Guide to MOIRCS Imaging Exposure Times
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) should be achieved when the sky background noise exceeds the read noise (15 e-) by a factor of 3.
Table 1: BLIP Operation
(1) The amplitude of the changes in the sky background level in near infrared is quite large. The change of the sky level by factor of 2-5 times (per night!) has ever been recorded. Values shown here are the approximate median during the 2009 - 2010 imaging observations.
(2) Note that the values here is based on the pure saturation. Fore reference, ~5% non-linearity level occurs around 35k ADU, roughly 60% of the saturation level (for ch1). We recommend to keep the sky level within 25000 ADU for science data. The typical unit exposure for faint objects would be 180-360 s for Y, 45-90 s for J, 10-30 s for H, 30-60 sec for Ks, respectively.
(3) See also the guideline for the choice of the NDR (multi-sampling) values for a BLIP operation which is described in the Multi-sampling Operation Page. Note that this is mostly for Y-band and the NB imaging observation.
Saturation magnitude is extremely sensitive to the seeing. The example of saturation magnitude level under good seeing condition (~0.4'', when about 3% of the total flux from an unresolved source falls in the peak pixel) tablated here is for old detector. We will update it soon.
For bright standard star observation we will take the data defocused.
Table 2: Saturation Magnitude
|Filter||Overhead||Typical exposure time (s)|
|Y||16 %||165 s (NEXPOSURE=1)|
|J||27 %||75 s (NEXPOSURE=2)|
|H||56 %||30 s (NEXPOSURE=4)|
|Ks||40 %||45 s (NEXPOSURE=3)|
[Overhead] = ([Total Observing Time] - [Total Integration Time]) / [Total Integration Time]
We use a circular pattern for dither for imaging observation, as shown below. Dither length (diameter of the circle) should be larger than the size of the largest object in the field, and the number of image by the dithered obs should be at least 5 for better median sky.
The same information for OLD MOIRCS before 2016 can be found here. We keep this for record.
Please note that all data on these pages are subject to change as the evaluation of the performance of MOIRCS progresses.