Celestron RASA Astrographs for Fast Deep-Sky Imaging

Celestron RASA – Purpose-Built Astrographs for Astrophotography

RASA stands for Rowe-Ackermann Schmidt Astrograph, a specialised optical system developed by Celestron specifically for astronomical imaging.

Unlike a traditional Schmidt-Cassegrain, a RASA is not designed as a general-purpose telescope for both visual observation and photography.

Its optical design is optimised for fast, wide-field astrophotography.

The camera is mounted at the front of the telescope at the prime focus, allowing an extremely fast focal ratio and a wide photographic field.

Why Is a RASA So Fast?

RASA astrographs operate at approximately f/2 or f/2.2, depending on the model.

A traditional Schmidt-Cassegrain telescope, by comparison, commonly operates around f/10.

The extremely fast RASA focal ratio allows the camera to collect large amounts of signal from extended deep-sky objects in relatively short exposure times.

This makes the system particularly attractive for wide-field nebula imaging, astronomical surveys, comet imaging and large mosaics of the night sky.

Wide-Field Deep-Sky Astrophotography

RASA astrographs combine relatively large apertures with short focal lengths.

This produces a large photographic field of view.

They are particularly well suited to imaging targets such as emission nebulae, reflection nebulae, dark nebulae, open star clusters, molecular clouds and extensive Milky Way regions.

Depending on camera sensor size and the individual RASA model, several galaxies or other objects can also be captured in the same frame.

For very small galaxies and planetary nebulae, a longer-focal-length telescope may provide a more suitable image scale.

RASA 8

The Celestron RASA 8 combines approximately 203 mm of aperture with an extremely fast f/2 focal ratio.

It is designed primarily for compact astronomical CMOS cameras, smaller CCD cameras and suitable mirrorless cameras.

Its combination of portability, speed and a wide field makes it particularly attractive for mobile deep-sky astrophotography.

RASA 11

The RASA 11 provides approximately 279 mm of aperture and operates at f/2.2.

Its 620 mm focal length combines substantial aperture with a remarkably wide photographic field.

The large corrected image circle also makes the RASA 11 particularly interesting for astrophotographers using larger imaging sensors.

It is an excellent system for wide-field deep-sky imaging where speed, aperture and sensor coverage are all priorities.

RASA 36 cm

The 36 cm RASA provides approximately 14 inches of aperture while retaining the fast f/2.2 focal ratio.

With a focal length of about 790 mm, it combines very large aperture with a wide field of view.

Its large image circle allows the use of large-format camera sensors.

This model is particularly suitable for advanced astrophotographers, observatories and astronomical applications requiring large aperture, wide fields and rapid image acquisition.

Front-Mounted Camera Design

A RASA does not direct the final light beam through the primary mirror to the rear of the telescope as a Schmidt-Cassegrain does.

Instead, the focal plane is located at the front of the optical system.

The camera is therefore mounted in front of the corrector plate.

This arrangement helps create the short effective focal length and very fast focal ratio.

Camera dimensions and mechanical adaptation should therefore be considered carefully because the camera itself is positioned in the incoming light path.

Compact astronomy cameras are particularly well suited to this design.

Can a RASA Be Used Visually?

RASA astrographs are designed for imaging.

Traditional visual observation with an eyepiece is not their intended application because the focal plane and camera connection are located at the front of the optical system.

If both visual observation and imaging are required, a Schmidt-Cassegrain, EdgeHD, Newtonian or APO refractor may provide greater versatility.

The RASA is a dedicated astrophotography instrument.

RASA vs APO Refractor

Both APO refractors and RASA astrographs can produce excellent deep-sky images, but they emphasise different strengths.

An APO refractor is generally mechanically straightforward and can often be used for both imaging and visual observation.

A RASA combines considerably larger aperture with an extremely fast focal ratio.

This allows the system to collect large amounts of imaging signal rapidly, making it particularly attractive for wide-field deep-sky photography.

RASA vs Newtonian Astrograph

Fast Newtonian telescopes are also popular astrophotography instruments.

Typical imaging Newtonians may operate around f/4, with specialised designs offering even faster focal ratios.

RASA systems operate at approximately f/2 to f/2.2 and provide a purpose-designed corrected photographic field.

Newtonian astrographs can offer excellent value and flexibility, while RASA systems are particularly attractive when maximum photographic speed and a large corrected field are priorities.

RASA vs EdgeHD

RASA and EdgeHD telescopes are both based on Schmidt-derived optical concepts but have very different purposes.

An EdgeHD telescope provides a long focal length and is suitable for visual observation, planetary imaging and high-resolution deep-sky astrophotography.

A RASA uses a much shorter focal length and an extremely fast focal ratio.

In practical terms, EdgeHD systems are particularly suitable for smaller objects and larger image scales, while RASA systems excel at fast, wide-field deep-sky imaging.

Short Exposure Times

One important advantage of a very fast astrograph is the ability to collect useful signal using relatively short individual exposures.

This can reduce tracking demands compared with much longer-focal-length and slower imaging systems.

A high-quality equatorial mount remains important, however, especially when many individual exposures are combined into a high-quality final image.

Does a RASA Need Autoguiding?

Whether autoguiding is required depends on the mount, exposure length, camera pixel size, RASA model and desired image quality.

Because RASA systems combine short focal lengths with very fast focal ratios, much shorter individual exposures can often be used than with slower telescopes.

Some setups may therefore operate without autoguiding.

Guiding can still be useful for demanding imaging projects or longer exposures.

Choosing a Mount for a RASA

The mount must safely support the complete imaging system.

In addition to the optical tube, the payload includes the camera, adapters, filters, cables and any additional equipment.

Mount requirements increase significantly with the RASA 11 and especially the 36 cm RASA.

For astrophotography, a high-quality equatorial mount with sufficient payload reserve is recommended.

Precise Focusing at f/2

The faster an optical system becomes, the more critical precise focus becomes.

At f/2, relatively small focus errors can noticeably affect star quality.

An electronic focuser can therefore be particularly useful for advanced astrophotography.

It allows precise focusing and can also support automated refocusing throughout an imaging session.

Sensor Tilt

Extremely fast optical systems are sensitive to mechanical alignment.

If the camera sensor is not precisely perpendicular to the optical axis, stars on one side of the image may appear less sharp than those on the opposite side.

This is commonly referred to as sensor tilt.

Accurate and rigid camera adaptation is therefore particularly important when using larger sensors.

Camera Spacing

Correct camera spacing is important for achieving optimum image quality with a RASA.

Adapters, filters and other accessories influence the optical distance between the telescope and camera sensor.

The manufacturer's spacing specifications for the individual RASA model should therefore be followed carefully.

Filters for Fast Optical Systems

Interference filters can behave differently when used with extremely fast optical systems.

Narrowband filters in particular are affected by the steep angle at which light passes through the filter.

Specialised filters are therefore available for optical systems around f/2.

This is especially relevant for narrowband imaging of emission nebulae.

RASA Under Light-Polluted Skies

A fast optical system records both useful astronomical signal and background sky brightness efficiently.

Suitable imaging filters can therefore be useful under light-polluted skies.

The optimum solution depends on the camera, target, local sky conditions and intended processing workflow.

Narrowband Imaging

Large emission nebulae are excellent targets for a fast wide-field system.

Suitable narrowband filters can isolate specific emission wavelengths and provide high-contrast images of nebulae.

When imaging at f/2, filters designed or specified for fast optical systems should be considered.

Comets and Sky Surveys

Large aperture, short focal length and a wide field make RASA astrographs well suited to projects that require efficient coverage of large regions of the sky.

Potential applications include comet imaging, sky surveys, variable-star projects and other wide-field astronomical imaging programmes.

Thermal Management

Astrographs also benefit from reaching thermal equilibrium with the surrounding air.

RASA systems use ventilation or active cooling features, depending on the model, to help reduce cool-down time and maintain stable imaging performance.

Choosing a Camera for a RASA

The ideal camera depends on the particular RASA model.

Important factors include sensor size, pixel size, camera body dimensions, mechanical connection and the available image circle.

Compact astronomy CMOS cameras are particularly attractive for the RASA 8.

The larger RASA models support significantly larger sensors and therefore enable exceptionally large photographic fields.

Celestron RASA Astrographs at Optikshop24

Optikshop24 offers Celestron Rowe-Ackermann Schmidt Astrographs for fast, wide-field astrophotography.

Choose from the portable RASA 8 at f/2, the powerful RASA 11 at f/2.2 and the large 36 cm RASA for advanced astronomical imaging applications.