An overview of the AEOS Burst CameraHeather Swan

University of Michigan

June 3, 2005

1

Outline

• Science Goals

• System design

• GRB Triggers/Response

• Grating analysis/Simulations

2

AEOSAEOSBurstBurstCameraCameraSensitivitySensitivity

ROTSE-I/TAROT

ROTSE-III

ABC w/o grating

Keck

Half of all GRBs have no optical counterparts

Could catch very fast faders (short bursts?)

High S/N for studying variability or spectral evolution

ABC w/ grating

3

1 min. 10 min.

ABC field of view is well matched to the Swift BAT error box

90% will be localized to a 3 arc minute radius

(Can see them with the ABC)

4

50% will be localized within 12 seconds

(Can see them promptly)

(From Fenimore, et al)

How many GRBs do we expect to observe in Maui?

• Swift promises 90/yr• 1/3rd of the time it is dark in Maui• 1/3rd of the time the GRB will have a high enough

elevation• 3/5ths of the time the weather will be good

• 1/15th of the bursts can be observed, or 6 bursts/year

That’s approximately the same number we’re allowed to observe per year (9)

55

The AEOS telescope is a large optical telescope used by the Air Force

Advanced Electro-Optical Systems Telescope (AEOS)

Largest ground based AF optical telescope (3.67m)

Designed to track satellites, can quickly (~20 sec) slew to coordinates

Located in Haleakala, Hawaii, at 10,000 ftABC

6

The AEOS Burst Camera (ABC) is attached to the AEOS

• Optics designed by Carl Akerlof

• Package designed by Alan Schier

• Camera built by Astronomical Research Cameras

Field of view 6' x 6'

F/# 4.5

Focal length of 16m

7

AEOS telescope image reducer design

ABC

8

ZEMAX point spread function

PSF gets bad near the edge of the FoV.9

CCD camera specs

• E2V 2kx2k back-illuminated CCD

• Cooled to –40 F

• CCD readout time ~6 seconds

• Typical exposure length ~10 seconds

10

Improvements to the ABC image quality

• The baffle– Attached to the secondary, blocks stray light

• Improvements in alignment

1111

The secondary baffle removes most of the stray light

•Image of M1 without a baffle •M1 with the baffle

12

13

The ABC took images of the Genesis probe, just hours before it reached the earth!

Genesis duringseparation

1414

Burst Filter

Fax::::::::::::::

GCN

SwiftGRB

User Interface

ABC Computers(Modified ROTSE

Software)

CD

The ABC will try to observe GRBs within minutes after they are localized

15

We filter GRBs from the GCN to determine

if the ABC should go after them We use the following criteria to determine if a GRB should be

observed in Maui:

1. Less than 1 hour old2. Localization should fit in our FoV3. Sun should be far enough below horizon4. Should be visible for at least an hour5. Should be 20 degrees above horizon6. Moon should not be too bright or too near7. Should be 20 degrees away from galactic plane

If a GRB passes all these cuts, it is automatically faxed to the AEOS control room (no humans required!)

1616

For GRB fax alerts

Non-GRB observationsThumbnail of last image

Camera Status

Removes current imagesfrom the queue, cancels currentset of exposures

ABC User Interface

17

Observation Timeline

T=0 sec Receive GRB coordinates from fax and pager

T=15 sec AEOS operator terminates current task

T=30 sec Operator moves telescope to GRB coordinates

T=45 sec Operator moves trunnion mirror to ABC position

T=60 sec Operator initiates data taking on ABC

T=10 min Operator notifies team of action on GRB

T=5 hrs Data taking concludes

T=12 hrs Collected data transmitted to U of M

18

We can also send manual GRB alerts

• ROTSE-III finds an optical counterpart– Average time between GRB localization and

ROTSE-III reporting afterglow, 30 minutes

• If It is still bright, and should be visible when Maui can see it– We send a fax w/coordinates and finding chart

• The ABC response won’t be prompt, but we know there is an afterglow!

19

ABC analysis pipeline

2020

Correct imageswith flat field and dark. Use SExtractor to

find objects

Download images from Maui

Match stars to catalog

Calculate magnitudes for each star

Find limiting magnitudes

Find spectra forobjects

Create lightcurves

And many otherthings!

GRB response

• 030329 – not prompt, but visible (t ~ 3 days)

• 030418 –not prompt, or visible (t ~ 8 days)

• 041006 – first image after 2.5 hours, bad pointing

No other “real” GRBs have been requested• (2 false HETE alarms, but the weather was

bad/mount was down)

21

030329

April 1, 2003

030418

April 25, 2003

041006: The pointing is off, how do we fix it?

The problem was known before this GRB

It’s a problem that is not easy to fix (hard to determine what is wrong)

Now we send a finding chart, and the operators use wiki stars to get the correct pointing offsets

We missed a GRB because of this problem!

Here there beDragons!

22

Test Burst response times

When the weather is bad, and the operators are in the room, sometimes the first images are taken seconds after the fax arrives!

23

Date Response Time (min)

May 30 9 Good images!

May 30 1 Good images!

May 20 3 Very cloudy

May 17 7 Crowded/correct pointing

May 16 7 Bad weather

Grating analysis/simulations• We installed a blazed transmission grating in Jan 2005

– 35 groves/mm, peak wavelength 640nm, blaze angle 2.2°– ~5 cm from CCD– R=/=8

• We’ve taken images of different types of objects– Red/blue stars– Quasars

• We can compare images to simulations

• Found limiting magnitudes for different exposure lengths

24

Stars look like blackbodies

Black body,

sun’s temperature

A star observed

with the ABC

25

Can differentiate between red and blue stars

Cooler temps

Hotter temps

26

Ra Dec g-r rmag

Red 126.00429 0.01915 1.4 18.1

Blue 126.01388 0.21709 0.2 15.2

We take a known spectrum of an object, and cram it through our simulator

• Start with spectrum from SDSS • Multiply by CCD and grating efficiencies• Use grating equations to see what happens to the light• Convolve with psf

SDSS spectrum of a quasar

What we expectto see with theABC

0th

order 1st

order

simulator

inte

nsi

ty

Image offset (pixels)0 50 100 150 200

Wavelength(nm)900800700600500400

Flu

x d

ensi

ty

27

Simulations look similar to actual data

• Quasars look “spiky”

What we expectto see with theABC

0th

order 1st

order

inte

nsi

ty

Image offset (pixels)0 50 100 150 200

What we actually seewith the ABC

This quasar has a z of 3.83, and anRmag of 18.53 (10 s image, taken at twilight)

28

Higher orders look like what we would expect

29

Limiting Magnitudes

Compare to SDSS images to find dimmest stars

(Gives a rough estimate of limiting mag)

Could be off- We have significant vignetting, and sparse fields

Don’t have many fields to get limiting mags from

Exposure Length (s)

Limiting Magnitude

10 17.4 19.1

15 17.7 19.3

Oth order 1st order

30

Conclusions• ABC is running well, operators know what to do

• The pointing is off, but the operators know how to correct for that

• The ABC should be able to get images a few minutes after the GRB is detected

• The spectral information is crucial for understanding the GRB progenitor

Questions?

31