396 



KNOWLEDGE & SCIENTIFIC NEWS. 



[April, 1906. 



course must be had to the telescope, which, as air, ail\ 

 indicated, should for piirelv photographic purposes 

 have an angular aperture of about f/4 or f/5. The 

 size of the image on the plate will determine whether 

 the focal length of the instrument shows the object on 

 a s',:lTiciently large scale to bring out the detail aimed 

 at. 



In presenting to the amateur the following hints re- 

 garding apparatus and methods of working, it ought 

 to be pointed out that these are for the most part merely- 

 explanatory of the writer's own experiments, as he has 

 never seen any photographic appliances used for astro- 

 nomical purposes, other than those in his own 

 pos.session. Satisfactory results can doubtless be ob- 

 tained by other methods, and an ingenious amateur 

 pos.sessed of a little mechanical skill will be able to con- 

 struct appliances in the shape of accessories, or adapt 

 those he may have already at hand, to meet his 

 individual requirements. 



Mounting CameraLS and Lenses. 



To bring such lenses as those referred to into use 

 they should be attached in the usual way to a suitable 

 camera — preferably one of the old-fashioned box type, 

 which secures the maximum amount of rigiditv. If the 

 lens is not fitted with rack and pinion the body of the 

 camera should be made in two portions so that one 

 part may be slipped, telescope fashion, into the other. 

 When the solar focus of the lens has been determined 

 previous to mounting a movement of about half an inch 

 will be quite ample. The camera should be firmly 

 bolted to the top of the telescope tube opposite the 

 Declination axis, a position which least disturbs the 

 balance of the instrument, and where any slight irregu- 

 larities in the driving are reduced to a minimum. 



With heavy lenses it is necessarv that the mount be 

 rigidly supported, otherwise the effect of flexure may, 

 when the exposure extends to several hours, be ap- 

 parent in the shapes of the star discs. A method which 

 is found to be very satisfactory in practice is to place 

 over the mount a flexible steel spring or strap tightly 

 stretched by a screw to the camera base, which is ex- 

 tended below the lens for that purpo.se. 



The circular caps, with which lenses are usually 

 fitted when .sent out by the makers, will be found with 

 large apertures to be exceedingly awkward to manipu- 

 late in the dark, and are otherwise unsatisfactorv. A 

 simple arrangement, which leaves nothing to be desired, 

 consists of a mahogany frame with shutter hinged to 

 open like a door. If the hinges are placed at the side 

 the shutter may be instantly clo.sed with a piece of 

 string, but, on the whole, it is, perhaps, preferable to 

 have the hinges below, and the shutter to open from the 

 top. 



Fig. I shows a doublet lens of 5I inches aperture 

 and 22 inches focus, mounted on a Newtonian reflector 

 with the appliances referred to placed in position. It 

 may be here incidentally pointed out that this is the only 

 form of telescope which the writer has had in actual 

 use in connection with photographic operations, and 

 certain of the accessori,\s described and methods of 

 using such are only applicable to this type of instru- 

 ment. 



Having decided to give an exposure of, say, two or 

 three hours, on a particular object, it will iiot infre- 

 quently happen in this changeable climate of ours that 

 before the desired limit has been reached operations 

 may be stopped by cloud, and provision must con- 

 sequently be made for completing the exposure on one 

 or more subsequent dates. The risk of disturbing the 

 position of the objects on the plate is thus considerablv 



nil iiased, and one of tlu- hki^i frequent sources of 

 trouble in this respect is due to replacing the shutter of 

 the dark slide. This may be obviated by attaching a 

 stout bar to the back of the camera, and if two screws 

 provided with cushions at the ends are pas.sed through 

 this bar so as to press firmly against the dark slide, no 



movement of the plate can take place. This arrange- 

 ment is shown in Fig. 2. 



Instances will occasionally arise where it is advan- 

 tageous to have a number of short exposures of an 

 object on the same plate. This is most easily done by 

 mfning the dark slide, and, in order to serure uniformitv 



in making the necessary movement, a long screw (also 

 shown in Fig. 2) may be attached to the side of the 

 camera. The end of this screw presses against the 

 slide, and by giving the requisite number of turns to 

 suit the requirements of the object, a row of images is 

 secured with exactly the same interval between each 

 exposure, if a lens of long focus is used a very inter- 

 esting .series of photographs might be taken in this 

 manner during the progress of a solar or lunar eclipse. 



