Guide to MOIRCS Spectroscopy Exposure Times
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.
(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.
(3) See also the guideline for the on-site NDR setting for the BLIP operation described in the Multi-sampling Operation Page.
OverheadsObject Acquisition Time:
It takes about 20 minutes for a MOS acquisition sequence. The acquisition time for Longslit also takes similar or a bit less (but the field/target confirmation generally takes LONG, especially if target is fainter, or if the finding chart is not well prepared). The time for mechanical movement from the start to finish loading mask requires about 4 to 5 minutes. If another MOS mask is on the focal plane, it will take additional 3.5 minutes to store it. If the elevation of target is low, we need the additional one minute to go to and back from zenith during the MOS mechanical move. During the MOS move, we move the telescope to the zenith for better repeatability of the mask position.
We recommend to execute the re-alignment of MOS and the alignment stars (or longslit and the target) when you execute the exposures for several hours, because the MOIRCS mechanical flexure or the differential atmospheric dispersion between optical (autoguider) and the NIR (MOIRCS) may cause the mis-alignment even though the AG guiding works fine. This will introduce the additional overhead (typically 3-5 min per realignment).
On cloudy condition, it could be happen that we lose the Autoguider star during the MOS observation. If we completely lose the AG star, we might have to do the target acquisition from the beginning, and you will lose significant time for redoing the alignment. Trying the MOS observation on cloudy condition has higher risk of losing significant observing time. Considering some backup imaging observation is recommended under poor condition.
The detail of the MOS acquisition process, please refer to the mos information page.
The overhead time associated with each exposure can be calculated using the Overhead Calculator. The observing efficiency under the typical exposure setting of 180 seconds exposure with 10 times multi-sampling for R500 grism is 83%. If 300 seconds is used, it goes up to 90%. However we recommend to keep the exposure short so that the affection of excessive cosmicrays and residual of night emission lines could be mild.
Exposure time for the VPH grisms are typically much longer than R500 case. The efficiency of over 90% is usually achieved.
Standard Star Observation:
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.
The time for standard star acquisition typically takes 15 minutes, if we have to remove the MOS mask for alignment. But we may reduce the acquisition time to 10 minutes if the slit for standard star observation is relatively close (~1 arcmin) to the pointing position, as we can use the Autoguider for the alignment instead of removing mos mask.
Usually the standard star data can work for both channel, if the dome flat is appropriately treated on the data. However, if you want to take a standard star data on both channels, overhead time by the target acquisition would be significant because we have to do the alignment again from the beginning for each channel. However, the blind offset from one channel to the other (without removing the slit mask) or the target acquisition by the AG described above might be possible if the MOS mask is prepared for the purpose. Note that even in such case, we still need some additional time to do the accurate alignment. If you want to do such operation, discuss well with the SA during the mask preparation.
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.
OLD MOIRCS Spectroscopy Exposure Times Page
The old MOIRCS Spectroscopy Exposure Times page before the 2016 upgrade can be found here.
Please note that all data on these pages are subject to change as the evaluation of the performance of MOIRCS progresses.