"Unsaturated exposures of fundamental standard BD+17 have been taken in all filters, with instrumental magnitude reproducible to 0.01 mag RMS for consecutive exposures on a cloudless night. Shutter-timing systematic error also <1% for these exposures. Can be executed by transferring BD+17 calibration to neighboring fainter stars, then measuring fainter stars on longer exposures."
The DES absolute flux requirement is relative to the flux of BD+17 4708; so, although observing this bright, northern star with DECam on the Blanco will be challenging, it will be worth the effort.
This test is also described in the DECam Commissioning Calibration Testing Plan Detailed Checklist (DES-doc#6449)
- Photometric conditions.
- Preferably with little or no moon (at least for g and r exposures( to help avoid saturating an already very bright observation.
- Due to its declination, BD+17 4708, never gets below an airmass of 1.5 at CTIO. We should try to observe BD+17 4708, near its lowest airmass. Its RA, DEC in J2000.0 coordinates is 22:11:31.4, +18:05:34 At CTIO, it is observable at reasonable airmass (X<2) only between mid-May and mid-November. Therefore BD+17 4708 might not be observable during DECam commissioning or Science Verification.
- Use exposure times of TBD.
- Complication: we may need to de-focus the DECam somewhat in order to get unsaturated images of BD+17 4708.
- Multiple observations of BD+17 4708 (say up to 10 observations in total over the course of several nights?) would be useful in the quality assurance of these observations.
- The observations of BD+17 4708 should be taken in conjunction with observations of other standard star fields (e.g., SDSS Stripe 82 standards and Southern u’g’r’i’z’ standards); the other standard stars will provide a measure of the first-order extinction.
- Other than the short exposure times, the processing of these images should be no different than for other DECam images; they can be processed as part of the standard DESDM processing. (Note: if there are noticeable variations in the effective shutter time from the shutter map measurements, the previous statement might not be completely true.)
- Initial synthetic AB magnitudes for BD+17 4708 have already been derived for the DES photometric system; they are (g, r, i, z, Y) = (9.62, 9.33, 9.25, 9.24, 9.24). (For improved synthetic magnitudes, we will want to have response curves generated by the DECal spectrophotometric system.)
- Comparison of the observed DECam magnitudes vs. the synthetic AB magnitudes will provide one estimate of the absolute flux calibration of the DES photometry.
The main goal of this test is to obtain good observations of BD+17 4708, the fundamental flux standard for DES, in order to tie other standards stars (say, those in SDSS Stripe 82) to its photometry. If we fail to obtain these data, we will need to make the attempt again in the future.
Since BD+17 4708 is only observable from CTIO at a reasonable airmass between mid-May and mid-November, it is possible we may not be able to collect these data during Science Verification if Science Verification is delayed past mid-November. If this occurs, we should attempt to obtain observations of other HST CalSpec spectrophotometric standards (see Test 5 from the DECam Commissioning Calibration Testing Plan Detailed Checklist (DES-doc#6449). (Observations of other HST CalSpec spectrophotometric standards is recommended in any case, whether or not we observe BD+17 4708.)