|
MOIRCS VPH Grism Information PageOpen with Limited Support: Observation under the Shared-Risk Policy We have two high-resolution VPH grisms (VPH-Y & VPH-KNote) for the open-use observations (Note: VPH-J and VPH-H were retired in 2020 following the arrival of the better performance LightSmyth Grisms). The grisms were originally carry-in grisms, and the PI of each grism kindly donated them for open use. We thank Prof. Toru Yamada (VPH-K), and Dr. Kazuhiro Shimasaku (VPH-Y), as well as the Dr. Noboru Ebizuka (manufacturing team leader) for their contribution to the community(note 1). It is strongly recommended that the observer who used these grisms would give acknowledgement to providers on your papers when the data based on the grisms are to be published. And also, please refer the paper by Dr. Noboru Ebizuka below.
We summarize the basic characteristics of these VPH grisms here. Users who would like to use the grisms should fully understand their basic characteristics. The use of these grisms is somewhat complicated: careless planning without understanding their characters may result in the failure of the observation. The VPH grisms have three characteristics the users should be aware of: (1) a relatively peaky (narrow) transmission curve, (2) the strong dependence of the transmission peak wavelength with the position of the slit, (3) a large shift of the spectra to the spatial direction (in VPH-K). And also, the spectra by VPH grisms basically covers the entire span of the detector in the dispersion direction. Thus the starting and the ending wavelength of the spectra will change with slit to slit (by the slit position) for the MOS data. These characteristics make the mask design for objects that distribute across the MOIRCS FOV fairly complicated, so careful preparation and planning are required. See the "The Observation" part below for more.
We started fabrication of the new "VB-K Grism" under the support of the NAOJ Leadership Funding Program. The new grism has similar spectral resolution but with high efficiency over much wider spectral coverage than the current VPH_K grism. Fabrication was ended in FY2022. Hopefully we can start the on-sky test with MOIRCS from fall 2023. If the overall performance is probed to be good, we are going to offer the use instead of the VPH_K grism from S24A. SpecificationThe table below describes the basic specification of grisms.
(2) The value will shift slightly (30-50A level) with the slit position. (3) In the preimage coordinate (=X_pre: the sign was changed on 9/30/2016). In the raw image coordinate, a positive shift in X_pre will be a negative shift in Y (raw). See the example for VPH-K case. (4) Estimated sensitivities listed are for a 5 sigma detection per pixel (in 1-D spectra) in 1 hour of on-source background-limited spectroscopy (0.5" slit and the object size are assumed: in vega mag). They are calculated using the measured MOIRCS efficiency and crude sky brightness between the OH night lines. (5) As of April 2014, the spectra show a tilt by about -1.5 deg and -1.0 deg for channel 1 and 2, respectively (clockwise rotation). Grism Efficiency and its CharacteristicsThe figures below is an example of the efficiency curve for each grism (from Figure 9 of Ebizuka et al.). The system efficiency here includes all the loss by telescope and MOIRCS.
The important characteristic of these grisms is that the sensitivity curve will change with the X-position (in raw image coordinates) of the slit. The behavior for VPH-J, VPH-H and VPH-K is each shown in Figure 2. The peak efficiency value of the transmission curve is also known to change with X-position. The shift of the peak wavelength of the transmission curves is summarized in Figure 3.
The figure below (Fig. 3) is the summary of the shift of the peak with the slit position.
The wmdp DesignThe above characteristics significantly limit the application of the VPH grisms for the MOS observation, because the target position virtually defines the observable wavelength range with high efficiency. The mask design will be complicated if the targets are scattered across the field of view. You will first have to consider the peak wavelength of the grisms at each target position, and then have to find out the targets that have a good match between the spectral feature of your interest and the peak efficiency. Unfortunately, it is currently impossible to display the peak and the observable spectral range effectively by wmdp_moircs.sav, the MOIRCS mask design software. Therefore, the observatory cannot support the design of the VPHs by the wmdp software. Those who go to observation must manage the design by themselves. Please use the wmdp_moircs.sav only for putting slits on the target and generating .sbr file. Note that you can change the spectral parameters manually by the menu ("Option" -> "Grism Parameter Setting") so you can make mimicked spectra of VPH. See section 3.7.4 of the manual. The ObservationWe can achieve the background-noise limited observation for the VPH grisms for normal slit width case within reasonable exposure times (<15 min per exposure) with 16-25 times multi-sampling CDS readout. A challenge related to the MOS observation by VPH would be the observation of the standard stars. A spectrum by the VPH grism basically covers the entire span of the detector in the dispersion direction. Thus the starting and the ending wavelength of the spectra will change from slit to slit (by the slit position) in the MOS data. So, the standard star data by a single representative slit (as we do for R500 grism) does not always work for other slits that are widely separated (in dispersion direction) from it. Thus we need to take the standard star data at a few positions to ensure the whole wavelength coverage. But, observing a standard star on several slits takes a long time, as we have to do the alignment at each slit. Doing the alignment by removing MOS mask for each slit position is not practical. We try to do this without removing MOS mask if possible. But unfortunately, it does not always work. As a compromise, we sometimes take standard star data by the slitless observation: we put the star at the slit position with the minimum (, middle), and the maximum X coordinates. A drawback of this is that it will make the resolution of the standard star data the seeing limited, and therefore the data will inevitably have a different resolution from the science data. And also, the standard star should be fairly isolated: unless otherwise the blending could ruin the data. If you don't need the calibration across the whole wavelength region of the spectra (e.g., emission-line measurement etc), these challenges can be greatly reduced. It is important to tell the SA about your intention on the data to avoid unnecessary overhead by standard star observation. Your feedback is greatly appreciated. The Old VPH Information Page (before 2016)We keep the old VPH information website before 2016 just for the record. Click here. Any questions should be directed to the Support Astronomer (Ichi Tanaka: ichi$naoj.org [change $ to @]). Last update: 2023-07-25 |