Alt Az from WCS

[tano...@gmail.com]

Hi and happy new year to everyone

I'd like to know if it is possible to compute azimuth and elevation from wcs info. 

I try to be clearer. I have many images acquired form the same sensor which is fixed (it is not installed on a telescope mount but on a simple trippod), so it observes always the same sky area. This means that each pixel corresponds to the same azimuth and elevation for each image. Now, if I solve the images I have the relation between pixel and celestial coordinates through the wcs. 

Can I use the wcs  info, such as image center, rotation matrix, field rotation angle and so on, to compute directly az el without passing through ra dec?

Regarding the field rotation angle: what does it represent? Is it the azimuth?

Thanks for the support

Cheers

Gaetano

[Michael Roberts]

Hi Gaetano,

You’ll need to know your geographic coordinates, WCS is purely a transform by mapping pixels to equatorial coordinates. You can’t ever know altitude and azimuth without knowing something about the observer.

Michael

On 10 Jan 2025, at 08:25, tano...@gmail.com <tano...@gmail.com> wrote:

Hi and happy new year to everyone

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[Gaetano Zarcone]

HI Michael,

thanks for the reply.

I know the observer position (latitude, longitude and height). 

So, If I understood correctly, I need to pass through ra dec to compute az el because the WCS mapps only in equatorial coordinates. Right?

Other questions. 

I have my coordinates ra dec in J2000 epoch. Now, I want to convert these coordinates to az el. Can I do this directly or before I have to correct the equinox, passing from J2000 to Jnow, and then convert to az el?

Does Az el depends on the epoch? I don't think so.

Thanks

[CaptAndy Sir]

Need time as well.

[Akarsh Simha]

In my understanding, any WCS will store the coordinates at the J2000 epoch. You can use astropy to easily convert from ICRS to Alt/Az for your observer and it'll take care of everything. There is an example on this website: https://astropy4cambridge.readthedocs.io/en/latest/_static/Astropy%20-%20Celestial%20Coordinates.html

If you want to write your own code from scratch, you'll need to calculate the precession matrix from J2000 to the current epoch, apply the precession conversion, then calculate the local sidereal time (LST) and use the standard formulas to convert from Hour Angle/Dec to Alt/Az. Depending on what you're trying to do, you may wish to include corrections for proper motion, nutation, parallax, aberration, figure of earth, refraction whatnot.

Regards

Akarsh

[Gaetano Zarcone]

Ok

Thanks for the support

Gaetano

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[Burger Oosthuizen]

Good day, I am new to all of this and am trying to understand how to calculate the Hour angle that appears in Stellarium with regards to the RA hours and minutes that appear in the plate solved image when I use Astrometry.net plate solver. My hour angle in stellarium is about 10 or so hours ahead here in New Zealand. Can someone help me?

THANK YOU!