Afternoon setup
- Review spectroscopy section of NIRC2 manual. Note
that the resolution is given per pixel. This resolution should be
divided by slit width to convert to resolution element. It is not
recommended that a slit width less than the diffraction limit of
the PSF be used. For example a slit width of 50mas or greater
should be used for K band.
- To help visualize and plan your setup, use
the Spectroscopy
Simulation Tool. It can be used in conjunction with the
offset guide star tool for planning combinations of slit
position and guide star position.
- Startup NIRC2 software
- Run the slitcol script. This is a wizard
style script that queries for various parameters. The script
converts user parameters into a machine parameter for slit position
known as slitmm, the slit position in mm in the focal
plane. slitcol sets the keyword slitmm or it can be set
directly with the command slitmm
- Setup NIRC2 to image the slit. Example:
filt k
camera wide
sampmode 2
tint .2
coadd 1
shutter open
pupil largehex
wait4ao off
grism clear
slitmm 5.0
slit 60
Note that the slit command reads the value of slitmm
and puts the slit selected at this position.
- Setup of the AO bench for the white light source. First check with the SA
that the bench is available for calibrations and run the command:
configAOforLamps
- Turn on white light source.
lamp white on
- Take image and measure slit position
goi 1
Note image number and slit position. Note that the
image will likely be saturated but should yield a good x centroid
for slit position. it is important to image the slit through the
same filter and camera that you will be aligning with later in the
night. There may be setups that require alignment in the wide
camera but the spectroscopy is done in the narrow or medium
camera. It is not recommend that narrow band filters be used since
they can create ghost reflections. Images with the white light
source off or one of the arc lamps may be more appropriate for
certain settings.
- Take a series of spectral flats, arcs, and darks for
calibrations. Confirm spectral setup with arcs and filter
transmission cutoff points.
Night time setup
- Enable DAR (differential atmospheric refraction)
compensation. It should be set to acquisition and tracking
during NIRC2 spectroscopy setup and integrations. The B-V value of
the guide star is also relatively important for best performance of
the DAR compensation. Verify with the OA that the DAR compensation
is enabled for acquisition and tracking.
Note: The array in NIRC2 has significant persistence. The is effectively
high dark current in areas where high flux levels have been incident on the
array. If there are bright objects in the field and the long integrations
on faint objects are planned, great care should be taken to avoid the bright
objects from being the same y location on the array prior to the long integrations.
The NIRC2 dark slide or shutter should be closed during slews for this reason.
For more information read about the NIRC2 persistence
characterization.
- Take image of object in same filter/camera
configuration used for step 8. of the afternoon setup above.
- Offset telescope so that the object is placed on the
same x pixel location of the slit. The mov command
is recommended to move the object from pixel location x1 y1 to
pixel location x2 y2.
mov x1 y1 x2 y2
- Repeat steps 1 and 2 until object it positioned on the
correct pixel.
Note: a useful technique is to
display the difference of the object image and the slit
image to confirm the setup. Use the math -> arithmetic menu in
quicklook to display the difference of any two
frames.
Other useful commands for making small adjustments include:
xy offsetX offsetY
units are arcsec, moves the telescope, thus object moves
opposite
pxy offsetX offsetY
units are pixels, moves the telescope, thus object moves
opposite
Note that in NGS mode, the accuracy of the offsets has an
uncertainty of +/- 40mas and thus offsetting can take several
iterations. This is due to the lack of precision in the FSM
control on the AO bench.
- Configure instrument for spectroscopy. Example:
filt k
camera wide
sampmode 3 16
tint 300
coadd 1
shutter open
pupil largehex
wait4ao on
grism medres
slitmm 5.0
slit 60
- Take integrations.
goi
Note that scripts exist for slit dithering
(example: sp2) but should be used with caution
due to the FSM problem noted above.
sp2 dy [n=]
Takes n pair of spectra seperated by dy arcsec. The spectra pairs are
centered about the current position.
To guaranty slit alignment in NGS mode, it is
recommended that the object be imaged between every slit
nod. Again, this is due the FSM inaccuracy problem. For
wider slits you may just monitor the spectral intensity and
realign if it drops significantly. LGS mode is
accurate to < 10mas and thus the slit dithering should
not be a problem.
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