focus jpg

Step Right Up

In order to focus precisely it is clearly mandatory to be able to make moves that are small enough to consistently arrive within the depth of focus band. The diagram below illustrates the all-too-common "Do you feel lucky?" large step size at the top and the "Required for precise control" smaller step size at the bottom. As you can see, it is necessary to know both the depth of focus of your system and the minimum step size of your system in order to be sure that you even CAN focus properly.

Depth vs. Step Size

The Real World

In the table below the first row restates the very liberal 2X depth of focus ranges from the table on page one. Bear in mind that using a depth of focus band half that size is more practical and safer. The second row lists the step sizes actually measured on three systems. These were measured as follows:

An extra long extension nosepiece was placed on the camera so that the camera could be positioned with the front face of the camera exactly one inch back from the rear of the focuser (measured with a machinist's gauge). The system was then focussed in that position using the primary or secondary mirror focus technique (depending on the scope). The position readout of the digital focus was noted. The camera was then slid forward so that the camera face was in contact with the rear of the focuser (exactly one inch). The system was then refocussed and the number of steps needed to do this was noted. A simple calculation then gave the step size for that configuration.

Instrument and F Ratio C8 at F10 RC at F9 RC at F7
Depth of Focus (Max. Inches) .01 .009 .007
Step Size (Inches) .008 .0008 .0005

As you can see, moving the primary mirror of the C8 with the manual DRO focuser was quite inadequate in terms of step size and was a lot like the top example in the diagram, whereas the electric secondary on the RC scope was more than adequate and more like the example shown at the bottom of the diagram. The important point here is that YOU REALLY NEED TO KNOW THESE NUMBERS FOR YOUR SYSTEM.

Next: The Great Truth (besides 42)


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