There is an easier method to obtain this angle if you assume that your optical center point is co-located with the Zenith point i.e. SkySafari for example lets you do this quite easily. You then have to identify and mark the center and reference point from the line above and readout the angle value. Open this application and set the location, date and time to the location, date and time the reference image was taken (if you have the FITS image file you can read this from the FITS header). You basically have to use an astronomical application which allows you to measure the sky angle between two points or stars. The angle is a bit more difficult to obtain. The distance is easy the get by using a measuring tool in the image manipulation application such as GIMP: For each of these point I then obtain the angle and distance from the center. I then mark a a number of points on that line which correspond to nearby stars that I can identify in an astronomical application. There are many way s to do this but I found it easiest to draw a line from the center to the edge which passes through a series of well defined stars. You can then open the image in an image manipulation program to obtain values. It's probably easiest to work with a jpg or png image, you can get such an image either through the 'Latest Image' function or through the 'image conversion function'. The values of pixel distance and angle need to be obtained from an image you have taken with your setup. The format of the string that needs to be set in the general settings is to separate the values in a pair with a comma and to separate the value pairs with a semicolon: AnglePointOne,PixelDistancePointOne AnglePointTwo,PixelDistancePointTwo AnglePointThree,PixelDistancePointThree and so onįor example the values I have determine will result in the following string:Ġ,0 9.7,97 22.3,216 42.5,403 55.5,515 69.8,635 The basic idea is that you define a string which contains pairs of values which describe a point on the image where: Because the application is used by many differing imaging systems it would be difficult to define a mathematical function to describe this relationship so I have instead opted to calculate this by measurements and interpolation. It basically describes how your optics project the incoming light onto the camera sensor. The series of points set here will tell the application how to convert the altitude of an object into number of pixels from the zenith point. Image Optics Transfer Function Points String If you find a good way please let me know! These values (x und y from the top left corner) should then be entered as 'Image Optical Center X' and 'Image Optical Center X' in the general settings section.įor setups that do not show the image circle it might be more difficult to work out where the optical center is.Maybe it might be possible to mark a point on the Perspex or adjust the lens to see some of the image circle. Hovering over the center point allows me to readout the x and y coordinates of that point at the bottom of the GIMP window. For example I have used Gimp to draw a rectangle into the shown imaging circle and used the corner diagonals to find the center point (there are various methods of doing this). Here you can estimate the center by trying to find the center of this circle as accurately as you can.įinding the optical center is probably easiest by using an image manipulation program which allows the readout of x and y coordinates of selected pixels. This will be easier for 180 degree lenses where the entire sky and surrounding edges are shown on the image. To roughly work out the optical image center, ideally you will have some sort of indication of where the image circle is on the image. So it is necessary to work out the x and y coordinates of the optical image center (measured from the top left corner) to allow the app to place the overlay correctly. For some cameras this might be close, for others it could be way off, especially if they are full 180 degree lenses which leave a lot of space surrounding the actual image. In an ideal world the optical center of the lens would coincide with the center of the camera chip and therefore the center of the created image. In GIMP the angle is give against a horizontal line so if we use this we need to calculate 90 minus the angle that is displayed i.e. This gives me a popup box which shows me the length in pixels as well as the angle. In my example below I have used the GIMP measure tool to draw this line.
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