Celestron RASA 6, 8, 11 and 36 cm: image circles, camera fit and f/2 imaging

The camera sits in front of the aperture

The RASA 8 page explains the layout: the design uses a prime-focus focal plane to reach f/2.0, so the camera mounts at the front of the tube. That also means no eyepiece and no visual use.

The lens assembly at the front already blocks the center of the beam: 77 mm on the RASA 6, 93 mm on the RASA 8, 114 mm on the RASA 11 and 158 mm on the 36 cm. A camera body narrower than that adds nothing. Celestron’s FAQ says a wider body blocks some light but the effect stays negligible up to about 120 mm on the RASA 8 and about 90 mm on the RASA 6. The same RASA 8 page also advises against bodies over 4 inches (101.6 mm), so the stricter number is the safer one to plan around.

Even with a small camera, the obstruction is large by refractor standards. Computed from the published diameters:

Model	Focal ratio	Blocked by area	Same clear area as	Focal length ÷ that aperture
RASA 6	f/2.2	26%	131 mm	2.56
RASA 8	f/2.0	21%	180 mm	2.22
RASA 11 V2	f/2.2	17%	255 mm	2.43
RASA 36 cm	f/2.2	20%	319 mm	2.48

The last column ignores transmission losses, but it shows that the nominal f/2 versus f/2.2 gap survives the obstruction.

Image circle versus sensor diagonal

Celestron publishes two diameters per model. The RASA 6 FAQ defines the image circle as the diameter the design is optimized for and the usable field as the diameter where performance is still excellent. Compared with four common sensors (diagonals from their pixel grids):

Model	Image circle	Usable field	Within image circle	Within usable field only
RASA 6	16 mm	22 mm	ASI533 (16.0 mm)	none of the four
RASA 8	22 mm	32 mm	ASI533	ASI294 (23.2), ASI2600 (28.3)
RASA 11 V2	43.3 mm	52 mm	all four, ASI6200 (43.3) at the edge	–
RASA 36 cm	60.1 mm	70 mm	all four	–

The angular figures Celestron prints next to each circle (2.74°, 3.15°, 4.0°, 4.4°) are 2·atan(circle ÷ 2f), the same formula the field table below uses. For the RASA 8, Celestron says full-frame sensors work but lose edge quality and illumination, so a full-frame camera belongs on the 11 or the 36 cm.

Back focus is fixed and short

With the included adapters, the RASA 6 needs 17.5 mm from the adapter to the sensor and the RASA 8 needs 25 mm; Celestron says a camera needing more than that cannot be used. ZWO lists 17.5 mm for the cooled cameras in the table below, so the RASA 6 needs no spacer and the RASA 8 needs 25 − 17.5 = 7.5 mm, following the subtraction the RASA 8 FAQ describes. The RASA 11 and 36 cm take 55 mm with their adapters; the RASA 8 FAQ cites the 55 mm a DSLR needs as one reason to choose the RASA 11 for DSLR imaging.

The spec table mixes reference points: the RASA 8, 11 and 36 cm back-focus values there are measured from the threaded collar, the RASA 6 value from its adapter. The back focus guide covers how to stack spacers to a target.

Filters at f/2

There is no room for a filter wheel. The RASA 8 FAQ says adding a filter in the light path at this speed degrades the image, so the optical window is removable and a filter can replace it; a wheel would sit in front of the optics and block too much light. The RASA 6 uses a drawer with a 2 mm clear filter that a 1.25″ or 2″ filter replaces. How bandpass filters behave in fast beams is covered in the narrowband and fast optics guide.

What f/2.2 buys at a given scale

At equal focal length, a faster system gathers more light per object. The RASA 11 at 620 mm and the Askar FRA600 at 600 mm frame nearly the same sky (2.17° × 1.45° versus 2.24° × 1.50° on an ASI2600MC Pro). The RASA’s clear area, 279² − 114² = 64,845 mm², is 5.6 times the FRA600’s 108² = 11,664 mm². By nominal focal ratio alone, (5.6 ÷ 2.2)² would suggest 6.5 times; the 114 mm central obstruction accounts for the difference.