Saturday, January 4, 2014

Legacy Series, how to analyse long exposure AZ data




Analysing Data Recorded with Azimuthal Setups

It is quite obvious that an azimuthal mount causes field rotation, when not used on the North- or the South-pole. For real long exposures (i.e. minutes to hours) this is utterly devastating for every image taken.
Here the abilities and power of CCDs are coming into play. With exposure times of a couple of seconds, field rotation does not play any role for the single frame. As long as the CCD amplifier ensures that the charge in a single pixel is high enough to result in a signal greater than the detection threshold we can integrate over several different frames. Now field rotation will return when not properly taken care of .

IRIS however is capable to compensate for rotation between individual frames during alignment. Lets see what is to be done, on a step by step basis, assuming the raw fits series (after conversion) is called r#.fits (for red), g#.fits (for green) and b#.fits (for blue). Furthermore assume that the data set contains 50 frames.
  • Get a “Display commands window” first.
  • If not using IRIS in the first place you most likely have to convert your AVI-file into a plurality of FITS-files. When going for color, every frame will be present in a red, a green and a blue channel in separate FITS-files. That means, that you will have to perform all following steps on a respective color files individually. Conversion done by: [click: File -> AVI conversion... ].
  • Now you will have to select two suitable objects (stars) by marking.
    For this you have to get one frame into the main window, preferably the first of a series: [type: load r1].
    Now mark two medium-bright stars which are not too close to each other: [click: Analysis -> Select Objects => with the funny mouse pointer click on two medium-bright stars].
  • If everything went fine so far, you will be ready for registering the frames yet: [type: rregister r rr 100 50].
    A new file-set with the name rr will be created. Forget about the third parameter for the moment, you will learn to use it with some practice.
    With the time it will become clearer which registration objects will give better results and what combination will not be so good.
  • Stacking registered frames, nothing easier than this: [type: add2 rr 50].
  • Now you certainly would like to save the result: [type: save red].
  • Do the same with the other two colors...
  • To combine the three images you could do all sorts of tricks with IRIS. A simple first glance could be: [type: trichro red green blue].
    Save your result by: [type: savebmp myimage].
Have a look at an example analysis done on M52 data. Both results came from the same data set, analysed in different ways.





M52, ETX-70, analysed with K3CCDTools, dark frame subtracted
M52, ETX-70, analysed as described above, no dark frame subtraction



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