ORCASat Scientific Objective
DEMONSTRATE NEW TECHNOLOGIES FOR CALIBRATING EARTH-BASED TELESCOPES BY PROVIDING A REFERENCE LIGHT SOURCE IN ORBIT
Why?
Ground-based telescopes measure how bright astronomical objects appear to be, not how bright they actually are. When observing astronomical objects, the light that is measured passes through the atmosphere and the optics of the telescope. These mediums cause the light to be attenuated, and the exact amount is difficult to determine. By the time the light reaches the telescope's sensor, there is an uncertainty on the measurement due to the attenuation of light from the atmosphere and optics. There are good atmospheric models which can predict how much light is lost, but there is still a significant component of the atmosphere which changes rapidly (aerosols) and its contribution to the attenuation of light passing through it is difficult to predict using existing models.
ORCASat will demonstrate that ground-based telescopes can be calibrated for this undesirable attenuation by providing a reference light source in orbit that can be viewed by a telescope on Earth. ORCASat is carrying two laser light sources and it is able to measure the amount of light (the radiant flux) that these light sources are emitting. Astronomers can measure how bright ORCASat appears, just as they would an astronomical object. At the same time that a telescope is measuring the light from ORCASat, the satellite will measure how bright it actually is. ORCASat also measures its altitude (how high it is above sea level), attitude (where it is pointing in space), position (where above Earth), health of the spacecraft, and health of light source. ORCASat is also carrying a GNSS receiver which allows it to sync the clock on the spacecraft to UTC to ensure the measurements from ORCASat have accurate timestamps. From this data, the astronomers can use models to determine how bright ORCASat should have appeared to the telescope on Earth. The difference between how bright ORCASat appeared to the telescope, and how bright it should have appeared is the amount of light that was attenuated by the atmosphere and/or the telescope optics. Once this is characterized, the telescope can then take more accurate measurements of the absolute brightness of astronomical objects. The ORCASat light source is SI traceable, meaning the absolute brightness and spectrum of ORCASat is well understood and characterized. This is the first satellite ever to carry a light source capable of performing this experiment to this level of absolute accuracy.
wHY IS ABSOLUTE BRIGHTNESS IMPORTANT?
The absolute brightness of astronomical objects is important to many areas of research, but one application of particular interest for the ORCASat mission is to reduce the uncertainty of measuring the accelerating expansion rate of the Universe. These measurements can be performed by surveying the absolute brightness of Type 1a supernova (SN1a). The brightness of these supernovae tell us how far away they are and from that we can determine how fast they are moving away from us. If we know how fast astronomical objects are moving away from us, then we can measure how fast the universe is expanding. ORCASat will be able to further reduce the uncertainties on how bright these supernovae are, and demonstrate that this method will increase the accuracy of universal expansion rates.
What is the light source onboard ORCASat?
ORCASat is carrying a custom-designed light source as a payload. This payload consists of:
Laser light source 1: 120 mW 660 nm laser (Oclaro HL65051DG), constant power driver.
Laser light source 2: 175 mW 840 nm laser(Thorlabs L840P200), constant power driver.
Integrating sphere: 5.08 cm (ID), 1.27 cm (ID) exit port, and a sphere multiplier of ~17.
Photodiode 1: Hamamatsu InGaAs G10899-03K photodiode.
Photodiode 2: Hamamatsu Si S12698-01 photodiode.
Temperature sensors: laser diodes, photodiodes, and electronics.
Laser current sensors.
Transimpedance amplifier: Custom transimpedance amplifier electronics to convert photodiode photocurrent into a digitized value that can be downlinked from the satellite.
The ORCASat payload light source has been calibrated at the National Research Council Canada via their Photometry and radiometry calibration service. The payload light sources are programmable and can operate for up to 30 seconds with the TIA sampling at 1000 kHz. Longer illumination times can be programmed if the sampling rate of the TIA is reduced. The payload can either pulse or leave both of the light sources individually or simultaneously. The pulse profiles for each laser can be different. The lasers can also be dimmed down to about 10% of their rated output power.
ORCASat Payload (annotated)
Where can I find more information?
Good question, you can visit the ALTAIR Project website, which is what ORCASat is inspired from. Co-investigator Prof. Justin Albert is using weather balloons to fly a larger and more sophisticated version of the ORCASat light source. Please visit the references below: