SSO Update
Published in the RSAA Lunations
Vol1 Issue22 1–30 November 2021
A new dome flat field illumination system has been installed and tested on the AAT. The current dome flat field is illuminated by a single lamp located at prime focus access. The lamp gives a good smooth broad spectrum at longer wavelengths above 5000A but is faint at shorter wavelengths. The illumination of the flat field from a larger angle also creates some unevenness of the illuminated dome flat field screen.
It was decided that a new illumination system should be mounted on the telescope and be brighter at wavelengths shorter than 4500A. A series of LEDs were carefully chosen for both their spectral coverage and their efficiency. The initial array of LEDs consisted of broadband IR LEDs from 10000A to 6000A. Red, Cyan and Blue LEDs in combination with White LEDs would make up 6000A to 4000A and a series of UVA LEDs would cover wavelengths from 4000A to 3500A.
Construction began with the IR LEDs, but when they were tested they were bright blue. A closer look at the datasheets revealed that these LEDs were an IR phosphor pumped with a very bright blue LED and their efficiency was very poor. The Blue pump source was 1000 times brighter than the IR output.
Testing of the visible LEDs was then commenced and these produced unsatisfactory results. Light baffles had been constructed to minimise stray reflections. The baffles reduced the stray light very well, however the spatial difference between the LEDs caused the light to be projected onto the flat field unevenly. Good for tourists, bad for astronomy. The baffles were removed and the spatial illumination was good but the spectral flatness was poor due to the narrow bandwidth of LEDs. Testing of a single halogen lamp showed better results than all the visible and IR's combined. Sometimes old school just works and the visible and IR LEDs were abandoned.
Finally, the UV LEDs were investigated and tested. Each LED has a narrow bandwidth of 100A much like the visible LEDs, but with recent developments UVA LEDs are available for almost any wavelength from 2500A to 4300A and at very reasonable prices. Seven 3W UVA LEDs combined with a 25W Halogen lamp make up the full spectrum of the illumination system. Four of these lamp assemblies were constructed by AAO mechanical staff and mounted on the telescope top end. The flat field illumination is controlled directly with software via DRAMA from the instrument console. A safety interlock system was also incorporated into the control system to prevent UV exposure to personnel at prime focus access. Measurements and calculations were also conducted to confirm exposure levels would be negligible even without the interlock.
The new illumination system produces comparable counts between the red and the blue ends of the spectrum, which to the eye results in a smooth pinkish purple glow inside the AAT dome.
Testing of the system has now started. The second key aspect of the system is uniformity. We are aiming for deviations from uniformity of less than a percent. This level of uniformity is critical for calibrating data obtained with Hector, which is due to be installed on the telescope later this month.
Ashley Anderson and the SSO team
