Orion #8296 10" F/3.9, #8297 8" F/3.9 Cooling the Telescope, Mirror Cooling Fan, Fan Installation

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

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.

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.

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