We present the design of a high altitude balloon payload which will perform high precision photometry on bright infrared sources. The payload uses an off axis Gregorian telescope with a 0.6 m clear aperture. Light from the telescope is split by a set of dichroics into individual bands to simultaneously record photometric flux in five different bands corresponding to roughly JHKL and M bands. Because we are observing isolated IR bright stars, we record the signal of a series of discrete diodes. This allows us to establish the IR colors necessary to characterize first-ascent red giants and asymptotic giant branch stars, or the spectral energy distributions of any ~1000-3000K objects, without the high cost and complexity of one or more IR focal plane arrays.
The payload is designed to address a glaring capability gap in our current portfolio. Many stars in the late stages of low- and intermediate-mass evolution, or indeed, nearby Milky Way stars or bright infrared sources in general, are saturated in modern survey instruments (e.g. 2MASS, WISE, or IRAC). Older instruments, like DIRBE, are generally confusion limited in the Galactic plane. This leaves us in the maddening position that the stars which appear to be the easiest to observe are among the most poorly photometrically characterized.
This is a particular problem because the highest resolution instruments—interferometers, exoplanet coronagraphs, and high resolution spectrometers among them—are best able to observe these brightest targets. This leaves a serious disconnect between the faint objects for we have high precision photometry and the bright objects for which we have high resolution observations. This project is designed to bridge this capability gap.