Scheiner method
The Scheiner method is a process for so-called "polar adjustment" or "northward alignment" with which an equatorial (parallactic) telescope mount can be precisely aligned with the celestial pole . The process is also called “shining” or “ shining in” . In the English-speaking world, the term "Drift Alignment Method" is common.
The method was developed by the German astrophysicist Julius Scheiner in connection with his participation in the international work on the photographic sky atlas. It was published in his book Die Photographie der Gestirne and in the Bulletin du Comité Permanent de la Carte du Ciel Photographique .
If you follow the instructions in the original publications exactly, the polar axis can be aligned to an accuracy of about 1 arc minute under optimal conditions in about 30 minutes. However, very many instructions for the Scheiner method do not use Scheiner's detailed description and sometimes deviate roughly and often incorrectly from it, which can lead to unnecessary expenditure of time up to hours or the failure of the method.
background
The Scheiner method (or other methods for exact polar adjustment) are the prerequisite for point-like star images in astrophotography . A mount that is not exactly aligned with the celestial pole leads to a slow "drift" of the observation object. The direction and strength of this drift depend on the size and direction of the polar axis misalignment, as well as the declination and the hour angle of the set object.
While the drifting of the stars, which is caused by inaccuracies in the drive, mechanical deflection, atmospheric refraction and the remaining polarity error, can be compensated for by the (manual or electronic) tracking correction, a deficient polar adjustment with long exposures causes a continuous and uncorrectable rotation of the image field . For this reason, the longer the exposure, the smaller the angle of view and the smaller the pixels of the image sensor, the higher the requirements for exact polarization.
Even good mechanical pitch circles can only be used meaningfully with a mount that is as precisely northed as possible ("incised").
The error in the alignment of the polar axis with the celestial pole ("pole error") can be split up into two components that are perpendicular to one another: the error in pole height and in azimuth . Most equatorial mounts have adjusting screws for this purpose, in order to enable fine correction of the polar adjustment in polar height and azimuth.
Scheiner recognized that the two components of the pole error in certain positions of the mount show up as a star drifting exclusively in the declination direction. Errors in the drive of the hour axis become ineffective because they only have an effect in the right ascension direction. In two different positions of the telescope, the azimuth errors and the polar height errors can be corrected in isolation from one another.
Another advantage of the Scheiner method is that the mount is aligned with the so-called "apparent" celestial pole, which is slightly higher than the true celestial pole due to the refraction . This is more advantageous because the refraction phenomena then have a somewhat less effect on the evenness of the tracking than when aligning with the true celestial pole.
Since the Scheiner method uses the direct effect ("drift") of the error to be corrected ("pole error") for measurement, it is - if applied correctly - almost foolproof, as it does not have any self-adjusting tools, such as a polar finder or partial circles , required.
Instructions (according to the original description)
First place a star in the meridian in the center of a crosshair eyepiece with the highest possible magnification (300 ... 600 times). To minimize refraction and deflection errors in the mount and the telescope tube, choose a star as close as possible to (or close to) the zenith . The eyepiece is rotated so that the threads are oriented precisely in the declination or right ascension direction. After a while, the azimuth error will become noticeable in a "drift" of the star on the declination thread. An independent drift in the right ascension direction is ignored.
The size of the drift in a fixed time interval (e.g. 10 minutes) is a direct measure of the azimuth error. Since an azimuth error of 1´ produces a declination drift of only 2.66´´ in 10 minutes of observation, the high magnification and the selection of a star close to the zenith are essential for a quick and reliable detection of the azimuth error. If the star deviates from the thread net in declination to the north (in the reversing telescope), the southern end of the hour axis points too far to the east (or to the west if the deviation was to the south).
For precise adjustment of the pole height , a star is set with an hour angle of 6 h or 18 h . To minimize the falsifying refraction phenomena, it is particularly important here to use a star near the celestial pole. The 6 h or 18 h hour angle means here a star exactly to the left or right of (and as close as possible to) the celestial pole.
The instruction that is often found to set a star in the east or west about 30 ° above the horizon for the polar height adjustment is extremely unfavorable, since the hour angle deviates significantly from 6 h or 18 h and the refraction and other harmful effects still occur are clearly pronounced. This incorrect instruction is probably responsible for the many uselessly spent adjustment hours with unsatisfactory results.
When turning the adjusting screws for polar height and azimuth, one should always keep an eye on the eyepiece and watch the movement of the control star when turning and finally tightening the adjusting screws after completing the polar adjustment. Due to the elasticity of the adjustment mechanism and the mounting head, linear behavior when slowly turning or when finally tightening the adjustment or counter screws cannot be expected. The guide star often moves abruptly, sometimes with an undesirable component perpendicular to the desired direction. You may therefore have to bring the control star back into the desired position with the opposite screw or with the screws of the other axis. A desired accuracy of, for example, one arc minute may correspond to only a few micrometers on the mounting head. For these reasons, it is advisable to carry out the procedure twice in succession or once more after tightening all adjusting and counter screws.
Extension of the Scheiner method
Scheiner himself pointed out that the necessary turns of the azimuth and pole height adjustment screws can be derived directly from the temporal declination drift of the star. However, this only works satisfactorily if a crosshair eyepiece with a measuring scale (or at least a “box division”) is used. Alternatively, the necessary correction in azimuth or pole height can also be read directly in the crosshair eyepiece, provided the crosshair eyepiece has scales in the X and Y directions (or alternatively, it is rotated by 90 degrees between drift measurement and correction setting). Detailed instructions can be found e.g. B. at Rhemann u. Kersche, Knülle, Leifert. Unfortunately, the wrong instruction with the star over the east or west horizon for the polar height adjustment creeps in here as well.
Instead of a crosshair or measuring eyepiece, an electronic CCD or CMOS camera is a considerable relief. This means that even the smallest drift rates can be quickly and easily recognized quantitatively and evaluated directly. There are specialized Scheiner programs for direct use with a large number of cameras or (autoguider) CCD cameras with integrated drift rate displays. Specialized apps for smartphones are also available.
Pole adjustment by means of electronic graduated circles and computer support
In the case of modern, computer-controlled amateur telescopes with Goto mounts or "digital partial circles", the computer built into the mount or on the hand control unit takes over these calculations. The prerequisite for this is that the date, time and geographical location are known (manual input or automatic determination by a built-in GPS receiver ) and the mount has already been aligned approximately to the north. The observer now only targets a bright star near the equator in the south meridian with a crosshair eyepiece. The computer then calculates the ideal position of the star and then moves it out of its original position. Finally, the user rotates the azimuth and pole height of the hour axis by hand until the star is back at the original position in the crosshairs. The control bridges the time until repositioning by constantly tracking the mount.
Newer go-to controls also know the "two-star method", in which stars are placed in two directions in the center of the field of view and the rotation matrix of the mounting axes is calculated from this. In the case of more expensive devices, there is even the “three-star method”, in which a third star is used to control and minimize residual errors.
The accuracy of these methods is limited to typically 10-30 arc minutes due to the resolution of the angle encoder used and other factors and often does not achieve the values required for long-term photography.
Individual evidence
- ↑ a b Julius Scheiner: The photography of the stars . 1st edition. Wilhelm Engelmann, Leipzig 1897, p. 99 ( archive.org ).
- ↑ a b Julius Scheiner: Sur une méthode très simple permettant d'orienter un instrument à monture parallacticque plus exactement . In: Bulletin du Comité permanent International pour l´Exécution Photographique de la Carte du Ciel . tape 1 . Paris, S. 385 .
- ^ Edward S. King: Forms Of Images in Stellar Photography . In: Annals of the Harvard College Observatory . tape XLI , no. VI . Cambridge 1902, p. 153-187 ( harvard.edu ).
- ↑ Gerald Rhemann, Franz Kersche: Astrophotography with portable devices . In: Stars and Space . tape 34 , no. 7 , 1995, p. 560 .
- ↑ Dr. Matthias Knülle: The Scheiner Method. December 3, 2000, accessed June 10, 2019 .
- ↑ Roger Leifert: The Scheiner method for polar adjustment. April 8, 1999, accessed June 10, 2019 .