Source position:   11:*6:07.41 +11:*1:25.7

The astromteric solution results:
GUI-acquired rough source positions: 
   Xm      Ym        Xwcs    Ywcs         RA_2000    DEC_2000
  386.3   290.2      108.6   176.1      11:*6:10.18 +11:*1:52.3
  258.8   160.2      192.6   166.0      11:*6:04.42 +11:*1:42.5
  274.0   363.1      113.6   113.6      11:*6:09.82 +11:*0:49.4
  277.5   460.9       75.8    89.0      11:*6:12.41 +11:*0:24.5
  537.1   287.9       66.9   233.0      11:*6:13.06 +11:*2:49.4
  505.9   348.3       53.7   205.2      11:*6:13.96 +11:*2:21.4
  428.3   360.3       70.7   173.0      11:*6:12.78 +11:*1:49.0

RMS(Ra,Dec):   0.2346   0.5642    n =    7

Figure 1: Using WCS from the above astrometric solution we can locate the known position of our LRS target. The purple circle above is a 6-sigma error region. Only one well-detected source (30 sec LRS) lies inside this area.

A modest observation

The majority of LRS fields would contain enough objects easily visible in a DSS image to provide a WCS solution good to a few tenths of an arcsec. If users provide RA,DEC in some known astrometric system, then some variant of CALIM could be used to greatly aid in the acquisition of faint LRS targets. The current approach of using paper finding charts, produced in a wide variety ways and qualities, should really become a thing of the past.

Photometric Calibration

Having derived a suitable astrometric solution, CALIM will next attempt to derive a photometric zeropoint for the image. Standard star magnitudes are read from a file of standard field photometry and matched up to the image sources via the new WCS. At present, the HET standard field set consists of BVR photometry of N7006, N7790, and N4147 (Odewahn etal 1993, PASP) and BVRI photometry of M92 (Christian etal 1985, PASP). For each source a set of aperture photometry data are computed and used to derive a photometric zeropoint. In addition, the local sky surface brightness is computed for each source. Some examples of figures generated with this portion of CALIM are shown below in Figures 2 through 4.

Figure 2: An LRS V image of NGC 4147. The position of each standard source is used as a starting-point in measuring photometry on the LRS image. After sky determination, intensity weighted moments are used to computed the position, size, shape and orientation of each source. Magnitudes are derived with a user-specified aperture size. The sizes of the source markers in the image above reflect the size of each image determined from the intensity-weighted second moment.

Figure 3: The V-band sky surface brightness (mag/sq.arcec) for each source measured in Figure 2. The sky brightness is slightly correlated with the source brightness. These sky measures are very bright due to the fact that a 90% illuminated moon was 75% away from NGC 4147 at the time of the observation.

Figure 4: The individual ZP estimated from the sources shown in Figure 2 as a function of distance from the LRS image center (in pixel units). Note the strong trend in ZP with radial distance caused by the strong vignetting function in an LRS image. Points, such as those encased in yellow circles above, may be isolated by the user for rejection prior to a linear regression.

CALIM upgrades - Oct2004

A number of refinements have been made to CALIM to allow the use of general HET LRS fields. Some initial work with this upgraded code to investigate rho position errors is summarized below.

                   HET Rho Measurements

 The following plate solutions for LRS fields were derived with CALIM
 using DSS images for an intial solution, and USNOA2.0 astrometry for
 the final least squares fits. The important column below is labeled
 DEL and is the difference between the HET (LRS header) PA (position
 angle) on the sky and the value for the same quantity I derive in the
 fit. In general, these numbers are low. The one high case (***) occurred
 when we were making changes to the rho zeropoint.

 I believe these results show that we can use CALIM to monitor rho
 positioning periodically. This method is not as accurate as the
 iraf method that Matt runs. That method uses bright cluster stars
 with good astrometry. I use USNO astrometry based on Schmidt plates,
 hence we see rms residuals on the order of a few tenths. However, we
 can use this method anywhere we take an image with LRS!


  #    scale    DEL    dRA   dDEC  skyPA_het    date    RA  Object

  S3  0.4670   0.03   0.276 0.290    242.2    20040504  SO  N4147
  S6  0.4680   0.11   0.464 0.575     96.3    20040811  S0  J2025+3343
  S5  0.4700   0.03   0.727 0.370    220.7    20040811  SO  J2120+0533

 ---------------------  PFIP Take-Down --------------------------------

  S7  0.4717  -0.15   0.471 0.576    134.6    20040904  MS  N7790
 S10  0.4671   0.29   0.313 0.441    175.8    20041010  SO  Munich
  S9  0.4683   0.60   0.458 0.663    -45.4    20041016  MS  N7790
 S11  0.4705   0.97   0.360 0.455    175.0    20041018  BR  Munich  (***)

 ---------------------  Final JRF Rho Fix -----------------------------

  S8  0.4670  -0.08   0.328 0.559    318.6    20041019  BR  N7790
 S12  0.4686   0.23   0.477 0.370     73.0    20041020  BR  J0742+4900

 scale     = derived pixel size in arcsec
 DEL       = skyPA_het - skyPA_fit  (in units of degrees)
 dRA,dDEC  = rms residuals (arcsec) in RA,DEC
 skyPA_het = position angle of LRS column on sky (from LRS header info)

(***) I did this fit many ways and continually came up with a 1 degree
      offset. I was concerned because I thought the mask was aligned
      and Jim's rho fix was in place. Neither was true! I ran MOSOBS
      on the setup image and found approximately a 1 degree rotation
      needed to align the mask! Also, this image was taken on 20041018
      (which is a day before JF's final fix).

      Note that my 20041010 Munich mask image gives 0.3 degree agreement
      between HET and fitted skyPA. This was about a week prior to the
      rho fix, and I am surprised the agreement was so go. I had to
      rotate the field by 0.8 degrees (from MOSOBS) to align the mask.
      Hence, what HET thought was at PA=175 had to be placed at 175.8.
      What is confusing to me is that my plate solution gives a close
      match to that at PA=175.5.

      Note that MS's 20041016 N7790 was also off by 0.6 degree, and this
      was what motivated the rho fix by JF. The initial fix was made in
      the wrong direction, and this accounts for the large shift needed
      for BR's 20041018 image. Once correct the rho sign was used (after
      20041018), we see that the later fields have HET and fitted sky
      PAs that agree well.