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ACAM first-light images
ACAM is a new high-throughput imager/spectrograph, permanently mounted at a folded-Cassegrain focus of the WHT.
The field of view in imaging mode is 8 arcmin (0.25 arcsec/pixel). A set of U B V R I Z filters is available by default, but almost any of ING's filters (including the TAURUS narrow-band filters) can be mounted in ACAM.
Low-resolution spectroscopy over ~ 3500 - 9000 A is provided by a VPH disperser. For 0.5- and 1.0-arcsec slits, the on-axis spectroscopic resolutions at 6000 A are R ~ 450 and 900 respectively.
ACAM was designed in-house, and the project was delivered in collaboration with Kevin Dee of Engineering and Project Solutions Ltd.
Below are a few images and spectra taken during, or soon after, the successful on-sky commissioning of ACAM 2009 June 9 - 12. The images are oriented with N up, E to the left, unless otherwise stated:
As of October 2009, ACAM has been used to obtain data for ~ 12 science programmes. The first publication in a refereed journal, based in part on data from ACAM, is: "Data mining for dwarf novae in SDSS, GALEX and astrometric catalogues" by Wils et al (2009), see under 'dwarf novae' below .
Pablo Rodríguez-Gil, Chris Benn, Andrew Cardwell
The image is centred near the zero of lunar coordinates, with N up, E left (lunar N is ~ 15 deg right of up). The dark area just above the centre is the Sinus Medii (a small mare).
The prominent crater with a central boss, slightly left of and below centre, is Herschel, diameter 45 km (140 ACAM pixels).
The image was taken through a narrow-band (5045-A) filter (and cloud!), with an exposure time ~ 0.1 sec.
This is the first-light spectrum of a science target:
The brightness of cataclysmic variable MV Lyrae varies dramatically between two levels (low state and high state), depending on the accretion rate from the donor star to the white dwarf. Recently, it has entered a very unusual intermediate state. This is the first spectrum obtained of the star in this state.
This image of the interacting galaxies NGC 5954 (left) and NGC 5953 (right) was created by combining images taken through V and R broad-band filters, and through [OIII], Hα and S[II] narrow-band filters, with red colour highlighting regions of Hα and [SII] emission. The image is ~ 2 arcmin across.
Both of these galaxies, located at a distance of some 25 Mpc (megaparsec), host an active galactic nucleus (AGN). NGC 5953 has been classified as morphological type SAa, whereas NGC 5954 is an SABcd galaxy. The strong patchy line emission is a result of both the active nuclei and of the massive star formation, stimulated by the galaxy-galaxy interaction. This image was obtained as part of a study of how episodes of massive star formation are triggered by gravitational interactions between galaxies.
Click on the image to see more of the field.
Johan Knapen (IAC), Nik Szymanek
The aim of the observations is to measure accurately the distance to the nova. This can be done by monitoring how the ionisation front caused by the nova explosion ionises the planetary nebula, i.e. by light-travel considerations. With an accurate distance, the evolutionary status of the stars forming the progenitor binary system can be explored. This is important, as only one other nova has been witnessed to explode within a planetary nebula, GK Persei. However, it's not yet certain that GK Persei is actually a planetary nebula.
Boris Gaensicke (Univ. Warwick), Pablo Rodríguez-Gil (ING/IAC), Albert Zijlstra (Univ. Manchester)
ACAM is designed to give excellent imalge quality over most of the field. At the very edge of the field, the point-spread-function from the optics is poorer, FWHM ~ 0.5 arcec (lab measurement, consistent with the ray-tracing prediction).
Testing the theory of stellar structure and evolution relies on model-independent measurements of masses and radii across the entire range of stellar masses and evolutionary stages. Such measurements can be obtained from observations of eclipsing binary stars.
ACAM was used to identify a new, faint (g=20), eclipsing white dwarf plus M-dwarf binary with an orbital period of 188 min, only the 15th such system known so far. The ingress and egress of the white dwarf eclipse last less than a minute, and resolving these features requires imaging with high time resolution. The ACAM observations were obtained with exposure times of 10s. Binning the camera 2x2 and windowing to a 2'x2' field of view decreases the dead time to 1.8s.
The light curve illustrates that the white-dwarf eclipse (at HJD=2454997.66 in the light curve above) is clearly resolved. The sinusoidal shape of the out-of-eclipse light curve is related to the non-spherical shape of the M-dwarf in this short-period binary. A flare is detected at HJD=2454997.61, indicating stellar activity on the M-dwarf. These observations demonstrate that ACAM on the WHT opens a new window into fast photometry of faint objects.
Boris Gaensicke (Univ. Warwick), Pablo Rodríguez-Gil (ING/IAC)
Among the many mysteries posed by the planetary nebula (PN) M1-75, one of the most striking is the apparent lack of a central star. The ACAM image above was taken through an I-band filter, minimised the contribution from the emission line nebula with respect to any central star candidate, and revealing star A (I ~ 19.5), offset from the geometrical centre of the PN and, 4.5 arcsec away, star B (I ~ 21.6), much fainter, but centred.
The extracted spectra of stars A and B (plus the unremovable PN) are shown above. Although further analysis is needed, star A seems slightly bluer (i.e. hotter), and the nebula around it seems brighter. Togther, these observations suggest that star B could be the (offset) central star of PN M1-75.
Wils et al (2009), astro-ph/0910.3218
Click on the image for a larger version (400 kbytes).
From left to right: Carlos Martin, Craige Bevil, Tibor Agocs, Kevin Dee, Chris Benn, Andrew Cardwell, Pablo Rodriguez, Don Abrams. Other members of the ACAM team are listed on the ACAM team page.
Photo credit: Pablo Rodriguez.
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