Fletcher Star Test for Microscope Optics - Advantages of Different Sizes of Artificial Stars

Sep 15, 2026

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The paper explains how the star test images for microscope objectives can be interpreted in terms of optical aberrations and why using different artificial star sizes is important. It compares star and Abbe inspection methods with resolution, interference, comparison, and Foucault knife-edge approaches, highlighting quantification difficulties and practical drawbacks. Construction methods for microscope star slides are reviewed, and guidance is given on choosing pinhole size and illumination to reveal coma and spherical aberration without masking effects.

LanguageEnglish

CategoryTechnology

FormatPDF

Pages6

Reading Time15 min

AccessFree to download

Summary

Methods for testing optical quality

Star test and Abbe test

Comparison and other test methods

Origin and purpose of the microscope star test

Producing microscope star slides

Choosing pinhole size and illumination strategy

Medium-size pinholes and Kohler considerations

Interpreting aberrations from star images

Coma and spherical aberration

The Star Test for Microscope Optics J.R.FLETCHER Depertment of Physics,Universlty of Nottingham,Univenity Park,Notingham NG72 RD. Summary The appearancesproduood byvarious on-axis andoff-axis aberrations of mieroscope objoctivesareshewn and described.The uses of diferent sizes of arificial stars are discussed,and the need forseveral types of test is empbasized. MANY METHODS have been devised for testing the quality of optieal systems,such as microscopes and telescopes,that are required to perform up to thetheoretical limits for their apertures: 1.Resolution tests involve examination of ruled gratings or periodicobjects of increasing fineness. 2.Interference tests show the shape of the wavefront emerging from theoptical system. 3.In the star test and Abbe test,visual inspection of the image of anartifical object under varying conditions of focus and illumination isinterpreted in terms of optical aberrations. 4.In the comparison test,images of a variety of objects are produced bythe lens under test and compared with those given by a lens believed tobe of good quality. 5.The Foucault knife-edge test is regularly used in the production ofastronomical optics,but has seldom been applied to the mieroscope. All of these testing methods are open to various objections.A graduatedseries of ruled gratings is difficult to obtain and natural periodic objects arevariable.Interference and Foucault tests require elaborate apparatus.It isdifficult to quantify the results of star or Abbe tests,and they are really asophisticated form of comparison.In the comparison test how can theobserver be certain that his standard lens is really of goodquality?All he cansay with certainty is that it is the best he has seen so far.The star test has theadvantage of that it can be quickly and easily used without the need for anyelaborate apparatus. The star test originated as a test for the astronomjcal telescope byobserving the focussed and defocussed imges of a star,and has beenthoroughly described by H.D.Taylor(1946).To a telescope,a star iseffectively a point source of light,being smaller than the resolution limit,andthe application of similar artificial stars'of small size to the testing ofmicroscopes has been described by Slater(1957)and Dade(1958).Thispaper will discuss the advantages of various sizes of artifical stars. A star slide for the microscope can be produced in several ways.Theopticiansof the last century used finely smashed mercury drops with obliqueight,but these are not permanent and mercury vapour is toxic.For thosewith laboratory facilities,aluminium can be evaporated onto a slightly dustyslide,leaving a film with pinholes.A similar result can be obtained usingpieces of Sellotape with metallic coating.This is sold for sealing Christmas parcels.The coating wil be found to have numerous pinholes and,withpersistence,a suitable sarmple can be found and stuck onto a slide.Itis usefulto have several pieces mounted under covers of differing thicknesses to testobjectives for cover correction.(This was suggested by DrG.Woolfe).Aneffective star slide can be produced by negatively stained bacteria.A toothScraping of plaque is mixed with Indian ink and spread over a slide to dry.The bacteria show as transparent holes in an opaque film.Finely crushed,crystalline calcite mounted in balsam also provides a star test slideExamined in polarized light between crossed polarizer and analyzer,thefragments appear as bright points on a dark background. Most descriptions of the test have assumed tha the pinhole 'star'should beso small that no structure in it can be resolved by the objective.This implies adiameter of less than 0.3μm There are serious disadvantages in using such asmall star.Critical illumination is needed with the lamp filament focussed ontothe pinhole by a corrected condenser to provide sufficient light.Theout-of-focus images of such a small source show concentric interference ringsand these often hide the effects of zo

FAQs

What illumination conditions are recommended for using medium pinholes?

The paper recommends illumination by a diffuse source with a condenser, avoiding Köhler illumination because it may produce anomalous out-of-focus images; immersion condensers are necessary for oil lenses.

Sep 15, 2026

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