PinPoint -vs- PlateSolve2


#1

I have been experimenting with PlateSolve2 and noticed an interesting discrepancy between it and PinPoint. A PinPoint solve of a plate produce a position angle of 272 degrees. When I solved the same plate with PlateSolve2, it reported 88 degrees. The 272 value is confirmed by Astrometry.net. RA and DEC values are nearly identical.

Testing with several other subs from different targets, I would see similar differences. Not sure why there would be these differences. Both products seem to be in wide spread use and I would expect both to report accurate results.

cm


#2

You have to report angle with reference to something. Each is simply referring to a different point. We standardize around PP. PS2 is using 360-angle.


#3

This brings up an interesting point – when PinPoint or Astrometry.net reports that a plate’s position angle is xx.x degrees, what is it actually telling us?

When looking at the FOV indicator of my QSI683wsg in TheSky6, it is shown with the short axis of the KAF-8300 chip as being aligned north-south and the long axis as east-west with the guide chip at the 180 degree position. This orientation agrees with what PinPoint and Astrometry.net report on my solved subs. Is that just a coincidence? Does TheSky6 just arbitrarily assign the short axis of the imaging chip to the north-south direction and then PinPoint looks at the short dimension of the image and also assigns it to the north-south direction?


#4

More info on my own question.

If you face south and look at a point in the sky on the equator, the celestial coordinate system is defined as north “up” (0 degrees); south “down” (180 degrees); east “left” (90 degrees) and west “right” (270 degrees). This is 0 to 360 degrees moving in a counter clockwise direction. A magnetic compass is 0 to 360 degrees moving in a clockwise direction.

Looking at the plate solve info from PinPoint:

WCS: Roll = 88.01 HScale = -0.549 VScale = -0.549
PA = 271.990°

PinPoint (and Astrometry.net) are reporting the orientation of the frame as rotated 272 degrees from north. The World Coordinate System is reporting the frame is rotated 88 degrees from north. PlateSolve2 also reports the frame as rotated 88 degrees. So we have a mix of position angles being reported as CCW from north or CW from north without an indication of which it is. At this point one assumes there is a standard.

While 88 degrees CW is the “same” position as 272 CCW, the 272 degree rotation is up-side-down compared to the 88 degree rotation. Perhaps this is the intent – since APOs, SCTs and similar OTAs invert the image, reporting a frame as rotated by 272 degrees makes it match the non inverted image as would be shown by a product like TheSky6 and other planetarium packages.

The bottom line is that mixing PinPoint / Astrometry.net with PlateSolve2 / WCS will produce conflicting results unless SGP understands that and reports PlateSolve2 position angles converted to conform with PinPoint and Astrometry.net.

The other question is what goes into the FITS header? I would have thought it would be the WCS value but it looks like my SGP images are getting the PinPoint value.

cm


#5

SGPro is aware of this and will force other solvers to conform to the same angle PP produces.


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