You are in: Instruments > NIRI > Performance and Use > NIRI Components > Science Detector > Saturation    

[NIRI internal CAD]

Exposure Time Limits due to Array Saturation

INTRODUCTION

Tables 1a and 1b below show the time taken for the background to reach 50% full-well capacity of the NIRI Aladdin array in imaging and spectroscopy modes. Tables 2a and 2b shows the brightness of a stellar source (in magnitudes) that will fill 80% of the well of a NIRI array pixel in a one-second imaging exposure (0.2 sec exposure for wavelengths longer than 2.5 microns for imaging, and for spectroscopy at M).

These estimates of the time taken for the background (sky + telescope) or point sources to be saturated for the current NIRI filters are based on calculations made using the NIRI Integration Time Calculator. Background measurements made with NIRI on the telescope are very similar to the adopted values used by the ITC, but obviously span a range of values. The ITC estimates presented below should therefore be used as guidelines only. For more information on the assumptions that go into these calculations, please see the "more info" links from the main ITC page. For these calculations we assume that L and M-band observations use the high-background bias voltage (deeper well); the low-background bias voltage (shallow well) is used for wavelengths shorter than 2.5 microns. These calculations assume median observing conditions and airmass <1.2. The thermal background depends sensitively on these assumptions, and the exposure time to saturation can differ significantly from the tabulated values.

SATURATION, EFFECTIVE WELL DEPTH, AND OBSERVING EFFICIENCY

Because the Aladdin array is read twice (reset-read-read) and the final image is the difference between the two reads, the saturation level in the output image is a function of the flux acquired during the first read. Thus an object or background that contributes a significant fraction of the full well depth before the first read is complete will show signs of saturation at a lower count level than the nominal saturation in the output image. In addition, increasing the number of reads to reduce read noise increases the time between the array reset and completion of the first set of reads, thus reducing the available well size.

Note also that, because the array is read starting from the corners and working inward to the center, the effective well depth is a function of location on the array (it is lower in the center than at the edges).

Because of the way the array is operated, progressively brighter sources will approach saturation and then begin to get fainter as the array approaches saturation in the time between the reset and the completion of the first read(s). Saturated stars typically appear as bright objects with central holes. Because the array reads from the corners to the center, along rows, when the whole array is saturated, values in the center will pass the saturation value and start approaching zero again. A gradient in the background with high levels near the edges and lower levels in the middle indicates a highly saturated array!

To minimize confusion due to an effectively variable saturation level and/or to observe efficiently (spending at least 90% of the exposure time actually exposing the array as opposed to reading it), we recommend exposure times greater than or equal to the recommended minimum exposure times. Note that the minimum and recommended minimum depend on the size of the sub-array, as the read times are shorter when reading out only a portion of the array.

 

Table 1a - Imaging: Time in seconds for BACKGROUND to reach 50% full-well depth
(Note that these are approximate times, as the background varies from night to night)
Camera J H K H21-0 S(1) L' M'
f/6 250 30 50 1000 0.031 0.011
f/14 1250 180 300 6000 0.21,2 0.071,2
f/32 7000 900 1800 36000 1.02,3 0.42
1Observing at f/6 and f/14 with the L' or M' filters is not possible with the full array, since the array will saturate in the minimum exposure time.
2Using deep well array configuration.
3If using Altair the maximum recommended exposure time is 0.11 seconds, which requires the 768x768 subarray.

Table 1b - Spectroscopy at f/6: Time in seconds for BACKGROUND to reach 50% full-well depth at brightest pixel1
Slit Width J1 H1 K1,2 L-L'2 M3
6 pix 10000 600 1000 1.0 0.3
4 pix 20000 900 1500 1.5 0.6
2 pix 30000 1800 3000 3 0.9
1In the 1.0-2.3um region the brightest pixel is usually the strongest OH line. The background between OH lines is typically 1-2 orders of magnitude less than the background in this pixel.
2Beyond 2.3um the background is dominated by thermal emission from the sky and telescope. For 3-4um spectroscopy, the brightest pixel / peak background (at 4.1um) is more than an order of magnitude greater than near 3.0um. Researchers may wish to choose longer exposures than those listed in the table if the longest wavelength portion of the 3-4um region is not of scientific interest. The NIRI integration time calculator can be used to check for saturation at the wavelengths of interest.
3Applies to 4.50-4.94um and several narrow wavelength intervals (bounded by opaque telluric H2O lines that saturate the array) between 4.94um and 5.20um.

Table 2a - Imaging : Magnitude of a point source filling 80% of the well
[for an exposure time of 1 sec (<2.5 microns) or 0.2 sec (>2.5 microns)1]
Camera J
mJ
H
mH
K
mK
H21-0 S(1)
mK2
L'
mL'
M'
mM'
f/6 10.05 10.35 9.75 7.23 - -
f/14 8.0 8.27 7.88 5.23 5.1 -
f/32 6.63 6.83 6.13 3.5 3.2 1.0

1 Observations at L' and M' will generally require individual exposures shorter than 1 second so as not to saturate on the thermal IR background.
2 K magnitude of a point source filling 80% of the well in the H21-0 S(1) filter

Table 2b - Spectroscopy at f/6: Magnitude of a point source filling 80% of the well1
[for an exposure time of 1 sec (0.2 sec in M band)2]
slit width J
mJ
H
mH
K
mK
L-L'
mL
M
mM
6 pix 5.5 5.5 5 4 1
4 pix 5 5 4.5 3.5 0.5
2 pix 4.5 4.5 4 3 0.0
1 Assumes nominal seeing.
2 Observations at M will generally require individual exposures of 0.2 sec or less and in many cases the use of a subarray so as not to saturate on the thermal IR background.

 

[Science Operations home] [NIRI home]


Last update 2007 February 26; Tom Geballe, Andrew Stephens, & Joe Jensen