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Astronomical Data Analysis Software and Systems VII
ASP Conference Series, Vol. 145, 1998
R. Albrecht, R. N. Hook and H. A. Bushouse, e
Ö Copyright 1998 Astronomical Society of the Pacific. All rights reserved.
ds.
Slitless Multiobject Spectroscopy with FOSC Type
Instruments
V. F. Polcaro and R. Viotti
Istituto di Astrofisica Spaziale, CNR, Frascati, Italy
Abstract. We present a simple, e#ective technique that allows the con­
temporary spectroscopy of all the objects, up to a limiting magnitude,
contained in a # 100 arcmin 2 field. This technique makes possible the
immediate and unambiguous identification of peculiar objects included in
the field. A data analysis procedure that produces the final results in a
very short time has also been developed. As a test of this technique, we
present the results # obtained for the peculiar young open cluster Berkeley
87. We show that this technique should also be quite useful in emission
line and WR star surveys, and illustrate the software procedures devel­
oped to make the wavelength and flux calibration of these spectra.
(*) Based on data collected at the Loiano Observatory
Modern hard X­ray satellites allow the real time positioning of transient
high energy sources with a precision of #10 # --30 # . In some cases, as for the
gamma­ray bursts, the fast identification of the optical counterpart is a crucial
item to unveil the physical nature of the emitting object. On the other hand,
no other way to make their identification in the optical range has been so far
found, but making the spectroscopy of all the objects included in the error box,
and looking at the objects which show a peculiar (e.g emission line) optical
spectrum. Of course, within a sky area of order of some 10 2 arcmin 2 , there
is an enormous number of objects and it is unlikely that we can obtain all
their spectra in a reasonable time period. This fact usually overcomplicates the
identification of the optical counterpart of the high energy cosmic sources, at
the risk of substantially diminishing the scientific potential of the imaging hard
X­ray and gamma­ray telescopes.
For this reason, we have developed a technique, that allows the simultaneous
spectroscopy of all the objects included in a square field of view with #10 # side,
using the widely used FOSC­type optical spectrometer and camera focal­plane
instruments. The technique is based on the use of a slitless low dispersion
grism coupled with a broad­band filter, which perform an ``objective prism like''
spectrogram of all the objects present in the field, without serious overlap of
adjacent spectra. For instance, the use of the very common grisms with #400­
800 nm bandpass and of the Johnson R filter will make an imaging spectroscopy
of # 200nm centered on H#, and emission line and peculiar spectrum objects
are easily identified.
We tuned this technique on the Bologna Faint Object Spectrometer and
Camera ­BFOSC­ instrument (Merighi et al. 1994), mounted at the Cassegrain
78

Slitless Multiobject Spectroscopy with FOSC Type Instruments 79
focus of the Bologna Astronomical Observatory ``G.B. Cassini'' 1.52 m telescope,
sited near Loiano (Italy) at about 800 m altitude on the Appennine Mountains.
In order to test this technique, we used the peculiar open cluster Berkeley
87, an intriguing object, that we have studied for more than 10 years (e.g., Norci
et al. 1988; Polcaro et al. 1989; Polcaro et al. 1991; Manchanda et al. 1996).
Actually, most members are young, heavily reddened OB stars, but a few are
much more evolved objects, such as the WO star Sand 5 and the M3.5I variable
BC Cyg making the evolutionary status of the cluster extremely uncertain. The
cluster member n.15 in the Turner and Forbes (1982; hereafter TF82) list is an
emission line star (V=11.8) also known as V 439 Cyg and MWC 1015 (Merril and
Burwell, 1949). This star dramatically changed its spectrum from late to early
type in a few decades; furthermore, some absorption lines that were still present
in 1986 and 1987 completely disappeared in 1988. The star is characterized by
a strong IR excess and a peculiar position in the HR diagram, suggesting that
it should be considered as an intermediate RGS/LBV star (e.g., Polcaro and
Norci, 1997 and references therein). Three other cluster members (no. 3, no. 9
and no. 38 in the TF82 list) are emission line stars.
On June 24, 1997 (23:11:55 UT), we took a 30 s R filter exposure of the
central (9 x 9 arcmin 2 ) region of Berkeley 87, shortly followed (23:18:34 UT) by
a slitless 300 s image through the R filter and the grism no. 4 (Fig 1). The H#
emission of stars no. 3, 9, 15 and 38 and the WR spectrum of Sand 5 where
immediately visible after a short tuning of the contrast. The direct overlap of
these two images using the IRAF task imarith on the recorded FITS files allowed
a prompt and unambiguous correlation between the objects and the spectra.
The whole procedure, including the overlap, took less than half an hour.
Notice also that our short exposure allowed the immediate identification
of the H# emission of the V=14 star no. 38 and of the WR spectrum Sand
5 (V=13.8) without saturating the H# emission of the V=7.81 star no. 3. In
absence of such a bright object (or if we do not care if we saturate its spectrum) a
longer exposure should allow the identification of emissions also in much fainter
objects.
A careful analysis of the slitless spectral image demonstrated that this tech­
nique has a field of application much wider than the fast identification of peculiar
objects described above.
Actually, after debiassing and flat­fielding (using standard IRAF proce­
dures) by means of a 10 s flat­field taken through the R filter, we obtained a
pretty clean image from which 25 good quality, not overlapped spectra of stars
up to V=16 were extracted. The comparison of this multiobject spectroscopy
with long slit spectra of a few objects in the same field taken on the same night
through the same grism, which were wavelength calibrated by means of a He­Ar
comparison lamp and flux calibrated using the standard star Kopf 27, shows that
the pixel­wavelength relationship was not a#ected by the lack of the slit and the
presence of the photometric filter, but just shifted in pixels, due to the di#erent
declination of the stars in the field, and cut between 550 nm and 780 nm due
to the combined e#ect of the filter and grism transmission curves. Thus, the
wavelength calibration is easily obtained, with a precision of ±0.5 nm from the
position of the stellar image, the 550 nm cut­o# and the red telluric absorptions.

80 Polcaro and Viotti
Figure 1. Figure 1: Slitless red spectral image of the open cluster
Berkeley 87. The field is 9 x 9 arcmin 2 . North is at the top and East
to the left. The H# emission line in star no.38 (top left) and n. 15 (top
centre) and the WR spectrum of star n.29 (Sand 5, bottom centre) is
clearly visible.
A further benefit was obtained from the slitless technique: the night sky
lines (and mainly the antropic lines that are quite strong at the telescope site,
due to the close proximity of the city of Bologna) as well as the numerous
nebular lines (mainly, but not only, H#) due to the intracluster matter (see e.g.,
Polcaro et al. 1991) result in a limited increase of the background level, so that
spectra was much cleaner than those obtained in previous runs by means of
slit spectroscopy. Furthermore, the absence of the slit eliminates the light loss
due to the poor seeing (3.5 arcsec in the night of our experiment) so that the
signal­to­noise ratio was much higher than that of ``classical'' spectra. In this
way we were able to unveil narrow H# emission cores in the cluster members
no. 26, no. 31 and no. 32. Furthermore, once the wavelength calibration of
each individual spectrum was achieved, we recognized (from the comparison of

Slitless Multiobject Spectroscopy with FOSC Type Instruments 81
the debiassed and flat­fielded slitless spectra in reduced CCD counts and of the
long slit flux calibrated spectra of the same objects) that, due to the linearity of
the CCD detector used in the BFOSC instrument, the flux calibration curve was
the same for the whole image. Thus the flux calibration of the slitless spectra
was also straightforward.
Our test demonstrates that the use of the slitless spectrometry technique
with the modern FOSC type instruments does not only allow the immediate
identification of spectroscopically peculiar objects in a field of the same order
of magnitude of that of a ``typical'' hard X­ray instrument. It also shows how
this technique can be used to obtain, without the need of either new hardware
or of large telescopes or high quality skies, much valuable scientific information:
for instance, we identified in a single short test­shot three previously unknown
emission line stars.
We now plan to apply the method to a search for emission line and WR
stars in young clusters.
We thank the night assistants of the Loiano observatory, Mr. G. Tessicini
and Mr. E. Delogu, for their help during our experiment
References
Manchanda R. K., Polcaro V.F., Norci L. et al., 1996, A&A, 305, 457
Merighi R., Mignoli M., Ciattaglia C., et al. 1994, ``BFOSC User's Manual'',
RT 09­1994­05, Bologna Astronomical Observatory.
Merrill, P.M. & Burwell, C.G., 1949 , Astrophys. J., 110, 387
Norci, L., Giovannelli, F., Polcaro, V. F., & Rossi, C. 1988, in ``Frontier Objects
in Astrophysics and Particle Physics'', F. Giovannelli and P. Mannocchi
(eds.), Italian Physical Society, Bologna, Italy, 7
Polcaro, V.F., Giovannelli, F., Norci, L., & Rossi, C., 1989, Acta Astronomica,
Vol 389, No. 4.
Polcaro V.F., Rossi C., Persi, P. et al. 1991, Mem.S.A.It. 62, 933
Polcaro V.F. & Norci L., 1997, Proc. of the UMIST/CCP7 Workshop on Dust
and Molecules in Evolved Stars, Manchester, March 24­27
Turner, D.G. & Forbes, D., 1982, Publ. Astr. Soc. Pac., 94, 789 (TF82).