1. 9/24/05LDP Prism description A Low Dispersion Prism for IMACS f/2 (LDP) A lower dispersion element affords more slits per mask at the expense of resolution (less pixels in a given wavelength range). For faint galaxy surveys, the primary observable per galaxy is redshift. –What’s the fewest number of pixels required to obtain a redshift to a specified precision? Crudely: Npix *  (z) ~ 1. Because of the f/2 wide F.O.V., grisms operating at 100 l/mm and below will be strongly affected by zero-order light (direct images of slits). A prism (or series of prisms) will disperse a very broad wavelength range (4000Ang-1 micron) over a small number of pixels. –No blaze condition, no preferred wavelengths. –But… a very non-linear dispersion pattern

2. 9/24/05LDP Prism description Current Prism Schematic (7/24/05) 83.66mm

3. 9/24/05LDP Prism description Ray tracing for IMACS f/2 rays from 400nm to 1000nm. 400nm 500nm 600nm 1um

4. 9/24/05LDP Prism description 3pix 4pix Resolution of LDP prism with IMACS f/2

5. 9/24/05LDP Prism description Why prism and not grism? Advantages of Prism High throughput, no blaze condition –Up to a factor of 2 more efficient Full wavelength coverage High slit density, up to 5000 per f/2 mask Advantages of Grism Linear wavelength dispersion Choice of wavelength range and blaze wavelength. Can resolve strong OH lines.

6. 9/24/05LDP Prism description Efficiency Curve for 300 l/mm transmission grating from Richardson Grating Lab website Expected Prism Efficiency Efficiency(%)

7. 9/24/05LDP Prism description PRIMUS survey with LDP+IMACS f/2 Burles, Eisenstein, Coil, Blanton, & Hogg Goal to observe 1 square degree down to I < 23.5 in 2006A to test target selection, optimal observing strategies and redshift success in well-tilled survey fields (i.e. CDF-S & COSMOS) Compare redshift precision with classic grating/grism surveys, as well as photometric-z surveys like COMBO-17 Will require some custom slitmask design and observing strategies, as well as some challenging data reduction and redshift identification algorithms. Our goal is to obtain 50,000 redshifts in Spring 2006, with time allocation through MIT & Arizona. If successful, we plan to embark on a larger multiple square degree survey to get underway in Fall 2006. This observing mode may be well suited to follow-up planned SZ fields.

8. 9/24/05LDP Prism description