Variations in temperature can effect observations in several ways. Relative thermal expansion shifts focus of the telescope, and surface of the mirrors can get distorted. To minimize these effects, temperature of the telescopes is kept 200 +- 30 C and the structure is made of Invar. The mirrors are mounted on flexible supports made of Invar to minimize thermal stresses. There is no provision for adjustment of the focus in orbit, but the effect of temperature variations on the focus is expected to be small.
Transmission of the filters/gratings and quantum efficiency/gain of the detectors too can change with temperature. The temperature of these elements is kept in range 150 C to 300 C. The thermal effects are not calibrated on ground. As the photon counting mode is not very sensitive to the gain of the detector, this mode is affected less than the integration mode.
Correction for the drift is based on assuming that there is no relative drift between the three channels. Temperature variations can lead to relative drift between the channels, and this can only be modeled with in- orbit data.
UVIT background has several sources. These are discussed below.
At 20 degree C the dark photon counts are (as per the test data from Photek, UK , who supplied the CPUs of the detector systems):
At angles less than 45 from the axis, bright earth can give background which is in excess of the Zodiacal light.
Zodiacal light is ~ mag 22 per sq arcsec in the V band. This corresponds to ~ 5 x10-18 erg/(s A cm2 arcsec2) or ~ 0.08 detected photons per second per sq arcsec for an effective area of 50 sq cm for a band 5500 A to 6500 A.
This background falls very slowly between 6000 A and 3000 A and it falls very fast at the shorter wavelengths. The photon flux per A is less by ~ 6 (300) at 3000 A (2000 A) as compared to that at 6000 A.
Geocoronal lines are excited by solar radiations and vary in intensity by large factors. Typical counts of detected photons in the detectors, without filter, are listed below:
It is clear that due to these lines observations cannot be made during the daytime in FUV channel. Further, the CaF2 filter (the widest band filter in FUV) could have leak of a few percent for Ly ά, and the nighttime counts could be in range ~ 1000/s to 5000/s.
Photon Counting and Integration Modes
The detectors can operate in two distinct modes: i) Photon Counting Mode
ii) Integration Mode
In the photon counting mode, a very high multiplication is obtained in the MCPs such that each photo- electron generated in the short exposure (<100 ms) is detected as a light pulse in the CMOS imager, and its centroid is found. Thus, for each frame of exposure a list of centroid-positions is obtained from the detector. The expected spatial resolution in this mode is < 1.8” FWHM.
In the integration mode, multiplication of the MCPs is kept low, and signals in all the pixels of the CMOS detector are obtained. In this mode, many (weak) pulses of light, from many photo-electrons, could fall at the same location; the signal at any location is a measure of how many photo-electrons were detected.
Photon counting mode gives a much better spatial resolution (<1.8” FWHM for FUV and NUV channels) as compared to the integration mode (~ 5” FWHM) . However, if in a single exposure two or more photon events occur within a separation ~< 3 pixels of the CMOS imager (~ < 10”), these are detected as a single event or rejected as unacceptable event (see later discussion on choice of modes).
For imaging full field, minimum exposure time is ~ 34 ms. Therefore, given the background levels, imaging in the VIS channel in photon counting mode would lead to overlapping photon events. Further, the total flux of visible photons is such that in photon counting mode life of the MCPs would be exhausted in about one year. Hence, imaging in the VIS channel is normally done in integration mode.
Avoidance of ram-angle, Sun, and bright-Earth for safety:
In order to avoid any damage to coating of the primary mirror, due to atomic oxygen, a minimum angle of 120 is kept between the ram direction and the roll-axis, i.e. axis of UVIT. In order to avoid damage/UV-assisted contamination due to radiation from Sun/bright-Earth, a minimum angle of 450 / 120 is kept between the axis and Sun / bright-Earth at all times even if UVIT is not observing.
Avoidance of bright objects:
Every field to be observed is checked for avoidance of very bright objects in a neighborhood of 20 radius; from 20., 1 part in 104 of the FUV flux could be scattered in the field.
Saturation with bright objects:
Any bright object which can give a detected photon count rate >1 count pr frame per ~ 10” X 10” would see serious saturation effects. Thus, to observe relatively bright objects in photon-counting mode a high frame rate can be used with partial field of view. (Minimum exposure time for the full field is ~ 34 ms, and it is ~ 2 msforafieldof6’X6’.
Choices in Photon Counting Mode and Integration Mode
The nominal mode of observations for the ultraviolet channels is photon counting. Due to its large background, nominal mode of observations in the visible channel is integration mode with a low gain of MCP. To observe bright sources in the ultraviolet, either a high frame rate with a reduced (windowed) field can be used or the integration mode can be used. (As mentioned earlier, the spatial resolution in the integration mode is ~ 5” as compared to < 1.8” in the photon counting mode.)
In the photon counting mode a set of choices are available, in the hardware of the detector system, for selecting: a) thresholds for signals, in the pixels of Star250, to define a genuine photon event, and thus reject noise; a large threshold would reduce effective quantum efficiency, while a small threshold would bring in more noise-events, b) choice of centroiding algorithm for the events from a 3 square/5 square/3 cross windows. In addition, during the ground processing, a threshold can be set for rejecting bright pixels at the corners of the events to eliminate close pairs of photon events – this is required to minimize errors in determination of position of the centroid.
In the integration mode, the hardware allows selection of the gain of MCP by control of the high voltage. Thus, for a very bright source a very low gain can be selected to avoid saturation of the pixels of Star250.
Observing in Partial Fields
For the full field (~ 28’ circle), the maximum double frame rate is ~ 29/s. For those sources which have an intensity > ~ 5 photons/s/(10” X 10”), many frames would have close double photon events at this frame rate. In order to avoid double photon events for such sources a higher frame rate can be obtained by selecting a partial field for observations: the field is selected in units of pixels of Star250. The maximum frame rate is roughly in inverse proportion to area of the field. Thus, given the full field as 512 X 512 pixels, a rate of >600 frames/s is obtained for a field 100 X 100 pixels.
Soft X-ray Telescope
Soft X-ray imaging Telescope (SXT) onboard Astrosat will be sensitive to soft X-rays in the energy range of 0.3 − 8 keV. X-rays in this energy range are amenable to focusing and will lead to: a) nearly 1000 times better sensitivity over non-focusing instruments of similar areas making over 10,000 sources detectable; b) separation of confusing sources; c) arc min imaging; d) spatially resolved spectroscopy; and e) variability studies.
SXT will cover a very important energy range of the broadband spectrum observed with Astrosat, and will be able to investigate the following scientific problems.
1. Resolving the K line emission from Si, S, Ar, Ca and Fe in hot thermal coronal plasmas, as well as
fluorescent line emission from these elements in the medium photo-ionized by strong X-ray continuum in accretion powered X-ray sources (neutron stars, stellar mass black-holes, supermassive black-holes etc.).
2. Carrying out spectroscopy of hot thin plasmas in galaxies, clusters of galaxies, nuclei of active galaxies, quasars, supernova remnants and stellar coronae.
3. Studying the physics of shocks and accretion disks, coronae, photo-ionized regions and their density, temperature, ionization degree, and elemental abundance.
4. Studying low energy absorption and the nature of absorbers, for example, whether these are cold (neutral) or warm (ionized).
5. Studying soft X-ray excesses due to a blackbody emission in AGNs, and in binary X-ray pulsars in conjunction with other higher energy X-ray instruments.
6. Carrying out Spatially resolved spectroscopy of Supernova Remnants and Clusters of galaxies.
7. Carrying out simultaneous wide-band spectral studies and time-resolved spectra of thermal as well as non-thermal plasmas in the universe using the unprecedented combination with
sensitive hard X-ray detectors.
The telescope consists of a tubular structure housing the X-ray reflecting mirrors and other components. There is a “Charge Coupled Device” (CCD) camera at the focal plane of the mirrors in order to image the cosmic sources.
The basic components of the telescope (apart from the mirror assembly) are given below (see Fig. 3.1).
a) A deployable cover/door at the top end of the telescope which covers the optical elements on the ground and protects them from contamination. It will be deployed in 4 to 6 weeks after launch. It can be closed only manually on ground. Once opened in space, it will be deployed at an angle of 270o.
b) A “Thermal Baffle” placed between the mirror assembly and the telescope door. All parts are made up of aluminum alloy 6061 T6. The function of thermal baffle is to protect the telescope from the Sun, and to provide a base for mounting the heaters to maintain the optics within a certain specified range of temperatures, and to block the unwanted area of the optics. The sun avoidance angle with the thermal baffle is ~45o.
c) A “Forward tube” made up of Composite Fiber Reinforcement Plastic (CFRP). Forward tube extends from the bottom of the “Top Lid”. It covers the thermal baffle assembly.
d) Ring 2 as an interface ring between the forward tube and the middle flange of the optics. Ring 1 provides an interface between the Rear tube-2 and the middle flange of the optics.
e) Rear tube-1 made up of CFRP is a hollow cylinder of diameter 343 mm ID and 347.8 mm OD. Rear tube-1 extends from Ring-1 to “Deck Interface Ring” (DIR). It houses 3α Optics while forward tube is the house for 1-alpha assembly (see below).
f) “Deck Interface Ring” (DIR) is made up of Al alloy 6061 and is used to assemble rear tube-1 and rear tube-2 to the main deck of the satellite.
g) Rear tube-2 made up of CFRP is a hollow stepped cylinder with a top portion thicker than the bottom portion to provide stiffness. Rear tube-2 extends from the DIR to the CCD interface ring.
h) CCD interface ring is provided to align the CCD Camera with the tubular structure to the desired accuracy. This is made up of aluminium
Using ASTROSAT as a broad-band Observatory
So far we have discussed the details of individual payloads on ASTROSAT and its capabilities. However, since all these experiment are co-aligned (except SSM) and mostly operates in unison, ASTROSAT can effectively conduct broad-band multiwavelength observation from Optical to hard X- rays for select sources in the sky. Here we explore briefly the nature of such potential investigations.
For correlated variability in soft and hard X-rays use time tagged photons data from SXT and LAXPC.
• Correlated variability in X-ray and UV bands require time tagged data from X-ray instruments and fastest possible photometric data from UVIT.
• To study simultaneous broad-band X-ray spectrum from 0.3-100 keV, combine data from SXT, LAXPC and CZT and perform simultaneous spectral fits in XSPEC (X-ray analysis package).
• Search of Cyclotron absorption feature (usually in ~10 keV to 60 keV ) needs well calibrated spectral data with high statistics from LAXPC and CZT instruments.
• For constructing multi-frequency spectra (Spectral Energy Distribution) of AGNs, CVs, SNRs etc, simultaneous observations with all 4 instruments over a time scale less than the variability time of a source is needed.
Observing with ASTROSAT
Introduction ASTROSAT orbit Observing Constraints
• Celestial constraints
• Sky Visibility
• Bright source avoidance
• Multiwavelength observations
Field of View of Instruments Instrument overheads
Observing Policies and Procedures
• ASTROSAT Time sharating plan
• Observing Cycles
• Proposal Types
• ToO proposals
• Proposal review process and time allocation