The Celestron 36cm/14" Rowe-Ackermann Schmidt Astrograph
In 2014 Celestron released a dedicated telescope that was capable of taking wide field of view images with short exposure times. The telescope has an aperture of 11" (279mm) and a (fast) focal ratio of f/2.2 and an optical system called the RASA: The Rowe-Ackermann Schmidt Astrograph. This RASA 11" has proved to be very popular amongst astrophotographers all over the world. Gary Palmer, one of the leading astro-imagers, uses a Celestron RASA 11" on a Celestron CGX-L mount and has obtained some very nice images: see for example his recent M33 in Triangulum and the Veil nebula in Cygnus
In addition to taking impressive wide-field images of celestial objects, the Celestron RASA telescope came to the attention of scientists that are involved in monitoring the near-space environment where our artificial satellites reside for potential hazards. This is called Space Situational Awareness (SSA). If you are interested in seeing more about the satellite and debris field that encompasses our Earth have a look at the interactive website: http://stuffin.space/
Screenshot from Stuffin.Space website showing the satellites and the space debris around the Earth. On the interatctive website, you can zoom in and see (and identify) what is overhead for your location.
A collaboration between Celestron engineers and SSA experts resulted in the development and production of the RASA 11's bigger brother - the RASA 36. This is a highly precise optical instrument with a large 36cm (14") aperture and fast f/2.2 focal ratio. The telescope has an image circle diameter of 60.1mm, a 4.3 degree field of view and an RMS spot size of <6.3microns across the field of view. The telescope is not only capable of detecting high speed space debris measuring less than ~1 metre in size, but can also be used as fast wide-field astrographic instrument that can also be used for various types of astronomical research and imaging.
The RASA 36cm OTA showing the front lens assembly and lens assembly holder (ready for camera attachment), dual 3"-wide CGE/Losmandy dovetail bars (top and bottom) and one of the two rear carry handles.
The Celestron RASA 36 cm aperture version is currently the largest of the RASA models that Celestron currently manufacture and is the biggest and fastest (f/2.2) optical instrument of its kind that is available commercially “off-the-shelf". In addition to its 11" aperture brother, the RASA range very recently gained a new family member - an 8" aperture model. This will be popular for a large number of amateur astronomers as the RASA 8 is smaller and lighter meaning that it will work well with affordable and portable mounts. We will cover more on the RASA 8" soon.
The optical design of the RASA 36 is shown below. The telescope features a StarBright™ XLT coated Schmidt corrector plate at the front end through which light first enters the telescope. At the "bottom" of the telescope sits a spherical enhanced aluminium coated primary mirror, where light is reflected up towards a 4-element Extra-low dispersion (ED) StarBright™ XLT coated lens group that focuses the light onto an imaging sensor (camera) that is attached to the lens group assembly holder. The result are images that are free of optical defects like false colour, field curvature, off-axis coma, astigmatism and also minimal vignetting. Furthermore, the RASA 36 cm focuses an extended 700-900nm (near infrared) spectral range (400-900 nm overall), allowing a brighter signal to be detected by an imaging sensor.
The Celestron RASA 36 Optical Design
The 4-element lens group housing also allows the attachment of an imaging camera with a (supplied) M48 camera adaptor and a knurled retaining ring with an internal teflon washer. The camera adaptor itself is shown below.
Celestron RASA M48 camera adaptor
The retaining ring is first placed over the camera adaptor and loosely threaded onto the lens group housing. The camera is then attached to the camera adaptor (e.g. using an M48 T-ring if using a DSLR) and, after making sure the adaptor is seated flat against the lens group housing the knurled ring can be tightened. The orientation of the camera can be adjust by simply loosening the knurled ring and re-tightening it after the desire orientation is achieved. The three elongated "slots" around the edge of the adaptor allows for easy access to the lens-assembly collimation screws. The image below shows a DSLR attached to the camera adaptor on the front of the Celestron RASA.
A DSLR camera attached to the front of the Celestron RASA
An optical window protects the front lens of the 4-element lens group. This can easily be removed should a filter (e.g. H-alpha ; Light pollution filter) need to be used in the light path, or if the imaging camera has an optical window, to maintain the best optical performance of the RASA optical system. The RASA 36 cm’s has a back-focus distance of 77.5 mm meaning that a wide variety of cameras and accessories (e.g. filter holders) can be used. On the outside edge of the lens assembly holder are screws that allow adjustments to the tilt of the lens assembly for the optical system to be collimated if, and when, required.
Front of the Celestron RASA, showing the lens assembly and the lens assembly holder with the collimation screws.
On the rear of the telescope (see image below) there are two carry handles for transportation. In the centre of the rear cell is a 12v DC input socket (AA power pack supplied - batteries are user supplied) that powers a centrally positioned MagLev cooling fan that, with two filtered tube vents, help the telescope optics cool to ambient temperature for best optical performance. In order to achieve focus, like their SCT/Edge HD cousins, the rear focus knob (centre-right) moves the primary mirror moves up or down within its rear cell. A completely redesigned focus mechanism allows for easy focusing and also minimises any image-shift of the primary mirror when focusing and during the course of any long exposure astrophotography. A motorised focuser add-on unit is available for the RASA 36 for automated or remote focusing - please contact us for more information on this unit.
Rear view of the RASA 36: The two 3" CGE/Losmandy dovetail bars and filtered tube vents can be seen top and bottom. In the centre is the 12v jack plug for powering the (central) cooling fan. To the right of centre is the focusing knob for the primary mirror.
The telescope tube comes with dual 3" CGE/Losmandy-style dovetail bar for mounting the RASA onto a mount (lower bar) and also for attaching additional accessories (upper bar: for e.g. finder, guidescope etc). The RASA 36 is 1055mm in length and 399mm in diameter and it weighs 34kg (75lbs). Its size and especially its weight, therefore means that for it to be properly supported, it does require a heavy duty mount capable of carrying this payload plus the payload of an imaging camera and any additional accessories used.
For a tour of the Rowe-Ackerman Schmidt Astrograph telescope (11"), and also to see the RASA 36 in action at Celestron, watch the two videos below.
So now we have briefly looked at the RASA36 - what sort of an image can it produce? Well, "a picture paints a thousand words", as they say, so here below is a beautiful image of part of NGC7000, the North American Nebula taken with a RASA 36. Enjoy!
For more information and specifications of the RASA 36 see >>HERE<