659 research outputs found
Resistive anode image converter
The invention of an apparatus for imaging soft X-ray and ultraviolet electromagnetic radiation and charged particles was described. The apparatus includes a pair of microchannel electron multiplier plates connected in a cascaded chevron configuration which intercepts an incident beam of radiation or charged particles. Incident photons or charged particles strike the front surface of the chevron configuration causing emission of electrons. The electrons are accelerated by a voltage gradient and strike the inner side walls of the individual channels, causing emission of secondary electrons. Accelerated and multiplied secondary electrons impinge upon a resistive anode after they transverse the chevron configuration. A pulse position circuit converts the magnitude or transit time of the currents flowing from the point of impact of the electrons on the resistive anode to four contact electrodes mounted on their periphery of the resistive anode into the spatial coordinates of electron impact
High-intensity source of extreme ultraviolet
High intensity ultraviolet radiation source was developed which is suitable for emission below 500 A. Source, useful for 100 to 1000 A range, is simple and inexpensive to construct, easy to operate, and very stable. Because of sufficiently intense output spectrum, source can be used with monochromator at wavelengths as low as 160 A
VINCI-VLTI measurements of HR 4049: the physical size of the circumbinary envelope
We present the first detection of the envelope which surrounds the post-AGB
binary source HR 4049. VINCI-VLTI K-band interferometric observations of this
source imply the existence of a large structure with a Gaussian angular FWHM
22.4 +/- 1.4 mas or uniform disk diameter of 34.9 +/- 1.9 mas. With the
Hipparcos parallax of 1.50 +/- 0.64 mas these values correspond to a physical
size of 14.9 (+11.1,-4.4) AU and 23.3 (+17.3,-7.0) AU, respectively. Our
measurements, covering an azimuth range of about 60 degrees, for the
sky-projected baseline, provide information on the geometry of the emitting
region and show that there is only a slight variation of the measured angular
values along the different directions sampled. Thus, our results are consistent
with a spherical geometry of the envelope. However, we cannot completely rule
out the existence of an asymmetric envelope (like the circumbinary disk
envisaged by some recent models) because of the limited spatial frequency and
azimuth range covered by the observations.Comment: 4 pages, 4 postscript figures, accepted letter for A&
Nearby main sequence stars with cool circumstellar material
The discovery of the so-called Vega phenomenon was one of the most important and unexpected results of the IRAS mission. Several nearby main sequence stars were found to possess clouds of solid grains emitting strongly in the far-IR. Three of these objects were marginally resolved by IRAS. This phenomenon appears to be widespread and not limited to proto-planetary epochs. Possible connection of this phenomenon to the existing of planets is discussed
On the accuracy of the S/N estimates obtained with the exposure time calculator of the Wide Field Planetary Camera 2 on board the Hubble Space Telescope
We have studied the accuracy and reliability of the exposure time calculator
(ETC) of the Wide Field Planetary Camera 2 (WFPC2) on board the Hubble Space
Telescope (HST) with the objective of determining how well it represents actual
observations and, therefore, how much confidence can be invested in it and in
similar software tools. We have found, for example, that the ETC gives, in
certain circumstances, very optimistic values for the signal-to-noise ratio
(SNR) of point sources. These values overestimate by up to a factor of 2 the
HST performance when simulations are needed to plan deep imaging observations,
thus bearing serious implications on observing time allocation. For this
particular case, we calculate the corrective factors to compute the appropriate
SNR and detection limits and we show how these corrections vary with field
crowding and sky background. We also compare the ETC of the WFPC2 with a more
general ETC tool, which takes into account the real effects of pixel size and
charge diffusion. Our analysis indicates that similar problems may afflict
other ETCs in general showing the limits to which they are bound and the
caution with which their results must be taken.Comment: 14 pages, 13 figures, to be published in PASP on July 200
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