Celestron 91071 Instruction Manual

2.2
6" OPTICAL TUBE ASSEMBLY
Instruction Manual
Model: 91071
Overview
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1 Schmidt Corrector
2 Lens Group
3 Lens Group Cell
4 Dovetail Mounting Bar
5 Cooling Fan
6 12V DC Jack for Cooling Fan
7 Focus Knob
The Rowe-Ackermann Schmidt Astrograph 6 (RASA 6) is an incredibly fast (f/2.2) 6"
aperture optical system that delivers a flat field without optical aberrations. It is capable
of capturing stunning deep-sky astronomical images without the difficulties presented
by slower and longer focal length telescopes. The astrograph can only be used with
a camera.
Mounting the Astrograph
The RASA 6 has a CG-5 dovetail mounting bar. This is compatible with mounts that accept
a CG-5 or Vixen dovetail. The astrograph weighs 8.4 lb. Choose a mount with enough load
capacity to accommodate the astrograph, your camera, and any additional accessories
you want to use.
Choosing a Camera
When selecting a camera to use with the RASA 6, there are several things to keep in mind.
1. Sensor size – The optical design is optimized for use with sensors that have a
diagonal size up to 16mm. It will still perform well with up to 22mm–diagonal sensors,
but field illumination in the corners will be reduced.
2. Pixel size – The RASA 6 works effectively with a wide range of sensor pixel sizes.
Unlike many telescopes, it can work well with smaller pixels (e.g., less than 3 μm
to its fast focal ratio (f/2.2) and 6'' aperture.
3. Obstruction – Since it mounts in front of the optics, select a camera that blocks as
little of the aperture as possible. Cylindrical cameras with an outer diameter measuring
less than 75mm work best. Many astronomical and "HyperStar-compatible" CMOS
cameras are especially well-suited to the RASA 6.
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8 Dust Cover
9 Battery Pack (Batteries not included)
10 Filter Drawer
11 Filter Holder with 2" Clear Filter (comes installed in Filter Drawer)
12 1.25" Filter Adapter
13 Camera Adapter (comes connected to Filter Drawer)
14 Retaining Ring (comes connected to Filter Drawer)
4. Backfocus – Backfocus, also known as flange focal distance, is the distance between
a camera's sensor and the point where the camera body connects mechanically
to a lens or telescope. For flat-field astrographs like the RASA 6, it's essential to
position the camera's sensor precisely to match the required backfocus, ensuring the
telescope's focal plane aligns with the sensor. Failure to do so may result in degraded
performance at the edges of the sensor, particularly with larger sensors. For the RASA
6, the correct backfocus distance is 17.5mm from the top flat surface of the camera
adapter (see drawing below). Cameras with sensors recessed more than 17.5mm into
their bodies are incompatible with the RASA 6, as their sensors cannot be positioned
at the proper focal plane.
Due to their large obstruction and long backfocus, DSLR cameras are not compatible
with the RASA 6.
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) due
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CAMERA ADAPTER
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14
IMAGE PLANE
17.50 MM
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