Available grisms and sensitivities
MOIRCS spectroscopy mode has been operating with two low-resolution grisms, a medium-resolution grism, and four high-resolution VPH grisms.
Two low-resolution "zJ500" and "HK500" grisms are characterized by its wide wavelength coverage and high throughput. The zJ500 grism is optimized for the observation of 0.9-1.78 micron regions, while the HK500 grism is optimized for 1.4-2.5 microns. Their resolution is designed to roughly R~500 under 0".5 slit width. Spectral resolution is defined by the slit width: higher resolution can be achieved for narrower slit width.
The medium resolution "R1300" grism is for a better observability of target lines between the atmospheric OH-line forest. The grism efficiency become worse for shorter wavelengths. Due to the intrinsically-broad line profile of the grism, the spectral resolution is roughly proportional only at the slit width larger than >0".7. This is found to be (mainly) due to the micro-cracking of the KRS5 prism used (see the website for more detail). Thus we currently do not recommend to use R1300 grism for science. We have a plan to update the grism in the future.
The VPH grisms have medium to high spectral resolution and good sensitivity at their peak. Their efficiency curve is in general fairly peaky, and has a dependence to the position of the slit you cut. Peak grism efficiency also has a strong dependence on the slit position on the image. These characteristics sometimes makes the mask design and the calibration data acquisition fairly complicated. Due to these special characteristics, the current mask design software does not support the VPH grisms. The use of the grisms is mainly for the experienced observers. Users who are considering the use of the VPH grisms, please visit the VPH Information Web page. All observation with the VPH grism is still under shared-risk policy.
Estimated sensitivities listed are for a 5 sigma detection per pixel in 1 hour of on-source background-limited spectroscopy (0.5" slit is assumed). They are calculated using the measured MOIRCS efficiency shown in Figure 5 considering the telescope efficiency and the atmospheric loss. The measured sky brightness between lines are used for calculation. Note that the majority of the spectra is under the OH night emission line for R500 grism, which makes the actual limiting magnitude around that wavelength regions significantly shallower (0.5 - 1 mag) than the values quoted.
(*1) For R1300 we use the 2nd-, 3rd-, 4th-order spectra for K, H, and J band observation, respectively. We use the imaging broadband filters for the order-sorting purpose. For K-band region you can use Ks-band filter instead (1.99 - 2.31 um range, with lower thermal background). The spectral resolution here is for 0.7-arcsec width slit. Note that due to the R1300 grism problem, ch2 grism resolution would not go better than this. The sensitivity is measured in 2006, not the current values. We do not recommend to use the grism due to the problem on the grism.
(*2) This is valid with the OC_ZJ order-sorting filter. We can observe below 9000A if we don't use the filter (to ~0.8um with lower sensitivity). However, you should be aware that the spectra will be contaminated by the second-order spectra at >1.5 um. The data without the order-sorting filter will also introduce the additional (but small) rise of the background noise due to the stray light around the bluest edge of the detector.
(*3) This is valid with the OC_HK order-sorting filter. When the OC1.3 order-sorting filter is used instead, the number will be 1.3 -- 2.5 um but with a significant rise of the thermal background noise.
(*4) At the straight-through wavelength. The spectral resolution changes with the wavelength proportionally. See the Figure below for the values at other wavelengths.
(*5) Straight-through wavelength is actually the function of the angle of incidence (= slit position on the detector). The measured offset for HK500 grism is about 170 A, with the shorter wavelength for the innermost slit (the shorter wavelength side) and the longest wavelength for outermost slits (longer wavelength direction).
(*6)The depth for R1300 is still for the previous detectors (for data before June 2015).
Figure 2(a): Total system throughput (telescope + instrument) for zJ500 grism (Left = channel 1 / Right = channel 2; red, May 2016 data). The system throughput for old detectors (measured using the same standard star in 2015) are also shown in blue for comparison purpose. The ripple pattern in the curve is from the order-sorting filter transmission curve (OC_ZJ: see the figure below). Thanks to the new detectors, the improvement of the sensitivity in YJ wavelength is evident. Note that the system throughput shown here includes the local variation of the detector QE, which is quite large for old detector. Also note that the degree of the inter-pixel crosstalk (IPC) on the gain values for old detector is unknown. Here we conservatively applied the same IPC correction for old detector data for a fair comparison purpose.
Figure 2(b): The same figures as Figure 2(a), but for HK500 grism (Left = channel 1 / Right = channel 2; red, May 2016 data).
Figure 3: Total system throughput (telescope + instrument) for R1300 grism with STD_K filter and comparison with the throughput of HK500 grism (Apr 2016 data). The system throughput of the grism with H-band or J-band filters are each roughly 40-60% (H) and 20-25% (J) of the STD_K case. Note that the use of the R1300 is currently not recommended due to the grism defects as described here in detail.Total MOIRCS spectroscopic efficiency for zJ500 / HK500 grisms. Channel-2 data with 2"-width slit is used for evaluation. Note that the efficiency curves around the wavelength ranges with high atmospheric absorption clearly suffer from the residual of the absorption correction. Also note that the turn-over feature seen the longest edge of the spectra of the zJ500 grism is due to the artificial pattern seen in the detector. Figure 5b(right): Total MOIRCS spectroscopic efficiency for the R1300 grism with the JHKKs filters. Channel-2 data with 2"-width slit is used for evaluation. The turn-over feature seen the longest edge of the spectra of the H+R1300 grism is due to the artificial pattern seen in the detector.
Figure 4: [Left] The order-sorting filters (OC_zJ for zJ500, OC_HK and OC1.3 for HK500). [Right] The YJHK imaging filters for R500/R1300/VPH grisms.
Although we have been accepting the user grisms, the port for new grism is not available any more. Enough discussion with the observatory is highly recommended if you want to make the new user grisms. The detail and the basic acceptance policy can be found in the User Filter/Grism Policy page.
OLD MOIRCS Grism Page
The old MOIRCS grism website 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.
Copyright © 2000-2016 Subaru Telescope, NAOJ. All rights reserved.