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.