Guide to MOIRCS Imaging Exposure Times

Background-Limited Operation

In order to achieve the maximum sensitivity for a given integration time, individual exposures should be long enough for the photon shot noise from the sky background to dominate the array read noise. In practice, background-limited performance (BLIP) may be achieved when the sky counts exceed the square of the read noise (40 e^- is applied here: see the information instrument parameter page for recent one) by a factor of 3 , as indicated by the following table.

Filter	Background		Min. BLIP time (s)	Maximum Exposure
	(mag/arcsec2)	e-/s/pixel
J	15.9	170	30	200
H	13.4	1800	3	20
Ks	14.1	510	10	80

Note:
(1) Minimum exposure time is 11.5 s (NDUMMYREAD=0 case: for NDUMMYREAD=2 it is 19.7 s) for the full-frame read mode (but see the footnote 7 of the MOIRCS Instrument Parameters page). Exposures shorter than this will be realized by using the partial readout mode (see the Partial Readout mode section).

(2) Sky background level in near infrared frequently changes within a night The variability amplitude level of 2-5 times has ever been recorded. The value shown here may be treated as just a nominal case. The range of actual values often used are, 50~200sec for J, 15~70sec for H, 35~100sec for Ks.

Saturation Magnitudes

The following table lists the point source "saturation" magnitudes in good seeing condition (0.3'', when about 12% of the total flux from an unresolved source falls in the peak pixel) with different exposure times. Here we use the word "saturation" as the level that the peak count of the pixels of interest exceeds ~1% linearity range, i.e. ~25000 ADU. To achieve short exposure time please consider the partial readout mode (below).

Filter	Saturation magnitude
	1 s	3.5 s	12.5 s	60 s	180 s
J	12.2	13.6	14.9	16.6	17.8
H	12.4	13.8	15.1	-	-
Ks	11.7	13.1	14.4	16.1	-

Partial Readout Mode

Note 1: The HAWAII-2 has four independent quadrants, and the 1-pixel-width rows/columns on each quadrant boundaries are not readable. A care should be paid for not entering into the unreadable rows/columns when taking standard stars. (Please refer to Figure 3).

Note 2 (!! Important !!): A systematic bias in the photometry is to be introduced when the partial-read mode is used for photometric calibration. The level of systematics is the strong function of the partial-read size as well as the input flux level. In the worst case, the sytematics introduced by it can be as large as 10% level. The use is not recommended now. See the Information Page for more detail.

Figure 3: An example of standard star observation with the partial read mode (1024 x 1024). Partial-read mode will be applied to both channel simultaneously, and regions around the center of each detector will be read out (the image center does not change). Please note that the distance between the center of each detector has 3.0 arcminutes projected on the sky (Please refer to Figure 1). A standard star (a star symbol) should be imaged within one quadrant, since the boundaries of the four quadrants are not readable. (Cross lines on the figure above indicates the boundaries of each quadrant).

Filter	Overhead	Typical exposure time (s)
J	22 %	180 s (coadds=1)
H	82 %	20 s (coadds=6)
Ks	42 %	50 s (coadds=3)

Note 1:
[Overhead] = ([Total Observing Time] - [Total Integration Time]) / [Total Integration Time]

Please note that all data on these pages are subject to change as the evaluation of the performance of MOIRCS progresses.

Updated 2009-06-11