Observing Procedures

Summary

Telescope Focus

The telescope is focused by the OA using the MIRA procedure with the slit viewing camera. Based on our commissioning data we recommend:
  • Perfom one MIRA at the start of the night close to the first target (within 20 degrees in elevation).
  • Perform another MIRA whenever the next target is farther than 45 degrees in elevation from the initial or subsequent MIRA positions. This is specially important if the initial MIRA was performed at high elevation. If any other target is observed below 45 degrees a new MIRA is strongly recommended. Remember to perform another MIRA if the next targets are at high elevation.
  • If there is a problem with MIRA, it is possible to run autofocus with the SVC (scam). In this case, the OA will start guiding with the scam. Be aware that focussing the telescope with the optical guider is not reliable. A somewhat faster method would be to run autofocus with the optical guider.

Definitions

Rotator

NIRES has an internal rotator to derotate the sky. It is controlled by the OA as it is a facility rotator. The observations can be performed with the rotator in one of the following configurations:

Rotator Stationary
In this mode, the external rotator mechanism does not move.
    Procedure
  1. Slew to your target, tell the OA you will be keeping the rotator stationary
  2. After the slew is completed, communicate the desired physical angle to the OA and verify it on the FACSUM (drive angle)
  3. One drawback of this method is the fact that the guider will rotate with the sky so the guide star can be lost during the observation.
Rotator Position Angle
This mode is needed to observe in the parallactic angle, to keep two objects on the slit simultaneously, when there are only specific PA for which good guide stars were found, or when using blind offsets to observe invisible objects.
    Procedure
  1. Slew to your target and inform the OA if you want to observe at a specific PA or at the parallactic angle. A specific angle was hopefully entered in the starlist.
  2. Verify in FACSUM that the rotator is at the requested PA.
  3. Check that the telescope is guiding and that the PO is either "NIRES" or "REFSLIT."

Pointing Origins

NIRES has several pointing origins. The following table describes them in detail. For faint targets we suggest to use the NIRES PO whereas bright targets can be send to the REFSLIT for a more efficient acquisition..

Guiding

The main guider is the optical (visual) guider . This guider is controlled by the OA and its images are displayed on Magiq.

The Slit-viewing camera (scam) can also be used as a guider. This is a backup guider that can be used if:

  • The optical guider is malfunctioning.
  • The target is located on a region of excessive obscuration in the visual and therefore guiding in the near-IR (K band) is the only option.
  • The target would need to be Observed""" for a long period of time (i.e., several hours) and there is concern between the flexure between the optical guider and the slit.

    More information about this type of guiding and its advantages/disadvantages can be found here

    Acquisition

    Bright object acquisition

    Objects brighter than about 12th magnitude in K do not need sky subtraction and the acquisition is done with the SVC.

    This procedure will take about 2 minutes plus slew time.

      Procedure
    1. Tell the OA your next target (via name, ID, or MAGIQ highlight)
    2. Ask OA to slew (they may go to a pointing star first)
    3. Tell the OA whether you will be in stationary or position angle (PA) mode on the rotator
    4. Prepare to ID the target (if a crowded field)
    5. After centering the object, the OA will send it to a pointing origin. Please tell the OA if you want the target on either the "NIRES" or the "REFSLIT" pointing origins. The "NIRES" PO is slightly offset from the slit.
    6. Use the SVC xterm or the "NIRES_SCAM_CONTROL" GUI to setup the SVC to take an appropriate integration time and coadds.
      Sit-Viewing Camera (SCAM) GUI
    7. Verify the telescope is under guider control
      • FACSUM will indicate a green LED for guiding,
      • or ask the OA.
    8. On the SVC xterm type 
       goiv
      or press "SINGLE EXPOSURE" on the "NIRES_SCAM_CONTROL" GUI. This script will take an object image and display it on the quicktool.
    9. Use the "Tel" --> "Move Telescope" feature of the Slit Viewer Display to center the object.
      • Click on the center of the star
      • Click a second time on the slit center (pixel 123,466)
    10. Take an image to monitor the position of the object in the slit. Repeat as needed. 
       testv
      or press "TEST" on the "NIRES_SCAM_CONTROL" GUI.
    11. Use the Slitnod Widget to finely center the object on pixel (123, 466)
    12. If in stationary rotator mode, ask OA to reset the SCOR to the current position.
    13. Observe

    Faint object acquisition

    For objects between 12th and 17th magnitude, your best option will be to use sky subtraction for the acquisition.

    This procedure will take about 5 minutes plus slew time.

      Procedure
    1. Tell the OA your next target (via name, ID, or MAGIQ highlight)
    2. Ask OA to slew (they may go to a pointing star first)
    3. Tell the OA whether you will be in stationary or position angle (PA) mode on the rotator
    4. Prepare to ID the target on the optical guider near the "REF" pointing origin
    5. After centering the object, the OA will send it to a pointing origin. Please tell the OA if you want the target on either the "NIRES" or the "REFSLIT" pointing origins. The "NIRES" PO is slightly offset from the slit.
    6. Use the SVC xterm or the "NIRES_SCAM_CONTROL" GUI to setup the SVC to take an appropriate integration time and coadds for the sky and target image. Verify this on the SVC status display.
    7. Verify the telescope is under guider control
      • FACSUM will indicate a green LED for guiding,
      • or ask the OA.
    8. On the SVC xterm type 
       snapi
      or press "SNAPI" on the "NIRES_SCAM_CONTROL" GUI. This script will:
      • take a target image
      • nod the telescope (default is 10 arcsec N and E)
      • take a sky image
      • nod back
      • display the sky subtracted image on the SVC display.
    9. Use the "Tel" --> "Move Telescope" feature of the Slit Viewer Display to center the object (the positive "white" image, not the negative "black" image)
      • Click on the center of the star
      • Click a second time on the slit center (pixel 123,466)
    10. If you have not taken a sky image with "snapi" you could take one manually.
      • Offset the telescope with 
        en 5 5
      • Take a sky image 
         goiv
        or press "SINGLE EXPOSURE" with the "NIRES_SCAM_CONTROL" GUI.
      • slew the telescope back 
         en -5 -5
      • store the sky image as a "sky image" for the rest of the NIRES software 
                  makeskynires last_sky_image
        (e.g. makeniressky 15)
    11. Setup the SVC to run continuously by using bgsubv (does not save images) of bgsavev (saves the images). 
       bgsubv -1 5
      This will start continuous obs mode or run 
       bgsubv
      to take individual images as needed. If one needs to stop taking these images, then use Control-C when not exposing.
    12. Use the Slitnod Widget to finely center the object on pixel (123,466)
    13. If in stationary rotator mode, ask OA to reset the SCOR to the current position.
    14. Observe

    Invisible object acquisition - Offset Stars.

    For objects dimmer than 17th magnitude, acquisition is more involved. We suggest you find an offset star within about 1 arcminute of your target and determine the offsets in arcseconds east and north. We will acquire the offset star, then offset the telescope to the science target.

    The rotator must be in PA mode for this type of observation.

    This procedure will take about 10 minutes plus slew time.

      Procedure
    1. Tell the OA your next target (via name, ID, or MAGIQ highlight)
    2. Ask OA to slew (they may go to a pointing star first)
    3. Tell the OA that you will be using position angle (PA) mode on the rotator
    4. ID the offset star on the guider near the "REF" pointing origin. If the offset stars are obvious, the OA may easily ID the field.
    5. After centering the object, the OA will send it to a pointing origin. Please tell the OA if you want the target on either the "NIRES" or the "REFSLIT" pointing origins. The "NIRES" PO is slightly offset from the slit.
    6. Use the SVC xterm or the "NIRES_SCAM_CONTROL" GUI to setup the SVC to take an appropriate integration time and coadds for the sky and target image. Verify this on the SVC status display.
    7. Verify the telescope is under guider control
      • FACSUM will indicate a green LED for guiding,
      • or ask the OA.
    8. On the SVC xterm type 
       snapi
      or press "SNAPI" on the "NIRES_SCAM_CONTROL" GUI. This script will:
      • take a target image
      • nod the telescope (default is 10 arcsec N and E)
      • take a sky image
      • nod back
      • display the sky subtracted image on the SVC display.
    9. Use the "Tel" --> "Move Telescope" feature of the Slit Viewer Display to center the object (the positive "white" image, not the negative "black" image)
      • Click on the center of the star
      • Click a second time on the slit center (pixel 123,466)
    10. If you have not taken a sky image with "snapi" you could take one manually. Note that by running snapi the sky image is automatically stored.
      • Offset the telescope with 
        en 5 5
      • Take a sky image 
         goiv
        or press "SINGLE EXPOSURE" on the "NIRES_SCAM_CONTROL" GUI.
      • slew the telescope back 
         en -5 -5
      • store the sky image as a "sky image" for the rest of the NIRES software 
                  makeskynires last_sky_image
        (e.g. makeniressky 15)
    11. Setup the SVC to run continuously by using bgsubv (does not save images) of bgsavev (saves the images). 
       bgsubv -1 5
      This will start continuous obs mode or run 
       bgsubv
      to take individual images as needed. If one needs to stop taking these images, then use Control-C when not exposing.
    12. Use the Slitnod Widget to finely center the object on pixel (123,466)
    13. OA marks base (zero the offsets)
    14. Ask the OA to apply offsets:
      • Offsets should be specified in the starlist (see: starlist format)
      • If not in the starlist, give the OA the offsets to the target: arcsec_east arcsec_north
    15. Start Science exposures Observe

    Note 1: Some observers choose to return to the offset star between each spectrum to perform the nod. While there is an efficiency hit, one will be certain that the object is in the slit each time.

    Spectroscopy Observations

    The spectroscopy observations can be started from either the command line of the SPEC GUI. Before taking images please verify that the H2RG detector settings are reasonable (i.e., exposure time, number of reads, and coaads). Please remember that aborting H2RG exposures might cause significant time loss due to a possible introduction of excessive noise. The only way to mitigate this noise is to restart the detector service which would take about 10 minutes. The settings can be changed using the command line scripts or the SPEC GUI (please remeber to press "CONFIGURE" to commit the changes.

    Command Line

    In order to start spectroscopy exposures, type 

     gois n
    on the niresserver2 xterm. n is an optional argument that defines the number of images to take (default is 1).

    There are several dithering scripts available for the observer: abba, abcpat, abpat, and abbaprime. These scripts will perfom a sequence of telescope moves and image taking. In order to understand what each script does please type 

     help script
    on an xterm.

    GUI

    Either press the "SINGLE EXPOSURE" button for single exposures or use one of the abba sequences. The "ABBA" button starts a single ABBA sequence whereas the "ABBA (seq + SVC)" button starts an ABBA sequence with syncronized spectrometer and scam images.

    Spectroscopy GUI
    In the latter case, please make sure that the overall exposure time in the scam is smaller than the SPEC observing time.

    Either press the "SINGLE EXPOSURE" button for single exposures or use one of the abba sequences. The "ABBA" button starts a single ABBA sequence whereas the "ABBA (spec + SVC)" button starts an ABBA sequence with syncronized spectrometer and scam images.

    In addition, the observer can also design any other dithering script by selecting the CUSTOM tab. These dithering scripts are simple ascii files. The name of this file is arbitrary. The first line contains the number of steps and the next lines the absolute dithering distance in arcsec (along the slit). At the end, the script commands the telescope back to base. For instance, an AB sequence that would move the target along the slit +2.5 and -2.5 arcseconds will consist on the following lines: 

    2
 -2.5
 2.5
    The observer has the two options as well: DITHER and DITHER (spec + SVC). Note that this file (named abpat.txt in this example) must be located in the home directory for the observer account.
    Spectroscopy GUI with CUSTOM tab selected

    Observing Considerations

    Do NOT abort exposures

    Aborting NIRES exposures is not recommended. We have found that aborting H2RG exposures produces significant time loss due to a possible introduction of excessive noise. The only way to mitigate this noise is to restart the detector service which would take about 10 minutes. Restarting the detector service is the suggested procedure after aborting exposures.

    So please check the configuration for number of reads, coadds, and exposure time before starting observations.

    Minimum exposure times

    SPEC: 1.4 seconds, sampmode=CDS

    SPEC: 4 seconds, sampmode=MCDS, 16 reads (minimum exposure time is approximately scaled by the number of reads (0.5 x 16 = 8)

    SPEC Minimum Exposure Times
    Sampmode Number of Reads Min. Time (s)
    2 n/a 0.5
    3 2 1.4
    3 4 2.4
    3 8 4.8
    3 16 9.6

    SVC: 1 second, sampmode=CDS

    Detector saturation level

    Spectroscopy: 20,000 DN per coadd

    SVC: 30,000 DN per coadd

    1% non-linearity in CDS or MCDS mode, BUT, see also the question about persistence below.

    How do I avoid persistent images?

    Avoid instant and/or prolonged persistence. Typically values over 4000 DNs would cause persistance on the H2RG. This persistance goes away with time (10-15 minutes). However, if you plan to observe bright targets be aware of the fact that the detector is continously reading out. So the detector can accumulate persistance if a bright source is left on the slit. Also, be aware that if a target will be observed repeatedly on the same pixels, then the persistence will last longer than described before.

    The dome arc lamps are faint and observing them for under 2 minutes does not cause persistance. .

    Pixel Scale

    DetectorPixel Scale (arcsec / pixel)
    Spectroscopy 0.15 (spatial axis)
    SVC 0.126
    Optical Guider0.22

    Slit center

    The nominal slit center on SVC is at pixel (123, 466).

    Slit Imperfection

    The slit has an imperfection located 6.2arcseconds from the slit center (approximately 0.5 arcsecond in size). The following link has a sketch of the defect. We suggest to avoid this sector of the slit. Therefore, please use an AB offset smaller than 11 arcseconds or larger than 14 arcseconds.

    Target Acquisition (two methods):
    Single Click
    1. Go to the TEL menu on the SVC Quicklook panel and select the "Slit Center" entry in that menu (just below "Move Tel")
    2. Left-Click once on the star

    No other input is needed. The telescope will then move the star to the slit center position located at pixel (123,466).

    If you are in rotator stationary mode, ask the OA to click "SCOR" or to stop and restart guiding once your object is centered on the slit.

    Two Click
    1. Go to the TEL menu on the SVC QuickLook panel and select the "Move Telescope" entry in that menu
    2. Left-Click once on the star
    3. then left-click once again on pixel (123,466).

    If you are in rotator stationary mode, ask the OA to click "SCOR" or to stop and restart guiding once your object is centered on the slit.

    Typical SVC exposure times

    Objects brighter than 12th should be visible in a 1 sec exposure.

    Maximum exposure times depend on the sky level which can vary during the night. Generally th emaximum exposure time should be under 40 seconds. Use coadds to go deeper, but also consider subtracting the background via SNAPI or bgsubv.

    Nothing in short (5s) SVC exposure, what should I do? (SNAPI)

    Take a difference of two displaced images: using the 

    snapi
    script run from the SVC xterm or press "SNAPI" on the "NIRES_SCAM_CONTROL" GUI. Please note this will move the telescope. You can go much deeper by doing a 9-point dither pattern: use the BOX9 script on the SVC xterm. In both cases, the telescope is returned to its original position after the dithers are done so that you can use Move Tel on the displayed image.

    SVC auto background subtraction (bgsubv)

    To set up SVC to periodically readout and subtract a background image:

      Procedure
    1. On the SVC xterm type 
       snapi
      or 
       goiv
      to save an image to use as the background and note its frame number. For example, if the image name is "s180120_0010.fits", the frame number is "10".
    2. Run 
      makeskynires 10
      on the SVC xterm where 10 is the frame number.
    3. Run 
      bgsubv -1 5
      on the SVC xterm. "-1" means run continuously, "5" is the time to sleep between images. I recommend at least 5 seconds of sleep time between images to minimize skipped subtractions. Even at 10 seconds between images, the sky subtraction will skip occasionally. The duty cycle will be itime x coadds + readout time + sleep time.
    4. Use "CTRL-C" to stop the script when not exposing.
    5. Note that the bgsubv script does not save any frames. If the observer would like to save the images then use bgsavev.

    Typical Spectroscopy exposure times

    There are many considerations for Spectroscopy exposure times so if you are unsure, please read through all of these points:

    Brightness of target
    Your data will be better if you do not saturate the H2RG detector or expose to a level that may cause persistence. If you are unsure of the target's brightness, take a 1 second test exposure with this script 
    	    tests
    on the spectroscopy xterm.
    Changing sky conditions
    The infrared sky changes on short time scales. With long exposures, you'll likely under- or over-subtract the sky lines. You'll have to get creative in your data reduction.

    Sky brightness (between OH lines) guidelines (N.B. The sky levels change during the night and night to night):

    • J : 1200 sec
    • H : 900 sec
    • K : 600 sec

    How can I see the spectrum taken?

    Each image taken is automatically displayed. Use a ABBA pattern and take the difference of the two A and B exposures using the Sdiff entry under the MATH menu in QuickLook. The Sdiff routine subtracts the most recent image from the one before (e.g., if the two most recent images are 34 and 35, Sdiff displays 34 minus 35), and the Arithmetic option allows any arbitrary image pair to be subtracted. Under the PLOT menu on QuickLook, you can use the Horizontal, Vertical or Diagonal entries to graph a cut through the displayed image.

    What is the format for non-sidereal targets?

    The differential rates required by the telescope are different that those most common (such as from JPL Horizons).

    Horizons gives you dRA*cos(DEC) and d(DEC)/dt in arcseconds per hour. At Keck, you must divide the RA rate by 15 into the psuedo-unit of seconds per hour.

    Note also that the values of dra and ddec as reported by DCS are per second, that is the values reported by DCS are the values from your starlist divided by 3600.

    Calibrations

    When do I need to take a set of arcs and flats?

    Arcs and flats can be taken in the afternoon.

    What is a good Cal star?

    The Cal star is needed to remove the atmospheric extinction (by division). The best Cal star is one with no (or very weak) intrinsic spectral features. Isolated features can be interpolated. A0V stars are useful (mainly in J and K); O stars are helpful for H-band where A-stars have too many lines. It is best to keep the airmass difference at 0.1 or less.

    How do I take calibrations?

    The Calibrations page describes how to take flat calibrations.

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    Last modified: 09/08/2023 11:44
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