# Presentation: 540627

## Panoramic  Survey  TelescopeandRapid  Response  System

- Pan-STARRS’ Role in the Dark Energy Mission
- Nick Kaiser
-  HEPAP
- DC July 14th, 2007

***Panoramic  Survey  Telescope******and******Rapid  Response  System***

## Outline

- Constraining Dark Energy with wide-field optical/NIR photometric survey missions
- General requirements for constraining DE from ground-based surveys
- Pan-STARRS System Design and Deployment Status
- Relation to other proposed experiments
- Role of Pan-STARRS in the Dark Energy mission

## Dark Energy from Wide-Field Surveys

- Goal:  determine equation of state w(z) = p / c2
- DE influences expansion history; geometry; growth of structure
- Need to exploit multiple, and complementary, methods
- Methods/techniques
- Weak Lensing (WL)
- Primary goal: measure evolution of P(k) with redshift
- Also: Geometrical tests
- Galaxy-galaxy lensing
- BAOs from WL; constraints from shape of P(k)
- Galaxy Clustering
- Primary goal: Use baryon oscillations (BAOs) as standard ruler
- Also: Evolution of P(k), shape of P(k)
- Hubble diagram from SN1a
- Standard candles
- Currently provides the best constraints on w(z) but
- Reaching limit of statistical (root-N) performance
- Follow up requirements for spectroscopy are formidable - need for techniques using only broad-band imaging
- Abundance of galaxy clusters
- Matched filter (“red sequence”) selection
- WL - mass selected samples

## Requirements

- Static sky studies (WL, galaxy clustering)
- Precision photometry 		(for photometric redshifts)
- Precision shape measurement 	(for WL)
- _Area_/depth 			(cosmic variance)
- Transient studies
- Precision photometry		(accurate light curves)
- PSF modeling			(for image differencing)
- Cadence				(sampling of light curves)
- Area/_depth_			(need to reach z~1)
- Flow down to top-level system design requirements for HW, SW and survey strategy.
- Common HW solution is adequate
- Much overlap in SW (e.g. image processing pipepine)
- Distinct survey strategies

## Pan-STARRS overview

- Time domain astronomy
- Transient objects
- Moving objects
- Variable objects
- Static sky science
- Enabled by stacking repeated scans to form a collection of ultra-deep static sky images

- Pan-STARRS observatory specifications
- Four 1.8m R-C + corrector
- 7 square degree FOV - 1.4Gpixel cameras
- Sited in Hawaii
- A  = 50
- R ~ 24 in 30 s integration
- -> 7000 square deg/night
- All sky + deep field surveys in g,r,I,z,y and w filters

## Test Cameras 1-2
Test Camera 3

Gigapixel Camera 1

Gpixel Cameras 2,3,4

- Test Camera 3
- Gigapixel Camera 1
- Gpixel Cameras 2,3,4

**PS HW Development/Deployment Schedule**

- Funded Sept 2002
- Development, infrastructure, and testing (2003-2005)
- PS1 Integration and Commissioning 2006-2007
- PS1 Science Mission
-       3.5yrs (2008.0-2011.5)
- 	PS4 (Development and Construction 2007-20010)
- PS4 10 yr Mission
-      (2011 – 2021)

*360 Mpix*

*1.4 Gpix*

**Notes:**
Pan-STARRS is a project which is developing very rapidly, and although the final goals are very ambitious, the stepping stones we shall traverse are achievable.
Obviously our crystal ball gets cloudier as we look more than one year into the future, but a notional idea of the development schedule is:

Jan 2005: Lot 1 (development) of OTAs, TC1 and TC2 (90 Mpix), Rev 0 controller, assembling the team of people and infrastructure
Jan 2006: Lot 2 (production) of OTAs, TC3 (360 Mpix), Rev 1 controller, PS1 telescope first light
Jun 2006: Lot 3 of OTAs, GPC1 (1.4 Gpix), PS1 telescope transitioning from commissioning to science
Jun 2007: Lot 4 and 5 of OTAs, GPC2 and GPC3, EIS complete, construction begins on PS4
Jun 2008: Lot 6 and 7 of OTAs, GPC4, PS1 retired, PS4 operations begin (we expect a 10 year lifetime for this project)

## PS-1 Optical Design - RC + 3-element wide-field corrector

## PS1 Telescope Views

## SDSS and Pan-STARRS Bandpasses

- Filters closely matched to SDSS -  no U-band, but added Y-band at 1 micron with
- exceptional quantum efficiency

**Notes:**
Our bandpasses are designed to match those of SDSS.  We will not attempt to do u band (our optics and detectors explicitly are permitted to be poor performers in u band), but our “glory bandpass” will be the “y” band at 1um, chosen to fit between the water bands at 950nm and the silicon cutoff.  We believe that we will be able to achieve a good enough sensitivity at 1um with our CCDs that we should be very sensitive to red objects.  The y band will permit us to find early-type galaxies with an H+K dropout to z=1.5 and Lyman-alpha dropouts to z=7 (which is virtually all of the visible universe, given the epoch of reionization).  We have elected to avoid the water band and to use a y filter which has a red cutoff because we intend to maintain excellent photometric accuracy in the y band.

## Pan-STARRS  Focal  Plane

**Pan-STARRS  Focal  Plane**

- Optics delivers a 3° field of view to meet science goals.
- PSF sampling of 0.26” required to match Mauna Kea seeing.
- Therefore each focal plane will have 1.4 billion pixels.
- Compared to existing cameras, goal was:
- 10x the pixel count
- 10x the readout speed
- 0.1x the cost per pixel

**Notes:**
The most challenging component of the Pan-STARRS project is the cameras.  The current state of the art in large scale imaging cameras (the various “mega”cams)  cost about 2 cents per pixel and read out in 20—30 sec.  This would be a 50% duty cycle since we are forced by moving objects to exposures of 30 sec, and therefore completely unacceptable.  Hybrid IR or Si PIN arrays on multiplexer substrates can get the data out much more quickly, albeit at higher noise, but cost 10 cents per pixel just for the detectors.  Very large monolithic CCDs cannot meet the simultaneous goals of low read noise and fast readout, and monolithic CMOS has not demonstrated the required noise, red sensitivity, and pixel uniformity needed for state of the art astronomical imaging.  Today’s large imagers are 0.3—0.4 Gpixel, but we require more than 5 Gpixel for our four cameras.  At the current state of the art this would cost 100—500 million dollars, and the combination of <5e- read noise and 3 second total readout time has not been demonstrated.
We therefore need to improve the state of the art in three different directions: we need to improve the readout time by a factor of 10, we need to increase the pixel count by a factor of 10, and we need to decrease the cost of the focal plane by a factor of 10.
We believe we can do this.  In collaboration with MIT Lincoln Laboratory we have designed and are producing a new type of CCD array, called the “Orthogonal Transfer Array” (OTA) which we think will meet the noise/speed/cost goals.  We are also developing a new controller to accommodate these devices.  This controller not only has to provide a large number of readout channels (512) to meet the speed goals, but it also has to be extremely compact because this focal plane is so large we must consider the controller to be  part of the tiled mosaic.  (If we do not do this we will have a nightmare of wiring and diseconomies of scale which would make it impossible to build so many devices into a single focal plane.)

## Orthogonal Transfer Array

- A new paradigm in large imagers.
- Partition a conventional large-area CCD imager into an array of independently addressable CCDs (cells).
- Massively parallel design allows rapid read-out -> rapid sky coverage

## OT Arrays

- Orthogonal Transfer Array
- A new pixel design to noiselessly remove image motion at
-     high speed (~10 usec)

*Normal guiding (0.73”)*

*OT tracking (0.50”)*

## GPC1

## Sub-system status

- Camera:
- 7 sq deg
- FOV

## OTA Quantum Efficiency

- OTAs demonstrate expected QE (-65oC)
- 70um thick devices have exceptional QE at 1um

**Notes:**
The QE of the 45um thick OTAs is virtually identical to CCID20 devices whose QE has been determined by us and others have measured with completely different apparatus.
However, the 75um thick OTA has a markedly improved red response beginning at about 750nm.  By wavelengths of 1um, the 75um device has a 50% higher QE than the 45um thick OTA, as expected from the increased pathlength for IR photons.

## PS1  System/Subsystem  Overview

**PS1  System/Subsystem  Overview**

**MOPS**

## Real-Time Image Analysis

- raw
- stacked
- image

- static
- sky
- image

- difference
- image

- -

- cleaned
- stacked
- image

- +

- The Static Sky

- PS-1 : N sequentially
- PS-4 : 4 simultaneous

- Image Combine
- &
- Image Difference

## Comparison of SDSS photometry with PSPhot

- R. Lupton (SDSS) has used PSPhot to perform photometry in the galactic plane.
- Color-magnitude diagram from M13 are shown (r vs g-r).
- left: SDSS Photo, right: PSPhot

## psastro + pswarp = quality image combinations

- Example of pswarp difference image applied to Megacam Taurus images.  The relative astrometric solution has a scatter of 6mas and yields an excellent subtraction.

## Pan-STARRS Basic  Data  Products

- 	-  Instrumental magnitudes, coordinates
- 	-  For precision astrometry/photometry
- 	-  Postage stamps for bright objects
-   Cumulative static sky images
- 	-  Signal + exposure maps
- 	-  Best + working + compressed intermediate saves
-   Static sky catalogs
- 	-  Includes time history of object magnitudes
-   Difference image detection stream
-   Recent (~1 month) source and difference images

## Auxiliary systems & control of systematics

- In addition to major system components (telescope-detector-SW), the PS system design features a number of advanced auxilary systems to control, reduce or ameliorate effect of systematic errors.
- Implemented features:
- Advanced tunable laser system for total system throughput calibration
- Wave-front sensing sytems for real-time control of optical alignment and collimation
- Imaging sky probe to monitor sky transparency
- Planned features:
- Atmospheric dispersion compensator (drop in replacement for lens-1)
- Spectroscopic sky probe - coupled with MODTRANS atmospheric model to provide accurate correction for sky absorption and subtraction of air-glow

## Calibration Screen(Chris Stubbs)

- Screen

- Calibration Screen(Chris Stubbs)

-  leaky fibre - monochromatic source tunable from 400-1000nm
-  measure system throughput
- Combine with spectroscopic sky probe -> synthetic fringe frames
-  Advantages
- Repeatability
- Uniformity
- Stability
- Shuttered light source

## Tunable laser (400 nm - 2 microns) 

- Second harmonic (532 nm) generator
- Mixer (to 355 nm)
- 1.064 micron NdYAG pulsed pump laser
- Tunable downconverter

## Shack-Hartmann Wavefront Sensing

- Deployable WFS extends out over focal plane to pick off a star for analysis
- Lenslet images and pupil images are parfocal with normal telescope images
- out of field of view for normal operations

**Notes:**
We will use two wavefront sensors: a deployable Shack-Hartmann apparatus illustrated above and a curvature sensing apparatus on the next slide.  The Shack-Hartmann optical path is folded up so as to bring the focus back to the nominal focal plane – it does not have its own sensor.  There are a number of advantages to this.  The S-H apparatus can be placed anywhere on the focal plane to analyze the wavefront at that field position.  The apparatus also produces an out of focus pupil image and will carry pinholes to examine scattered light.  

## Curvature Wavefront Sensing

- Curvature sensing design
- 2—4 locations, above focal plane but outside 3 deg FOV
- Converging lens and block of calcite
- Two images of every star provide above and below focus donuts
- Difference is quite sensitive to wavefront aberrations
- Operates automatically and continuously with every exposure, no special pointing or overhead.
- Results available within 30 sec.

**Notes:**
The light intensity of an out of focus image is related to the Laplacian of the wavefront error.  The difference between above- and below-focus images is very sensitive to wavefront errors and is extremely valuable for focussing and collimation.  We will use a block of calcite oriented so as to exploit the difference in index of refraction of the ordinary and extraordinary polarization and simultaneously produce above- and below-focus images for every star.  By proper choice of the optical axis of the calcite, we can arrange for the two star images to be adjacent to one another at a convenient separation.  Since the introduction of the calcite would ordinarily put the balanced pair of donuts below the nominal focal plane, and since we want this apparatus to be parfocal with the rest of the field of view, we introduce a converging lens as well.  There are obviously large aberrations and chromatic terms from this setup, but since we expect to place it outside of the region of good focus the telescope optics will create aberrations as well.  Nevertheless, the middle lower panel shows what the difference between the two images will look like, and although they do not precisely cancel for collimated optics, decollimation by enough to increase the image size by 0.5” FWHM makes very noticeable residuals.  The operational plan is to collimate the optics using the Shack-Hartmann apparatus, and then use that to determine what the nominal curvature sensing donuts should look like when the telescope is or is not in collimation.  Extensive ray tracing calculations of the calcite design indicate that this will work well.

We have built a calcite apparatus before and the lower right image shows a focus sequence of the UH2.2-m without (left) and with (right) the calcite block.  The collimation is obviously wretched because the UH 2.2-m floats off its hardpoints particularly at the high airmasses beloved of planetary astronomers unless the airbag pressure is kept very low, which then results in trefoil astigmatism.

## Wavefront Sensing

- Secondary has
-       x,y,z, tip, tilt
- Primary has
-      x,y,z,tip,tilt
-      and 36 actuators
-      along optical axis
-      to correct for gravity
-      and astigmatism
- Need to correct
- 	often to maintain
-      image quality over
-      large field of view

## PS1 Imaging Sky Probe - monitors sky

- Transparency in real-time

- SkyProbe @ CFHT

- large detector (20482)
- back-side illumination
- larger aperture (120 mm)
- 5 filters (*grizy*)
- Good sampling (5 arcsec)
- controlled focus
- CCD -> OTA TEC package

## spectroscopic sky probe

- 3 fiber fed spectrographs
- measure sky emission and stellar continuum
- 16” telescope
- ~30 sec exposures
- targets: F and earlier type stars  9th magnitude
- all-sky bright star spectra catalog
- Measure low-res sky spectra (sky & stars)
- Generate high-res model (MODTRAN)
- Synthesize “fringe frame”
- Not currently implemented

## Spectroscopic sky probe can provide accurate calibration of absorption of atmposphere and subtraction of air-glow 

- Spectroscopic sky probe can provide accurate calibration of absorption of atmposphere and subtraction of air-glow

## Atmospheric Dispersion Compensator

- M1

- Siloxane fill view port
- Prism rotation motor

- M1 baffle mounting surface
-  Custom ball bearings

- ADC-L1
- ADC-P
- ADC-L2
- L2

- Cable wrap energy chain
- L1-L2 rotation motor

-  Variable strength prism corrects for refractive effect of atmosphere
-  Drop-in replacement for 1st element of wide field corrector (L1)
-  Not currently implemented for PS1

## Relation of Pan-STARRS to other proposed DE missions

- Ultimately, space-platforms will be needed/preferred
- But wide area needed for WL currently only affordable from ground
-  PS - LSST
-  Limiting magnitude in ~30s exposure
-  PS1  r~=23	  PS4  r~=24	    LSST  r~=25
-  e.g. number of galaxies ~ 1/flux limit & precision ~ 1/sqrt(N)
-  SW/data processing requirements are identical
-  PS1 development has provided basic image processing system
-  major input from key LSST scientists
-  PS1 has ~200 scientist consortium w. teams tasked to develop science infrastructure SW & refine/debug pipelines
- Deployment time-scales and site choices complementarity

## Funding requirements

-  Order of magnitude increase in performance over state-of-the-art wide field imaging surveys (e.g. SDSS, CFHT/megacam Legacy)
-  PS1 starting now - PS4 could start as soon as 2010
-  PS1  PS4  PSnn?/LSST? Is a rational progression
-  Common SW infrastructure needs
-  limits are from systematic errors - need to learn from experience
-  PS4 is funded year-on-year via the AF at the ~$9M/year level, but
-  no funds for operation (~$30M for 10 years of ops)
-  current funding profile will delay deployment
-  Accelerated construction to be operational by 2011 requires an injection of ~$20M (based on PS1 actual costs)
- Highly leveraged investment on AF funding (no commitment to sponsor for scientific deliverables)

## Pan-STARRS in a Nutshell

- Telescopes
- Four 1.8m R-C + corrector
- 7 square degree FOV
- Sited in Hawaii
- A  = 50
- R ~ 24 in 30 s integration

- Detector and controllers
- 1.44x109  0.26” pixels per camera
- Image motion compensation
- 512 channel controller
- few second readout
- 6e- read-noise
- Operation mode:
- Broad band optical imaging
- Four telescopes view the same field to detect transient or moving objects and build up a deep image of the sky
- Data-Processing System
- Multicolor summed images
- Difference images for detection of moving and variable objects
- Catalogs of static, moving, transient objects
- Survey Capability (for Dark Energy):
- All-sky (3 pi steradian) to r~26 for WL, BAOs and cluster abundance
- Medium & ultra deep surveys for SN1a

## The End