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™ were ordered from RPC
The distances from the diffuser filter surfaces to the focal plane are
117.00 mm when mounted in FASUB and 100.50 mm when mounted in FASUA.
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 one
of the new diffusers, please acknowledge Dr. Carolina von Essen from
the Stellar Astrophysics Centre, Aarhus University, Denmark.
In 2018 we had a diffuser on loan from Dr. Gudmundur Stefansson (Pennsylvania
State University), which has the same characteristics as Diffuser#1,
see table above, albeit of a smaller size. With ALFOSC, this diffuser
provided 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 in
Stefansson et al. 2017 ApJ 848. When using the test diffuser on loan
during 2018, please acknowledge
Dr. Gudmundur Stefansson from the Department of Astronomy & Astrophysics of
The Pennsylvania State University, USA, in any publication.