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too large density for photo mode
03-09-2008, 14:58
Post: #1
too large density for photo mode
Hi,

i've to search for dark nebulas in the taurus-auriga-region.
For this, I use the images from DSS Sky Survey.
I want to use SExtractor on this images with the Photo Mode, but if I start the process, I get this error: "The density range of this image is too large for photo mode".
I would be glad if someone could help me which value I have to change to get rid of this image.

Greetings,
Julian
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03-13-2008, 14:23 (This post was last modified: 03-13-2008 14:24 by Emmanuel Bertin.)
Post: #2
RE: too large density for photo mode
Hi Julian

The PHOTO mode expects input images to be in photographic densities (which does not have to be ANSI). As explains in page 36 of the SExtractor manual, these densities are exponentiated before being integrated in flux and centroid measurements. Being used as an exponent means they have to be sufficiently small, of the order of a few units (photographic densities are typically measured in the range 0.-5.). I suspect that DSS pixel data are not rescaled and represent straight ADC output values in the range 0 - 2^N-1. Is it the case? If yes you should adjust the FITS BSCALE header parameter to keep image data within a reasonable range.
- Emmanuel.
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03-15-2008, 10:40
Post: #3
RE: too large density for photo mode
Hi all,
I use SExtractor with scanned photographic plates in the CCD mode. IMHO, there is nothing to afraid of. I correct for the non-liner magnitude scale later...
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07-22-2016, 16:10
Post: #4
RE: too large density for photo mode
Hi,

I have the same problem as Julian, but changing my BSCALE to a low number didn't quite fix it. I am using plates from the POSS-I survey.
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