NIRSPEC
Stationary Mode Guiding

Please Note

This page was originally written to guide observing with a broken image rotator, which was the case during early 2000. The image rotator is now working again, and normal "Position Angle" mode observing is the norm. There are still occasions when observers may want to work in stationary mode, such as when they want to preserve a fixed relation to the dewar window instead of a fixed angle on the sky. This mode may be desired by observers taking spectra in the thermal IR, who want to be able to subtract off "features" due to the dewar window.

Please note also that this observing mode is very inflexible. The chief limitations are:

This note was last updated 5 March 2001, by David Sprayberry.

Object Acquisition, Nodding, and Guiding

  1. Preliminary
    1. Observer: choose the menu entry NIRSPEC Control Menu - Enable Night-time Mode. Complete the question-and-answer script that pops up.
    2. Observer: In XNIRSPEC Instrument Control select Irot - Tracking On.
    3. Observer: In the image rotator GUI, set the rotator physical angle to 0.00 degrees by typing 0.00 in the lower-left text entry box and clicking the "Set" button next to that box..
    4. Observer: In XNIRSPEC (or using the EFS GUI in "Setup Only" mode) select the 0.43x24 arcsecond echelle-mode slit.
    5. OA: proceed to check pointing and run MAlign as usual, following the start of night procedures.
    6. OA: Move to the desired star or object, and point it to REFA (if you are using the NIRSPEC guider) or REF (if you are using the SCAM guider). Use "Adjust Pointing" on XGuide if the object is visible on the guider.
    7. OA: Point the object to the desired pointing origin on the slit, e.g., HNOD1 or HNOD2.
    8. OA: Select NIRSPEC as the guide camera on XGuide. NOTE: this step is essential if you have been using the SCAM guider to position the object; the following steps will not work with SCAM as the selected guider.
    9. OA: On XGuide,
      1. Enter the command hfudge 1 1 in the XGuide command box
      2. Select "HIRES mode guiding"
      3. Enter the object pixel x and y on the PXL camera as determined by the pointing origin in use (see table).
      4. Begin guiding on the NIRSPEC annular guider.
      PLEASE NOTE that these steps must be done with the "NIRSPEC" annular CCD guider as the selected guide camera. The hfudge and x y pixel entries will not "take" if you have selected SCAM as the guider when you enter them.

  2. Standard faint source acquisition
    1. Observer: Verify that you have control of SCAM by checking the "IR Guiding" status box on the top XNIRSPEC panel; it should say "off".
    2. Observer: Make sure that a filter is in place that allows SCAM to see light, e.g., KL.
    3. Observer: Take a SCAM image (with Test or Go or Snapi) to determine the object position.
    4. Observer: If the object is not properly centered on the slit, center it on the slit using the NIRSPEC Slit Nod Widget or the Tel -> Move Telescope feature on QuickLook.
    5. Observer: Take another SCAM image to verify the position. You can use the previous SCAM image as the sky to subtract, via Math -> Sdiff.
    6. Observer: Fine-tune the centering of the object using the NIRSPEC Slit Nod Widget as necessary.
    7. Observer:
      • If you are taking spectra with the L-prime, M-prime, or M-wide filter, select that filter now using XNIRSPEC. Then you can start your exposure directly using the XNIRSPEC SPEC exposure control window.
      • Alternatively, select your desired filter, grating settings, etc. on the EFSgui, and use Nod Pattern - Stare in EFSgui to begin the spectral exposure, after entering the desired exposure time, object name and co-adds.
    8. Observer: SCAM exposures can be taken periodically to determine if the guiding is good or if small slit nods are necessary. Do not use Snapi, which would offset the telescope for the sky exposure!
    9. Observer: When you want to "nod" back to the other position along the slit, ask the OA to stop guiding and move to the other pointing origin by name (e.g., HNOD2 if you have just finished on HNOD1).
    10. OA: Carry out the following steps:
      1. Stop guiding.
      2. Point the object to the new pointing origin.
      3. Enter the pixel x and y on the PXL camera as determined by the newly selected pointing origin (see table).
      4. Start guiding on the NIRSPEC annular guider.
      5. Tell the observer when guiding is running again.
    11. Observer: Return to step 1 on this list and check centering on the slit at the new pointing origin.

  3. Bright object acquisition and guiding

    If the science object is bright enough to guide on (mag. approx. brighter than 13.5), or the object is a calibration star, then we recommend guiding directly on-slit, which allows nodding to occur automatically via EFSgui scripts. Of course, this method can only be used when observing with a filter that allows the SCAM detector to receive light at a wavelength of less than 2.5 microns (e.g., K or KL). In these cases, the OA will take control of the SCAM, and set up and control guiding. The object should be placed at the spatial center of the slit prior to starting nod scripts with the EFSgui.

  4. Imaging

    For imaging on SCAM, the field rotation should not affect individual exposure times of common length (less than 2 minutes), except when observing near the keyhole. However, over the course of a box9 set of exposures, the field rotation means that simple shift and adding of the image set will cause the resulting mosaic to be blurred, as is done automatically at the end of the box9 script. Observers should note that additional post-processing to de-rotate the images will be required.

    For imaging, the REF pointing origin is recommended, because it is off the slit(s).

  • Blind object acquisition

    In this case a star with know offsets to the science object should be utilized. The distance to the offset star should be minimized since offsetting must be done open-loop (while guiding is off). Treat the star as the science object in IV, steps 1-5 above. Guiding must be stopped before the offset can be made, and then the OA or observer can apply the offsets to the science target. Restart guiding ASAP, and commence the spectral exposure. To nod (point) for a second or succeeding exposure, stop guiding, reverse the offsets, and then point the telescope to place the offset star at the new nod position (pointing origin). The accuracy of guiding and/or offsetting open-loop in this fashion can be estimated by noting where the offset star returns when the offsets are reversed.

  • Offset guiding on SCAM

    It is not possible to guide on an offset star on the SCAM guider. SCAM guiding with the rotator stationary can only be done on-slit, on the science object.

    Established Pointing Origins (nod positions)

    There are 6 pointing origins that observers would commonly use:

    1. SLIT Low resolution slit, near center.
    2. LNOD1 Low resolution slit, upper position.
    3. LNOD2 Low resolution slit, lower position.
    4. HNOD1 High resolution slit, right position.
    5. HNOD2 High resolution slit, left position.
    6. REF Near image center, off slit (for box9 initial position).
    The SCAM and PXL pixel positions for these pointing origins are in the following table. Following the table are images with the locations annotated.

    Pointing Origin PXL Diff "Object" SCAM Pixel Position
    REF 526, 497 125, 135
    SLIT 544, 508 131, 125
    HNOD1 552, 499 158, 131
    HNOD2 509, 509 107, 119
    LNOD1 532, 467 134, 168
    LNOD2 528, 541 129, 81

    Formula for conversion of Xim,Yim to PXL coordinates:

    Xpxl = 531.75 + 6.519*Xim
    Ypxl = 504.77 - 6.519*Yim

    Pointing Origins on 0.430x24 arcsec echelle slit:

    See the table above for current SCAM pixel positions associated with these pointing origins.


    Pointing origins on 42x0.570 arcsec low resolution slit:

    See the table above for current SCAM pixel positions associated with these pointing origins.

    List of other slits that may be used in this mode

    As noted above, the pointing origins needed for this mode were calibrated with the 0.430x24 arcsec echelle mode slit, and the 42x0.570 arcsec long slit. The other slits fall in different places in the focal plane, and so they are not guaranteed to work with these pointing origins. However, some of the other slits are very close to these two calibrated slits, and may be used successfully. The table below gives a list of all the NIRSPEC slits, with an estimate for each as to how close the relevant pointing origins are to the slit. This is a rough guide only; use any of these slits at your own risk!

    Slit name Distance from P.O. Relevant P.O.'s
    42x0.760 0-1 pixel LNOD1, LNOD2
    42x0.380 0-1 pixel LNOD1, LNOD2
    0.576x24 0-1 pixel HNOD1, HNOD2
    0.720x24 1-2 pixels HNOD1, HNOD2
    0.288x24 1-2 pixels HNOD1, HNOD2
    0.720x12 3 or more pixels HNOD1, HNOD2
    0.576x12 1-2 pixels HNOD1, HNOD2
    0.432x12 1-2 pixels HNOD1, HNOD2
    0.288x12 3 or more pixels HNOD1, HNOD2
    0.144x12 3 or more pixels HNOD1, HNOD2

    Re-defining a Pointing Origin Real-time

    Note: This procedure will semi-permanently change a pointing origin's position. It should only be followed after consulting with the Instrument Specialist on duty.

    1. The observer or OA takes control of the SCAM. Acquire a bright star, point to REF. Adjust pointing.
    2. Point to desired PO. Note that you will be re-defining this named PO's position.
    3. Using the telescope handpaddle in Instrument or RA/Dec frame, (or SCAM's Tel - Move Tel) steer the object on to the SCAM pixel already defined for this PO (see table). Zero offsets (set base) when in position.
    4. Change the handpaddle to XIM,YIM frame. Steer the star to the new pixel position for the pointing origin.
    5. When satisfied, save the position in the DCSgui table of pointing origins.
    6. You can test the new definition by pointing to REF, say, steer the star to pixel 125,135, zero offsets, then point to the "new" PO. Make another XIM,YIM offset to redefine if necessary.

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    Last modified: Tue Jul 26 16:49:55 HST