Adventures in Classical PSF Subtraction - PowerPoint Presentation

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Adventures in Classical PSF Subtraction

C. Grady Eureka Scientific & GSFC&Marshall Perrin STScI

Sagan Summer Workshop 2014

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Need for Coronagraphy

Circumstellar Disks, exoplanets, stellar companions are often inconveniently close to a bright object (host star)Exposing sufficiently deeply to detect the object of interest can mean that you overexpose the instrument you are using and swamp the signal of interest

This talk will focus on using a simple coronagraph, and how best to separate the signal of interest from light from the star

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The PSF

Optical systems, and HST is no exception, typically spread the light from an unresolved source due to diffraction, scattering in the telescope, and in the science instrument, and in some cases within the detector system. This is termed the point spread function (PSF). For the majority of circumstellar disks and

exoplanets signal in the wings of the PSF>>signal of interest.

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Classical PSF Subtraction

Simplest of the techniques that will be covered in this hands-on demonstrationUse a suitably chosen other observation as an estimate of the light from the star that you want to get rid of. Need to match the science observation in terms of factors affecting the shape of the PSF, and those affecting temporal variation in the measured PSF.

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Shape of HST PSF depends on Source Spectral Energy Distribution

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Targets for

coronagraphic

observation are

t

ypically on the bright side for direct

Imaging.

STIS CCD used for

coronagraphic

imaging

Has throughput from 0.2-1.0 microns

Effective wavelength of the image is

a

strong function of

Teff

.

Here we see the observed response for

a

white dwarf (Feige 110) – bulk of signal is at wavelengths <5000 Å

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A1V simulation

Sagan Summer Workshop 2014By A1V, the bulk of the signal is in the Conventional optical, but with broad

Wings.

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M2 Simulation

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By early M, the effective wavelength

o

f the STIS image is 7700

Å

.

Sensitivity to color of source decreases

a

s bandwidth decreases, but is typical

o

f all 3 HST coronagraphs.

JWST will have other issues, namely

a

sensitivity to thermal emission from

The inner disk (IR excess), and the fact

t

hat diskless stars will be blue

c

ompared to any system with an IRE. 7Slide8

And also on factors affecting focus

•HST is in a low-Earth orbit and experiences changing thermal conditions. •Scheduling for HST does not include thermal effects•differences between focus conditions for the target and what you are using as an estimate of the PSF result in radial streamers – differences in the dispersed speckles

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HST STIS Coronagraph

Very broad band (2,000-10,000 Angstroms)

50 mas/pixelPair of occulting wedges to allow choice of occulter width

No

apodization

,

no

Lyot

masking of diffraction spikes

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Case 1: Use the same star, different roll of spacecraft on sky

AU Mic – obiw36030_flt.fits

Subtracting obiw35030_flt.fits

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Introduction to IDP3

For exploring PSF subtraction you need a display-oriented tool that will let you play with registering, scaling, and subtracting one image from another, while allowing you to tweak the display scaling, color, etc. to your preferences.Various tools used by the HST IDTs, and other teams, but we have chosen IDP3 developed by the NICMOS team

Stobie et al. 2006Starting point is file in the detector frame

with NO geometric distortion correction or mapping to the sky.

Filenames

ending in “_

flt

” in HST STIS pipeline terminology

.

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Starting IDP3

Log into the VNC sessionOpen a terminal (from Applications, System Tools menu )> cd Tue/Part1

> idl

IDL> idp3

A large

window should appear with File, Images, Adjust, Edit, Spitzer, and Help on menu

bar

Note that you may need to resize windows and use scroll bars; idp3 was designed for larger screens.

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Loading Data

ChoicesSTIS data are typically multiple sub-exposures to facilitate cosmic ray removal. Can load the first image of a set, or the full

set – we will load only the first image.Under file, go to the load images item

To read the first readout, go to load image on the 2

nd

menu,

to

read in a set, go to Load

Multiaccum

(NICMOS terminology).

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Data for Case 1: AU Mic

M dwarf with edge-on debris disk at 10 pcUses 1.0

arcsec wide wedge positionSame star at different orientation will be

used as PSF reference.

Exposures from two adjacent orbits of Hubble, with the telescope rolled around the optical axis.

1630 s per exposure, rectangular

subarray

Load these files:

obiw35030_flt.fits

obiw36030_flt.fits

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Selecting the file to be loaded

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Showing the images

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Adjusting the Display

You can adjust the image dynamic range, color table, and scaling (linear, log, square root) using adjust> display menu item. HST coronagraphic images typically have a large dynamic range, so I find log scaling works well. You can also shrink the window to the actual data size using the resize display option.

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Starting Point: The pipeline processed image

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Raw Data (

flt

file) linear stretch

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Raw Data log Stretch

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Image Subtraction

Make sure you have loaded images obiw35030_flt and obiw36030_flt (IDP3 in this window replaces .fits with 1fits)

all other images should either not be listed in Show Images or should be turned off

Select the image that you want to act on

(register to the other image, scale, etc.)

by clicking on its name in the show images list

. You should see an * to the left of the name.

Decide whether you want this image to be in positive (add) or negative (subtract). Click on the add button and select the entry from the pull down menu. If you select add for one image, you want to select subtract for the other image.

Set obiw35030 to Subtract and obiw36030 to add

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The disk

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Residuals on one side of star indicate that image

Registration is needed

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Image registration

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Image Registration

In Show Images window select the image to be subtracted as the one with *Now go to the Adjust>Adjust Position menu

A dialog box will appear – see next slide. Use the multiple arrow panel to “drive” the image to be subtracted so that there is not a significant dark/light asymmetry in the image and the diffraction spikes are largely

nulled

Default increment is 1.0 pixels (0.05” for STIS),

I find 0.05-1 pixel works better

In this case since we were using data for the same star taken in consecutive orbits, we did not need to adjust the flux scaling for the net image.

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Adjust Position Tool

Arrow pad moves around the selected imageOffset Increment adjusts the size of the

steps in pixels. Set to no more than 0.1

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Registering the Images

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Positive/Negative Structure in the

Spkes

means you need to

Register images using the image offsets.

Try 0.1 pixel increment, and push the arrow button until the

negative feature merges with the positive and the spike gets

m

uch fainter – this is an example of bad registration

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Good registration – this is roll-differencing (

Lowrance

et al.

2005) or angular differential imaging in its most primitive form

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Saving Your image

You can save for use within IDP3 Or write a file for external use or bothGo to File>Save Display

menu, give a file name and path. Congratulations, you now know the basics of classical PSF subtraction.

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Saving Your Image

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Now for a face-on disk…

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HD 181327 v11 – v12

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Strengths and Weaknesses of ADI-like strategies

Best for point source detection and edge-on disks such as AU MicCan eliminate any signal which is azimuthally-symmetric over the roll angle – face on disks can be eliminated. Can end up with a mess if the nebulosity is very structured

Need alternate robust technique for removing the PSF, which conserves flux

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Case 2: Non-Contemporaneous PSF template observations

Science target HD 181327, mid-F member of β Pic moving groupDisk marginally resolved by Herschel (Lebreton et al. 2012)Disk resolved in scattered light by NICMOS and ACS (Schneider et al. 2006)Location of bulk of debris constrains SED modeling; asymmetries can constrain planets.

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Using archival data for HR 4413 as template

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HD 181327v12-hr4413

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Using HD 134970 as template

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What you get from such processing

DetectionBulk of signal in ring – inner edge, outer edge, inclination, can compare with predictions for location of debris belt from FIR data assuming that the grains are large, compact grains (gray or blackbody). Exterior to ring, additional, azimuthally asymmetric nebulosityThere are residuals which depend on choice of PSF template data, and this is a bright disk. Residuals become more of a nuisance as the surface brightness of the disk decreases.

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Data for Case 2: HD 181327

F star with circumstellar ring debris diskUses 1.0 arcsec wide wedge positionDeep exposures which are saturated just on either side of the wedge. Ignore those regions.Files:

obiw11040_flt.fits

obiw12040_flt.fits

And several PSF stars listed on following slides

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Case 2 hands-on

Turn OFF the AU Mic images by toggling the on/off buttonsLoad files obiw11040_flt.fits

– HD 181327 obiw12040_flt.fits “

Now try subtracting 12040 from 11040

using the same approach as for AU

Mic

– one file should be add, the other subtract

Y

ou

should see very little

– roll

differencing a face on disk – not a great

idea.

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Case 2 hands-on

Now load some additional PSFsobiw13040_flt.fits – psf contemporary with the HD 181327 data

obiw17040_flt.fits – same PSF star, taken at a later date

Now try subtracting 13040 from 11040 or

12040

(11040 or 12040 add, 13040 subtract) – you will need to flux scale as well as register.

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Flux scale adjustment is just below alignment.

Iterate adjusting the alignment and flux scaling until you are satisfied. Slide38

Case 2 hands-on

Now load some additional PSFsobiw13040_flt.fits – psf contemporary with HD 181327

obiw17040_flt.fits –

psf

taken with the second visit set

Now try subtracting 13040 from 11040 or

12040

(11040 or 12040 add, 13040 subtract) – you will need to flux scale as well as register.

obiw93040_flt.fits

-

psf

– another star

obiw97040_flt.fits

-

psf

- another star

Try using one of these observations in place of 13040 or 17040

If we can get everyone to this stage, we will have a poll to see which PSF data everyone thinks works best.Sagan Summer Workshop 2014

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Case 3: Color-matched, contemporaneous PSF data

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Comments on color-matched, contemporary PSF subtraction

Suppress residuals, since have allowed HST to come to a quasi-equilibriumNote large number of hot pixels, cosmic ray events, etc. – these can be removed by combining a suite of science target-PSF data where HST is rolled between observations – reduce STIS wedge to a quasi-circular occulted zone with r=0.35”, and can median filter to remove hot pixels. – requires creating masks: too time consuming for demo here.

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HD 181327 – smaller obscuration

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Image Credit Schneider et al. 2014

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HD 181327 after merging images

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Image Credit Schneider et al. 2014

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Deprojection

and compensation for r-2 illumination gradient

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Image credit Schneider et al. 2014

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Further improvements & Summary

Can largely remove the remaining residuals using filtering techniquesNow are at the point that you can begin science analysis – see Stark et al. (2014) for detailsPSF subtraction with HST requires choice of suitable template targets, planning the observations so that the template is taken as close in time to the science data as feasible, and straightforward data reduction.

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Loading multiple reads

File>load image>load multiaccum

To load all reads from an image, type *; or you can specify individual reads by number

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