Preliminary APART Modeling Results

(Last updated: July 8, 1998)

Comparison of AutoCAD, Code-V, I-DEAS input 3-D models and APART output 3-D models.

Input 3D model information came from the following sources. The 3D positioning of all the optical and mechanical elements traces directly to either the Code-V model of the IDM (Instrument Development Model) or the I-DEAS drawings that were used to fabricate the IDM.

Code-V IGES files
I-DEAS 3D Model Files (IGES transfer)
An AutoCAD 3D Model synthesized from the above, plus miscellaneous viewgraphs. This formed the basis for input to the APART 3D model.

The following images are plots generated by APART from the basic input 3D model data. They consist of the basic optical system only. The analysis has only been done between 0.25 to 3.5-degrees off axis. Added detail to the model will not contribute to stray light in this angular region. Objects modeled are:

SIRTF Primary
SIRTF Secondary
IRAC Pick-off Mirror (with edge masks to account for APART using square mirror for optical surface)
Pick-off Mirror Diffraction Edges
"White Throat/Wall" (100% diffuse reflectivity)
Lens Diffraction Entrance Aperture
Lens A (Aspheric/Flat)
Lens B (Spherical, Double Concave)
Beam Splitter
Filter
Pupil Stop
Detector

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Side View: Diffraction Edges, "White Throat" - Isometric View : Diffraction Edges, "White Throat"

Top View: Diffraction Edges, "White Throat"

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Side View of Thermal Sources - Close-up of Thermal Sources (Isometric View)

Single Side View Comparing AutoCAD, Code-V, I-DEAS and APART

Image Surface Irradiance Distribution

- Thermal Sources (1, 2, 3, 4, & 6 arc-min off-axis)

- Point Source (0.25, 0.5, 0.75, 1.00, & 1.25 degrees off-axis)

- Point Source (1.5, 2.0, 2.5, 3.0 & 3.5 degrees off-axis)

BRDF Data Set for IRAC (with data from other sources for comparison)

IRAC Performance Summary - APART Analysis

PST is presented in PSNIT units. This is determined by the APART program input request.

PST=Point Source Transmittance

PSNIT=Point Source Normalized Incidence Transmittance. PSNIT=(POWER OUT/UNIT AREA) / (POINT SOURCE IRRADIANCE AT THE INSTRUMENT)

Power In is 1 unit of power per unit of area perpendicular to a collimated source. This can be scaled or be considered as 1 watt/sq. inch for the input power.

Power Out is in units of power on the detector. For 2.0 degrees it is nominally 0.90E-3 units or watts.

The Area of the input aperture is 776.87306 square inches. [DISK 50 10 -1 0.000 16.920 6.245 *Primary Mirror] pi x(16.920²-6.245²)= 776.87306 sq. inches

The Area of the Detector used was 0.102 square inches. (Conversion round-off errors from square to circle) [RECT 198 8 1 5.590896-.340 0.1614175] = (2 x 0.1614175)² = 0.10422244

At 2-degrees, the output power is 0.90E-3 units. The PSNIT = (0.91E-3 /0.102) / (1) = 0.0089215686; Log PSNIT = ¯2.0495588

The input power from a point source is 777 units. If that power were focused on a 3x3 pixel area, the flux would be 86 units/pixel. The residual flux at 2 degrees off axis is spread over a 256 x 256 pixel detector. The flux per pixel is 0.9E-3 /(256 x 256) = 1.37E-8 units/pixel. This is equivalent to a point source signal reduction of about 1E-10. If the source were the sun, we would still have about 10 orders of magnitude to go to be background limited. The GOES 30 cm diameter telescope can detect 6^th magnitude stars with a SNR of 2 with the sun about 6 degrees away from the optical axis.

Primary & Secondary Mirrors (Value used by J. Fleming for SIRTF). COATING 22 MIRROR 0.30 SLOPE -1.06 MIN BRDF 5.E-3 MAX BRDF 1.0 *HDOS + CL600

Pick-off Mirror BRDF is 10 times higher than the DIRBE coating. From the DIRBE data, the diamond turned mirrors were about 10 times worse than the polished mirrors (at 10.6 microns). COATING 23 1.2305E-02 -1.30173 *30 A Mir-10xDIRBE

Point Source Normalized Incidence Transmittance.

[Cycle] Angle Log PSNIT

[1] 0.25 -1.00
[2] 0.50 -1.32
[3] 0.75 -1.51
[4] 1.00 -1.65
[5] 1.25 -1.76
[1] 1.5 -1.87
[2] 2.0 -2.05*
[3] 2.5 -2.22
[4] 3.0 -2.36
[5] 3.5 -2.48

2.0-degree Log PST Required -1.3

Percent Contribution from Top Ten Paths at 2.0-degrees off-axis
(Contribution from 0.25 to 3.5-degrees off-axis is basically the same)
Edge Diffraction is included, but negligible!

Primary (50) 16.98%
Secondary (60) 77.64%
Pick-off (101) <0.00%
Lens-A (144) <0.00%
Lens-B (147) <0.01%
Beam Splitter (158) 2.85%
Filter (167) 2.47%
Pupil Stop (182) 0.05%

There were 25 paths that had power to the detector.
Throat Wall had 100% Lambertian Surface and less than 0.00% contribution.

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