A Novel Double-Imaging CAMera (ANDICAM)

The ANDICAM is an imaging instrument capable of simultaneously recording images at an optical and an infrared wavelength. The primary purpose of the instrument is for microlensing event follow-up to look for the presence of planets and other other anomalous behaviour. There are two copies of the instrument built by Ohio State: ANDICAM, which is used on the Yale/CTIO 1-m telescope in Chile, and DANDICAM, which is used on a 1-m telescope in South Africa. The ANDICAM was funded in part by the U.S. National Science Foundation. The DANDICAM by the Dutch ASTRON and the Kapteyn Insitute (part of Rijksuniversiteit Groningen); the name stands for the Dutch ANDICAM. The instruments will become part of a large international collaborations known as PLANET, which stands for Probing Lensing Anomalies NETwork.

The scientific goals of the instrument demand the highest possible photometric precision at a wide range of wavelengths. Most of the projects also require a field-of-view large enough to include many comparison stars, to insure accurate relative photometry even in marginal conditions. Accordingly, the instruments were designed to produce a plate scale of 0.3" per pixel onto both optical and infrared detector arrays so that a stellar point-spread-function is very well sampled. Also, the optics will fully illuminate a 2048x2048, 15 micron pixel CCD and a 1024x1024, 27 micron pixel infrared array, which should guarantee sufficient comparision stars are available.

Both ANDICAM and DANDICAM share optical, mechanical, electronic, and software designs. Both have similar CCDs and IR array detectors. The CCDs were obtained from a national distribution of scientific CCDs sponsored by University of California Observatories/Lick Observatory and the NSF (see here for more details about this program). The CCDs were processed by the Steward Observatory CCD Laboratory. The quantum efficiency and cosmetics of the CCDs are excellent. The IR detectors are hybrid HgCdTe arrays from Rockwell Science Center.

The optical design of the instruments is similar, but not exactly the same. The slight difference is dictated by the different f/# of the two telescopes (the CTIO/Yale 1-m is f/10; the SAAO 1-m is f/15), since we wanted to preserve the same pixel plate scale at both telescopes.

The basic optical design is an Offner relay, which produces excellent 1:1 re-imaging. The Offner design is convenient because it provides a reasonable image of the telescope pupil at the secondary. A cold stop at that pupil eliminates excess thermal emission from the telescope, which is crucial to reduce the infrared background. The stop will also dramatically reduce scattered light at all wavelengths and should improve the precision of flat fields. After the Offner, depending on the channel and telescope, there are lenses that modify the focal length to produce the scale. The following figure shows a schematic light path for the ANDICAM (which requires no lens in the visible channel and a negative lens in the infrared channel, since the CTIO/Yale 1-m is f/10). Note the cylindrical mirror in the infrared channel. This is required because a flat plate of glass (such as the dichroic) introduces astigmatism to a non-collimated beam; the cylindrical surface on this mirror compensates for this effect. The images produced by the optical design are excellent: essentially diffraction limited in the infrared channel and with 80% encircled energy of less than one pixel in the visible channel.

Schematic of light path through the ANDICAM.

Of course, the actual lay-out of the instrument is more complex, since the instrument must fit within a restricted volume. The instrument is also designed for ease of maintenance in the field (i.e. the vacuum jacket can be removed quickly and easily). Click here to see a 3-dimensional lay-out of the ANDICAM.

The throughput of the instrument should be excellent at all wavelengths from U to K. The U sensitivity in particular should be very high, since the CCD quantum efficiency is very good at U and the instrumental optics are designed to work well at U. The instrumental throughput at U is about 20% (including telescope and atmosphere), and >35% for BVRIJHK.

The mechanical, electronic, and control system designs of the instruments are similar to those already in use on other instruments built by our program (see the descriptions of MOSAIC and the IFPS). In particular, mechanism designs, array controller architecture, and control software is the same for ANDICAM and DANDICAM as is used in other instruments.

There are three important mechanisms in the instruments: a visible channel filter wheel, an infrared channel filter wheel, and an internal chopping mirror in the infrared channel that allows dithered infrared imaging while taking a long CCD exposure. The dithering of infrared images is important to remove bad pixels and the relatively large sky emission. All these mechanisms are driven by stepping motors mounted directly to the cold optical bench and are operated by our control software (see the PROSPERO home page for a description of the user interface).

Cold work surface with optics and 8-position filter wheel mounted.

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