Subject: Solicitation of opinion: SDSS z filter for NIC-FPS

From: Jon Holtzman

Submitted: Mon, 17 Mar 2003 14:22:45 -0700

Message number: 663 (previous: 662, next: 664 up: Index)

To: 3.5m users
From: Jon Holtzman, Jon Morse
Re: SDSS z filter for NIC-FPS camera

As most of you know, CU is building a near-IR camera (+ Fabry-Perot) for the
3.5m. A relatively small amount of 3.5m CIF funds have been set aside to
provide some "enhancements" to the baseline instrument.

At the Seattle AAS, significant interest was expressed to obtain a SDSS z
filter for the new camera, and CU would like to proceed with the purchase
of such a filter. (NIC-FPS filters are 65mm round and operate at 77K.
CU already has in its possession Mauna Kea consortium J, H, and Ks filters.)

However, some choices are required about the details of such a filter. In
the SDSS survey, the long-wavelength end of the z filter is defined by
the CCD response, which gradually tails off from 0.9-1.1 microns; the
short-wavelength end is relatively sharp and defined by the filter. In a
near-IR camera, the filter must provide the long-wavelength cutoff. At
the short wavelength end, the bandpass will be defined either by the
filter or by the short wavelength cutoff of the IR detector; the quoted
short cutoff response of the Rockwell detector appears to match the
short wavelength cutoff of the SDSS z filter rather well.

This leads to a choice:

 1. Try to construct a filter that mimics the response dropoff of the SDSS
    CCDs as well as possible in an attempt to match the SDSS bandpass.

    Advantages: potentially allow NIC-FPS z measurements to be accurately
      transformed to the SDSS system, although how accurately this can be
      done is still somewhat questionable

    Disadvantages: expensive (preliminary quote from Barr of ~$20k),
      likely difficult to closely match SDSS to within a few percent,
      gradual slope of long wavelength cutoff results in lower throughput

 2. Construct a simpler (square) filter with a sharper long-wavelength cutoff.

    Advantages: cheaper (~$11k), provides "cleaner" bandpass, e.g. for use in
      photometric redshifts, higher throughput (depending on choice of
      long wavelength cutoff

    Disadvantages: likely to be difficult to obtain good transformations to
      standard SDSS z, especially for objects with non-standard colors

Please note that this is a SDSS z filter in a near-IR camera that might
be used when extending studies into the near-IR; a SDSS z filter already
exists for use in SPICAM, which is likely to be a reasonable match
to the SDSS z bandpass.

We solicit input about this decision; please send any comments you have to both and

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