6. Explore data

Once we have reduced our data, we probably want to look at. In fact, even before we start the reduction, we should look at the raw data in case of issues.

DRAGONS is not an analysis package. But it does offer a basic way to display images.

The display and inspect primitives can be used to visually inspect the data with ds9. Those are primitives not recipes, yet reduce can run individual primitives.

Note

ds9 must be launched by the user ahead of running the primitive.

6.1. Display

To display an image to ds9:

reduce -r display N20160102S0271_stack.fits

By default, the saturated pixels are marked. This can be turned off by setting the threshold parameter to “None”.

6.2. Inspect

It is recommended to inspect the raw data prior to reducing the data. A quick visual inspection can catch obviously bad data, or flag some frames for more detailed inspection. Having to run display on each frame individually would be painful. The primitive inspect can be used here. It accepts a list of file and will display each frame in sequence, adding a short pause between the frames. The length of the pause can be controlled with the pause parameter.

reduce -r inspect @stdstar.lis

Exercise - Explore 1

  1. Display N20160102S0271_stack.fits to buffer 2 and turn off the saturation marking.

  2. Reduce the time between frames to 1 second in inspect.

Hint: reduce file.fits -r primitive -p primitive:parameter=value

[Solution]

6.3. Accessing table content

Some output files will have table information. Once written to disk as a FITS file, the tables are accessible as any FITS files. A common table is a list of sources detected by the primitive detectSources.

Let say that we want to get position of the sources in the flux standard stack we produced in the previous chapter. First, we run detectSources since it was not run after the stack was created.

reduce -r detectSources N20160102S0296_stack.fits

This will add a table with object coordinates, magnitudes, etc. The output file is N20160102S0296_sourcesDetected.fits. Use your preferred tool to access the FITS table named OBJCAT from that output. Or you can use DRAGONS’ astrodata to access the table as an Astropy Table.

1import astrodata
2import gemini_instruments
3
4ad = astrodata.open('N20160102S0296_sourcesDetected.fits')
5ad.info()

Filename: N20160102S0296_sourcesDetected.fits
Tags: GEMINI IMAGE NIRI NORTH PREPARED SIDEREAL

Pixels Extensions
Index  Content                  Type              Dimensions     Format
[ 0]   science                  NDAstroData       (1195, 1195)   float32
          .variance             ndarray           (1195, 1195)   float32
          .mask                 ndarray           (1195, 1195)   uint16
          .OBJCAT               Table             (29, 43)       n/a
          .OBJMASK              ndarray           (1195, 1195)   uint8

6ad[0].OBJCAT

<Table length=29>
NUMBER  X_IMAGE   Y_IMAGE        ERRX2_IMAGE       ... REF_MAG REF_MAG_ERR PROFILE_FWHM PROFILE_EE50
int32   float32   float32          float64         ... float32   float32     float32      float32
------ --------- ---------- ---------------------- ... ------- ----------- ------------ ------------
     1 638.74554   7.358276  9.678123990684229e-06 ...  -999.0      -999.0       -999.0       -999.0
     2   52.5378  15.138286  0.0007821264390706773 ...  -999.0      -999.0          0.0    3.5255525
     3 500.59454  1146.9167   0.012262252608200411 ...  -999.0      -999.0     4.513517    6.1514697
     4 879.47473  1098.4614  2.762548642098426e-05 ...  -999.0      -999.0     4.222008    5.0135765
     5  99.23588  1012.4829  7.044837003541005e-06 ...  -999.0      -999.0    3.5682483    4.5748672
     6   67.8556 1023.36334  0.0036609541234138973 ...  -999.0      -999.0    3.7424104    4.9484925
     7   654.096  918.19916 2.5297123639012174e-05 ...  -999.0      -999.0    4.6524267     4.829588
     8 855.02795   830.9594  6.732778544641382e-06 ...  -999.0      -999.0     6.675581    6.2657185
     9 1026.4033    835.157     0.0433908185530153 ...  -999.0      -999.0     4.068429    12.851359
    10  380.1143  726.31464 0.00041709156417067195 ...  -999.0      -999.0     4.370194    4.4451947
    11 994.73755    721.932   0.012686314238205688 ...  -999.0      -999.0    4.6524267      9.30962
   ...       ...        ...                    ... ...     ...         ...          ...          ...
    18  854.6116  352.42896 1.5660364975379077e-05 ...  -999.0      -999.0     4.222008     4.193287
    19 522.56934  317.01556  0.0012430236331253412 ...  -999.0      -999.0    3.5682483     3.738548
    20 936.59576  242.70079  7.264929326425364e-06 ...  -999.0      -999.0     4.068429    4.1155357
    21  811.8014  225.92892   8.23835737153326e-05 ...  -999.0      -999.0    3.9088202    4.1611533
    22 478.92426  217.57622    0.02797088088581828 ...  -999.0      -999.0     8.812923       -999.0
    23  479.3753   206.3997   0.022581652547939684 ...  -999.0      -999.0    12.257336    12.230416
    24 478.64758  194.86026   0.009814622938623417 ...  -999.0      -999.0    2.7639532    15.229653
    25 521.57794  209.82019   0.029336651973282495 ...  -999.0      -999.0    3.1915383    16.864668
    26  479.6435  152.71979 3.3380840555892644e-06 ...  -999.0      -999.0    3.9088202    3.7304442
    27 955.26416   150.0823  7.081766055952015e-05 ...  -999.0      -999.0    3.7424104     4.108046
    28 561.57697  151.73418   0.021476893612064937 ...  -999.0      -999.0     6.675581     9.169356
    29 129.58281  107.06496  0.0009807660866281128 ...  -999.0      -999.0    3.3851376    3.6767333

See the astropy documentation for information about how to operate on Table objects: https://docs.astropy.org/en/stable/table/.

For more on astrodata, see the Astrodata User Manual.