ISO 9642:2020 pdf free.Optics and photonics一Optical transfer function一Application一Telescopes.
The specimens considered are telescopic observational instruments with direct view used for viewing remote objects and include many instruments such as telescopes, binoculars, telescopic sights or spotting scopes.
Ideally, instruments would be best with no astigmatism and no field curvature coupled with good chromatic correction but frequently compromises as mentioned above shall be tolerated.
Many optical systems include roof prisms to give a compact instrument. However, the image produced by such systems is basically made up of two superimposed images and the accuracy with which they match will depend on the accuracy with which the roof edge has been constructed. In such cases the orientation of the roof edge shall be noted (see 5.5).
In use, the eye is coherently coupled to the telescope, so it may be contended that the only valid test would be one that included the eye: reference is made to the case of cascaded optical systems in the introduction to Iso 9334. However, in observer tests using telescopes, improved performance has been obtained with instruments with better measured OTF performance in a variety of tests, including contrast sensitivity using sinusoidal grating targets, which confirms the value of OTF tests.
OTF tests also enable performance to be compared with that computed by the telescope designer and provide effective quality assurance tests of production specimens.
When considering the details of tests, some features of the eye need to be borne in mind, especially its ability to accommodate for varying object distances and to adjust the working aperture, varying the iris size, according to the ambient illumination. Thus firstly, unlike the photographic lens testing case, refocusing for off-axis tests is necessary. Secondly, the working aperture of the telescope, i.e. the exit pupil diameter, will need to match the receiving eye pupil, which generally has a range of 7 mm down to 2 mm diameter for different ambient illumination levels. The size of the evaluation pupil for the MTF measurement (receiving pupil in the test setup) is specified in the corresponding imaging state tables.For case 5J a) direct measurement of the frequency response to targets of different spatial frequencies, a typical test setup is shown schematically in Figure 1. The separation between the test pattern unit and the collimator is adjusted to give an infinite conjugate for the test. The separation between the image analyser collimator (“decollimator”) and the image analyser needs to be adjustable by a suitable micrometer, operating on the image analyser focus slideway, to position the image analyser at the image of the test pattern.
When the object generator assembly (test pattern unit and collimator) and the image analyser assembly (image analyser collimator and image analyser) are aligned, without the optical system to be tested, the micrometer setting for optimum response of the test system will be the datum. When the optical system to be tested is positioned for an on-axis test, refocusing of the image analyser is needed and any change from the datum setting gives a measure of the on-axis dioptre setting of the system being tested. In off-axis tests, a different setting from that for on-axis tests will be found and the new change from the datum will give the dioptre setting for the particular field point and azimuth of the test; the difference from that of the on-axis test gives a measure of the field curvature.ISO 9642 pdf download.