A Motor-Driven Scotch Mount (Barn Door Mount)- Construction Details - Issue 3.1 (correction to Fig.6 of issue 3)
By Alan Jefferis

Others have often described the simple-to-construct Scotch Mount (sometimes called a barn-door mount) in which the rotation of a screwed rod at a given rate (e.g. half a turn every 20 seconds) turns the camera during time exposures to follow the rotation of the stars and thus avoid trailing. The mount described here follows the same principle but includes a motor, gears and electronic timer. All the parts (except the finder telescope and ball-and-socket head) are readily obtainable from Maplins and any good hardware shop. This note is intended to give enough information for its construction.

Figure 1 illustrates the overall construction. The numbered parts are:

1. Pan-and-tilt or Ball-and-socket head for camera (eg. Jessops £9.99)
2. Top board (9mm ply, 230 x 125mm). Note that precise length may need to be cut at final adjustment stage, so start with it protruding slightly beyond the main board.
3. Flexible plastic sheet (50 x 50x1.5mm)
4. Rider - 10 x 15 x 50mm hardwood with embedded threaded bush
5. Supplementary board for motor and gears (12mm shaped as per Fig.3 )
6. Main board (9 or 12 mm ply, 300 x 125mm)
7. Pan and tilt head on the tripod
8. On/off switch
9. Finderscope
10. Vertical support (22mm thick softwood or hardwood, 65 x 125mm) on which the electronics timer circuit is also mounted
11. 3” brass hinge or two 1.5” hinges or piece of piano hinge

The flexible plastic sheet has two slots rather than holes to attached it to the rider. Two M4 screws in the rider with wing nuts (or better still knurled bushes) enable it to be readily detached for the rider to be returned by spinning it down the drive screw when it reaches the top.

The holes in the top board and main board for the ball and socket and pan-and-tilt heads should be drilled when the rest of the device is completed so that they can be positioned for rough balance and clearance from other parts. Best balance is with these fixings being fairly close to the hinge and, if the device is to be used at medium latitudes, towards the edge of each board (tripod near side, camera far side). The type of bolts needed for these will depend on the particular head. The standard thread for tripod fixing is ¼” BSW. In the case of the lower tripod fixing if the screw in the tripod pan-and-tilt head can be replaced with a longer screw and a nut (¼” or 6mm) then a simple clearance hole is needed. If however the tripod screw is captive then the best way is to embed a ¼” nut in the underside of the board (hole cut to shape of nut to prevent rotation) and trap it with a thin metal cover plate. The fixing to the top ball-and-socket head needs to be a ¼” bolt at least ¾” long.

Motor and Gearing
This uses all Maplin parts except for the shaped disc. The motor and gears could be mounted on the main board but are probably best assembled separately on a separate small board of 9 or 12mm ply and this board glued or screwed to the main board on completion. A piece of 2mm or 3mm thick rigid plastic is on top of the part of the board carrying the gears to give a smooth running surface and to enable accurate drilling of holes. Figure 2 shows the arrangement. The worm drive set, A, gives 40:1 reduction. The remaining gears are 48/12 ratio gears. The first, B, is not visible in Figure 2 as it is attached under gear A. Gears B to C give 4:1 reduction while C to D have 1:1 ratio. Each of the gears and gear assemblies run on 3mm (M3) screws as bearings. The screws pass through a 3mm diameter hole in the plastic and self tap into a 2.5mm diameter hole in the wood.

Preparation of the gear-set baseboard

The holes for the screws holding the gears must be drilled with some precision so Figure 3a is intended for use as a template and gives precise dimensions for drilling the holes for the gears. Make sure your printout is at exactly the same scale as the dimension shown (e.g. by printing the actual JPG picture). Simply prick through the centres of the holes using a plastic headed pin, then enlarge this mark with any sharp point to give a clear start position for the drill. Drill the two holes shown as “fixing” holes with a 3mm drill through both the plastic and the shaped piece of plywood and put 3mm screws and nuts to hold the two parts firmly together. All the other holes should then be drilled with a 2.5mm drill taking great care of the accurate spacing for the three gear positions. Also make sure the holes are truly perpendicular to the base by using a bench drill or getting a helper to view the drill alignment. Finally, enlarge the 2.5 mm holes to 3mm for the thickness of the plastic only.

Preparation of the gears
The green and orange gears have slightly different diameter center holes but they can be considered interchangeable in this application. The 12-tooth parts of C and D must be cut off (eg, with a small hobby type hacksaw) so they can be mounted close to the board. File these surfaces so they end up flat.

The Maplin gears are moulded with a small circumferential ridge near the outer edge. This must be also filed off all the gears (including F) to give flat faces.

Two opposite holes of gear B need to be countersunk on the underside so the screw heads do not foul gear C. The same needs to be done on the underside face of gear D. In this case make sure you go deep enough for the heads to lie below the surface.

The part of the screw on which each gear will turn should be lightly filed just to remove the sharp crests of the thread but not to significantly reduce its diameter.

Check that gears B, C and D engage smoothly and with no risk of slipping by temporarily fitting them using an M3 screw just a few turns into the appropriate hole in the baseboard.

Enlarge to 4mm one of the outermost holes in gear A. This to provide screwdriver access to the fixing screw of gear C in case it should need adjusting later

Cut and file the shaped disk E from a piece of 2mm or 3mm thick rigid plastic using the template in figure 3b.

Assembly and fitting of gears in order of fitting - refer to exploded views in Fig.4 below

Gear C.
A felt pad is placed under C to permit control of the degree of free running. At this stage insert its screw until the gear just tightens onto the felt pad sufficiently for it not to be able to spin freely, and then one turn more.
Gears A and B
Gear B is attached under gear A with two countersunk-head 3mm screws. There are four nuts between the wheels, two on each screw. This is to ensure clearance from other parts and to put wheel A level with the worm gear.

Gear D, shaped disc E and support disc F
Insert the two csk-head screws that join D and E putting a nut between D and E so that E will run clear of C. Tighten down with the first nuts on top of E. Attach the D+E assembly to the board at this stage using a panhead screw and an M4 washer underneath. It should rotate freely and engage with gear C.

Another gear is used as the disc F, serving simply as a rigid disc to carry the M4 threaded drive screw. It is best to tap the hole in F with an M4 thread but if that is not possible simply enlarge the hole with a 4mm drill. The drive screw is a piece of M4 studding 60mm long locked into the disc F by a nut above and below and both tightened as much as you dare. Disc F and the drive screw is then attached to the two screws protruding from the D+E assembly, with nuts underneath and on top. It may be necessary later to drill a hole in the top board to clear the top end of the drive screw.

When all the gears are in place check that when A or D is turned by hand the rest turn smoothly and are very lightly braked by the felt disc under C. If necessary tighten or loosen C's screw via the hole on A that was enlarged for the purpose.

I found the microswitch holes for 2.5mm screws could be safely drilled out to 3mm. The screws will self tap into the 2.5mm hole in the board. Make sure the microswitch is the right way round as shown in Fig.2 (roller on the left). It will need a 3 or 4mm spacing piece (of plastic or wood) underneath the microswitch for the roller to ride properly on the shaped disk. Check that it operates positively and releases positively as the shaped disk is rotated clockwise.

Push the worm gear onto the motor shaft. Position the motor on the part of the baseboard not covered by the plastic plate and adjust the position so the worm engages with gear A with enough clearance to permit positive but free running. Mark positions for the motor's fixing screws. Drill the holes and fix the motor with at least two csk-head screws from underneath the board and nuts on top.

Rider and plastic connecting piece
Fix the plastic to the rider using two M4 screws in tight fit holes through the rider and with wing nuts or knurled threaded bushes on the outside. Make slots in the plastic rather than holes so it can be released for manually winding the rider back to the bottom.

Fix the plastic to the top board using M3 screws self tapped into 2.5mm holes in the wood. However, first make sure the length of the top board is such that the plastic will be neither bent in nor out when the rider is about 10mm from its bottommost position on the drive screw. If the top board is too long cut it back. If too short insert a spacer.

Calculation of Rotation Rate and Timer Circuit Values

The methods of calculating the rotation rate and the values of the resistors in the electronic timer are given here in case anyone wants to use different dimensions etc. If not then you do not need to follow the calculations - just use the values in this design (which give an adjustment range of around +/- 20% anyway).

The sky rotates through 360 degrees in 1436 minutes (one sidereal day). The corresponding motion at the end of a radial arm of length r is 2
pr. So one minute of earth's rotation corresponds to a segment equal to p/718 times the radius. In my case the radius of the rotation (measured from screw thread to hinge axis) is 216mm so the screw needs to move by 216 / 718 = 0.945 mm per minute. The pitch of the M4 screw is 0.703 mm so each turn needs to last for 0.703 / 0.945 x 60 = 44.6 seconds.

The basic method of operation is that the motor is fed from its 3v battery via the microswitch such that the motor will run until the microswitch contact lever reaches a raised segment of the plastic disc. The next brief pulse from the timer then operates the motor again and once the microswitch lever reaches the next recess the microswitch keeps it running until the next raised segment. Thus, if the timer gives a pulse every 11 seconds and there are 4 recesses on the shaped disc the drive screw will rotate once every 44 seconds. The frequency of the timer and the duration of the short pulse are determined by the values of the resistors RA and RB (each includes both the fixed resistor and the value of the variable or trimmer resistor).

The duration of the timer cycle in seconds is given by         T = C ( RA+2RB ) / 1.44
And the pulse duration    t = C. RB / 1.44

The short pulse must be long enough to move the raised part of the disc away from the microswitch lever but not long enough for the next raised part to reach and pass it, even allowing for motor overrun. Around 0.3 seconds should be safe on both counts. With C = 150 microfarad RB then needs to be about 3k. For 11 sec cycle duration RA needs to be about 100k. Using 82k fixed resistor and 50k trimmer for RA and 2k fixed plus 5k variable resistor for RB gives an appropriate adjustment range for both.

The timer electrical circuit (see Figures 5 and 6)

The circuit is assembled on the PCB stripboard using about a quarter of the board as purchased. Each row has copper strip right across except for a break at the central hole. When soldering the components in place do not let the components get too hot. Observe correct polarity for the diode, the LED and the 100 and 47 microfarad capacitors. Use single-strand insulated wire for the wired interconnections and single or stranded wire for connections to components off the circuit board. Do not solder to battery box connections with batteries in (plastic will deform). Double check the wiring before connecting the batteries. Also check the underside of the circuit board with an eyeglass to ensure solder has flowed onto the wire of every connection (dry joints are the commonest cause of trouble).

The drive screw should turn clockwise. If not reverse the wires to the motor.

[Note for anyone who has already made a unit based on a previous version of these notes but is modifying it to this version. The microswitch now makes its connection during the recess, not the raised portion of the disc, so use terminals 1 and 3 not 1 and 2]

On/Off switch - I mounted the on/off slider switch in a piece of plastic cut from an old printer tray. The slider switch mounting holes are tapped for M3 thread. Tip - when packing the device for transport put tape over the switch as these switches are quite easily accidentally pushed on (it has already happened to me and to another user!).

Finder telescope
This may be the most difficult component to obtain cheaply. I happened to have a Telrad finder going spare, which is ideal. A new Telrad may cost £50. You may wish to build your own following Graham Wood's design described on his website (www.gcw.org.uk <http://www.gcw.org.uk>). I have also purchased a very cheap conventional finder from Broadhurst, Clarkson and Fuller at Astrofest (£5 including bracket) - I don't know if they also have them at the shop. Or you might even wish to try without a finder - eg a simple sighting arrangement using circular electric cable clips or by using a protractor with plumline or hanging pointer and compass for (for correct altitude and azimuth respectively). Beware compass error due to steel components in the pan and tilt head and aquaint yourself with the local true north/magnetic north correction. I have used this successfully in the southern hemisphere.

Camera Viewfinder
At certain pointing directions it may be difficult to look through the viewfinder of the camera. A simple sighting aid consists of a flat piece of thin aluminium bent as shown in Fig.7 and shaped to fit in the camera accessory shoe. Cable clips again serve as the sighting aid.

Adjustments and Tips on Use

To set the correct timing adjust the blue 50k trimmer with a small screwdriver until the motor starts every 11 seconds. Time it over at least a minute for an accurate setting. When the second adjustment (below) has been done and the unit is fully operating the timing should be checked again and adjusted if necessary.

The 5k potentiometer adjusts the duration of the pulse that starts the motor. If it is too long the motor may drive the raised segment of the disc right past the microswitch lever. If it is too short the motor may not drive the disk far enough for the microswitch lever to reach the next recessed segment. It is important to do the adjustment of pulse duration with the platform fully loaded by a camera and with the platform tilted at the approximate operating angle (your latitude). Hold the microswitch lever away from the disc so it is continually off. Adjust the potentiometer spindle until the motor runs about one and a half segments (counting each raised or recessed part as one segment). I.e, six bursts should take it just over one complete turn. Turn the potentiometer clockwise to increase the pulse duration and counterclockwise to reduce it. If the batteries are new you can err above this but make sure it does not exceed two complete segments, i.e. keep it so 4 bursts turn it just under one revolution.

Note that when the batteries run down the motor may not enable the disc to always turn fully to the next recess segment. At this time it is only necessary to replace the two motor batteries, not the other four.

The important thing about the mounting of the finderscope or other sighting arrangement is that its axis must be parallel to the axis of the hinge. Then, if the finder is pointing at the pole nothing else matters (eg. whether the tripod is level). If using heavy lenses on the camera it is worth checking polar alignment after pointing the camera to the desired portion of sky.
The device can easily be used in the southern hemisphere simply by reversing the direction in which the finder or alignment mechanism points. Compass and protractor provide the best means of south polar alignment.

Some tips on taking pictures

  1. The less automated (i.e. the older!) the camera is the better. It must have the ability for the shutter to be held open i.e. a B setting on the exposure scale and a cable release with lock screw or ring. Some modern cameras are discharging the battery for the duration of the exposure.

  2. Slide (transparency) film is preferred over normal print film, even if you have a film scanner (well, that's based on my own experience). Kodak Elite Chrome, ISO 400 is a popular favourite. If you use the equivalent processed-paid Fuji film you can have the images digitized onto CD for computer processing. This is also available with the process-paid Agfa slide film which is ISO 200 maximum but can be stretched to 400 or 800 in processing upon request and a small additional payment.

  3. Suggested exposures are from about 4 minutes up to 15 minutes.

  4. Wide angle lenses generally have larger aperture (lower f number) than long focus lenses and so need shorter exposure to capture the same detail. The effect of any tracking error (e.g. due to poor polar alignment) is also less with shorter focus lenses. 35mm and 50mm lenses are quite good choices. Nevertheless longer focus lenses can be used to capture a smaller part of the sky. If your lens is a cheap one you may find it best to use one f stop away from full aperture.

Parts List

Maplin parts (with Maplin codes) Hardware

555 timer ic QH66W
relay UG36P
microswitch GW70M
capacitors 100, 50, 0.1uF KQ70M, KQ68Y, BX03D
resistors 2.2k, 27k, 82k G2K2, G27K, G82K
trimmer 50k WR43P
potentiometer 5k JM70M
diode QL73Q
LED (red) WL32K
switch FH36P
battery holder (2AA) CL17T
battery holder (4AA) CL19V
circuit board FL17T
motor HA83E
worm gear set WC80B
ratio gear set WC78K

1 hinge (e.g. piano hinge or 3” brass or two 1.5” brass)
2 M4 x 20mm panhead machine screws (rider to plastic piece)
1 M4 internally threaded spacer (embed in rider)
2 M4 wing nuts or threaded knurled studs
1 M4 studding - cut to 60mm
2 M4 nuts (disc F on drive screw)
1 M4 washer (under gear D)
7 M3 x 16mm csk machine screws (A to B, D to E to F, C to board, motor fixing)
2 M3 x 8mm or 12mm csk machine screws (switch)
4 M3 x 16 or 20mm panhead machine screws (microswitch, plastic to top board)
1 M3 x 25mm panhead machine screw (A and B assembly to board)
16 M3 nuts
¼” BSW bolt at least 5/8” long (holds ball-and-socket head) and nut to ensure correct protruding length if bolt is long. Second ¼” BSW nut (or 6mm nut and bolt) for lower tripod fixing - see text.

Alan Jefferis
February 2004