1 space-based galaxy survey exposure time calculator version 10 user manual & code description point of contact: christopher m. Hirata caltech m/c 350-17, pasadena, ca 91125, usa chirata@tapir.Caltech.Edu march 28, 2012 table of contents 1. Overview....................................................
2 f. R outines to compute the modulation transfer function (mtf) ...................... 35 g. R outines to compute galaxy image properties ................................... 35 h. R outines to compute cosmological information .................................. 37 i. R outines to compute properties ...
3 data such as background levels and counts per exposure from objects of various magnitudes so that the user can diagnose this situation should it occur. Suggestions for improvement, or additional effects that should be included, can be sent to the point of contact (christopher hirata). This manual ...
4 switches the bao snr computations from a cut on observed snr to a cut on expected snr. In v10 and later, the cut on snro is default and this option makes snre available. In v9 and earlier, all computations used a cut on snre. Options related to the ecliptic longitude of observations: -din_dlon the...
5 -dno_obj_continuum turns off the source continuum contribution to the noise model (usually a minor effect, of order ~1% since in wide-‐bandwidth slitless spectroscopy the galaxy continuum is almost always far below the sky background). -dlogical_read_floor this option changes the default behavior...
6 ./baoetc.Exe enter telescope configuration [0=generic, 1=from file]: 0 enter aperture outer diameter (meters): 1.5 enter central obscuration (fractional - linear): 0.5 enter pixel scale (arcsec): 0.37 enter throughput (total, all orders): 0.6 enter throughput (1st order): 0.6 the aperture outer di...
7 • the etc does not currently include the production of multiple charges per photon, which may be significant at very blue wavelengths. This should not be significant in the range of wavelengths recommended for the wfirst wl and bao programs, λ > 1 μm. If the wfe_input option is used (it is on by d...
8 and the number of grid points n at which the throughput curve is defined. Following this is a table with n lines each with three numbers: the wavelength λ in μm; the total throughput f all (λ); and the desired-‐order throughput f 0 (λ) or f 1 (λ) (for imagers or spectrometers, respectively). The ...
9 enter single exposure time (s): 150 enter read noise floor (effective e- rms per pixel): 5 enter dark current (e-/pix/sec): 0.05 enter ecliptic latitude (degrees): 45 enter galactic reddening, e(b-v) (magnitudes): 0.05 enter number of exposures: 6 the currently available detector types (and the in...
10 the position of the observation on the sky is required in order to calculate the zodiacal foreground radiation. This in principle requires the ecliptic latitude β and the difference in ecliptic longitudes of the observation and the sun, λ−λ . In default mode the latter is set to 90°, but can be i...
11 even if a galaxy is well-‐resolved, it will not be usable for wl if its snr is too low to recover a shape. For this reason, we include a maximum ellipticity error σ e,max . The computation of the ellipticity uncertainty assumes an exponential profile galaxy and does a full integration over scale...
12 the galaxy catalog cmc1.Dat is used for most analyses is extracted from the cmc (taking r eff >0 and z>0 objects). 9 it is based on the (non-‐randomized) magnitudes for a suite of 7 filters ranging from 0.43—2.16 μm. If the -dout_wl_cat option is specified, the code will ask for an output file n...
13 used based on the original version of the cmc. This older model is still available by setting j = 0. Hα luminosity functions: the majority of the luminosity functions are in the schechter form. This form supposes that the probability of finding a galaxy per unit comoving volume per unit l hα is €...
14 table 1: the schechter function parameters of the sobral et al (2012) hαlf (model i = 3), and the corrections applied. Z = 0.40 z = 0.84 z = 1.47 z = 2.23 faint end slope α −1.60 −1.60 −1.60 −1.60 original log 10 ϕ * [mpc −3 ] −3.12 −2.47 −2.61 −2.74 original log 10 l * [erg s −1 ] 42.09 42.25 42...
15 € log 10 r eff arcsec = −0.62 + 0.25 19 + log 10 f h α w m -2 ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ± 0.2. F. Pzcal-specific inputs there are no inputs unique to the pzcal mode. In general the inputs are identical to the bao mode, except that the galaxy population model is not required. 4. Code outputs: wl etc the w...
16 however, the etc does not penalize surveys with inadequate sampling. It is the user’s responsibility to independently check that their chosen dithering pattern will be successful. The minimum usable galaxy size according to the user-‐specified cut on resolution factor is given, as are the sky ba...
17 = 0.2, up to a maximum of z = 3) and total surface density are returned. There are two columns, distinguished by n versus n eff . Here n is the total number of galaxies with measured shapes; n eff is defined by down-‐weighting galaxies with non-‐negligible shape measurement noise, i.E. € n eff ...
18 ii. The wavelength λ (in μm) at which hα is observed. Iii. The ee50 radius of the psf (in arcsec). Iv. The conversion from area to comoving volume, dv/dz/da (in mpc 3 deg −2 ). V. The mdlf for a source with r eff = 0.3 arcsec (in w m −2 ). 19 vi. The comoving density of detectable galaxies n (in ...
20 λ−λ . The brightness i λ at λ = 0.5 μm was interpolated from table 17 of leinert et al. 23 the sky brightness at redder wavelengths was determined using the solar spectrum 24 and the correction for the color of the zodiacal light (relative to the solar spectrum), eq. (22) of leinert et al. A simi...
21 the effective noise for a galaxy in the absence of source continuum is computed using: € noise 2 = n exp Ω eff σ eff 2 p 2 , where € σ eff 2 is the effective noise variance per pixel per exposure; Ω eff is the effective solid angle of the source; and p is the pixel scale. The effective noise has ...
22 where the centroiding integral is € i c = 2 π u 1 ˜ Π (u) 2 d 2 u ∫ . This is used in the routine get_gal_centroid_integral, but is not used for any code outputs at present. E. Expected vs. Observed snr the bao program often works at low snr (e.G. A reasonable detection significance is 7σ), in wh...
23 € snro = 1 a max λ f 0 t( λ 0 ) t( λ ) + ε t( λ ) [ ] . As long as the maximum is near λ 0 (i.E. For correctly identified lines), we may taylor-‐ expand the terms in the maximum. To second order in ε and Δλ = λ−λ 0 , we find € snro = 1 a max Δ λ f 0 t( λ 0 ) t( λ 0 ) + ε t( λ 0 ) + f 0 t( λ 0 ) ...
24 where x is the value of snre satisfying € snre + 1 2 snre -1 − z = 0 (to second order in the noise: € x = z − 1 2 z −1 ), Θ is the heaviside step function, and δ is the dirac impulse function. 30 in the third line we have used the moments of the snro distribution and performed a formal taylor exp...
25 here i λ,sky is the sky brightness in units of power per unit area per unit solid angle per unit wavelength; and f all (λ) is the all-‐order throughput, i.E. The fraction of the photons entering the geometric area that produce collected electrons in any order [note: generally f all (λ)≥f 1 (λ)]....
26 € σ eff,src 2 (x 1 , x 2 ) = f d p 2 af 1 f ν t 1 hd θ Π( ʹ′ x 1 , x 2 )d ʹ′ x 1 ∫ , where f ν is the source continuum flux (in w m −2 hz −1 ), d θ is the spectral dispersion (in arcsec per Δ ln λ), (x 1 ,x 2 ) are spatial coordinates in the dispersion and cross-‐dispersion directions, and f d i...
27 g. Thermal background model thermal emission from the telescope may be a significant noise contribution for wfirst. It can come from several sources, including: 1. The “telescope” (primary and secondary mirrors and associated structures). 2. Additional optics upstream of the filter, such as the t...
28 € Ω t = π 4 f 2 (2 ε − ε 2 )(1 − υ 2 ) (where υ is the linear central obscuration) for cases with a pupil mask, and € Ω t = π 4 f 2 (2 ε − ε 2 )(1 − υ 2 ) + υ 2 + ρ 2 −1 [ ] (where ρ≥1 is the linear ratio of the beam size that passes through the limiting mask at the exit pupil to the actual size ...
29 • temperatures for telescope and aft optics: t tel = t aft = 230 k. • cold pupil mask not included, ρ = 1.05. • emissivity per surface ε = 0.02, aft optics have 4 reflections ε aft = 0.0776. • downstream component thermal background i down = 0.005 e − /pix/s (imager) or 0.03 e − /pix/s (spectrome...
30 € f (u) = 1 2 π d ϕ ˜ g (ucos ϕ ,usin ϕ ) 2 0 2 π ∫ . For this reason, we compare to the “noise-‐effective mtf” € ˜ g eff (u) = [ f (u)] −1/ 2 ; in each of the cases considered, the fitting formula used was either conservative or within example of such a comparison is shown in figure 1. Emission...
31 the small variations with source color must be considered in data analysis, but will not have a significant effect on the exposure time calculation. The bao calculations include a contribution added to the jitter to account for the smearing of the galaxy by the finite spectral resolution. This co...
32 aberrations: defocus, astigmatism, coma, and trefoil; and cases with 25:75, 50:50, and 75:25 mixtures of defocus with the other three aberrations. Figure 2: the rms mtf for an unobstructed aperture with aberrations, relative to the unaberrated case at rmswfe = 0.2λ. The solid curve is the fitting...
33 € f pen = Ω eff,s Ω eff ; the formula for the ellipticity error then reads σ e = 2f pen1/2 /snr. The penalty factor is 1 for a well-‐resolved gaussian, and r −2 (where r is the resolution factor) for general gaussian psfs and galaxies. It always approaches ∞ for point sources. Note that for non-...
34 #define n_throughput_max 512 typedef struct { int n; /* number of points in interpolation grid */ double lambda[n_throughput_max]; /* wavelengths (in microns) of nodes */ double throughput[n_throughput_max]; /* throughput values */ } throughput_data; the throughput curve is defined by linear inte...
35 d. Routines to set up detector parameters void configdetector(int det_type); this routine assigns values to the global variables describing the detectors: pixsize_phys, pix_cd, and (for nir detectors) t_frame and var_read_frame. It takes as input a detector type: the current options are specified...
36 double get_gal_omeff(double reff, psf_data *psf); returns the effective solid angle Ω eff of an exponential profile galaxy with effective radius r eff convolved with psf. All radii are in arcsec, and the solid angle is in arcsec 2 . Double get_j_couplingintegral(double reff, psf_data *psf); compu...
37 returns the limiting ab magnitude for shape measurements, as a function of r eff (in arcsec; assumes exponential profile), the psf, the noise variance per exposure (in electrons 2 arcsec −2 ), the calibration (in electrons per exposure for an ab magnitude 0 source), and the maximum ellipticity er...
38 get_n_detectable includes a correction for the conversion from snre to snro. (that is, get_p_detectable always cuts on the expected snr, but get_n_detectable cuts on either the expected or observed snr as requested by the user.) double get_n_galaxies(double z, psf_data *psf, double var_1exp, doub...
39 double get_zodi_bkgnd(double ecl_lat, double ecl_dlon, double lambda_min, double lambda_max, throughput_data *t); the zodiacal light intensity in photons/m 2 /s/arcsec 2 between λ min and λ max , observed through the throughput curve t. The ecliptic latitude β (ecl_lat) and longitude relative to ...
40 double getthermalbkgndtot(throughput_data *thr, double lambda_min, double lambda_max, double filter_throughput, double fratio, double centobs, thermal_data *thermal, int component); this routine computes the total thermal background in the detector from all three sources (telescope; aft optics up...
41 n. I/o and associated routines void error_check_thermal(thermal_data *thermal); this routine checks a thermal_data structure for unusual or illegal values. The program exits if an illegal value is found, and issues a warning if unusual values are found. Void read_thermal_data(char data[], thermal...
42 included code to catch incompatible compilation options. Introduced -dout_wl_cat and -dwfe_override options. Added independent snr cut for wl source galaxies. Added finite line width smearing in bao mode. Added more realistic model for noise variance on nir detectors (v6 and previous had behavior...
43 11. Acronyms bao baryon acoustic oscillations ccd charge coupled device cds correlated double sample cmc cosmos mock catalog ee50 50% encircled energy radius etc exposure time calculator ew equivalent width fomswg figure of merit science working group (2008) fwhm full width at half maximum hαlf h...
