# Presentation: 801320

## The HESSI Imaging Process

## Purpose

- 	HESSI will make observations in space of the X-rays and gamma-rays emitted by solar flares in such a way that pictures of the flares can be reproduced in computers back on the ground. The purpose of this presentation is to explain how HESSI makes these observations and how the information is used on the ground to make the images.

## This image is a layout of HESSI’s  nine detectors
The precise orientation of the grids in space is recorded using the Solar Aspect System (SAS) and the Roll Angle System (RAS).

- The precise orientation of the grids in space is recorded using the Solar Aspect System (SAS) and the Roll Angle System (RAS).

## First, consider a steady point source on the Sun
Photons are emitted in all directions.  HESSI is only concerned with those that reach the instrument.

- Photons are emitted in all directions.  HESSI is only concerned with those that reach the instrument.

## All photons that reach HESSI can be considered as traveling on parallel paths since the Sun is so far away (93 million miles or 1.5 x 1011 m) compared to the diameter of the HESSI grid tray (~1 m). The largest angle between any 2 photons detected would be
	360/(2 pi 1.5 x 1011) degrees = 1.2 x 10-6 arcseconds

- 	360/(2 pi 1.5 x 1011) degrees = 1.2 x 10-6 arcseconds

## The resolution of HESSI’s finest grids is 2 arcseconds
This is several orders of magnitude greater than the possible 1.2 x 10-6 arsecond angle between photons

- This is several orders of magnitude greater than the possible 1.2 x 10-6 arsecond angle between photons

## Components of Animations

**Front****Grid**

**Rear ****Grid**

**Detector**

- These are the main parts of the Flash animations that illustrate how HESSI’s rotating grids work.

## Next, consider how the grid pairs modulate a parallel beam of photons
Imagine yourself riding along one of these photons
The accompanying animation shows what you would see assuming you were traveling exactly along the HESSI spin axis. 

- Imagine yourself riding along one of these photons
- The accompanying animation shows what you would see assuming you were traveling exactly along the HESSI spin axis.

- All Nine Grids RotatingPoint Source Located on the Spin Axis

## Only the slats of the front grid would block your view of the detectors
The thermal insulation blankets and the cryostat cover used to keep the instrument cool are transparent to all but the lowest energy X-rays.

- The thermal insulation blankets and the cryostat cover used to keep the instrument cool are transparent to all but the lowest energy X-rays.

- All Nine Grids RotatingPoint Source Located on the Spin Axis

## Transmission Fraction

- The area of each detector (yellow) seen tells you how many of the photons riding along with you in the virtually parallel beam from your common point source will be recorded by HESSI at any given time.
- The number of photons on your parallel beam that will be recorded by HESSI changes between 50% of the photons (when the slats of the front grid do not block any part of the rear slits) and no photons (when the slats of the front grid completely cover the slits of the rear grid).

- 50% transmission

- 0% transmission

## Another Consideration...

- As photons travel from the front grid to the rear grid, HESSI is rotating at 15 rpm.  However, the time it takes a photon to travel this length is negligible since the distance between grids is only 1.55 m and the photon is traveling at the speed of light (186,000 miles per second or 3 x 1010cm s‐1).  It only takes 5.2 x 10-9 s for light to travel 1.55m.  As shown by the equation:		(1.55 * 102)/(3 * 1010)s = 5.2 * 10-9s
- At the 15 rpm rotation rate, even the grids near the edge of the trays move a very small distance in this time (2 pi x 50 x 5.2 x 10‐9/4 cm = 0.004 microns) compared to the  finest slit width (20 microns).

## All Nine Grids Rotating Source Offset from the Spin Axis

## Source Looking Along the Spin Axis: Grid #9

- Shows what a photon emitted from the HESSI Spin Axis would see
- No modulation of the visible portion of the detector
- All detectors record a steady rate of photons equal to a fraction of those that passed through the front grid.

## Source Looking Along the Spin Axis: Grid #9

- If grids were concentric, 50% of photons would be detected
- Grids are offset slightly in reality
- Exact offset to be determined from the first flares detected

## Source Offset From the Spin Axis: Grid #9

- Front and Rear Grids no longer appear concentric
- Area of detector visible through grids changes with time (modulation occurs)

## Lightcurve

- Graph generated by the modulation of the are of the detector visible to the source as a function of rotation angle
- Takes 4 seconds to generate a lightcurve	(15rpm=one 360 degree rotation every 4 sec)
- Illustrates the transmission fraction changing between 0 and 0.5

## These 5 animations represent the full process of HESSI back projection imaging

## Box 1 represents the area of the detector seen by the source: the white is the portion of detector, black is combined front and rear grids that block a certain portion of detector, leaving between 0 and 50% of the detector visible to the source

## Box 2 is what the detector sees as a function of time: white = optimum transmission black = no transmission

## Box 3 is a lightcurve created by the modulation of the number of photons absorbed by the detector

## Box 4 is a probability map of the sky where photons could and could not have originated
In the bottom left hand corner of Box 4 is the location of the point source. In the pink lines optimum transmission is achieved, in the dark lines 0% of the photons are detected from source

- In the bottom left hand corner of Box 4 is the location of the point source. In the pink lines optimum transmission is achieved, in the dark lines 0% of the photons are detected from source

## Box 5 is the final reproduced image of the source
The “ghost” source (upper right-hand corner) occurs because the same modulation pattern could have occurred for a source exactly 180 degrees opposite the actual one. The slight movement of the spin axis blurs the ghost source leaving only the image of the true source in reality

- The “ghost” source (upper right-hand corner) occurs because the same modulation pattern could have occurred for a source exactly 180 degrees opposite the actual one. The slight movement of the spin axis blurs the ghost source leaving only the image of the true source in reality

## Back Projection

- The accompanying animation shows the possible paths where photons could have originated
- Corresponds with probability map of the sky animation (Box 4)