A TRAVEL ASTROGRAPH
The purpose of this article is to describe a small travel astrograph that, easily disarmed, can easy be taken in an airplane in a small
Suitcase. It allows to make long exposure photographs with any camera with focal lengths until about 200 mm, and also to the use of small telescopes.The idea of its construction had been born in distant 1975 with the desire to photograph the Kohoutek comet, because it did not have any telescope and I needed with urgency an equatorial mount.
The luck wanted that it did not arrive in time, because when the comet arrived the mount was not still ready, and I had to conform myself to photograph
Although in this first version of the mounting the latitude is fixed, it can be used in any latitude with the use of a good tripod. To such aim, a screw of 1/4 of inch is provided in the inferior part of the base.
In the left part of the base is located the power unit, that works with a 6 volts battery. In center it is the support for the polar axis and the worm gear. For a preliminary regulation of the speed, a potentiometer is also provided.
The control of the speed is very simple but totally efficient, and consists of regulating preliminary the speed in so an way, that it is always a little slower then necessary. A hand controller, that allows to make bridge on the potentiometer, allows to accelerate the motor whenever it is necessary, and to obtain therefore the correct speed. For an automatic guiding, a generator of impulses could easily be added for the control of the speed.
The polar axis - a stainless steel tube of 12 mm diameter - is mounted on ball bearings. The hole axis allows - but only in the North hemisphere - to orient the mount observing the pole star. For a precise orientation, an interposed prism turns aside the polestar towards the pole of the necessary amount. Since the angle (azimuth) depends on the date and the hour, a round slide rule is provided to such aim (round small box on the left side) that allows to calculate the angle in fast and easy way.
The declination axis is also mounted on ball bearings. For the crossing of axes I used an hydraulic TEE, opportunely adapted, that gave optimal results.
The mount also has right ascension and declinations circles, and an index that marks the hour and the hour-angle.
Other characteristics of the mount can be seen observing the photos, like for example the possibility of using a small 50 mm refractor, that can easily reach and surpass 100x, and whose magnification- and resolving power (2 arcsec.) can integrally be exploited, being the small lens practically insensible to the turbulence. The Cassini’s Division on the Saturn ring can, for example, be easily observed with this small device.
Finally, a small
guidescope can easily be leaned to the same camera.
In order to make the reticle, a microscope cover glass was used, on which a cross with a ball-point pen has been drawn up. As each one can verify, when a line is drawn up on a glass surface with a ball point pen, the pen produces in fact two very thin and very close parallel lines (tenths of mm) that can act so well as required.
As example of which it is possible to do with this small instrument, see the photo of
Venus, obtained with the small 50 mm-refractor when Venus was very distant, and the one of Altair (my first long-exposure photograph), obtained with a Pentax 3.5/135mm with a green-yellow filter and 20 minutes exposure on Kodak 103a-F: in this photo can be counted more than 2500 stars, where with the naked eye no more than 10 can be seen!