MOIRCS VPH Grism Information Page
Open under the Shared-Risk Policy during S08B-09A
We will start the operation of two high-resolution VPH grisms (VPH-J and VPH-H) for the open-use users from S08B under shared-risk policy. The grisms have been prepared by Prof. T. Ichikawa (Tohoku Univ: the PI of MOIRCS project) and his VPH team, and all the basic characteristics has been examined by the team. The basic data was mostly taken inside the dome, and the performance verification is still not well done yet due to the bad weather during Engineering nights. Characterization is not fully done yet at the time of wrighting (Jan 2008).
We will summarize the basic characteristics of these VPH grisms in the webpage below. One important note is that currently the setting of VPH-J grism is not optimized for observation across full J-band window, as the grism has been prepared as the "user grism" and is tuned for their needs. Currently the peak sensitivity wavelength is shifted near the short wavelength edge of J-band window under the use with the Long slit. We are planning to make a special long slit that will enable us to choose the appropriate slit position so that we can observe the best effeciency at the wavelength of the interest. Users who would like to use the grisms should fully-understand their basic characteristicsby reading this page, as well as his reports (in Japanese only) if possible. Please explicitly describe about how your observation (scientific goal) is robust and executable under the current characteristics.
Specification
We list the basic specification of grisms in the table below. Note that these data was taken using the long slit (LS200709). The x coordinates (raw image) of 0".5-width slits (used for evaluation) are 1170 pixel and 823 pixel for channel 1 and 2, respectively.
| VPH-J | VPH-H | |
|---|---|---|
| Resolution(1) | 3050 | 2940 |
| Straight-through Wavelength [A/pix](2) | 12347 (Channle 1) 12357 (Channel 2) | 16687 (Channle 1) 16462 (Channle 2) |
| Pixel Scale [A/pix] | 0.955 (Channle 1) 0.958 (Channle 2) | 1.317 (Channle 1) 1.314 (Channle 2) |
| Shift of spectra in spatial direction [pix](3) | -40 (Channle 1) +52 (Channle 2) | +135 (Channle 1) +80 (Channle 2) |
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(1) Measured under 0".5-width slit, and at 1.25um and 1.65um for VPH-J and VPH-H, respectively.
(2) The value will shift slightly along with the slit position used (x coordinate in raw image).
(3) In raw image coordinate. In the preimage coordinate, positive shift in y(raw) will be negative shift in X (preimg).
(4) The result of the linear fit for the pixel-wavelength relation is as follows (in unit of angstrom and pixel in raw image x coords).
| VPH-J (Ch1) | Lumda = 0.955 x + 11227 |
| VPH-J(Ch2) | Lumda = -0.958 x + 13146 |
| VPH-H (Ch1) | Lumda = 1.317 x + 15152 |
| VPH-H(Ch2) | Lumda = -1.314 x + 17543 |
Grism Effeciency and its Characteristics
The effeciency of the grisms have been measured using the standard star data, which is shown in the following figures. The effeciency here includes all the loss by telescope and moircs as well as the atomosphere. The peak effeciency of VPH grisms only has been estimated to be 0.7-0.8.
Figure 1a: Effeciency of VPH-J grism for long slit (CH-2 data: with the slit position x=821 pixel in raw image). Waterly-blue line indicates the J-filter transmission curve for reference. Note that we can use OC_ZJ order-sorting filter (0.9--1.78um) with the grism for observation shorter than J-band window.
Figure 1b: Effeciency of VPH-H grism for long slit (CH-2 data: with the slit position x=821 pixel in raw image). Faint green line indicates the H-filter transmission curve for reference.
The important characteristics of these grisms is that the sensitivity curve will shift from the one shown in Figure 1 when the slits at different x position (in raw image) is used. The behavior is schematically shown in Figure 2 (note that the curve shown here is the designed one and different from the current setting). You can see that the move of the curve is actually large For example, the J-band sensitivity of VPH-J grism for slit at x~300 pixel in chip2 (near pointing coordinate) will be almost 0. For the case for the MOS use, the sensitivity curve for each slit will be the strong fuction of the x-position of the slit. Users who plan to use the grism for MOS should check the expected sensitivity for each target.
Figure 2: The shift of effeciency for VPH-J grism with the slit x coordinates. The curve shown here is not the actual data but the designed curve. The level of relative offset is correct, and the level of the shift can be approximated linearly.
As is already described at the top of the page, we may try to change the peak sensitivity position of VPH-J to longer wavelength than the current value. The chance of the work will be the same as the chip-1 replacement work. Currently, the situation is still unclear whether we can get new detector by July 2008. If it becomes available and we work for these issues, the sensitivity curve for S08B will change.
Original VPH report(Japanese only: by courtesy of Prof. T. Ichikawa)
The link below lists the reports prepared by Prof. T. Ichikawa. We would like to thank him for allowing us to open the original reports.
Any questions should be directed to the Support Astronomer (Ichi Tanaka: e-address is below).
Last update: 2008-07-31
by Ichi Tanaka (ichi$naoj.org [change $ to @])