Stationary Mode Guiding

• Introduction and Cautionary Notes
• Bright Object Acquisition and On-Slit Guiding
• Faint Object Acquisition and Offset Guiding (Sidereal Objects Only)
• Faint Object Acquisition and Offset Guiding (For NON-Sidereal Objects)
• Established pointing origins, including SCAM and PXL pixel locations
• List of other slits that may be used in this mode

Introduction

This guiding mode was originally developed to allow 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 for most programs, at least at wavelengths in K-band or shorter. There are still occasions when observers may want to work in stationary mode. Here are some situations where stationary mode guiding may be desirable.

• When the observers want to preserve a fixed relation to the dewar window instead of a fixed angle on the sky, to better subtract off "features" due to dewar window contamination. This will be especially important for science programs in the thermal IR where the "features" show up in emission.
• When the observers need to observe non-sidereal science targets while guiding on the PXL offset guider. Observations of a comet in the thermal IR, where SCAM is not useable, are the best example of this situation.
• When the observers want to eliminate any possibility of echelle grating shifts due to rotator vibration. See this note in the FAQ list for more information about grating movements.

Observers working with sidereal objects are now advised to follow the newer procedure for smoother and more efficient operations. Many Thanks to Joel Aycock for suggesting this method, testing it, and writing the first draft of this description. Mahalo, Joel!

The old procedure is still available. Although it is very inefficient, cumbersome, and error-prone, it is the only way to obtain accurate guiding and tracking on non-sidereal objects. Observers with very slowly moving non-sidereal objects (e.g., Kuiper Belt Objects) may want to try the more efficient newer procedure, but should do so understanding that the guiding performance may be less than optimal.

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

• This mode is guaranteed to work only at rotator physical angle=0.0 degrees. The pixel positions of the pointing origins on the PXL guider have been calibrated only for the one rotator position of 0.0 degrees.
• This mode has been calibrated only with the 0.43x24 arcsec echelle-mode slit and the 42x0.57 arcsec low-res slit. The other slits are in slightly different locations in the focal plane, so the pointing origins needed for this method are not guaranteed to work with other slits. Please see this note for a rough guide to the use of this mode with other slits.

Bright Object Acquisition and On-Slit Guiding. Highly Recommended!

On-Slit guiding with SCAM is by far the easiest, most efficient, and most trouble-free way to observe with a stationary rotator. No special actions are required of either the observer or the OA. With the rotator in stationary mode, the OA simply takes control of the SCAM guider, acquires the object on SCAM on the SLIT pointing origin, centers the object on the slit and starts on-slit guiding as usual. The observer then runs the normal nod sequences from the EFS gui as always.

However, to use this mode, three conditions must be met by the observing program:

• It must involve taking spectra at a wavelength where SCAM guiding can be used. The KL filter is the longest-wavelength filter that allows SCAM to work.
• The science targets must be bright enough to permit on-slit guiding. A rule of thumb is that the object should be not fainter than about 13.5 mag in K band. If a narrow-band filter is in use, the object must be correspondingly brighter to compensate for the narrower bandpass on SCAM.
• The observing program must not require saving of simultaneous SCAM images and spectra. The observer cannot save SCAM images while the OA is using SCAM as a guider.

Faint Object Acquisition and Offset Guiding. (Sidereal Objects Only)

During testing in March 2001, the following procedure resulted in excellent guiding and tracking, with minimal overheads. It was also MUCH less exhausting and less error-prone than the "old" offset guiding procedure of changing pointing origins by hand, both for the observer and for the OA. This mode can be used when the object is too faint for on-slit guiding, or when the spectral observations are at a wavelength where SCAM is insensitive (for example, M-band), or when the observer needs to use SCAM to save science images. Otherwise on-slit guiding is recommended as being more efficient and easier to set up.

Please note: This offset guiding procedure should work with any of the "on slit" pointing origins: SLIT, HNOD1, HNOD2 in high-resolution echelle mode; or SLIT, LNOD1, LNOD2 in low-resolution mode. However, due to limited availble engineering time, it has only been rigorously tested on the LNOD2 pointing origin. The procedure below is written for the SLIT pointing origin in confidence that it will work there. If any problems are encountered, please notify David Sprayberry or Grant Hill by email.

Procedure:

• Observer: Select a filter that allows SCAM to see light. If your science spectra will be at M-band, change to the KL filter now.
• OA: Tell the observer you are taking control of SCAM, and select the SCAM guider.
• OA: Set PO = REF, use "Adjust Pointing" to collimate carefully on a nearby GSC star.
• OA: Set PO = SLIT.
• OA: Move to science object.
• OA: If science object is visible, center at SCAM pixel for the chosen pointing origin (see table); if not,tell the observer to center at that pixel before starting his NOD script.
• OA: Select "Nirspec" PXL annular guider.
• OA: Set DIFF Guiding to "ON"
• OA: Click "S" (send) on sEL and sParan, also on "Wavelength" (0,0)
• OA: Enter the PXL pixel X and Y coordinates for your chosen Pointing Origin (from the table), into "OBJECT X/Y" fields,
  and click on "OBJECT".
• OA: Start guiding on the PXL.
• Observer: Center the science target on SCAM pixel for the chosen pointing origin (see table) if the OA has not already done so; use the "Snapi" or "Box9" buttons on the SCAM control panel of XNIRSPEC if you need deeper, sky-subtracted images to see the object.
• Observer: Change filters and make other instrument reconfigurations as needed for the science observations.
• Observer: Proceed as usual to take the science observations using the EFS gui to run whatever nod sequences are desired.

Xguide currently leaves "DIFF" mode "ON" when switching between cameras (ie from PXL to Scam), even tho the GUI will show "DIFF OFF"... You MUST click "DIFF Guiding OFF" after switching back to the Scam guider, even tho it appears to already be turned off.

Faint Object Acquisition and Offset Guiding. (NON-Sidereal Objects)

1. 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.
2. OA: Point the object to the desired pointing origin on the slit, e.g., HNOD1 or HNOD2.
3. 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.
4. OA: On XGuide,
   1. Enter the values sEl = 1 and sParange = 1 in the appropriate in the XGuide entry boxes
   2. Select "DIFF 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.
5. Observer: Verify that you have control of SCAM by checking the "IR Guiding" status box on the top XNIRSPEC panel; it should say "off".
6. Observer: Make sure that a filter is in place that allows SCAM to see light, e.g., KL.
7. Observer: Take a SCAM image (with Test or Go or Snapi) to determine the object position.
8. 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.
9. Observer: Take another SCAM image to verify the position. You can use the previous SCAM image as the sky to subtract, via Math -> Sdiff.
10. Observer: Fine-tune the centering of the object using the NIRSPEC Slit Nod Widget as necessary.
11. 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.
12. 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!
13. 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).
14. 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.
15. Observer: Return to step 5 on this list and check centering on the slit at the new pointing origin.

Established Pointing Origins (nod positions)

There are 6 pointing origins that can be used for offset guiding, although SLIT should be the most commonly used, as the EFS nod sequences assume the science target is centered along the length of the slit.

1. SLIT Low or High 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).

This table last updated on 26 July 2005.

4 August 2001 Formula for conversion of Xim,Yim to PXL coordinates:

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

25 July 2005 Formula for conversion of Xim,Yim to PXL coordinates:

Xpxl = 512 - 6.517*Xim
Ypxl = 156 + 6.517*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!

• Slits that are 0-1 pixels off the pointing origin should work well.
• Slits that are 1-2 pixels off the pointing origin may work for shorter integrations, but will probably require more setup time and more care in monitoring to make sure the object stays on the slit.
• Slits that are 3 or more pixels away can be expected not to work. The object will probably not stay on the slit for long enough to be useful.