The following text is extracted from T. Pettauer "The Kanzelöhe Photoheliograph"
in Publications of Debrecen Heliophysical Observatory, Vol. 7, p. 62 - 63
Sketch of the telescope.
|
The heliograph is of the classical design with an enlarging lens and cross-hairs in the
prime focus for orientation.
The front element of the Jena AS-objective lens (130/1950 mm) is coated with a transpaÂrent
goldfilm, to get, with full aperture, exposure times in the millisecond range and to
prevent eccessive heatload on the interference filter . The goldfilm, evaporated in high
vacuum on the air gap side of the front lens transmits 9 % in the green. With this method
no additional optical element is necessary, and the soft film is well protected against
mechanical damage. Goldfilms are very stable in the atmosphere and have additionally
a sharp cut-off in transmission in the green to longer wavelength.
The interference filter of image quality has a HBW of 1O nm at 546 nm. It is placed
about 150 mm in front of the prime-focus with a small tilt, to reflect the offband light to the
well blackened (3M black velvet) and baffled telescope tube.
|
The "camera-unit" with the magnifying lens and shutter is screwed to the telescope
tube. The alignement of the camera in respect to the telescope tube is given by a ground
steel plate, which defines a reference plane and a mechanical ( = optical) axis. Exactly
centered on this axis are: the cross hairs necessary for precision position measurements
and the magnifying lens, a modified orthoscopic eyepiece. This magnifying lens has
considerable distortion and will be replaced. The diameter of the enlarged solar image is
87 mm. Three invar rods define the film plane parallel to the reference plate (and focal-
plane) better than ±0.02 mm, therefore plate tilt is neglible. Vacuum film holders
prevent wrapping of the 13x16 cm film (Kodalith Ortho 3).
|
The shutter unit consits of a commercially available magnetic Compur-shutter and a
rotating disk shutter. The latter defines the exposure time and has a trapezoidal shutter
profil with appr. 80% complete opening. A microprocessor is used for the correct timing
of the shutter sequence and control of the motor speed of the disk shutter. The exposure
time is set manually or measured electronically.
The cross-hairs in the prime focus serve as reference marks on the heliograms. To
get accurate positions, the orientation of one of the cross-hairs on the sky has to be
known very precise. Normally the daily motion of the sun is used for this purpose. With
this heliograph a simple and accurate orientation method is used.
If one cross-hair is exactly in the east-west direction, than the angle it includes with
the horizon is the parallactic angle (eta in the Debrecen notation). In practice the crossÂ
hair deviates from the east-west direction due to errors in telescope alignment, mounÂting
of the camera etc. The orientation error at the time of exposure is obtained by
comparing the true, measured angle of the cross-hair with the calculated angle.
The device to measure the inclination of the cross-hair consists of a spirit level
(20" sensitiÂvity per part) mounted on an incremental encoder to measure the angle with one arc
minute resolution. The "P-meter" is firmly attached to the reference plate of the camera
and forms a compact unit with the cross-hairs.
|
Spirit level. |
 |
The instrument is completly calibrated in the laboratory with the camera mounted on
an exactly leveled granit-table with a horizontal camera axis. First the cross-hair is
aligned in the east-west direction to a few tens of a degree, than the cross-hair is brought
into a perfect horizontal position by turning the whole camera around its axis. This
position defines the zeropoint of the P-meter.
The true angle cross-hair/horizon on the instant of exposure is obtained by interpolaÂtion
of P-meter readings before and after the exposure. The times of the P-meter
readings and exposure are automatically registered and are accurate to 1 second.
Errors of the P-meter:
1. The axis of the spirit level is not normal to the rotation axis
of the angle measuring device. - If the rotation axis was horizontal during calibration,
the correction is proportional to tan(elevation). By reading the P-meter in the normal
and reverse position it is possible to measure this correction directly.
2. The rotation axis of the encoder is not parallel to the optical axis of the heliograph.
- This error can be determined by reversing the telescope (if possible). There is an error
component in declination and hourangle. It is possible to determine both errors in the
laboratory.
With the P-meter the orientation of a single heliogram is known to an accuracy of
1 minute of arc.
This instrument was brought into routine operation in August 1989, to supplement
the sunspot observations for the Debrecen Photoheliographic Results.
Tube of the vacuum film holder.
|
|
