Knifeedge and Ronchi focusing In theory

A time-honored way to find the focal plane of a telescope is to remove the eyepiece, or set it out of focus, and run a knife edge across the focal plane (Figure 8.6 d). While doing so, look at a star through the telescope; you will see a large, out-of-focus disk or doughnut.

If the knife edge intercepts the focal plane precisely, then the big, blurred star will suddenly wink out all at once. If the knife edge is ahead of or behind the focal plane, the knife edge will sweep across the blurred star from one side to the other (Astrophotography for the Amateur, 1999, p. 86).

Instead of a knife edge, you can use a Ronchi grating, which is a piece of glass with a series of black stripes.1 In that case, the stripes appear superimposed on the out-of-focus star, and as you approach the focal plane they seem to get farther and farther apart. This makes it easy to approach correct focus from a grossly incorrect setting; a knife edge doesn't tell you how far you are off, but a Ronchi grating does. At perfect focus, the edge of a stripe acts like a knife edge.

1 Invented by Vasco Ronchi (1897-1988). The name is pronounced RON-kee.

Ground glass

Eyepiece or magnifier

Double image if out of focus

Double image if out of focus

Scheiner disk

Crosshairs or no screen Eyepiece

Known distance

Figure 8.6. Four ways of focusing: (a) ground glass or SLR focusing screen; (b) Scheiner disk; (c) parfocal eyepiece (not recommended); (d) knife-edge or Ronchi grating. From

Astrophotography for the Amateur.

Eyepiece optional with this method

(if used, put it out of focus)

Figure 8.6. Four ways of focusing: (a) ground glass or SLR focusing screen; (b) Scheiner disk; (c) parfocal eyepiece (not recommended); (d) knife-edge or Ronchi grating. From

Astrophotography for the Amateur.

Knife-edge or Ronchi focusing gives a very precise result - you can't be off by even 0.1 mm - and is totally unaffected by the focus of your eyes. You need not have sharp vision at all; if you need glasses, you can leave them off, and the results will be the same.

In practice

The trouble with Ronchi or knife-edge focusing is that you can't do it with your DSLR in place. You must remove the DSLR and substitute a film camera body with no film in it, then run the knife edge across the film plane or put the grating there. The two camera bodies must match perfectly - but a film SLR body is unlikely to be a perfect match to a filter-modified DSLR because changing the filter changes the focal plane.

In place of a second camera body, you can use a Stiletto focuser (www.stellar-international.com). This is a convenient, ready-made device that goes in place of a camera and has a knife edge or Ronchi grating at the right position. It has its own eyepiece so that you view from the same position each time.

Many astrophotographers feel that the Stiletto or an equivalent device is the gold standard of focusing. I don't use one myself for two reasons. First, anything that requires me to swap camera bodies will increase the risk of getting dust on the sensor. Second, it is easy to sample the actual DSLR image electronically, and that's what I'd rather use for my final focusing.

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