What telescope owner has never thought about photographing the wonders he sees in space? Modern technology makes it possible to take photographs using a smartphone, attaching it to the eyepiece of a telescope. However, there is also a special device for astrophotography – the astrograph.
What is an astrograph?
An astrograph is a camera with a guide telescope attached parallel to it. To avoid image stretching, the camera must follow the diurnal rotation of the sky. The camera can be driven by an automatic mechanism or manually. It is difficult to build a good clockwork, and it is difficult to obtain a high-quality image with manual control. Especially when it is cold outside.
Today we will consider the construction of a simple astrograph on an equatorial mount. The camera of this astrograph has a small focal length and therefore you can rotate it following the daily movement of the sky not quite accurately and even with interruptions. It is only important that for the time interval between rotations the images of the stars are displaced by 0.025 mm.
With this small and simple instrument, you can make the fantastic shots of stars up to magnitude 8, bright nebulae, comets and meteors. The astrograph's field of view covers an area of the sky with an area of about 860 square degrees (at a focal length of 50 mm).
How an astrograph works
Equatorial astrograph mount consists of two panels. Do it from textolite. You can also use a plywood sheet with a thickness of 10-12 mm. If you decide to use plywood, then it must be treated with a protective compound.
The bottom panel has three set screws in the feet. Attach the upper panel to it at an angle that is complementing the latitude to 90°. This panel has a built-in bearing in which the polar axis rotates. An L-shaped camera bracket should be screwed to its top edge.
Put a driving circle on the lower end of the polar axis. It should precisely rotate on the polar axis, and when fixed with a clamping nut, they should rotate together with the polar axis. It is highly recommended to make the polar axis and the driving circle on a lathe with special precision, because the quality of photographs depends on theirs.
A thin steel tape is attached to the drive wheel (this can be replaced with a typewriter tape). The other end of the strap is connected to the lead screw nut. A steel circle with divisions is put on the head of the lead screw. How it works? When the drive screw is turned, the nut moves along the thread and winds the tape from the drive wheel. Together with the driving circle, the polar axis and the camera rotate.
The radius of the leading circle depends on the time interval through which the camera should turn, on the lead screw pitch, and on the number of seconds in sidereal days. Calculate the radius of the driving circle with this formula: 2πRn /h = S / t.
Here, π is the number of divisions on the drive rotor circle. It is recommended to turn the screw by 9° every 6 seconds, then n will be equal to 40.
It is important to make accurate calculations when making the polar axis and driving circle. If the calculations are inaccurate, the camera, when accompanying the stars, will be ahead of them or lag behind them. As a result, the images will be stretched.
For more comfortable work, you can equip your structure with lighting.
A telescope with a crosshair in the eyepiece is mounted on the top panel parallel to the polar axis. It helps to establish the astrograph in the meridian. The objective of the tube can be a simple converging lens with a diameter of 20-25 mm and a focal length of 60-65 mm, and a 10-fold magnifier as an eyepiece.
PHOTOGRAPHING THE SKY
How to use an astrograph?
It is better to make observations with an astrograph on a dark moonless night. First of all, you should install the device in the meridian. To do this, using the set screws in the legs, we adjust the pipe to the North Star.
Then, lowering the clamping nut, we first direct the camera along the hour angle and fix the desired position with the same nut. Then, slightly loosening the camera mounting nut, we aim it along the declination angle. Without removing the lens cap, open the shutter and turn on the illumination of the circle with divisions and the stopwatch.
After removing the lens cap and holding it in front of it for a few seconds so that the vibration dies down, we proceed to photographing. Do not forget to rotate the circle every 6 seconds by one division, equal to 9°. If the focal length of the lens is 100 mm, then the circle must be rotated after 3 seconds by half a division.
You can also try to use an astrograph with astronomy binoculars.