Imaging

FLAMINGOS-2 covers a circular 6.1 arcmin field with an average plate scale of about 0.179 arcsec / pixel. We present here:

• the Image quality
• the Field distortions

Image quality

The specifications for the FLAMINGOS-2 image quality are 50% encircled energy (EE) within 25 μm diameter, and 80% EE within 43 μm at the detector (18 μm / pixel). Using images of a pinhole grid mask, we obtained a full-width-half-maximum (FWHM) of 0.36 arcsec across the 6 arcmin field-of-view, meeting the specifications.

Fig.: FLAMINGOS-2 measured FWHM, as a function of distance from center. Due to an optical misalignment, FLAMINGOS-2 showed increasing PSF values in 2013, and was brought back to specifications after an intervention in 2014. Horizontal lines mark the FWHM values in various observing condition constraint bins (Image Quality 20%-ile = IQ20, etc.).

Field distortions

The plate scale of FLAMINGOS-2 ranges from 0.1792 arcsec / pixel on-axis to 0.1787 arcec / pixel at the outermost radius. Even though modest, these distortions must be taken into account when coadding sets of dithered images to avoid image degradation.

Fig.: Optical distortions (plate scale variations) for FLAMINGOS-2, determined with Scamp from 85 dithered Ks-band observations of a dense stellar field.  Note that the corners are extrapolated, because they do not contain data due to the circular field-of-view. The small spatial offset of the distortion pattern from the center of the square detector array is real.