FOXSI-4 Instrumentation 🦊

Details on the make-up of the FOXSI-4 instrument. This page will help a user decide what response components they want when using the response files and/or Response-tools code.

Overview

The FOXSI-4 instrument is composed of seven independent X-ray telescopes. Each of these seven includes a direct-focusing X-ray optic module, a focal plane detector, and some attenuation material along the optical path. The following figure summarizes the focusing optics, attenuation, and detector elements that comprise each telescope:

Light enters the top of the figure and is detected at the bottom. The telescopes are identified by their position number (0 through 6). This nomenclature for telescope positions is used in FOXSI documentation throughout this package and beyond.

This package provides access to the response for each telescope (columns in the above diagram). Documentation of the API used to retrieve the ARF and RMF for each telescope is linked in the diagram, along the bottom.

If desired, you can also access the response for individual components of each telescope (each box in the above diagram). The links in each box point to API documentation for retrieving the components of each telescope’s response.

FOXSI-4 telescopes

If you’d like to do some analysis on FOXSI-4 data, which telescope(s) should you use? This table may help you make up your mind:

Telescope	Detector	Optics	Collimator	Attenuation	Energy range	Energy resolution	Spatial resolution (FWHM)
0	CMOS 1	MSFC 2-shell	AR = 1:290	OBF + Prefilter	0.8–10 keV	0.4 @ 10 keV	3.4”
1	CMOS 2	Nagoya 1-shell	AR = 1:200	OBF + Prefilter	0.8–10 keV	0.4 @ 10 keV	1.5”
2	CdTe DSD 4	MSFC 10-shell	—	381 µm Al	4–20 keV	0.8 @ 14 keV	7.2–11.4”
3	CdTe DSD 2	MSFC 2-shell	—	Pixelated attenuator + Mylar	4–20 keV	0.8 @ 14 keV	6.1–10.8”
4	CdTe DSD 3	Nagoya 1-shell	—	127 µm Al	4–20 keV	0.8 @ 14 keV	5.3–10.3”
5	CdTe DSD 1	MSFC 10-shell	—	Pixelated attenuator + Mylar	4–20 keV	0.8 @ 14 keV	7.2–11.4”
6	Timepix [1]	MSFC 2-shell	—	Mylar	5–20 keV	6 @ 60 keV	6.9””

You may have an idea of which FOXSI-4 telescopes you’d like to use for analysis. To work with per-telescope response products, you can use the convenient Level 3 API to obtain response products for your chosen telescope(s).

If you are also interested in working with the response data for individual components of a telescope (e.g. the position 2 optic response, or the position 1 detector response), there is a convenient Level 2 API available as well! And detail on those components below.

FOXSI-4 components

Each telescope in FOXSI-4 is built from several optical elements. Each of these elements has its own response. The following sections break down the response by element type.

Optics

Three types of X-ray optics were flown in FOXSI-4: 1. Single shell, high-resolution optics from Nagoya University; 2. Double shell, high-resolution optics from MSFC; 3. 10 shell optics from MSFC.

The response of each of the seven optics modules flown was characterized experimentally in extensive beamline campaigns [2] [3]. Currently, only effective area information for each optic is included in the response. Future updates will add vignetting information and PSF for each optic.

Retrieving the optic response for a particular telescope will return the unique, measured response for that optic module.

Collimators

The two CMOS, soft X-ray telescopes (positions 0 and 1) include collimators to eliminate ghost rays [4] which would contaminate the detector field of view. Each collimator has a different open area ratio and a different aspect ratio [5].

Retrieving the collimator response for a particular telescope will return an estimated open area value.

Pixelated attenuators

A new attenuator technology, developed for X-ray spectroscopy applications, was demonstrated on FOXSI-4. These pixelated attenuators are produced by etching μm-scale holes into a silicon wafer using conventional microfabrication methods. The hole pitch, radius, and depth are tuned to achieve the desired transmission response.

Traditional, monolithic attenuators are used in solar hard X-ray spectroscopy to remove the excess of soft X-ray photons produced in flares and avoid saturating detectors with soft X-ray photons. But monolithic attenuators do this so effectively that there is little soft X-ray flux left to measure. Pixelated attenuators allow for a tuned transmission response—reducing the soft X-ray flux from a flare without eliminating it completely.

Two such pixelated attenuators were flown on FOXSI-4, in front of CdTe DSD detectors (in positions 3 and 5). Both were characterized in post-flight beamline tests, and their measured responses were found to match the modeled prediction very well.

Retrieving the pixelated attenuator response for a telescope will return a modeled transmission curve.

Filters and other attenuators

X-ray flux is attenuated along each telescope’s optical path by a variety of monolithic filters:

• Thin, metalized foil filters in the CMOS detectors’ optical paths.

  These are used to prevent visible light from contaminating the detector. These are referred to as the OBFs (for optical-blocking filter) and prefilters.

• Aluminized Mylar filters in front of the position 3 and 5 CdTe detectors.

  These have have a small impact on the transmission for telescopes 3 and 5, they are included primarily to protect the detector surface during integration.

• Thin aluminum filters in front of the position 2 and 4 CdTe detectors.

  These are included to bring the observed count rate into an acceptable range for the CdTe detectors.

• Multilayer insulation (MLI) blankets in front of the position 2–6 optics.

  These blankets shield the optic modules from intense infrared radiation and heating; they transmit X-rays well.

Detectors

Three types of detectors were flown on FOXSI-4:

1. CMOS soft X-ray detectors in positions 0 and 1,

2. CdTe double-sided strip detectors in positions 2–5,

3. Timepix detector in position 6 [1].

The following table summarizes the characteristics of each detector:

Detector	Energy range	Energy resolution	Pitch
CMOS	0.8–10 keV	0.4 @ 10 keV	11 µm
CdTe	4–20 keV	0.8 @ 14 keV	60–100 µm
Timepix	6–20 keV	6 @ 60 keV	55 µm

Prior to the flight, all detectors underwent calibration campaigns involving sealed radioactive sources [6] and beamline testing [7].

For the CMOS detectors, the response information made available through this package was constructed from experimental data taken with a representative detector. The detector used to construct the response was not flown on FOXSI-4; the detector response is applied to the CMOS detectors in positions 0 and 1 identically.

For the CdTe detectors, the response information available through this package was constructed from experimental data taken with each specific detector, in combination with GEANT4 simulation. The response for CdTe1–CdTe4 is unique to that detector.

[1] (1,2)

No flight data from Timepix/position 6 is available.

[2]

See Ampuku et al. 2024, DOI: 10.1117/12.3018109

[3]

See Baumgartner et al. 2023, DOI: 10.1117/12.2680486

[4]

See Buitrago-Casas et al. 2020, DOI: 10.1117/12.2561578

[5]

See Sato et al. 2024, DOI: 10.1117/12.3018219

[6]

See Minami et al. 2024, DOI: 10.1117/12.3018742

[7]

See Shimizu et al. 2024, DOI: 10.1117/12.3017009