In stock & shipping from Oregon. New: Wanderer Astro Filter Cube baseplate for existing 90 mm cages →
In stock & shipping from Oregon. New: Wanderer Astro Filter Cube baseplate for existing 90 mm cages →
Home / Learn / Special RASA optical train setups
The RASA 11 and RASA 8 need special care in the optical train. Whether you run M42, M48, M54 or M68 can make or break these fast scopes.
Plenty of telescopes claim a fully illuminated full-frame field. A full-frame sensor has a diagonal of about 43 mm, so a 48 mm optical train should cover it, right? Not always.
At f/2 the light cone from the RASA to your sensor is far steeper than from an f/7 refractor. That steeper cone is where the speed comes from, but your optical train has to make room for it. A 48 mm train can still clip a full-frame sensor.
So how big an optical path does a RASA 11 with a full-frame sensor need?
We built 3D models of the RASA 11 light cone at f/2.2 across a 72.8 mm backfocus, covering a 43.3 mm full-frame sensor, to see exactly how the camera mount, rotator and filter slider affect it.
An M68 camera mount is wide enough at the base to stay clear of the cone. With 50×50 mm square filters on the Photon Cage, the filter mounts and screws sit outside the cone on the perpendicular axis, with spare room at the sensor for the screw cross-section.
Next, a cross-section through the sensor’s widest point, the diagonal. Neither the M68 base nor the 50×50 mm filter system touches the light cone anywhere.
The result: full corner-to-corner performance from an f/2.2 RASA 11 with a full-frame sensor.
The same diagonal cross-section with a 2-inch filter shows significant obstruction from the filter assembly at the height the filter sits.
Filter trays on most full-frame setups sit 16–18 mm from the sensor, since most full-frame ZWO cameras have 12.5 mm of backfocus. A camera with more backfocus, like the QHY600, pushes the filter further out and makes it worse.
Both are stacked, dithered images from a RASA 11 with a full-frame sensor, so single-sub, guiding and tilt issues are out of the picture.
Taken with 2-inch round mounted filters and an M54 camera mount. Quality holds up until the very tips of the stack, where corner stars show artifacts from the obstruction.
Some corner vignetting is expected on any RASA; Celestron notes some light loss in the corners regardless. Flats handle it well, and a light crop removes the edge stars.
Same scope and sensor with our M68 camera mount and a 50×50 mm Astronomik MaxFR Ha filter. The corner stars are round and symmetrical, in a system with almost no tilt or backfocus error.
Performance is consistent on all four edges, and the frame is usable with no cropping beyond the dither zone.
Our opinion, from running a lot of RASA configurations. Your results may vary.
PRO Edition · M68. If you can, go M68 mount, M68 rotator and the M68 PRO filter slider with 50×50 mm filters. You’ll be at the limit of the scope, with its own vignetting but no obstruction from accessories. M54 works too, with slight vignetting and corner coma; pair it with 2-inch filters and good flats.
Standard M54 or PRO M68. An excellent match. M54 with 2-inch filters and good flats gives quality images to the edges. M68 with 50×50 mm filters helps all the way through, but isn’t as necessary as on full frame.
Standard Edition · M54. The stock M48 with 2-inch filters works fairly well, but M54 with the Photon Cage gives almost no vignetting and very flat images. The stock felt-backed M48 mount is prone to tilt.
Not recommended. The RASA 8 is designed for up to APS-C. Use the stock M42 adapter, or our 2-inch slider and Photon Cage built for its very short backfocus.
RASA 8 Edition with slider. Very good with 2-inch filters on our modular base, made for the 2600 on the RASA 8 with removable filter sliders. A huge f/2 field with the flexibility to run mono.
Standard Photon Cage + slider. The stock M42 camera mount with our standard slider and 2-inch filters gives very flat, vignette-free images. An excellent combination.
Astro PM maps tilt and curvature across your subs, so you can see where your field is off and check each adjustment you make.
The Photon Cage gives you 4-corner tilt and independent backfocus on almost any telescope.
