Orion #8297 8" F/3.9 Collimation Cap, Primary Mirror Center Mark, Aligning the Secondary Mirror

The Collimation Cap

Your f/3.9 Newtonian Astrograph comes with a “quick collima- tion cap” (Figure 9). This is a simple cap that fits on the fo- cuser drawtube like a dust cap, but has a tiny hole in the cen- ter and a reflective inner surface. The collimation cap helps center your eye over the focuser drawtube so that aligning the optical components is easier to achieve. The reflective sur- face provides a distinct visual reference that is helpful in cen- tering the primary and secondary mirror reflections. Figures 8b through 8e assume that you have the collimation cap in place.

We strongly recommend the use of a laser collimating tool such as the Orion LaserMate Deluxe or LaserMate Pro to aid in collimating the optics. A laser collimator will ensure a more precise collimation than you can usually achieve with the included collimation cap. With fast optics like those of the f/3.9 Newtonian Astrographs, getting a very precise collimation is critical for obtaining the sharpest, crispest images, so a laser collimator is a worthwhile (and small) investment. However, for the purposes of this instruction, we’ll assume that you don’t (yet) have a laser collimator, and that you’ll be using the supplied collimation cap.

The Primary Mirror Center Mark

You’ll notice that the primary mirror of your f/3.9 Newtonian Astrograph has a tiny ring (sticker) marking its center. This “center mark” allows you to achieve a very precise collimation of the primary mirror; you don’t have to guess where the exact center of the mirror is.

NOTE: The center ring sticker need not ever be removed from the primary mirror. Because it lies directly in the shadow of the secondary mirror, its presence in no way adversely affects the optical performance of the tele­ scope or the image quality. That might seem counter­ intuitive, but it’s true!

Preparing the Telescope for Collimation

Once you get the hang of collimating, you will be able to do it quickly even in the dark. If you’re new to It, it is best to col- limate in daylight, preferably in a brightly lit room and with the scope aimed at a light-colored wall. It is recommended that the telescope tube be oriented horizontally. This will prevent any parts from the secondary mirror from falling down onto the primary mirror and causing damage if something comes loose while you are making adjustments. Place a sheet of white paper inside the optical tube directly opposite the fo- cuser (Figure 10). The paper will provide a bright “back- ground” when viewing into the focuser during collimation.

Aligning the Secondary Mirror

To adjust the secondary mirror collimation, you will need both a small and a large Phillips screwdriver.

You will need to check, and adjust if necessary, four aspects of the secondary mirror’s alignment:

Figure 9. The quick collimation cap, which features an inner reflective surface, helps in centering reflections of the optics in the focuser during the collimation process.

Figure 10. Placing a piece of white paper inside the optical tube opposite the focuser provides a bright background when viewing into the focuser.

1.The secondary mirror’s axial position

2.The secondary mirror’s radial position

3.The secondary mirror’s rotational position

4.The secondary mirror’s tilt

The first three will probably only need to be checked and (possibly) adjusted once. Chances are, however, that they are set correctly from the factory and will not need any adjusting. We recommend that you do not make the adjustments in 1 to 3 above unless you first determine that they absolutely need it! It is usually only the secondary mirror’s tilt that will need to be adjusted occasionally.

Adjusting the Secondary Mirror’s Axial Position

With the collimating cap in place, look through the hole in the cap at the secondary (diagonal) mirror. Ignore the reflections

8