MOIRCS Multi-Object Spectroscopy Mode

The Layout of the Slit Mask

Figure 9: A layout of the slit masks.

Usually we ask all MOS observers to do the mask design based on the shallow MOIRCS images (the "preimage") of your fields. We take your preimages when your MOS targets are available at > 50 degree in elevation during the morning time, which will occur roughly 2 to 4 months before your observation. The PIs granted MOS time should start the preparation of the preimaging coordinates soon when you are announced getting the time. Sometimes we may ask you to submit the field coordinates in a short notice, as the chance for preimaging obs may come rather abruptly. Please prepare the coordinates and PAs carefully, because we usually cannot take any compensation data of the failure unless it is by our fault. Basic information and the preparation tips for pre-imaging obs can be found in the Pre-imaging Information page.

The observatory has a policy to ask ALL observers with MOIRCS MOS times to surrender a part of your time for MOS pre-imaging for other observers. Pooling pre-imaging times this way is necessary as your pre-images are also prepared by using observation times of other observer. All spectroscopy observation is requested to be completed one (1) hour before sunrise. Please note that from S08B this guideline is to be considered as the duty of all MOS observers by the recommendation of the 14th SAC Meeting. The actual loss by this operation rule may not be much large, as the autoguider (AG) will become unavailable soon due to the quick rise of the sky level in twilight. Please make sure that your observation program is completed by the time including the time for the mask store.

The actual operation under the above rule can be changed/revised by the situation, and the observers and the support scientist may need to negotiate about the actual plan. For example, you may be able to continue the exposure as long as the AG works if we do not have much pre-imaging fields. If you have a bit of non-target time during midnight, we may execute the preimage acquisition instead of the morning time. Please ask the SA for the plan for specific mornings when your observation become near. During the season when weather is generally bad, we may also have to ask to increase this pre-imaging pool time to more than one hour, and we will greatly appreciate it if you can kindly accept it.

By default, the pre-imaging will be executed in 30-sec exposures in Ks band with five to seven dithers with dither length of ~15 arcseconds. This may not be deep enough for your targets. What we request observers in such cases is to register your deep science image to the pre-image of the field taken by MOIRCS and choose the targets from the superposed science image, not from the pre-image itself. Therefore, it is strongly recommended that all applicants for MOS observation can prepare deep imaging data of your target fields in FITS format before the mask design. For more detail, please see the Pre-imaging Information page.

If you need much longer exposures for preimaging data than described above, or if you need special setting for preimage that may take long time, you must explicitly request the time for imaging observaton with the description about the detail (including the reason for special requirement) on your proposal (technical detail section). We execute it only when it is approved by the TAC. Any small change (e.g. change of the filter etc) may be okay if you send the SA about the change well before your preimaging date.

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. Therefore, all MOS observers are recommended to prepare for situations where pre-images by MOIRCS are not available. For example, you can use the Supreme-Cam or FOCAS data as a substitute of the MOIRCS preimage. If your own data is properly distortion-corrected, and if the pixel scale of the image is accurately known, you may use it for preimage, though the alignment accuracy may be limited. In the worst cases, you may have to switch your observation to the long-slit or imaging mode.

Usually the SA will provide the pre-image data soon after they are acquired, together with the quick data reduction scripts (or we may send the processed data instead). Each MOS observers has to prepare mask design files by about one month before your observation. Mask design files are called .mdp files, and have the same format as those for FOCAS MOS observation. The mask design program for MOIRCS called wmdp_moircs^(Note) based on the FOCAS MDP software (wmdp4) has been prepared with the help of the FOCAS SA. The Users Manual for MOIRCS MDP has been prepared.

Note:
The IDL-based program wmdp_moircs.sav can be downloaded from the website here.

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

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

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

Created by Yuka Katsuno Uchimoto (Feb 2006)
Revised by Ichi Tanaka (2008-06-09)