Large Field

The Large Field of SOFI gives a scale of about 0.292 arcsec/pix and a field of wiev of almost 5 arcmin. Due to a not perfect allignement of the Large Field objective the images are radially elongated in the left (north) part of the field. This elongation affects a vertical strip of about 150 pixels and smoothly disapears moving toward the center of the field. An alternative to the Large Field is the use of the Spectroscopic objective. This was initially designed to work with the echelle mode of SOFI that, in the end, has not been implemented and has a higher quality yielding to a scale and field that are similar to those of the Large Field, mainly 0.276 arcsec/pix and 4.7 arcmin. The Spectroscopic objective is slightly chromatic. The next image compares the image quality in the Large Field (top pannel) and Spectroscopic objective (bottom pannel).

Small Field

Images taken with the small field objective of SOFI suffers from image distortion near the lower and upper edges of the field of view. These regions are unsuitable for PSF fitting routines. An alternative to the SOFI small field is the large field in combination with the focal elongator in the grism wheel. Thes gives a pixel scale similar to that of the small field objective, a more stable PSF across the field of view, but a slightly lower throughput. The next image compares the image quality in the Small Field objective (top pannel) and Large Field + Focal Elongator (bottom pannel).