Telescope

Offsets

The units for offsetting the telescope are arcseconds on the sky. The following approximation is sufficient for arcminute moves:

Right Ascension (RA)
offset [arcsec] = [RA1 - RA2] * 15 * cos(Dec1)
Declination (Dec)
offset [arcsec] = [Dec1 - Dec2]

Focus

The telescope should be focused to NIRSPEC at the start of each night. If there is a large temperature change as the night proceeds, we recommend doing a additional telescope focus later in the same night.

Initial Focus
This takes 10-15 minutes, Filters in the K-band (e.g. Nirspec-7) make this easier to do in twilight
  1. Observer sets up NIRSPEC in a JHK-like filter
  2. OA finds a 12th mag star and runs MIRA (telescope focus that takes about 10 minutes)
  3. OA runs a guider autofocus routine to focus the MAGIQ guide camera to NIRSPEC and the telescope (takes about 2 minutes)
Subsequent Focus
If ambient temperature changes significantly, another focus later in the night is reccommended.
  1. Observer requests another focus
  2. OA find suitable star and runs another MIRA (10 minutes)

Acquisition

Rotator

NIRSPEC has an internal rotator to derotate the sky. It is controlled via the Rotator GUI.

The Rotator GUI shows the current state of the mechanism. There are 3 panels left to right: The left panel shows the physical position of the rotator mechanism, with limits denoted in red. The center panel shows the position angle on sky for SCAM and the right panel shows the position angle on sky for the Slit. The rotator MODE is selected by a pull down menu located below the right panel. Physical angle or Slit PA angle destinations are entered into the boxes below the selected mode.

Rotator Stationary
This mode is typically used by thermal observers and those who require the highest precision radial velocities. The internal rotator mechanism does not move in this mode, and the sky field will rotate about the target that is positioned in the slit as the telescope tracks. Note that this is the default rotator mode and no action is needed unless a physical angle other than 0 is needed.
    Procedure
  1. Initialize the rotator from the Rotator GUI: Click the "Init" button.
  2. Slew to your target, tell the OA you will be keeping the rotator stationary
  3. Select the stationary mode from the pull down menu
  4. Type the desired physical angle destination value into the appropriate box in the Rotator GUI
  5. Click the "Apply" button.
  6. You will see rotator position of the Physical Angle moving to your inputted value. A progress bar for the movement is shown at the bottom of the GUI.
  7. Once the rotator is in position, the arrow movement in the left panel and progress bar will show no activity.
Rotator Position Angle
This mode is needed to keep two objects on the slit simultaneously or when using blind offsets to observe invisible objects. The rotator can slightly shift echelle grating position, at most by 1-2 pixels due to vibration. This is not an issue in low resolution mode.
    Procedure
  1. Initialize the rotator from the Rotator GUI: Click the "Init" button.
  2. Slew to your target, tell the OA you will be setting a position angle for this object
  3. Select the position angle mode from the pull down menu.
  4. Type the desired position angle value into the Slit PA destination box of the Rotator GUI
  5. Click the "Apply" button.
  6. Notice if the physical angle is approaching one of the red limits. If so, then change your desired PA by 180 degrees
  7. The rotator is in position once the SCAM and Slit PA dials stop moving, and the phyical angle settles at position that is not at or very near a red limit.
Rotator Verical Angle
This mode would be used to keep the slit orientated along the elevation axis of the telescope. Dfferential atmospheric refraction (DAR) is generally not an issue at infrared wavelengths, except possibly when observing at extremely low elevations (airmass > 3.0). This mode is presently not implemented.

Bright object acquisition

For objects brighter than about 13th magnitude, targets will be aquired on SCAM and aligned to the slit. Guiding with SCAM is not yet available, but the object will be clearly visible in the SCAM field with the slit. Ask the OA to acquire and guide on the annular guider.

This procedure will generally take less than a minute plus slew time.

    Procedure
  1. Tell the OA your next target (via name, ID, or MAGIQ highlight)
  2. Tell the OA that your object is bright
  3. Ask OA to slew (they may go to a pointing star first)
  4. Tell the OA whether you will be in stationary or position angle (PA) mode on the rotator
  5. If in PA mode, set PA during the telescope slew, or before Guiding is started
  6. Prepare to ID the target (if a crowded field)
  7. Once the OA has placed the object at the SCAM pointing origin, the observer will acquire it in the slit
  8. From a nirspec xterm, set up SCAM to take multiple images at the minimum exposure time with one coadd
  9. Acquire the object at the slit center
  10. Tell OA to "mark base" and "reset the SCOR" to the current position
  11. Starting Observing sequence using EFS "GO"

Faint object acquisition

For objects between 13th and 17th magnitude, ask the OA to acquire and guide on the annular guider. The SCAM guider will be under observer control for fine acquisition and verifying proper guiding.

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. Tell the OA what your approx. target vmag is
  3. Ask OA to slew (they may go to a pointing star first)
  4. Tell the OA whether you will be in stationary or position angle (PA) mode on the rotator
  5. If in PA mode, set PA during the telescope slew, or before Guiding is started
  6. Prepare to ID the target (if a crowded field)
  7. Once the target is identified, the OA places the object at the SCAM pointing origin & starts guiding with the Offset Guider.
  8. The observer will acquire the target in the slit, after guiding is established (observer first confirms with OA)
  9. From a nirspec xterm, set SCAM exposure time and coadds to their minimum values and take a test exposure
  10. Measure the background (and object, if visible) counts on SCAM
  11. Scale tint2 up to a higher SCAM inegration time (stay below 15,000 ADU/coadd for linearity)
  12. Increase coadds2 above 1, if needed
  13. Take a background subtracted frame with SCAM: From a nirspec xterm, type "snapi"
  14. Take continuous SCAM exposures with background subtraction
  15. Acquire the object at the slit center using the differenced frame display
  16. Tell OA to "mark base" and "reset the SCOR" to the current postion
  17. Starting Observing sequence using EFS "GO"

Invisible object acquisition - Offset Stars.

For objects dimmer than about 18th 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.

This procedure will take about 10 minutes plus slew time.

    Procedure
  1. Tell the OA that you will be aquiring an offset star to do a blind acquisition (Select the offset star from your starlist)
  2. Tell the OA what your approx. vmag is for the offset star
  3. Ask OA to slew (they may go to a pointing star first)
  4. Tell the OA whether you will be in stationary or position angle (PA) mode on the rotator
  5. If in PA mode, set PA during the telescope slew, or before Guiding is started
  6. Prepare to ID the target (if a crowded field)
  7. Once the target is identified, the OA places the object at the SCAM pointing origin & starts guiding with the Offset Guider.
  8. The observer will acquire the target in the slit, after guiding is established (observer first confirms with OA)
  9. From a nirspec xterm, set SCAM exposure time and coadds to their minimum values and take a test exposure
  10. Measure the background (and object, if visible) counts on SCAM
  11. Scale tint2 up to a higher SCAM inegration time (stay below 15,000 ADU/coadd for linearity)
  12. Increase coadds2 above 1, if needed
  13. Take a background subtracted frame with SCAM: From a nirspec xterm, type "snapi"
  14. Take continuous SCAM exposures with background subtraction
  15. Acquire the object at the slit center using the differenced frame display
  16. Tell OA to "mark base" and "reset the SCOR" to the current telescope position
  17. Ask the OA to apply offsets:
  18. Start Science exposures Observe

Note: You may wish to go back to the offset star after a single ABBA nod sequence to verify it is still in the slit.

Observing

Use the EFS to Abort Exposure Sequences

You may use the abort button on the EFS to abort an observing script that is in progress. Doing this will allow the current exposure to finish (which is strongly recommended). After the current exposure reads out, you will need to re-acquire the target to the slit center again. If you are worried about excessive counts on SCAM from the current exposure in progress, you may move the star off the slit while you are waiting for the exposure to end.

.

Minimum exposure times

SPEC: 1.5 seconds, sampmode=CDS

SPEC Exposure Times w/ReadMode
Sampmode # Read Pairs Exp. Time Range (sec)
CDS 1 1.5 - 2.9
MCDS 2 3.0 - 4.4
MCDS 3 4.5 - 5.9
MCDS 4 6.0 - 60
MCDS 16 61 and longer

NOTE: expsoures when commanded from the EFS will automatically set readout parameters according to this table.

SCAM: 0.67 seconds, sampmode=CDS

Detector saturation level (non-linearity begins)

SPEC: 25,000 DN per coadd

SCAM: 15,000 DN per coadd

1% non-linearity in CDS or MCDS mode. We have NOT seen evidence for persistence on the new SPEC or SCAM detectors

New NIRSPEC Detectors do NOT show persistance

A test of a 900 sec dark frame with SPEC that immediatetly followed an image of an arc lamp frame that showed strongly saturated lines exhibited NO evidence of persistance. SCAM images of bright targets are regularly saturated during acquisition, but also do not show persistance afterward.

What is the Thin Blocker?

The Thin blocker is a thin piece of PK50 glass used to block thermal emission. The NIRSPEC-N custom filters have small leaks in the thermal IR, so the Thin blocker is used to block this emission. However, in some filters (e.g. NIRSPEC-1), the thin blocker does add fringing at the ~50 adu level. Some observers may opt to select "Open" instead of "Thin" when configuring for their setup up to avoid this fringing, at the expense of elevated background in some (~1/3 of the orders) on the detector. In General: