IAU, Commission 9, Working Group on Sky Surveys
Newsletter 12 (2000) 15-19
Observations with Rozhen Schmidt telescope and ST6 ccd camera
Tsvetan B. Georgiev
Rozhen NAO, Bulgaria
Abstract
The equipment and the progress of the observations by
means of the Rozhen 50/70/172 cm Schmidt telescope and SBIG ST6 CCD
camera are described. The original scale and field of the CCD
frames are 2.7"x3.3"/pix and 16.9'x13.3', respectively. Using 1.5 x
Barlow lens and suitable image processing methods the scale may
become 1"/pix. Examples of observations and reductions, as well as
a revealing Arp's ring of M 81, are given.
Contents
The physical size of CCDs currently still restricts the field
of view of the many research telescopes. However, for various
investigations a field significantly larger than typical 15-20' is
needed. Field size >1°, suitable for surveys and investigations of
large comets, star clusters and galaxies can be obtained with
Schmidt type telescopes. Successful works in this direction are
reported e.g. by Dettmar et al. (1993) and Bauer (1994).
In the period 1979-1995, the 50/70/172 cm Schmidt telescope of
the Rozhen NAO provided more than 7000 plates (see e.g. Mutafov et
al. 1993). The high quality of this telescope, proved by Golev et
al. (1982) and Tsvetkov et al. (1987), makes it suitable for CCD
observations with scale about 1"/pix.
One SBIG ST6 CCD camera with computer were kindly leaved to
Rozhen NAO by EAS/ESO support of the astronomy in the central and
eastern European countries in 1992. In the period 1993-1996 the CCD
ST6 was attached to the 2/16 m Rozhen RCC telescope. In 1995 and
1996 successful probe observations by the CCD ST6 with the Schmidt
telescope were made. Hoping to equip the telescope with large CCD,
we began regular observations and testing of various observational
methods in 1998. The equipment and examples of observations are
described in the present paper.
The CCD ST6 is mounted instead of the new 16 cm plate holder of
the Rozhen Schmidt telescope, changed the original 13 cm one
(Tsvetkov 1984). The original bottom cover of the CCD box was
changed with such one having a screwing hole and the interface
coupling is mounted on the side of the new cover. The original
frontier cover of the CCD was also changed by larger and thick one
for better cooling and for filter keeping. The diameter of the CCD
ST6 is 15 cm, so the light obscuring is negligible.
Normally the two stage Peletier cooling of the CCD ST6 ensures
the temperature of ~40°C below the surrounding. Because of the more
efficient cooling through the iron of the telescope and
through the new massive frontier cover, the working temperature now
is ~ 50°C below the surrounding. So, in the winter we could
suppress efficiently the dark current. Unfortunately, the absolute
limited minimum of the cooling of the CCD ST6 is -50°C.
The scale of the Schmidt telescope is 120"/mm and the CCD ST6
with pixel sixe 23x27 µm gives original resolution of 2.7"x3.3"/pix.
The size of the field is 8.5x6.5 mm, 375x242 pix or 16.9'x13.3'.
The maximum of the sensitivity of the CCD is ~ 70% at l ~ 650 nm
and the readout noise is ~ 30 e . The CCD ST6 has not UV
sensitivity. More detail information about this kind of CCD is
given by Pravec (1993).
In normal conditions the Rozhen Schmidt telescope provides
stellar images with FWHM of ~3" and using CCD ST6 we have the case
of "big pixel". Therefore, the original set-up is not suitable for
accurate photometry of faint stars and distant galaxies. To
decrease the pixel size we use one 2 x Barlow lens, originally
belonging to one Carl Zeiss (Jena) 18/180 cm telescope "Meniscus".
We place the lens ~10 cm from the CCD and achieve 1.5 times scale
increase (and 1.5 times field size decrease). The vignetting
effect restricts reaching better resolution. Fortunately, the
shifting range of the focusing device of the telescope is enough
for observation both in direct mode and using Barlow lens.
The use of CCD with a Schmidt telescope needs high accuracy
focusing and guiding. The original mechanics of the telescope
occurred enough good for this purposes. Additional frequency
generator permits fine correction of the main telescope moving.
In a clear winter night the temperature in Rozhen NAO, placed on
1750 m above the see level, is about -10°C. The ST6 CCD is well
designed for such conditions, however, the computer and the power
supply device need warm places. For this reason we situated the
auxiliary equipment in the small dark room, built earlier inside the doom
for photograph handling.
Normally one person may work with the whole equipment alone.
Having big pixel we prefer to form wide and sensitive
photometric bands rather than narrow and accurate ones. The broad
band BVI and GRZ photometric systems are realized by Schott filters
with total thickness of 2 mm, as follow: B: 1BG12+1BG39, V:
1GG495+1BG12, I: 2RG9, G: 1BG14+1BG39, R: 1OG570+1KG3, Z: 2RG760
(or Z'-2RG830). The first system is composed for stellar photometry
and the second one -- for surface photometry of galaxies. The color
coefficients of the system BVRI almost coincide with those ones,
found for the case of the 2 m RCC telescope, equipped with
CCD ST6 (Georgiev et al. 1994).
Because of the high dark current and cosmic hits we apply up to
20 min single exposure times. To achieve similar observing deepness
in various bands the next set of exposures may be recommended: 40
min in B, 20 min in G and V, 10 min in R and I, 20 min in Z and 40
min in Z'. Using such exposures for direct observations we reach
the most faint objects which are visible on the digitized POSS
frames - about 20.5 mag in V band. Using the Barlow lens we must
apply > 2 times longer exposures.
The original software of the CCD ST6 provides frames in FITS
format. We prefer to reduce the frames using the Rozhen software
(Georgiev 1995), written on Microsoft C 6.0 language as extension
of the PCVISTA package (Treffers & Richmond 1989, 1997). It
performs various space, amplitude and scale transforms, filtering,
rebinning, mapping, photometry and shape analysis of images. In the
case of observations with the ST6 CCD we use also special procedures
for making dark masters and flat masters, as well for simultaneous
dark subtraction, flat fielding and scaling of the program frame.
Example processing of R frames of the galaxy M 51 with exposures
5 min each is shown in Fig.1.
The used dark and flat master frames
are composed from 5 source frames each.
Fig.1a shows the result
from two direct frames after dark subtracting, flat fielding,
rebinning and adding with scaling to 2.7"/pix by X and Y axes. The
FWHM of the seeing seems to be about 2 pix or 5.4".
Fig.1b shows
four frames, obtained using the Barlow lens with 1.48 times
increase of the resolution, processed in the same manner as for
Fig.1a. The estimation of the FWHM now is 3.2 pix or 5.8".
Further, the four frames are rebinned again by means of drizzling
(see below)
with scaling to 1"/pix. The central part of the result frame is
given in Fig.1c.
In the end, the frame from Fig.1c is smoothed
with sliding regression surface of 4th degree, using round window
with size of 9 pixels. The applied method (Georgiev 1996) does not
decrease the resolution. The result is given in
Fig.1d. The FWHM
in Fig.1c and 1d is 4.1".
One variant of the method for drizzling or reconstruction of
shifted frames (Hook & Adorf 1995) is applied to produce the image,
shown in Fig.1c, as follows (Georgiev 2000). All frames are rebinned
to one of them and added.
The linear coordinate transforms between each input
frame and the output frame, comprehending translation, scaling and
rotation, is found by the coordinates of ~10 common reference
stars, using the MLS. The scaling coefficient (here 1.82) is used
for generation of a new, finer output grid. Then each input
frame is transformed to the new fine grid as follows. Each pixel
from the output grid is projected onto the input grid and the
nearest four pixels from the input grid forms weighted mean value
for the output pixel. The weights must be proportional to the
reciprocal value square of the distances r to the pixels, but
because of the difficulties in the case r=0 we prefer Gaussian
weights using Gaussian sigma e.g. 1/10 of the pixel size. In the
end all transformed frames are added. The applied method decreases
slightly the point spread function of the frame.
The current set-up of the Schmidt telescope plus CCD ST6 was
originally designed for surface photometry of large galaxies, but
it occurred very powerful also for observations of the solar system
bodies. Generally, the equipment occurs handy and useful for
various observing tasks. The following examples demonstrate this.
The ring of Arp (1965) is a faint halo-like optical structure,
found just north of the nearby spiral M 81. Its most bright
eastern part coincides with a dense HI cloud in the eastern outer HI
spiral arm of M 81 (Yun et al. 1994). Efremov et al. (1986) found
there blue diffuse and starlike objects and considered them as star
forming appearance. Generally the Arp's ring is extended and
extremely faint detail, which has not good surface photometry yet.
Fig.2
presents the Arp's ring from one 10 min direct exposure in
R band. The observations are made in the beginning of 2000 under
good atmosphere transparency. After removing the stars and
smoothing the frame we estimate the surface brightness of the ring
in R band to be less than 25 mag per square arcsecond. The ring is
well visible also under 20 min exposures in G and Z bands.
The investigations of the small bodies of the Solar system is
another important possibility of the Rozhen Schmidt telescope with
CCD ST6. The realized equipment plus a suitable image processing
provides possibilities for astrometry of asteroids and comets with
accuracy of 0.001s by R.A. and 0.01" by DEC (Radeva et al. 2000).
The exposure times for asteroids of 16-20 mag and comets of 13-16
mag, without filter, are 30-600 sec.
Fig.3 presents the 60 s images
of the asteroid 2000 D01 (d=0.048 AU, r=1.037 AU, UT=2h 09m 04s,
March 3, 2000, V=15 mag) and the comet C/1999 S4 (LINEAR) (d=3.026
AU, r=2.445 AU, UT=19h 53m 49s, March 7, 2000, V=13.3 mag).
The author expresses gratitude to the technical staff of the
Rozhen NAO
for the support in the mounting and exploring of the ST6
CCD at the Schmidt telescope and to Drs.
R. West and
M. Tsvetkov for
the discussions about the present set-up. The author is especially
grateful to Dr. R. West for the
ESO
support in the development of the
equipment for observations and image processing in Rozhen NAO.
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Electronic version created by Petra Böhm
(
), 2000-08-23