IR Imager for SOAR

IR IMAGING with SOAR

Richard Elston
and
S. M. Simkin

Introduction

What follows is a melding of discussions during a telephone conference with Elston on 07/11/97 and discussions with the MSU astronomers about the type of IR imaging they would like to do with SOAR. In that discussion we put forth the following possibilities for an IR imager:

Resolution 0.07$"$ to 0.10$"$/pixel.

Wavelength range(s) 1.0 to 2.5$\mu$m (with 2.5 - 5 $\mu$m given second priority).

Wavelength resolution - provision for JHK filters plus narrow band filters (R = 100 to 200) with possible upgrade to Fabry-Perot and possible provision for a grating grism.

FOV, as large as possible

I have expanded on this below where I have tried to let the instrument specs flow from the science. Section 2. is a summary of the science, section 3. discusses the resolution, FOV issues for SOAR, section 4. is a summary of the basic requirements which flow from these with possible priorities noted.

Basic Science

This is arranged according to distance, from nearby to most distant. These mostly reflect the use of SOAR as a survey instrument.

Deep field survey for M dwarfs Needed: JHK photometry down to a faint limiting magnitude (eg. K= 20 - 22). Followup observations (for spectral classification) at R=500.

Survey for low mass stars in clusters Needed: JHK photometry down to a faint limiting magnitude (eg. K= 20 - 22) : Wide field imaging (4' or more):

Followup observations to the 2MASS and DENIS sky surveys Needed: JHK photometry down to a faint limiting magnitude (eg. K= 20 - 22).

Surveys for young stellar objects with extended disks Needed: Wide field imaging (4' or more): Filters to pick out promising emission features for these warm stellar disks (eg. Pa$\alpha$ $\lambda$ 1.87 $\mu$m)

Surveys for red supergiant Carbon stars near the galactic center Needed: wide field imaging (4' or more): Filter (R=100) at 1.8 $\mu$m and grism with R=80 to 200 to identify these, faint limiting magnitude (eg. K=20 to 22).

Metal abundance in AGB objects in star clusters and Visual and K-band Photometry of Star Clusters in the Magellanic Clouds. Needed: wide field imaging (4' or more): JHK photometry to a faint limiting magnitude (eg. K=20 to 22). Follow up with filter imaging (R=100 tp 300) at 1.8 m$\mu$ and grism with R=80 to 200 to identify the type of stars (Oxygen or Carbon rich).

Imaging of nearby AGN hosts Needed: imaging in JHK and broad IR emission lines with filters at R=100 and spatial scales of 0.1" with FOV = 1' to 2. '

Surveys for agns in distant clusters of galaxies Needed: wide field imaging (4' or more) : Grism with R=80 to 150, Filters to isolate night sky emission.

Search for emission line objects associated with distant QSOs Needed: wide field imaging (4' or more) : Grism with R=80 to 150, Filters to isolate night sky emission (Or narro band filters at redshifted H$\alpha$, [OIII] $\lambda$ 5007, [OII] $\lambda$ 3727

Fit to SOAR Imageing and possible AO upgrades

At K the SOAR Airy disk size will be roughly 43 m$\mu$ or 0.14$"$. If we assume we can come close to this limit then we need resolutions of 0.07$"$. (This is also roughly 1/3 of the 0.20$"$ telescope image expected for the best 25\% of the tip-tilt corrected images.) For accurate photometry of stars in crowded fields (the cluster, galactic center, and young stellar objects projects above) this is the type of resolution we will need (3 pixels). On the other hand, several of the projects (eg. surveys for red dwarfs, surveys for young stellar objects, metal abundance of AGB stars in clusters, and emission line objects in distant galaxy clusters) require as wide a field of view as possible. FOV of 4$'$ or more are desirable. If it is possible to build a 4KX4K mosaic array then 4$'$ will be achieved with 0.10$"$ pixals. and this would still be acceptable for stellar photometry most of the time. There are several ways to approach the survey question: One possibility is narrow band filters, another is a filter (to lower the sky background) - grism combination (the technique now used in Schmidt surveys). The latter might possible if the camara mirror in the imager could be interchanged with a concave, low order grating (to focus and disperse the light). Finally, low order AO promises to yield the defraction limit at K band and provisions for upgrading to this should be included in the imager design. (If the output of the AO is a different f ratio from the original f16 then it might be possible to design the IRimager to be adapted to this different beam.)

Summary

For present the priorities (based on the above) seem to be:

Resolution 0.10$"$ (with 0.07$"$ desirable but not at the expense of FOV).

Wavelength range(s) 1.0 to 2.5$\mu$m (with 2.5 - 5 $\mu$m given second priority).

Wavelength resolution - provision for JHK filters plus narrow band filters (R = 100 to 200) with possible provision for a grating grism and possible upgrade to Fabry-Perot and the f-ratio of the AO upgrade output.

FOV: 4$'$ if possible