process is much faster than

trying to focus the scope on the extended object, especially if poor atmospheric seeing causes the image to

be distorted differently in successive images.

Visual. Many users will find that the RoboFocus allows them manually to control the focus while avoiding

touching the telescope (which induces vibration). In this case, the eye provides the indication of focus.

Using the dual rate focusing, you can make the tiniest corrections to focus settings. If you do much visual

focusing, you may want to use the Manual Menu or RFCP configuration screen to slow the stepping

(longer step periods).

Photographic. When using film, the focusing is normally done with a single lens reflex or other camera

with a ground glass screen. Once the focus is determined, the RoboFocus allows the user to return to that

position using the computer control.

CCD Camera. The CCD camera and software provide several different methods of focusing. Usually,

focusing is best done by taking successive images of a medium bright star. The CCD software then

provides several measures of the quality of the stellar image. An easy measure to use is the peak brightness

(one adjusts focus for maximum brightness). Another measure is the Full Width Half Maximum (a

measure of the image width in pixels, which one focuses to minimize).

Many astronomers employ a variety of devices to assist in finding the best focus. These are usually masks

with holes or wires that fit onto the front of the scope. These introduce image artifacts that one observes

when adjusting focus. An obvious problem with these devices is that they are difficult to employ in a

remote operation. However, the good news is that most users find that the peak or FWHM methods work

as well or better.

If you are new to CCD imaging you may be aware that most cameras and software allows you to select a

small area of the image to use, usually from within a “focus” procedure in the software. This is an

extremely important procedure to expedite focusing as it greatly speeds up downloading. For example, a

typical focusing routine would be:

• Take an image using X3 binning, full frame

• Use your mouse to draw a box around a star

• Goto X1 binning

• Take images of the star, and adjust focus.

This procedure minimizes wasted time spent in downloading. With many cameras, you can do an

image/focus cycle in 30 seconds so that your total time spent focusing will be only a few minutes.

Schmidt –Cassegrain Scopes

Achieving excellent focusing results with a Schmidt-Cassegrain scope requires some understanding of the

characteristics of the scope that affect good focusing. For your reference, as noted above, our convention

for S-C scopes is that OUT on the RoboFocus corresponds to CCW focus knob which moves the focus

position OUTward. For the 8-inch f/10 LX200, approximately eight (8) microsteps are required to move