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FAQs about the Kuhn Telescope Images


Using these images for personal purposes

All Kuhn Telescope images may be used freely provided that such use is NOT FOR PROFIT and that credit is given to the Orange County Astronomers. The OCA provides these images as part of our continuing efforts to promote public awareness and understanding of astronomy.

How these images are taken

Click here for a detailed explanation.

Length of the exposures

The exposure length varies depending on the subject. For faint, deep-sky objects (eg galaxies), the typical time is 60 seconds.

For bright nebulae and star clusters, it can be anywhere from 5 seconds and up, again depending on the brightness.

With very bright objects, such as planets and the moon, the exposures drop to very small fractions of a second (1 millisecond or less).

Format used for these images

Unless otherwise stated, all images in the Image Gallery are in JPEG format. JPEG supports image compression and results in relatively small file sizes. These images are generally between 10KB and 30KB.

Original data format

The images in this Gallery are not the original files taken by our CCD camera. The original files are in a 12-bit proprietary format known as IMG. (Note: there are several other image file formats that also bear the name "IMG"; our IMG format is unrelated to any of these.)

Availability of the original data

At present, our original data files, which currently number in the thousands, are not available to the public. However, we would like to start making them available -- but it all depends on our work loads (we're a volunteer group, and always looking for more able bodies!). Stay tuned for future developments!

How these images are processed

The raw IMG files are first converted to FIT (Flexible Image Transport) format. FIT (or FITS) is a fairly universal format for astronomical data, and can generally be read by any software application designed to process astronomical images. This conversion is done with a C language utility program (written by OCA/EOA member Mike Silveus). Once the FIT file is created, it is processed with SkyPro software to stretch and/or scale the data to bring out detail. For a composite image, SkyPro is also used to register and add together the constituent images. In either case, the result is saved to a GIF file. Further processing is done using commercial software such as PhotoShop or Picture Publisher. The sophisticated image enhancement capabilities of these applications are used to bring out further detail. The dynamic range of the image may also be further adjusted. Last, the result is saved to a JPEG file, which further compresses the data.

Scientific value of these images

Since these images are the result of compressing and processing the original data, they cannot be considered to be a reliable source of scientific data.

Artifacts: dark halos, stars with black centers, crosses/spikes around stars; uneven image brightness

Dark "halos": An artifact often seen as a result of unsharp masking, an enhancement technique used to bring out fine detail. Simply put, unsharp masking sharpens detail and exaggerates contrast. The edge of a star (white) against the sky (almost black) has high contrast to begin with; unsharp masking exaggerates this difference, so that the area immediately around the star looks like a pure black "halo".

Stars with black centers: While moderate use of unsharp masking can cause dark halos around stars, extreme use can blacken the centers of stars. To see a demonstration of what excessive unsharp masking can do, click here.

Crosses/Spikes: This is an effect known as diffraction spikes. Normally, this effect is barely noticeable in an image. However, sharpening (especially unsharp masking) tends to exaggerate the effect.

Uneven image brightness: If the central part of the image appears brighter than the edges, this is due to vignetting, explained here. If there is a brightening towards one edge or corner of the image, this is due to an external source of light hitting the CCD camera (as from a red LED) during the exposure.


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