Ground Calibration

A summary of the ground calibration activities are listed below. For details, please refer to following publications on the UVIT ground calibration and references therein.


Detectors and filters

The detectors were calibrated at the University of Calgary for the following:

  • characteristics of the signal of photon events, in photon counting mode, in CMOS imagers
  • settings of the thresholds for detection of photon events
  • systematic errors in estimating centroids (by the on-board hardware) of the photon events and flat field effects, i.e. pixel-to-pixel variation in the sensitivity, for FUV and NUV detectors in photon-counting mode (Postma et al. 2011)

It was found that, subject to any errors due to low-frequency variations in the beam used, the flat field effects were less than 15% p-p. The errors due to low-frequency variations in the beam are to be estimated from observations in orbit.

The following calibrations for the detectors were done at IIA:

  • quantum efficiency as a function of wavelength (Stalin et al. 2010)
  • distortion (Girish et al. 2017) - this calibration determines deviations of the scale from linearity, caused by the fibre-taper
  • transmission of the filters as a function of wavelength

The properties of all the filters are presented in Table 1. Based on the ground calibrations of the detectors and filters, and the reflectivity of the mirrors, effective areas can be estimated as a function of wavelength. The calculated effective areas were verified for the assembled telescopes by observations with a collimated beam generated from a calibrated point source. These matched the estimates within 25%.

Adopted name FilterWavelength (A)Bandwidth (A)
F148 W CaF2-11480 500
F148Wa CaF2-2 1485 500
F154 W BaF2 1540 380
F172 M Silica 1716 125
F169 M Sapphire 1607 290
N242 W Silica-1 2418 785
N242Wa Silica2 2418 785
N245 M NUVB13 2447 280
N263 M NUVB42632 275
N219 M NUVB15 2195 270
N279 N NUVN2 2792 90
V347 M VIS1 3466 400
V391 M VIS2 3909 400
V461 W VIS3 4614 1300
V420 W BK7 4200 2200
V435ND ND1 4700 1600

Assembled telescopes

The assembled telescopes, with the detectors and the filters, were calibrated with a collimated beam for variation in focus with temperature, and FWHM for a point source and its variation with filter and position in the field. Typical FWHM of the image (including the effects of any aberrations in the collimated beam) was found to be ~1.5" for FUV and NUV in photon counting mode. The variation in focus with temperature was found to be < 0.032 mm/oC. Therefore, the contribution of de-focus to the PSF would be < 0.15" rms for the variability of < 3oC in the temperature expected in the orbit.

Assembled payload

After assembly of the payload relative alignments of the three detectors were tested. The alignments were found to be within 50" (observations in orbit showed an angle of ∼70" between the centres NUV and FUV fields). The payload was also tested for vibrations and for thermal effects (in vacuum) as per the standard protocol of ISRO (Kumar et al. 2012). After these tests the payload was integrated with the satellite and further tests were done before the launch. In order to check the overall performance for imaging, simulations were done for imaging of a galaxy in FUV and NUV and the results showed that FWHM < 1.8" could be obtained (Srivastava et al. 2009).