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Near­Infrared Photometry
at the Gornergrat Infrared Telescope
L. K. Hunt
Infrared Group
C.A.I.S.M.I.--C.N.R.
Arcetri Technical Report, N ffi 15/1991
Firenze, 22 October 1991

NIR Photometry at the TIRGO 1
This manual is designed not to be an exhaustive reference, but rather to provide a
``recipe book'' for performing near--infrared (NIR) photometric measurements at the
Infrared Telescope at Gornergrat (TIRGO). To this end, brief descriptions of the
use of the wobbling secondary and television acquisition systems are given, followed
by the characteristics of the Arcetri InSb photometer. Observational procedures
are then outlined. For further information, Salinari (1982) and Saraceno et al.
(1982) give detailed descriptions of the telescope and wobbling secondary system,
respectively. The charge amplifier system of the photometer is described in Hartill
et al. (1986). Details of the TIRGO standard star network are given in Hunt et al.
(1987).
For convenience, a list of the TIRGO calibration stars is provided in Appendix
A. Note that a few of the secondary standard magnitudes given in Appendix A differ
with respect to Hunt et al. (BS 4392, BS 6707, and BS 7504). We will provide a
complete updated version of the secondary standard star magnitudes with the com­
pletion of the proposal approved for Winter 1991 for the refinement of the TIRGO
photometric calibration.
To accurately subtract the NIR sky background, it is necessary to alternate source
with sky measurements at frequencies ? 2 Hz. Beam switching the telescope effec­
tively eliminates both spatial and temporal linear variations in sky + telescope emis­
sion (see Appendix B). These operations are accomplished at the TIRGO through
the use of a wobbling secondary system and an automatic beam switching algorithm
incorporated in the data acquisition program.
The direction of the secondary modulation is adjusted through a toggle switch
on the telescope control panel; the position is reflected in a numerical readout on
the telescope status screen. Numerically, an EW direction is roughly 1120 with
beam B to the left of beam A on the TV acquisition video; NW corresponds to
about 2390 with beam A below beam B on the TV. These numbers are not exactly
reproducible and, if required, should be checked for accuracy. The above mentioned
numerical fiducials correspond to about 14.1 cts/degree. It is therefore possible to
set the secondary chopping direction according to the desired position angle, taking
into account the orientation of the TV acquisition video.
The amplitude of the secondary modulation (``throw'') is set by means of a
potentiometer on the rack above the telescope control panel. 100 potentiometer
units correspond to roughly 19.3 arcsec. This number can be checked by measuring
on the TV a given displacement in mm and converting it to arcsec.

2
In the thermal bands (L and M), it is possible (and may be necessary) to minimize
offset (difference in sky background between the two reference positions) by adjusting
the center position of the secondary. This is effected by adjusting (on the same rack
as the amplitude potentiometer) a similar potentiometer that indicates the nominal
secondary centering (usually 500 along the telescope optical axis). This should be
done while in ``FREE RUN'' with the telescope pointed to an empty sky field.
Beam switching is defined by an option in the data acquisition program after
the direction and amplitude (and centering) of the chopping secondary are set.
When this is done, a measurement always starts with a positive beam on the source
(``A''); after a given time interval (also set in the acquisition program), the program
automatically moves the telescope to the alternate beam (``B''). A complete beam
switching cycle consists of four beams, ABBA. In theory the centroid of the two
beams should lie on the optical axis of the telescope, but in practice it is common
to center positive beam A on the axis. This should not be a problem in the NIR as
the unaberrated field is sufficiently large.
Two television cameras are available: one with a large field of view (FOV) (¸ 1 ffi )
is mounted on the telescope structure and the other (total FOV ¸ 12 arcmin) is
mounted on the instrument cube. The second TV is equipped with two magnifica­
tions (f/10 and f/20) and is the one preferentially used for data acquisition. The
FOV of the TV monitor is roughly 3 2 4 arcmin with the f/10 magnification and
the magnitude limit for point sources (with no moon) is ¸ 16 mag.
The CRT tubes of both TV cameras are powered up by switches on the TV
control panel to the right of the telescope console. Shutters and filters should be
checked and placed in the desired position. It is very important that the shutters
on both TV cameras be closed if the dome is not dark. The TV screen can monitor
either the image of the camera mounted on the telescope or that mounted on the
instrument cube; this is controlled by a toggle switch on the TV control panel.
Before observing (see preliminaries), the focal plane position of the TV mounted
on the cube should be reset and brought to the nominal position of the detector.
This position should be given on the telescope control panel.
When the dichroic is in place, the position of the star on the TV screen is
displaced relative to that when the dichroic is not inserted in the optical path. The
direction (either horizontal or vertical) of the displacement depends on the face being
used. This should be remembered when centering sources by eye.
The TV system is also provided with an Arlunya image intensifier capable of
enhancing images up to a factor of 10 (depending on the sky background, presence
of moon, etc.). The control panel for the Arlunya is above the telescope console
and can be adjusted to optimize the identification of faint sources. In addition, the
Arlunya can be used to draw boxes of any size on the monitor screen (control box
to the right of the TV control panel). This avoids parallax errors when centering
sources in small diaphragms.

NIR Photometry at the TIRGO 3
The detector is a single element chip maintained at solid nitrogen temperature. This
temperature is indicated by the PT100 readout installed to the left of the console;
solid nitrogen temperatures correspond roughly to 10 --
13\Omega of the PT100. This
temperature should be frequently checked whenever problems are suspected, or after
refilling the internal chamber.
Nominal values for filters and diaphragms are given in Tables 1 and 2, respec­
tively. The measured filter transmission curves are given in Figure 1.
Table 3 gives approximate magnitude limits obtained with the current detector
(mounted in January 1990). Of course, these depend on sky conditions and should
be considered only a rough guide. Since sky magnitudes depend on lunar phase and
proximity as well as air mass, the values in Table 3 are approximate. (They are
representative of grey time and intermediate air mass.)
Table 1
InSb Filters
Name Symbol
a – [¯m]
a 1– [¯m] a Max. OE [arcsec]
J J0 1.25 0.28 28
H H0 1.65 0.34 28
K K0 2.24 0.38 28
L 0 L0 3.83 0.63 28
M M0 4.74 0.62 20
H(large) HG 1.65 0.33 42
CO CO 2.35 0.10 28
CVF1 -- 1.40 ­ 2.55 14
CVF2 -- 2.50 ­ 4.50 14
CVF3 -- 4.4 ­ 5.6 14
a 50% transmission.
Table 2
InSb Diaphragms
Name
a Diameter
a [Arcsec] [mm]
D1 7 1.0
E1 10 1.5
D2 14 2.0
E2 17 2.5
D3 21 3.0
E3 24 3.5
D4 28 4.0

4
Figure 1

NIR Photometry at the TIRGO 5
Table 3
Limiting Magnitudes
Band Limit a Sky Magnitude [mag arcsec 02 ]
J 15.4 13--14
H 15.25 13.5
K 14.4 12.5
L 8.5 3
M 5.5 0
a 1 oe in 1 sec at low gain.
As noted in Hunt (1986), the detector functions differently in observing condi­
tions (AC), than in the laboratory (DC). This difference results in an inability to
observe with the same calibration bright objects (K ! 3) and faint ones. Because
of the difference in detector sensitivity, the corrections given in Hunt (1986) are not
applicable to data acquired after January 1990. Therefore, if only bright objects are
observed, it is advisable to use only bright calibrations stars; conversely, if program
objects are faint, only faint calibration stars should be used. Caution should be
exercised when comparing photometrically bright objects and faint objects. This
problem has been addressed, although the solution has not yet been implemented.
Before beginning science observations, it is important to perform the preliminary
operations outlined below. To minimize dome seeing, it is best to open the dome
and turn on the ventilator fans an hour or two before beginning observations. If the
fans will not be used, then the trap door isolating the dome from the building should
be closed. It is assumed that the telescope pointing has been set and that the dome
has been enabled and the mirror covers opened. The photometer should be ON (i.e.,
switches on the electronics box in the dome), and the various panels (modulating
secondary, TV acquisition system/image intensifier, etc.) should be powered up. It
is assumed that the latest beam profiles have been checked for symmetry.
Preliminaries
Reset the TV acquisition video, then position at nominal values for the
InSb detector. The nominal position should be given on the telescope control
panel for the photometer.
Reset filter and diaphragm wheel. This is advisable to ensure the acquisition
computer is apprised of the position of the filter and diaphragm encoders.

6
Set filter and diaphragm. Preliminaries are usually done in H or K to maximize
the instrumental count rate. Initially, it is advisable to use a 28 arcsec diaphragm
(D4) as it will facilitate finding the position on the screen corresponding to the
location of the detector.
Set dichroic to appropriate face for photometer. The setting should be indi­
cated above the dichroic indicators (telescope control panel).
Point to a bright star. It is usually advisable to point to a star near the zenith
initially. At the beginning of the night, a pointing initialization is usually performed
(LAD) so as to correct for any temporal (or other) error accrued relative to the
previous observations.
Set the amplitude of the modulating secondary. This amplitude need not
be that required for the science observations; in fact, for focusing and aperture
definition, it is probably better to use small throws (at least a diaphragm diameter).
Define the aperture to be used for science observations. This is typically
done using the FREE RUN option of the data acquisition program which gives a sort
of analog display (digitized) on one of the terminals. When the source is centered
in beam A, counts should be positive; if counts are negative (and large), the source
is probably centered in beam B. Dimensions of the diaphragms should be checked
both for symmetry (equal diameters in NS and EW), and for correspondence with
the nominal dimensions. If multi--aperture photometry is to be performed, then
each diaphragm to be used should be defined and checked as above. In addition,
the diaphragms should be concentric; if not, then each diaphragm requires a new
center position.
Focus the telescope. There are many different methods to focus a telescope; the
technique usually adopted at the TIRGO involves minimizing the dimension of a
point source, as measured by the width of the rise in a beam profile. This procedure
can be performed either in FREE RUN or by using the strip chart recorder and
passing a star (typically in declination to avoid correcting for cosffi) in guide across
the diaphragm. When using FREE RUN, the width of the seeing disk is measured
by counting the number of points needed to go from minimum to a stable maximum.
These are then converted to arcsec by measuring the time difference between each
point (a function of the integration time and chopping frequency) and multiplying
the time corresponding to the number of points from minimum to maximum by the
number of arcsec per unit time (guide speed). This then is the full width of the seeing
disk at zero intensity. Typical focused secondary positions corresponding to about
6 ffi C in the dome are 1540 (read off the telescope status panel). When temperatures
are warmer, the numbers decrease (about 20 cts per degree). Absolute numbers
change every time the instrument is remounted so these should only be used as a
relative guideline. Typical seeing at TIRGO is around 3 arcsec (in K).

NIR Photometry at the TIRGO 7
Define telescope beam switch. This is performed by an option in the data
acquisition program. Before doing this, the amplitude and direction of the wobbling
secondary should be set to the program values. The procedure involves centering
(by eye and by looking at the ``analog'' (digitized) output) a star first in beam A,
then in beam B. During this operation, it may be helpful to disable the pointing
corrections (TRK OFF). For large beam throws, it is helpful to define the telescope
beam switch on a faint star so as to best match the actual observing conditions.
Otherwise, the beam switch (defined on a bright star) probably will not be precise
for program objects (if they are faint) because the static and dynamic secondary
positions differ slightly and the difference varies with chopper frequency (which is
slower for fainter sources). Slight discrepancies from the defined beam switch can
be adjusted for by lightly tampering with the modulating amplitude potentiometer.
Note that the parameters of the chopper throw (amplitude and direction) are not
read by the acquisition program, so if desired, they should be introduced explicitly.
Update parameters for pointing corrections. Since accurate pointing depends
on the temperature and pressure (refraction index) and the central secondary po­
sition, it is wise to introduce these parameters explicitly into the file read by the
pointing algorithm. (Presently, this is accomplished in the CAT program.)
Check for automatic dome control. Sometimes the positioning algorithm for
the dome puts the dome in a position that is 180 ffi away from the correct one. This
should be checked if there seem to be problems. When this occurs, it is helpful to
perform a pointing reset (LAD) on a star to the south and east while the dome is
stationary. (This problem should be resolved with the new pointing control system.)
Note that a LAD should never be performed while the dome is in motion.
Program observations
After setting up the telescope and ascertaining focus, instrument function, etc.,
standard star and science observations can begin. After pointing the object:
Set filter and diaphragm.
Perform the automatic scaling that sets the instrument parameters. This
is done using an option in the data acquisition program; by sampling source + sky
and sky flux, the scaling algorithm sets up the gain, integration time, chopping
frequency, and temporal parameters of the instrument.
Set desired S/N, duration of a single beam, and minimum and maximum
number of ABBA cycles. It is advisable to perform at least two ABBA cycles
even on bright sources as the first integrations can be systematically different from
the others.

8
Start observation. Once the measurement is started, it is important to check
for large offsets that retain the same sign in all filter bands. These could indicate
unwanted objects in the reference beam.
Suggestions for extended source observations
We advocate the use of offset pointing to ensure accurate telescope positioning at
the nominal position of the object to be observed. This is effected by selecting either
a SAO or an HR star as close as possible to the nominal coordinates of the program
object, pointing the telescope to the star, then fixing the coordinates by performing
a ``LAD'' with the star perfectly centered (with dichroic in). In these conditions,
the pointing accuracy of the telescope is better than 2 -- 3 arcsec rms.
The program object can then be pointed with a reasonable probability that the
dominant pointing error is the error in the coordinates, and not telescope pointing
error. It may be possible to center by eye on the optical center (using the lowest
magnification of the TV), although guiding will be difficult if integrations are long
and there is no guide star in the field. If necessary, a guide star should be found
by moving the television camera while keeping the telescope fixed at the nominal
position.
Having positioned the guide star on the screen, the program object, if the signal
is sufficiently intense, can be centered by maximizing the NIR signal. For galaxies,
H or K are the best bands to use as the sky counts are such that the integration
times are shorter. The NIR signal is maximized using FREE RUN and watching
the counts as a function of position of the guide star when the telescope is displaced
from the nominal coordinates. Alternatively, the strip--chart recorder can be used
but by connecting the ``InSb OUT'' cable to the input and disabling the chopping
(detaching the cable from the input signal). Note that the ``analog out'' output on
the detector is the correct one for this option.
If the object is too faint for this method to be effective, the data acquisition
mode can be used to maximize the signal. In the menu where the measurement is
begun, the minimum and maximum number of ABBA cycles should be set to 1. It is
typically necessary to perform at least five measurements positioned at the nominal
coordinates, and equidistant on each of the four sides; each measure lasts from 30
-- 60 sec according to the duration of each beam. The procedure should be iterated
until the center measurement exceeds that of the surrounding regions.
Note that after restoring the television camera to its nominal position, it is
advisable to recheck the diaphragm definition (in case of slippage).
The author would like to thank C. Baffa, G. Calamai, S. Gennari, F. Lisi, L. Mor­
bidelli, P. Ranfagni, and R. Stanga for useful comments, additions, and clarifications.

NIR Photometry at the TIRGO 9

10

NIR Photometry at the TIRGO 11
To be defined.

12
References
Hartill, D., Lisi, F., and Bettarini, A. 1986. Applied Optics, 25, 1701.
Hunt, L. K. 1986. Rapporto Interno, Osservatorio Astrofisico di Arcetri.
Hunt, L. K., Calamai, C., and Oliva, E. 1987. Rapporto Interno, Osservatorio As­
trofisico di Arcetri.
Salinari, P. 1982. Second ESO Infrared Workshop, Garching, eds. A. F. M. Moorwood
and K. Kj¨ar, p. 45.
Saraceno, P., Messi, R., and Orfei, R. 1982. Second ESO Infrared Workshop, Garch­
ing, eds. A. F. M. Moorwood and K. Kj¨ar, p. 45.