Guide to MOIRCS Spectroscopy Exposure Times
!! The information below are for the previous detectors (for data before June 2015) !!
The actual values for the current page is to be updated after the on-sky test in May 2016. Sorry for any inconvenience.
For the proposal preparation, please use the current performance. You may also consider 30-40% increase of the sensitivity in YJ band if necessary.
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 | 15 s | 60 s | 300 s | 600 s | Max exposure time (s)(2)(3) | Min BLIP time (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:
- 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.
- The values here are 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.
- 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
Object Acquisition on Slits:
It takes about 25 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 7-9 minutes. If another MOS mask is on the focal plane, it will take additional 7 minutes because we first have to store it. If the elevation of target is low, mechanical move time gets a few minutes longer, because we go to zenith during the MOS mechanical move for safety.
If you need to take AN OBJECT with both channels (for example, you may want to take standard star data on both channel) by the longslit, overhead time will be almost doubled because we have to do the alignment again from the beginning. We may try to move the target from one channel to the other blindly if you request. We still need some additional time to measure the accurate offset value. Trying the jump for faint target will be risky.
Since 2009 we have been trying a more efficient acquisition technique for longslit acquisition. The new method will reduce the target acquisition less than 10 minutes for bright (< 15 mag) one, or 15--20 minutes for faint one. For more detail, please contact the SA.
On cloudy condition we may lose the Autoguider star during the observation. If it happens, we may have to do the target acquisition from the beginning, which will add another ~30 minutes overhead. Trying the MOS observation on cloudy condition has high 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.
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 differential atmospheric dispersion between optical (autoguider) and the NIR (MOIRCS) may cause the shift of guide stars 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 atmospheric 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.