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Proceedings Paper

Impact Of Chopping On Image Quality In The SIRTF Telescope
Author(s): Murk Bottema
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Paper Abstract

We compare two methods of spatial chopping, namely, (a) symmetrical chopping and (b), asymmetrical chopping. In method (a) the IR object and the sky background are alternatingly observed at equal and opposite chopping angles. In method (b) the IR object is observed on axis. In both methods chopping is implemented by articulation of the secondary mirror. The basis of comparison is the achievable size of an imaging field in which the angular resolu-tion is diffraction-limited at 2 pm. The comparison is made for telescopes with fixed diam-eters of 850 mm, but with f numbers ranging from f/15 to f/30. In method (a) we find usable fields between 2.2 arc min at f/15 and 1.7 arc min at f/30, in case the secondary mirror pivots near its vertex. The field diameters can be increased to 6.0 arc min at f/15 and 4.1 arc min at f/30, if the pivot center of the secondary mirror is placed at the so called "neutral point". However, this has some severe technical disadvantages. In method (b) the size of the imaging field is not limited by aberrations, but by the restriction that it can not be larger than one third of the total field diameter. The latter is defined by the ac-quisition field of the Fine-Guidance System (FGS) and is equal to 15 arc min. With regard to figure imperfections in the cooled mirrors, method (b) offers a much larger margin than method (a). It also offers superior image quality in the FGS field. For further develop-ment of SIRTF we recommend method (b), but consider method (a) acceptable, if the secondary mirror pivots near its vertex.

Paper Details

Date Published: 1 February 1985
PDF: 11 pages
Proc. SPIE 0509, Cryogenic Optical Systems and Instruments I, (1 February 1985); doi: 10.1117/12.944977
Show Author Affiliations
Murk Bottema, Ball Aerospace Systems Division (United States)


Published in SPIE Proceedings Vol. 0509:
Cryogenic Optical Systems and Instruments I
Ramsey K. Melugin, Editor(s)

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