CELESTRON 91078 Telescope Instruction Manual

CELESTRON 91078 Telescope

Overview

The 36 cm Rowe-Ackermann Schmidt Astrograph (RASA 36 cm) delivers a flat field with no coma or astigmatism across a large (60 mm) image circle and extended spectral range (400-900 nm). The astrograph can only be used with a camera.

Mounting the Astrograph

The RASA 36 cm has a CGE dovetail mounting rail. This is compatible with mounts that accept a CGE or Losmandy-D style dovetail. The astrograph itself weighs 75 lb., so a mount with a load capacity more than that is recommended. This will ensure good performance of the mount when a camera and any accessories are added.

Because of its size and weight, moving and lifting the RASA 36 cm onto a mount will take two people. Use the handles on the rear cell as needed.

For custom mounting applications, there are two sets of two M8x1.25 mounting holes in the rear cell. These utilize threaded inserts for extra reinforcement.

Installing a Camera

The supplied M48 camera adapter can be used to attach a DSLR camera. It will require a 48 mm “wide T-ring” for your make of DSLR to connect to the adapter.
If you are using an astronomical camera with a smaller than “full frame” (43 mm diagonal) sensor, you may also be able to use the supplied M48 camera adapter. This likely will require utilizing optional adapters, such as an M48-to-M42 adapter along with M48 extension rings, to connect your camera and place your camera’s sensor at the proper backfocus distance. The backfocus distance when using the M48 adapter is 55 mm.

Cameras with larger sensors will require a custom camera adapter to best illuminate the sensor. Refer to the “Camera Adapter Requirements” drawing on Celestron’s website, located on the 36 cm Rowe-Ackermann Schmidt Astrograph webpage under the “Support and Downloads” tab, to determine the design needed for your camera.

Note : Due to the steep light cone provided by the f/2.2 optical system, camera housings can be a source of vignetting. Despite the astrograph having a large illuminated field, vignetting can occur with large sensor cameras when the sensor is deeply recessed within the camera body. This is common with full-frame DSLR sensors.

To attach your camera:

1. Place the knurled retaining ring over the camera adapter and loosely thread it onto the lens group housing. The Teflon washer goes between the retaining ring and the camera adapter.
   

2. Thread the camera body onto the camera adapter.

3. Carefully hold hold the camera so the camera adapter seats flatly against the lens group housing.
   

4. Finish threading the retaining ring onto the lens group housing. Rotate the ring until the camera adapter is snug against the housing. Do not over tighten! Camera orientation can be adjusted later.
   Caution : Overtightening the retaining ring can cause the lens assembly to rotate relative to the Schmidt corrector. Keep the camera adapter snug against the lens cell, but do not overtighten 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 overtighten the ring.

Cooling Down and Fan Operation

Optical performance of the RASA 36 cm 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 to keep it outside for an hour or so before imaging. The RASA 36 cm rear cell is equipped with a cooling system containing a fan to facilitate cooling the 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 the optics have cooled down and you are ready to image, unplug the battery pack to turn the fan off.

Focusing

The focuser moves the primary mirror within the rear cell. The RASA 36 cm V2 uses the Ultra-Stable Focus System (USFS) that minimizes unwanted shifting of the primary mirror when focusing and during long exposures.

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 travel. In this case, rotate the knob in the opposite direction until focus is achieved. If you turn the focus knob too quickly, you may 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.

An essential accessory you’ll likely want to add is a focus motor. The Celestron Focus Motor is fully compatible with the RASA 36 cm V2. Please visit the focus motor webpage at celestron.com for more information.

Attaching Accessories

The RASA 36 cm includes a CGE dovetail accessory rail that is also compatible with most accessories which work on Losmandy-D plates. The dovetail accessory rail can be used to mount an optional guidescope.

A Celestron findescope 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.

Removing the Optical Window

If using a filter somewhere in the light path, or if the camera has an optical window, we recommend that you remove the optical window in the front lens cell. This will maintain the best optical performance of the astrograph. The optical window utilizes M110 x 0.75 threads on the exterior of its housing.

To remove the optical window:

1. With the camera and camera adapter removed, carefully grab hold of the knurled cell of the optical window.
2. Carefully unthread and remove the optical window.
   

Dew Prevention

The Schmidt corrector is especially susceptible to condensation forming because the glass is exposed to the outside air. 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 14” dew shield which shrouds and insulates the corrector. If conditions are more severe, a dew heater can be added to apply slight heat to the corrector lens to keep it dry all night.

Celestron has a full suite of dew prevention products that are fully compatible with the RASA 36 cm. Visit celestron.com for more information.

Do not store the astrograph if covered in dew. Allow it to dry fi rst without the dust cover on. Storing optics when wet promotes mold growth on the optical surfaces.

Cleaning of the Optics

Dust, debris, and fingerprints on the optics will generally have little effect on imaging with the RASA 36 cm. However, if the external surfaces of the Schmidt corrector and the lens group become excessively dirty, they should be cleaned. Dust can be removed with a blower bulb or a camel hair 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. Low pressure strokes should go from the center of the corrector to the outer edge. Use a new tissue for each stroke so as not to spread any oils or debris. Do not rub in circles.

You can use a commercially made lens cleaning solution or mix your own. A good cleaning solution is isopropyl alcohol (99% pure) mixed with distilled water. The solution should be 3 parts isopropyl alcohol and 2 parts distilled water. You can also use soft white tissue instead of lens cleaning tissue, as long it doesn’t have added lotions or perfumes.

To minimize the need to clean your astrograph, replace the dust cap once you have finished using it.

Cleaning the internal optical surfaces should only be done by the Celestron repair department. If your RASA 36 cm needs internal cleaning, please contact Celestron at celestron.com/pages/ technical-support for a return authorization number and price quote.

Collimation

The RASA 36 cm optics are factory aligned, and should not normally need collimation adjustments. However, adjustments can be made to the tilt of the lens assembly, if needed, to collimate the entire optical system. The primary mirror and corrector are permanently aligned at the factory, and cannot be adjusted.

The astrograph must be collimated with a camera installed. Using a camera that does not obstruct the light path (i.e. the aperture of the Schmidt corrector) is recommended, as this makes it easier to see concentricity when stars are defocused.

Equipment needed:
2 mm Hex key
3 mm Hex key
Mount and camera setup, 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 allow access to the collimation screws. Retighten the retaining ring.
   

3. Point the astrograph at a bright star and focus the camera. Center the star in the camera’s field of view. Use Polaris, if possible, as it won’t move much over time.

4. Defocus the star by about 1 turn of the focus knob.

5. Inspect the defocused star pattern. When collimated, the pattern should look like a concentric doughnut. If the pattern is concentric, no adjustment is necessary. If the “donut hole” is not centered within the pattern, then some adjustment is needed.
   

6. There are three pairs of collimation screws. Each pair works together as a push-pull tilt adjustment. Adjust the collimation by loosening and tightening one of the pairs of collimation screws. Always loosen one of the screws first, then tighten the other one. Adjust one pair of screws at a time.
   

7. If the star pattern is thin on one side, adjust the collimation screws so that the star moves towards the thin side, then adjust the position of the astrograph to re-center the star. Repeat this until the defocused star image is concentric.
   

Specifications

FOV
**| FCC NOTE:** This equipment has been tested and found to comply with the limits for a Class B digital device, pursuant to part 15 of the FCC Rules. These limits are designed to provide reasonable protection against harmful interference in a residential installation. This equipment generates, uses, and can radiate radio frequency energy and, if not installed and used in accordance with the instructions, may cause harmful interference to radio communications. However, there is no guarantee that interference will not occur in a particular ninstallation. If this equipment does cause harmful interference to radio or television reception, which can be determined by turning the equipment off and on, the user is encouraged to try to correct the interference by one or more of the following measures:

• Reorient or relocate the receiving antenna.
• Increase the separation between the equipment and receiver.
• Connect the equipment into an outlet on a circuit different from that to which the receiver is connected.
• Consult the dealer or an experienced radio/TV technician for help.

This product is designed and intended for use by those 14 years of age and older.

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| WARRANTY
celestron.com/support/warranties
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• All rights reserved
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2835 Columbia Street
• Torrance, CA 90503 USA 0222

References

• Celestron - Telescopes, Telescope Accessories, Outdoor and Scientific Products
• Celestron - Telescopes, Telescope Accessories, Outdoor and Scientific Products
• Technical Support | Celestron
• Warranty | Celestron

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