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 Mak1250mm15mm0.69°*25mm 50° Plossl
C127 Mak1500mm27mm1.03°32mm 50° Plossl
C51250mm27mm1.24°32mm 50° Plossl
C61500mm27mm1.03°32mm 50° Plossl
150 Mak1800mm30mm0.96°ES 70° 25mm; Panoptic 27
C82032mm37mm1.04°Panaview/SWA 32mm; ES 68° 34mm; Ethos 21
C9.252350mm46mm1.12°Panaview/SWA 38mm; ES 68° 40mm; Panoptic 41
C112800mm54mm1.1°0.94° /w 2" Panaview/SWA 28mm etc; 1.07° /w 3" ES 100° 30mm
C143910mm54mm0.79°0.67° /w 2" Panaview/SWA 28mm etc; 0.77° /w 3" ES 100° 30mm

*Due to the short tube you can apparently reach 1.3° using a 32mm Plossl with some vignetting.

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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2 thoughts on “Maximum field of view for Celestron SCT and Maksutov

  1. John holgate says:

    Interestingly I just tried an ES 68\34mm in the 127 Mak and despite the baffle size in the Mak, the field stop is clear and sharp and I can’t detect any vignetting. It’s a much better view than the 32mm plossl. I also had a 27mm panoptic which also seemed to work well. Just goes to show, never let a good theory get in the way of reality!

    • ecuador says:

      Thanks for the feedback. Well, theory and reality do not clash in these things, there is vignetting, but what you are saying is that it is too little for you to notice with these eyepieces. It’s similar to how we can’t just attach wide field 2″ eyepieces on 1.25″ focusers and expect to get the same benefit. If you take an image with a large sensor it is easier to see that the vignetting will softly start at the 1.03° circle. Otherwise, it is indeed supposed to be clear and sharp of course, as this is an f/11 scope and the baffle size does not affect that, but it is good to hear the 34/68° is pleasing to use in terms of vignetting. You could give it a try during the day too, it might be easier to discern the edge vignetting.

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