5
Cooling the Telescope
All optical instruments need time to reach “thermal equilibri-
um.” The bigger the instrument and the larger the temperature
change, the more time is needed. Allow at least 30 minutes
for your telescope to cool to the temperature outdoors. In very
cold climates (below freezing), it is essential to store the tele-
scope as cold as possible. If it has to adjust to more than a
40°F temperature change, allow at least one hour. You can
use the telescope while it’s cooling down, just note that you
may see “tube currents,” which interfere with the telescope’s
ability to resolve a sharp image. Tube currents are essential-
ly heat waves exiting both the optical components (such as
the primary mirror) and the telescope itself. The effect seen
through the eyepiece is much like looking above a hot surface
or fire.
Mirror Cooling Fan
Your f/3.9 Newtonian Astrograph comes with a cooling fan
that attaches to the rear of the primary mirror cell. Using the
fan reduces the amount of time required for the primary mir-
ror to reach thermal equilibrium with the ambient air. The fan
is powered by 12-volts DC. The included battery holder holds
eight D-cell alkaline batteries (not included). Alternatively, the
fan can be powered by a 12-volt DC field battery, such as the
Orion Dynamo Pro.
Fan Installation
1. Place the fan on the rear of the mirror cell of the tele-
scope and line up the holes in the fan with the threaded
holes in the mirror cell. Make certain the label on the fan
is facing the primary mirror.
2. Place the fan cover (wire grille) over the fan so the holes
in its corners line up with the holes in the fan and mirror
cell (
Figure 4a). The fan cover should be oriented so that
its corners seat flush onto the fan.
3. Thread a screw, with a washer attached, through the fan
cover and fan and into the mirror cell
(Figure 4b). Firmly
tighten the screw, but be careful not to overtighten and
strip the threads. Repeat this for the other three screws
(and washers).
4. Insert eight D-cell batteries (not included) into the battery
pack. Orient the batteries as shown on the plastic battery
holder.
5. Plug the cord from the battery pack into the fan. The fan
should begin rotating. If it doesn’t, check the connections
and orientation of the batteries and try again. To turn the
fan off, unplug the cord from the fan.
Using the Cooling Fan
The cooling fan should be turned on as soon as the telescope
is brought outdoors prior to your imaging or observing ses-
sion. It should run for approximately 15-30 minutes to properly
cool down the telescope. When the telescope is in use, the
fan should be turned off. This is because even though it runs
free from vibration, the air currents it generates in the optical
tube will degrade image quality. Once the mirror has cooled
to the outdoor ambient temperature, the fan will likely not be
needed again for that session.
Imaging with the f/3.9 Newtonian Astrograph
This instrument has fast f/3.9 parabolic optics, which produce
bright images and allow short exposure times. Fast optics
also inherently produce some coma, or distortion of star
images toward the periphery of the field of view. Therefore, to
achieve the best possible images, we highly recommend use
of a coma corrector (sold separately) designed for use with
f/4 Newtonians, or for a range of focal ratios that includes f/4.
Many common coma correctors have a 2" diameter housing
and T-threads for attachment to a camera. Typically, the coma
corrector is attached to the front of the camera body via its
T-threads (for DSLRs, to a compatible T-ring), then inserted
into the 2" accessory collar of the focuser drawtube. Use of a
coma corrector will allow you to utilize the entire imaging area
of your camera without the need to crop the edges of your
astro-images due to optical distortion. Check Orion’s website
for compatible coma correctors.
Figure 4a.
Line-up the holes in the fan and fan cover with the
holes in the rear of the primary mirror cell.
Figure 4b.
Attaching the fan to the mirror cell.
