Scotch Mount for Camera
Used for long exposure photgraphy of the sky.
We perceive the general 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 equatorial mount. In this mount one of the axis of the mount is made parallel to the axis of the earth. This axis which is naturally aligned with the north and south celestial poles is referred to as polar axis. The advantage of this type of mounts is, by rotating the polar axis from east to west, 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 made a provision to move the telescope quickly from one object to the other and to make small changes in the speed of the clock when ever required. Multiple refraction of the light due the atmosphere causes the image of the star (or the other celestial body) 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 of the camera (that is area of the sky covered) is quite large (of the order of a few tens of degrees) than one can build a very low cost hand operated drive.
The first of its kind was made in 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. And this is why - in a letter to Arvind Paranjpye, dated 8th July 1996 he attached this note why he chose to call the mount 'The Scotch Mount'.
A note of Explanation
I first made the door-hinge mount for the 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.
This simple device or mount became very popular. Many amateur astronomers all over the world made this mount and improved it. For some reason its original name too was changed – one of the most popular name 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 that to use two wooden planks joined with a hinge. and mounted 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
We are presenting a similar mount here which you can make it quite easily if you are a handy man yourself and improve upon the design. Or get it fabricated from a carpenter.
Material required for the basic mount
1. Wooden planks - 150 X 350 x IS mm: two nos.
2. Bolt (20 TPI quarter inch, 2 inch long) : one no.
3. Bolts (20 TPI quarter inch, 4 inch long) : two nos.
4. Door hinge 4 inch long: one no.
5. Required screws and nuts.
6. 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 camera and camera stands.
Then for T = 1min, P = 20th of an inch = (1/20) x 2.54 = .17
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 plank exactly 29 cm from the hinge center. Now thread the nut on 20 TPI bolt half way. Insert this bolt in the hole till the nut touches the plank and tap lightly with a hammer. Impression of the nut will be left on the plank. Take out the nut and bolt and remove the nun from the bolt. Scoop out the wood over this area so that the nut can snugly slide inside this cavity.
Put the nut inside this and glue it, taking care that the glue doesn't touch the inside threads of the nut. When you thread the bolt through the nut, it will control 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, the planks should be inclined to the horizon by 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 take your first astrophotograph. Find a dark enough place. Put the drive and the mount on 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 full open. Place your analog wrist watch with seconds 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 seconds arm move. Take a few shorts of 5, 10 and 15 minutes of durations each and get the film developed. Most likely the film processing lab owner or the technician working there in 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, tricks and traps
- Use a colour negative film, so that you can get it developed in 'one hour labs'.
- Glue a small metallic rod to the turning bolt to help you follow the seconds arm. Strap a wrist watch to the board and' use a dim torch covered with the red cellophane tape to read time.
- Instead of continuously moving the bolt, rotate it by 90 degrees every 15 second.
- 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.
- 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.
- 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 mount that I made long ago and used it regularly.
In this A and A are the wooden planks, B is the 20 TPI bolt used for tracking. The door hinges are 29cm from this bolt (hidden in the picture), C is the bold for adjusting the latitude so that axis of the hinges point to the North Celestial Pole.
The camera is CCD ST6. We also added a small telescope for basic alignment.
Note that for the Scotch Mount for at latitudes closer to equator are 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 equatorial mount.