Locked History Actions

Mock LSST Catalog w/GalSim

The recipe for creating a mock Galsim image from the LSST schema:

  1. Calculate the flux [ergs/s/Hz/cm2] of the galaxy in a band-pass given the apparent magnitude provided by the schema

    The flux is given by: f = 3631 Jy *10-0.4 m; where m is the apparent magnitude and 1 Jy = 10-23 ergs/s/Hz/cm2

  2. Multiply the flux by F_nu, the width of the respective band-pass filter

    The width of the band-pass filter is given by: c*d_lambda / lambda2 ; where d_lambda is the width of the filter and lambda is the central value.

    • For the SDSS filters:
      • Filter

        mean lambda [nm]

        d_lambda [nm]



















  3. Convert flux from [ergs/s/cm2] to [photons/s/cm2]

    The average energy of the photons in a band-pass is: 1.984*10-9/lambda[nm]; where lambda is the central wavelength of the filter in nm.

  4. Multiply the photon flux by the aperture of the LSST telescope

    The effective clear aperture of the LSST telescope is 6.7 m. Thus the effective collecting area is 1.41 * 106 cm2

  5. Set the flux of the image based on the gain (band-pass dependent?) and exposure time (possibly other factors such as telescope throughput and atmospheric transmission)
  6. Use other schema parameters to make the galaxy: ra, dec, pa, inclination (for both disk and bulge components)


Note: The draw() command in Galsim currently has two normalization options: 'flux' (default) and 'sb' (surface brightness8). The former outputs an image with the sum of the pixels equal to the total flux, the latter outputs an image with units of flux/arcsec2 (i.e. Sum(image)*pixel_area = flux). The draw() command also has gain [photons/ADU] built into it.

Bright Galaxy Tails


The question as to how far out the tails of bright galaxies extend is partly a question on what the detection limit of the instrument being used is. This is due to the fact, that no disk edge has been seen due to unreliable photometry at such low levels (i.e. there is an instrument or time limitation). Given an instrument with optimum qualities and seeing conditions, the size of the tails is proportional to the integration time.

A case in point is the study of NGC 300 (see above Refs.). An exponential disk (Sérsic n=1) surface brightness profile was used to model the galaxy out to a surface brightness of 30.5 mag/arcsec2 which covered a radius of 24', corresponding to an effective radius of ~17. There was no evidence for truncation of the stellar disk.

Tal & van Dokkum support using Sérsic profiles, showing the light profiles of massive ellipticals are well described by a single Sérsic component out to 8 r_e, with evidence for additional flux beyond these radii possibly related to unresolved intra-group light.

According to Kelvin et al. "It is clear that opinion is divided amongst the community as to how a galaxy behaves below the typical limiting isophote, particularly so in the case of a disk galaxy. Each of these studies does however suggest a more complex structure at large radii than a Sérsic profile extrapolated out to infinity would imply. In order to account for the lack of reliable profile information at large radii, Sérsic magnitudes require some form of profile truncation so as not to extrapolate flux into regimes of which we know little. Two schools of thought exist in terms of appropriate truncation methods, extrapolating flux down to a fixed surface brightness limit or integrating under the profile out to a fixed multiple of the half-light radii. A constant surface brightness limit is more closely related to galaxy gas content, and so has physical meaning. However, this method introduces a redshift dependence on truncated flux, causing different fractions of light to be missed at different redshifts. Truncating at a given multiple of the effective radius assumes that the effective radius is well understood prior to truncation, which owing to the inter-dependency between output Sérsic parameters, is not always evident. It does not display any redshift dependence however, and is trivial to subsequently recorrect if desired. Corrections are typically minor for most galaxies, becoming most acute in high-index systems."

An important note is that, according to Kelvin et al., the SDSS model magnitudes employ a smooth truncation at 3r_e down to zero flux at 4r_e for exponential (Sérsic n = 1) profiles and 7r_e down to zero flux at 8r_e for de Vaucouleurs (Sérsic n = 4) profiles. Adopting a sharp truncation radius of 10r_e for all Sérsic indices, which corresponds to an isophotal detection limit of μ_r = 30 mag/arcsec2, the limit to which galaxy profiles have been studied seems to be a safe model to adopt. A 10r_e truncation gives a negligible magnitude offset for Sérsic n = 1, and an offset of delta_m = −0.04 for Sérsic n = 4, with larger corrections for higher Sérsic indices. Given a 10r_e truncation, 100% of the pretruncation flux is retained for Sérsic n = 1, reducing to 96.1% for Sérsic n = 4.

Not all galaxies are as simple to model, however. From a sample of 90 face-on late-type galaxies, Pohlen & Trujillo (2006) confirm the accuracy of Sérsic profiling down to μ = 27 mag/arcsec2, and suggest up to 10% of their sample show evidence for a deviation from a standard n = 1 Sérsic fit (Type I), instead showing a broken exponential profile. These breaks appear in the form of either a downbending (Type II; steeper flux drop-off) or upbending (Type III; shallower flux drop-off) with increasing radii. Importantly, their study also suggests this observed feature is independent of local environment.

Given that elliptical galaxies, which are normally well modeled by a de Vaucouleurs (Sérsic n = 4) profiles, are estimated to have an apparent magnitude of 25.7, 25.6, 25.6, 25.2, 24.5, 23.6 in the u, g, r, i, z, y band passes respectively (see Table 3.1 of LSST Handbook) at z =2 and the the expected ten year depth of the LSST stacked images is only r = 28, it seems reasonable to either truncate or smooth the profile tails at either a ratio of the total radius or at a surface brightness limit.

Other notes:

  • Dust extinction in edge-on galaxies may mimic a break radius
  • The size of a galaxy will differ, at least slightly, in different band passes
  • There are many correlated galaxy parameters in the Fundamental Plane: luminosity, effective radius, mean surface brightness, metallicity, velocity dispersion, and dispersion v. rotation supported.

Minimum Surface Brightness Ratio

Tabulated below is the expected median zenith sky brightness for LSST, assuming mean solar cycle and three-day old Moon, in mag/arcsec2 (from Table 3.2 of the LSST Handbook), the typical apparent magnitude of elliptical galaxies at z = 2 (from Table 3.1), and the ratio of M/(mu*A) where M is the apparent magnitude, mu is the limiting surface brightness which we'll assume is the same as the sky brightness, and A is the pixel area (0.2")2.


Sky Brightness [mag/arcsec2]

Apparent Magnitude


























ImSim vs. GalSim

The following ImSim comparison is based largely on work done by Chihway Chang (see her notes here).


Above, is the central chip (4000 x 4072 pixels) image of one of the LSST photon simulation images generated using GalSim drawShoot(). You'll notice some artifacts in the image. For instance, some of the bright galaxies (one towards the bottom-right and two near the top-left) did not have large enough bounding boxes and now appears as squares. These were fixed by Chihway using wmult=5 when drawing the galaxies in GalSim. The image includes noise.

storedproc_field.png Above, the same field produced from the University of Washington databases catalog and using GalSim draw(). This image does not include stars in the field, just galaxies with no noise added. Notice that the tails of the brightest galaxies are very extended and wash out much of the field.


Above, the same field generated by ImSim. Notice that the bright star to the top-left has what looks like diffraction spikes. These artifacts are not present in the GalSim version of the image.

The following image is the difference in the above image generated by ImSim and the one generated from the UW database using GalSim. diff_im_eimage.png

One important difference between the two images, which may be hard to see above, is the fact that the position angle of the galaxies do not match up. Chihway's image has an additional pi/2 rotation in the position angle of galaxies. The two images below point out this issue.

chihway_gal.png our_gal_noise.png

Above left, is a galaxy taken from the photon simulation catalog and generated in GalSim using draw(). The total flux of the image is 33144.324 photons. Note that Chihway includes what she calls a "fudge factor" of 0.76 in her calculation of the galaxy flux, which she may have confused for the gain. The image is only for one visit of 15 seconds. Above right, the same galaxy taken from UW database, also produced in GalSim using draw(). The image corresponds to a total of 460 visits (6900 seconds); the total flux of the image is 211098824.567 ADU.

As one can see the two galaxies have different position angles, however, they would have the same position angle if the pi/2 rotation is not there. Chihway added this knowing it would change for future ImSim work noting "this will change in the next phosim version!!" However, the database at UW and the ImSim catalog agree on position angles now and this should not be an issue any longer. The galaxies also have different normalizations. The difference in the two images is shown below. The normalization of the UW GalSim version is higher than that of the ImSim by a factor of 6369. However, the UW GalSim image is assuming 460 more visits than the ImSim version. The images also have different units one is in photons the other ADU.


Above, the difference image of a galaxy drawn in GalSim using the photon simulation catalog and the same galaxy drawn in GalSim using the UW database.

The tails of bright galaxies are more extended in the image taken from the UW db. This is probably due to the fact that Chihway drew the galaxies in GalSim using drawShoot() whereas we used draw() to generate the image.

our_other_gal_field.png chi_other_gal.png

Above left, a galaxy from the UW database drawn using GalSim. Above right, the same galaxy taken from the ImSim catalog also drawn in GalSim.

GalSim Chip Image Issues

The following image represents a single chip image (4096 x 4096 pixels) taken from a larger (1 degree square) field named OneDegSq.dat.


The image has a few artifacts, most notably are the postage stamp looking galaxies and the long vertical dotted line in the upper lefthand side. Each of the artifacts are discussed in turn below. All of the following images come from using the "--stamps" feature of galsimcat.py and were taken using DS9. The scale for all of them is linear/min-max.

Postage stamp with negative flux

This galaxy is from line 832624 of OneDegSq.dat. Below, from left-to-right, is the galaxy without a psf convolution, the galaxy with psf convolution , and the mask of the galaxy.

zero.png zero_psf.png zero_mask.png

Postage stamp with small bounding box

This galaxy is from line 26168 of OneDegSq.dat. Below, from left-to-right, is the galaxy without a psf convolution and the galaxy with psf convolution.

topleft.png topleft_psf.png

Note that the mask for this galaxy is all 1's so it was not included here.

Another example of this type of artifact is line 212966 in OneDegSq.dat

Dotted vertical lines

This galaxy is from line 17977 of OneDegSq.dat. Below, from left-to-right, is the galaxy without a psf convolution, the galaxy with psf convolution , and the mask of the galaxy.

dots.png dots_psf.png dots_mask.png