Tests of photometry in test field 418

Michael Richmond
Sep 6, 2006
Sep 12, 2006

Waqas Bhatti gathered a nice, big chunk of data from the SDSS for tests of photometry.

Attached is the data for Field 418, Run 94 (Justin's second report). I used the following criteria:


(Flag descriptions at: http://cas.sdss.org/runs/en/help/browser/enum.asp?n=PhotoFlags )

The stars fall within a box with rough limits 37.35 < RA < 37.51 and -0.21 < Dec < +0.01 The photometry comes from repeated scans of the celestial equator, some as part of the SDSS Supernova Survey. These scans were made over the past few years, with Julian Dates ranging from 2,451,075 (Sep 18, 1998) to 2,453,641 (Sep 27, 2005).

Here's a picture of the field, taken from the POSS-I plates, courtesy of Aladin.

I used my ensemble photometry package (with very minor modifications) to

My primary tool for gauging the quality of a solution was the scatter-versus-magnitude relationship: if the scatter of most of the stars is relatively small, then the solution is good. Since stars at the bright end might be saturated, I looked at them carefully, and marked as "variable" any which showed a scatter larger than expected; these "variable" objects are not used to determine the photometry solution, but do appear in the output.

I will show the results for the g'-band, r'-band and i'-band photometry only in a series of graphs below. Each displays scatter from the ensemble mean magnitude on the vertical axis, and the mean ensemble magnitude on the horizontal axis. Note that the brightest stars tend to show elevated scatter (in some cases too high to appear on on the graph); almost all of these are, I believe, due to saturation. I show on each graph the level corresponding to a scatter of 0.01 mag = 1 percent. There are black squares showing the estimated RMS for stars, based on theoretical calculations of the telescope and filter throughputs, etc. from Gunn et al., AJ 116, 3040 (1998).

I will use only the "fiber" magnitudes, since we have learned that these aperture measurements are more stable from night to night than the PSF-fit magnitudes.


The first pass of the ensemble solution yields the following graph of "sigma-vs-mag":

Let's zoom in on the interesting regions.

There are several features we can see in a glance:

  1. the "noise floor" for the brightest stars is around 0.01 mag, or 1 percent. This is far above the theoretical photon noise, but that's no real surprise
  2. the actual scatter is pretty close to the theoretical prediction for faint stars; again, no big surprise

There are a few outliers with scatter much larger than one would expect. Let's look at a few of them. The three outliers in the closeup figure are at roughly

  A:  rmag = 16.928, rsig = 0.038    37.39122,  0.66545  8658170690058453028
  B:  rmag = 18.454, rsig = 0.096    37.37436, -0.08688  8658170688984711242
  C:  rmag = 18.636, rsig = 0.043    37.43294, -0.58280  8658170688447840380
I'll also include a "constant" star, one chosen at random which has a small scatter from the mean. Here are the light curves of these stars:

Based on this very small sample -- but also experience with other sets of photometry in the past -- I would note

The "sigma-versus-mag" plot is one way to look for evidence of variability. It may not be the best tool, especially for our purposes.


The first pass of the ensemble solution yields the following graph of "sigma-vs-mag":

This shows the same behavior as the r'-band photometry. Fine.

Once again, I'll concentrate on the three stars which had higher-than-expected scatter in the r'-band photometry. I'll also include the same "constant" star, one chosen at random which has a small scatter from the mean. Here are the i'-band light curves of these stars:

Aha. It turns out that one of our three variable candidates does NOT show any significant variability in i'-band.

  A:  imag = 16.777,  isig = 0.008    37.39122,  0.66545  8658170690058453028
  B:  imag = 17.976,  isig = 0.100    37.37436, -0.08688  8658170688984711242
  C:  imag = 18.618,  isig = 0.034    37.43294, -0.58280  8658170688447840380

Star A looks constant in the i'-band, which suggests that its r'-band scatter was due to a single bad measurement. On the other hand, the scatter, and the detailed light curves, of stars B and C are similar in both passbands. We may then conclude that stars B and C may really be varying (though in different ways).

Well, I guess this was obvious :-)


The first pass of the ensemble solution yields the following graph of "sigma-vs-mag":

This shows the same behavior as the other passbands.

Night-to-night adjustment factors

Another of the ensemble photometry method's results is the "adjustment" for each image (or each night): the amount by which all the measurements on that image (or night) should be shifted to bring them into agreement with the other images (or nights).

Let's look at the adjustment values for the ensemble solutions in r'-band and then i'-band. First, the r'-band result:

Now, the i'-band result:

Finally, the g'-band result:

You can see similar patterns in all passbands. The big outliers are likely due to thin clouds. The long-term change from the first night to all the others may be due to gradual buildup of dust on the mirrors, or some long-term change in detector properties.

Ensemble output datafiles

I provide below some of the output files from the ensemble solutions. The ".sig" files look like this:

     0 13.193  0.158  0    0.25     35.32486      7.03099
     1 11.360  0.073  0   -0.26     35.33325      6.72861
     2 12.190  0.078  0   -0.75     35.33335      7.18730
     3 13.695  0.137  0   -0.84     35.34351      6.66053

The columns in this file are

  1. star index number
  2. "true" magnitude in the ensemble solution
  3. uncertainty in the "true" magnitude
  4. variability flag: 0="not variable", 1="variable"
  5. "variability score": how much above or below the typical amount (for stars of similar brightness) did this star vary?
  6. RA coordinate, shifted to match template position
  7. Dec coordinate, shifted to match template position

Below are files showing the full light curve of each star (well, actually, the values for all stars in a passband are gathered together in a single file). You can find the format of these files described in the ensemble package documentation; look at the section describing solvepht.out files.