A unique focal reducer for the Steward Observatory 2.3 m Ritchey-Chretien telescope on Kitt Peak was designed and built. It increased the field of view on a 2048 x 2048 charged coupled device (CCD) with 15 mu m pixels from 5.1' x 5.1' to 23.9' x 23.9'. The inverted-Cassegrainian, two-mirror design permits high efficiency of ultraviolet (UV) wavelengths and good broad band image quality from 3500 A to 8500 A without significant chromatic aberrations. Other features include a coronagraph and a filter slide in the slow f/9 beam which permits the difficult and useful combination of narrow band imaging with a wide field of view. The image quality is seeing limited in the central 12', and degraded by about 0.8" in the central 18', and degraded by 1-2" at the edges of the field. Vignetting due to a field lens misalignment and poor filter uniformity cause flat field variations of +/-5%. The development of the optical system prescription is presented. Very narrow bandpass interference filters of 15 A bandwidth (FWHM) for Hα and 30 A for (S II) were tilted to accommodate various red shifts up to 1500 km/s. Useful models estimating changes in filter center wavelength (CWL) and bandwidth due to tilt in collimated and converging beams are presented. The current understanding of the recently discovered warm ionized diffuse gas phase (WIM) of the interstellar medium (ISM) and the motivation for observations of it in external galaxies are discussed. Continuum-subtracted Hα and (S II) images of the edge -on galaxies NGC 4244 and NGC 5907, taken with the focal reducer, are presented. The images show H II regions and the WIM to be more confined to the plane of the disk for both galaxies than in our Galaxy or in NGC 891. Interesting filamentary structure is evident. Calculations based on the images quantify the distribution of ionized gas in NGC 4244 and NGC 5907.
- Pub Date:
- January 1996
- FOCAL REDUCER;
- Physics: Astronomy and Astrophysics, Physics: Optics