Copy position to clipboard in degrees

[MalcolmP]

Can this be set to be the epoch(equinox?) of the observation instead of J2000 ?
thanks

[han.k]

The epoch is always the time of observation. Moving stars/asteroids are at position of the epoch of the observation.

So for example the Tycho star database is in J2000, epoch 1991 because that was the time the satellite took the observation and moving stars are in position 1991.

The equinox so the coordinate system is in J2000 because the database is J2000. When you get the positions of stars and deepsky object it is also always J2000 otherwise the positions would be outdated after one day.

The sky is pretty static. The only thing what changes is the wobbeling axis of the Earth but that is
only relevant for telescopes following the static sky. Why would want the object position in JNow?

Cheers, Han

[MalcolmP]

Thanks Han,
Yes, precession is my problem !
Why do I want to, well it is a long story ! :)

I am using ASTAP with Stellarium as a "push-to" system with a DSLR and 135mm telephoto lens on a pan/tilt tripod. There is not always a bright known star visible in the lcd screen and I easily get lost !
So, I transfer an image to my pc and solve it, rightclick on a star or near a group of stars ( it is amazing how astap can pull stars out of a featureless sky due to twilight or thin clouds ! )
Then I give that position -(I modify the astap string into a format acceptable to stell. using a small javascript routine that i have made)- to stellarium using a core.move command which takes the display near to but not on the star. By guesswork I found that if I set stellarium to near 2000 it all came good :) Vice versa, if I first precess the given location to 2024.7, using Mel Bartell's online calculator, I can leave stellarium at the current time.
Sorry, it is a long story ! there are work-rounds like giving stell. a nearby found ngc or similar deepsky object. This works better, quicker, than stars which are not always identified or identifiable with "simbad star annotation"
cheers,
Malcolm.

[han.k]

You could convert the J2000 position to Jnow in a few lines of code.

Note there is also a set target option for push-to systems:

Success, Han

[MalcolmP]

Ha, you have been looking over my shoulder ! It all started as a good reason to learn javascript, a few weeks later and much hair-pulling over Meeuse' Astro Algorithms and I have a version of his 'simple' method which is off as well, or I have made a mistake ! I am not sure if a few lines (and my brain cell) will be enough to do the rigorous method !
Thanks for the hint about Target, I'll give that a try.

[han.k]

Here a simple procedure in Pascal. For jd (Julian day) you could fill a fixed value something like 25 {years since 2000) x 365.25+ 2451545

procedure precession(jd, ra1,dec1 : double; out ra2,dec2 : double); {precession correction, new Meeus chapter precession formula 20.1}
var
t,dra,ddec,m,n,n2 : double;
begin
t:=(jd-2451545)/36525; {time in julian centuries since j2000 }
m:=3.07496+0.00186t;{seconds}
n:=1.33621-0.00057t; {seconds}
n2:=20.0431-0.0085t;{arcsec}
dra:=(m + n sin(ra1)tan(dec1))pi/(360012);{yearly ra drift in radians}
ddec:=n2cos(ra1)pi/(3600180); {yearly dec drift in radians}
ra2:=ra1+(drat100);{multiply with number of years is t100}
dec2:=dec1+(ddect*100);
end;

[han.k]

The forum remove stars. Attached the same as a text file:
Again, now correct

[MalcolmP]

Thank you Han, that is very neat and compact !
I will have a go at translating it into JS and compare with my version of that chapter.
Seeing your "double" in there reminds me to check up on the precision of variables in JS.

many thanks,
Malcolm.

[han.k]

Double is nothing special. It is a "8 byte float" and standard for 64 bit cpu's .

[MalcolmP]

Update : I have suspended my efforts to fix my precession problems by installing HNSKY instead !

The "move-to" in HNSKY is excellent and enables the quick and simple entry of the solved position from ASTAP with just one paste and a click to go there, easy peasy :) thank you Han for both softwares.

cheers,
Malcolm.