When you face north and observe the stars you will notice that their position is rotated in the east-west direction. Since a full 360o earth rotation requires 24 hours, in 1 minute the stars shift by 1/4o. If you are looking through a telescope with a field of 1-2 degrees you need a star tracking mechanism to keep your field of view.
The same is true if you want to photograph the sky for a long time. The professional way to do this is to mount your camera on a telescope motorized base. The cheap way to do the same will be discussed in this instructable.
The design is known from the 80’s as a barn door star tracker or a scotch mount. There is a lot of information in the internet where you may find sophisticated designs that try to minimize the systematic errors of the first design.
I will describe the simplest version of this device with a small improvement compared to other designs, a movable axis that allows calibration. The capability of this system is limited to 20 minutes exposures but this is satisfactory for a DSLR camera with an ISO sensitivity ≥800.
Step 1: Equipment/ Materials / Tools
a. A DSLR camera with ISO > 400 and the option of keeping the shatter open for several minutes (BULB). My Olympus E420 has a maximum sensitivity of 1600 ISO and an open shutter limit of 30min. In the case of my camera a remote control was required to operate the BULB exposure.
b. A steady photographic tripod
c. An adjustable base for mounting the camera. Your camera deserves a good quality one.
d. A telescope finder. Can be substituted with a simple tube, even a soda straw for those with good vision. Ι used half of a broken set of 10x25 binoculars to make mine.
a. Two pieces of plywood 15cmx30cm, thickness 12mm
b. A piano type hinge of length 14-15cm . The axis of the hinge should be as thin as possible. I used two larger hinges of length 7.5cm
c. A piece of 5cm x 15cm alluminum sheet 2mm thick.
d. A 6mm metric screw of length 8cm. This has a 1mm pitch (M6x1).
e. Common wire 1-2mm thick
f. Pieces of scrap wood and metal for the rest.
Common wood working tools, saws, a drill , a vice etc. Additionally you may need to use a tap of 1/4" to prepare threaded holes on a plate to mount the construction on the tripod. This type of thread is standard for camera equipment.
Step 2: The Principle of Operation
A full 360o rotation of the earth requires 24 hours, that is in 1 minute the sky rotates around the polar axis by 1/4 of a degree. If the base of the camera is also rotated in the direction East-West with the same velocity the camera will follow the movement of the sky.
The hinge is directed to Polaris with the help of an aiming device such as a telescope finder, We rotate the camera base with the help of a driving bolt. It is convenient to adjust the position of the axis in such a way as to cover the 1/4 of a degree in 1 minute. The bolt is turned manually by making reference to a watch showing the seconds.
The distance of the bolt from the hinge is related to the step fo the screw. I used a 6mm screw with a 1mm step therefore the distance is 229.2mm. Depending on what you are using this calculation has to be modified.
Step 3: Construction
1.Prepare the positions and the holes that will be needed in sequence on the wooden plates.
2. Prepare the wooden angle with the hinge. This has to be done carefully. Set up the wooden plates vertical on a flat surface. Mount the hinge on one of them and the other part on the wooden spacer. First use glue to attach the spacer on the other plate and take care that both plates remain vertical on the surface and touch uniformly on it. When the glue is dry, secure the second plate with scews on the spacer.
3. Prepare the driving bolt. Using a file form its end in a conical shape. The wooden disk should have a minimum diameter of 60mm. mark 12 sectors on the disk.
4. Mount the supports of the axis as shown in the photographs below. The supporting point should be allowed to slide +- 10mm in respect to the expected distance from the hinge of 229.2mm.
5. Prepare and mount the camera base, the finder and the base plate for the tripod.
The finder (or simple tube) should be aligned parallel to the edge of the wooden plate.
Step 4: Calibration
Now you are ready for calibration.
1 .Move the axis as close to the hinge as possible and secure the screws on both plates. Position the wooden angle on a sheet of paper on a flat surface. Then trace the angle that results after 20 turns (20 minutes) of the bolt . This should be exactly 5 degrees but normally it should be larger (in the diagram below this is the point 5.26degrees,88mm ). Measure the angle from the traced triangle better using trigonometry. Repeat several times and take the mean value.
2. Do the same for the other extreme point. In the diagram this resulted in the point 4.92 degrees, 81mm.
3. Find the correct point for the axis by extrapolation. In my case this gave a distance of 82.5 cm.
The accuracy of the measurement is (5.02+- 0.01) degrees corresponding to a deviation of 7 seconds in 20 minutes measurement and to only 1-2 pixel shift for a photograph of a 70 degrees field, 3450 pxls wide.
Step 5: Preparation of the Camera
1. Turn the knob to MANUAL (what else did you expect?)
2. Select the aperture as small as possible for maximum luminosity (F4 in my case)
3. Select a high sensitivity ISO 400 or higher
4. Select the RAW format if your camera has it (typical in DSLRs)
5. Natural light colour scheme .
6. If your camera has a built in Noise Reduction feature use it. However this will result in slower processing (some minutes). Otherwise take several exposures.
7. Before doing anything focus manually on a distant light or star holding your camera at hand.
8. Learn how the BULB feature works. Olympus E420 needs a remote control for this.
9. Last but not least do not forget to remove the cap!
Step 6: Setting Up the System on Location
1. Charge the camera battery before going there. You may be working for an hour or more.
2. You definitely need a small torch. I use a rechargable one.
3. Take binoculars with you
4. You also need a piece of white cloth to cover your camera for "flat" exposures.
5. Take a watch and attach it on the tracker as shown in the photograph.
Step 7: Orientation
First you have to look north and locate Polaris, the tail star of the Small Bear. The actual position of the north pole is approximately in the mid distance of Polaris (alpha Ursus Minor) and lambda Ursus Minor. The distance of the two stars is about 1.5 degree. If the sky is clear enough lambda should be visible in your finder. Try to center the mid point of the two stars. However for large angle astrophotography, centering Polaris should be adequate.
The photograph below was taken using ISO 400 and 60 sec exposure. The image was turned to grayscale and colors inverted to show the stars in black. The north pole position is shown with a cross.
Step 8: Photographic Procedure
1. Take several field photos for a fixed ISO and a fixed time. Try 2,4,8 minutes
2. Take several photos with the camera covered with its own cap. These are called "dark" exposures.
3. Take also several photos with the camera covered with the white cloth (no rincles). This is useful to eliminate stray light from the ground or vignetting etc. These are called "flat" exposures.
Now you need a stacking programme for astrophotography.
My preference is DeepSkyStacker (freeware) but there are others like GIMP.
Install one of these and feed it with all your exposures. The programme will do the rest.
Step 9: Example 1: the Milky Way and Sagittarius
The photograph of the milky way to the south and Sagittarius ( a part of which is the teapot, lower left) was a single 8 minutes exposure at 800 ISO with flat and dark exposures, all processed with DeepSkyStacker. The starting photograph material was in RAW format while the final output of the program was in TIFF. What you see here has been compressed in JPEG.
The famous star clusters and nebulas in Sagittarius are clearly visible. The brightest part is in the direction of the center of our galaxy! The two stars that are close (bottom center-right) are the cat’s eyes in the tail of Scorpion.
The camera was facing south in very clear night , no moon or lights.
Step 10: Example 2: Andromeda's Galaxy
This is a part of a larger photograph the result of the superposition of 3 x 4 minute exposures using ISO 800.
It is worth noting that the faintest stars mapped are of magnitude 8.5 while 6 is the limit of the naked eye and 4-5 is what we usually see in a light polluted sky.
The camera was facing North-East high.