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Object through slit
Two possible situations:
target is a relatively bright object and you can see it in imaging mode with
exposure times typically shorter than ~10 seconds. Use "centering on the slit"
(mode 6 of the GUI) button and press "start
exposure". This first removes the slit from the field of view and takes
continuously images with the choosen exposure time. You should see the object
moving toward the slit shaddow. This is usually done by the night assistant.
He/she has a direct view on your screen and is able to move the field to have
the target at the slit location you indicate him/her.
If using the T193 "bonnette" guiding, it is recommended to check about every
15min if your target is still within the slit ("Search Slit Position" mode 5 of
the GUI) due to telescope mechanical flexures
(see also the
Cook Book and the
Test report).
A long spectroscopic exposure is therefore preferably split
into several ~15mn exposures at the moment (until the new guiding system
directly attached to Mistral will be available).
The target is too faint to be detected with short exposure times (~10seconds).
The previous process will then become very uncomfortable and you will need to
use a differential positioning on the slit. For this, you first need to make a
deep enough science image (mode 3 of the GUI:
"Science Image"+"Star exposure") to see the target. On this image you note the
target's x,y coordinates. You then compute the difference [
(xslit-xtarget)*spatial_sampling] in x position between the object and the slit,
in order for the night assistant to know by how much he has to move the
telescope in RA. You do the same [ (500-ytarget)*spatial_sampling] in Y position
to approximate the DEC shift to apply. The value of the spatial sampling is 0.48
arcsec. This process assumes that the field rotator is oriented north-south.
This process is not possible for now if other orientations are set. At this
step, it is highly recommended to cycle a second time the whole process: deep
image without slit, locate the target coordinates, recompute the offset
(xslit-xtarget)*spatial_sampling, and apply eventually the RA shift; and same
for DEC.
Given the MISTRAL imaging capabilities, trying to observe an object not visible
in 10-second exposures will be, however, pretty rare.
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