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  1. Data Management
  2. DM-12207

Add color selection for photocal

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      Description

      Stars of certain colors (e.g., the upper branch in the gri diagram) are not suitable for use in measuring the photometric zeropoint. We know the color of stars in the reference catalog, so we can exclude them using that.

      This involves putting in a suitable Config entry to select the appropriate stars, and implement the selection within photocal.

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            Hide
            price Paul Price added a comment -

            Masayuki Tanaka writes:

            Starting with a background, we have used all bright stars within
            each CCD to calibrate the photometric zero-points. But, red stars
            tend to have a large intrinsic color scatter due to metallicity
            variations. For the photometric calibration, we would like to use
            stars with a small intrinsic scatter only. I have looked into
            the color range we should use to select stars for the photometric
            calibration.

            I used SRCMATCHFULL*fits from the S17A processing run. Somewhat
            arbitrarily, I picked the D-COSMOS field, where the Galactic
            extinction is small and the area is relatively large. The exposure
            time is slightly longer than the Wide exposure, but we are talking
            about very bright stars and that is probably not a concern.
            I used the visits 24318-24332; this is 50%:50% combination of
            30:270 sec exposures. This is the i-band data, but I realized
            I should probably look at some y-band data as well to see if
            the following results do not have strong filter dependence.

            The first plot [gri.png] shows r-i plotted against g-r. PS1 stars are in
            black. The original PS1 catalog seems to have a lot of contamination
            from galaxies, but the good star/galaxy from HSC makes the matched
            catalog mostly clean, although there are some outliers from
            the stellar sequence and they may still be galaxies. Most of
            the stars here are i<~21. The red squares are synthetic PS1 colors
            from the Pickles stellar library just for reference.

            As you see, the width of the stellar sequence increases around
            g-r~1.2 and r-i>~0.5. This area is dominated by M-stars.
            I would propose to use stars with g-r>0 and r-i<0.5 shown as
            the solid line. For reference, a K6V star has g-r~1.2 and r-i~0.5.
            So, I am cutting stars later than mid-K. I am not sure if we need
            to cut on the blue edge, but that probably does not matter too much
            because we have few stars earlier than A0V. About 40% of
            the selected stars satisfy this color cut, and the number of stars
            in each CCD is shown in the 2nd plot [hist.png]. The open histogram shows
            all stars in each CCD and the filled histogram is for stars within
            the color box.

            On average, we have ~20 stars and I think that is sufficient.
            COSMOS is a low stellar density field and so this is probably
            a lower limit, but general-use observers may take a much shorter
            exposure under poor conditions and run into a trouble. So, we may
            want to have a fallback mechanism, e.g., first apply the color cut,
            but if that results in too few stars (e.g., <5 stars), do not apply
            the cut and do the calibration with all available stars.

            Show
            price Paul Price added a comment - Masayuki Tanaka writes: Starting with a background, we have used all bright stars within each CCD to calibrate the photometric zero-points. But, red stars tend to have a large intrinsic color scatter due to metallicity variations. For the photometric calibration, we would like to use stars with a small intrinsic scatter only. I have looked into the color range we should use to select stars for the photometric calibration. I used SRCMATCHFULL*fits from the S17A processing run. Somewhat arbitrarily, I picked the D-COSMOS field, where the Galactic extinction is small and the area is relatively large. The exposure time is slightly longer than the Wide exposure, but we are talking about very bright stars and that is probably not a concern. I used the visits 24318-24332; this is 50%:50% combination of 30:270 sec exposures. This is the i-band data, but I realized I should probably look at some y-band data as well to see if the following results do not have strong filter dependence. The first plot [gri.png] shows r-i plotted against g-r. PS1 stars are in black. The original PS1 catalog seems to have a lot of contamination from galaxies, but the good star/galaxy from HSC makes the matched catalog mostly clean, although there are some outliers from the stellar sequence and they may still be galaxies. Most of the stars here are i<~21. The red squares are synthetic PS1 colors from the Pickles stellar library just for reference. As you see, the width of the stellar sequence increases around g-r~1.2 and r-i>~0.5. This area is dominated by M-stars. I would propose to use stars with g-r>0 and r-i<0.5 shown as the solid line. For reference, a K6V star has g-r~1.2 and r-i~0.5. So, I am cutting stars later than mid-K. I am not sure if we need to cut on the blue edge, but that probably does not matter too much because we have few stars earlier than A0V. About 40% of the selected stars satisfy this color cut, and the number of stars in each CCD is shown in the 2nd plot [hist.png]. The open histogram shows all stars in each CCD and the filled histogram is for stars within the color box. On average, we have ~20 stars and I think that is sufficient. COSMOS is a low stellar density field and so this is probably a lower limit, but general-use observers may take a much shorter exposure under poor conditions and run into a trouble. So, we may want to have a fallback mechanism, e.g., first apply the color cut, but if that results in too few stars (e.g., <5 stars), do not apply the cut and do the calibration with all available stars.
            Hide
            price Paul Price added a comment -

            John Parejko may be interested in using this information and/or code in jointcal.

            Show
            price Paul Price added a comment - John Parejko may be interested in using this information and/or code in jointcal.
            Hide
            price Paul Price added a comment -

            John Parejko, thanks for agreeing to review this. I hope that there are some elements in here that will be useful to you for your work on jointcal.

            Show
            price Paul Price added a comment - John Parejko , thanks for agreeing to review this. I hope that there are some elements in here that will be useful to you for your work on jointcal.
            Hide
            price Paul Price added a comment -
            Show
            price Paul Price added a comment - Jenkins passed .
            Hide
            price Paul Price added a comment -

            I've revised everything according to the existing comments, but as far as I can tell meas_algorithms hasn't yet been reviewed.

            Show
            price Paul Price added a comment - I've revised everything according to the existing comments, but as far as I can tell meas_algorithms hasn't yet been reviewed.
            Hide
            Parejkoj John Parejko added a comment -

            Yes, I'm still finishing the meas_algorithms review.

            Show
            Parejkoj John Parejko added a comment - Yes, I'm still finishing the meas_algorithms review.
            Hide
            price Paul Price added a comment -

            Thanks John. I think you're done reviewing now and have only forgotten to switch the status here.

            I've tweaked meas_algorithms as well and am running everything through Jenkins. I would like to merge in the next day or so, so please shout soon if there are any objections to the current state.

            Show
            price Paul Price added a comment - Thanks John. I think you're done reviewing now and have only forgotten to switch the status here. I've tweaked meas_algorithms as well and am running everything through Jenkins. I would like to merge in the next day or so, so please shout soon if there are any objections to the current state.
            Hide
            price Paul Price added a comment -

            Jenkins is green, merging soon.

            Show
            price Paul Price added a comment - Jenkins is green, merging soon.
            Hide
            Parejkoj John Parejko added a comment -

            Hey, I just responded to a couple of your points, otherwise I think you're good.

            Show
            Parejkoj John Parejko added a comment - Hey, I just responded to a couple of your points, otherwise I think you're good.
            Hide
            price Paul Price added a comment -

            Thanks John.

            Merged to master.

            Show
            price Paul Price added a comment - Thanks John. Merged to master.

              People

              • Assignee:
                price Paul Price
                Reporter:
                price Paul Price
                Reviewers:
                John Parejko
                Watchers:
                John Parejko, Paul Price
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                  Updated:
                  Resolved:

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