Maximum field of view for Celestron SCT and Maksutov

January 27, 2017 // by ecuador

When I got my first 127mm Maksutov, I had not done my research and so I was a bit disappointed by the maximum possible field of view it could provide. You see, its baffle tube diameter was 27mm, which is the same as the maximum field stop for a 1.25″ eyepiece, so even with a reducer I could not get more FoV than what a 32mm Plossl provides – which is not much at a 1500mm focal length (1.03 degrees). A wider baffle tube allows you to either use 2″ eyepieces that can have a larger field stop, or a reducer without vignetting.

In the following table, I have used the baffle tube diameters from the Celestron knowledge base to calculate the maximum field of view for each of their Catadioptric OTAs (if you want to do it yourself, the formula is 57.3*field stop/focal length). The Maksutov numbers should also be valid for the Skywatcher and Orion models. I also list the eyepiece that will provide the maximum field of view without vignetting. In the case of the C11-C14, their baffle tube is larger than the maximum possible field stop for 2″ eyepieces. In the case of the C90, the tube is tiny in diameter, but not very long apparently so if you don’t mind vignetting you can almost go to the 1.3° that Celestron advertises with a 32mm Plossl.

OTA Focal Length Baffle Tube Max FoV Example Eyepiece Max Fov with no vignetting
C90 Mak 1250mm 15mm 0.69° 25mm 50° Plossl
C127 Mak 1500mm 27mm 1.03° 32mm 50° Plossl
C5 1250mm 27mm 1.24° 32mm 50° Plossl
C6 1500mm 27mm 1.03° 32mm 50° Plossl
C8 2032mm 37mm 1.04° Panaview/SWA 32mm; ES 68° 34mm; Ethos 21
C9.25 2350mm 46mm 1.12° Panaview/SWA 38mm; ES 68° 40mm; Panoptic 41
C11 2800mm 54mm 1.1° 0.94° /w 2" Panaview/SWA 28mm etc; 1.07° /w 3" ES 100° 30mm
C14 3910mm 54mm 0.79° 0.67° /w 2" Panaview/SWA 28mm etc; 0.77° /w 3" ES 100° 30mm

As you can see, one more reason I like my C9.25 is that it can give you the maximum apparent field of view possible for a 2″ eyepiece (which goes up to a 46mm field stop), so it can actually provide a wider true field of view than either the smaller C8 or the larger C11. The C11 and C14 can take advantage of eyepieces larger than the 2″ standard to go closer to their max field of view, but you’d have to find an external focuser to take an eyepiece such as the 3″ Explore Scientific 100° 30mm.

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