the present invention is a microfiche recorder , the basic function of which is to record a plurality of photographic images of data on a sheet of film . the schematic of fig1 illustrates the basic operation of such a microfiche recorder . a sheet of film 10 is held to the undersurface 14 of platen 12 by vacuum source 15 communicated to platen 12 by vacuum hose 17 . a reference plate 20 having a precision ground flat top surface 22 is mounted to a fixed supporting structure 25 . surface 22 is normally horizontal , and will be assumed so in the remainder of this specification . a pair of l shaped legs or rails 30 maintain film 10 at a separation from surface 22 , thereby preventing scratching of the delicate film emulsion . additionally , legs 30 have inwardly protruding portions 32 which prevent the film from dropping down when not held in place by the vacuum . surfaces 33 of legs 30 confront surface 22 of reference plate 20 . thus , the l shaped legs in conjunction with the undersurface 14 of the platen define a film guide which permits longitudinal but not lateral movements of the film in the platen . reference plate 20 has a rectangular aperture 35 through which an image of printed data is projected . a typical projection system includes a cathode ray tube 37 and imaging optics 40 . the location of cathode ray tube 37 and optics 40 is such that the image is formed in the plane of film 10 . the location of the image on film 10 will clearly depend on the position of platen 12 relative to aperture 35 . one aspect of the present invention is the provision of improved means for moving platen 12 over reference plate 20 in a rectilinear fashion so as to record a plurality of images in a rectilinear array on the film . to prevent wear on reference plate surface 22 or on platen leg surfaces 33 , legs 30 are preferably constructed of a low friction material such as a teflon impregnated plastic . surface 22 may also be coated with a low friction material . fig2 a is an overall plan view of the microfiche recorder . for convenience it is useful to define a coordinate system having mutually perpendicular x and y axes 42 and 43 parallel to the plane of surface 22 . broadly , x and y servo motors 44 and 45 impart motion along the x and y axes to x carriage 50 and y carriage 70 respectively , thus moving platen 12 in the desired fashion . the servo motors are standard articles of commerce such as a dc servo motor with an optical tachometer manufactured by the electro - craft corporation of hopkins , minn . a servo control is suitably programmed or actuated by a main computer operated control unit to drive platen 12 toward its home position near the upper left corner ( with respect to fig2 a ) of reference plate 20 . arrival of platen 12 at its home position is sensed independently as described below . thereafter , and in coordination with the formation of images on crt 37 , x and y servos 44 and 45 are intermittently energized to advance the platen for linear recordation of images in rows and columns along the x and y axes respectively . the normal format has data recorded sequentially within a given column so that most of the platen movements are along the y direction . only one movement in the x direction is required for each column , while every image requires a movement in the y direction . the detailed construction of x and y carriages 50 and 70 is best understood by reference to fig2 a , 3a , and 4 . x carriage assembly 50 is free to move along a direction parallel to the x axis . this degree of freedom is defined by elongate cylindrical rod or bar 52 ( hereinafter x bar 52 ) which is fastened at both its ends to the fixed structure . carriage assembly 50 is provided with journal bearings 55 which snugly engage x bar 52 and are free to slide along it . x carriage 50 is an open generally horizontal frame defined by a pair of arms 57 which extend in the y direction away from journal bearings 55 . the arms are joined at their ends remote from x bar 52 by end portion 60 . arms 57 are also joined near journal bearings 52 by a cross member 62 . x carriage 50 is provided with an elongate cylindrical rod or bar 65 ( hereinafter y bar 65 ) which extends along the y axis perpendicular to x bar 52 . y bar 65 is mounted to downwardly extending y bar mounts 67 and 68 which depend from end portion 60 and cross member 62 respectively . each y bar mount has a downwardly facing v groove into which y bar 65 is seated and bolted . y carriage 70 is provided with journal bearings 72 which snugly engage y bar 65 and are free to slide along it . y carriage 70 has a generally tubular main member 75 and paired outwardly extending wings 77 . each wing 77 is provided with a downwardly protruding pin 80 for coupling to platen 12 as will be described in more detail below . thus it can be seen , that platen 12 can have motion along the x axis imparted to it by moving x carriage 50 along x bar 52 . motion of platen 12 along the y axis is provided by causing y carriage 70 to move along y bar 65 . it can be seen that the platen will be in its home position when x carriage 50 nears its leftmost position along x bar 52 and when y carriage 70 nears its uppermost position along y bar 65 . these conditions are sensed optically . detector 81 mounted near the end of x bar 52 is provided with a light source and a photodetector in optical communication . an interruptor such as a blade 82 is mounted on x carriage 50 and is positioned to interrupt the optical communication when the carriage is at the desired position . similarly , detector 83 on x carriage 50 , and interruptor 84 on y carriage 70 sense the y motion . the mechanism whereby rotary motion of servo motors 44 and 45 is transmitted to produce linear motion of x carriage 50 and y carriage 70 along x bar 52 and y bar 65 respectively is shown in fig2 a , and more clearly in fig2 b . independent pulley and cable systems are employed . fig2 b shows the pulleys and cables with the carriages in phantom . turning first to the x carriage driving system , x carriage 50 has mounted thereon paired x pulleys 85a and 85b . the fixed structure has mounted thereon pulley 87 and x drive pulley 90 , while pulleys 85a and 85b are mounted to the x carriage . the ends of an x cable 95 are anchored to the support structure at anchor point 97a between pulley 85a and pulley 90 , and anchor point 97b between pulley 85b and pulley 87 . tension in cable 95 is maintained by an anchor spring 100 , shown near anchor point 97a . proceeding from anchor point 97a , x cable 95 engages pulley 85a , pulley 90 , pulley 87 , and pulley 85b , whereupon it is anchored at point 97b . cable 95 is preferably wound around pulley 90 an extra revolution . pulley 90 is driven by x servo motor 44 . pulleys 85a , 90 , 87 and 85b define x cable segments 95a , 95b , 95c , 95d and 95e . segments 95a , 95b , 95d and 95e are parallel to the x axis . assuming a clockwise rotation of pulley 90 , it can be seen that cable segments 95a and 95b are shortened , with the slack being taken up by a corresponding increase in length in cable segments 95d and 95e . cable segment 95c between pulleys 90 and 87 remains constant . thus , x servo motor 44 is able to impart x directional motion to x carriage 50 . the y carriage driving system is somewhat more intricate due to the requirement that the activation of the x servo motor only , with corresponding motion of the x carriage , must not result in any movement of the y carriage in the y direction . a first pair of pulleys 110a and 110b are mounted to cross member 62 of x carriage 50 . a second pair of pulleys 112a and 112b are mounted to end portion 60 . a third pair of pulleys 115a and 115b are mounted to y carriage 70 . a fourth pair of pulleys 117a and 117b are mounted to arms 57 of x carriage 50 . y drive pulley 120 and pulley 122 are mounted to the fixed structure . y cable 125 is anchored to the fixed support structure at anchor points 127a and 127b , which are relatively closely spaced and on the same side of x carriage 50 . it is convenient to regard y cable 125 as comprising a first strand between anchor point 127a and y drive pulley 120 , the first strand including segments 125a , 125b , 125c and 125d between adjacent members of the group consisting of anchor point 127a , pulley 110a , pulley 115a , pulley 110b and pulley 122 . the second strand includes segments 125e , 125f , 125g , 125h , 125j , and 125k , the segments being those portions between adjacent members of the group consisting of anchor point 127b , pulley 117a , pulley 112a , pulley 115b , pulley 112b , pulley 117b and pulley 120 . y cable segments 125a , 125d , 125e and 125k are parallel to the x axis ; segments 125b , 125c , 125f , 125g , 125h , and 125j are parallel to the y axis . in this configuration , movement of x carriage 50 without movement of y drive pulley 120 results in no movement of y carriage 70 along y bar 65 . when y drive pulley 120 is driven by the y servo motor , motion of y carriage 70 along y bar 65 results . fig3 a is a sectional view taken through line 3a -- 3a of fig2 a in which x bar 52 , reference plate 20 , and supporting structure 25 are shown in section . x carriage 50 , except for certain cutaway portions , is shown in elevation . x carriage 50 , which extends generally in the y direction away from x bar 52 has a downwardly protruding lever 140 which is spring biased in a clockwise direction ( as seen in fig3 a ) to thereby bias the x carriage toward reference plate 20 . lever 140 is fitted with a sleeve 142 through which a horizontal pin 145 passes . the pin extends in the y direction and is bifurcated and fitted with a ball bearing 147 which is free to rotate about a vertical pin 150 passing through the bifurcations . ball bearing 147 confronts a horizontal bar 152 which extends in the x direction . the bifurcated end of pin 145 is biased away from lever 140 by spring 153 . in this fashion , the compression of the spring 153 biases the x carriage downwardly . since ball bearing 147 rotates , there is only rolling friction with bar 152 as the x carriage moves along the bar 52 . in this fashion , a limited amount of rotation of x carriage 50 about x bar 52 is possible . at least initially , i . e . when platen 12 rests on support surface 22 , the rotational movement of the x carriage is in the z direction , i . e . substantially perpendicular to both the x and the y directions . referring back to fig2 b , it can be seen that those segments of cables 95 and 125 that run between a point on the fixed structure , e . g . an anchor point or a pulley mounted to the fixed structure , and a point on the x carriage , i . e . a pulley carried thereon , are relatively close to , and are parallel with x bar 52 . thus , a rotation or pivotal movement of x carriage 50 through a limited arc ( sufficient to decouple the platen 12 and y carriage as discussed below ) does not result in appreciable misalignment of the components of the x and y drive systems ( so that they continue to function properly even though the precise alignment of the components might be lost . fig3 b is a detailed cross sectional view of a preferred mounting for x bar 52 to the fixed structure . x bar 52 rests on horizontal surface 162 of fixed structure and is loosely located in a groove defined by vertical flanges 165 and 168 . a wedge 170 adjacent flange 168 confronts x bar 52 and holds it snug against surface 162 and flange 165 . minor adjustments in the vertical and horizontal directions can be accomplished by placing shims between the x bar and flange 165 or between the x bar and surface 162 . as shown in fig3 b , a vertical shim 172 has been inserted between flange 165 and x bar 52 in order to reliably displace the x bar by the thickness of the shim and thereby adjust the position of the x bar in the y direction to accurately , yet inexpensively align it in the x direction . wedge 170 is held in place by a bolt 175 . the coupling between y carriage 70 and platen 12 is best shown in fig3 a and 4 . each of the paired wings 77 of the y carriage has a downwardly protruding pin 80 provided with a rounded end . the upper surface of platen 12 has two pin receiving pads 180 and 182 . pad 180 has a conical hole 185 while pad 182 has a v - shaped groove 187 extending away from pad 180 in the x direction . spring 153 ( shown in fig3 a ) biases the x and y carriages downwardly and thereby engages the rounded pin ends with the conical hole and the v - shaped groove . in this way , platen 12 is effectively constrained to the y carriage so that it follows motions of the latter in the horizontal plane because the interengagement of the sloped surfaces of the hole , the groove and the rounded pin ends causes the transmission of horizontally acting forces between the platen and the y carriage . the angle of the sloping surfaces , and particularly the angle of the conical hole and the v - shaped groove is chosen so that the horizontal forces which are encountered during normal operation are positively transmitted . it is further chosen so that the y carriage and platen will decouple if excessive forces are generated between them . in the preferred embodiment the included angle in conical hole 185 and v - shaped groove 187 is about 60 °. referring to fig1 and 5a platen 12 has a film confronting side of surface 14 provided with a central channel 180 to which a vacuum is communicated via a plurality of orifices 182 . extending outwardly from vacuum channel 180 in both directions are oblique channels 186 which provide a herringbone - like pattern of channels to communicate the vacuum uniformly over platen side 14 . the platen side is shown partially cut away to reveal its rigid supporting structure 188 . a downstream end 190 of the platen includes lateral cutouts 195 which overlap a portion 192 of film 10 to facilitate the withdrawal of exposed film from the platen as is more fully discussed below . fig5 b is a fragmentary sectional view and shows the general contour of the vacuum channels . in particular , it can be seen that each channel has a generally semicircular bottom portion 197 and straight sides 198 . another aspect of the invention is improved film transport mechanism for loading film in and unloading it from platen 12 . referring to fig2 a and 6 , a supply of film in roll form is stored in a film supply box 200 . film from the film box is advanced in a downstream direction towards the platen positioned on reference plate 20 by paired rollers 202 , 205 , 207 , 210 which extend over the width of the film . it should be noted that upper rollers 202 and 207 have circular guides 212 spaced apart by the film width to prevent lateral motion of the film , i . e . motion of the film along the axis of the rollers . one of the circular guides or flanges 212 of , say , the upper roller of each pair , is fixed and defines an index for one of the longitudinal film edges . the other circular flange of each of these rollers is axially movable on the roller and biased towards the fixed flange . in this manner the indexed film edge is always at the same relative location , which is of importance for aligning the film with the platen , while the roller can accommodate the normally encountered variations in the film width . only one of the rollers , e . g . roller 207 is driven while the others are idlers . all rollers include a resilient , e . g . rubber backing so that they can grasp film without damaging it . between rollers 207 , 210 and reference plate 20 is a film cut off defined by a transverse , downwardly moveable upper blade 215 and a transverse , fixed lower blade 220 . when it is desired to load platen 12 with film , platen 12 is driven in the normal fashion to a position immediately adjacent cutter blades 215 and 220 . it is important to align one of the grooves in platen rails 30 with the indexed film edge . this is done by correspondingly adjusting the optical position indicator 84 . once the platen is properly aligned , roller 207 is actuated for a number of revolutions sufficient to feed a length of film , i . e ., the standard length of a microfiche in the x direction over the reference plate 20 and into the grooves of platen rails 30 . contact between the film and the reference plate is prevented by an air bearing generated by pressurized air escaping from a multiplicity of orifices 225 in reference plate 20 and arranged along the direction of travel of fresh film as it moves into the platen . when the proper length of film has been advanced , vacuum is communicated to platen 12 so as to bias the film upwardly against film confronting side 14 and upper blade 215 is moved downward to sever the advanced length of film or microfiche from the supply . the film withdrawal system is located downstream ( to the left as seen in fig6 ) of reference plate 20 . paired upper rollers 230 and lower rollers 232 located on shafts 235 and 237 respectively , are relatively narrow and separated by a distance that is at least as great as the width of the downstream edge 190 of platen 12 , but less than the overall width of film 10 . thus , side portions 192 of the film which overlap cutouts 195 in the platen are engaged by roller pairs 230 and 232 when the platen is in its home position immediately adjacent said roller pairs as controlled by position indicators 83 and 84 . suitable film guides 240 and 242 support the edges of the film once it has passed out of platen 12 . when it is desired to remove film 10 from platen 12 , platen 12 is driven in the normal way to a position immediately adjacent roller pair 230 and roller pair 232 . these rollers engage the film , but are driven with a sufficiently low torque that no lateral movement of the film from platen 12 occurs so long as vacuum is still maintained . as soon as the vacuum to platen 12 is released , the rollers pull the film out of the platen . the tendency of the film to sag and contact reference plate 20 is countered by the provision of an air bearing just as is supplied when the film is being loaded into platen 12 before vacuum is applied . once the exposed film has been removed from the platen , it conventionally passes to a processor where it is typically developed , fixed and dried . the film is then ready for use and / or duplication .