# Presentation: 005203

## SNAP R&D

**Chris Bebek**
**19 February 2004**

## Instrument Working Concept

**Instrument Working Concept**

## Focal Plane Concept

- All instruments coalesced on one focal plane.
- Common 140K operating temperature.
- “Bolted” to telescope structure.
- Photometer sensors in one focal plane, example.
- 36 2k x 2k, 18 μm HgCdTe NIR sensors.
- 36 3.5k x 3.5k, 10.5 μm CCD sensors
- Spectrograph mounted to focal plane.
- Two channel spectrograph with light access port in the focal plane.
- Objects dropped into spectrograph light port by steering the satellite.
- Fixed filter mosaic
- 3 NIR bandpass filter types organized in 3 x 3 arrays.
- 6 visible bandpass filter types organized in 6 x 6 arrays.
- Guide off the focal plane during exposures.
- 4 regions of star guider CCDs.
- Telescope I&T and on-orbit calibration hardware

## Focal Plane Concept

**Focal Plane Concept**

- Guider

- Spectrograph
- port

- Visible

- NIR

- Focus star
- lamps

- Calibration lamps

- Spectrograph

- Thermal links

- Focus star
- projectors

- Calibration projectors

- rin   =   6.0 mrad (129 mm)
- rout = 13.0 mrad (284 mm)

## Instrument R&D Responsibilities

## Impact of detector characteristics

- Detector characteristics that determine signal to noise for a given exposure time or, conversely, determine the exposure time to achieve a specified signal to noise.
- Dark current (e/pixel/s)
- Read noise (e/pixel/read)
- Quantum efficiency
- Number of pixels to sum (charge diffusion effects + diffraction)
- Visible – 1, 2, and 3 will not be a problem, modulo radiation damage effects
- NIR – all four not simultaneously achieved

## Near infrared detectors

- All technologies are hybridized arrays of photodiodes on a CMOS multiplexer – format is 2kx2k, 18-20 μm pixels.
- Photodetector material can be HgCdTe or InGaAs (for SNAP).
- Ratios of elements determines high-side wavelength cutoff – we want ~1.7 μm to minimize counting thermal photons from a warm telescope.
- Rockwell made 1.7 μm HgCdTe for the HST WFC3 upgrade – a long struggle that has probably solved the dark current issue but leaves QE and readnoise anti-correlated.
- Raytheon is re-entering low cutoff HgCdTe business for astronomy.
- InGaAs has large industrial base of optical communications and NIR industrial imaging.
- Contracts in place
- Rockwell – two lots of 2kx2k format to continue work on QE and readnoise made by MBE
- Raytheon – two lots of 1kx1k format made by LPE
- Rockwell/Sensors Unlimited – 1kx1k InGaAs device

## NIR detector status

- Rockwell HgCdTe
- Lot 1 complete – reproduces WFC3 results in 2kx2k format
- Lot 2 nearing completion – addresses decoupling readnoise and QE
- Raytheon HgCdTe
- 3 parts made, 2 delivered
- Very large dark current; supposedly improved 2.5 μm being tested by people we are in contact with.
- QE looks good
- Rockwell/Sensors Unlimited InGaAs
- Photodiode array complete
- Assembly in progress

## NIR testing

- Things to measure
- Dark current vs temperature
- Readnoise and readnoise scaling with multiple samples
- Quantum efficiency
- Spatial response
- Uniformity of all the above
- Three collaborator test sites
- U of Michigan – HgCdTe
- Two test stands for general measurements – dark current, readnoise, ...
- Equipment to do absolute QE measurement
- Pin hole projector for intra- and inter-pixel response
- Cal Tech – HgCdTe
- Test stand for general measurements – dark current, readnoise, ...
- Calibration lamps for QE
- JPL – InGaAs
- Test stand for general measurements – dark current, readnoise, ...
- Calibration lamps for QE

## NIR tests - QE

- Note, only the Rockwell results have been measured on true astronomical hybrids under typical astronomical observation conditions.

## NIR tests

- Dark current
- Determines operating temperature
- Determines temperature stability
- Read noise
- Our target is ~5 e
- Devices are typically 10-20 e
- Prove that rt-n works
- Determines shutter closed time
- Spatial response
- Determines number of dither samples required

## Visible detectors

- Focus is on LBNL CCD technology
- Extended red response – overlaps NIR detectors
- Control of charge diffusion – makes small pixels possible, hence a smaller focal plane.
- Enhanced radiation tolerance – high resistivity, n-type silicon
- Other CCD characteristics are typical
- Challenges
- Robust fabrication process still be developed
- Packaging challenging due to silicon thickness
- Testing at LBNL and Yale
- Alternatives
- Venders interested in adapting LBNL technology
- Silicon PIN diode arrays
- Same multiplexer, package, and readout at NIR
- Only available in large pixels – expensive development to get to ~10 um
- Uncertain dark current and spatial response
- Have acquired one to understand state of the art

## LBNL CCDs

- DALSA processed wafer with four SNAP CCDs – 3.5kx3.5k, 10.5 μm pixels.

- S.E. Holland et al., “Fully depleted back-illuminated p-channel CCD development,” SPIE conf. 5167 talk 07, 2003.

- High resistivity, p-channel silicon, over depleted.

## CCD testing - QE

- LBNL CCDs have extended red response, >50% at 1 μm.
- The thickness of the CCD reduces interference fringing at long wavelengths.

## CCD testing - charge diffusion

**CCD testing - charge diffusion**

- Over-depletion voltage and silicon thickness can control diffusion.
- Our measurements indicate that 80 V and 200 μm will give ~4 μm charge diffusion – 10 μm then make sense.
- Typical CCDs have diffusion of 10-15 μm.

- ~6 μm scales to 4 μm at 200 μm thickness.

- 290 μm thick, 15 μm pixel

## CCD testing - radiation tolerance

**CCD testing - radiation tolerance**

- CCDs in space have proven to be very sensitive to CTE degradation and hot pixel formation due to radiation – mostly protons.
- Proton radiation testing at room temperature shows >10 more radiation tolerance than previous space CCDs.
- Program starting at Yale van de Graaff to do heavier ions.
- FNAL using GEANT and MARS codes to design a radiation shield.

- CTE radiation tolerance (12 MeV protons)

## CCD electronics

**CCD electronics**

- Have built a four channel dual integration correlated double sampler CRIC.
- Operates at 140 K so it can be co-located with CCD.
- Meets specs, even after cold irradiation to >150 krad
- Will submit version with ADC in March.

- Image acquired with CRIC mounted next to an LBNL CCD operated at 140K.

## CCD Packaging

**CCD Packaging**

- Four side abuttable package to allow nxm arrays
- Provision for ASIC mount on back of package

- Yale and LBNL working  with Imaging Technologies Lab, University of Arizona

## Filters

- 4” silicon wafer with V-band filter

- Discrete filter mounting

- 1 mm thick, 1 mm spaced filter

- Star fluxes have >106 range on the focal plane.
- Discrete filter  mounting can be a source of stray light and ghosting.
- SSL and Michigan studying impact of suspended, discrete filters.
- Indiana exploring direct deposited filters with Barr Assoc.

## SNAP Spectrophotometric Calibration

**Standards**

**SNAP Science**
**Measurements**

**Fundamental**
**Standards**

**Secondary**
**Field**
**Standards**

**Calibrate **
**Instruments**

**Calibration Transfer**

_**Calibration Transfer:**_
**MKS calibration**
** ****Fundamental Standards**
**   ****observed spectroscopically**

-  **Primary Field Standards**
-    **observed spectroscopically**

-  **Primary Field Standards**
-    **observed photometrically**

-  **Secondary Field Standards**
-    **observed photometrically**

**SNAP Spectrophotometric Calibration**

***SNAP Spectrograph required to calibrate fundamentals and transfer calibration to primaries and to imager***
***=>Results in well calibrated spectrograph and imager***

**MKS **
**calibration**

***1***

***3***

***5***

***4***

***2***

## Calibration Error Budget Framework

**Color **
**calibration**

**Zero point **
**(sets scale) **

**Secondary star field calibration**

**Flatfields**

**Primary standards **
**calibration**

**Aperture **
**corrections**

**Scattered  light**

**Detector **

**Telescope **

**linearity**

**intra-pixel **

**Calibration Error Budget Framework**

- How do calibration transfer errors propagate down the network?
- How well do filters+detectors, integral field spectrograph need to be calibrated?

**Optics**

**Filter **

**Imager/**
**Spectrograph**

**band pass shifts**

**non-uniformity**

## Simulation

- Left: The grand scheme of a typical simulation run has a mirrored structure. First there is the simulation phase, to the left of the red line, where *truth* is generated. A *realization* of the data and noise are created and thrown “over the wall” to the right side. Calibration and Analysis consist of a new set of models of the sources and instruments, which generate the *fit*, expected data to be compared with the realization.
- Right: A Hubble curve generated by the simulation.

- Simulation impact on detector R&D is establishing minimum performance requirements.
- This will make detector fabrication more achievable and provide margin to increase yield.

## system engineering

- Focal plane impacts
- Optics that accommodates detector size and resolution
- Partitioning of cold and warm elements
- Packaging with final integration and test in mind
- Partitioning of instrument electronics
- Radiation and thermal shielding
- .......

## Conclusion

- Large effort in demonstrating detector technology.
- This will have to be followed by demonstration of large scale manufacturability.
- Simulation and calibration studies will generate the least restrictive set of requirements for the detectors to achieve the above.
- System engineering will produce an optimized concept for the telescope, focal plane, instrument electronics, spacecraft, and operations.
- With the above, our response to a JDEM announcement of opportunity will be credible and defensible.