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ACAM spectroscopy - manual acquisition of targets
Targets are usually acquired onto the ACAM slit using the ACAM target-acquisition tool, commissioned in October 2010. Should this tool for any reason become unavailable, the target must be acquired manually, using the recipe below.
ACAM has no equivalent of the slit-viewing camera used with ISIS. Target are acquired onto the slit in imaging mode. The procedure is as follows:
1 - Point the telescope
TO> gocat target-name &
Ask the telescope operator to set the rotator angle to the required position angle (usually parallactic). The operator will then also look for guide stars.
2 - Measure x,y of slit
TO> acamimage SloGunR 1
TO> glance acam 5 (usually enough to see the slit, even on a moonless night)
NB the same filter should be used throughout steps 2 - 6 of the acquisition sequence (changing filter may change the position of the image on the CCD by a few arcsec).
Use iraf's imexam option 'k' to find the center of the slit in the 'y' direction. Note that due to the tilt of the slit you need to do this at the x-position where you want your star to land. It's also best to measure this with the telescope pointing at the target of interest - after large movements of the telescope and Cassegrain rotator, the position on the CCD may change by a few pixels.
3 - Measure x,y of target
TO> acamimage SloGunR (or TO> acamslit 1)
Take an image of the sky, and display it:
TO> glance acam 10 (should suffice to reach mag ~ 21)
ecl> display s1 1 l+; imexam
If the target is starlike, use imexam option 'r' to measure it's x,y position. If it is not starlike, measure x,y using the cursor. The default WHT pointing should put the target near the rotator centre (~ 1080,1900 on the unwindowed CCD).
4 - Offset the telescope
Ask the TO to offset the telescope the required amount right/left and up/down on the CCD (the telescope control software takes any non-zero sky PA into account).
Check that the target has arrived at the correct x,y position:
TO> glance acam 5
If not, offset again. More than two iterations should not be needed.
5 - Check the through-slit image
Insert the required slit in the light-path, e.g.
TO> acamimage SloGunR 1
inserts the 1-arcsec slit.
Take a through-slit image. A 15-sec exposure works well for a 20th-mag object:
TO> glance acam 15
Display the image using iraf, but turn off the automatic scaling. Otherwise only the slit will be visible, not the star on the slit. E.g:
cl> displ s1 1 fill+ zs- zr- z1=4000 z2=15000
displays the image with black corresponding to 4000 counts, and white corresponding to 15000 counts.
If, at this point, the target is not well-centred on the slit, the operator will need to tweak it in. If this requires more than one iteration, it's probably best to window the CCD to image only ~ 200*50 pixels around the target, and start a multglance of short exposures so that the TO can use the RTD display as a slow acquisition camera, with the image looking something like this:
6 - Blind-offset, if required
7 - Autoguide
8 - Insert the VPH
TO> acamspec v400 1 (for a 1-arcsec slit)
9 - Expose
TO> run acam 1800 "M51 OIII"
10 - Record an acquisition image
TO> run acam 60
If no blind-offsetting is involved, this could be done after step 4 above, but otherwise it's probably best to leave it until after the science exposure, to avoid moving components in and out. Take an arc first though, if wavelength scale is important.
The x position of the spectrum on the CCD will be slightly different from the x position on the image. The difference dx (spectrum - image) in June 2009 was:
x 400 520 630 1100 1600 1700 dx -9.1 -13.1 -6.9 -1.9 7.5 8.4
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