Radiometric errors due to focusing collecting optics at varying object distances

Area of Science:

• Optical instrumentation
• Radiometry and photometry

Background:

• Collecting optics in radiometric and photometric instruments exhibit distance-dependent étendue variations for near objects.
• This variation in effective collecting area and solid angle introduces significant errors in measurements.

Purpose of the Study:

• To analyze and compare the étendue variation errors for different optical systems, specifically a thin lens and a Cassegrainian system.
• To investigate methods for minimizing these errors, focusing on the impact of secondary mirror movement and focal length ratios.

Main Methods:

• Comparative analysis of étendue variation for a thin lens and a Cassegrainian telescope.
• Modeling the effect of moving the secondary mirror for focusing on étendue stability.
• Evaluating the influence of the primary to secondary focal length ratio on field of view variation and depth of focus.

Main Results:

• Moving the secondary mirror for focusing significantly reduces étendue variation with distance.
• The ratio of primary to secondary focal length critically impacts the variation in the field of view.
• An optimal focal length ratio can minimize étendue variation, thereby enhancing the depth of focus.

Conclusions:

• Adjusting the secondary mirror position is an effective strategy to mitigate distance-dependent étendue errors in optical instruments.
• Careful selection of the primary to secondary focal length ratio is crucial for optimizing instrument performance, particularly depth of focus.
• These findings are valuable for extending the operational range of instruments and enabling accurate calibration using near-field sources.