990227s UT Test of Leach Electronics at Southen 2MASS Facility

The new Leach electronics were installed at the Southern 2MASS Facility on 990226 UT. Two dark scans were taken and the results were analysized by Mike Skrutskie and were also run through the normal DARKS subsystem in 2MAPPS. Mike reported that the darks appeared similar to those produced when the electronics were tested in the lab. The only exception was some minor amplifier glow in the corners of the arrays. He stated that the amplitude of this glow should be well below the level to impact the noise in the darks. The images showing the difference between the darks taken with the Leach system and recent souther darks taken with the GATIR system show the glow in the corners (see below). The amplitude of the signal in the corners relative to the middle of the arrays is ~50-60 DN in J, ~40 DN in H and ~30 DN in K_s.

The dark frame statistics reported by DARKS is as follows:

  Band/ReadType:      J/1      H/1      K/1      J/2      H/2      K/2 (evening)
 Avg Dark Sigma:     4.57     4.31     4.24     2.20     2.24     1.94
St Dev of Above:     0.85     1.38     1.28     0.58     2.82     1.41
 No. Masked Pix:        0        0        0        6      138       24

The images below compare the new dark frames (on the left) with recent southern canonical darks (on the right), taken from the night of 990217s UT.

J-band (left-990226s right-990217s)

H-band (left-990226s right-990217s)

K_s-band (left-990226s right-990217s)

These three images show the differences between the 990226s and 990217s dark frames in each, band. All are shown with a fixed display range between 0 and 70 DN.

J-band

H-band

K_s-band

The electronics were installed on the telescope for night of 990227 UT. A full night of observations were made under nominally photometric conditions. Two calibration scans, scans 045 and 046 covering the S312-T cal field, and one survey scan were ftp'd to IPAC and processed using 2MAPPS v2.1. The pipeline output from these runs was evaluated and summarized below.

• Visual inspection of the individual flattened frames for cal scans 045 and 046 revealed no unusual source-related artifacts in J, H or K_s. Known detector-related artifacts such as latent images appear nominal. Also, no diagonal fixed-pattern noise is evident.

  There is no evidence of the "anti-ghosts" seen near bright stars when the Leach electronics were first installed on the northern telescope.

  From Mike Skrutskie:
  
  

  I was able to generate the equivalent of flat frames
  locally and they look perfect.  The only problem that
  could arise now would come from the statistics of
  source counts and uncertainties, but that is unlikely
  at this point. ...
  

  From Gene Kopan:
  
  

          Southern telescope with Leach electronics installed. 
          Looked at elvis:/k4/OPS/990227s/s046-s047, r1 noise & r2 flattened
  frames look ok, these scans are fairly high density ( ~5000 extrations- cal
  scans) so subltle effects in the r2 may not be obvious, & noise is pumped
  up by confusion noise.
   
          Sawtooth pattern in r1 frame medians, peak-to-peak:
   
          j    1.0 dn
          h    0.5 dn
          k    1.5 dn
   
          Solos about normal for this density, frexas residuals look ok. No
  obvious problems. It would be good to check the r2 noise on a low density
  scan.
  
  

• The median frame backgrounds from the two cal scans and the survey scans appear reasonably well behaved. The J-band backgrounds show some evidence for the <1DN "oscillation" frame-to-frame, but it appears to be less significant than in the northern data. H and K_s are stable, but note that the K_s backgrounds were high during these two scans.



• Coadds appear nominal in all bands. None of the diagonal fixed-pattern noise that was seen in the northern tests is noted. There is no evidence for the banding in J-band, or any other, in these two short scans. We will run the banding diagnostic shortly to get a quantitative measure, though. Coadd noise ranges from 0.7-0.8DN in the J-band, 1.0-1.1 in the H-band and 1.2-1.4 in the K_s-band.



• The photometric zero point offset for the two scans was measured using the single calibration star in the S312-T field. The instrumental magnitudes for the star and offset values are summarized below:

                         S045                         S046
      Mcat          Minst         ZP            Minst         ZP
  -----------------------------------------------------------------
  J  11.949   11.988 +/- .021   -0.039    12.106 +/- .029   -0.157
  H  11.669   11.642 +/- .031   +0.027    11.632 +/- .021   +0.037
  Ks 11.609   11.580 +/- .029   +0.029    11.580 +/- .027   +0.029
  
  

  The zero point offsets for this cal scans are slightly higher, but generally consistent with recent values measured at CTIO. The zero point offsets from the night of 990217s are: -0.187 mag, -0.072 mag, and -0.057 mag at J, H and K_s, respectively.

  The H and K_s instrumental magnitudes repeat extremely well, but the J magnitudes differ by almost 10% between the two scans. The figure below shows the magnitude difference between all stars common to the two scans, plotted versus their average magnitude. The heavy lines show the trimmed-average magnitude difference within 0.5 magnitude bins. The vertical error bars show the RMS magnitude difference in each bin. There is no statistically significant bias in the instrumental photometry between the two scans. Note that no effort has been made to remove confused objects, and this is a moderately dense field. Note that confusion noise has moved the SNR=10 level close to the Level 1 Req. brightness.

  Magnitude differences plotted versus mean magnitude for all stars in common between S045 and S046
  
  
  



• A full calibration scan set was ftp'd to IPAC on the morning of 1 March, and run through the standard pipeline. This cal set also covered the S312-T calibration field. The figure below shows the photometric repeatability for all sources detected in at least 7 out of the 8 scans of S312-T (scans 015-020 and 045-046). The photometric repeatability is nominal, and the sensitivity is well within the Level 1 Requirements for the survey.
  

  Photometric Repeatability from 8 scans covering the S312-T Cal Field
  
  
  

• The band-band photometric performance appears nominal, as shown by the color-color plots for scans S040, S045 and S046, shown below. These are made using instrumental magnitudes, only, so there is some slight offset in J-H due to the relative zero-point offsets. (flagged artifacts have been removed, but no attempt has been made to remove confused sources). In all of these plots, objects with SNR(K_s) > 20 are shown in blue.

  Instrumental color-color plot for S040
  
  
  

  Instrumental color-color plot for S045
  
  
  

  Instrumental color-color plot for S046
  
  
  

• Tracey Evans reports that the bright star artifacts in the new data appear nominal. The standard MAPCOR algorithms appear to detect them as usual. The high source densities in these scans result in lots of artifacts, of course.



• Howard McCallon reports that the reconstructed position of the first frame of survey scan S040 is within a few arc-seconds of the expected location. Thus, no adjustment of the first-frame timing seems necessary.