We perceive the motion of celestial bodies from east to west because of the rotation of the Earth on its axis from west to east. To follow a particular object in the sky with ease astronomers use what is called an equatorial mount. In this type of mount, one of the axes is made parallel to the axis of the earth. This axis is referred to as the polar axis. The advantage of this type of mount is, by rotating the polar axis from east to west, at the rate of (nearly) one rotation per day the telescope can be continuously kept pointing towards any celestial objects, excluding of course the solar system object.
Such mounts are made in mechanical workshops, for the mount should not only have a precise clock drive to rotate the polar axis but there should also be a provision to move the telescope quickly from one object to the other and to make small changes in the speed of the clock whenever required. Multiple refractions of the light due to the atmosphere cause the image of the star (or the other celestial body) to shift continuously and the provision should be made in the mount to correct this error in pointing.
However, if one is planning to take photographs of short duration, (of the order of 15 minutes) and the field or area of the sky that camera will cover is quite large, of the order of a few tens of degrees, than one can build a very low-cost hand operated drive for astrophotography.
The first of its kind was made in the early 1970s by a Scottish gentleman George Y. Haig. He published an article in British Astronomical Association Journal [B.A.A. Journal, volume 85 no. 5 (1975) page 408] about a simple hand-driven tracking system for astronomical photography.
Mr Haig called the mount Scotch mount. He explained in a latter to me in a later date 8th July 1996 why he chose to call the mount ‘The Scotch Mount’. (See below.)
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A note of Explanation I first made the door-hinge mount for use in the winter of 1972 – 73. Since it was neither a German nor an English equatorial, it could have been described as a Scottish equatorial; however, the word “Scotch” rather than “Scottish” seemed to me to convey a hint of that comic parsimony (allegedly associated with Scotsmen) which I felt was appropriate for such a cheap and simple device. Then of course there is a brand of Scotch whisky made by Haig (not a relation of mine, unfortunately!) so the epithet is also an oblique (perhaps even obscure) allusion to my own surname. G.Y.H. |
This simple device or mount became very popular and many amateur astronomers all over the world made this mount and improved it. For some reason, its original name was also changed. One of the most popular names has been barn-door mount.
Mr Haig also experimented with taking spectra of stars using a prism mounted in front of the camera lens.
Making the Scotch Mount
The basic technique is to use two wooden planks joined with a hinge. and mounted in such a way that the axis of the hinge is aligned with the axis of the earth. A bolt of suitable pitch ‘P’ (the distance between the adjacent thread on the bolt) is attached to one of the planks at a distance ‘X’ (from the axis of the hinge, in such a way that when the bolt is rotated at a certain speed ‘T’ (normally one rotation per minute) it pushes the planks away from each other with the angular change corresponding to the rotation of the earth.
The formula is
X = 228.5 x P x T (here X is in mm, P is in mm and T is in minutes)
We are presenting a similar mount here which you can make quite easily if you are a handyman yourself and improve upon the design. Or get it fabricated by a carpenter.
Material required for the basic mount
- Wooden plank – 150 X 350 x 15 mm: one nos.
- Wooden plank – 160 X 350 x 15 mm: one nos.
- Bolt (20 TPI quarter inch, 2 inches long): one no.
- Bolts (20 TPI quarter inch, 4 inches long): two nos.
- Door hinge 4 inches long: one no.
- Required screws and nuts.
- Epoxy glue, such as Araldite or M seal
The 20TPI quarter-inch bolt was suggested by Mr Haig in his original article. This is also the standard size used in cameras and camera stands.
Then for T = 1min, P = 20th of an inch = (1/20) x 2.54 = .127
X = 288.5 x 0.127 = 29.0195cm or about 290 mm
Making the mount
Join the planks with the hinge. Drill a quarter-inch hole on one of the planks exactly 29 cm from the hinge centre. This is our top plank. Attach the nut to the 20 TPI bolt and push the bolt inside the hole that you have drilled until the nut touches the plank. Now tap the bolt on its head lightly with a hammer. It will leave the impression of the nut on the plank. If the impression is not deep enough tap on the bolt again.
Next, take out the nut and the bolt. Using a sharp knife or blade scoop out the wood where the impression of the nut was made until the nut can snugly slide inside this cavity.
Apply the epoxy glue to the side of the nut and to the cavity and push the nut inside the cavity. (Trap: Take care that the glue doesn’t touch the inside threads of the nut.) When you thread the bolt through this nut it will rest on the lower plank. As you rotate the bolt clock or anti-clockwise it changes the separation between the two planks.
On the two ends of the bottom plank put two more additional nuts and bolts to control the inclination of the planks. As shown in the figure below. The planks should be inclined to the horizon by an angle equal to the latitude of the place. Make the length of the board aligned east-west the axis of the hinge parallel to the axis of the earth. If you rotate this bolt at one rotation per minute, it will compensate for the rotation of the earth. Now make a suitable fixture for mounting the camera.
It is now a simple matter to start with your astrophotography. Find a dark enough place. Put the drive and the mount on a firm table or ground. Aligned it east west north south. Put the camera shutter speed on ‘B’, so that the shutter will remain open as long as you keep the shutter release liver or the button pressed. Keep the aperture fully open. Place your analogue wristwatch with the second arm next to the 20 TPI bolt. Load the camera and direct it to some prominent constellation. Open the shutter and rotate the bolt as the second arm move. Take a few shorts of 5, 10 and 15 minutes duration each and get the film developed. Most likely the film processing lab owner or the technician working there will not make any prints saying that the negative has only some dots. But insist on prints and examine those. If you have done everything correctly you will have a fine photographic star map for yourself.
Some tips and tricks
- Trick: Glue a small metallic rod to the turning bolt to help you follow the second arm. Strap a wristwatch to the board and use a dim torch covered with red cellophane tape to read the time.
- Trick: Instead of continuously moving the bolt, rotate it by 90 degrees every 15 seconds.
- Tip: While rotating the bolt, hold a black paper cover in front of the camera lens. So in case you disturb the setup you can abort the exposure.
- Trick: Just in case you missed the time or turn, don’t start where you left, keep covering the lens and move the bolt to the position where it should be at that time.
- Trick: Glue a piece of metal to the lower plank just where the bolt rests over it. It will protect the wood.
Here is a picture of the scotch of the mount that I made long ago and used regularly.
In this A and A are the wooden planks, and B is the 20 TPI bolt used for tracking. The door hinges are 29cm from this bolt (hidden in the picture), and C is bold for adjusting the latitude so that axis of the hinges points to the North Celestial Pole. 
The camera is CCD ST6. We also added a small telescope for basic alignment.
Note that the Scotch Mount for latitudes closer to the equator is quite stable and the rotating bolt at top of the board works better. But for higher latitudes more than 40 deg. the inclination of the axis is quite large and needs better support for the wooden planks.
BTW – if you are at the North Pole – our altazimuth mount (like Dobsonian) is an equatorial mount.
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35 Dalmahoy Crescent,
Renfrewshire PA11 3JB
Scotland, U. K.
8th July 1996
Mr. Arvind Paranjpye,
Inter-University Center,
for Astronomy & Astrophysics,
Ganeshkhind.
Dear Mr. Paranjpye,
Your letter to Mr. Mobberley has just reach me – I am pleased to learn that you have been using the Scotch mount with satisfactory results. He tells me that he has posted a copy of the JBAA article to you; I enclose some additional information which you may find useful.
Now for the biographical information you require. I was born in 1928, educated in Kirkcaldy (a town in the country of Fife, Scotland) and at the University of Edinburgh, where I graduated with honors in physics in 1949. My first employment was in the research department of Messrs, Babcock and Wilcox at Renfrew (near Glasgow) where I worked on furnace temperature measurement,
acoustic imaging and electronic process control. In 1955 I left this firm to join the Glasgow firm of Barr and Stroud as an optical designer, first spending a year at Imperial College (London University) to gain a post-graduate diploma in Technical Optics. As head of the Optical Computing Group at Barr & Stroud, I was involved in the design of submarine periscopes, infra-red viewing systems, and aircraft and satellite optics.
In 1962 I joined the staff of Paisley College (now the University of Paisley) as a lecturer and subsequently senior lecturer in Physics, teaching Mathematical
Physics, Electromagnetic Theory, and Astrophysics. I retired in 1993, having spent my last year as Acting Head of Department. One highlight of my time at Paisley occurred in the early eighties, when at the request of British Council I made several visits to Nepal as leader of small teams of lecturers, to conduct Physics Teaching Seminars at Tribhuvan University in Kathmandu.
I have been a member of the B. A. A. since 1950, and a Fellow of R.A.S. since 1965. An interest in astronomy education motivated me to construct a simple planetarium projector in 1972; the design has since been developed to the stage that a projector for public displays was recently installed (1994) in Paisley’s Coats Observatory. Other interests are music (I play the organ in a local church) and the design of astronomical clocks, of which I have made three different prototypes. My wife is very tolerant of the clutter of books, tools and gadgets which fill my study and often overflow into other living area!
Many thanks for your interest, and best wishes for your own work,
Yours sincerely,
George Y. Haig.
DIYA 