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Plate solving and WCS?

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Hi!

I'm a bit confused by how Ekos handles WCS in the sense of when and how the fits header of the images are updated with WCS. I do variable star observations, and up until recently, the software VPhot that I use for measurement (AAVSO online tool) recognized my images from Ekos as having useful WCS info in the fits header. However, recently, that is no longer the case. I have to manually plate solve before measuring. 

Now, I am not fully sure which information in the fits header that really is the WCS info, or if anything in this has changed. Or if this is a matter of changes in VPhot. But it would be useful, I think, to have a clear description of what Ekos does in terms of this, and what is saved in the fits header and when. Anyone could help me out with this?

Magnus
2 years 11 months ago #69984

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Magnus, do you have example images of those that work and those that don’t work? Perhaps a comparison of the FITS header will show what changed. And did you update KStars/Ekos/INDI in the mean time?


Wouter
2 years 11 months ago #69986

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Replied by Jasem Mutlaq on topic Plate solving and WCS?

WCS information is only present in the FITS files after the first solve is successful. If this is not the case, then there is a bug.
2 years 11 months ago #69993

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Hi!

OK, så here is the fits header of an image that is NOT recognized by VPhot as being plate solved (below). I can not decode if there is WCS-info in that or not - is there?

I always do alignment, either manually or via Scheduler. I typically slew to a new target, align, take a few images, then slew to another and repeat the process. So there should in principle be WCS information in all images. Right?

Magnus

SIMPLE  = T                                 / file does conform to FITS standard
BITPIX  = 16                                     / number of bits per data pixel
NAXIS   = 2                                                / number of data axes
NAXIS1  = 1677                                           / length of data axis 1
NAXIS2  = 1264                                           / length of data axis 2
EXTEND  = T                                / FITS dataset may contain extensions
COMMENT FITS (Flexible Image Transport System) format is defined in 'Astronomy
COMMENT and Astrophysics', volume 376, page 359; bibcode: 2001A&A...376..359H 
BZERO   = 32768                    / offset data range to that of unsigned short
BSCALE  = 1                                             / default scaling factor
ROWORDER= 'TOP-DOWN'                                                 / Row Order
INSTRUME= 'Atik 383L'                                                 / CCD Name
TELESCOP= 'Losmandy Gemini'                                     / Telescope name
OBSERVER= 'Unknown '                                             / Observer name
OBJECT  = 'BZ_UMa  '                                               / Object name
EXPTIME = 3.000000E+02                                 / Total Exposure Time (s)
CCD-TEMP= -2.50E+01                                  / CCD Temperature (Celsius)
PIXSIZE1= 5.400000E+00                                  / Pixel Size 1 (microns)
PIXSIZE2= 5.400000E+00                                  / Pixel Size 2 (microns)
XBINNING= 2                                            / Binning factor in width
YBINNING= 2                                           / Binning factor in height
XPIXSZ  = 1.080000E+01                          / X binned pixel size in microns
YPIXSZ  = 1.080000E+01                          / Y binned pixel size in microns
FRAME   = 'Light   '                                                / Frame Type
IMAGETYP= 'Light Frame'                                             / Frame Type
FILTER  = 'V       '                                                    / Filter
FOCALLEN= 1.76E+03                                           / Focal Length (mm)
APTDIA  = 2.79E+02                                     / Telescope diameter (mm)
SCALE   = 1.268093E+00                                       / arcsecs per pixel
SITELAT = 5.554810E+01                 / Latitude of the imaging site in degrees
SITELONG= 1.336470E+01                / Longitude of the imaging site in degrees
AIRMASS = 1.212951E+00                                                 / Airmass
OBJCTRA = ' 8 53 43.47'                               / Object J2000 RA in Hours
OBJCTDEC= '57 48 41.20'                            / Object J2000 DEC in Degrees
RA      = 1.334311E+02                              / Object J2000 RA in Degrees
DEC     = 5.781144E+01                             / Object J2000 DEC in Degrees
EQUINOX = 2000                                                         / Equinox
CRVAL1  = 1.3343110868E+02                                              / CRVAL1
CRVAL2  = 5.7811444955E+01                                              / CRVAL1
RADECSYS= 'FK5     '                                                  / RADECSYS
CTYPE1  = 'RA---TAN'                                                    / CTYPE1
CTYPE2  = 'DEC--TAN'                                                    / CTYPE2
CRPIX1  = 8.3850000000E+02                                              / CRPIX1
CRPIX2  = 6.3200000000E+02                                              / CRPIX2
SECPIX1 = 1.2680933633E+00                                             / SECPIX1
SECPIX2 = 1.2680933633E+00                                             / SECPIX2
CDELT1  = 3.5224815648E-04                                              / CDELT1
CDELT2  = 3.5224815648E-04                                              / CDELT2
CROTA1  = 2.7607910000E+02                                              / CROTA1
CROTA2  = 2.7607910000E+02                                              / CROTA2
DATE-OBS= '2021-04-12T22:57:03.102'              / UTC start date of observation
COMMENT Generated by INDI                                                     
END                                                                           

 
Last edit: 2 years 11 months ago by Magnus Larsson.
2 years 11 months ago #69998

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Yes, the WCS information is in the FITS header. You can identify it on this lines:
OBJCTRA = ' 8 53 43.47'                               / Object J2000 RA in Hours
OBJCTDEC= '57 48 41.20'                            / Object J2000 DEC in Degrees
RA      = 1.334311E+02                              / Object J2000 RA in Degrees
DEC     = 5.781144E+01                             / Object J2000 DEC in Degrees
EQUINOX = 2000                                                         / Equinox
​​​​​​
2 years 11 months ago #70009

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Hi!

OK, thanks. That one is NOT recognized by VPhot as having WCS, The one with the fits header below is - that one is OK with VPhot. Is there a difference in the WSC-sections? (there are stuff added by AstroImageJ here that I don't understand). 

Magnus

SIMPLE  = T                                      / Created by ImageJ FITS_Writer
BITPIX  = -32                                    / number of bits per data pixel
NAXIS   = 2                                                / number of data axes
NAXIS1  = 3354                                           / length of data axis 1
NAXIS2  = 2529                                           / length of data axis 2
EXTEND  = T                                / FITS dataset may contain extensions
COMMENT FITS (Flexible Image Transport System) format is defined in 'Astronomy
COMMENT and Astrophysics', volume 376, page 359; bibcode: 2001A&A...376..359H 
ROWORDER= 'TOP-DOWN'                                                 / Row Order
INSTRUME= 'Atik 383L'                                                 / CCD Name
TELESCOP= 'Losmandy Gemini'                                     / Telescope name
OBSERVER= 'Unknown '                                             / Observer name
OBJECT  = 'IY_Uma  '                                               / Object name
EXPTIME = 6.000000E+02                                 / Total Exposure Time (s)
CCD-TEMP= -3.00E+01                                  / CCD Temperature (Celsius)
PIXSIZE1= 5.400000E+00                                  / Pixel Size 1 (microns)
PIXSIZE2= 5.400000E+00                                  / Pixel Size 2 (microns)
XBINNING= 1                                            / Binning factor in width
YBINNING= 1                                           / Binning factor in height
XPIXSZ  = 5.400000E+00                          / X binned pixel size in microns
YPIXSZ  = 5.400000E+00                          / Y binned pixel size in microns
FRAME   = 'Light   '                                                / Frame Type
IMAGETYP= 'Light Frame'                                             / Frame Type
FILTER  = 'V       '                                                    / Filter
FOCALLEN= 1.76E+03                                           / Focal Length (mm)
APTDIA  = 2.79E+02                                     / Telescope diameter (mm)
SCALE   = 6.329659E-01                                       / arcsecs per pixel
SITELAT = 55.5481                           / geographic latitude of observatory
SITELONG= 13.3647                          / geographic longitude of observatory
AIRMASS = 1.0024698847910976                    / Target airmass at mid-exposure
OBJCTRA = '10 43 55.94'                               / Object J2000 RA in Hours
OBJCTDEC= '58 07 31.88'                            / Object J2000 DEC in Degrees
RA      = 1.609831E+02                              / Object J2000 RA in Degrees
DEC     = 5.812552E+01                             / Object J2000 DEC in Degrees
EQUINOX = 2000                                                         / Equinox
CRVAL1  = 1.6098310369E+02                                              / CRVAL1
CRVAL2  = 5.8125522265E+01                                              / CRVAL1
RADECSYS= 'FK5     '                                                  / RADECSYS
CTYPE1  = 'RA---TAN'                                                    / CTYPE1
CTYPE2  = 'DEC--TAN'                                                    / CTYPE2
CRPIX1  = 1.6770000000E+03                                              / CRPIX1
CRPIX2  = 1.2645000000E+03                                              / CRPIX2
SECPIX1 = 6.3296592027E-01                                             / SECPIX1
SECPIX2 = 6.3296592027E-01                                             / SECPIX2
CDELT1  = 1.7582386674E-04                                              / CDELT1
CDELT2  = 1.7582386674E-04                                              / CDELT2
CROTA1  = 8.7754800000E+01                                              / CROTA1
CROTA2  = 8.7754800000E+01                                              / CROTA2
DATE-OBS= '2021-02-14T23:42:41.269'              / UTC start date of observation
COMMENT Generated by INDI                                                     
JD_SOBS = 2459260.487977639                   / Julian Date at start of exposure
JD_UTC  = 2459260.491449861                  / Julian Date (UTC) at mid-exposure
HJD_UTC = 2459260.4953886443             / Heliocentric JD (UTC) at mid-exposure
BJD_UTC = 2459260.49622639                / Barycentric JD (TDB) at mid-exposure
ALT_OBJ = 85.97138249500703                    / Target altitude at mid-exposure
AZ_OBJ  = 49.891940098519115                    / Target azimuth at mid-exposure
HA_OBJ  = -0.38823878790103805               / Target hour angle at mid-exposure
ZD_OBJ  = 4.028617504992965             / Target zenith distance at mid-exposure
RAOBJ2K = 10.732206666666666           / J2000 right ascension of target (hours)
DECOBJ2K= 58.12551944444444              / J2000 declination of target (degrees)
RA OBJ  = 10.754719617321642             / EOD right ascension of target (hours)
DEC OBJ = 58.013652798461955               / EOD declination of target (degrees)
HISTORY Previous Filename = IY Uma_Light_V_600_secs_2021-02-15T00-52-52_001.  
HISTORY ... fits                                                              
HISTORY Bias corrected with ATIK2_BIAS_median_2021-01-18_bin1.fits            
HISTORY Dark corrected with ATIK2_DARK_2020-11-27_300_sec_median_bin1.fits    
HISTORY and exposure time scaling factor = 2.0                                
HISTORY Flat corrected with ATIK2_FLAT_2021-02-15_C11_V_mflat_bin1.fits       
END                                                                           
2 years 11 months ago #70013

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Replied by Richard Beck on topic Plate solving and WCS?

Two possibilites looking at your headers:
1) It may be that VPhot isn't handling the blank in OBJCTRA = ' 8 53 43.47'.
2) The image that worked had the fields RAOBJ2K and DECOBJ2K

Do you have any other single digit RA images to check if the work with VPhot? If they others work, look at the fields I highlighted to see the difference.
2 years 11 months ago #70019

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Hi!

I have now found out that VPhot looks for the field CD1_1 to determine whether an image is plate solved or not. And my images from Ekos does not contain this field. 

Am I missing something, or is this the way it is supposed to be? Could I do something to make the field CD1_1 (along with the data and subsequent fields) appear in the header?

Magnus
2 years 11 months ago #70331

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Replied by Hy Murveit on topic Plate solving and WCS?

Magnus,

I've recently run across this too, in looking at polar alignment issues. I believe (but am not positive) that CD1_1, CD1_1, CD1_1 & CD1_1 are the elements of a 2-D matrix which defines the angle of rotation and pixel scale of the image. They are the "modern" way of doing this (i.e. as of a long time ago) but we seem to use the previous setup, where we use older versions of the keywords (that should still be respected).
It seems that we use CDELT1, CDELT2, CROTA1, CROTA2.

If you research the proper values of CD* given CDELT1,2 CROTA1,2, then I'd be happy to try and submit a merge request adding those keywords.
You might start with www.atnf.csiro.au/people/mcalabre/WCS/ccs.pdf

Hy
2 years 11 months ago #70348

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Hi!

Well. Not trivial at least. 

However: I plate solve my images, after downloading, with ASTAP. But I also use ASTAP for aligning in Ekos. Could not the same process that produce the correct fits headers in the later plate solve process be used in Ekos, as it is (at least as an option) the same program doing the plate solve?

Magnus
2 years 11 months ago #70360

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