Knife Edge Focusing with Nikon Type C Screen

This method was developed to circumvent the annoying task of changing focusing devices and cameras to achieve precise focus with Schmidt-Cassegrain telescopes. It works when using Nikon cameras with Type C focusing screen mounted. This screen consists of a finely ground glass all over the surface, except for a 4mm diameter central circle of clear glass with a cross hair, and fits perfectly the purposes of frame composition and knife edge focusing. The combination Type C Screen/DW-4 Viewfinder is a good choice for astrophotography.

The method is outlined below:

- Attach the camera with the C screen to the telescope.
- Select a magnitude 2 or brighter star.
- Coarsely focus it using the matte portion of the screen.
- Turn the focus knob 180º clockwise to take the star out of focus.
- Place the star inside the clear glass central circle at the position shown below (near the border).

-   Remove the DW-4 or any other viewfinders.
-   Place your eye as near the C screen as you can, looking at the star. You will see the telescope's secondary mirror evenly illuminated by the star.
-   Now move the star toward the border of the central circle with the declination knob or motor. You will see a shadow move toward the center of the secondary mirror. Notice your screen is outside focus. When it is inside focus the shadow will move in the opposite way.
-   Turn the focus knob slightly counterclockwise and repeat the procedure above.
-  When you reach perfect focus the secondary mirror will darken evenly all over, like in the Foucault’s mirror test.
-  Now the center of the field is in perfect focus. To place the focus midway between the center and the edge, turn the focus  knob 1/36 to 1/24 of a turn counterclockwise. This is needed to compensate for the small curvature of the field every SCT suffer of.
       It takes more time to read than to do this routine. With very little experience one will be able to reach perfect focus in less than a minute.

Click here to see a photograph taken by means of this method.

Marco Antonio De Bellis
Rio de Janeiro, Brazil
E-mail: marco.db@globo.com