SEE_ALSO

## NAME

jimage -- Convert jitter information into an image.

## USAGE

`jimage intable output nx ny`

## DESCRIPTION

This task reads U2 and U3 coordinates of the HST pointing from two columns in a table and creates an image that reflects that "jitter" information. Different rows of the input table give the HST pointing at different times. The image is intended to show what would be observed for a point source using a telescope with no aberrations or diffraction. The image is normalized to a volume of one. The coordinate information put into the image header gives the translation between pixel coordinates and (U2,U3).

It is assumed that the input table contains the (U2,U3) coordinates
where the HST is pointing at different times.
If the rotation (`angle`) is given as zero,
the values in the `u2col` column are converted to the first image axis,
and the negative of the values in the `u3col` are converted
to the second image axis.
The information from OMS (e.g. the .cmj table)
may contain columns with names implying
that they contain (V2,V3) coordinates.
These may be the same as the (U2,U3) where the telescope is pointing,
or they may be opposite in sign,
or they may differ by 90 degrees, or worse.
At the time of writing this is still uncertain.

For computing the CD matrix for the output image, the column units are gotten from the input table. If the units are "degrees", "arcseconds", "ms" or "milliarcseconds" (minimum match is allowed), then the appropriate scale factor is applied so the CD matrix will be in degrees per pixel. If the unit strings for the two columns are not the same, or if the units are not recognized, then arcseconds will be assumed.

Several keywords are written to the output image header.
These include the name of the input table,
the names of the columns, the column units,
and the task parameters `angle` and `flip_u3`.
The total number of rows from the input table
(i.e. the non-INDEF rows) is saved as NTOTAL,
while the number of those points
that were off the edge of the image is NMISSED.
NTOTAL is also used for normalization;
if NMISSED = 0 then the sum of the pixel values in the image will be one.
Two values of RMS of the jitter data are computed,
since in general the jitter ball is not circularly symmetric.
These are the RMS along the major and minor axes of the jitter ellipse,
and they are saved as SIGMA_A and SIGMA_B respectively.
The position angle of the major axis is also computed and saved as POSANGLE.

## PARAMETERS

- intable [file name]
- A table containing U2 and U3 positions as a function of time.
The U2 and U3 coordinates are taken from columns with names
given by the parameters
`u2col`and`u3col`respectively.

- output [file name]
- The name of the image to be created.

- nx = [integer, min=1, max=INDEF]
- Size of the X dimension of the output image.
The image to be created will have size
`nx`by`ny`.

- ny = [integer, min=1, max=INDEF]
- Size of Y dimension of output image.
The image to be created will have size
`nx`by`ny`.

- (pixsize = 0.005) [real, min=0., max=INDEF]
- The size of a pixel of the image to be created. The units are the same as the units of the U2, U3 values in the table, such as arcseconds. The default (in arcseconds) is about equal to the RMS jitter. The pixel size for FOC f/96 is 0.02237 arcseconds, for example.

- (angle = 34.56) [real]
- The angle in degrees measured clockwise from the image X (sample) direction to the U2 direction as projected on the sky. Some values are: FOC f/96, 34.56; FOC f/48, 159.42.

- (u2col = "si_v2") [string]
- Column name in
`intable`for the U2 values.

- (u3col = "si_v3") [string]
- Column name in
`intable`for the U3 values.

- (flip_u= yes) [boolean]
- Flip the U3 coordinates? If the input jitter data are U2 and U3
(or V2 and V3), then
`flip_u3`should be set to yes; the U3 coordinates will then be "flipped" by subtracting from the median of the U3 values.

## EXAMPLES

1. Create a jitter image "jb.hhh" for FOC f/48 from table "jb.tab" but with pixels 10 times smaller than the actual pixels (0.045 arcseconds) in order to see the structure of the jitter.

pl> jimage jb.tab jb.hhh 100 100 pixsiz=0.0045 angle=159.42

## BUGS

## REFERENCES

This task was written by Phil Hodge. The algorithm for computing the RMS and position angle of the jitter ellipse was based on code written by Kavan Ratnatunga.

## SEE ALSO