Shaping the PSF to nearly top-hat profile: CHEOPS laboratory results

Abstract

Spreading the PSF over a quite large amount of pixels is an increasingly used observing technique in order to reach extremely precise photometry, such as in the case of exoplanets searching and characterization via transits observations. A PSF top-hat profile helps to minimize the errors contribution due to the uncertainty on the knowledge of the detector flat field. This work has been carried out during the recent design study in the framework of the ESA small mission CHEOPS. Because of lack of perfect flat-fielding information, in the CHEOPS optics it is required to spread the light of a source into a well defined angular area, in a manner as uniform as possible. Furthermore this should be accomplished still retaining the features of a true focal plane onto the detector. In this way, for instance, the angular displacement on the focal plane is fully retained and in case of several stars in a field these look as separated as their distance is larger than the spreading size. An obvious way is to apply a defocus, while the presence of an intermediate pupil plane in the Back End Optics makes attractive to introduce here an optical device that is able to spread the light in a well defined manner, still retaining the direction of the chief ray hitting it. This can be accomplished through an holographic diffuser or through a lenslet array. Both techniques implement the concept of segmenting the pupil into several sub-zones where light is spread to a well defined angle. We present experimental results on how to deliver such PSF profile by mean of holographic diffuser and lenslet array. Both the devices are located in an intermediate pupil plane of a properly scaled laboratory setup mimicking the CHEOPS optical design configuration.

Toward Space-like Photometric Precision from the Ground with Beam-shaping Diffusers

“…Engineered Diffusers TM are available with direct etching of the diffusing surface on fused silica, making it suitable for space applications due to its radiation resistant properties. As such, Engineered Diffusers TM have been considered in the lab for space use for the CHEOPS mission by Magrin et al (2014). Their aim was to shape the PSF to a nearly top-hat with 30" FWHM, corresponding to a 30pixel FWHM due to the CHEOPS plate scale of 1".0/pixel.…”

“…Their aim was to shape the PSF to a nearly top-hat with 30" FWHM, corresponding to a 30pixel FWHM due to the CHEOPS plate scale of 1".0/pixel. Magrin et al (2014) studied in the lab the PSF shaping capabilities an off-the-shelf Engineered Diffuser TM (part number: EDC-0.25-A-1r), with an opening angle of 0.25 • . This is the same 0.25 • polymer diffuser pattern as we studied (see Table 2), replicated on a 1" circular substrate instead of the 2"x2" square substrate diffuser we studied.…”

“…This technology is versatile, offers broad-band compatibility, and is capable of stabilizing stellar PSFs with diffusers placed in either converging or collimated beams. While such diffusers have been briefly explored in the context of precision photometry for the upcoming CHaracterising ExOPlanet Satellite (CHEOPS) mission (Magrin et al 2014), they were not part of the final CHEOPS design. This work represents the first published results of detailed characterization, testing, and on-sky results using diffuser-assisted photometry.…”

CHEOPS: a space telescope for ultra-high precision photometry of exoplanet transits

“…A figure of merit to evaluate the performance has been derived by considering the optical PSFs, the jitter effect and the flat field performances. The result of the simulations has pointed out that a defocused PSF gives sufficient performance to meet the requirements (see Figure 2, [13]). PSFs have been generated at the defocused focal plane (about 3.5 mm from the nominal focal plane) with a flat spectral wavelength between 400 and 1100 nm with a spectral sampling of 50 nm.…”

CHEOPS: a space telescope for ultra-high precision photometry of exoplanet transits

“…The result of the simulations has pointed out that a defocused PSF gives sufficient performance to meet the requirements (see Fig. 3, [5]). PSFs have been generated at the defocused focal plane (about 3.5 mm from the nominal focal plane) with a flat spectral wavelength between 400 and 1100 nm with a spectral sampling of 50 nm.…”