The units for offsetting the telescope are arcseconds on the sky. The following approximation is sufficient for arcminute moves:
The telescope must be focused to NIRSPEC at the start of each night. The MAGIQ system will eventually allow real-time focus correction, but that is still in testing. For now, observers must request additional telescope focus runs.
NIRSPEC has an internal rotator to derotate the sky. It is controlled via the Rotator GUI.
The upper panel of Rotator GUI shows the current state, the lower panel is for setup. There are 3 columns from left to right: The left column shows and controls the physical position of the rotator mechanism. The center column shows the position angle on sky for the slit and the right column shows the position angle on sky for SCAM.
For objects brighter than about 12th magnitude (and not in M-band), your best option will be to use the SCAM guider for acquisition and guiding.
The observing assistant will take control of SCAM and use it as a guider. While SCAM is being used as a guider, the observer will not be able to take SCAM images via keywords, scripts, or XNIRSPEC.
This procedure will take about 2 minutes plus slew time.
For objects between 12th and 17th magnitude, your best option will be to 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.
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.
Note: 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.
SPEC: 0.25 seconds, sampmode=CDS
SPEC: 4 seconds, sampmode=MCDS, 16 reads (minimum exposure time is scaled by the number of reads (0.25 x 16 = 4)
SCAM: 0.1 seconds, sampmode=CDS
SPEC: 18,000 DN per coadd
SCAM: 25,000 DN per coadd
1% non-linearity in CDS or MCDS mode, BUT, see also the question about persistence below.
Avoid saturation and over-exposure. Taking arc lamps in any mode can cause persistence. Please see the discussion on the troubleshooting page.
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 needed with them. Here's the rule:
If there is a menu entry for a Thick blocker, ignore it. The former Thick blocker has been removed and replaced with the AO pupil stop.
| Detector | Pixel Scale (arcsec / pixel) |
|---|---|
| SPEC (high-res) | 0.190 (along the slit) |
| SPEC (low-res) | 0.144 (along the slit) |
| SCAM | 0.178 |
| NIRSPECM (annular) | 0.200 |
N.B. these are approximate values as calculated by the EFS.
| Low Resolution | |||
|---|---|---|---|
| Filter | Grating Order | Dispersion (μm/pix) | Coverage per 1024 pix (μm) |
| N-1,N-2 | 4 | 2.1093e-4 | 0.216 |
| N-3,N-4,N-5 | 3 | 2.8123e-4 | 0.288 |
| N-6,N-7 | 2 | 4.2189e-4 | 0.432 |
| KL,M-wide | 1 | 8.4373e-4 | 0.864 |
The high resolution wavelength dispersion for NIRSPEC is a linear
function of wavelength:
Dispersion [μm/pix] = λ [μm] × 1.44654e-5
| High Resolution | |||
|---|---|---|---|
| Filter | Mean λ (μm) | Dispersion (μm/pix) | Coverage per 1024 pix (μm) |
| N-1 | 0.95270 | 1.3779e-5 | 0.0141 |
| N-2 | 1.19050 | 1.7225e-5 | 0.0176 |
| N-3 | 1.24909 | 1.8072e-5 | 0.0185 |
| N-5 | 1.58733 | 2.2976e-5 | 0.0235 |
| N-7 | 2.17687 | 3.1493e-5 | 0.0322 |
| KL | 3.04954 | 4.4089e-5 | 0.0451 |
| M-wide | 4.76205 | 6.8898e-5 | 0.0706 |
The nominal slit center on SCAM is at pixel (131,125).
The nominal slit center on NIRSPECM (annular) is at pixel (521,517).
Go to the TEL menu on the SCAM QuickLook panel, and select the "Move Telescope" entry in that menu. Left-Click once on the star and then left-click once again on pixel (131,125). If you are in rotator stationary mode, ask the OA to stop and restart guiding once your object is centered on the slit.
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:
Take a difference of two displaced images: use the SNAPI button on the SCAM control panel of XNIRSPEC. Please note this will move the telescope. You can go much deeper by doing a 9-point dither pattern: use the BOX9 button on the SCAM control panel of XNIRSPEC. 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.
To set up SCAM to periodically readout and subtract a background image:
There are many considerations for SPEC exposure times so if you are unsure, please read through all of these points:
Sky brightness (between OH lines) guidelines (N.B. The sky levels change during the night and night to night):
Each image taken is automatically displayed. Use a NOD2 pattern and take the difference of the two 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.
Depending on whether NIRSPEC is behind the AO system or not, and on what filter and blocker combination is used, echelle spectra may exhibit an underlying sinusoidal ripple. The effect is particularly pronounced in AO mode, where the slow, almost parallel beam (f/160) induces a Fabry-Perot pattern as it encounters the dewar window. In non-AO mode, interference occurs between the NIRSPEC order sorting filters and their associated blockers near the pupil plane. This effect can be almost completely removed with the application of a fourier domain filter, such as hanning. In non-AO mode, the fringing effect is usually less than a few percent. This may be reduced by taking out the long wavelength blocker, although care should be taken to characterize the impact of the red leak if the blocker is out.
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.
Invisible object acquisition is outlined here.
Especially when observing extremely faint (invisible) objects, one must consider his or her data reduction plan before observing. Spectra on NIRSPEC are tilted with respect to rows and coloumns on the SPEC detector. If your object has no continuum, you must correct for this tilt in another way.
The EFS will "gong" at the end of each script. If you prefer not to use the EFS (running via XNIRSPEC or keywords), but wish to have an end of exposure noise or sound:
You must take arcs and flats BEFORE you change the settings of either of the two Gratings or the Slit wheel.
Please note that taking arcs will cause localized persistence on the SPEC detector. Further discussion is on the troubleshooting page.
A useful observing pattern is: setup 1, object, cal star, arcs, setup 2, object, cal star, arcs, etc.
Another option is: setup 1, object, arcs, setup 2, object, arcs, etc. then repeat the sequence for the Cal star.
Maybe. Spectral shifts of up to +/- 2 pixels may occur because of vibration from image rotator when observing in slit P.A. mode or slewing to a new rotator angle. Night sky OH lines inherent to the spectra may be used for internal calibration if there is a discrepancy with the observed arc lamp lines.
Alternatively, rotator stationary can eliminate the movement of the echelle grating by eliminating image rotator motion and hence image rotator vibration.
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.
The Calibrations page describes how to take calibrations using either the the EFS or the CAT.
The EFS will give you minimum cals needed to calibrate your data:
The CAT gives you more flexibility in number of flats and lamps off frames.
If you wish to get pure dark frames, please follow this procedure:
NIRSPAO throughput is lower by a factor of 5 to 10 in spectroscopic mode and 3 to 4 in imaging mode.
Each pixel is about 10.6 times smaller in AO mode:
Only those filters in filer wheel 2 are available (see Specifications page).
You do not use the internal NIRSPEC rotator with AO and you will not see the Rotator GUI. Instead, you will set the AO rotator via the command line:
Also known as the "LUI", this displays the loop status, the laser state, and the guide star counts/Rmag.
The "targwave" is the rough central wavelength of your science setup.
To help the AO system with differential atmospheric refraction (DAR), please start the AO Targwave GUI (if it does not start automatically: "NIRSPEC Control Menu" --> "Restart AO TARGWAVE GUI"
The "targwave" is the rough central wavelength of your science setup.
Note that when the MAGIQ ObserverUI is launched, you will be viewing the ACAM guider rather than the usual NIRSPEC annular guider.
The ACAM field of view is 2 x 2 arcmin. The pixel scale is 0.125 arcsec/pix. There is a vertically offset ghost reflection in the camera optics--the upper one is real.
The NIRSPEC flat lamp intensity is manually controlled via a switch on the instrument. If your counts are low, it's possible this switch did not get set to the AO/high setting. Ask your support astronomer for help.
Under AO control, typically we do not specify differential rates for non-sidereal objects, rather we let the AO system guide on the non-sidereal object. This may change as development continues.
Please do not load non-AO scripts into the EFS or run non-AO scripts from the command line. This will affect the position of the AO pupil stop and you will lose valuable observing time.