How much magnification and true field of view does the NexImage camera have when placed in my eyepiece holder?
August 15, 2006
The approximate magnification of an astronomical imaging camera is equivalent to that of an eyepiece with the same focal length in millimeters as the camera’s film or chip diagonal size. Since the chip size of the original #93712 NexImage camera is 3.6mm x 2.7mm, it has a 4.5mm diagonal. So it is equivalent in magnification to a typical 50-degree apparent field of view 5mm eyepiece. To determine approximate magnification when using this NexImage, divide the focal length of your telescope (in mm) by 5.
With the new #93711 NexImage 5, the chip size is bigger, 5.7mm x 4.3mm, with a diagonal of 7mm. It is thus equivalent to a 7mm eyepiece. So it has a wider field of view than the original NexImage camera. Divide the focal length of your telescope (in mm) by 7 to determine NexImage 5’s magnification.
To accurately calculate any camera’s true field of view (TFOV) for any optical system, you can use the drift method. On the celestial equator, a star will drift 15 seconds of arc for each second of time.
With the drive of your scope off, let such a star drift completely across the camera's field of view. Multiply this time in seconds by 15 to get the TFOV in arc-seconds. Multiply the time by 1/4 to get the TFOV in arc-minutes. A drift time of 30 seconds corresponds to a 450 arcsecond TFOV or 7.5 arcminutes.
You can also geometrically derive the field of view. Divide the diagonal dimension of the chip of either camera by the focal length of your optical system and then multiply by 206265. The resulting number will be the TFOV in arcseconds. If you are interested in the length and width instead, use those dimensions instead of the diagonal.
Example: A C11 with a 2800 mm focal length has a TFOV using the original NexImage at prime focus of 265x199 arcseconds. For comparison, the full moon is 1800 arcseconds across.