| In our series about colleagues who are still active at 70, this is a somewhat unusual picture of Hans. Normally, he appears to be a severe man, with much to be severe about. This causes some people to be a little intimidated sometimes. However, this is unjustified, because, according to him, this picture is much closer to how he actually feels. Keep that in mind, the next time you meet him in the corridor.
Hans' long association with the Westerbork Synthesis Radio Telescope can only be called a Labour of Love. He started in Leiden the early 70s, reducing the data for the mob of frantic WSRT users, who were busily skimming the cream with this wonderful new telescope. Many a famous radio astronomer vividly remembers his morning entry into the coffee-room, contemptuously flinging the new maps onto the ping-pong table amidst an eager stampede.
After the move to Dwingeloo, Hans designed and largely implemented the on-line software system for observations with the new Digital Line Backend (DLB) of the WSRT. He then gradually extended the system to deal with the Digital Continuum Backend (DCB), several series of front-ends, new dish controllers, VLBI backends, etc, etc. He developed robust position and pointing calibration, and pioneered the holography method for measuring dish surfaces. And it all worked, down to the 16th decimal if necessary. Hans was the one who knew the WSRT from beginning to end, and who could quickly implement new observing modes, e.g. for high time-resolution solar observations, or mosaicking for the WENSS survey.
In all those years, Hans had one brief fling with another telescope. Together with Rudolf le Poole he developed some clever calibration software for SCUBA, a bolometer array for the JCMT mm telescope on Mauna Kea. Unfortunately, unanticipated microphonics in the SCUBA hardware turned out to be the limiting factor.
During the 90s, the WSRT was upgraded with a new (DZB) backend, new (MFFE) frontends and an entirely new Telescope Management System (TMS). Hans loyally helped to finish the TMS and get it to work, and quietly re-introduced some of his old diagnostic tools when some of the new software did not materialize quickly enough. Despite being pensioned off, he still shows up for a few days a week, and he still is the one who knows the WSRT system best.
We are now on the verge of yet another WSRT software system, to deal with the focal-plane arrays of the Apertif project. Designing and implementing such a system is a daunting task, in which Hans will obviously only play an advisory part. But it is often forgotten that keeping it running reliably, day after day, year after year, is at least as big a challenge. SRZM, NFRA and ASTRON have been lucky to have Hans around.