Celestron 91073 Instruction Manual page 2

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
The Rowe-Ackermann Schmidt Astrograph 8 (RASA 8) is an incredibly fast (F/2.0) 8" 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 8 has a CGE dovetail mounting bar. This is compatible with mounts that accept a CGE or
Losmandy-D style dovetail. The astrograph itself weighs 17 lbs., so a mount with a load capacity greater
than that is required to ensure good performance when a camera and any accessories are added.
Choosing a Camera
When choosing a camera to use with the RASA 8, there are several things to keep in mind.
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1. Sensor size– The optical design is optimized for use with sensors have a diagonal size up
to 22 mm. It will still perform well with up to 32 mm diagonal sensors, but field illumination in
the corners will be reduced.
2. Pixel size– The RASA 8 works effectively with a wide range of sensor pixel sizes. Unlike many
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telescopes, it can work well with smaller pixels (i.e. less than 5 μm ) due to its fast focal ratio
(F/2.0) and short focal length (400 mm).
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3. Obstruction– Since it mounts in front of the optics, use a camera blocks as little of the
aperture as possible. Best results will be obtained with cameras that have a cylindrical design
and an outer diameter of less than 100 mm. Many astronomical CMOS and "Hyperstar
compatible" CCD cameras are especially well-suited to the RASA 8.
4. Backfocus– For all flat-field astrographs, including the RASA 8, it is important that the
camera's sensor is positioned at the proper distance from where it mounts to the telescope.
Otherwise, performance at the edge of the sensor, especially larger sensors, may suffer. For
the RASA 8, this location is 29 mm from the top surface of the lens group cell. A camera's
backfocus, also known as its flange focal distance, is the distance between the camera sensor
and where the camera body mechanically connects to a camera lens or telescope. Therefore,
cameras with sensors deeply recessed within their body having more than 29 mm of backfocus
cannot be used with the RASA 8 as the sensor cannot be placed at the proper location.
DSLR cameras cannot be used with the RASA 8 due to their large obstruction and long backfocus.
Installing a Camera
If using a camera that mounts with M42 threads, the supplied M42 camera adapter can be used.
This will likely require utilizing optional M42 extension rings to place the camera sensor at the
proper location. When using the M42 camera adapter, the required backfocus distance is 25 mm.
So, for example, if the backfocus of a camera is 17 mm, then 8 mm of M42 extension ring(s)
between the camera and M42 camera adapter will be needed. For best results with larger sensors,
get the backfocus within 1 mm of the required distance.
The supplied C-thread camera adapter can be used to attach any camera which mounts with
a standard C-thread. If the camera's sensor spacing meets the C-thread standard (i.e. 17.5 mm
backfocus), no additional adapters are required.
For use with Canon or Sony mirrorless cameras, Celestron offers optional RASA 8 camera adapters.
These have a bayonet mount, and position the camera sensor at the proper location.
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7 Focus Knob
8 Dust Cover
9 Battery Pack (Batteries not Included)
10 C-Thread Camera Adapter
11 M42 Camera Adapter
12 Retaining Ring
Other cameras will require a custom camera adapter. Also, cameras with relatively large sensors
may benefit from a custom camera adapter to best illuminate the sensor. Refer to the "Camera
Adapter Requirements" drawing at Celestron.com (located on the RASA 8 webpage under the
"Support and Downloads" tab) to determine the custom camera adapter design.
To attach your camera:
1. Place the knurled retaining ring over the camera adapter.
Retaining
Ring
Camera
Adapter
2. Thread the retaining ring clockwise onto the lens group cell at the front of the RASA 8.
Make sure the camera adapter seats flatly on the lens cell and tighten the retaining
ring until the camera adapter is snug against the cell. Do not over-tighten the ring.
3. Thread the camera body onto the camera adapter. Camera orientation can be adjusted later.
Alternatively, the camera adapter, with retaining ring attached as in step 1, can be first connected to the
camera. Then, while holding the camera, thread the retaining ring onto the lens group cell.
CAUTION: Over-tightening the retaining ring can cause the lens group cell to rotate
relative to the Schmidt corrector, which may affect optical alignment. Keep the camera
adapter snug against the lens cell, but do not over-tighten the ring.
Adjusting Camera Orientation
Once the camera is installed, you can adjust its orientation by slightly loosening the retaining ring.
Loosen the ring no more than 1/8th turn, rotate the camera as desired, and retighten the ring.
Remember not to over-tighten the ring.
Cooling Down and Fan Operation
Optical performance of the RASA 8 will be best when it has reached thermal equilibrium with the
outside air. The greater the temperature difference between the astrograph and the outdoors, the
longer it will take to cool down. It is a good idea to store the astrograph in a cool area and keep it
outside for an hour or so before imaging.
The RASA 8 rear cell is equipped with an air cooling system utilizing a fan to facilitate cooling the
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optics to the ambient temperature. The fan pulls air through the mesh vents located on the perimeter
of the rear cell. The mesh prevents dust from entering the astrograph. The supplied battery pack
powers the fan. It requires 8x AA batteries (not included). Install the batteries as indicated in the
plastic mold of the battery holder. Plug the battery pack into the jack near the fan to turn it on. When
ready to image, unplug the battery pack to turn the fan off.
Alternatively, the fan can be powered with a 12V DC power source supplying at least 100mA.
Connect to the fan jack with a 5.5 mm/2.1 mm tip positive DC power plug.
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Focusing
The focuser moves the primary mirror forward and backward within the rear cell. The RASA
8 utilizes a redesigned focus system, called the Ultra-Stable Focus System, which minimizes
unwanted lateral movement of the primary mirror when focusing or slewing the astrograph.
To focus, turn the focus knob until the image appears sharp in your camera. If the knob will no
longer turn, the focuser has reached the end of its mechanical travel. In this case, rotate the knob in
the opposite direction until focus is achieved. A single turn of the focusing knob does not move the
primary mirror much; it takes about 30 turns to go through the entire range of focus. If focusing
on a dim object and the focus knob is turned too quickly, it is possible to go right through focus
without seeing the image. To avoid this problem, try first focusing on a bright star so the image is
visible even when out of focus.
Celestron offers a focus motor which is compatible with the RASA 8. A focus motor can be a great
convenience when astroimaging, as the focusing can be done from the same computer
controlling the camera.
Attaching Accessories
The RASA 8 includes a CGE dovetail accessory bar which is also compatible with accessories
which work on Losmandy-D plates. The dovetail accessory bar can be used to mount an
optional guidescope.
A Celestron finderscope can be attached using the pairs of threaded holes in the rear cell of the
astrograph. These holes have Phillips-head screws in them when not in use.
Mounting Holes
For Finderscope
Removing the Optical Window
If using a filter somewhere in the light path, or if the camera has an optical window, it is
recommended to remove the optical window in the lens group cell. This maintains the best
optical performance of the astrograph.
To remove the optical window:
1. Remove the camera and camera adapter.
2. Carefully grasp the knurled edge of the optical window cell within the lens group cell, and begin
rotating it counter-clockwise. If the edge of the window cell cannot be grasped initially because
it is too recessed, use the slots in the window cell to begin rotating it.
Optical
Window Cell
3. Unthread the window cell completely, and carefully remove it from the lens group cell.
4. Store the optical window in a place where it will not get scratched or dirty.
Celestron offers a light pollution filter for the RASA 8 which conveniently mounts in place of the
optical window. This improves image contrast in skies which have significant light pollution.
Dew Prevention
The Schmidt corrector is especially susceptible to condensation because the glass is directly exposed
to the night sky. If the temperature of the astrograph drops below the dew point, dew may form on the
corrector within minutes. The easiest way to prevent dew is to add an optional 8" dew shield, which
shrouds the corrector and slows radiant cooling. If conditions are more severe, a dew heater can be
added to apply slight heat to the corrector to keep it dry all night.
Do not store the astrograph if covered in dew. Allow it to dry first without the dust cover on. Storing optics
when wet can promote mold growth on the optical surfaces.
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Cleaning of the Optics
Dust, debris, and fingerprints on the optics will generally have little effect on imaging with the RASA 8.
However, if the external surfaces of the Schmidt corrector or the lens group become excessively dirty,
they should be cleaned. Dust can be removed with a blower bulb or an optical cleaning brush. Then, use
optical cleaning solution and lens cleaning tissue to remove any remaining debris or stains. Apply the
solution to the tissue and then apply the tissue to the lens. Use low pressure strokes, do not rub in circles.
When cleaning the corrector, strokes should go from the center to the outer edge. Use a new tissue for
each stroke so as not to spread any oils or debris.
To minimize the need for cleaning, keep the dust cover on the astrograph when it is not in use.
Cleaning the internal optical surfaces should only be done by the Celestron Repair Department. If your
RASA 8 needs internal cleaning, please call Celestron for a return authorization number and price quote.
Collimation
The RASA 8 optics are factory aligned, and should not normally need collimation adjustments. However,
adjustments can be made to the tilt of the lens group, if needed, to collimate the entire optical system.
The primary mirror and corrector are permanently aligned at the factory, and cannot be adjusted.
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The astrograph must be collimated with a camera installed. Using a camera
light path (i.e. the aperture of the Schmidt corrector) or that has a cylindrical body is recommended;
this makes it easier to see concentricity of the defocused star diffraction pattern.
Equipment needed:
3 mm Hex key
2 mm Hex key
Mount and camera set up, ready to image
1. Install the camera on the astrograph.
2. Slightly loosen the retaining ring and rotate the camera adapter so the three slotted
openings in the adapter allow access to the collimation screws. Retighten the retaining ring.
does not obstruct the
Collimation
Screws