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siaper -- Draw the scientific instrument apertures for Hubble Space Telescope.


siaper image


The siaper task draws the science instrument apertures of the Hubble Space Telescope (HST) on the specified graphics device. This task can be used to create overlays of the HST Field-of-View (FOV) onto images.

Aperture plots may overlay an image on the output device (using the imdkern task in the IRAF plot package). This is done by first displaying the image with the display task, and then drawing the aperture with siaper.

There is no real communication between display and siaper. Therefore, you must tell both tasks how to position the image and the overlay. The disconlab task in the STSDAS graphics.sdisplay package can be used to combine the aperture plot with other graphics, simplifying the overlay process.

The aperture specifications are those defined in the Project Data Base (PDB) Science Instrument Aperture File (SIAF). STSDAS is shipped with a FITS file containing this aperture file---it should have been converted to a STSDAS table during installation. Refer to the examples on how to access this file.

The naming scheme of the apertures is non-obvious due to a 10 character limit imposed by the PDB. In general, each aperture contains a substring indicating which instrument it belongs to, beginning with the single letter designation of the instrument. The general categories are as follows:

	ota          - HST FOV
	fgs          - FGS
	xf96, x96    - FOC f96
	xkf96, xk96  - Post-COSTAR FOC f96
	xf288, x28   - FOC f288
	xkf288, xk28 - Post-COSTAR FOC f288
	xf48, x48    - FOC f48
	xkf48, xk48  - Post-COSTAR FOC f48
	ybl, yrd     - FOS Blue/Red detectors
	ykbl, ykrd   - Post-COSTAR FOS Blue/Red detectors
	z	     - GHRS
	zk	     - Post-COSTAR GHRS
	v	     - HSP
	wwf, wpc     - WF/PC
	uwf, upc     - WF/PC II

There are over 586 apertures specified in the SIAF. The SIAF itself is used by the HST ground system specifically for target acquisitions and interactive offset information. The SIAF also contains older definitions of some of the apertures for historical purposes. Finally, some apertures have never been rigorously defined due to instrument problems or simple lack of use. Hence a number of the apertures are not useful for overlay purposes. Below is a discussion, per instrument, of the "real" physical apertures.

Two files, found in stsdaslib$ and distributed with STSDAS, contain lists of some of the "real" apertures. The file siaper_def_apers.txt contains the major apertures for Post-Costar. The file siaper_def_apers_pc.txt contain the major Pre-Costar apertures.

ota - HST Field-Of-View (FOV)
This aperture specifies the limit of the HST FOV.
fgs - Fine Guidance Sensors
The apertures "fgs1", "fgs2", and "fgs3" are the "pickles" of the corresponding FGS'. Simply specifying "fgs" in the aperture parameter will draw all three FGS apertures.
xkf96, xk- FOC f96
The aperture "xkf96" defines the circular entrance aperture to the f96 camera.

"xk96n512", "xk96n256", and "xk96n128" are the normal or unzoomed 512x512, 256x256, and 128x128 image formats. The zoomed formats replace the "n" with "z".

xkf288, xk- FOC f288
The aperture "xkf288" defines the circular entrance aperture to the f288 camera.

"xk28n512", "xk28n256", and "xk28n128" are the normal or unzoomed 512x512, 256x256, and 128x128 image formats. The zoomed formats replace the "n" with "z".

xkf48, xk- FOC f48
The aperture "xkf48" defines the circular entrance aperture to the f48 camera. Note that the actual entrance aperture is not circular. However, the SIAF table represents apertures using only simple geometric forms. Since this aperture is not an image mask, the circular approximation is acceptable.

"xk48n512", "xk48n256", and "xk48n128" are the normal or unzoomed 512x512, 256x256, and 128x128 image formats. The zoomed formats replace the "n" with "z".

FOC Occulting Fingers and Slit
FOC entries which end with "_4" or "_8" are apertures used when the target is to be placed behind the 0.4 or 0.8 occulting fingers for the f96 and f288 cameras.

FOC entries which end with "a4" or "a8" are apertures used when acquiring targets for occulting or spectroscopy modes of the FOC for the f96 or f288 cameras.

FOC f48 entries which end with "s" are apertures used to acquire and place targets on the slit.

ykbl, ykrd - FOS Blue/Red detectors
All apertures are identified by their pre-COSTAR size, where the decimal point is replaced by and underscore, "_". For example, the blue side 4.3 aperture is specified as "ykbl4_3".

The paired apertures have either "pra" or "prb" appended to the name corresponding to the pair identification, A or B.

The bar apertures have "bar" in their designation.

Finally, the apertures "ykbl" and "ykrd" alone define the circular entrance apertures to the FOS in the HST FOV.

The GHRS apertures start with "zk" (or just "z" for pre-COSTAR). The numbers "1" and "2" correspond to the detector. "zk2" is the circular aperture for the GHRS but is not an image mask.

"zk1n2_0" and "zk2n2_0" is the Large Science Aperture (LSA) for each detector.

"zk1n_25" and "zk2n_25" is the Small Science Aperture (SSA) for each detector.

The "n" can be replaced by "a" to use the apertures related to the attenuated mirrors. The other apertures are used for engineering purposes.

"upc1" is the PC.

"uwf2", "uwf3", and "uwf4" are the wide-field chips.

Note that the chips overlay each other when all plotted. This is due to the fact that the edges near the pyramid do not appear on the sky. The SIAF has defined the full sizes of the chips but has positioned them such that their appropriate areas are illuminated by the sky.

The optimum apertures for the methane quad filters are identified by names which begin with "ufqch".

The optimum apertures for the polarizer have "pol" in their names.

The optimum apertures for the redshifted OII filter are identified by names beginning with "ufqu".

The Woods filter optimum apertures begin with "uf160".

Aperture String Matching
Due to the large number of apertures, a regular expression matching technique is used. In general a string of characters will match to any aperture that contains the string of characters. For example, "xyz" will successfully match the strings "xyz", "abcxyz", "xyzabc", or "axyzb". The pattern matching commands are as follows:

	^     - Matches the beginning of the line.
	$     - Matches the end of the line.
	*     - Matches zero or more occurrences of the 
		proceeding character.
	?     - Matches any single character.
	[xy]  - Match a character if it appears in the list of
		characters "xy".
	[^xy] - Match a character if it does not appear in the
		list of characters "xy".

See the EXAMPLE section for use of pattern matching.

SIAF Table
There are 125 columns defined in the SIAF table. The only column the user needs to be aware of is the column SIAP_ID. This column contains the name of the apertures. To examine the table for aperture names, use the command tread as follows:

	cl> tread stsdas$data/scidata/cosiaf columns=siap_id

To get a printout of the column, use tprint:

	cl> tprint stsdas$data/scidata/cosiaf columns=siap_id


image [file name]
An image containing the world coordinate system (WCS) information needed to properly scale and rotate the HST apertures. Usually the image on which the apertures will be overlaid. If blank, the WCS information will be taken from the parameters in the wcspars pset.
(apertures = "@stsdaslib$siaper_def_apers.txt") [string]
A list of aperture names to be plotted. The names may be separated by spaces or commas. See the Aperture description above for a list of common aperture names.

The names can also be placed in a text file. To specify the file name as the parameter value proceeded with the "@" character. The default value "@stsdaslib$siaper_def_apers.txt" specifies a file.

(center_ap = "ota") [string]
The aperture which will be located at the specified right ascension and declination. The default aperture is the ota, or the V1 axis. Any aperture can be specified. This parameter does not support pattern matching; a exact aperture name needs to be specified.

The center aperture will not be drawn. If the center aperture is to be plotted, it must also be included in the apertures parameter.

(ra = "") [string]
The right ascension (in H:M:S format) at which the center aperture will be located. The default RA is the reference RA in the image's WCS.
(dec = "") [string]
The declination (in D:M:S format) at which the center aperture will be located. The default DEC is the reference DEC in the image's WCS.
(roll = 0.) [real]
The spacecraft roll measured in degrees east from north. The default is zero degrees roll.
(wcspars = "") [pset]
Parameter set (pset) for specifying a different WCS if the image is not specified or does not contain WCS information. Type "help wcspars" for more information.
(siaf = "") [file name]
The STSDAS table containing the PDB SIAF file. This file contains the aperture names and positioning information needed to draw the apertures.
(dvpar = "") [pset]
The dvpar pset contains generic parameters specifying the plot device, location of plot on the device, and whether to append this plot to a previous plot. The parameters used from the dvpar pset are described below:
(device = "stdgraph") [string]
The graphics device used to display the apertures. Usually, if this is being overlaid on an image, the device is one of the "imd" devices. (Type "help plot.imdkern" for details).
(left = INDEF), (right = INDEF), (bottom = INDEF), (top = INDEF) [real]
The device viewport, in normalized device coordinates (from 0.0 to 1.0 inclusive). If not specified, a viewport is chosen that will fill the graphics display such that a circle will appear as a circle.
(append = no) [boolean]
Overlay aperture on previous graph without initializing graphics device?

If the graphics are to be overlaid on an imtool display, append should be set to "no".

(fill = no) [boolean]
Fill the viewport to the size set by the left, right, bottom, and top parameters?

If fill = no, the default, then the viewport is sized to preserve the aspect ratio of the image.

(sky_project = yes) [boolean]
Invert the HST V3 axis for sky projection? If yes, the HST FOV is inverted, as it is when projected through the HST OTA onto the sky. For situations where input WCS does not require this inversion, such as a unitary WCS or to simply produce plots of the HST FOV, one can set this to "no" to simplify the WCS creation.


The next few examples demonstrate general usage of the task.

1. Getting the default SIAF table. In general, the table should be located in stsdas$data/scidata/cosiaf.dat. It is possible that the file was not created by the STSDAS system administrator when STSDAS was installed.

If this is the case, the distributed FITS file should be located in stsdas$data/scidata/fits/cosiaf.fits. A user can use this file to create their own version of as follows:

	cl> cd home$
	cl> strfits stsdas$data/scidata/fits/cosiaf.fits "" \
	>>> oldirafname+

If the STSDAS administrator has "stripped" the installation, the FITS files will no longer be present. In this case, contact STScI on how to obtain a new copy.

2. Display the default apertures on an image displayed on either Imtool, SAOimage, or Ximtool. The V1 axis, center of the HST FOV, will be placed at the reference coordinate in the image's WCS. This position is specified by the keywords CRVAL1 and CRVAL2.

	cl> display dev$wpix 1 xsize=0.8 ysize=0.8 fill+
        cl> siaper dev$wpix device=imd left=.1 right=.9 \
	>>> bottom=.1 top=.9

Note that the task "" automates display and overlay of apertures on an image. The above could be accomplished by:

	cl> disconlab dev$wpix cont=no label=no doapers=yes

Note that the image dev$wpix covers an area just larger than the WF/PC II, hence most of the other apertures fall outside the image.

3. On dev$wpix, place the FOC f96 on the reference coordinate of the image WCS. Show only the 512x512 normal format.

	cl> display dev$wpix 1 xsize=0.8 ysize=0.8 fill+
        cl> siaper dev$wpix apertures=xkf96,xk96n512$ \
	>>> center_ap=xk96n512 \
	>>> device=imd left=.1 right=.9 bottom=.1 top=.9

4. Same as example number 3. However, place the aperture on the object located at R.A=13:28:01.2, Dec=47:29:49.7. Also, roll the HST by ~35 degrees to place the 512x512 aperture parallel to North and East. Do this with only the relevant image section.

	cl> display dev$wpix[1:160,380:512] 1 xsize=0.8 ysize=0.8 fill+
        cl> siaper dev$wpix[1:160,380:512] apertures=xkf96,xk96n512$ \
	>>> center_ap=xk96n512 ra=13:28:01.2 dec=47:29:49.7 roll=35 \
	>>> device=imd left=.1 right=.9 bottom=.1 top=.9

The following examples demonstrate how to specify apertures, in particular using the regular expression syntax.

Note that all the following examples do not use an image, but the contents of the pset wcspars. The default "wcspars" pset is not used, but a pset called "siaper_defwcs". This pset defines a WCS transformation for a region, measured in arcseconds, which just covers the HST FOV. These are examples of simply creating plots of the HST FOV without an image.

5. Display every aperture found in the SIAF file.

	cl> siaper "" apertures=* wcspars=siaper_defwcs \

6. Specify a file as the source of aperture names. The file in the example, stsdaslib$siaper_def_apers_pc.txt, is distributed with STSDAS. It contains a list of apertures prior to the first Service Mission, SM94, or Pre-COSTAR.

	cl> siaper.wcspars=siaper_defwcs
	cl> siaper.sky_project=no
	cl> siaper "" apertures=@stsdaslib$siaper_def_apers_pc.txt

7. Matching groups of apertures. All the apertures for a particular instrument will be plotted on separate graphs. Since each instrument is identified by the leading character, the match needs to be forced at the beginning of line. The use of the "^" accomplishes this task. Also, for each plot, the ota will be included. Each command will plot, respectively, the apertures for the FGS', FOC f96, FOC f288, FOC f48, GHRS, and WF/PC II.

	cl> siaper.wcspars=siaper_defwcs
	cl> siaper.sky_project=no
	cl> siaper "" apertures=ota,^fgs
	cl> siaper "" apertures=ota,^xk96,xkf96
	cl> siaper "" apertures=ota,^xk28,xkf288
	cl> siaper "" apertures=ota,^xk48,xkf48
	cl> siaper "" apertures=ota,^zk
	cl> siaper "" apertures=ota,^u

8. Matching an exact aperture. Since the default matching will match a string to anywhere in an aperture name, it may be necessary to further constrain the match to get an exact aperture. For example, to plot the 512x512 image format of the FOC F96, the end of line match needs to be specified, "xk96n512$". If one just specified, "xk96n512", the following apertures would be matched: xk96n512, xk96n512_4, xk96n512a4, xk96n512_8, and xk96n512a8. Compare the plots from the following two commands.

Note: the "=gcur" command can be used to zoom the plots to see more detail.

	cl> siaper.wcspars=siaper_defwcs
	cl> siaper.sky_project=no
	cl> siaper "" apertures=xk96n512$
	cl> siaper "" apertures=xk96n512

9. Complicated matching. Draw all the FOC f96 512x512 formats, both zoomed and normal. This happens to be two, xk96n512 and xk96z512. Since the single character, "n" or "z", is the wildcard, use the "?" pattern match. Also match the end of line to avoid the occulting finger apertures.

	cl> siaper.wcspars=siaper_defwcs
	cl> siaper.sky_project=no
	cl> siaper "" apertures=xk96?512$

The following examples discuss the use of the wcspars parameters.

10. This example demonstrates the use of the wcspars parameters. This example will use wcspars to reproduce the information that is found in image dev$wpix. This should reproduce example 2. Note that no image is specified to siaper and the default wcspars pset is used (wcspars="").

	cl> display dev$wpix 1 xsize=0.8 ysize=0.8 fill+
	cl> wcspars.ctype1 = "ra---tan"
	cl> wcspars.ctype2 = "dec--tan"
	cl> wcspars.crpix1 = 257.75
	cl> wcspars.crpix2 = 258.93
	cl> wcspars.crval1 = 201.94541667302
	cl> wcspars.crval2 = 47.45444
	cl> wcspars.cd1_1 = -2.1277777E-4
	cl> wcspars.cd2_2 = 2.1277777E-4
	cl> wcspars.log_x1 = 1
	cl> wcspars.log_x2 = 512
	cl> wcspars.log_y1 = 1
	cl> wcspars.log_y2 = 512
        cl> siaper "" wcspars="" device=imd left=.1 right=.9 \
	>>> bottom=.1 top=.9

11. This examples explains the values found in the siaper_defwcs pset. The goal of this transformation is to create a plot of the HST FOV in V2-V3 coordinates. The "logical" or "pixel" space will be V2-V3 in arcseconds. Since the V1 axis is at the center of V2-V3, both the CRPIX and CRVAL values are 0.0, the origin of V2-V3.

The CTYPE values are set to "linear". There is no need to complicate things any more.

siaper assumes the WCS is a transformation between degrees and "pixels". Since our "pixels" in this case are arcseconds, the transformation found in the CD matrix is the conversion from degrees to arcsecond, 1/3600. Since no rotation exists, the cross terms of the CD matrix are zero.

Finally, the size of the "logical" space needs to be defined. The HST FOV is just less than 1800 arcseconds in diameter. Since V2-V3 is centered on (0,0), the logical domain to be covered is from -950 to +950. One may note that the Y axis is reversed. This is because, as convention in most HST documentation, V2-V3 is shown with V3 increasing downward.

The effect of this transformation is to cancel the transformation in siaper from arcseconds to the assumed units of degrees of an image's WCS. If one were to use "=gcur" to query the positions of the plotted apertures, the reported positions will be the V2-V3 coordinates as specified in the SIAF.


The SIAF is not the best suited for overlay capability. The information is present, but the large number of apertures and lack of definition provided for each aperture is not user-friendly. Unfortunately, this file is the only official source of aperture characteristics at the Institute.


Jonathan Eisenhamer, STSDAS


dvpar, wcspars, display, imdkern,tread

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