Orion #8297 8" F/3.9 User Manual download pdf (Page 7)

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.

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).

drawtube

Reflection

of primary

mirror clip

Primary mirror

center mark

Reflective surface

of collimation

cap

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Orion #8297 8" F/3.9 User Manual Page 7

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