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NOTES ON ROKCAM (GJH updated 12/95)
ARRAY CHARACTERISTICS
The new array has similar QE to the old one, but is cosmetically a bit
inferior. It has 2 dead spots, numerous bad pixels, and high dark count pixels.
Gain is between 5.8 and 6.8 electrons per data number.
Read noise is 30 electrons.
Non linearity sets in at about 10K counts
Array should use rk1 microcode which enables continuous flushing of the
array. This removes much of the large initial hysteresis encountered previously.
But there is some hysteresis still associated with badly saturated stars. As
a result it is better to use the CCD acquisition camera to check the pointing
on bright stars rather than let the light fall on the array. The boresight
between the center of the array and the CCD acquisition camera can be set up
on a fainter star such as an Elias standard.
INSTRUMENT NOTES
Dewar flexure
Tests show a small residual flexure in spite of stiffening measures.
The flexure is typically 1 pixel for 45 degrees zenith distance and appears
to be due to the folding flat mirror in the snout.
This will result in poor cancelation of dust specks on the field lens
when the data is flat-fielded.
There are several specks on the lens that cannot be removed. These do not
produce significant features in the flat field.
If this is important, keep the lens cap on as much as possible, and
try taking dome flats at the same positions as your data.
Note that the lens cap is difficult to remove if it is fully pushed onto
the snout, so attach it lightly.
If there is a lot of dust on the field lens, it can be cleaned (carefully!).
To do this the snout with the field lens and fold-flat should be removed.
The two small access caps cover captured bolts that hold the snout to the
body of ROKCAM. Undo the bolts and remove the snout. It is pinned so that
it will go back into the same place again. Dust may now be blown off the
field lens with dust-off (be careful not to shake the dust-off can while
doing this). If a more thorough cleaning is needed, the lens can be
Removed from the snout, but this should not be attemptd by novices as
improper installation of the field lens can ruin the effectiveness of the
pupil stop. I will explain how to disassemble the snout to David Doss, so
ask him if you need this doing.
Hold time
The dewar takes about 10 hours to get to about 83 K, but about 24 hours to
reach 78 K, which is the coldest the temperature sensor reaches.
The temperature sensor can be read with the voltmeter from the Visiting
Astronomer's Lab (1st floor), set to DC volts. The BNC socket giving the
voltage is clearly marked onthe ROKCAM penthouse. The conversion of volts to
temperature is indicated on the side of the penthouse.
Once cold, the outer can seems to hold for >8 <9 hours. The inner can can last
at least
another 8 hours probably, but we haven't tested it. You can let the outer can
go dry during the day, but it must have LN2 in it for observing otherwise the
radiation shield warms a bit increasing the dark counts.
Be very careful moving to declinations above 55 degrees. The outer can dumps
LN2 at high declinations, so it will need refilling after observations in
the north. In addition, it is possible that the LN2 can pur directly into
the snout, pool on the field lens and crack it. We do not have a good solution
for this, so if you have to work in the north a lot, you should consider
rotating the ATOG 180 degrees so the fill tube for the outer can is to the
south rather than the north.
Background Counts
The following numbers are useful:
at 41 F (XX C) 5s exposure in K gives about 4200 DN, 10s max exposure
at 32 F (0 C) 10s exposure in K gives about 4500 DN, 20s max exposure
at 41 F (XX C) 12s exposure in Kshort gives about 6600 DN, 19s max exposure
At K, 5000 DN/sq. arcsec/s in the sky corresponds to K=12.0 mag/sq. arcsec.
(measured at T=41F)
A K=6.45 star gives 7.9e5 DN/s (5.39e6 electrons per second).
A K=0.0 star has 6.3e-24 W/m^2/Hz at a mean wavelength of 2.2 microns. This
flux corresponds to 1.1e10 photons incident onto the 2.7-m primary.
This implies 0.19 efficiency for the ROKAM+telescope+atmosphere system.
Telescope has .88^2*.75 throughput = 0.58, so ROK is 33% efficient.
This is roughly what you'd expect from the optics plus detector!
Second calculation based on data taken at 0 deg C:
450 DN/pxl/s in sky or 2800 DN/sq. arcsec/s.
5.12e5 DN in 1 sec from K=7.04 mag star or 3.5e6 electrons/s (with gain=6.8)
note this 16% more DN than in the previous measurement probably because the
statistics box is larger this time.
So sky background is 12.74 mag/sq arcsec at 0 C and 12.11 mag/sq arcsec at XX C,
which is excellent!
Second efficiency calculation based on the nite2 data, gives 21% on the sky
for gain=6.8 or 18% on the sky for gain=5.8 e/DN. Hence ROKCAM has between
31 and 36% intrinsic efficiency.
Flat fields
The following method is recommended for taking dome flat fields. Set up the
telescope pointing to 40 min East and 30 deg dec. Move the upper windscreen
so it covers the flat field lamp on the dome (controlled bu 'A' on the remote
control), and so the telescope is pointing at its top portion. Turn on the
lamp. The diffusing effect of the wind-screen will reduce the counts to a
suitable level for 0.4 sec exposures. Then take set sof flats with the
lamp on and with it off. Process the data by subtracting the median 'off'
exposure from the median 'on' exposure, thereby removing the dark and other
systematics simply.
SOFTWARE NOTES
See ICE manual (I'm not going to copy it here!)
ICE saves the data as long signed integers in 2-dimensions only (the various
ARGUS options are gone (thank god)
Note: bug in the header - The filter name, AM, UT are NOT in the header. Make
sure you record these in a log. This will be fixed.
Display defaults to South right, West down in ximtool
specific ROKCAM features:
grid: takes 9 images spaced on a 3x3 grid, you specify grid
size in RA dec.
skysub: takes sky-object-object-sky four frames and then displays the
sky subtracted result. May be sequenced multiple times.
Note: The NOVA with LCS210 program needs to be running to offset the telescope
for grid and skysub. The switch under the table must be at "NOVA".
Note: bug with grid and skysub - the name of the object is not put in the
header, instead "grid" or " " is the object name. This will get fixed.
nframes in detpars: uses syntax "sum n" or "ave n" to sum or
average n frames into a single output frame. This works with
grid and skysub.
gotchas: ctrl-C is bad if done during a sequence. ICE loses the filter
wheel if you do it. Cure is to logout of ICE and IRAF and
restart. The filter will need to be homed again.
The frame number ("sequence" in obspars)
will have to be set to an appropriate value as it is not
updated until the sequence finished gracefully.
When you start ICE home the filter with "h" for the filter in instrpars.
This uses a microswitch to sense zero, and you can use this as often as you
wish. The home position is nominally at J which is nominally
0000 on the counter. However, note that the counter sometimes slips so the
most reliable thing is to use the home feature. If you end up beyond the blank
position (XXXX on the counter) it may be risky to home. If this is the case
manually move the filter wheel back to less than XXXX, and then home.
To start atog from left hand mouse pull-down, add:
"ATOG6u8" !"xterm -T atog -n atog -e /usr/bin/vt52 /dev/atog1 &"
to:
Menu "UtilityMenu"
in .tvtwmrc
Bug: when you start it the refresh doesn't work, so in the atog window type
exit
then start it again with
atog6u8
SETTING UP THE CABLES FOR TELESCOPE AND ATOG
(may not need to know this and it may be out of date!)
1. ATOG
The connection goes from port 4 on Atlas' serial port extender to the
cable that plug sin behind the ATOG monitor that normally is plugged into
the "ATOG" port on the back of the ABC Switch Box. The cable is labelled
"Switchbox ATOG" it is line 8251
2. TELESCOPE MOVEMENT
Connects from port 2 on the serial port extender through a null modem
to the ribbon cable that goes in "input/output" on the Data Transfer
Switch box above the Nova terminal. This i a temporary measure as it should
ultimately go into port C on the same box when an adaptor is found.