MOIRCS grisms

Available grisms and sensitivities

MOIRCS spectroscopy mode has been operating with two low-dispersion grisms and four medium-dispersion grisms.

Two low-dispersion "zJ500" and "HK500" grisms are characterized by its wide wavelength coverage and high throughput. The zJ500 grism is optimized for the observation of 0.9-1.8 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 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. Note that the VPH-J and VPH-H grisms were replaced to a better performance LS-J and LS-H grism.

A team of scientists in Tohoku Univ, Riken, Subaru Telescope, Tokyo Univ have developed the new grisms ("LightSmyth J/H"). They will have similar spectral resolution to the VPH-J/H grisms, but with higher and flatter transmission curve. The dependence of the sensitivity with the slit position is minimal so should be much easier to use than VPHs. When you publish the result based on the grisms, please give citation to the SPIE paper.

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) (*1)
Straight-through
Wavelength [um](*4)
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
VPH-Y
VPH-K
0.9 - 1.1
1.8 - 2.5
3180 @ 1.02um
2680 @ 2.2um
See the VPH webpage for more detail(*5).
LS_J
LS_H
1.05 - 1.4 @ LS-J
1.4 - 1.9 @ LS-H
~3000 See the LightSmyth Grism webpage for more detail.

      (*1) At the straight-through wavelength. The spectral resolution changes with the wavelength proportionally. See the Figure below for the values at other wavelengths.

      (*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) 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).

      (*5) VPH-J/H grisms were decomissioned from MOIRCS in 2021. They were replaced to the LightSmyth J and H grisms.


General Comments:

  • 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 decomissioned the R1300 medium resolution grism in 2020 due to the technical issue (see the website for more detail).
  • 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).

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 3: [Left] The order-sorting filters (OC_zJ for zJ500, OC_HK and OC1.3 for HK500). [Right] The YJHK imaging filters which can be used with R500/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 2024-1-25


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