Originally from ticket #26651
I tried to set up FOV circles and found that the calculations of FOV and Mag are not correct. For example, if I match my Evo 9.25 with my Panoptic 27, SS says the FOV is 0.56 and Mag is 120.4x. According to Televue’s calculation site, the FOV is 0.74 and Mag is 87x. For my TV 55 plossl, SS says the FOV is 0.85 and Mag is 59.1x. Televue says FOV is 1.1 and Mag is 42.7x. Why the discrepancy?
Keiron Smith replies:
We believe you have something entered incorrectly (reviewed by SkySafari developer Bill Tschumy)
We get the FOV = 0.78º and the Mag = 87. This closely matches TeleVue's info.
The slight discrepancy in FOV is due to the fact that TV is using the field stop diameter for the calculation which can slightly cut off the FOV.
Since most manufactures don't publicize the field stop diameter, we use the AFOV which is always publicized and it gives basically the same result.
FL = 2350
Aperture = 235 mm
FL = 27
AFOV = 68º
I have double checked my inputs, and even deleted and re inputed the info, Pan 27, FL 27 and AFOV 68. The info on the Display page still says FOV .56, Mag 120.4. The same thing with the TV 55 plossl. Same problem with my ES 100/20. Astrocalc says for my 9.25 TFOV is .85, Mag is 117.5x. SS Display page says Fov = .62, Mag = 162.5x. Why would I be getting a different answer from you?
Bill Tschumy replies:
Can you send me screen shots of the Equipment screens that you have for the scope and the eyepiece? Also send the screen shots showing the incorrectly reported FOV.
Right now I can't say why you are getting different results.
These are the screens from my iPad. As you can see, the inputs are correct but the outputs on the Display page aren’t correct.
Bill Tschumy replies:
You have the data incorrect for the scope. According to:
The aperture is 235 mm and the focal length is 2350 mm. You have the first two digits reversed for both of these.
Aha!! I guess I transposed the numbers one place and just copied it everywhere else. Maybe I was dreaming of a bigger scope, although the 9.25 is plenty big for me. Thanks for solving the mystery.
While I have you, can you explain the field rotation angle for me and how it would be useful in observing?
Also, any word on whether or when Starry Night might get synced with SkySafari?
Thanks for your help.