BUSCA: An instrument for simultaneous direct imaging in 4 colour bands.

BUSCA was developed as a cooperative effort at the Astronomical Institutes of Bonn University as a common user instrument for the 2.2m telescope at Calar Alto observatory. The instrument was realized in part through grant 05 2BN114 of the Federal Ministery of Education, Science, Research and Technology (BMBF). It offers direct imaging of the same field in four colours simultaneously. It is based on dichroic beam splitters and four large area CCD systems.


In April 2001 BUSCA was commissioned at the 2.2m telescope at Calar Alto.

Since the observing periode 2001/2002 observers can apply for observing time with BUSCA.

Find out more about BUSCA in the report about the first Calar Alto campaigns.


The optics are designed for 4kx4k CCDs with 15µm pixels to cover a field of view of 12'x12' at the 2.2m. The four colour bands, as defined by the dichroic beamsplitters, are at lambda < 430nm, 430nm < lambda < 530nm, 530nm < lambda < 700nm and lambda > 700nm. The band edges have been choosen, so that the Strömgren as well as the Thuan-Gunn filter systems can be used and no prominent emission line falls on them. The first available filter system will be the Strömgren system. The filter wheels in the 4 colour channels carry up to 4 filters each. Various combinations of broadband and narrowband line filters are possible.

The dichroic beam splitters are made of plano-parallel glass plates operating at an angle of 45 degree. This causes astigmatism in the converging beam. An opto-mechanical layout was therefore choosen which cancels the astigmatism (to the greatest part) by the introduction of another plano-parallel glass plate of suitable orientation into the optical train. TEST a telescope/star simulator (f/8) has been build to verify the imaging quality of the optical setup and to prove the promising results of the optical design calculations.

The CCD control electronics and the data acquisition system will be able to master 4 CCD systems each having up to 4 read out channels (2x2 CCD mosaic or large CCD's with multiple read out amplifiers). However the first version of the instrument will be equipped with "conventional" 2kx2k LORAL CCD's one of which is a backside illuminated thinned device with high QE below 400nm. This part of BUSCA is closely related to the BOCCIA project.


A publication to appear in SPIE, Vol. 3649 as HTML document
Last Update: March, 2002
Klaus Reif (E-mail: reif@astro.uni-bonn.de)