The image above was calculated using a flat, lambda-dominated cosmology with Ho = 65, Omega_o = 0.41, which has an age of 13.3 Gyr. The K band counts agree more or less with Djorgovski etal. The assumed aperture consisted of 6 hexagons in a ring, having a total area of 45 m2. The diffractive PSF of this aperture was convolved with the simulated image, along with an 0.05" FWHM Gaussian blur. A careful examination of the cores of images will reveal a hexagonal diffraction pattern. The three colors in the picture are 5, 3.162 and 2 microns for red, green and blue. The assumed array has 1/8" pixels, but the image was generated using a 3x3 pattern of substeps, so the spacing of the pixels in the picture is 1/24". The full image is 1440x1440 pixels or 60"x60". The assumed noise levels per pixel in each of the 9 frames were 0.6, 0.16 and 0.19 nJy, corresponding to a 10,000 second integration in each of the 9 frames. Filter bandwidths of 20% in wavelength were assumed, along with a dark current of 0.1 e/sec, read noise of 10 e rms, and 90% QE.
A higher resolution version of the same field observed with 1/24" pixels and no sub-stepping. The noise levels at 2, 3.2 and 5 microns were 0.07 nJy, 0.07 nJy and 0.2 nJy per pixel for 10,000 seconds of integration. There is only one frame so the total integration time is 10,000 seconds per color or nine times smaller than the substepped image shown above.
The "hole" in the center of the bluish galaxy images is an artifact of the method used to combine the three monochromatic images into a color picture. Because diffraction is much less for the 2 micron (blue) image, the center of bright objects is quite blue, so the RGB picture switches over to a fairly pure blue. But pure blue is a dark color, producing the appearance of a hole.
The same field with stars included, using 1/24" pixels and no substepping. The total exposure time is 10,000 sec per color so the noise level is the same as before. The brightest brown dwarf expected in 15'x15' has been moved to the center of this frame, but there are several other BD's in the picture. All these BD's have color temperatures of 462 K, appropriate for dusty BD's with L = 1E-6 L(sun), M = 0.05 M(sun), 10 Gyr age and Teff = 500 K. While these are very tough targets, the assumed density was 0.01 M(sun)/pc3 which is probably too high according to MACHO. The aperture used for the PSF calculation in this frame had 7 hexagons with a 1 meter diameter hole in the middle hexagon, and a total area of 45.2 m2. The brightest BD in the center has a K magnitude of 27.21 but a "large" (for NGST) 5 micron flux of 2 microJy due to the low color temperature.
A 380 kB 1440x1440 JPEG showing the same 1'x1' FOV at 0.8 microns
with HST resolution and 1 sigma point source noise of IAB = 30,
which is close to the HDF sensitivity.
The PSF of NGST at 5 microns, 1/48" pixels, 6 decade log scale, total FOV 10.7"
