radial velocities

March 12 2014

IMACS takes 6.5 in square x 12 mm filters. 

Newport sells 6.5 inch square filters

These folks sell Schott glass: http://www.vpglass.com/filter_glass.php

We want a cut-on at around 830 nm, cutoff around 880 nm. That's same as z on blue side, so use it for now. 

Dispersed imaging exposure time calculation. 

If we use z band then disperser reduces throughput by, say, 30% compared to direct imaging. Sky noise stays the same. Flux spreads across N pixels. 

300 l/mm grism produces 1.25 A/pixel. Effective passband is around 100 nm so that means signal spans 125 pixels. Assuming flux is flat photon SED, compared to 0.2 arc sec/pixel, in one arc sec seeing we go from 5 pixels across to 25 times as many. So signal surface brightness is 25x lower and sky is just as bright. That amounts to star being 3.5 mag fainter. So we can compute direct imaging SNR at 23.5 for equivalent. But we also need to include grism efficiency of 

Your observation will require approximately  seconds to complete. 
Star counts:        Sky counts/pixel:  Noise Contributions: Star     Sky     CCD