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Exposure times

The following five tables provide various exposure time limits. Constraints are due to a number of factors, more than one of which may need to be considered for a given NIRI configuration:

  • Minimum possible exposure time due to array electronics (Table 1)

  • Efficiency - ensuring that a much larger percentage of the time is spent exposing the array than reading it out (Table 2)

  • Time required for noise from fluctuations in the sky+telescope background to equal read noise (Table 3)

  • Maximum possible exposure time before array is saturated by background (Table 4)

  • Maximum possible exposure time before saturation on a bright star (Table 5)
  • The minimum possible exposure times, which are not recommended, are given in Table 1; they are the times required to read the array. Using the minimum possible exposure time, the observing efficiency is 50%, but this does not including the 2-3 second required to write the frame to disk. Additional activities such as nodding, jittering, or changing the NIRI configuration further reduce the efficiency. Note that each of these minimum times depends on the size of the sub-array, as read times are shorter when reading out only a portion of the array. In general, multiple coadds should be used for exposure times less than a few seconds in order to keep the efficiency high.

    Table 2 shows minimum recommended exposure times for NIRI, for the three read modes used for different background levels. These times allow efficient observing (spending at least 90% of the exposure time actually exposing the array as opposed to reading it). Note that with the f/32 camera, the background is low enough that it is comparable to the read noise, even for broad-band JHK observations. The decision of whether to use low or medium read noise should be based on which yields higher S/N for a given amount of observing time, including all overheads. It is important to note that under some conditions (e.g., high background or bright stars), the minimum recommended exposures can result in saturation of the array. In such cases shorter exposures than those recommended in Table 2 must be used.

    Table 3 shows the time required for the square root of the background to equal the readout noise in each NIRI filter/camera imaging combination for medium and low read noise modes. The medium read noise mode corresponds to a 16 times digitally averaged single Fowler pair (double correlated sampling) and should be used for f/6 broad-band JHK imaging. The low read noise mode uses 16 digitally averaged sample pairs and is optimized for f/32 observations and narrow band work. All L and M-band exposures should be taken in high read noise mode, which uses a single Fowler pair and no averaging, as they are always background-limited.

    Table 4 shows the time taken for the background to reach 50% full-well capacity of the NIRI Aladdin array in imaging mode. Table 5 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). The values are based on calculations made using the NIRI Integration Time Calculator. Background measurements made with NIRI on the telescope are 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.


    Table 1: Minimum Possible Exposure Times (sec)

    Array Size Low Background Medium Background High Background
    1024 x 1024 8.762 0.548 0.179
    768 x 768 4.980 0.313 0.106
    512 x 512 2.276 0.144 0.052
    256 x 256 0.654 0.043 0.020

    Table 2: Minimum Recommended Exposure Times (sec)

    Array Size Low Background Medium Background High Background
    1024 x 1024 44 2.7 0.9*
    768x768 20 1.4 0.5
    512 x 512 11 0.7 0.25
    256 x 256 3 0.25 0.1
    * This exposure time saturates the array at M'; typically 0.5 sec or less must be used.


    Table 3: Time (sec) for sqrt(background) = read noise for low (medium) read noise modes - Imaging only

    Camera J H K H21-0 S(1) L'* M'*
    f/6 1 (5) <0.1 (0.5) 0.2 (1) 2 (25) <0.1 <0.1
    f/14 4 (60) 1 (10) 1.5 (15) 10 (150) <0.1 <0.1
    f/32 30 (300) 3 (40) 5 (70) 50 (700) <0.1 <0.1
    *high read noise mode L' and M' only

    Table 4 - 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 5 - 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