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Exposure Time Estimation

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Integration Time Calculator (ITC)

The NIRI Integration Time Calculator (ITC) can be used to determine limiting magnitudes, exposure times, S/N ratios, background levels, etc. for a wide range of source properties, observing conditions, and NIRI configurations. 

The ITC has been used to generate tables of sensitivity estimates, which usually agree with values measured on the telescope to 20%. However, because some conditions vary unpredictably (e.g., the OH sky emission, which dominates the 1-2.4μm background, can vary by a factor of two from hour to hour), one should consider the ITC predictions to be accurate only to within a few tens of percent.

The ITC is not available for AO imaging at wavelengths beyond the K band. However, it is possible to use ALTAIR for imaging in the 3-4μm region. For AO imaging at L', throughput is reduced by a factor of about 7 because the Altair dichroic was not originally designed for thermal observations. Experiments show that with ALTAIR the background per pixel at L' increases by 10X (3.2X increase in noise). Thus, signal to noise (S/N) is reduced by a factor of ~20 with ALTAIR due to the dichroic and warm optics. ALTAIR does improve a 0.3" FWHM image of a point source at L' to ~0.1" FWHM, implying a S/N increase of 9. Thus, overall, there is about a factor 2 reduction in S/N with the use of ALTAIR in L'. However, there is an overall gain in angular resolution, which may be useful for some programs involving bright targets which require improved angular resolution.

NIRI is only available for imaging due to problems with the focal plane mask and beam splitter mechanisms. The spectroscopic setups are provided in the ITC for use in archival data reduction.

In the four sections of this form, select the appropriate astronomical source, telescope and instrument configuration, observing conditions, and observation parameters. Click on the calculate button (calculate button) at the bottom of any section to submit the parameters from all the sections to the server or the reset button (reset button) to reset all parameters to their defaults. The results are reported in a separate web page that can be resized and printed.

See the ITC Help for general guidance on use of the Integration Time Calculators, and the (more info) links for specific help on each section of the form.

WARNING: A bug has been reported in the NIRI ITC when performing calculations on extended sources with uniform surface brightness and using Altair for guiding and the "optimum S/N" software aperture size. Until the problem is resolved, please manually select the software aperture size (under "Analysis method") when using extended sources with uniform surface brightness and Altair.

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Exposure time limits

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:

Table 1: 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.

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 the table above must be used (see Table 5). 

Table 2: 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

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 3 - 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 4 - 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
H21-0 S(1)
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

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 5: 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

The minimum possible exposure times are not recommended. 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.

Exposure Time Estimation | Gemini Observatory


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