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SI BROWSE FILES

The IUE SI browser images have been created from the original IUE NEWSIPS SI files. A SI browse file consists of a single gif file and some additional information.

The SI browse gif files contain a color display of the SI image, a color bar showing the Flux Number (FN) color scale, an FN histogram, and an NU Flag bit histogram. In addition the SI browse html files contain selected header information.

The original IUE SI images have undergone considerable massaging to produce the SI preview images. The SI browse images are mainly intended to be used to check the quality of the data (Is the observation overexposed, affected by cosmic rays or data dropouts, etc.) and should not be used for direct scientific data analysis.

Many different conditions (some intentional, some instrumental, and some accidental!) could affect the quality of IUE data. During the creation the preview images a "Rogue's Gallery " of images was assembled. This page contains a brief description of various 'problems' that might affect IUE data and example IUE SI browse images for each 'problem'.

LOW DISPERSION SPECTRA

The SILO browse images are displayed with a 60 level color table. There are 640 x 80 pixels in an SILO image. A total data range of 800 (SWP), 600 (LWP), and 800 (LWR) Flux Numbers (FNs) are displayed in each SILO browse image, with the minimum and maximum FN of the display allowed to increase or decrease slightly depending upon the general background level of the SILO image. Note that the minimum and maximum possible FN values in an SILO image are -1024 FN and +1024 FN, respectively. Generally, the number of extremely low or extremely high pixels in an SILO image are relatively small. Thus, in order to prevent these extreme pixel FN values from dominating the color scale, the full range of FN values in a SILO image are not displayed in these browse images. Pixel FN values less than the minimum display FN were set to the minimum display value (color=black) and pixel FN values above the maximum display FN were set to the maximum display value (color=red). The maximum and minimum displayed FN values are listed above the color bar. Saturated pixels are displayed with a yellow color and pixels affected by telemetry data loss are set to the FNmin (black) value.

The minimum FN value of the color scale was determined using the following method. First the average background was determined from two narrow strips above and below the spectral data. Only 'good' (NU flag = 0) data was used to determine the average background. The minimum FN value displayed (color=black) was given by:

  FNmin = <average background> - 0.05 * FNrange

The maximum FN displayed (color=red) was given by:

  FNmax = FNmin + FNrange

Where:

  FNrange(SWP)=800FN, FNrange(LWP)=600FN, and FNrange(LWR)=800FN.

Typically, the minimum of the display varies from about -40 FN to +50 FN. Note that the overall pedestal of the cameras decreased with time, thus later images may have a negative FN pedestal (especially ones with short exposures). Images with high radiation backgrounds may have significant positive pedestals.

The minimum FN displayed for the wavelength calibration images (object class 98) and flat fields (Object class 99: Tungsten Floods, UV Floods and Nulls) was determined by:

   FNmin = 0 - 0.5*FNrange

Where the FNrange is given above.

The low dispersion wavelength scale in Angstroms is displayed below each SILO image.

FN Histogram

An FN histogram (below the color bar) shows the number of pixels inside the target ring at each of the possible FN values. The green 'X' on the left side of the FN histogram shows the total number of pixels with FN values less than the minimum display FN value. The green 'X' on the right side of the FN histogram shows the total number of pixels with FN values greater than the maximum display FN value. The yellow diamond shows the number of saturated pixels inside the target ring. Note that if there were no pixels at a given FN value the histogram point was set to -1 to prevent arithmetic overflow.

Nu Flag Histogram

On the lower right side of the browse image is the NU Flag histogram showing of the number of pixels inside the target ring which are flagged with each of the possible NEWSIPS quality (NU) bit values. Note that the following NU bit values are not shown in the histogram [Bit=15: 'Pixels not photometrically corrected', Bit=13: 'Reseau in the ITF', Bit=10: 'Warning track - near edge of photom region] because they either would dominate the scale of the histogram and/or because they are only applicable to pixels outside of the target ring. The number of pixels inside the target ring flagged as having no problems (NU flag = 0) are also displayed in the histogram but are truncated in the display (again to prevent this flag from dominating the histogram ).

The minimum and maximum FN data value in the SILO file and the total number of pixels inside the target ring are shown along the bottom of the gif file.

"Inside the Target Ring"

Only pixels "inside the target ring" are displayed in the browser images and included in the FN and NU bit histograms. Pixels inside the target ring were determined by masking out all pixels with a NU bit flag of 15 (generally the black border around the image) and with NU bit flag of 10 (Warning track). In addition, the IDL function 'ERODE' was used to slightly decrease the size of the remaining image for display.


HIGH DISPERSION SPECTRA

SIHI Browse Image Format

The SIHI browse images are displayed with a 60 level color table. There are 768 x 768 pixels in an SIHI image. A total data range of 600 (SWP), 400 (LWP), and 500 (LWR) Flux Numbers (FNs) is displayed in each SIHI browse image, with the minimum and maximum FN of the display allowed to increase or decrease slightly from a 0 pedestal, depending upon the general background level of the SIHI image. Note that the minimum and maximum possible FN values in an SIHI image are -1024 FN and +1024 FN, respectively. Generally, the number of extremely low or extremely high pixels in an SIHI image are relatively small. Thus, in order to prevent these extreme pixel FN values from dominating the color scale, the full range of FN values in a SIHI image are not displayed in these browse images. Pixel FN values less than the minimum display FN were set to the minimum display value (black) and pixel FN values above the maximum display FN were set to the maximum display value (red). The maximum and minimum displayed FN values are listed adjacent to the color bar.

For the SIHI image of all 3 cameras, the minimum of the color scale was determined from a region near the 'top' or short wavelength part of the image, which was approximately the region of lowest sensitivity on the camera. The minimum FN displayed is equal to the average FN value minus 3.5 (SWP), 1.75 (LWP), and 5.75 (LWR) times the standard deviation of this low sensitivity region. The maximum FN displayed was set equal to the minimum FN displayed plus 600 FN (SWP), 400 FN (LWP), and 500 FN (LWR). Typically, the minimum of the display varies from about -40 FN to +50 FN.

Note that the overall pedestal of the SWP camera decreased with time, thus later images may have a negative FN pedestal (especially ones with short exposures). Images with high radiation backgrounds may have significant positive pedestals.

Saturated pixels are displayed with a yellow color.

FN Histogram

An FN histogram (to the right of the color bar) shows the number of pixels inside the target ring at each of the possible FN values. The green 'X' on the bottom of the FN histogram shows the total number of pixels with FN values less than the minimum display FN value. The green 'X' on the top of the FN histogram shows the total number of pixels with FN values greater than the maximum display FN value. The yellow diamond shows the number of saturated pixels inside the target ring. Note that if there were no pixels at a given FN value the histogram point was set to -1 to prevent arithmetic overflow.

Echelle Order and Wavelength Information

Every fifth echelle order and its central wavelength are marked on the left side of the browse image. The first and last echelle order numbers present in the SIHI file are also labeled on the left side of the browse image.

Nu Flag Histogram

On the lower right side of the browse image is the NU Flag histogram showing of the number of pixels inside the target ring which are flagged with each of the possible NEWSIPS quality (NU) bit values. Note that the following NU bit values are not shown in the histogram [Bit=15: 'Pixels not photometrically corrected', Bit=13: 'Reseau in the ITF', Bit=10: 'Warning track - near edge of photom region] because they either would dominate the scale of the histogram and/or because they are only applicable to pixels outside of the target ring. The number of pixels inside the target ring flagged as having no problems (NU flag = 0) are also displayed in the histogram but are truncated in the display (again to prevent this flag from dominating the histogram ).

The minimum and maximum FN data value in the SIHI file and the total number of pixels inside the target ring are shown along the bottom of the gif file.

"Inside the Target Ring"

Only pixels "inside the target ring" are displayed in the browser images and included in the FN and NU bit histograms. Pixels inside the target ring were determined by masking out all pixels with a NU bit flag of 15 (generally the black border around the image) and with NU bit flag of 10 (Warning track). In addition, the IDL function 'ERODE' was used twice to slightly decrease the size of the remaining image for analysis. This was done to exclude a number of saturated pixels in the upper left hand corner of the SWP SIHI image which were evidently part of the target ring that had not been flaged with a NU bit of 10.


[ADF] The IUE browse files were conceived and originally implemented by Dr. Derck Massa, Dr. Nancy Oliversen, 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.