TAUVEX Ground Calibration
From Tauwiki
This document gives an overview of the various tests that will be performed as part of the ground calibration of TAUVEX.
Contents |
[edit] SPECTRAL RESPONSE
The purpose of the test is to determine the absolute sensitivity of TAUVEX for different wavelengths and filters. A monochromatic source is placed at the focal plane of the detector. A calibrated photomultiplier is used to measure the total counts from the source which is then compared with the total counts in the TAUVEX images. This is repeated for all filters.
Given below is the equation for calculating the sensitivity of the instrument.
Sensitivity (throughput) = N*Fi*A *QEph/Nph events/(photon/cm2)
where,
Nph = number of counts in the calibrated photomultiplier (measured during the test)
N = number of events in the image in any telescope
A = area of photomultiplier
QEph = Quantum efficiency of calibrated photomultiplier at wavelength ~k (from calibration curve)
Fi = the ratio between “the number of counts in the calibrated photomultiplier at the image location” (which is the average value across the entire TAUVEX aperture) and “the number of counts in the calibrated photomultiplier at the photomultiplier location” measured during the mapping step.
[edit] DARK NOISE
This test is conducted in order to create dark noise maps for each telescope of TAUVEX. It is performed without illumination with the HVPS at nominal setting. Two sets of images are taken for all three telescopes for 30 minutes each. From each image, a map of the intensity of the dark noise of each telescope is obtained. From this we can get the dark noise count rate for each telescope.
[edit] DISTORTION AND ANGLE CALIBRATION
This test is performed in order to map the distortion in the TAUVEX telescopes and detectors and to verify that it is according to the required specifications.
A 200-hole target is illuminated with a white light source. The image of the target is recorded in all three telescopes with different filter combinations. Each spot on the image will be analyzed to find its centroid.2. The coordinates of the spot positions will be matched to a best fit array of equally spaced points, and the deviation of each spot from the corresponding spot of this array will be measured. The R.M.S. value of the ratio of deviations to the distance to the center of the image will be determined.
This map will be used for correcting the images. A comparison will be done between the maps obtained for different filters in order to evaluate the spectral dependence of distortion.
[edit] FLAT FIELDING
This test is performed in order to determine the flat field characteristics of TAUVEX. Non-uniformity maps of each telescope will be created using all the filters.The test is performed under ambient conditions as well as in vacuum. The telescopes atleast two) are uniformly illuminated with a Xenon lamp with the Wide Aperture Integration Sphere (WAIS) at the collimator focal plane, one by one and with different filters. In the case of ambient conditions, this test is performed for approximately 3.5 hours such that a total count of 280 xl0^6 is obtained at a rate of 24000counts/sec for each telescope and filter. From this a 3D map of intensities is obtained from which a response map is created for each pixel.
In the vacuum case, a Deuterium lamp is used with the integrating sphere in the collimator focal plane. Images are recorded with about 180 xl06 events at a rate of 24000counts/sec. In this case, there is only partial illumination of the central telescope and the entire FOV is covered in two steps.
From the images taken in air and vacuum, flat fields are created.
[edit] LINEARITY
This test is performed in order to
1. determine the linear portion of TAUVEX II response and
2. evaluate the non-linear portion of TAUVEX II response and its saturation level
The test is performed under ambient conditions and the source of illumination is a white light source (Halogen lamp) with an integrating sphere. An attenuator is placed between the lamp and the sphere. Events are recored for different attenuations and for different levels of illumination. For each point of interest of the telescope, plots are made of the count and event rates as a function of the illumination level. In the low illumination regime the response should be linear and saturate beyond some illumination level. These are measured and verified.
The same test is repeated using a two-hole-target and the change in the PSF with count rate is measured.
[edit] LOS and FOV
The LOS and FOV performance of TAUVEX is tested under ambient conditions. A single-hole target is used for the purpose together with a A white light source (Halogen lamp).
[edit] RESOLUTION
The resolution of TAUVEX is tested under ambient conditions as well as in vacuum. A 25-hole target is used for the purpose and is illuminated with a white light source. Images are recorded for each telescope with different filters. The image which consists of a pattern of spots is analysed and the FWHM of each spot and hence the spot size is determined.
[edit] STRAYLIGHT
The purpose of the test is to obtain the stray light attenuation as a function of angle under ambient conditions. The BBF filter is used. A collimator with a tungsten halogen lamp near the focal plane is used to point at TAUVEX at any desired angle. All other light sources are closed and TAUVEX is surrounded with black curtains to block any reflected light.
Measurements are taken initially with no illumination and then with illumination at different angles.From these images, the the stray light attenuation is computed.
