Orion #8296 10" F/3.9, #8297 8" F/3.9 Visual Observing with the f/3.9 Newtonian Astrograph

drawtube

Reflection of primary mirror clip

a.

Reflective surface	Primary mirror
of collimation	center mark
cap

b.c.

d.e.

Figure 8. Collimating the optics. (a) When the mirrors are properly aligned, the view down the focuser drawtube should look like this. (b) With the collimation cap in place, if the optics are out of alignment, the view might look something like this. (c) Here, the secondary mirror is centered under the focuser, but it needs to be adjusted (tilted) so that the entire primary mirror is visible. (d) The secondary mirror is correctly aligned, but the primary mirror still needs adjustment. When the primary mirror is correctly aligned, the center “dot” of the collimation cap will be centered, as in (e).

cal tube in front of the focuser. Both the 8" and 10" models have added length in front of the focuser, compared to a stan- dard Newtonian, to prevent any possibility of incoming light from impinging directly on the secondary mirror or entering the focuser drawtube. The baffle rings and the interior of the optical tube are both painted flat black to further absorb stray light. These enhancements ensure the best possible contrast when observing and photographing faint celestial objects.

Visual Observing with the f/3.9 Newtonian Astrograph

The 8" and 10" f/3.9 Newtonian Astrographs are optimized for photographic imaging, but they can also be used of visual observing of ccelestial objects. (Because the field of view in a Newtonian reflector is rotated from right-side-up, this type of telescope is not recommended for terrestrial observing.) The tall 1.25" eyepiece holder adapter that comes installed in the focuser from the factory should allow you to achieve focus with most 1.25" telescope eyepieces (Figure 7).

For 2" eyepieces, you will likely have to use a 2" extension adapter to achieve focus with a 2" eyepiece. Two such adapt- ers are included with your telescope (Figure 7). One is a thread-on adapter that adds 30mm of extension, the other is a slip-in adapter that adds 36mm of extension. You may need one or the other adapter depending on the amount of exten- sion required for your eyepiece.

To install the thread-on 2" extension, first remove the 2" accessory collar on the focuser drawtube by turning it coun-

terclockwise until it comes off. Then thread the 2" extension adapter onto the focuser drawtube until tight. Insert a 2" eye- piece into the extension adapter and secure with the two thumbscrews. To install the slip-in 2" extension adapter, just insert its tapered base into the 2" accessory collar of the fo- cuser, then tighten the two locking thumbscrews to secure the adapter in place.

Collimating the Optics (Aligning the Mirrors)

Collimating is the process of adjusting the mirrors so they are aligned with one another. Precise collimation of the optics is especially critical for fast Newtonian optics such as in the f/3.9 astrographs. If the mirrors are even slightly misaligned, image quality will suffer. So you should check the collimation before every observing or imaging session, to make sure it is dead- on. The process of collimation is a relatively easy and can be done in daylight or darkness.

Your telescope’s optics were collimated at the factory, and should not need much adjustment unless the telescope was handled roughly in transit. To check collimation, remove the eyepiece and look down the focuser drawtube. You should see the secondary mirror centered in the drawtube, as well as the reflection of the primary mirror centered in the sec- ondary mirror, and the reflection of the secondary mirror (and your eye) centered in the reflection of the primary mirror, as in Figure 8a. If anything is off-center, proceed with the following collimating procedure.

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