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1. Where can I find information about the GALEX Guest Investigator program?

2. What is the schedule for the GALEX mission and Guest Investigator program?

The GALEX primary mission started normal operations in July 2003 and will take at least 28 months to complete. A NASA Research Announcement released in late January 2004, solicits Guest Investigator proposals to use the GALEX observatory and archival data; proposals for Cycle 1 are due on 16 April 2004 and future cycles are expected to follow approximately the same cycle. GI observations will begin in late 2004. Further information can be found at http://galexgi.gsfc.nasa.gov/.

Visit the GALEX Guest Investigator program web site at http://GALEXgi.gsfc.nasa.gov or contact the GALEX GI helpdesk (Feb 1 to April 16 2004) or call 301-286-3623. For technical or scientific questions, contact the help desk or GALEX Mission Scientist, Dr. Susan Neff (at GSFC, 301- 286-5137). For programmatic questions, contact the GALEX Program Scientist, Dr. Zlatan Tsvetanov (at NASA/HQ, 202-358-0810).

4. The GALEX team has reserved all the targets of interest to me can I still propose to the GI program?

Because the GALEX Field-of-view is so large (1.2^o), many targets of interest to different audiences may be in the same field. You may propose for a Guest Investigation as long as the science you are proposing does not overlap any of the GALEX Science Team's primary science investigations (http://galexgi.gsfc.nasa.gov/piscience). 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. Where is a web site with more information on GALEX so friends or colleagues can learn about GALEX?

3. Why does GALEX observe only at night, whereas FUSE observes in all parts of the orbit?

GALEX can only observe when it is in the earth's shadow, or eclipse, because on the day side of the orbit atmospherically-scattered sunlight and airglow would swamp and might damage the detectors (especially the NUV detector). The GALEX field of view is 1.25 degrees in diameter. Even the small amount of residual atmosphere at the 700-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 100,000 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 905-1195 Angstroms. GALEX operates at 1350-2800 Angstroms, closer to the peak of the sun's illumination. Also, during data reduction is not possible to remove atmospheric lines from the slitless spectroscopy GALEX uses, in contrast to the slit spectroscopy of FUSE.

4. How long is an orbit? How long is an eclipse? How long is an observations?

GALEX orbits the earth every 98.6 minutes. Approximately 1/3 of this time is spent in eclipse, defined as the sun being below the depressed horizon. 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.

5. What is the schedule for the GALEX mission and Guest Investigator program?

The GALEX primary mission started normal operations in July 2003 and will take at least 28 months to complete. A NASA Research Announcement released in late January 2004, solicits Guest Investigator proposals to use the GALEX observatory and archival data; proposals for Cycle 1 are on 16 April 2004 and future cycles are expected to follow the same cycle. GI observations are expected to begin in late 2004. Further information can be found at http://galexgi.gsfc.nasa.gov/.

Instrument and Operations

Fundamental detector safety requirements limit observations of bright targets. Currently, point sources, with flat spectra, may not be observed (imaging or grism) that are brighter than:

Bright and / or crowded fields may not be observed if they exceed total brightness levels of :

0.75^o for an object with F_l = 1 x 10^-12 erg cm^-2 s^-1Å^-1, or m_AB = 10.8 (NUV, ~ 2300 Å ) (5000cts/s)

0.88^o for an object with F_l = 1 x 10^-11 erg cm^-2 s^-1Å^-1, or m_AB = 8.3 (50,000cts/s)

1.00^o for an object with F_l = 4 x 10^-11 erg cm^-2 s^-1Å^-1, or m_AB = 6.8 (200,000cts/s)

1.50^o for an object with F_l = 1 x 10^-10 erg cm^-2 s^-1Å^-1, or m_AB = 5.8 (500,000cts/s)

2.00^o for an object with F_l = 2 x 10^-10 erg cm^-2 s^-1Å^-1, or m_AB = 5.0 (1,000,000cts/s)

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. Finally, the GALEX detector active area is 65 mm in diameter, ideal for this survey mission. CCDs are available in neither the requisite size nor shape, and CCD mosaics have gaps.

GALEX has a peak spectral response (effective area) of approximately 22 cm² in the FUV and 49 cm² in the NUV. The mean response in the FUV between 1350 and 1800 Angstroms is 13 cm². The mean response in the NUV between 1800 and 2800 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 1.6 Angstroms per arcsecond (range 1.2 to 1.9). The mean dispersion for the NUV in 1st order is 4.0 Angstroms per arcsecond (range 3.3 to 4.3). With a 5 arcsecond FWHM PSF, a point source would yield a FWHM resolution of approximately 8 Angstroms in FUV, and 20 Angstroms in NUV. A plot of the dispersion function for the most significant orders are shown. Note that, in direct image mode, the source would appear at approximately the position of offset=0 in grism mode.

This is not a supported GALEX observation mode. 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.

6. 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 global effects are corrected in the pipeline, and amount to a correction as high as 40% for the highest allowable global rates (100,000 cps). The local linear range of count rates has also been tested on the ground for the two detectors. We found that for isolated stars, the FUV detector is linear to about 100 cps (m~14) and the NUV detector is linear to about 1000 cps (m~12.5). 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.

The point spread function or psf is determined by the microchannel plate detector PSF, as well as the GALEX (Ritchey-Chrétien) optical design and the as-built tolerance errors. 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.

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.

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 0.2, which produces the FUV scallop. A more detailed discussion of image artifacts may be found in Section 7 of the GALEX Pipeline Data Guide.

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. In "single-visit" or "stare/dither" mode, only one field center is observed for an entire eclipse. In "sub-visit observations", or "AIS (All-sky Imaging Survey) mode", several (typically 10-12) contiguous field centers are observed during one orbital night. Grism observations are always done in "single visit" mode; multiple observations are made at different grism orientations and then combined. GI observations may only use these standard GALEX observing modes.

Yes. The grism has 872 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

A detailed discussion may be found in the GALEX Pipeline Data Guide, but in short, the pipeline converts GALEX photon lists, satellite telemetry data and any necessary corollary data into calibrated images and catalogs. The GALEX Science Operations Center (SOC) receives data from the satellite and unpacks it into time-tagged photon lists, instrument/SC housekeeping and satellite aspect information, and uses it to generate images, spectra and source catalogs. 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.

2. How are the data flat fielded? Background subtracted? Will there be improved processing?

The data are initially being flat-fielded using ground measurements of the system throughput, and this yields relative photometry on the order of 25% repeatability. 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.

Details of GALEX calibration may be found in the GALEX In-Flight Calibration Plan. 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. DR1 will be calibrated with in-flight parameters.

Galex was calibrated on the ground during the spring of 2002, in thermal vacuum, using a Roper Scientific Acton Research VM-502 0.2-m vacuum monochromator with a deuterium lamp to provid UV illumination. 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. Results of this calibration may be found in the GALEX Observer's Guide.

· Several "calibrator" fields containing well-known stars (eg LDS749b) will be observed in different GALEX field locations to refine absolute calibration

· Relative calibration will be obtained with multiple observations of rich fields.

· Numerous detector corrections (wiggle, walk, distortion, flat field, ACS dither) will be refined with in-flight data

· Initial indications are that GALEX throughput is close to the value measured in ground calibration: NUV zero ~ M20, FUV zero ~M19.

· Current photometry accuracy is +/- 0.10Mab FUV, +/-0.05Mab NUV. Goal is +/-0.05 both channels

· Pipeline PSF performance is now approaching optimal values with the recent addition of a global detector "wiggle" correction.

· Detector flight background count rates are comparable to ground measurements, with total backgrounds of 76 counts/sec (FUV) and 189 counts/sec (NUV). These may be compared to typical global flight count rates of ~3000cps FUV and ~20000 cps NUV

· GALEX flat fields are being refined using sky background data; this is likely to improve photometric repeatability at the finest levels

· Astrometric performance is +/- 1.2" FUV, +/- 0.8" NUV; goal is +/- 0.5" both channels

· Spatial non-linearity currently limits resolution at the field edges, affecting approximately the outermost 10' significantly. The non-linearity map will be refined using star fields measured at different angles.

Data Products

· FUV and NUV intensity image (FITS), 3480 x 3480 pixels, 1.5" pixels, J2000 WCS, in counts

· Basic matched catalogue of extracted sources (FITS and text); including matched FUV+NUV sources, R.A., Dec., magnitudes and errors, fluxes and errors, E_B-V, sizes, and data flags.

· The full pipeline output is also available, although most users will not want this

· 1-D extracted combined spectra (FITS), 1300-3000 Å, in units of photons sec^-1 cm^-2 Å^-1 vs. Å, on a linear dispersion scale; including a number of estimated source properties.

· The full pipeline output is also available, although, again, most users will not want this

DR1 will include all of the above, but calibration will be based on in-flight calibrations, improved flat fields and artifact removal, measures, spectral image strips will be included as images, and several additional fields will be included in the source catalogues.

Early Release Observations (ERO) were released on 1 December 2003, and are available, together with a query server and other tools for efficient data retrieval, at the Multimission Archive at Space Telescope (MAST),

Data Release 1, containing ~10% of each of the GALEX surveys, will be released on 1 October 2004

Data Release 2, containing the rest of the GALEX survey data, is currently planned for June 2006.

All GALEX data is distributed to the astronomical community through the GALEX archive within the Multimission Archive at Space Telescope (MAST), using protocols similar to those used for distribution of HST data.

4. How does the quality of the ERO data compare to GALEX observations so far, and to the planned complete mission?

The ERO imaging data quality should be typical for GALEX observations so far. The satellite performance has been stable, and the pipeline has been revised twice to account for on-orbit performance. We anticipate that further improvements in the pipeline may improve the PSF and flat-field moderately, and may handle detector artifacts more automatically. The catalogues are expected to improve substantially in future data releases. A principle deficiency in the early pipeline is the handling of the most extended sources, which tend to be shredded by the detection algorithms. Faint sources near the confusion limit will probably require refined processing to achieve the theoretical sensitivity limits. In particular, the ERO spectroscopic products (catalogues, extracted spectra) have received substantially less attention than the images, and we expect significant improvements in future releases.

The ERO data are in the public domain. However, the principal goal of this release is to provide potential Guest Investigators information about GALEX data properties. The lean GALEX Science Operations and Data Analysis Team has concentrated the majority of its resources in the early mission months on producing high quality images, the fundamental input to downstream pipeline products. Catalog and spectroscopic products have received less attention to date. Calibration observations and analysis continue, but are not complete. The documentation provided with the ERO data is limited. Thus we caution that science analysis using the ERO data set may require revision, possibly significant. The first major public data release (DR1, 1 October 2004) will be of sufficient and known quality to support most science investigations.

Starting January 15, 2004, the Multimission Archive at Space Telescope (MAST) will have the ERO images and catalog data on-line, with a query server and other tools for efficient data retrieval. ERO data may also be found at

7. What are the sensitivity limits, completeness, and reliability vs. exposure time and background level for the GALEX ERO catalogs?

The completeness and reliability of the GALEX catalogs are functions of the sensitivity limit, the exposure time, and the background level. The local source density is of course an additional important factor. The exact relationships are still a topic of investigation by the GALEX Science Team. Some estimates have been established using the initial three months of survey data: completeness and reliability have been investigated using multiple visits to the same fields, using MIS and DIS observations of AIS fields, and by using artificial subvisits created from MIS visits; reliability has been studied using combined FUV and NUV catalogs, and by comparing GALEX with other catalogs such as SDSS.

The following plots give preliminary completeness results for exposure times of 400-1600 seconds in the relatively low background Groth DIS region. NUV is left and FUV is right. All GALEX magnitudes are in the AB system.

Sensitivity vs. exposure time for low background targets (DIS, which have low diffuse galactic light and zodiacal background) is shown in the Figure below. At these background levels, imaging surveys are background limited for exposures longer than 2 ksec [NUV] and 10 ksec [FUV] respectively. Background levels may be as high as 3-5 times these, with corresponding reduction in the transitional exposure time.

Catalog reliability has been measured by comparing detected GALEX sources with SDSS DR1 sources. GALEX sources without a SDSS DR1 within 6 arcsec radius are considered spurious, and reliability is calculated as R=1-[#GALEX w/ NO SDSS]/[#GALEX]. Reliability of 90% is achieved in the AIS at m_FUV~21 and m_NUV~22. In the MIS, 90% occurs at m_FUV~23.25 and m_NUV~23.25. A small fraction in the MIS NUV sources may indeed have missed detection by SDSS, so this may be an underestimate of the MIS NUV reliability.

GALEX	AIS [3 deg^2]				MIS [1 deg^2]
mag	#		Fraction spurious		#		Fraction spurious
m	#FUV	#NUV	FUV	NUV	#FUV	#NUV	FUV	NUV
13.75	0	1	0	0
14.25	0	2	0	0	0	2	0	0
14.75	0	4	0	0	0	2	0	0
15.25	0	8	0	0	0	1	0	0
15.75	1	9	0	0	1	7	0	0
16.25	1	14	0	0	1	1	0	0
16.75	2	12	0	0	0	4	0	0
17.25	3	22	0	0	1	6	0	0
17.75	2	19	0	0	0	6	0	0
18.25	4	46	0	0.022	1	18	0	0
18.75	8	56	0	0	7	20	0	0
19.25	24	77	0	0	9	29	0	0
19.75	42	124	0.071	0.008	9	47	0	0
20.25	66	187	0.045	0.016	33	78	0	0
20.75	119	281	0.076	0.025	52	96	0.019	0.021
21.25	213	437	0.174	0.025	78	210	0.013	0.01
21.75	128	484	0.266	0.064	162	283	0.037	0.018
22.25	6	129	0.5	0.109	269	520	0.026	0.021
22.75	0	2	0	0.5	386	789	0.028	0.046
23.25					326	835	0.092	0.098
23.75					16	238	0.188	0.181
24.25					0	2	0	0
TOTAL	619	1914			1009	2119

Data Interpretation

Artifacts are in general very distinguishable from real sources, notably with visual inspection. A detailed discussion of known artifacts, with examples, may be found in the GALEX Pipeline Data Guide. Most artifacts are associated with bright stars, and/or particular regions of the field of view, and can therefore be anticipated and removed or ignored. The pipeline currently recognizes and flags many, but not all artifacts. Many FUV artifacts are apparently immediately from the lack of a corresponding NUV source, which should be present in almost all cases when real FUV sources are detected.

3. I see halos around bright stars. Do only bright stars have these?

All sources have halos with amplitude proportional to the flux from the source. The bright-source halos are just more obvious.

The following table gives conversions from GALEX countrates to AB and other magnitudes.

Parameter	Description	FUV	NUV	Units
	effective wavelength	1516	2267	Å
	Pivot wavelength	1524	2297	Å
	Average wavelength	1529	2312	Å
	rms bandwidth	114	262	Å
	FWHM bandwidth	269	616	Å
	effective bandwidth	268	732	Å
Uresp	unit response (1 cps; m_GALEX= 0)			erg s^-1 cm^-2 Å^-1
f₀	f_GALEX(1 cps; m_GALEX= 0)	108	36	[10^-29erg s^-1 cm^-2 Hz^-1]
m₀(AB)	m_AB-m_GALEX	18.82	20.02	magnitudes
m₀(STLAM)	m_STLAM-m_GALEX	16.04	18.12	magnitudes
m₀(AB) m₀(STLAM)	m_AB-m_STLAM	2.78	1.90	magnitudes

Magnitude errors are a determined by the Source Extraction (Sextractor) routine using simply Poisson errors expected from the number of photons in the source and background (i.e., errors do not include errors in the determination of the background (which is based on a large-scale smoothed backround image) or on any systematic component.

In the FUV channel, diffuse galactic light (DGL) from dust-scattered starlight dominates the background. DGL varies from 300 ph cm^-2s^-1A^-1sr^-1 (PU) to > 2000 PU, depending largely on the dust column density (and to some extent on the local stellar radiation field), which is well correlated with extinction and HI column density. Some contribution to the FUV background comes from H₂ fluorescence, HII 2-photon emission, and line emission from ionized gas in the 10⁴-10⁶ K interstellar medium. In the NUV channel, zodiacal light dominates the background, with a substantial contribution from DGL as well. Detector background is very low in comparison (<1%), except in local "hot spots" which are masked in pipeline processing. Nightglow from residual atmosphere at 700 km altitude produces a modest amount of background in both channels, which increases at the beginning and ending of any eclipse as the zenith angle increases (see eclipse profile in Section 2 of the GALEX Observer's Guide).

The point spread function (psf) is determined by the microchannel plate detector PSF, as well as the GALEX (Ritchey-Chrétien) optical design and the as-built tolerance errors. 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.

The main limitation on the astrometric repeatability is our knowledge of the fine-grained distortion map for each detector. As fields are observed at different roll angles, errors in the distortion map, especially at the field edges, may creep into the astrometric solution. Flight data is currently being used to refine the ground-data-generated distortion map, and several special observations may be planned for the purpose of observing a large number of FUV stars in order to complete this task. Currently, the astrometric precision is of order 1", but there may be isolated areas at the edge of the field where the error is significantly larger. More details are given in Section 6 of the GALEX Observer's Guide.

A gnomonic projection maps a sphere onto a plane by projecting all points on the sphere radially, from the sphere's center, onto a plane that is tangent to the sphere. This projection distorts both angle and area, but it has the useful property that all great circles are projected into straight lines. One can see this by noting that all great circles lie in planes containing the center of the sphere, and that the projection will therefore be the line of intersection between the plane containing the great circle and the plane of the projected map. For more detail, see for instance: http://mathworld.wolfram.com/GnomonicProjection.html

The GALEX spectroscopic mode employs slitless spectroscopy. This provides an image of the sky in which every object is spread out into a spectrum similar to what one sees when viewing the sky directly through a prism or transmission grating. GALEX uses a transmission grism, a ruled prism. This has the advantage of providing spectra for all the objects in the large GALEX field of view but the disadvantage of overlapping spectra in crowded fields. For this reason we take spectra of each part of the sky at different spectral position angles on the sky. This permits us to remove the confusion caused by overlapping spectra. An image strip is a two-dimensional spectrum. As in slit spectroscopy, one dimension is the spectral dimension and the other is the spatial dimension. In slitless spectroscopy however, the spectral dimension is also a spatial dimension, thus a single point on a GALEX grism-mode image represents various wavelengths depending on the source position in the sky, along the spectral-dispersion dimension.

The image strip is a portion of the (corrected) two-dimensional detector image (in grism mode) representing the locations where photons from the spectrally-dispersed source reach the detector. The image strip size is usually about 80 by 600 arcseconds (default scale is 1 arcsecond/pixel), but this size can vary depending on the source flux. The image strip is long enough to include the primary grism orders (1,2 for NUV and 2,3 for FUV) and is wide enough to include all the background used for background subtraction during spectral extraction. The source is centered on the middle row of the image in the dimension orthogonal to the dispersion. The blue end of the spectrum is to the left or lower column numbers. Multiple grism orders may be present--usually 1st&2nd for NUV, and 2nd&3rd for FUV. An image strip, as described above, contains photons from a single visit during a single eclipse, at a single grism position angle. Image strips can also contain photons from multiple visits at various positions angles, by taking either a sum or a median at each pixel in the individual image strips. The image values are scaled integers with the zero point and scale factor given in the header for each source. Negative values in the image strips indicate masked pixels which are ignored in the final spectral extraction.

The following two images show image strips from a single source (5917) from NUV (top) and FUV (bottom). The extracted spectrum is given in the bottom panel. More information on GALEX spectroscopic data processing may be found in Section 3 of the GALEX Pipeline Data Guide.

The fastest way to do this is to use ds9, the image display program freely available from http://chandra-ed.harvard.edu/install.html . Open the image of interest in ds9 (the image must have a J2000 WCS), then go to Region (on main menu), Load Regions, and find the ttttttt_vvvv-[f,n]d-cat.ds9reg file. GALEX detection ellipses determined by the object size and position angle will be displayed as green ellipses.

There are various methods for extracting columnar data from binary fits tables. In IDL, the mrdfits astro lib function reads fits tables into structures with the appropriate tag names. fv is a useful utility for viewing fits tables and can be downloaded here.

This website is kept for archival purposes only and is no longer updated.

GALEX Frequently Asked Questions

Guest Investigator Program

Mission

Instrument and Operations

Pipeline processing and calibration

Data products

Data interpretation

Answers

Mission

Instrument and Operations

Pipeline Processing and Calibration

Data Products

Data Interpretation