How to simulate PSFs with Tiny Tim¶

Step 1¶

The following sequence of commands will generate a PSF with the requested conditions. First, choose the name of the PSF, e.g. psf0

$tiny1 psf0

When prompted, choose the desired instrument, e.g. 23 for the WFC3/IR channel.

$ 23

For the position, we can choose the middle-ish pixel, i.e.

$550 550

Next is the filter passband corresponding to the instrument, e.g. F160W for the WFC3/IR channel we chose.

$f160w

We will then select the spectrum from which 13 wavelengths will be used to generate the PSF. Let’s select one from a list.

$1

A table of stars and their colors will be printed. Let’s choose AOV.

$7

For the PSF diameter, simply choose a reasonable diameter that will fit within your frame, e.g.

$5

I don’t know what the focus, secondary mirror despace parameter does. Let’s just not use it.

$0

Choose a rootname of PSF image files.

$test

The first step is complete!

Step 2¶

The second step is simply calling tiny2 followed by the PSF name.

$tiny2 psf0

tiny2 will tell you that the intermediate PSF dimensions are 108 by 108, and proceed to compute the PSF at 13 wavelengths. It outputs an intermediate file called <root name>00_psf.fits, e.g. test00_psf.fits in our case, as well as a template optional parameter file required by tiny3 called <root name>.tt3, e.g. test.tt3. The intermediate PSF is an undistorted PSF with the pixel size of 0.046241 arcsec.

Step 3¶

tiny3 resamples and distorts the intermediate PSF from tiny2. If we run:

$tiny3 psf0

with no arguments, the output PSF, named <root_name>00.fits (test00.fits in our case), is 46 by 46 pixels. But say we want to apply a super-resolution (subsampling) factor of 4. Then run:

$tiny3 psf0 sub=4

This will yield a PSF with 184 (46 times 4) pixels on each side.

Now we are ready to rebin. See How to rebin and drizzle PSFs.