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How To Perform Stray Light Analysis
- By Dan Hill
- Published 12 September 2005
- Stray Light
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Stray Light From the Moon
(Q1) So, how much stray light from the moon is incident on the camera?
From the Detector Viewer, it is clearly visible that the majority of the energy is at the centermost pixels of the detector. However, even on a log scale, it is very difficult to discern how much total power is associated with the rays which fall outside this central region.
To determine the fraction of total power which is associated with the moonlight, let's first isolate the rays from the moon. To do so, we may set the "# Analysis Rays" for Source Object 1 to zero and re-run the Ray Trace, or we may utilize the filter string to only display those rays which originated from a specific source. For the purposes of demonstrating the effectiveness of the filter strings, we will choose the latter of the two.
Filter strings may be applied to 3D Layouts, Detector Viewers, and (as we have already seen) within the Ray Database Viewer. Within the settings of each of these analysis tools, the desired filter string may be typed into the "Filter" entry. We may further demonstrate some of these capabilities using the current example file.
Within the settings of the Detector Viewer, select the previously saved .ZRD file from the "Ray Database" pull down menu. Once the desired Ray Database is selected, the "Filter" entry becomes enabled.
The "On" filter string flag will filter the database for only those rays which originated from source number n. Since we wish to only display those rays which originate from the moon (Source Object 2), the filter string "O2" can be applied. Note that the "O" is the letter O, not the number 0.
Depending upon the size of the saved ray database and the complexity of the string, it may take ZEMAX some time to process the data. Once complete, the Detector Viewer should show the filtered ray data.
Note that the data in the bottom portion of the Detector Viewer displays both the chosen ray database file and the filter string applied. Therefore, the settings need not be re-opened to verify which filter has been applied to the data you are viewing.
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From the filtered Detector Viewer in the present example, the "hot spot" is no longer present, which was undoubtedly the image of the star (rays originating from Source 1)
For the current ray trace, roughly 8.531E-006 Watts of the Moon's initial total power reaches the camera. But, as the moon is much brighter than the star, 10-5 rejection against stray moonlight may be inadequate, so it may be desirable to reduce the amount of stray moonlight to achieve better resolution of the image of the star. There are many methods which could be used in attempt to reduce the stray light from the moon. However, these topics go beyond the scope of this article, and will not be covered: we will concentrate on identifying the sources of the stray light in this article.
From the Detector Viewer, it is clearly visible that the majority of the energy is at the centermost pixels of the detector. However, even on a log scale, it is very difficult to discern how much total power is associated with the rays which fall outside this central region.
To determine the fraction of total power which is associated with the moonlight, let's first isolate the rays from the moon. To do so, we may set the "# Analysis Rays" for Source Object 1 to zero and re-run the Ray Trace, or we may utilize the filter string to only display those rays which originated from a specific source. For the purposes of demonstrating the effectiveness of the filter strings, we will choose the latter of the two.
Filter strings may be applied to 3D Layouts, Detector Viewers, and (as we have already seen) within the Ray Database Viewer. Within the settings of each of these analysis tools, the desired filter string may be typed into the "Filter" entry. We may further demonstrate some of these capabilities using the current example file.
Within the settings of the Detector Viewer, select the previously saved .ZRD file from the "Ray Database" pull down menu. Once the desired Ray Database is selected, the "Filter" entry becomes enabled.
The "On" filter string flag will filter the database for only those rays which originated from source number n. Since we wish to only display those rays which originate from the moon (Source Object 2), the filter string "O2" can be applied. Note that the "O" is the letter O, not the number 0.
Depending upon the size of the saved ray database and the complexity of the string, it may take ZEMAX some time to process the data. Once complete, the Detector Viewer should show the filtered ray data.
Note that the data in the bottom portion of the Detector Viewer displays both the chosen ray database file and the filter string applied. Therefore, the settings need not be re-opened to verify which filter has been applied to the data you are viewing.
From the filtered Detector Viewer in the present example, the "hot spot" is no longer present, which was undoubtedly the image of the star (rays originating from Source 1)
For the current ray trace, roughly 8.531E-006 Watts of the Moon's initial total power reaches the camera. But, as the moon is much brighter than the star, 10-5 rejection against stray moonlight may be inadequate, so it may be desirable to reduce the amount of stray moonlight to achieve better resolution of the image of the star. There are many methods which could be used in attempt to reduce the stray light from the moon. However, these topics go beyond the scope of this article, and will not be covered: we will concentrate on identifying the sources of the stray light in this article.