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5 9 G L I H T N O V E M B E R 25TH, 1943.

Reflector Gun SightsTwo Typical Models for Movable nd Fixed Quns escribed

IN the- old davs , the s tandard gun s ight in use in the

Royal Air Force was the Ring-and-B ead Sight, w hichconsisted of a central bead in the field of view.of thegunner and a concentric ring so ar ranged as to subtenda fixed angle at the eye. The bead served to indicate thedirection in which the guns w ere pointin g w hilst the r ingformed a s tandard from w hich to judge ang ular allowancesand could also be used to estimate the range of the target .

This method of sighting, however, was open to seriousobjections in t h a t it was necessary for the eye to be heldin a fixed position relative to the ring in order to achieveaccurate s ighting. Any wandering of the eye off the sightingaxis resulted in inaccurate aim, whilst the purpose of the,ring in subtending a fixed angle at the eye held only solong as the eye remained at a constant distance from thering.

Free Gun Reflector Sight Mk. IIIATo overcome these difficulties, the Reflector Sight was

developed. The F ree Gun Reflector Sight Mk. I I I A * isa typical sight, which will serve to i l lustrate the principlesinvolved. As its nam e implies, it is used w ith free guns ,an installation on a Vickers G.O. gun being show n below.

The sight consists of a diffused m onoc hrom atic lightsource which illuminates a graticule consisting of a translu-cent r ing and bead pa ttern etched upon an opaque ground,a lens system forming an image of the graticule patte rnat infinity 500 yards) . A tra ns par en t reflector, inclineda t 45 deg. to the optical axis and through which the targetis viewed, reflects the luminous graticule image whichappears superimposed upon the target . A dimm ing screenconsisting of a neutra l t inted optical f lat for enhancing con-

trast between the luminous image and brigh t skies and ahood complete the basic com ponents of the sight. The optical system is contained within a case of moulded

plastic or a die-cast box into the base of which is fittedthe detachable lamp holder. The outside diame ter of thecentral, portion of the body of the optical system is

R E F L E T O R

RE F L E CTOR CARRI E R

REFLECTOR SEATING

RETAINING PLATE

SIGHT BODY

BASE RANGE IMAGE

IMAGE HOLDER

CARRIER BUSH

BINDING SCREW

PLUNGER BALL SPRING PLUG

SCALE PINION

SCALE RACK

SECURING RING

DOUBLE FILAMENT LAMP

G R AT I C U L E H O L D E R -

CONCAVE P L ANE L E NS -

HOLOER RETAINING SCRE

BEZEL RING

RETAINING SCREW

RANGE SCALE-

RANGE HEAD

BASE HEAD

BASE SCALE

L AMP l i OL 0F R.L E F T

HOLDER SPRIHOWITH FINGER PIECE ^ C A B L E T R 1C EL L

PACKING RING

Sectioned d rawing of Mark II fixed-gun reflector sight.

machined spherical to provide, in conjunction with acorresponding socket on the mount ing bracket , a monnt-ing which is adjustable for harmonisat ion .

The hood, which is mounted on the top of the opticalsystem, carries the reflector and the dimm ing screen. Italso serves a useful purpose in protecting the reflector andoptical system from mechanical damage, and protectsthe graticule from the effects of the sun s ray s whichwould otherwise be focussed on the graticule by the lenssys tem.

The graticule pattern is etched upon an opaque glassdisc which is interposed between the graticule lens and thecollimating lens system. The d iameter of the ring whichprojected by the optical system on the ta rget pla ne subtends1a constant angle at the eye, its radius representing the cor-rect deflection for a target having an ap par en t crossing speedof 50 m . p . h . For crossing speeds other tha n 50 m.p.h. ,the pilot must mentally construct an ima gina ry concentric

A 0 J U S T I N 6 PIN CRANK

OP E S AT I NS HE AD

PLUNGER KJR P l AT t

Mark IIIA reflector sight fitted to a Vickers K gun. The Mark IIIA free gun reflector s ight dissembled.

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\OVEMBER 25TII, 1943 L I H T 9

-:.IQ of the appropr iate radius as.ormal practice for ring and

. i sights.\ disability of the reflector

sight is the lack of contrast be-tween the lumino us graticuleand very bright backg round s. Insuch circumstances, therefore, it

is necessary to enhance the con-trast by interposing a d immingscreen between the sight and thetarget. The dim ming screendots not, as m igh t be t hough t ,dim the graticule image, as theimage is in fact a vir tual imageformed by light ray s reflecteddirect from the optical unit tothe obs erver s eye. The inter-position of the dimm ing screen,therefore, dims only the targetand background, the graticuleimage appearing bright by contrast.

To operate the sight, the lam])is switched on and its brightnessiSbtrolled by a dimmer switch.It is then used in j u s t the sameway as the o lder Ring-and- BeadSight, with the great advan tagethat, provided that the sight iscorrectly focused, small varia-tions of the eye posit io n relativeto the sight do not affect itsaccuracy.

Improving upon the above principles, the P i lo t s Re-flector Gun Sight Mk. II has been developed. This sight,which is i l lustrated oppo site, is used for fixed gun installa-tions and is, therefore, r igidly attached to the aircraft,usually behind the windscreen and in the line, of sight ofthe pilot. The ma jor differences from the Mk. I l l A* freegun sight described above is t h a t a range estimator isincorporated and the hood is dispensed w ith.

The optical system is contained within a cast light alloybody, at the upper end of which is mounted the reflectorand dimming screen, and at the lower end the detachablelamp holder. As before, the central portion of the bodyis externally m achin ed spherical in order to provide anadjustable mounting.

Pilot s Mark II reflector gun sight.

The graticule differs in t h a t itis etched on to the flat or lowersurface of an opaque concaveplane lens and consists of ntranslucent r ing and bead andtwo cross-wires. The cross-wires are interrupted centrally,the vertical wire stopping shorta t the ring and the horizontalone extending nearly to thecentre, its inner e nds bein^masked to a varying degree bythe base /rang e mech anism. Thediameter of the r ing when pro-jected on to the target planesubtends a constant angle andi ts radius represents the correctdeflection for a target crossingspeed of ioo m . p . h .

Immediately below the grati-cule is a base/ ran ge m ask whichis rotatable by the range andbase heads situated just below

the spherical mou nting. Rota-tion of the heads causes themask to increase or decrease thegap between the two horizontalwires. The sett ing of the maskis differentially controlled by thetwo heads, increments of rangeprogressively reducing the gapand increments of base, i.e..apparent span of targ et, increas-ing it. The purpose of the

range /bas e m echanism is primarily to obviate the tendencyto open fire while still out of effective range; it is notintended to function as a range-finder in the accepted sense.

To operate the sight, the graduation mark on the basehead is set to an estimate of the targe t span based onrecognition of the target type and paying due regard toany obliquity of aspect of the target . The range head isse t to the range at which it is desired to engage the targetand the lamp is then switched on, its brightness beingcontrolled by the dimmer switch. Fire is then withheldwhile the range closes un til the target fills the gap betweenthe horizontal cross-wires.

W e are indebted to the Ministry Of Aircraft Prod uctionfor the above notes and i l lustrations.

RUSSI N DIVE BOMBER

A PE 2 taxying to the take-off point of Voroshilovgrad airfield. The PE 2 so named after its designer Petlyakov) is a high-speedlight bomber which can be used for either precision or dive bombing.