GALEX FUV PDF

The star, named Mira pronounced my-rah after the latin word for "wonderful," is shedding material that will be recycled into new stars, planets and possibly even life as it hurls through our galaxy. The close-up picture at bottom gives a better look at Mira itself, which appears as a pinkish dot, and is moving from left to right in this view. Shed material appears in light blue. The dots in the picture are stars and distant galaxies.

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GI observations will begin in late If there is any appearance of duplication, you will need to explain carefully feasibility section in your proposal why your proposed investigation does not duplicate the science objectives of the PI team. You may not need to obtain new observations; you may be able to use the data that will be released in Data Release 1 DR1 for an Archival Proposal.

Mission 1. Even the small amount of residual atmosphere at the km GALEX orbital altitude scatters significant flux into the telescope. FUSE, the Far Ultraviolet Spectroscopic Explorer also has to contend with atmospherically-scattered sunlight and airglow, but its field of view covers about , times less sky, so much less of the scattered light enters the spectrograph. There are other details in the way the two instruments operate that make GALEX more susceptible to atmospherically-scattered sunlight background.

These include their wavelengths. FUSE operates at Angstroms. How long is an orbit? How long is an eclipse? How long is an observations? GALEX orbits the earth every The actual time available for an eclipse observation is less, and is determined by a combination of observational constraints sun-angle, zenith-angle, location of SAA, moon-angle and the observation initiation sequence, which starts with a slew from solar pointing solar arrays to the final target pointing and roll angle twist.

During this time the high voltage is ramped from the intermediate value to nominal levels which takes 2 minutes.

Ramping can only start after the satellite enters the umbra. GI observations are expected to begin in late Instrument and Operations 1. What are the brightness limits? Fundamental detector safety requirements limit observations of bright targets. Pointing centers must be separated from bright stars by : 0. The microchannel-plate MCP detectors that GALEX uses have intrinsically low red leak so they reject longer-wavelength light that is outside the nominal bandpass.

This is important in the ultraviolet since the sky is much brighter in the visible redward than in the UV. To avoid red leaks, CCDs require special filters that are difficult to make and prone to pinholes. In addition, CCDs require cooling, which greatly exacerbates the difficult contamination control necessary for ultraviolet instruments.

Next, MCP detectors detect and time tag each photon. This permitted us to save in development cost by using looser satellite pointing requirements and reconstructing the image using software after the data is telemetered to the ground. Were data taken on a CCD detector with the same satellite pointing, the image would be blurred. The mean response in the FUV between and Angstroms is 13 cm2. The mean response in the NUV between and Angstroms is 35 cm2. Plots of the response of the most significant spectral orders in each band are shown.

The mean dispersion for the FUV in 2nd order is 1. The mean dispersion for the NUV in 1st order is 4. A plot of the dispersion function for the most significant orders are shown. Since GALEX counts individual photons, a moving target may be allowed to drift across the field, and the image may be reconstructed using the time-tag photon list.

Guest investigators wishing to observe moving targets will be given time-tag data and will be responsible for reconstructing the images themselves. What is linearity range of count rates?

Has linearity been tested? The GALEX detectors have a non-linear response at high count rates due to both local effects attributable to the intensifiers and global effects due to the electronics. The local linear range of count rates has also been tested on the ground for the two detectors. This difference is attributable to the proximity focus method of the NUV detector, which spreads the PSF out over a larger area on the intensifier surface, reducing the current density.

We now have a wealth of flight data and are using it to refine the linearity calibration across each detector field of view.

Why does the psf vary across the image, and from image to image? The detector psf is determined by the position digitization process, which is analog and subject to random noise. The psf varies across the image due primarily to gain variations lower gain regions having a broader psf.

Other effects that affect the wings of the psf include surface roughness of the optical surfaces, ghosts from multiple reflections in refractive optical elements, and grazing reflections from baffles or struts in the optical beam path. Most optical design aberrations cause the psf to vary radially over the field of view, but those associated with the dichroic beamsplitter cause variation along the satellite X-axis, which can vary in sky coordinates, depending on the satellite orientation around the telescope optical axis.

Thus, in general a given source in a repeated observation of the same part of the sky will have a different psf if the satellite orientation is different around the telescope optical axis. Is the psf variable as a function of counts non linear? Yes, the PSF will change under intense illumination as the intensifiers exhibit a phenomenon known as "gain sag" whereby the central region of bright star images will be flattened and then eventually cored out as the intensity of the star increases.

Why are there notches and scallops in the images? There is a notch in FUV images due to a high detector background emission "plume" at one edge of the detector. Depending on the roll angle a target is observed, it may appear anywhere on the perimeter.

The FUV detector also displays some areas of low gain and efficiency. The pipeline masks images in regions were the relative efficiency falls below 0. All science data collection uses a spiral dither, to prevent bright-star-induced fatigue of localized regions on the detectors and to improve image flat-fielding.

Grism observations are always done in "single visit" mode; multiple observations are made at different grism orientations and then combined. Can I pick the orientation s of the grism? The grism has possible grism angles selectable. In the target list you should specify the PA on the sky along which you would like the dispersion to run. If you wish to do spectroscopy in crowded fields, using more than about 6 grism exposures, then it is advisable to let the planning software choose random grism angles to improve observing efficiency.

Pipeline Processing and Calibration 1. An astrometric module corrects the photon positions for detector and optical distortions and determines an aspect solution using star positions from the time-tagged photon data, a photometric module accumulates the photons into count and intensity maps and extracts sources from images, and for grism observations a spectroscopic module uses image source catalog inputs to extract spectra of individual sources from the multiple slitless grism images.

How are the data flat fielded? Background subtracted? Will there be improved processing? Work is currently being done to refine the flat field based on much higher resolution flight data, and we expect this to improve the relative photometry substantially. The Early Release Observations use ground-based corrections. The first major data release DR1 will have improved processing. Calibration steps are summarized below.

The initial calibration was done on the ground, before flight. Data acquired in-flight are being used to improve and refine calibration parameters.

The ERO data were calibrated using the pre-flight ground calibration. Ground calibration: Galex was calibrated on the ground during the spring of , in thermal vacuum, using a Roper Scientific Acton Research VM 0.

Top priority calibration items were relative sensitivity versus wavelength, flat field, imaging-to-spectroscopic differential sensitivity versus wavelength, and spatial nonlinearity. Middle priority were absolute sensitivity measured at 3 pencil beam locations in the aperture , grism dispersion function compared to imaging, high count rate tests local and global , and a sky-simulation target. Lowest priority were PSF characterization, near-angle stray light, deuterium spectrum monochromator at zero order , and detector background.

The in-flight calibration procedure and some results were presented at the Atlanta AAS meeting The non-linearity map will be refined using star fields measured at different angles. The first major data release DR1 will be on 1 October and will have improved processing. Data Products.

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The instrument is a 50 cm Ritchey-Chretien telescope with selectable imaging window or objective grism feeding a pair of detectors simultaneously with a multilayer dichroic beamsplitter. A cross section of the instrument is shown in Figure 2 below. The adopted design meets the challenging requirements of providing moderate-resolution optics with a high throughput and large field of view in four different optical paths two UV simultaneous channels, with imagery and slitless spectroscopy modes , while keeping the instrument compact and simple to build and adjust. As needed for a SMEX program, special attention was paid to keep the concept tolerant to component positioning, especially for the moveable parts. Its reflecting entrance side corrects the FUV channel, whereas the exit side cancels the entrance side effects and brings in the required amount of correction for the NUV channel. Allowance has been made for a small wedge on this aspheric window to compensate for the coma it induces in the NUV convergent beam. Its wedge angle is adjusted to correct for the coma induced in a converging beam, simultaneously for orders 2 and 1 spectra of the FUV and NUV channels respectively.

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GALEX FUV PDF

GI observations will begin in late If there is any appearance of duplication, you will need to explain carefully feasibility section in your proposal why your proposed investigation does not duplicate the science objectives of the PI team. You may not need to obtain new observations; you may be able to use the data that will be released in Data Release 1 DR1 for an Archival Proposal. Mission 1. Even the small amount of residual atmosphere at the km GALEX orbital altitude scatters significant flux into the telescope. FUSE, the Far Ultraviolet Spectroscopic Explorer also has to contend with atmospherically-scattered sunlight and airglow, but its field of view covers about , times less sky, so much less of the scattered light enters the spectrograph. There are other details in the way the two instruments operate that make GALEX more susceptible to atmospherically-scattered sunlight background.

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