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Beam-shaping diffusers for ALFOSC to do high-precision photometry

To observe planet transits, high-precision photometry is needed. A standard technique is to defocus the telescope, such that the star image is spread over many pixel. This allows to detect more photons from the object without saturating the detector, increasing the observing efficiency, but more importantly, it limits flat field errors as they are averaged over a larger area, and are less affected by changes in atmospheric seeing. However, the resulting shape of the stellar profile is very irregular, and variable, still effected by atmospheric effects. The use of diffusers can effectively scrambled the incoming light, providing a more constant and optimal (`top-head’) shape, with minimal light loss, significantly reducing any systematic effects in the photometry. Dr. Carolina von Essen from the Stellar Astrophysics Centre, Aarhus University, Denmark, has obtained funding from IDA in Denmark, to purchase such diffusers for use with ALFOSC.

Two Engineered Diffusers have been ordered from RPC Photonics, and will arrive at the NOT in March. Currently, we have a diffuser on loan from Dr. Gudmundur Stefansson (Pennsylvania State University), which has the same characteristics as one of the two diffusers on order. With ALFOSC, this diffuser provides a very stable stellar image with a width (~FWHM) of 5.46 arcsec (see for some test results here). Detailed results from data obtained with this diffuser at the ARC 3.5m telescope can be found here. The second diffuser on order is expected to give a stellar image in ALFOSC with a width of 8 arcsec.

Name ID number Diameter Angle (deg) FWHM (FASUA) FWHM (FASUB) Available Documentation
Diffuser#1 664 90 mm 0.35 4.8" 5.3" Since Aug 2018 Diffuser#1 sheet and Spectral Coating curve
Diffuser#2 665 90 mm 0.50 TBD TBD Since Oct 2018 Diffuser#2 sheet and Spectral Coating curve

To use the diffusers together with a filter, the script "alfosc.wheels" has been updated to take the ID number of the diffuser as an input with the "-d" flag.
For example the items Diffuser#1 and the R-band (#76) filter are entered with:

        alfosc.wheels -d 664 -f 76

The diffusers are open for use to all observers at the NOT. When using the current test diffuser on loan, please acknowledge Dr. Gudmundur Stefansson from the Department of Astronomy & Astrophysics of The Pennsylvania State University, USA, in any publication. When using one of the new diffusers, please acknowledge Dr. Carolina von Essen from the Stellar Astrophysics Centre, Aarhus University, Denmark.

Back to top Last modified: 16-Oct-2018