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IUE "Rogues Gallery"

Many different conditions (some intentional, some instrumental, and some accidental!) could affect the quality of IUE data. The following page contains a brief description of various 'problems' that might affect IUE data and example IUE SI browse images for each 'problem'.


Cosmic Ray 'Hits'

Higher energy cosmic rays would interact with the camera targets resulting in 'hits' in the image. The 'hits' could look comet-like or more circular depending on the angle that the cosmic ray went through the camera target. In some cases the cosmic ray hits could be confused for extra emission from the target if it landed in or near the spectral order. The NEWSIPS processing system has software to flag these cosmic ray 'hits' in all the NEWSIPS data and to attempt to remove them from the low dispersion extracted data. Note that the cosmic ray hits could occur at any point in the orbit and were not necessarily associated with the high backgrounds seen when the IUE spacecraft entered the Van Allen Belts during the US2 shift. See the sample images: SWP01550 and LWP29076.


Data Dropouts

Portions of an image would be permanently lost, if the telemetry lock was lost during the 5 minute period of time necessary to read down a camera. This typically happened if the IUE spacecraft's S-band antennas drifted into an unfavorable angle with respect to the ground receiving station. The telemetry could also be lost if the ground receiving antenna hardware or its control computer malfunctioned and/or the ground operational computers suffered a 'serious' crash during a camera read. (In many cases the telemetry was recoverable from ground system computer crashes.) Note that the low dispersion and echelle order format was not alligned with the read scan direction, thus in the SIHI and SILO images data dropouts appear at an angle to the dispersion direction. Note also the 'scalloped' effect in the LWR SIHI data dropout lines. This is due to the distortion from the geometric correction and echelle order straightening done to create the SIHI images. See the sample images: SWP01336, LWP04487, LWR01305, and SWP16337.


Double Dispersion Exposure

Occasionally, a low and high dispersion exposure were acquired on the same exposure. Often, this was done at the request of the Guest Observer, or it could have been a operations error. See the sample double dispersion images (processed as a high dispersion exposure): SWP03643, SWP41049 and LWP10870.


Exposures of Extended Objects

IUE's spatial resolution was approximately 3-4 arcseconds. The large aperture shape was an 10 by 20 arcsec oval with the low dispersion direction approximately parallel to the short axis of the aperture. Thus, in low dispersion, it was possible to obtain spectral information in the spatial direction of extended objects. See for example, LWR15908, which is an exposure of Comet Iras-Araki-Alcott.


Geocoronal Lyman Alpha Emission

Geocoronal Lyman Alpha emission was usually superimposed on all SWP images. The emission rate was variable and was most noticeable in 'longer' exposures. It's signature is the prescence of an oval-shaped emission feature near 1215 Angstroms. Note that objects which have intrinsic Lyman Alpha emission or interstellar absorption require specialized extraction methods in this region. See for example: SWP13840, and SWP17475.


High Radiation Background

During about a third of the orbit ('US2' shift) the IUE spacecraft passed through the outer Van Allen Radiation Belts. Cherenkov radiation from high-energy electrons entered the ultraviolet converter section of the camera producing increased phosporescence. This occurred only while the camera was exposing. This increased the average background level on the entire image also causing it to be significantly more noisy. A Geiger counter particle monitor, known as the Flux Particle Monitor (FPM), with a threshold of 960 kev for electrons and 15 Mev for protons, measured the strength of this particle field. FPM values ran from 0.08 to 3.60. The flux particle monitor (FPM) malfunctioned in May 1991 and it was turned off in October 1991. Therefore, only images taken prior to that time have valid radiation values.

The rate at which the background level accumulated on the most sensitive part of the detector was an exponential function of FPM, and was given by

DN/hour = A x 10FPM,

where A = 1.35 for the LWP camera, A = 0.73 for the SWP camera , and A = 1.00 for the LWR camera.

See the sample images: SWP01428 and LWP04090.


LWR 'Flare'

A number of the LWR images were affected by a bright extended spot at the lower edge of the SEC target that was visible in long exposures. The 'Flare' first appeared sometime between March 30 and April 14, 1983. The intensity of the spot increased as a linear function of the exposure time. The maximum intensity of the spot (and consequently its extension) linearly increased with time at a rate of 2.17E-3 DN/minute per day. It was thought that the spot was due to a flare in the UV converter. Consequently, after (October 1983) the project began using the LWP regularily instead of the LWR and the LWR was only used with a reduced UVC voltage. See the sample images: LWR18000 and LWR18487.


Microphonics Noise

A significant proportion of the spectral images obtained by IUE were affected by periodic noise artifacts (often called "microphonics"). This noise was different for each camera.

  • In the LWP camera, the noise was introduced by the reaction roll wheel speed change during maneuvers. However, only the portion of the image which was read down at the time the roll slew was in progress was affected.
  • In the LWR camera, microphonics noise (or a "ping") was typically confined to a relatively small band in the image. The interference had the characteristics of a damped electronic oscillator and pointed to an instability in the camera head preamplifier as a possible source of the problem. In order to minimize the effects of the LWR ping a 4-minute Heater warmup procedure was developed. The heater of the read section of the camera was turned on for 4 minutes prior to initiation of the read scan. This effectively displaced the ping such that it usually occurred above the spectral image. The length of the LWR heater warm up time was variable, but 4 minutes was most common. A ping is shown in the sample image: LWR01315.
  • In the SWP, microphonics noise was introduced by the roll wheel speed change during maneuvers and by the roll wheel spinning below +/- 100 rpm or above +/- 400 rpm. In SWP most, if not all, of the image was affected but normally with a lower amplitude than the LWR. See the sample image: SWP07482.
  • Note that for most GSFC-obtained images spacecraft maneuvers were typically initiated only after the read beam had scanned past the spectral image, thus minimizing microphonics affects on the SWP and LWP cameras.
  • Many early SWP images were affected by strong SWP microphonics noise caused by the Panoramic Area Sensor being left on in scan mode. The PAS was subsequently turned off since it was only needed during initial attitude acquisition after launch. This microphonics noise also caused problems for the NEWSIPS processing which had trouble finding and registering on the relatively faint fixed pattern noise in the image. See the sample image: SWP01465.


Multiple Exposures in the Large Aperture

Occasionally the Guest Observer would choose to obtain more than one exposure in the large aperture before reading the camera down. There were several possible reasons for doing this. This was a way to do a 'pseudo-trail' and increase the signal-to-noise on objects that were too faint to employ the standard trail method. Multiple exposures in the large aperture were also occasionally used on selected variable stars in order to obtain spectra at time scales less than the standard read-prep time of 25 minutes. Multiple exposures in the large aperture could also be obtained in crowded fields. Specialized software is needed to extract the individual spectra. See for example: SWP13351, SWP17013, and SWP19000.


Overexposed Data

IUE raw Data Numbers (DN) ran from 0 to 255. A 'good' exposure was typically about 180 to 220 DN. Exposure levels above about 220 DN would fall on the extrapolated portion of the Intensity Transfer Function (ITF). [The ITF was used to convert the raw DNs to the linearized Flux Number (FN) scale of the SI files]. Raw data values saturated at 255 DN. In addition, occasionally individual pixels could also effectively saturate at values below 255 DN. Of course, it was possible to have portions of a given image well-exposed and other portions overexposed, due to variation of the sensitivity of the cameras with target position and the object flux levels as a function of wavelength. See the overexposed sample images: SWP01464, LWP04250. LWR18312, LWR11965.

One side effect of a heavily overexposed spectrum was that negative residual image may be left on the pedestal of subsequent image(s) which may affect the extracted flux levels. See the sample image: SWP45189, which is an 36x overexposure of an FIV star. This exposure was followed by three 'null' exposures (SWP45190, SWP45191, SWP45192). A null exposure was produced by reading the camera after a SPREP or a XSPREP.

Another possible side effect of a heavily overexposed spectrum is that a positive residual image of the overexposed object may be superimposed on subsequent images.


Preparation of the Cameras

During most of the life of IUE the cameras were prepared with an 'SPREP' (standard prep) or 'XSPREP'. After an image was read down, the camera was 'PREP'ed for the next exposure. For the SPREP, the target was first overexposed with a 200% tungsten flood lamp with the camera SEC gain at the MAXG setting, then scanned with a defocussed read-rate erase scan (768 x 768 pixels), then again exposed to a 50% tunsten flood at the MEDG gain setting, and finally scanned with an oversized (804 x 804 pixels) defocussed read-rate scan. Note MEDG=MAXG/4. This left a relatively consistent pedestal on the target for the next exposure. Note that a read followed by an SPREP typically took about 25 minutes of time.

An XSPREP consisted of an XPREP followed by an SPREP. XSPREP's were typically used after the cameras were overexposed by 4 times or more. An XPREP consisted of an 800% tungsten flood lamp exposure at the MAXG gain setting followed by 3 fast wipes (reads) of the target. The XSPREP mostly removed the effects of the overexposure from the cameras although they tended to have increased phosphorescence backgrounds for some time after the XSPREP, limiting observations of faint targets immediately following the XSPREP. An example of an image which was only XPREPed: LWR04007.

An FPREP (fast Prep) was faster than an SPREP, but the camera target was incompletely cleaned. An FPREP consisted of a 200% tungsten flood lamp exposure at the MAXG gain setting followed by three fast wipes (reads) of the target. See the sample images: SWP01258.


Small Aperture Exposures

The size of the IUE small aperture was about an 3 arcsecond diameter circle. The throughput of the small aperture was approximately 50% that of a similar exposure in the large aperture. However, the exact throughput varied depending on how well centered the object was in the small aperture. Large and small aperture exposures were often obtained on the same image in order to scale the small aperture flux level. Thus, the standard NEWSIPS (and IUESIPS) merged extracted small aperture fluxes may vary by about a factor of 1-2 times. See SWP13336 for an example of a large and small aperture exposure on the same image.


SWP Fiber Optic 'Kink'

A 'Kink' in the spectrum was present at the long wavelength end of the SWP. This was thought to be due to a misallignment or shearing of some of the fibre optic bundles in either the output window of the UltraViolet Converter (UVC) tube and/or the input window of the Secondary Electron Conduction (SEC) Vidicon tube of the SWP camera. See the sample image: SWP39519.


References:

  1. IUE Spacecraft Operations Final Report, ESA SP-1215, Sept. 1997.
  2. "The History of IUE", A. Boggess and R. Wilson, in Exploring the Universe with the IUE Satellite, 1987, p. 3.
  3. "The Event Round Robin IUE VICAR Image Labels", M. E. VanSteenberg, NASA IUE Newsletter, No. 40, Dec 1989, p. 37.
  4. "IUE Observing Guide", NASA IUE Newsletter, No. 47, August 1992.


[ADF] The IUE browse files were conceived and originally implemented by Dr. Nancy Oliversen, Dr. Derck Massa, and Ms. Patricia Lawton, then members of the GSFC Astrophyics Data Facility (ADF) staff under direction of Dr. Michael Van Steenberg. Some modifications have been made as part of the transition to MAST maintenance.