Binospec / Hectospec ETC help
Main information page: Binospec information page
Please use the new exposure time calculator for Binospec, Hectospec, MMIRS, and GMACS at New Binospec exposure time calculator. This help page was originally written for the old ETC described at older Binospec ETC. Most of the fields described below apply to the new calculator as well.
Description of the options
Instrument and Mode: Choose instrument and grating (in lines/mm, eg 270). This will update the angstroms/pixel. The angstroms/pixel and arcsec/pixel give approximately the spectral resolution for a given slit (eg 1.3 A/pixel and 0.267 arcsec/pixel mean that a 1 arcsec slit has FWHM of 3.75 pixels = 4.9 A.)
Spectrum type: Choose a category of spectral templates. A second menu will then appear that lists templates in that category (eg for Kinney-Calzetti templates, choose among bulge, elliptical, spirals, starbursts). Note that the wavelength coverage of these templates varies and this can be important for higher redshifts.
Redshift: Redshift to shift the template to.
AB Mag in observed band: Normalization of the spectrum, eg if you want to calculate for a galaxy with observed r=23, put 23 here and choose ‘r’ in the ‘Bandfilter’ selection below.
Source effective radius: Half light radius of the source. This is convolved with the seeing and then sampled by the extraction aperture to compute how much light from the source is in your aperture.
Exposure time: in seconds
Moonage: 0 is dark (new moon), 14.7 is full moon, “Moonage Daydream” is a David Bowie song. Used to estimate the sky brightness.
Seeing: this is convolved with the object radius and then sampled by the aperture. Choose something like 1.0 arcsec, the default of 0.4 arcsec is optimistic in the optical at MMT.
Bandfilter: filter where the spectrum is normalized to the selected magnitude. The template spectrum is convolved with this filter to synthesize a magnitude. The options are g, r, i, because this filter should overlap with the wavelength coverage of Bino/Hecto. If the template does not fully cover the selected filter, the normalization can be off.
Aperture type and width: Select the shape and size of your spectral extraction aperture. For example you might choose ’round’ and 1.5 arcsec for the Hectospec fiber, or ‘rectangular’ and width=1.0 arcsec, height=2.0 arcsec for a Binospec 1 arcsec slit on a small object where you expect to extract in a window of 2 arcsec in the spatial direction.
The width should match your slit width. Larger height will include more light from the object but also more noise from sky flux.
Airmass: airmass of observation (affects extinction).
Plot spectrum / flux : you will get a plot of SNR per resolution element (ie per slitwidth) vs wavelength and a plot of either counts or flux vs wavelength. It often takes a couple of seconds for the plot to refresh.
Tips
It is best to use a template that fully covers the region you want to observe and the bandfilter you are normalizing the spectrum with. For example, if you take an SDSS-gals spiral spectrum (starts at 3800 A in rest frame), put it at redshift 0.7, and try to normalize it to mag=22 in the g-band, it may not work, because the redshifted spectrum doesn’t cover the g filter. But if you normalize it to mag=22 in r-band, it should work.
If you’re having trouble with the wavelength coverage for redshifted objects, try using the Kinney-Calzetti templates, as they extend well into the rest UV.
Credits / questions
ETC calculator code by Michael Kurtz, kurtz@cfa.harvard.edu. New ETC calculator code by CfA/SAO staff.
MMT help page maintained by Ben Weiner, bjw@mmto.org.