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Topics - CraigNZ

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General / Image Solver/Focal length calculation
« on: 2015 September 02 17:42:20 »
Using a sample image I get this report from Image Solver:

Referentiation Matrix (Gnomonic projection = Matrix * Coords[x,y]):
           +0.000190468       -5.53464e-008           -0.319207
          +8.53177e-008         +0.00019046           -0.241338
                     +0                  +0                  +1
Projection origin.. [1676.282351 1266.380252]pix -> [RA:+22 09 12.72 Dec:-09 51 54.40]
Resolution ........ 0.686 arcsec/pix
Rotation .......... 179.980 deg
Focal ............. 1624.44 mm
Pixel size ........ 5.40 um
Field of view ..... 38' 18.4" x 28' 56.1"
Image center ...... RA: 22 09 12.710  Dec: -09 51 54.66
Image bounds:
   top-left ....... RA: 22 07 54.907  Dec: -10 06 22.64
   top-right ...... RA: 22 10 30.546  Dec: -10 06 21.61
   bottom-left .... RA: 22 07 54.987  Dec: -09 37 26.60
   bottom-right ... RA: 22 10 30.398  Dec: -09 37 25.57

The formula for calculating FL from an image is:

FL (mm) = 206265 * pixel size (mm/pixel) / plate scale (arcsec/pixel)

I carefully measured a few star distances on the plate and found a plate scale of 0.6865 arcsec/pixel which is in agreement with Image Solver.

The camera is a SBIG STF-8300M with pixel size of 5.4 u (0.0054 m)
substituting into the above equation yields:
FL = 206265 * 0.0054 / 0.6865 = 1622.48 mm

Image Solver shows 1624.44 mm

Why the difference between the two numbers?  Both are using the same Pixel size and plate scale.
Curvature of the plate?

I have been using the trial version of PI for two weeks with no problems.  But in the last two days when I run the program I get this error.  If I close the error and try again sometimes it works okay and other times I have to try and run it three or four times before it finally works okay.

Can someone explain why I get the error and then when I try again it works okay?  Something wrong with the server or the network?


General / Open DSLR raw file shows wrong pixel values
« on: 2015 July 08 23:47:54 »
I am using a Canon 60D and acquired some RAW images at ISO 800.
I set Format Explorer|DSLR_RAW to Pure Raw.

When I open a .cr2 file (canon raw) and use Process|Image|Statistics|14 bit setting it shows the mean as 528.6 and maximum as 3826.6

When I open the same image with AIP4WIN and again use RAW with no conversion I get mean as 2114.36 and maximum as 15307.

The AIP4WIN figures are correct.  These are the raw pixel values as verified by several other programs and makes sense because Canon has a bias of about 2000 on each pixel.  The max value is also correct because a saturated pixel will be just shy of 16383 the maximum value for a 14 bit camera.

So why is PI showing a completely different value?  I am guessing that it is applying several correction factors that appear in the cr2 file supplied by the camera rather than the real raw pixels.  I believe the fault is in DCRAW and maybe some setting not being set.  Maybe a Hint setting would fix this?

The reason I bring this up is because I took images at 30 sec, 60 sec and 300 sec and they all show the same detail after calibration, debayering and integration.  The statistics report shows the same values with a very slight difference.  That means there is no point in combining the images to expand the dynamic range because they are all the same.

I believe all the pre-processing of the raw images should be done with true raw pixel values so that down the stream we have the full dynamic range available using something like HDR.

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