Re: Question about ASTAP plate solve

Dale Ghent

On Sep 24, 2020, at 7:08 PM, uncarollo2 <> via <> wrote:

Ok, thanks. Just trying to understand how some of these programs interface with a mount. Apparently this one does not, directly.

I have used Maxim's Pinpoint to analyze an image and it gives very quick response. It appears to be a simple and fast way to see where your image is pointed in the sky. There are two problems with this simple program. The output is in the form of a number that appears on the screen, and since the mount doesn't have any eyeballs, it cannot make use of it (mounts can't read). While the Dec accuracy appears to the 1 arc second resolution, the RA is limited to 1 second of time, which is 15 arc seconds. This unfortunately is useless for knowing precisely where the image is pointing to arc sec levels.
Ok, I see. The important distinction here is that the RA that that is *displayed* is of a lower resolution than what it *solves*. Getting a more precise display of RA coordinates is basically just asking Han Kleijn to display the RA with fractional seconds out to whatever precision is desired. I'm sure he would oblige and I can make that request for you if you'd like. But if you want to get that info right now? You'll need to use the information that ASTAP generates in the image header.

So here's one example of the solve info that ASTAP generates in the form of WCS (World Coordinate System) FITS keywords, taken from an imaging session I did a few nights ago:

CTYPE1 = 'RA---TAN' / first parameter RA , projection TANgential
CTYPE2 = 'DEC--TAN' / second parameter DEC, projection TANgential
CUNIT1 = 'deg ' / Unit of coordinates
CRPIX1 = 4.788500000000E+003 / X of reference pixel
CRPIX2 = 3.194500000000E+003 / Y of reference pixel
CRVAL1 = 3.179473196381E+002 / RA of reference pixel (deg)
CRVAL2 = 5.995875589269E+001 / DEC of reference pixel (deg)
CDELT1 = 3.657940337146E-004 / X pixel size (deg)
CDELT2 = 3.658289760477E-004 / Y pixel size (deg)
CROTA1 = -8.826178511599E+001 / Image twist of X axis (deg)
CROTA2 = -8.825277468135E+001 / Image twist of Y axis (deg)
CD1_1 = 1.115309975310E-005 / CD matrix to convert (x,y) to (Ra, Dec)
CD1_2 = -3.656606402582E-004 / CD matrix to convert (x,y) to (Ra, Dec)
CD2_1 = 3.656239645690E-004 / CD matrix to convert (x,y) to (Ra, Dec)
CD2_2 = 1.109666085332E-005 / CD matrix to convert (x,y) to (Ra, Dec)
PLTSOLVD= T / ASTAP internal solver
COMMENT 6 Solved in 0.5 sec. Offset was 0.009 deg.

These are standardized keywords that pretty much any solver out there generates, and the standard can be found here:

So what do you look for in that mess of numbers? For simplicity's sake and since we just want RA and declination of the center pixel of the image, we look to the CRVAL1 and CRVAL2 keywords, which are RA and declination respectively. The units for both are decimal degrees. Naturally, the decimal degrees for RA will need to be converted to HH:MM:SS format if that's what is desired. J2000 is assumed.

CRVAL1 has a value of 3.179473196381E+002, or 317.9473196381 degrees RA.
CRVAL2 has a value of 5.995875589269E+001, or 59.95875589269 degrees declination.

If you don't have a converter handy for converting RA decimal degrees to H:M:S format, just pop the value of CRVAL1 into Wolfram Alpha:

The answer: 21 hours, 11 minutes, 47.3567131 seconds RA. Arc-second precision out to 7 decimal places. And, yes, I was imaging SH2-129.


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