Thursday, August 15, 2013

Focal Reducer

I've played around with our Opticstar CCD cameras enough now that I feel limited by their relatively small field of view (FOV). For the 14-inch telescopes the FOV w/ CCD is approximately 8 x 6 arcminutes. Saturn and the Ring Nebula (and other planetary nebulae) fit just fine, but I'd also like to image larger objects, like globular clusters.

The easy solution is a focal reducer. We recently purchased what seems to be the only focal reducer in the universe that will work with our CGE Pro EdgeHD telescopes. It's a beast of an accessory at 3.25 pounds, so today we balanced one of the scopes with the focal reducer and camera.

The reducer will reduce (go figure) the the focal length by 0.7. By my calculations, the new field of view (for the CCD camera) should be about 12 x 9 arcminutes. It's not a huge gain, but it will allow me to fit globulars like M10 and M12 (constellation Ophiuchus) in the field. The Hercules Cluster, M13, may be too big, but I'll likely try to image the core anyway.

Focal reducer at base of 14-inch w/ Opticstar CCD camera
BTW, I calculated the FOV in arcminutes using this:

FOV = (S x 3438) / f

where S is the size of the CCD chip in mm, and f is the focal length of the telescope in mm. Exciting, I know.

I don't plan on changing out the focal reducer anytime soon since balancing the scope is a pain in the keister. What will the FOV be with a regular eyepiece? A rough calculation gives me 32 arcminutes for our 26 mm eyepiece and 44 arcminutes for our 40 mm eyepiece. Big, but not quite big enough to fit the 60 arcminute Brocchi's Cluster. We may have an eyepiece (a Panoptic?) with a larger apparent FOV, though. I'll have to check.