Shooting at F/2 With The Celestron 8-Inch RASA!
The 8-inch, 400mm focal length Celestron Rowe-Ackermann Schmidt Astrograph (RASA) is a veritable Hoover for photons. With a focal ratio of F/2 (actually, the math works out to more like F/1.97), subframes are down to 60 seconds or less.
Celestron 8-Inch RASA
RASA Cutaway (Celestron White Paper)
Unlike a traditional Schmidt Cassegrain telescope, the RASA is built specifically for astrophotography. The camera mounts to the front with its sensor facing the inside of the tube. The 8-inch model will only accept smaller astronomical cameras. A DSLR would be too larger and block part of the aperture. Spacing is critical between the camera and the camera adapter. You’ll need about 18-19mm with a ZWO cooled camera to be in the sweet spot. Unfortunately, the best I could do with what I had on hand was about 28mm. Yikes! I’ll have that sorted out soon enough.
Focus is achieved by moving the mirror with the typical Celestron SCT-style focus knob. I’ve attached the new Celestron focus motor to my RASA. If there’s anything I hate, it’s leaving my toasty-warm home office to refocus in the dead of winter!
8-Inch RASA on a Celestron CGX Mount
A word about guiding: I see plenty of comments about not needing to guide with such short exposure times. Personally, I like to guide. It keeps my subject framed and I’m a stickler for dithering. I think dithering is especially needed with color cameras. So yes, I guide.
I finally got a slight break in our February weather and was able to get the RASA up and running for a couple of hours before the clouds rolled in again. I aimed for the Horsehead and Flame nebulas. Seeing turned out to be horrible. I was shooting through a thin cloud layer and had to toss a good third of the images. In the end, I came out with 20 minutes of data.
Everything was against getting results. My camera spacing was bad, seeing was worse and I got very few usable frames. I was going to chalk it up as a practice session because I didn’t expect anything out of it. Even with all that, I got a pretty decent image! It won’t win any awards, but it shows the potential of F/2 astrophotography.
Horsehead and Flame Nebulas in Orion
Equipment
Optics: Celestron 8-Inch RASA
Mount: Celestron CGX
Camera: ZWO ASI533MC Pro
Focuser: Celestron Focus Motor
Guide Camera: ZWO ASI120mm-s
Image
Optolong L-eNhance Filter: 20 x 60s
Dark Frames: 25 x 60s
Flat Frames: 25
Capture Software: Sequence Generator Pro
Guide Software: PHD2