The basic physical problem
The refraction of the atmosphere causes a small rotation of the field of view. That means that the true deviates from the apparent North direction. The apparent North direction is that what we would see e.g. in a telescope. It is now necessary with a decent choice of the position of the hour axis to minimize the field rotation. In case of a field of view of about 1 square degree this choice can be the apparent pole under most circumstances.
In the contemporary literature two aspects are treated in an inadequate manner: the alignment of the hour axis and the effects of the refraction of the atmosphere on a celestial photograph.
There is only little known what the accuracy of the position of the hour axis should reach in order that the rotation of the field can be minimized effectively. And that is astonishing. Today often the position of the hour axis is far away from the nominal value and the field rotation is dominated by it.
The practical problem
The first celestial photographs are often disappointing because there are several problems to cope with. Once the decision for a certain telescope and detector is made, e.g. a conventional, CCD-, web- or video camera, then it is necessary to get step by step into their peculiarities. Even if one is used to the equipment there is no guarantee for pleasing celestial photographs.
Figure
1: NGC 2683, These five enlarged insets represent the center and
the four corners of an ordinary
slide taken with an Astrophysics Starfire EDF. The focal
length of this triplet refractor is 1260 millimeters and the
diameter of the objective is 180 millimeters. The picture was
taken at the local hour angle -3h10m and
lasted for 70 minutes.
In
the investigations, which are presented here, it is shown that the
alignment is as important as the optical quality of the telescope
and the mechanical of the mounting. Or in other words the
alignment of the hour axis contributes a great part to a good
celestial photograph. The negative effects of the atmosphere can
be compensated to a large extent if the hour axis points to the
appropriate place.
If the hour axis does not point within narrow limits to a certain position then even an automated guiding system can not do much. Despite these systems compensate all irregularities of the drive and the movement of the guiding star due to the refraction only the place of the guiding star remains unaffected. How the field stars move relative to this point is at first unclear.
A certain idea how the image of the field stars are deformed is presented in fig.1. The directions and the length of the star trails are different over the field and neither the optics nor the seeing are the limiting factors. The star trail in the middle is about twice as long as wide. The fine details of the galaxy NGC 2683 are washed out and the detected amount of light falls below threshold in the outer parts.
There is no additional equipment necessary for the precise alignment of the hour axis except a stop watch, a eyepiece with reticle and a programmable pocket calculator. What is surely needed is the time to get used to the alignment process. If this initial step is mastered then the alignment of the hour axis will become a routine process which needs about an hour to perform and ends with precision of about better than 30 arc seconds.
Comments, questions, corrections: markus.wildi@one-arcsec.org