MOIRCS Multi-Object Spectroscopy Mode

The Layout of the Slit Mask

The layout of a long-slit mask and a multi-object slit mask projected on the sky are shown in the following diagram. Also, please refer to the FOV page.


A layout of slit masks.



Preparation of Imaging Data for Mask Design

Our MOS mask design software requires a FITS image for design. The observers are encouraged to prepare the image for design beforehand. The image has to be distortion-free, with the pixel scale accurately to 0.1170"/pix (NOT 0.1160"/pix!). Nowadays most imaging data reduction pipelines from the major observatories do a 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 good imaging data for the design, we recommend using 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.

    --> There is a recipe for converting your own or the PS1 images to the moircs preimage.

What we suppose you to do by the PS1 image is, not picking up your targets from the PS1 image, but finding some bright stars for the MOS mask alignment. We suppose that you have your own deep science images for picking up your targets. You'll first register your own deep images to the PS1 cut-out images by e.g., the IRAF geomap/geotran tasks so that your images can be converted to distortion-free data with 0.1170"/pix scale. Please contact the primary SA when you are not familiar with the image register. Unfortunately you cannot do the MOS mask design directly from the wcs information only.

If you have a deep image that is reliably distortion corrected, you can directly use the image after converting the pixel scale to 0.1170"/pixel (see the above recipe for conversion). However, there are two things you should be careful about. Firstly, you should check if the image is not too old. If it was taken more than 10 years ago, the position of the stars (for alignment) may noticably be different from the current position due to the proper motion. If you use such stars for the alignment, it could introduce the large alignment error. Secondly, be careful with the saturation of the alignment stars. Sometimes the stars in your deep images for science suffer from significant saturation. Saturation of alignment stars makes the centering of the software inaccurate. This again may result in a poor alignment.


Preimage acquisition 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 ("preimage observation") is an option. However, you should ask for the preimaging observation explicitly when you submit your proposal (you can put it in the Technical Justification Section with the reason). Once it is approved by the TAC, we will try to take your preimages. We do this when your MOS targets become available at > 50 degrees in elevation during the morning time, which will occur roughly 2 to 4 months before your observation. The chance for preimaging observation is quite limited, so we unfortunately cannot guarantee the acquisition of your preimages. The PI should start the preparation of the exact coordinates and the PAs of the preimaging fields immediately when your program is accepted. For more detail, please read the following Pre-imaging Information Page carefully.

As an observatory policy, we will ask ALL MOIRCS MOS observers to donate a part of your time for pre-imaging for other observers whenever necessary. We 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 determined to be the duty 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 the figure below).

    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<17 (Vega mag). If alignment stars are brighter than K=11, it may cause saturation on the alignment 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. It is always a good idea to check the magnitudes of the alignment stars using the 2MASS point source catalog during the mask design.

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.



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


About the Number of MOS Masks for Users...

Currently, 17 out of 21 ports of the MOS stocker are available to the open-use observers. As we need at least 4 days for exchanging the MOS masks (2 days for warm-up and 2 days for cooldown), 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 masks will be fixed once it is assigned by TAC's critical review. The request for any backup masks 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.

If you want to execute a MOS observation for your filler or "Sukima Jikan" program (a special program during the time your main targets are not available: see the Policy), please consult to the MOIRCS primary SA well beforehand. We may accept it only if (1) you do not require preimaging observation, and (2) if there is enough room for additional MOS masks in the mask stocker. The request have to 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 2023-02-05


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