Camera Focusing

There are several methods used in astrophotography:

• Ground glass focusing with a magnifier as commonly seen on standard camera bodies supplemented with a focusing magnifier or microscope (10x or greater).
• Aerial focusing with a short focal length eyepiece as employed with the Taurus astrocamera system.
• Knife edge (or Ronchi grating) focusing which uses a sharp edged mask to cut the light cone of a point source. The Ronchi grating serves to provide convenience by creating multiple parallel knife edges.

Knife Edge Focusing

Of these 3 methods, the last is generally accepted to be the most accurate method of assuring focus. The way this works is that when the knife edge is cutting the light cone of a point source at the exact focus, if your eye is positioned behind the focus point in the diverging light cone, the source's light will be extinguished suddenly. If the knife edge is inside or outside the focal point, the light source will gradually be extinguished as the knife edge cuts across the light cone.

In practice, one way to implement this is to use two compatible camera bodies - one being used to hold the film for photos, and the other used empty, just to supply the focal plane surface for the knife edge or Ronchi grating. The scope is focused using the empty camera body, then it is replaced with the other body for shooting a photo.

The other implementation is to buy or make a focusing device which provides a knife edge focusing reference surface at the same place as the film plane in the camera. Generally, the procedure is to knife edge focus the empty camera body to insure that the scope is accurately focused, then replace the camera with the focusing device and adjust it until it also indicates exact focus. Once this is done, the focusing device is locked so that it can be used to focus the scope once the camera has been loaded with film.

Generally these knife edge (or Ronchi) focusing devices can be purchased for 35mm cameras, but for medium format or larger cameras, they need to be custom made. Below are two custom implementations for the Pentax 67 camera:

At left and also immediately below is a custom-machined focuser belonging to Jim Janusz. It is designed to thread into an AP 2.7" focuser (or 4" focuser via an adapter), thus circumventing the problem of making a coupler compatible with the camera being used. Jim writes:

My usual procedure is to put the camera on and center a star. Then I focus the camera by eye and move it slightly out of focus in a known direction, (out) then I remove the camera and flattener and replace it with the adapter setup. I knife edge on that and I'm focused. Once or twice a year I knife edge on the camera rails and then check the adapter setup, it has always been right on since I first set it. Then I never use the camera, only the adapter.

Note that Mark adapted a 35mm focuser for use with a Pentax 67 by using the endcap for a Pentax lens. This endcap has the proper bayonet coupling to replace the camera body. Although it is plastic, it is stiff plastic, plus it is reinforced on the inside of the cap with a metal ring (see photo below).

The basic design is the same as Jim's. Two pieces can move relative to each other and then locked into place once the focal point has been calibrated to the camera back.
Because of the difficulty of sighting directly through the focuser for knife edge focusing, Mark told me that once he calibrated it against the camera back, he only used it to verify that he was achieving good focus with his normal ground glass focusing. In his case he was using an eyepiece turned backwards as his focusing magnifier.