NIRI Exposure Times

Table 1 below shows minimum recommended exposure times for NIRI for the three read modes used for different background levels. The minimum recommended exposure times allow efficient observing (spending at least 90% of the exposure time actually exposing the array as opposed to reading it). The minimum possible exposure times, which are not recommended, are given in Table 2; they are the times required to read the array. Using the minimum possible exposure time, the observing efficiency is 50%, not including the 2-3 second overhead 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.

Note that with the f/32 camera, the background is low enough that it becomes 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 1 must be used.

Table 3 shows the time required for the background to equal the square of 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.

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.

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 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'^*
f/6	1 (5)	<0.1 (0.5)	0.2 (1)	2 (25)	<0.1
f/14	4 (60)	1 (10)	1.5 (15)	10 (150)	<0.1
f/32	30 (300)	3 (40)	5 (70)	50 (700)	<0.1

^*high read noise mode L' only

The estimates of the time to be background (sky + telescope) limited for the current NIRI filters are based on calculations made using the NIRI Integration Time Calculator. For more information on assumptions that go into these calculations, please see the "more info" links from the main ITC page.