Friday, November 26, 2010
Framing the Image
If you are new to astronomical imaging, there are a few tricks that can make your life easier. One of these is being able to work out the orientation of the imaging camera’s (CCD device) field of view. For many objects, we don’t particularly care. The object is centered in the field of view and so long as it is not to big (so that it overflows the imaging space) it will be fine.
But if you want to make a mosaic of an object over a larger area, or if you want to image two interesting objects which aren't close to the center of the FOV, then the orientation of the CCD imaging device is important.
Image of comet 103P Hartley near Mel 71 taken with the global-rent-a-scope G14 instrument, click to embiggen to compare the chart above with the actual image.
If the CCD imager is square, like the STEREO spacecraft's H1A imager, no problem, but if the imager is significantly rectangular, like the GRAS 14 instrument at Global Rent a Scope which has a field of view (FOV) of 155.8 x 233.7 arc minutes, or my Philips ToUCam webcam (roughly 20x 10 arc minutes) then you have to be aware of the orientation of the FOV, or your image won't include what you want.
With my web cam it's fairly simple, I can just grab it and rotate it until I have the desired FOV orientation (and since I almost always do planetary imaging with it and have live preview, this is fairly easy to do, except when I was imaging Vesta). With remote scopes it's a bit trial and error, and you may have to do a trial shot to see which way the CCD FOV is oriented. For example, with global-rent-a-scope the imager G14 FOV long axis is oriented North-South, while the G5 imager FOV long axis is oriented East-West (with regard to the star chart orientation, see chart and image above).
Useful information, but what to do with it? Well, all decent planetarium programs can place CCD frame outlines on the maps they have drawn (for example, in SkyMap the CCD frame tool is in the Insert | Camera/CCD frame menu item, in the freeware Cartes du Ciel it's Chart | Lines / Grid | Show Mark). Most will have predefined frames, but will also allow you to draw your own CCD frames. In SkyMap I've created my own frames fo the STEREO H1A imager, my webcam and GRAS12 and GRAS5. For SkyMap you have to convert the arcminute FOV widths in the GRAS website to degrees, minutes and seconds if you are using the GRAS scopes.
All you have to do is get the program to draw the frame for you, and now you can work out what to image.
Well, not quite. There's a few quirks along the way which you have to watch out for. In SkyMap for instance, if you have the screen in RA/Dec mode, the frames will be in the correct orientation for G5, but wrong for G12. To get frames in line for G12, you have to labouriously hand rotate the screen so that the North-South axis runs left-right, then the first frame you put down will have the correct orientation (all subsequent ones will be in the WRONG orientation though, unless you revert to RA/Dec mode, then the subsequent frames follow the first frame orientation ... don't ask me why).