WCS Plate solving status question

British Astronomical Association, Variable Star Section (BAA-VSS)
Thu, 11/09/2023 - 22:39

The WCS column in the Image list.

This Plate Solving, is this something I have to actively do as part of image calibration?

I don't see anything written in the FITS file header.

How do I ensure my raw images comply please?


American Association of Variable Star Observers (AAVSO)


You have seen the two buttons in the VPhot image list. CAL indicates that you have conducted image calibration. WCS indicates that you have plate-solved your images. They are two separate processes that are not related so they are conducted separately, although some software will do both.

Calibration corrects your raw fits images for read noise (bias), thermal noise (dark) and pixel non-uniformity (flat). For VPhot, the CALSTAT=BDF header should be present. Some software does not include this header but may note that calibration was conducted in a HISTORY header. IF you know that your images have been calibrated you can ignore the red CAL button in VPhot due to the missing CALSTAT header.

Plate-solving maps the camera xy pixel location to RA/DEC location on the image so that photometric software, like VPhot, can identify where to place your targets and comps. This process requires initially comparing your image to a star catalog (e.g., GSC11, UCAC4). PinPoint is used by VPhot to conduct this process. There are many headers that are generated to allow this mapping to be applied to your image. They may include headers like :

PA          = 6.77389907785E-002 / [deg, 0-360 CCW] Position angle of plate       
CTYPE1      = 'RA---TAN'           / X-axis coordinate type                         
CRVAL1      = 2.94430802576E+001 / X-axis coordinate value                        
CRPIX1      = 7.68000000000E+002 / X-axis reference pixel                         
CDELT1      = -4.02917964181E-004 / [deg/pixel] X-axis plate scale                 
CROTA1      = -6.77389907785E-002 / [deg] Roll angle wrt X-axis                    
CTYPE2      = 'DEC--TAN'           / Y-axis coordinate type                         
CRVAL2      = 3.80731226497E+001 / Y-axis coordinate value                        
CRPIX2      = 5.12000000000E+002 / Y-axis reference pixel                         
CDELT2      = -4.02665768279E-004 / [deg/pixel] Y-Axis Plate scale                 
CROTA2      = -6.77389907785E-002 / [deg] Roll angle wrt Y-axis                    
CD1_1       = -4.02917682590E-004 / Change in RA---TAN along X-Axis                
CD1_2       = -4.76058911177E-007 / Change in RA---TAN along Y-Axis                
CD2_1       = 4.76357074358E-007 / Change in DEC--TAN along X-Axis                
CD2_2       = -4.02665486864E-004 / Change in DEC--TAN along Y-Axis                
TR1_0       = 7.68000123138E+002 / [private] X-axis distortion coefficients       
TR1_1       = 1.53599999457E+003                                                  
TR1_2       = 1.25820152088E-001                                                  
TR1_3       = -9.12923231438E-002                                                  
TR1_4       = -1.60647584151E+000                                                  
TR1_5       = 8.26964083014E-001                                                  
TR1_6       = -3.00730532799E+000                                                  
TR1_7       = 1.15751238146E+000                                                  
TR1_8       = -1.16495392245E+000                                                  
TR1_9       = 3.90705116977E-001                                                  
TR1_10      = 1.18341257771E+000                                                  
TR1_11      = 6.99113356668E+000                                                  
TR1_12      = -6.44961802131E+000                                                  
TR1_13      = 6.25652602705E+000                                                  
TR1_14      = -1.08834634459E+000                                                  
TR2_0       = 5.11999999503E+002 / [private] Y-axis distortion coefficients       
TR2_1       = 1.71077261173E-003                                                  
TR2_2       = 1.02400000042E+003                                                  
TR2_3       = 3.45866336714E+000                                                  
TR2_4       = 9.13988125996E-001                                                  
TR2_5       = -4.56716140393E-001                                                  
TR2_6       = 1.10214592009E+000                                                  
TR2_7       = -9.01809200186E-001                                                  
TR2_8       = -5.91446733765E-001                                                  
TR2_9       = 1.11267609190E+000                                                  
TR2_10      = -1.31065909807E+001                                                  
TR2_11      = -1.21629295435E+000                                                  
TR2_12      = -4.92837565453E+000                                                  
TR2_13      = -8.95082678646E+000                                                  
TR2_14      = 1.87392816068E+000                                                  
HISTORY       WCS added by PinPoint 7.0.0 at 2023-10-28T06:03:16                     
HISTORY       Matched 162 stars from the Gray GSC-ACT Catalog                        
HISTORY       Average residual was 0.39 arc-seconds                                  
PLTSOLVD    = T / Plate has been solved by PinPoint

HTH, Ken

British Astronomical Association, Variable Star Section (BAA-VSS)

Thanks Ken,

Yes that helps.

You may have followed on other posts that I am using ASTAP after having played with AstroImageJ, AIP4Windows and several other packages.

I found the others overwhelming in terms of the parameters needed to enter (many default to some standard value).

ASTAP also does have an entry for CALSTAT.

It also informs users that they need to install a star catalogue (which I haven't done yet and the calibration worked fine)

Independent processes as you said. I just tried to Plate Solve and bingo it prompted the need for a star catalogue.

I thought there might be something in a FIT header that indicated a plate solved image, but nothing jumps out.