Photometric Calibration CookBook

Point-source Photometry for NIC1 and NIC2

The newest values (March and May 1998 delivery) of the photometric keywords supplied in the headers of the NICMOS images for cameras 1 and 2 have been obtained in a specific way. The photometric keywords refer to a nominal infinite aperture defined as having 1.15 times the flux in an aperture with 0.5 arcsec radius, irrespectively of filter.

A brief summary of the steps needed to perform absolute photometric measurements in NIC1 and NIC2 images follows:

  1. Calibrate the image using the newest dark and flat reference files and photometric table.
  2. Perform photometry of the objects in the image using aperture photometry, PSF-fitting photometry or any other technique.
  3. Correct the measured total countrate to a 0.5 arcsec radius aperture (size of the aperture used for the standard star aperture photometry measurements).
  4. Multiply the countrates by 1.15 (i.e. correction to a nominal infinite aperture).
  5. Convert countrates into flux using the photometric keywords.
  6. Apply the flux to magnitude conversion using the fluxes given for a zero magnitude star (see values listed in column ZP(Vega) in section Current Values for NICMOS Photometric Keywords and Zeropoint in VEGAMAG System), if magnitudes are desired.

Point-source Photometry for NIC3

Accurate absolute photometry with NIC3 is more challenging than NIC1 and NIC2 photometry, in particular for data taken with the camera out of focus, i.e. outside the January and June 1998 campaigns.

The NIC3 photometric keywords listed in the newest photometric table (May 1998) refer to a nominal infinite aperture defined as having 1.075 times the the flux in an aperture with 1.0 arcsec radius, irrespectively of filter.

A brief summary of the steps needed to perform absolute photometric measurements with NIC3 images taken during the campaigns:

  1. Calibrate the image using the newest dark and flat reference files and photometric table.
  2. Perform photometry of the objects in the image using aperture photometry, PSF-fitting photometry or any other technique.
  3. Correct the measured total countrate to a 1.0 arcsec radius aperture (size of the aperture used for the standard star aperture photometry measurements).
  4. Multiply the countrates by 1.075 (i.e. correction to a nominal infinite aperture).
  5. Convert countrates into flux using the photometric keywords.
  6. Apply the flux to magnitude conversion using the fluxes given for a zero magnitude star (see values listed in column ZP(Vega) in section Current Values for NICMOS Photometric Keywords and Zeropoint in VEGAMAG System), if magnitudes are desired.

NIC3 images obtained outside the January and June 1998 campaigns are out of focus, the exact amount depending on when the data were taken. The TinyTim software allows to generate NIC3 PSFs for PAM positions not equal to best focus. These modeled PSFs can be used to compute the necessary aperture corrections when performing absolute photometry.