The Visible Emission Line Coronagraph (VELC), which serves as the main payload for the Aditya L1 mission, India's first space mission on solar research, will transmit 1,440 images every day to the ground station for analysis once it reaches the designated orbit.
The largest and most technically demanding payload aboard Aditya-L1 underwent integration, testing, and calibration at the CREST campus of the Indian Institute of Astrophysics (IIA), in Hoskote, near Bengaluru. This process involved significant collaboration with ISRO. Aditya L1, launched on the PSLV-C57 rocket, carries seven instruments for solar study, with four dedicated to observing solar light and three focused on measuring plasma and magnetic field parameters directly at the source.
Aditya-L1 will be positioned in a halo orbit encircling the Lagrangian Point 1 (L1), located approximately 1.5 million kilometres away from Earth, which is just about 1 percent of the distance between Earth and Sun, aligned to the Sun's direction. It will maintain a constant relative position with respect to the Sun, ensuring continuous and uninterrupted observation of the Sun.
"From the continuum channel, which is the imaging channel, an image will come - one image per minute. So, approximately 1,440 images for 24 hours, we will be receiving at the ground station," Aditya L1 Project Scientist and Operation Manager for VELC Dr Muthu Priyal said.
She explained that the IIA will serve as the hosting site for the VELC Payload Operations Centre (POC). It will receive raw data from ISRO's Indian Space Science Data Centre (ISSDC), process the data to prepare it for scientific analysis, and then return the processed data to ISSDC for distribution.
"Also, a unique software has been developed by the IIA to detect automatically the occurrence of coronal mass ejection and the time it happened, which will be provided to the science community within 24 hours - whether it is directed towards the earth or whether it is a very energetic event, if the speed is high, will it hit the earth or not, etc - all those information will be made available," she added.
As per statements from IIA officials, the 190-kg VELC payload is expected to transmit images throughout the satellite's nominal five-year lifespan. However, its operational duration could extend further, contingent on factors such as fuel usage.
IIA’s K Sasikumar Raja said that out of four channels, the continuum channel will run independently and send 1,440 images per day, while the other three spectroscopy channels will also provide images, but that is dependent on how the observer wants.
The IIA Scientists expect the first images to be available towards the end of February. "The satellite is expected to be put into the orbit in the middle of January and then we will do the test if all the systems are working properly and by the end of February we expect to get the regular data. It will take time and we have to test instrument by instrument. First we will test the smaller instruments, and the VELC's shutter will be opened last, by middle of February," stated Prof Jagdev Singh.
"We call it cross contamination. On reaching L1 orbit, the other payloads will be starting first, so that the outgassing coming from the other payloads, should not go and deposit on the primary mirror, which is very super polished and any deposition on the primary mirror will lead to scattering, which can mask the complete coronal mass, which is the field of interest," S Nagabushana explained why the VELC shutter will be opened at the last.
The Sun's outer atmosphere, known as the Corona, becomes visible to us during a total solar eclipse. Instruments like VELC, a coronagraph with 40 different optical elements (mirrors, gratings, etc.) inside it, are designed to block out the intense sunlight from the Sun's surface, enabling continuous imaging of the significantly fainter corona, IIA scientists explained.
The primary scientific objectives of the Aditya-L1 mission are to gain insights into the source, movement, as well as the behaviour of Coronal Mass Ejections and to contribute to resolving the longstanding mystery of why the Sun's corona is so much hotter than its surface, a phenomenon known as the Coronal Heating Problem.
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