Frank Höller

Absolute measurement of non-comatic aspheric surface errors

An absolute test for rotationally symmetric aspheric surfaces was developed. All non-rotationally symmetric surface errors apart from those of comatic nature were determined interferometrically. For this purpose, the asphere was measured 12 times under different angles of rotation. The difference of a single interferogram and the average of all 12 interferograms gave the non-rotationally symmetric errors in an absolute manner apart from the comatic errors. With the purpose of noise reduction, the so-determined 12 interferograms of the non-rotationally symmetric errors were appropriately rotated and subsequently averaged. The rotationally symmetric errors were detected via stylus profilometry. Repeatabilities of the so-obtained aspheric surface errors are presented.

Extended depth of focus as a process of pupil manipulation

During the last few years, the understanding of pupil plane manipulation capabilities to achieve an enhanced image formation process and its theoretical limitations becomes more and more important. Particularly the increased potential of computer calculations and digital signal detection makes it possible to simulate this kind of manipulations very fast and efficient. At the same time there is the opportunity to change the optical system such, that a digital reconstruction of the image gives a higher amount of information. An analysis of the optical transfer function as an important parameter of imaging quality with special interest in extended depth of focus is presented. The performance of different pupil plane masks is illustrated in comparison with standard optical systems. This means the basic features like depth of focus, resolution and contrast were derived and the limitations are shown. The mathematical principle of extended depth of focus with pupil manipulation is described and demonstrated with exemplary calculations. Furthermore, the relation between a given optical transfer function and the matching pupil function is shown. A robust and iterative algorithm is presented to calculate a pupil mask for a desired optical transfer function.

The cold focal plane chopper of HERSCHEL's PACS instrument

HERSCHELs 3.5 m primary mirror will be passively cooled to T ~ 80 K in the L2 orbit. In order to reduce the effects of the remaining high thermal background on the sensitive far infrared detectors (60..210 μm), a focal plane chopper is a vital element in the entrance optics of the imaging and spectroscopic instrument PACS. A gold coated 32 × 26 mm2 plane mirror, suspended by two flexural pivots and driven by a linear motor, allows for precise square wave chopping with up to 9° throw at a frequency 10 Hz with a position accuracy of 1 arcmin. The power required at T ~ 4 K is about 1 mW. The chopper has undergone an extensive qualification programme, including 650 million cold chop throws, 15 cold-warm-cold thermal cycles, 3-axis 26 G-vibration at T ~ 4 K etc. Five models were built and thoroughly tested; the flight model of the chopper is now integrated into the flight model of PACS, ready for the HERSCHEL/PLANCK launch in 2008 by an ARIANE5 rocket and the following 5-year mission.