MOIRCS grisms

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.



Grism
name
Operating
range [um]
Resolution
(0.5'' slit) (*4)
Straight-through
Wavelength [um](*5)
Dispersion
[A/pixel]
Sensitivity (Vega magnitude)
[mag/arcsec2]
zJ500 0.9-1.78(*2) 464 (ch1)
463 (ch2)
1.125.65 (ch1)
5.66 (ch2)
Y=21.1
J=20.4
HK500 1.3-2.3 (*3) 571 (ch1)
531 (ch2)
1.9337.91 (ch1)
8.51 (ch2)
H=20.1
K=19.2
R1300 (*1) 1.18 - 1.33 @ J
1.49 - 1.78 @ H
2.0 - 2.4 @ K
1100 @ J
1050 @ H
920 @ K
1.14 @ J
1.50 @ H
2.24 @ K
1.91 @ J
2.61 @ H
3.88 @ K
J=17.8(*6)
H=16.7
K=17.1
VPH-Y
VPH-J
VPH-H
VPH-K
0.9 - 1.15 @ VPHY
1.05 - 1.4 @ VPHJ
1.4 - 1.9 @ VPHH
1.8 - 2.5 @ VPHK
3180 @ Y
3020 @ J
2940 @ H
2680 @ K
See the VPH webpage for more detail.

General Comments:

  • Note that some spatial shift of the spectra (with respect to the slit position) is observed for the channel-1 R1300 grism spectra as well as all VPH grisms. The shift is +22 pixels to the spatial direction (+y direction on raw image, -x direction on preimage: in wmdp_moircs software it is not displayed!). For other grisms the shift is negligible.
  • TheYJHKKs imaging filters are also available for the R500 spectroscopy. They can be useful for more multiplicity. A care should be paid to the location of the second-order spectra for zJ500+Y setting (it will appear at 1450 pixel from the slit position).
  • You can take the H-band spectral data by both zJ500 and HK500 gratings. Sensitivity is 10-40% better for HK500 grism especially for red side, though the spectral resolution is a bit better for zJ500 grating.
  • We replaced the old OC_1.3 order-sorting filter to new one in October 2007, because the old filter introduced a significant second-order light longer than 2.35um. The order-sorting filter for HK500 is with thermal cut (OC_HK: 1.3--2.3um), which effectively suppress the background noise. Currently the use of OC_HK filter is the default order-sorting filter for HK500. If your science requires the data longer than 2.3um, you should use OC1_3 order-sorting filter instead.
  • We changed the turret position of order-sorting filters (OC_zJ and OC_HK) in July 2008. The relative sensitivity between channel 1 and 2 and flat patterns should have changed slightly from the previous layout, though they should be small.
  • The installed direction of all the VPH grisms were rotated by 180 degree in late 2015 so that the dispersion direction of them to become the same as the other grisms.


Figure 1: (Left) The efficiencies of the zJ500 and HK500 grisms (by the datasheet supplied from the company: the source from Tokoku et al. 2007 Ph.D. thesis). (Right) The spectral resolution of R500 grisms as a function of wavelength (0.5"-width slit is assumed). Note that the spectral resolution is inversely proportional with the slit width, up to 0.3".




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.



User 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.

Updated 2016-11-16


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