Celestron RASA 8” Reviewed: Recommended OTA

Optics

The Celestron RASA 8” is an 8-inch RASA type telescope. RASA stands for Rowe-Ackermann Schmidt astrograph, which was a relatively unknown telescope design until RASA telescopes became standard products by Celestron. Here I will list some of the essential specs you’ll find in this scope.

The aperture is 8 inches, or 200mm. This is a decent size for the amount of light you are getting, but it is nothing extravagant or tiny. The focal length of this telescope is 400mm, thus making the focal ratio (FL / Aperture) F/2. This is the main feature that makes the RASA different from the competition. Typical F/5 8-inch scopes will be 1000mm long and have a focal length of 1000mm. Compare this to assuming that we use an identical camera. A wide field of view allows you to capture larger objects in the sky, such as nebulae or constellations, rather than being limited to smaller nebulae and galaxies with longer F/ratio telescopes.

A fast focal ratio allows you to take in much more light in a given area at one time. This is a huge benefit when you are doing deep-sky astrophotography, as you can cut down your exposure times greatly and still produce excellent images with shorter exposure times, or you could take images with the same exposure times and be able to capture extremely faint targets.

The RASA, as stated before, is a relatively new telescope design. It contains the original primary (main) mirror from the C8 SCT and the original corrector plate at the front, but removes the secondary mirror and instead places a corrector lens assembly. The telescope is F/2, but still remarkably maintains a perfectly flat, huge field! I will explain how this telescope manages to utilize all these features to pull off this amazing feat.

The light traveling towards the primary mirror first is refracted through the corrector lens at the front of the telescope. The corrector lens minorly adjusts the light incoming and does not do much actual collection of light. The corrector corrects for the spherical shape of the mirror and acts as a coma corrector that is built into the telescope to correct for the large amount of coma expected at f/2.

Speaking of the primary mirror, it is a relatively simple spherical mirror that has an incredibly short 400mm focal length, which is extremely difficult to manufacture and is really only used for telescopes such as SCT’s. The primary mirror is the main collector and focuser of light, while the other optics only correct the image. The primary mirror then reflects the light up into the center of the corrector plate.

In the middle of the corrector plate, there is a lens assembly that corrects various errors and also allows you to use a very wide field of view camera with pinpoint stars across the frame.

Mechanics

The RASA has an extremely small body, similar to that of an SCT telescope. This means that you will be able to move the telescope around without much strain as the length of the telescope is not much of an issue. It also weighs only 9 kg (22 lbs), making it quite light for its aperture, which makes it easy on you and your telescope mount. The telescope features an integrated fan system to quickly equilibrate the mirror with the outside air temperature, allowing you to take sharp images without turbulence inside the tube.

Due to the nature of these types of telescopes, the focuser is at the back of the telescope and moves the primary mirror. The camera is also screwed in at the front of the telescope because that is where the light from the primary mirror is reflected to make a fast telescope like the RASA, but this has some disadvantages, which I will discuss later.

The telescope, while being an amazingly fast scope, has many disadvantages and critical features that had to be sacrificed to create the flat-field astrophotography telescope. Its speed also comes with some disadvantages, which you must consider when picking cameras to use with the telescope. 

The first thing you will notice when looking at a setup using the RASA is the position of the camera: in front of the aperture! You probably will be thinking: “Doesn’t that block the light?” and you would be correct. The unfortunate placement of the camera limits your choice of cameras greatly and is a factor in why cylindrical cameras are becoming more popular. It also means if you wish to use a filter wheel, that is pretty much out of the question, as is an off-axis guider.

Using a filter wheel is out of the question with the RASA, as the large size of the filter wheel would obstruct the telescope or produce optical artifacts. Using an off-axis guider would also block a fair amount of light.

The telescope comes with a filter system, which I will discuss later, but you may be forced to use a color camera, which comes with its own disadvantages. Using a color camera means that you can’t use narrowband filters, for example, and if you use RGB, the sensitivity is greatly reduced.

That also means USB/power cables have to go over the aperture, unfortunately creating ugly spikes around stars in your images, but if you align them right you can negate spikes or create Newtonian telescope style spikes if you like them.

The fast F/2 speed means that you will be required to collimate the telescope very often, and with an extreme degree of accuracy. Collimation is changed on the RASA is done via the lens assembly on the corrector, which is extremely sensitive as it is very close to the sensor.

Collimation is required when the mirror and lens shift slightly. This happens with use and with the movement of the telescope. Usually, this is less of a problem with slower scopes, but with fast f/ratio telescopes, the degree of precision required in positioning the mirror is huge. Collimation is also very difficult on the RASA, as there is no center to the primary mirror to aim a laser at like in typical telescopes.

And obviously, you are unable to use this scope for visual purposes as your head would block most of the light! Very important to remember as you do not want a dud scope!

Mount Recommendations

The RASA 8” works well on any autoguider-ready, GoTo German equatorial mount with a payload capacity of 30 lbs or more, such as the Celestron Advanced VX, CGEM II, or CGX, as well as mounts like the Sky-Watcher EQ6Ri Pro and Orion Atlas EQ-G. 

Should I buy a Used Celestron RASA 8”?

The RASA 8” is a complex and rare telescope, and we would recommend you purchase it only from someone who has actually used one for imaging or from a reputable third-party vendor of used gear.

Aftermarket Accessory Recommendations

A cooled one-shot color astronomical camera, guide scope, and guide camera, as well as a Bahtinov mask for focusing are essential to get the most out of an 8” RASA. You will also probably need to add a second dovetail bar on top of the optical tube in order to attach your guiding setup and any dew controllers or cable routing that you may have. To avoid condensation forming on the corrector plate, reduce glare, and help with routing your cables, a dew shield is necessary. In addition, a dew heater might be beneficial. A motor focuser (offered by brands such as Celestron, ZWO and Pegasus) is highly recommended as well, since focusing the RASA by hand accurately enough can be difficult.

Astrophotography Capabilities

The RASA 8” being a fast telescope has not very much back focal distance for accessories. You may not know what BFD is, so I will break it down for you. Because of this, the RASA requires separate accessories for filters and camera mounting equipment.

First off, the back focal distance is the distance between the threads on the telescope and the camera’s sensor that keeps the image flat out to the corners. Typically, this is around 55mm, but for the RASA 8″, as it has a much shorter focal length, it has a much closer back focus than other telescopes; in fact, the back focal distance is only 29mm.

The limited back focus means that you will not be able to use cameras with long back focal distance requirements, and not monochrome cameras with filter wheels unless you manage to find an extremely low profile filter wheel pairing it with a low back focal distance camera like the ASI1600.

Luckily, the RASA 8” comes with its own filter system, but it is extremely limited as it means you have to unscrew the entire camera, unscrew the filter, change filters, and reattach the camera if you wish to change filters quickly. Some manufacturers make filter slide systems that make filters much easier to remove and swap, which can ease some of the difficulty when changing filters. Unfortunately, neither of these solutions is automated, making image sequencing with a monochrome camera unfeasible.

Unfortunately, because of this extremely small back focal distance, using a DSLR camera is also out of the question, as the distance to the sensor with a DSLR is 55 mm, a good 26mm too long. You are effectively forced to use a color astrophotography camera with this telescope.

The RASA 8” comes with an M42 threaded mounting system for you to place your camera on. M42 works for under APS-c sized cameras but can block some light with full-frame cameras. Luckily you can purchase a custom camera adaptor for full-frame cameras.