Guide to MOIRCS Spectroscopy Exposure Times

Saturation Magnitudes

The following table lists the point-source saturation magnitudes for various exposure times and grisms, with 1-arcsec-width slit under good seeing condition. Also listed are the times taken to achieve background limited performance (BLIP) with MOIRCS for a 1'' slit (the time is directly proportional to the slit width). BLIP may be achieved when the sky counts exceed the square of the read noise (14 e^- by using the multi sampling) by a factor of 3. Note that these values are based on simple calculation and should be treated as crude estimates.

Grism	Saturation magnitude				Max exposure time (s)(2)(3)	Min BLIP time (s)
	15 s	60 s	300 s	600 s
zJ500-J	9.8	11.3	13.0	13.8	800	300
HK500-H	9.4	10.9	12.9	13.9	600	50
HK500-K	8.4	9.9	11.7	12.5	600	200
R1300-J	6.8	8.3	10.1	10.8	TBD	4200
R1300-H	7.3	8.8	10.6	11.3	2800	300
R1300-K	7.3	8.8	10.5	11.3	2500	500

Note:


(1) All numbers described here are based on the simple calculation under the assumption that the observed sky brightness is continuum, so the minimum BLIP times should be considered as the lower limit. The values based on the actual data will be updated in the near future. Also, the stray light component is not included in the calculation.

(2) The values here is based on the single observation data. Since the night sky emission lines are used for the wavelength calibration, they should not be saturated and the maximum time has been set as such. Saturation of the night sky lines will also make the sky subtraction difficult. The mechanical flexure of the instrument may also become prominent under long exposure condition, which also makes the sky subtraction difficult. Therefore we recommend to keep the exposure time short within the acceptable level.

(3) The strength of the night sky lines changes hourly/daily/seasonally with the amplitude of several. Also, sky lines generally become brighter at low elevation.

Overheads

It takes about 25 to 35 minutes for a MOS acquisition sequence. Acquisition time for long slit also takes similar (20 to 45 minutes), and it generally takes longer if target is fainter. The time for mechanical movement from the start to finish loading mask requires about 7-9 minutes. If another MOS mask is on the focal plane, it will take longer because we first have to store it to the mask stocker. If the elevation of target is low, mechanical move time gets a few minutes longer, because we usually go to zenith during moving MOS mechanism.

If you need to take an object with both channels (for example, standard star observation), overhead time will be doubled because we have to remove mask from the focal plane to check the position of the object after the observation by one chip. And also, we may have to do the target acquisition again from the beginning if we lose target/Autoguider star etc during the observation/alignment, and it means we need more 25-35 minutes for acquisition. Thus, the overhead tends to become very large during the spectroscopy. Less-tight and flexible bservation plan is welcomed.

The detail of the MOS acquisition process, please refer to the mos information page.

Spectroscopy:

The overhead time including readout time and dither time is about 80 seconds for each frame under 8 times multi-sampling. A typical individual exposure time is roughly 300 to 600 seconds for both zJ500 and HK500 (for faint object observation). In this case, we expect the overhead of 26% (300sec) to 13% (600sec) for each exposure.

We recommend to execute the re-alignment of MOS and guide stars when the target observation become long, because the MOIRCS mechanical flexure or the differencial atomospheric dispersion between optical (autoguider) and the NIR (MOIRCS) may cause the shift of guidestars from the mos alignment hole. This will introduce the additional overhead.

And also, do not forget considering the time for standard star observation. As the mos exchange process and the acquisition of the standards on the slit also takes long, we recommend to consider the use of a slitlet on your MOS plate for standard observation. If your target field has a star with appropriate magnitude, you may put a slitlet on the star and observe it with your targets. Then the spectra of the star may also be used as the reference of the atomospheric absorption correction although the accuracy may be limited.

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

Created by Yuka Katsuno Uchimoto (Feb 2006)
Revised by Ichi Tanaka (2008-02-07)