Stephane Plaszczynski

previously:

• CosmoDC2
• Run2.1(DR1) caracterization
• Run2.1 x CosmoDC2
• DC2 validation paper
• macroscopic validation (all data at once: Run2, cosmoDC2,stars)-> convenient with Spark , needs DPDD files.
• reporting on DC2-validation channel
• data mining (see cosmoDC2 post) -> you can then use GCR to understand things better

1.  Size

cumulative cuts (except mag_i vs mag_i_cModel)

measured Area (nside=2048) =304.1 deg^2

2.  tracts/patches

• 166 tracts

should have 49 patches each but on the borders : ideed (in red non 49 ones)

3.  density

• quality cuts : extended=1 & blendedness<10**(-0.375) & snr_i_cModel>5

(nside=2048)

• i<24

• the peak density is ~ good, we expect/measure ~17.4 on cosmodc2 (see here)

pb on a single patch for v1 seems fixed: see https://github.com/LSSTDESC/DC2-production/issues/408

4.  cosmoDC2 galaxies

• run2 galaxies= good+clean+ extended==1 + snr_i_cModel>5 + blendedness<10**(-0.375)

using the run2.2 footprint,on cosmoDC2:

around 3 more galaxies/pixel in run2, slightly more peaked.

Note I changed the color range to have similar dynamic.

So there is an excess of ~3 galaxies/pixel in run2 with the structure shown above. Not clear what it is

5.  stars

here is the stars density that has been injected (truth)

there is a gradient due to the milky way drection

and here histograms of mag_i compared to run2 sources (good+clean+extendedness=0)

some are missed around 20. They have been probably flagged as EXTENDED and might represent teh object excess, which explains the residual gradient.

Is this due to blending? I don't known: here my galactic choice includes a samll blendedeness so blended stars should be removed....

6.  Photometry

6.1  stars cross-match

• stars from refcat_v3_dc2_r2p1i with "isresolved"===false : 2.14M
• run2: mag_i<23 + good+ flags: 2.86M
• crossmatch (<0.1 arcsec+ single candidate) 0.52M (25%). given Run2 110 deg^2 + larger stars footprint: OK
• 1.8% were flagged as "extended" (removed)

RUN 2.1	RUN 2.2

The left tail comes from mag_i~16 and was also present in run2.1.

probably saturated stars not properly flagged.

testing the error

histgram of the pull ie. \Delta(mag)/magg\_err note : I cut above mag_star_i>17 to avoid the tail. (in blue expected normal distribution). since it is know mags are not (at all) gaussian same with flux

Error too low by a factor 2-3, better at high mag.

6.2  galaxies (cosmoDC2 cross-match)

run2 quality cuts:

• good and clean
• mag_i_cModel<25.3
• snr_i_cModel>1
• extended=1
• no Nan's for cModel_i

=>19.2M

Cross-match with r<1 arcsec + single candidate => 14.4M (75% , slightly low maybe due to border tracts)

SNR higher than in run2.1, still breach at 10 due to multiband switch.

• from Mike Jarvis: you expect a roughly horizontal cutoff when the y axis is mag_*_cModel. Which implies a diagonal cut when the y axis is mag_*
• possible explanation of this cut from Dan Taranu : you should be aware that there's a pileup of low S/N objects (hopefully mostly galaxies) at the size prior mean.

resolution

with SNR>10 and i<24 (waring in mmags and mag_i("true")/mag_i_cModel("rec") have been switched)

• the maximum probability ("mode") is shifted by ~50 mmag
• the bias ("mean") is about 5 mmags
• the fwhm/stddev ~ 200 mmags (non gaussian)

pull

with SNR>10 and i<24

that's bad. supposed to have stdev~1 : meaning errors are underestimate dby a factor ~5-7 (depending on how you count)

7.  Astrometry

cross-match with CosmoDC2 (SNR>1)

7.1  astrometric resolution

When cross-matching catalogs one can reconstruct the \Delta x,y positions in the transverse plane.

The bidim is log scale, not the single dx one:

Amazing precision (better than 0.1 arcsec) , no bias.

7.2  mean PSF

This astrometric resolution is related to the PSF by (\Delta x)^2 \simeq 2 \frac{PSF^2}{SNR^2} (seeLupton's note) so that we can reconstruct the PSF map

• PSF_x=\Delta x~SNR/\sqrt{2}
• PSF_y=\Delta y~SNR/\sqrt{2}

Plotting the bidim histogram wrt r=\sqrt{PSF_x^2+PSF_y^2} and normalizing the zero point to 1

one measures (half) the FWHM by drawing the horizontal line at 0.5 : we obtain 0.6 so <FWHM>=1.2

And here is what is in the catalog (drawing the vertical line at 1.2)

(warning this is the "object" PSF not the model one, its name will change)

mean is 1.14: very nice.

7.3  sample PSF

One can also test the estimated psf by sample. Here is how:

• The previous distribution was fitted approximately to a Moffat distribution of factor \beta=2.5.
• the distribution of the variable "r" follows r Moffat(r)
• working out the maths, the maximum ("mode") of the distribution lies at

r_{max}=\frac{a}{\sqrt{2\beta-1}} with a= \frac{fwhm}{2\sqrt{2^{1/\beta}-1}}) which is 0.44 fwhm for \beta=2.5

• Then if we histogram r/psf_fwhm_i it should peak at 0.44 if psf_fwhm_i is correct.

The mode is slightly higher (0.55) but given the various approximations (sqrt(2) factor from gaussian, poor Moffat fit at low r) this is quite fair.

NEW

A more accurate way to test the error is to reconstruct the pdf of the rescaled variable R=\frac{r}{a} (a is the classical Moffat normalization realted to the FWHM by a= \frac{fwhm}{2\sqrt{2^{1/\beta}-1}}). If the FWHM is properly estimated the pdf should be: f(R)=2(\beta-1)R\left(1+R^2\right)^{-\beta}

8.  Summary

• galactic density looks OK up to i<24 (and maybe more)
• be careful about a discontinuity near SNR_i=10
• astrometry
  • resolution better that 0.1 arcsec resolution in most cases.
  • mean and sample PSF properly estimated
• photometry
  • stars
    • bias decreased from 8-> 2 mmags
    • some saturated stars leaking near i~16
    • magerr to low by a factor ~3 increasing for high luminosities
  • galaxies
    • resolution about 0.2 mag (i<24) and a possible +0.05 mags shift
    • errors too low by a factor 5-7