urn:nasa:pds:context:instrument:ctio-cerro_tololo.soar_4m1.soi 1.0 1.15.0.0 Product_Context 2023-02-28 1.0 Initial export from OLAF urn:nasa:pds:context:telescope:ctio-cerro_tololo.soar_4m1 instrument_to_telescope SOAR Imager (SOI) Imager SOAR Imager (SOI) The text was adapted from the SOAR/SOI website. The SOAR Optical Imager (SOI) is a bent-Cassegrain mounted optical imager using a mini-mosaic of two E2V 2k x 4k CCDs to cover a 5.25 arcminute square field of view at a scale of 0.0767"/pixel. It was designed, built, and integrated at SOAR by a NOIRLab/CTIO team lead by Drs. Alistair Walker and Hugo Schwarz. Optics: The SOI optics consist of a 6 element focal reducer and field corrector, that converts the f/16.63 beam of the SOAR Telescope to f/9.82, preceded by a linear, "trombone style", Atmospheric Dispersion Corrector (ADC). These optics were designed to deliver images of < 0.18 arcsec FWHM (equivalent to 80% encircled energy within D80 < 0.27 arcsec) at the zenith and <0.34 arcsec FWHM (D80 < 0.51 arcsec) at 70deg zenith distance, in each of the U, B, V, R, I broad band filters, and over the entire field. The glasses selected, and the use of SOLGEL over MgF2 coatings on all external surfaces ensure high transmission over the entire 310nm-1050nm passband. CCDs: The SOI focal plane is imaged onto a mini-mosaic of two CCDs with charge transfer efficiency of 99.9995 and dark current of 0.01e/pix/hr. CCD mounting gap: The two CCDs in the SOI are mounted with their long sides parallel and spaced 102 pixels apart, resulting in a 7.8" gap between the individual CCD images. The gap can be filled by taking dithered images. We recommend taking at least 3 images with 10" steps to produce a complete combined image with no gap. Flat Fields and Fringing: The CCDs show fringing in the I, i' and z' bands at the 10%, 3% and 12% level, respectively. In the U band some hatched structure is seen but this flat fields out perfectly. Ghosts, stray light etc..: The use of interference filters - which select their passband by reflecting the rest of the light - results in haloes around bright sources. These haloes look like images of the entrance pupil of the telescope with the secondary mirror spider clearly visible, especially when the seeing is good. We have some stray light from encoders in the SOI rotator on the CCDs in the I band. The level is about 0.2 ADU per second, or less than 1% of the dark sky background. We are working on eliminating this stray light. Read-out: The chips are read out by two amplifiers each. Various read-out and binning modes will be available. Note that a seeing disk of 0.45" is still well-sampled by 2x2 binned pixels, so that only for the very best seeing and using the tip-tilt mode should the CCDs be used in unbinned mode. At present, the tip-tilt mode is not yet offered. SOI is no longer offered in slow read-out mode (as of 2008 Nov).