Celestron 8" Rowe-Ackermann Schmidt Astrograph (RASA): An Introduction

Celestron 8" Rowe-Ackermann Schmidt Astrograph (RASA): An Introduction

Would you like to to take images like that shown below quickly and easily? ... then read on about the new Celestron RASA 8" telescope that can help you do that!

Celestron RASA 8" Image of Horshead Nebula

Horsehead and Flame nebula taken with RASA 8" by Michael Jager

A few months ago, Celestron announced a smaller aperture astrographic telescope in the form of an 8"/200mm Rowe-Ackermann Schmidt Astrograph (RASA). 

Celestron RASA 8" OTA

Celestron RASA 8" OTA

The RASA telescopes are purely for imaging and allow you to capture wide field deep sky images with its incredibly fast f/2.0 optical system. This 8" telescope is a new model in the Celestron RASA lineup, and goes alongside the very successful and popular 11" model and also the (relatively) newly introduced 14" version. With the introduction of this 8" RASA version, Celestron have opened up the door of fast wide field imaging to a much wider audience. The RASA 8" will be very affordable (price to be finalised soon) and its weight of only 7.7kg/17lbs means that it can be mounted on a wide variety of highly popular low-cost mounts capable of carrying such a relatively low payload.

In this first blog, we'll introduce the telescope and outline some of its features. In a later blog we will look more at the technical side of the RASA 8" and then its use practically "in the field".

About the RASA: The RASA optical design is a development of Schmidt Cassegrain optics consisting of an aspheric (Schmidt) corrector plate and spherical mirror. However for the RASA there is no secondary mirror. In its place are a series of lenses, that refract the light to produce a field of view across an imaging chip that is flat, and also free of coma, astigmatism and chromatic abberation (false colour). A schematic of the RASA optical design is shown below.

Celestron RASA Optical Design

Celestron RASA Optical Design Layout.

The RASA telescopes are purely an astrophotographic instrument - they cannot be used for visual purposes. In order to use the telescope for imaging, a camera is attached to the lens assembly holder using one of the supplied camera adaptors. Focusing, like for SCT/Edge HD telescopes, is achieved by moving the primary mirror up/down in inside the telescope tube by a rear-mounted focus knob.

 

Lets look at the telescope: Externally, the telescope looks like a slightly stretched-out 8" Edge HD OTA, with its light grey paintwork, side mounted primary rear cell filtered air vents, carry handle (for easy transportation and mounting the OTA on a mount) and orange Losmandy dovetail bar on its underside. However that's where the similarity ends.

 Side view of the RASA 8"

Side view of the RASA 8" showing the orange Losmandy dovetail bar (bottom), filtered side vents, focus knob and the carry handle.

On the rear of the telescope there is a 12V DC socket that powers an internal MagLev (MAGnetic LEVitation) fan to provide vibration free cooling for the primary mirror and, with the two side vents, provide good airflow whilst filtering out any dust. An external battery pack (with pouch) to power the internal fan is supplied and takes 8xAA batteries. You can power the fan using any other 12V DC supply (e.g. Celestron PowerTank). A rubberised focus knob that, when rotated, moves the primary up/down inside the telescope tube, allows focusing to be accomplished. Celestron have designed a new mechanical focus system for the RASA 8", called the Ultra-Stable Focuser System. It comprises two sets of precisely aligned precision bearings to virtually eliminate image shift. This system has been extensively tested and has been found that, for this 8" model, mirror locks are not required with this new focuser.

Celestron RASA 8" Rear View

Rear view of the RASA 8" showing the central fan opening with 12V DC input socket, carry handle (bottom) and the black focus knob.

Focusing can also be achieved electronically using Celestron's newly announced Focuser Motor for SCT, Edge HD and RASA telescopes (see above). With this accessory you can precisely control the focusing from a Celestron NexStar+/StarSense handset, if using a Celestron mount, or via a PC using the supplied focuser utility software or a number of 3rd party programs (e.g. Sequence Generator Pro, Maxim DL etc).

Celestron Focuser Motor

Celestron Focuser Motor for SCT, Edge HD and RASA Telescopes.

On the base of the telescope there is a 3" CGE/Losmandy dovetail bar to mount the OTA onto a suitable mount. The CGE/Losmandy bar is much more rigid than a (narrower) Vixen dovetail so considerably reduces the chances of any "flexing" which can cause "star trails" in your images. The length of the bar is near that of the length of the OTA itself so allows easy forward/backward balancing in Declination when setting the telescope up with a camera attached. On the top of the OTA are mount points for a finderscope, Celestron StarSense (if using the RASA 8" with a Celestron GOTO mount; or Celestron StarSense for SkyWatcher if using a SkyWatcher GOTO mount), and for an additional dovetail bar to mount a guidescope for example. 

Turning to the front of the telescope, there sits the Schmidt corrector plate and its retaining ring that proudly displays the Rowe-Ackermann name, and that its an F/2.0 Schmidt Astrograph with Celestron's StarBright XLT coatings (applied to all refractive optical surfaces) too. In the middle of the corrector plate, in place of the secondary mirror and its mirror holder usually found on SCT's/Edge HD telescopes, there is instead a 4-element lens group, for producing a flat abberation free image across an imaging sensor (see later), all housed in a lens group holder. 

Celestron RASA 8" Front View showing corrector plate and corrector lens assembly

Front view of the RASA 8" showing corrector plate, lens group and lens group holder.

One the front plate of the lens group holder, there are 3 pairs of screws at approx 120-degrees apart for tilt adjustment of the whole lens group for collimation purposes, should collimation need to be done. Collimation would be performed with a camera attached (whilst running the camera "in live view") using one of the supplied camera adaptors (see later). The camera adaptors have long slots cut into them for easy access to the collimation screws with a hex-allen key, when fitted to the RASA.

Around the edge of the lens group optics is a small knurled retaining ring that holds in place a removable optical window that covers the front lens element of the lens group. This can be simply unscrewed by hand or by using an adjustable lens opening tool if its too tight (or too recessed). If a camera is to be used that already has a built-in optical window, then the RASA 8's optical window can be very easily removed. Furthermore, with the optical window removed, Celestron have produced a suitable imaging light pollution filter for the RASA 8" with an integrated filter mount that screws directly in place of the optical window, so imaging can be done under light polluted skies.

RASA 8" with LPR fitted and with spectral responseImage showing the Celestron RASA 8" Light Pollution Filter (LPR) fitted in place of the supplied optical window. Inset: spectral transmission curve for the LPR filter. 

To attach your camera to the RASA 8" you would use one of the two adaptors supplied - an M42 adaptor and a C-thread adaptor - (or one of the optional mirrorless or a custom-made adaptor) that would be appropriate for your camera. The camera adaptor is attached and secured to the lens assembly holder using a screw-on retaining ring. This retaining ring screws onto the external threads on the outside of the lens assembly holder and clamps the camera adaptor in place against the lens assembly face-plate. Once your camera is attached you can slightly loosen the retaining ring to orientate your camera to suit the object you are imaging and then re-tighten.

 

So what cameras can you use with RASA 8"?: The RASA 8" design was optimised to be used with cameras that have sensors with a diagonal size of up to ~22mm which gives an image circle of 3.15¬į. This means that the RASA 8" will works extremely well with a very large range of the highly popular CMOS deep sky cameras such as those from e.g. ZWO (e.g. ASI183, ASI1600) and from other brands too.

Celestron RASA 8" with Astro Camera attached

Celestron RASA 8" with popular sized astroimaging camera attached

A DSLR will not work with the RASA 8" as the working back focus distance from the camera adaptor to the imaging sensor of 25mm is less than the flange to chip distance of popular DSLR's (e.g. Canon is 44mm).¬† However, it is possible to use one of the sensitive Canon/Sony mirrorless APS-C cameras (with one of the optional mirrorless adaptors) as their flange to chip distance is only ~18mm and that their APS-C size sensor (~27mm diagonal) is within the usable field of view of 32mm/4.6¬į (that comes with only a small loss of performance near the edge of the field).

Celestron RASA 8" with APS-C Mirrorless Camera attached

Celestron RASA 8" with an APS-C Mirrorless Camera attached

As the aperture of the RASA 8" and the central obstruction is relatively small, compared to the size of the body of some cameras, Celestron have recommended that camera bodies should be less than approximately 100mm/4" in diameter. 

 

What accessories come with the RASA 8"?: Out of the box the RASA 8" telescope comes with

  • Corrector lens cap to keep the corrector plate clean when not in use
  • M42 Camera adaptor for CMOS/small CCD cameras
  • C-thread camera adaptor for smaller cameras
  • Battery pack with pouch for powering the internal primary mirror fan

Celestron RASA 8 included accessories - front dust cover, camera adaptors and 12v fan power pack

 Standard accessories supplied with the RASA 8"

All you need to supply is a suitable mount, power and a camera.

Finally, these telescopes come with a 2 year guarantee for peace of mind. 

In the next blog we will look a little more at the RASA 8". However, in the meantime....here is another example of what can be achieved with the RASA 8"...

 Celestron RASA 8" Image of the Rosette Nebula.

Celestron RASA 8" Image of the Rosette Nebula by Michael Jager

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