Celestron 6” Rowe-Ackermann Schmidt Astrograph (RASA 6) Optical Tube Assembly (CG-5 Dovetail)

A visual observer looking at the Celestron RASA 6 for the first time usually asks the same question: where does the eyepiece go? The answer is that there isn't one. There's no diagonal, no conventional eyepiece holder, and no way to look through the telescope in the usual sense. Instead, a dedicated astronomical camera attaches to the front of the tube, directly in the incoming light path. It seems like an unusual arrangement until you understand what Celestron was trying to accomplish: a six-inch astrograph working at an extraordinarily fast f/2.2 focal ratio.

For someone accustomed to imaging through an f/5 or f/7 refractor, that speed can change the way an evening of astrophotography unfolds. In a basic focal-ratio comparison for extended objects, f/2.2 delivers roughly five times the image brightness per unit sensor area of f/5 and ten times that of f/7. Actual exposure requirements depend on the camera, image scale, sky brightness, and other factors, so it isn't a promise that every five-minute exposure becomes thirty seconds. But the advantage is substantial. Instead of spending much of the evening collecting long individual exposures, you can gather useful data through shorter sub-exposures and often complete a project in less time.

The RASA design has its roots in the Schmidt cameras that professional observatories used for decades to photograph large areas of the sky. Celestron's Rowe-Ackermann optical system combines a Schmidt corrector and primary mirror with a specialized three-element lens group near the camera to deliver a fast, well-corrected field. The RASA 6 brings that approach to a 152mm aperture and 335mm focal length in an optical tube weighing just 8.4 pounds. It isn't a general-purpose telescope, and it doesn't pretend to be. It's an instrument built specifically for capturing astronomical images efficiently.

At a Glance

• 152mm aperture at f/2.2: Extremely fast optics for short-exposure deep-sky imaging and live stacking.
• 335mm focal length: A wide-field astrograph particularly well suited to nebulae, large galaxies, and rich star fields.
• 16mm optimized image circle: With a usable field extending to 22mm for compatible astronomical camera sensors.
• Integrated filter drawer: Accepts 2" filters and includes an adapter for 1.25" filters with appropriately small sensors.
• Lightweight optical tube: Just 8.4 pounds, with a CG-5/Vixen-style dovetail bar.

What f/2.2 Means Under the Night Sky

Astrophotographers spend a remarkable amount of time waiting for light. A faint nebula may be visible in a test exposure, but bringing out its delicate structure and color can require hours of integration. That's particularly frustrating when the weather doesn't cooperate, the Moon rises before you're finished, or work the next morning limits the time you can stay outside. The RASA 6 is designed to make better use of those available hours. Its fast optical system forms a bright image at the sensor, allowing shorter sub-exposures and, under appropriate conditions, less total imaging time than a substantially slower instrument.

Shorter exposures bring another practical advantage: they can be more forgiving of small tracking errors. A mount that struggles to produce consistently round stars during ten-minute exposures may perform much better when individual frames last thirty or sixty seconds. That doesn't mean polar alignment stops mattering or autoguiding becomes unnecessary. The mount still has to track accurately, and a poorly aligned system can still produce elongated stars. But the shorter exposures made practical by f/2.2 give the imager more flexibility, particularly with a compact telescope carried to a dark observing site.

The speed is also appealing for electronically assisted astronomy, or EAA. Combine the RASA 6 with a sensitive CMOS camera and live-stacking software, and you can watch an astronomical image develop on a computer screen as exposures accumulate. A faint nebula that appears as a gray suggestion in the first frame can begin showing color and structure as additional images are added. For outreach events, or for someone who enjoys sharing the night sky with family and friends, that can be a wonderful experience. Rather than describing what an object looks like through an eyepiece, you can watch its image take shape together.

Choosing the Right Camera

The camera is the most important purchasing decision you'll make after selecting the RASA 6. Because the sensor sits at the front of the telescope, the camera body becomes part of the central obstruction. Celestron recommends compact cylindrical cameras less than 75mm in diameter for the best arrangement. The optical system already has a 77mm central obstruction, so a camera smaller than that generally doesn't add to it. Celestron indicates that camera bodies approaching 90mm can still be usable with relatively little additional effect, but smaller cameras remain preferable. Traditional DSLRs and mirrorless cameras are unsuitable because their bodies obstruct too much light and their sensors generally cannot be positioned at the required focal plane.

The RASA 6 is optimized for a 16mm image circle, making many smaller astronomical CMOS cameras natural companions. Celestron also specifies a usable field extending to 22mm, so a Four Thirds sensor can be used near the outer limit of that field, although some reduction in illumination and optical performance should be expected toward the corners. An APS-C or full-frame sensor extends beyond the intended usable field and isn't a sensible match if you expect good coverage across the entire sensor. A camera's body dimensions and mechanical spacing matter just as much as its sensor size, so compatibility should be checked before purchasing.

Back focus is particularly important. With the supplied M42 camera adapter, the RASA 6 requires 17.5mm between the adapter's flat mounting surface and the camera sensor. If your camera has a shorter sensor-to-mount distance, an appropriately sized M42 spacer can make up the difference. If the sensor is recessed more than 17.5mm, the camera cannot be positioned correctly using the standard adapter. Celestron recommends keeping the spacing within 1mm of the specified distance for best performance with larger sensors. At f/2.2, accurate spacing and careful focusing are essential to getting the sharp field this telescope was designed to produce.

Filters, Focusing, and Cooling

Celestron has built a filter drawer into the front imaging assembly, allowing filters to be changed without dismantling the entire camera connection. A removable 2" clear filter is supplied and should remain installed when no other imaging filter is being used. The RASA's optical system is designed around that thickness of glass in the light path. When installing an optional filter, you replace the clear filter rather than adding another piece of glass in front of it. That preserves the intended optical arrangement and helps maintain image quality at such a demanding focal ratio.

The drawer accepts 2" filters and includes a 1.25" adapter, although Celestron specifies that the smaller filter format should be used only with sensors measuring less than 10mm diagonally to avoid substantial vignetting. Narrowband imaging also deserves special attention. The steep light cone at f/2.2 can shift the effective passband of interference filters, reducing transmission at the wavelengths you're trying to capture. For emission nebulae, choose filters specifically intended to perform with very fast optical systems rather than assuming an ordinary narrowband filter will behave the same way it does in an f/7 refractor.

The RASA 6 uses a traditional moving-primary-mirror focusing mechanism, with a generous adjustment range that allows small changes to be made carefully. It also includes a 12V MagLev cooling fan that draws air through screened vents to help bring the optical system closer to the surrounding temperature. The fan can operate from the supplied battery pack, which requires eight AA batteries, or from a suitable external 12V supply. Cooling and focus are closely related: as the telescope changes temperature during the evening, the best focus can shift. Starting the fan early and checking focus periodically is a good habit, particularly when imaging with small-pixel cameras.

StarBright XLT coatings are applied throughout the optical system to help maximize light transmission. Celestron specifies an RMS spot size of less than 1.5 microns across the optimized 16mm image circle, with 91% illumination at a point 8mm off-axis. Those figures reflect an optical design intended to produce small, well-corrected star images across the field rather than merely collect light quickly. As with any demanding astrograph, actual results still depend on sensor spacing, focus, collimation, and atmospheric conditions.

Under the Night Sky

The RASA 6 is especially attractive for the large emission nebulae and wide star fields that can be difficult to frame through longer-focal-length telescopes. With a typical Four Thirds camera, its 335mm focal length gives a field of approximately 3 degrees by 2.2 degrees, broad enough to photograph objects such as the Rosette Nebula in Monoceros, the Orion Nebula and Running Man together, and much of the intricate nebulosity surrounding the North America Nebula in Cygnus. The California Nebula in Perseus is another appealing target, although its full extent requires careful framing. The generous field allows these objects to appear in their surroundings rather than filling the photograph so completely that their larger structure is lost.

Summer offers the Cygnus Wall, the Crescent Nebula, and the Elephant's Trunk region of IC 1396, while winter brings the familiar emission nebulae of Orion. The Pleiades become a particularly interesting project from a dark site, where their delicate blue reflection nebulosity can be recorded around the brighter stars. Andromeda is another excellent wide-field subject, although its extended outer regions make careful framing important even with a Four Thirds sensor. The RASA 6 isn't intended for tiny planetary nebulae or small galaxies that demand a great deal of image scale. Its strength is showing large astronomical subjects with enough surrounding sky to appreciate their size and structure.

With a smaller 1-inch-format sensor, the field is approximately 2.3 degrees by 1.5 degrees, making objects such as the Rosette, Crescent, and Orion Nebula particularly appealing. The smaller sensor also stays closer to the optimized image circle, although the exact field and corner performance depend on the particular camera. It's worth thinking about the subjects you most want to photograph before choosing the sensor, rather than simply selecting the camera with the largest possible chip.

Observing Tip: Get Focus Right the First Time

At f/2.2, the tolerance for focus is extremely small. A star that looks sharp when you first set up can become noticeably softer as the telescope cools, especially during evenings with a substantial temperature change. Turn on the cooling fan as you begin assembling your equipment, then focus carefully using a bright star, a Bahtinov mask designed for the telescope, or the focusing tools in your capture software. Check the focus again after the optical tube has had time to approach the outdoor temperature, and whenever conditions change significantly. A compatible motorized focuser can make this easier, particularly if your imaging software supports automated focusing during a session.

Know Before You Buy

• Photography only. The RASA 6 cannot be used with a conventional eyepiece or diagonal. It is designed exclusively for astronomical cameras.
• Camera compatibility matters. Compact cylindrical CMOS cameras are preferred. Celestron recommends bodies under 75mm in diameter, while some larger bodies approaching 90mm may still be suitable.
• 17.5mm back focus. Measured from the supplied M42 camera adapter. Cameras with greater sensor-to-mount depth are incompatible with the standard configuration.
• 16mm optimized image circle. The usable field extends to 22mm, with some falloff toward the edges when larger sensors are used. Full-frame and APS-C sensors exceed the intended field.
• Filters need careful selection. Use filters compatible with f/2.2 optics. The 1.25" filter adapter is intended only for sensors smaller than 10mm diagonally.
• No conventional filter wheel. A front-mounted wheel would obstruct too much of the incoming light. The built-in filter drawer is the intended solution.
• Short focal length. At 335mm, the RASA 6 is excellent for wide-field imaging but isn't intended for small planetary targets requiring substantial magnification.
• Fan batteries are not included. The supplied battery pack requires eight AA batteries. A compatible external 12V power source can also operate the fan.
• Camera and mount sold separately. The optical tube includes a CG-5/Vixen-style dovetail and weighs 8.4 pounds before adding your imaging equipment.

What's Included

• Celestron RASA 6 optical tube assembly, 152mm f/2.2
• CG-5/Vixen-style dovetail mounting bar
• M42 camera adapter
• Integrated filter drawer with removable 2" clear filter
• 1.25" filter adapter
• Cooling fan and battery pack (requires eight AA batteries, not included)
• Front dust cover
• Instruction manual

FAQ

Can I look through the RASA 6 with an eyepiece?

No. The RASA 6 forms its image at the front of the telescope, where a camera is mounted. It is designed exclusively for astrophotography and electronically assisted astronomy, not visual observing.

What cameras work with the RASA 6?

Compact astronomical CMOS cameras with suitable sensor dimensions, body diameter, and back focus. Celestron prefers cylindrical camera bodies under 75mm in diameter and specifies a maximum back-focus distance of 17.5mm with the included M42 adapter. Sensors up to 16mm diagonal are within the optimized image circle, while sensors up to 22mm diagonal can use the larger specified field with some corner falloff. DSLR and conventional mirrorless cameras are unsuitable.

Can I use a Four Thirds camera?

Yes, provided the camera body and back-focus requirements are compatible. A typical Four Thirds sensor has a diagonal of approximately 21.6mm, which falls within the RASA 6's 22mm usable field. Some reduction in corner illumination and performance should be expected compared with a smaller sensor.

How does the RASA 6 compare with the RASA 8?

The RASA 8 provides 203mm of aperture and a 400mm focal length at f/2, along with a larger optimized image circle and usable field for bigger sensors. It also weighs substantially more and requires a more capable mounting system. The RASA 6 is the smaller, lighter option for observers using compatible compact astronomical cameras.

Should I choose the RASA 6 or a small apochromatic refractor?

A small refractor is generally more flexible. Many can be used visually and can accommodate larger cameras with relatively straightforward spacing. The RASA 6 is much faster than typical f/5 to f/7 refractors and is designed specifically for wide-field imaging. If short exposure times and rapid data collection are your priorities, the RASA 6 is worth considering. If you want one telescope for both visual observing and photography, a refractor is the more versatile choice.

Do I need an autoguider?

Not necessarily for every imaging session. The fast focal ratio makes shorter exposures practical, so some imagers can obtain good results without guiding when using a well-aligned mount. Autoguiding remains valuable for maintaining accurate tracking, particularly during extended sessions or when conditions are demanding.

Can I use narrowband filters with the RASA 6?

Yes, but the filters should be designed for very fast optical systems. Ordinary interference filters can experience a shift in their effective passband at f/2.2, reducing transmission at important emission wavelengths. The integrated filter drawer accepts 2" filters, with a 1.25" adapter for appropriately small sensors. Replace the included clear filter rather than stacking an additional filter over it.

Is the RASA 6 suitable for live stacking and EAA?

Yes. Its fast optical system can collect useful image data in short exposures, making it well suited to live-stacking software and electronically assisted astronomy. With a compatible camera and computer, you can watch deep-sky images develop as additional frames accumulate.

What mount do I need?

The RASA 6 weighs 8.4 pounds and uses a CG-5/Vixen-style dovetail bar. Celestron recommends a mount capable of carrying at least 12 pounds, but you should account for the camera, dew equipment, and other accessories. For astrophotography, selecting a mount with comfortable capacity above the total equipment weight is advisable.

Does the RASA 6 require special focusing equipment?

No. The built-in moving-mirror focuser can be adjusted manually. However, because f/2.2 has a very small focus tolerance, careful focusing is important. A compatible motorized focuser is a useful optional upgrade for automated imaging sessions.

Accessories

Compatible astronomical CMOS camera (sold separately): Required for imaging. Check the camera's sensor diagonal, back focus, mounting threads, and body diameter before ordering.

Celestron 6" Dew Shield or Dew Heater Ring (sold separately): Helps protect the front Schmidt corrector from dew during humid nights.

Fast-optics-compatible dual-band or narrowband filters (sold separately): Useful for recording emission nebulae, particularly from light-polluted locations. Select filters specifically rated for very fast optical systems.

Celestron Focus Motor (sold separately): Provides electronic focus adjustment through compatible software, making repeatable focusing and automated imaging more convenient.

Celestron RASA 6 Carry Bag (sold separately): Provides a convenient, protective way to transport the optical tube to an observing site.

Explore our astrograph collection for additional imaging telescopes and accessories.

Final Thoughts

The RASA 6 is a telescope for someone who has decided that astrophotography is what they want to do and would like to spend less of their limited clear-sky time waiting for exposures to finish. It won't accept a conventional eyepiece, it won't work with a DSLR, and its unusually fast optical system demands careful camera selection, spacing, and focusing. Those limitations are real, but they come from a design built around a very specific purpose rather than one trying to accommodate every kind of observing.

In return, you get a six-inch f/2.2 astrograph with a wide, well-corrected field, an integrated filter drawer, active cooling, and an optical tube weighing only 8.4 pounds. For large nebulae, rich star fields, and live-stacked images, that combination can make a remarkable difference in how much you accomplish during an evening. If your biggest frustration with astrophotography is having more targets than clear hours, the RASA 6 deserves a serious look.