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.
- Calibration and Performance of the Photon-counting Detectors for the Ultraviolet Imaging Telescope (UVIT) of the Astrosat Observatory
- Tests and calibration on ultra violet imaging telescope (UVIT)
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 | Filter | Wavelength (A) | Bandwidth (A) |
| FUV: | |||
| F148 W | CaF2-1 | 1480 | 500 |
| F148Wa | CaF2-2 | 1485 | 500 |
| F154 W | BaF2 | 1540 | 380 |
| F172 M | Silica | 1716 | 125 |
| F169 M | Sapphire | 1607 | 290 |
| NUV: | |||
| N242 W | Silica-1 | 2418 | 785 |
| N242Wa | Silica2 | 2418 | 785 |
| N245 M | NUVB13 | 2447 | 280 |
| N263 M | NUVB4 | 2632 | 275 |
| N219 M | NUVB15 | 2195 | 270 |
| N279 N | NUVN2 | 2792 | 90 |
| VIS: | |||
| 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).
