| Description: | What does a telescope pupil look like at a wavelength of 9 microns? The answer can be found in today's image as it shows the pupil of ESO's VLT UT3 (Melipal) as seen by the ASTRON-built spectrometer of VISIR. As VISIR works in the thermal infrared (wavelength 7.5 - 28 microns), the instrument sees all parts of the telescope at "room" temperature as hot glowing surfaces. The different hot parts are labeled in the figure. To avoid straylight from outside the telescope mirrors M1 - M3, a slightly undersized cold (about 35K) diaphragm is placed in the re-imaged telescope pupil. The figure shows that cold stop and telescope pupil are not aligned perfectly, but the straylight is dominated by radiation from spiders and hole of the primary mirror. |
