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Pixel scales, FOV, and Field Orientation

Pixel Scales1 and Field of View2

Camera Pixel dimension (arc sec) Field of View (arc sec)
f/6 0.1171 119.9 x 119.9
f/14 0.0499 51.1 x 51.1
f/32 0.0219 22.4 x 22.4
f/32 + Altair Field Lens 0.0214 21.9 x 21.9

1NIRI with Altair has been measured to have radial barrel distortion at f/32.  This is most prominent for programs performing large offsets.  The formula for removing the distortion is r' = r + k*r*r, where k = (1.32 ± 0.02) * 10^-5 . r is the uncorrected distance from the field center in pixels and r' is the corrected distance from the center in pixels.  Thus, at a distance of 500 pixels from center, an object will appear about k*500*500 = 3 pixels closer to the center of the array than reality. This correction is available in an IDL program, and PIs interested in high-precision astrometry are encouraged to contact either the NIRI or Altair instrument scientists to discuss this issue.

2For full (1024 x 1024) array 

Field Orientation

The NIRI field orientation is determined by which telescope port (bottom or side) NIRI is installed on, and if on the bottom port, whether or not Altair is used. For most cases, at a PA = 0, North is up and East is to the left, however, when installed on the bottom port and used without Altair, East is to the right:

Port Altair Non-AO
Bottom (1)
Side (3 or 5)