498 Transactions. 



Art. LI. — The Star Test for Telescopic Mirrors. 



By T, Allison. 



[^Read before the Wanganui Philosophical Society, 17th May, 1915.] 



Ix the manufacture of mirrors for telescopic work the usual test is that 

 known as Foucault's shadow test. This test used in connection with the 

 zonal one is very convenient for workshop use, but it has its drawbacks. 

 It will be found that the depth of the shadow varies according to the length 

 of focus of the mirror, and it is affected also by the size of the hole through 

 which the light shines and by the amount of light in the room. It is very 

 easy, therefore, to be deceived by the shadow test : it takes, also, some 

 experience to know what shadow should be seen. Theoretically, the zonal 

 test should be satisfactory, but the necessary measurements are so small 

 and delicate that I should not care to depend on it. There therefore re- 

 mains one test for the final figure of a mirror, which I think is supreme : 

 this is the star test. I will endeavour to describe it, premising that, as a 

 mirror is specially made for viewing the stars, testing it on the stars seems 

 quite a rational proceeding. 



Focus the mirror on to a bright star and then rack it slightly out of 

 focus both inside and outside the focal point. The image swells into a ring 

 of light with a dark centre, the shadow of the flat. This should be exactly 

 similar at equal distances on each side of the focal point. If, on the other 

 hand, with the eye-piece slightly outside the focus the ring of light with the 

 dark centre is seen, but with it slightly inside the focus there is a disc of light 

 with a star-like point in the centre, the figure of the mirror is elliptic. If 

 the reverse appears — that is, with the eye-piece inside the focal point there 

 is a ^disc of light with a dark centre, and with it outside the focal point the 

 telescope shows a disc of light with a bright star in the centre — -the figure 

 is a hyperbola. On throwing the eye-piece still farther out of focus it will 

 be found, if the figure is elliptical, that inside the focal point the disc of light 

 expands with a fairly large dark spot in the centre, while outside the focal 

 point, at an equal distance, the disc of light will have a small black spot 

 in the centre. If the mirror should unfortunately prove h\'perbohc, outside 

 the focal point the disc of light will have a large black spot, while inside the 

 focal point, at a similar distance, the disc of light will have a small black 

 spot. If i}he expanded disc is hairy or ragged when inside but well defined 

 outside the focus, the edge of the mirror is turned down. 



With a perfect mirror, throwing the eye-piece out of focus at equal 

 distances on each side of the focal point, the mirror should show the out- 

 side edge of the disc slightly heavier than inside ; and it should contain 

 concentric rings of light each slightly fainter than the one immediatel}' 

 outside it, and in the centre there shoidd be a black spot. Each image at 

 an equal distance from the focal point should be an exact copy of the other. 

 In making these experiments the eye-pieces should, if possible, be achro- 

 matic, and for the results to be critical a bright star, such as Sirius, Canopus, 

 Arcturus, or either of the Pointers, should be used, and an eye-piece of fairly 

 high power. I used in these experiments three achromatic eye-pieces — ^180, 

 260, and 360 — and also a low-power negative one of about 100. The mirror 

 can only b3 perfectly balanced with one power. Any power from 180 to 

 260 would be a good power to finally correct it with. If the series 

 oi rings do not shade down uniformly it shows that there are zones in the 



