419 



KDooiledge & Selentifie Neais 



A MONTHLY JOURNAL OF SCIENCE. 



Conducted by MAJOR B. BADEN-POWELL, F.R.A.S., and E. S. GREW, M.A. 



Vol. III. No. i8. 



[new series.] 



MAY, 1906. 



r Entered at -1 

 LStationers' Hall.J 



sixPEN'ci: m:t. 



CO\TENTSSee page VJI. 



Astronomical 



Photography. 



Hints to Amateurs Regarding Appliances 

 and Methods of Working. 



By Alexander Smith. 



II. 



Adapling Telescopes for Photographic Purposes. — Both 

 refracting and reflecting- telescopes may be utilised for 

 photographic purposes, but unless the former type of 

 instrument is specially corrected for this kind of work 

 the chemical and visual foci will not be coincident. The 

 difference may be ascertained by making a number of 

 short exposures, and noting the plane at which best 

 definition is obtained. Photographic achromatism is 

 sometimes secured by altering the distances between 

 the crown and flint lenses, and sometimes b\' using a 

 third lens specially corrected for the purpose. Triple 

 object-glasses suitable for both visual and photographic 

 work are now procurable, but are very expensive. The 

 reflector is, however, the type of instrument which ap- 

 peals to the ordinary amateur, who is desirous of taking 

 up astronomical photography, as perfect achromatism 

 is .secured at a comparatively small cost. To adapt a 

 well-mounted clock-driven instrument of this class for 

 such work, few additional appliances are required. A 

 guiding telescope — either reflector or refractor — will 

 have to be provided, and the aperture should preferably 

 be not less than four or five inches. A plate-holder 

 with suitable carrier will also be required. The latter 

 may be attached to the telescope with a piece of brass 

 tubing of the requisite diameter to slide firmly into the 

 eye-tube. To one of its ends a flange is fitted, by 

 means of which the tube is attached by screws to the 

 carrier, the necessary focussing adjustment being made 

 by the rack of the telescope. 



With large reflectors, used solely for photography, a 

 flat is usually dispensed with, and the plate placed in- 

 side the main tube at the primary focus of the mirror. 

 The plate thus practically takes the place of the flat, 

 and is supported in a similar fashion by three thin 

 pieces of steel. The plate-carrier is attached to a short 

 length of stout brass tube, inside of which another 

 tube, worked by rack and pinion, slides, and by which 

 the final focussing adjustments are made. The dark- 

 slide is fitted wltii a shutter, which folds back in the 



line of focus, where it is held in position by a spring 

 catch. In some cases a flap-shutter is also fitted to the 

 carrier, and is raised or lowered by means of two 

 strings, a small weight attached to either string keep- 

 ing it in position. In such equipments the mirror has 

 usually a central aperture, through which the brighter 

 objects on the plate can be directly examined with a 

 small telescope, and, in the case of long exposures, it 

 can also be ascertained by the same means whether anv 

 displacement of the speculum has taken place. 



Enlarging Lenses. — With instruments, such as are 

 generally to be found in the hands of amateurs, the 

 discs of the sun, moon, and planets at the primary 

 focus of the mirror are too small, particularly in the 



case of the phsncts, to exhibit much in the shape of 

 detail, compared with the results which are obtained 

 \isually when an eye-piece is employed. At the same 

 time it is astonishing how much detail can be shown 

 by enlargements from such photographs, if the method 

 afterwards dcscrilx^d is adopted. It must, however, be 

 conceded that a negative obtained by enlarging the 

 primary image by a lens in the telescope will exhibit 

 less of " grain " than if the image is directly transferred 

 to a photographic plate and afterwards enlarged to the 

 same dimensions. 



Different forms of enlarging lenses mav be success- 

 fully employed, the most popular probably being the 



Rarlow " and low-power positive eve-pieces. X'erv 

 fine results may also be obtained with the negative 

 components of telephoto len.ses of the better ^lass, 

 which give a very flat field, and have the additional ad- 

 vantage that only a short extension of camera is re- 



