How to build an Astrograph

Nowadays most people are surrounded by street lights and can’t do much astrophotography because of the orange sky. This article describes how to make an Astrograph that can be taken out into the country and used on a car roof, or any flat stable surface. When I say it only costs a few pounds to build, I exclude the price of a 35mm camera and the film. These items I leave to the constructor. However, the faster the film the better.

The Objective
As most people realise, the sun rises in the East and sets in the West. This is also the case with the stars. Due to this motion, the instrument must follow them if the stars are to remain as points of light on the photograph.

What is Required
1/ A piece of 10mm chipboard about one metre square.
2/ A 40mm long M3 bolt.
3/  Two hinges.
4/ 2off  M6 bolts about 15mm long with wing nuts.
5/ A small spring.
6/ Two screw in curtain eye hooks.
7/ 50mm angle iron about 30cm long and 1mm thick.
8/ A PP9 battery.
9/ 1 PP9 battery connector.
10/  Two 820 ohm half watt resistors.
11/  Four rubber feet.
12/ Two red light emitting diodes.
13/  A dual spirit level about 50mm square.
14/ An old fashioned watch with a second hand.
15/  One switch.
16/  Two garden bricks.
17/ Various assorted bolts, nails, glue etc.

Construction
The two bricks are put together to provide a steady structure that is vibration proof. Each brick measures 200x100x65mm and are put so that the dual brick is 200x130x100mm.

1/ Cut the chipboard:    1 off  150x115mm
                                     1 off  220x165mm
                                     2 off  200x130mm
                                     2 off  200x120mm
                                     2 off  150x120mm
                                     1 off  300x220mm

2/ Cut the 150x115 piece along the diagonal, giving two triangular pieces. This must be done accurately as it sets the 53 degree angle for the latitude of Lincoln.

3/ Drill one hole in each triangular piece (Fig 1a and 1b). This should be the same size as the Light Emitting Diodes.

4/ Take the two triangular pieces and one 200x130 piece. Glue and nail together as shown in Fig 2.

5/ Take both 200x120 pieces and the 150x120 pieces. Glue and nail these around the outside of the 200x130 piece with the triangles outermost. Let this dry.

6/ Take the bricks and put these in the box. Slide in the remaining piece  (the bottom of the box) until it holds the brick firmly. Glue and nail. Trim the outside of the box until smooth.
(You should now have a heavy box with two triangular pieces of wood on top)

7/ Screw three of the feet to the bottom of the box directly under the ‘A’s in Fig 2.

8/ Take one of the 300x220 pieces and drill a hole as shown in Fig 3. This must be smaller than the 40mm long bolt (item 2). Screw the bolt in to the hole. This should be a tight fit but the bolt should turn with little effort.

10/ Attach the two hinges to the longer edge of the 220x165 piece; one near the top, the other towards the bottom.

11/ Now attach the other parts of the hinges to the sloping board ( Fig 4). The distance between the centre of the bolt and the centre of the hinge (115mm) is only for an M3 bolt. The tolerance on this measurement is 1mm as it determines the accuracy of tracking the stars.

12/ File the end of the M3 bolt to a point and add a screw to the inside of the opening board so that the bolt touches the face of the screw rather than the centre slot (Fig 4).

13/ Add the spring to the centre of the opening edge (fig 4).

14/ Screw in the curtain eyes, one near the top, the other near the bottom. These must be along a line parallel to the hinges. (Fig 4).

15/ Remove the bolt and wrap a small nail around it. Lock this into place with a washer and two nuts (Fig 5). Screw into the hole and turn. The boards should now open as the screw is turned clockwise.

16/ Cut the angle iron into 50mm, 50mm and 100mm lengths.

17/ Taking a 50mm section, drill 3 holes as shown in Fig 6. Also drill a hole to take the bolt and wing nut on the other face. Screw this to the board. Drill one hole in the centre of each face of the other 50mm length.

18/ On one face of the 100mm section drill a hole to take the bolt for the camera. This should be measured
with the camera back resting on the other face of the angle iron (see Fig 7). Attach the angle iron together using the bolts and wing nuts. (fig 7).

Wiring

Before installing the wiring it is recommended that the circuit in Fig 8 is assembled on a table and tested in a darkened room. The Light Emitting Diodes (L.E.D) only work in one direction and by doing this, that direction can be determined.

Setting Up

The setting up must be done as accurately as possible. Place a ruler along the edge of the two curtain eyes. Adjust the position of one of the eyes until the ruler is parallel to the centre of the hinges. Take the astrograph out on a clear night (you may have to wait a long time). Put it on a level elevated surface eg a table. Look through the two curtain eyes from about 6 inches below the lower one. Adjust the Astrograph until the pole (not the pole star) is in the centre of both of them (see Fig9). Pack the spirit level until both the bubbles are centralised.

The astrograph is now set up.
 

Using the astrograph

The instrument is used by aligning the car East West and standing on the North side with the astrograpgh just in front of you. By going around the far side of the car, imagine a line up through the eyes to the pole star and adjust the position until aligned. Going back to the the North side of the car, put the remaining foot under the other and move the instrument (on the curve of the car roof) until the spirit level bubble is central in both directions. Place the watch under the right L.E.D and switch on. After aiming the camera at the sky, open the shutter, using the cable release. Turn the handle on the astrograph to follow the second hand of the watch. After five minutes close the shutter. On 500 ASA film, stars to magnitude +10 or +11 can be reached. This is over 30 times fainter than the naked eye can see. The instrument can also be used on any flat surface. If it is possible to look through the curtain eyes from a few inches below the lower one, this should be done as the accuracy of the instrument is greatly improved. It is not possible to detect any star trails on a five minute exposure on a car roof with a 50mm lens (usually fitted, but if a 135mm lens is used, setting up must be done within 0.25 degrees, which is not possible without looking through the curtain eyes.