Step-by-Step procedure for transferring SNAP calibration

Jan 15, 2002
Jan 31, 2002


We assume that
  1. the SNAP field is the North Ecliptic Pole (we ignore the southern SNAP field for now)
  2. the SNAP field is one square degree
  3. the primary standard(s) have been chosen
  4. the primary standard(s) are within 10 degrees of the SNAP field
  5. the primary standards have been measured with an accurate spectrometer

Pick field(s)

The actual SNAP field should not have bright stars in it; they will contaminate measurements of the faint, V=25 SNe. We assume that no stars brighter than V=11 fall within the SNAP field itself. We expect to find roughly 10 K giants with V<15 in the SNAP field.

Step 1: Transferring Primary (outside SNAP field) to Secondary (outside)

Step 2: Transferring Secondary (outside SNAP field) to Tertiary (inside)

Step 3: Transferring Tertiary (inside SNAP field) to Quaternary (inside)

Step 4: Transferring Quaternary (inside SNAP field) to Final (inside)

Internal checks

We can very easily build two aperture masks for the WIYN 0.9-m telescope: one with a diameter about 0.9 meters, one with a diameter about 0.2 meters. These will allow us to check the shutter motion and the linearity of the CCD. They will also permit us to take images of stars as bright as V = 6 with 10 second exposures. If necessary, the smaller mask could be made 0.15 or 0.10 meters in diameter to accomodate the primary standards.

Building an aperture mask for the WIYN 3.5-m telescope is not so easy, because the darn structure is too large, and sits far above the ground. It might be relatively easy to place a mask of some sort into the light beam at the Nasmythe focus. However, the beam has converged a lot by that point, and a simple circular hole will not dim all stars in the field by the same amount. I have been thinking about various kinds of obstructions (wire mesh screens, plates with many holes drilled into them), and haven't found one that will meet our needs yet. It is possible that we might use a neutral density filter at this point to perform certain internal checks, even if we don't use it in calibration observations. The narrower our filters, the better the neutral density filter will work for us.

In any case, we should do at least the following tasks to verify our ground-based calibration.

  1. measure the shutter motion of the all cameras we use (MOSAIC on 0.9-m, miniMosaic on 3.5-m)
  2. measure the CCD linearity of all the cameras we use
  3. compare relative magnitudes derived from WIYN 0.9-m to relative magnitudes derived from WIYN 3.5-m, for at least the Secondary-Tertiary step
  4. place limits on the variability of all secondary, etc. standard stars
  5. acquire spectra of all secondary, etc. standard stars


Which passbands should we choose when comparing one set of stars to another? The choices are:

I believe we should use a set of narrow-band filters for the first transfer of primary-to-secondary at least. The narrow filters will allow us to use large telescopes and reasonable exposure times to look at stars as bright as V = 10.

Some of the filters which are available for the WIYN telescopes and appear to suit our needs are shown below.

wiyn # cwl fwhm  %T maxsize         name   comments       date_measured age
    16 6618   72     874x4   UWisc                        1-97          1997
    17 6725   70     874x4   UWisc                        1-97          1997

    19 4063   56     634x4   comet set 2 C3               3-97          1997
    20 4448   62     744x4   comet set 2 Blue Cont        3-97          1997

    25 7026  190     874x4   comet set 2 H2O+             3-97          1997
    26 7121   60     874x4   comet set 2 Red Cont         3-97          1997

I have checked to make sure that the filters above do not fall across any strong spectral features in K giants. The filter number 15 is close to H-alpha, but may be okay.

The result

After all the photometric observations and reductions, we end up with the following information:

primary standards:       know relative intensity as a function of wavelength
                               from visible to near-IR, due to balloon-borne

all other standards:     know the flux integrated over several passbands
                               relative to the flux of the primaries,
                               integrated over the same passbands
                         know the flux of each star, integrated over several
                               passbands, relative to the flux of itself,
                               integrated over other passbands

In other words, for the secondary standards, we do NOT know the ratio (for example)

              flux at 5000 Angstroms
             flux at 14000 Angstroms
Instead, we know only these ratios (where V and J are examples of the several passbands we may choose to use for calibration):
          flux of self integrated over V band        
          flux of self integrated over J band        

          flux of self integrated over V band
          flux of primary integrated over V band

The narrower the passbands we use for calibration, the smaller the uncertainty in these ratios (I claim).

In other words, if we pick a set of narrow-band filters and observe all standards through them, we will end up with known ratios of flux at a number of wavelengths. We can then calibrate an observed spectrum by forcing it to pass through these points.

In order to perform cosmological tests of nearby SNe versus distant SNe, one must know

  1. the shape of the spectra of SNe
  2. the shape of the passbands used to calibrate the SNAP standards
  3. the shape of the passbands used to observe the nearby SNe
  4. the shape of the passbands used to observe the distant SNe

Note that the passbands in items 2 and 4 above WILL NOT BE IDENTICAL, because the instruments on board the SNAP spacecraft will not be the same ones used to calibrate the standard stars.