Polar alignment of equatorial mountings with a camera and a spreadsheet

By Juergen Kahlhoefer

Introduction

This new method for polar alignment of equatorial mountings is based on a star trail image taken with a turn of the camera. [1, 2] A spreadsheet has been developed for the evaluation of the images. Input data are the rectangular coordinates of three stars on the image at both ends of their trails, the right ascension and declination of the stars, the longitude and latitude of the observatory, and the date and time the image was taken. Results are the necessary corrections of azimuth and polar altitude in degrees. These can be converted into the necessary number of turns of the adjustment screws.  

The spreadsheet and detailed instructions are available free at the download link:

            Download:    Polar alignment with Excel (140)

 

Features of this method

The method is quantitative and exact. In the calculation, an accuracy of 1 minute of arc is attainable. Altitude is corrected for the influence of atmospheric refraction. The method is applicable both in the northern and southern hemisphere. And it is safe, because faulty coordinates or false identification of a star will produce an error warning.

 

Necessary software

A spreadsheet program (Microsoft Excel or Open Office) is absolutely necessary. The spreadsheet has been developed in Open Office and converted to Excel format

 An image processing program (notably Fitswork) is recommended for the measurement of the coordinates of the stars.

 A planetarium program (e. g. Stellarium) helps with the identification of the stars and serves as a source for right ascension and declination values. The data for a small selection of stars are already provided in the sheet.

 

Photography

A digital camera with a standard lens or a short telephoto lens is fixed near the front end of the telescope tube or on the mounting and directed at the celestial pole. Then a star trail image is made with rotation of the camera. The beginning and the end of the star trails must be clearly marked with a few seconds of static exposure.

 

Evaluation

On the star trail image, the rectangular X and Y coordinates of 3 selected stars are measured at both ends of their trails and entered into the spreadsheet. (X1, Y1 and X2, Y2 for each star) The calculation in the spreadsheet program compares the triangle of these three stars on the image to their corresponding triangle in the sky, calculated from their right ascension and declination data. Results are the necessary corrections of azimuth and polar altitude.

 

Literature:

[1] Luethen, Hartwig: Scheinern war gestern. Sternkieker 43: 109-110, (2006)

[2] Luethen, Hartwig: http:\www.gva-hamburg.de\scheinernistout.htm

 

Download:         Polar alignment with Excel (140)