MOIRCS Multi-Object Spectroscopy Mode

The Layout of the Slit Mask

Layout of a long slit mask and a multi-object slit mask projected onto the detectors of MOIRCS is shown in the following diagram. Also, please refer to the FOV page.

Figure 9: A layout of the slit masks.

Preparation of Imaging Data for Mask Design

Our MOS mask design software requires a FITS image for design. The observers are enconraged to prepare the image for design beforehand. The image have to be distortion-free, with the pixel scale accurately to 0.1170"/pix. Nowadays most imaging data reduction pipelines from the major observatories do reasonably good job for it. For example, the HSC data reduced by the HSC pipeline, the PAN-STARRs data release 1 (DR1), and the Hubble Space Telescope Imaging data all works great for the design purpose. If you don't have a good imaging data for the design, we recommend to use the imaging data from the PAN-STARRs DR1 (PS1 image). Thanks to the nearly full northern sky coverage of the PS1 data, you can make the MOS design image from the PS1 image cut-out service for most cases.

What we suppose to do by the PS1 image is, not to pick up your targets from the image, but to find some bright stars for the MOS mask alignment. We suppose that you have your own deep science images for picking up the individual targets. During the mask design, you will find your targets from the image transposed on the PS1 image. In short, all applicants for MOS observation should already have your own deep imaging data of the target fields in FITS format beforehand. We do not accept cutting the MOS mask based on the wcs information only.

If you have the deep image that is reliably distortion corrected, you can directly use the image after converting the pixel scale to 0.1170"/pixel. However, there are two things you should be careful. Firstly, you should check if the image is not too old. If it was taken more than 10 years ago, sometimes the position of the stars (for alignment) are different from the present position significantly due to the proper motion. If you choose such stars for the alignment marker star, the alignment accuracy could be significantly bad. Secondly, be careful to the saturation of the alignment stars. Sometimes the deep image you have for the faint target research suffer from significant saturation of brighter stars which we can use for the alignment. Saturation of stars makes the centering of the software quite inaccurate, which again may result in a poor alignment of the MOS mask to the sky.

Preimaging for mask design: Guideline

If you have difficulty in finding the image you can use for the MOS mask design, taking shallow images of your science fields by MOIRCS beforehand ("preimaging observation") would be an option. It is highly recommended to ask the preimaging observation explicitly when you submit your proposal, in the technical justification section. Once it is approved by the TAC, we try to take your preimages. We do this when your MOS targets become available at > 50 degree in elevation during the morning time, which will occur roughly 2 to 4 months before your observation. The PI should start the preparation of the exact coordinates and the PAs of preimage fields immediately when your program is announced. It is however important to aware that the chance of taking preimages is very limited, and thus we unfortunately cannot guarantee the preimaging data acquisition. For more detail, please read the following Pre-imaging Information Page carefully.

As an observatory policy, we will ask ALL MOIRCS MOS observers to surrender a part of your time for pre-imaging for other observers whenever necessary. We really appreciate your cooperation. Therefore, all spectroscopy observers should be prepared to complete one (1) hour before sunrise(*1). Please make sure that your observation program can be completed by the time including the time for the mask store.

Please be aware that there is always a risk of failing the preimaging data acquisition due to bad weather, instrument problems, or any other unforeseeable causes, even though we make our best to obtain your pre-image in time for the mask preparation.

    (*1) This guideline has been considered as the burden of all MOS observers by the recommendation of the 14th SAC Meeting since S08B.

Preimaging data and the mask design software

The preimages (including the design images converted from the PS1 data etc) will usually be the shallow (a few minutes exposure) data. This is usually not deep enough to pick up your targets. What we suppose is that you superpose your own deep science images to the preimaging data by yourself, and choose targets from the superposed science image, not from the pre-image itself. Therefore, all applicants for MOS observation should already have your own deep imaging data of the target fields in FITS format before the mask design.

For the mask design you can use the dedicated software provided by us. The software, wmdp_moircs(Note), is the IDL-based program (you can run it with a free virtual machine). The mask design file has the same format as those for FOCAS: i.e., it is so called .mdp file. You can download the most recent version of the wmdp_moircs.sav from the website below.

    (Note) The software wmdp-moircs.sav is based on the FOCAS MDP software (wmdp4).

MOS Acquisition

The MOS acquisition sequence is summarized as follows (see Figure 10).

    1. Pointing
    2. Imaging the target field
    3. MOS mask install (at zenith position)
    4. Imaging the MOS mask
    5. Measure the position of the alignment stars and holes
    6. Offset calculation, and moving Telescope
    7. Re-imaging with MOS mask.
    8. Alignment check
    9. Final adjustment of telescope pointing
    10. Final imaging for confirmation
    11. MOS exposure

Typical magnitudes of alignment stars are roughly 11>Ks>16. If alignment stars are brighter than K=11, it may cause saturation on the alighment image and may introduce the position measurement error (though we can reject the star during the alignment). If the stars are too faint (or if you choose a galaxy instead of a star), again the measured position may become less accurate. A guideline is that you choose the stars for alignment from the 2MASS point source catalog.

Selecting more than three alignment stars for each channel is strongly recommended. It takes about 20 minutes for MOS acquisition (see the Guide to Exposure Time section).

The alignment accuracy is better than ~0".1 RMS, enough small compared to the typical slit width for a MOS observation of 0".8.

Figure 10: The MOS acquisition sequence (Tokoku 2006, Ph.D. Thesis)

About the Number of MOS Mask for Users...

Currently 15 out of 21 ports of the MOS stocker are available to the open-use observers. As we need at least 3 days for exchanging the MOS masks (a day for warm-up, and two days for cool down), these user ports have to be shared between all observers during a contiguous MOIRCS observing period. If you want to use more MOS mask than available for the period, your MOS observing run will be split into two or more periods that are separated by more than three days.

Please note that the number of MOS mask will be fixed once it is assigned by TAC's critical reviewing. The request for any backup mask is basically not permitted due to the limited availability of the slot. However, you can ask the observatory for additional masks, if there are any special reasons that were unpredictable during the proposal submission.

Similarly, you may want to execute MOS observation during the time your main targets are not available ("Sukima Jikan"). We may accept it only if (1) you do not require preimaging observation, and (2) if there is enough room for additional MOS mask in the MOS stocker. The request should be done at least a month before your observation.

Please note that all data on these pages are subject to changes as the evaluation of the performance of MOIRCS progresses.

Updated 2017-07-27

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