Seconds

[Tony Cooper]

Right Ascension is hours:minutes: seconds

Declination is degrees:minutes: seconds

Angles are degrees:arcminutes: arcseconds (sky-linear by definition)

15 seconds of RA is 1 arcsecond (at the equator)

1 second of Dec is 1 arcsecond (anywhere)

Easy so far, now down to the nitty-gritty

If D is the declination then

RA/cos(D)  is 1 arcsecond sky-linear  (at Declination D)

So the question is: PHD2 just considers correction in sky-linear arcsec and isn't concerned with RA seconds or Dec seconds right? If we want to do, say, comet tracking we need to enter into PHD2 correction of arcsec per hour. 

So if we go to JPL Horizons which gives us RA and Dec corrections we need to convert those to PHD2 arcsec per hour. Right?

Dec is easy - 1 second = 1 PHD2 arcsec. 

But RA is tricky. JPL gives us "d(RA)/dt is multiplied by the cosine of declination to provide a linear rate in the plane-of-sky." So I would expect to enter the JPL values into PHD2 because the tracking history is sky-linear (right?)  So corrections should be sky-linear (right?). But PHD2 appears to want non-linear values here - it appears to want us to divide the JPL values by cos(D). Now I have a headache. The only agreement between JPL and PHD2 is that they both use arcsec / hour.

PHD2 documentation: "If you are getting the rates from the MinorPlanetCenter web site, you should choose the option for 'Separate RA and Declination coordinate motions'.  PHD2 will automatically adjust the rates to compute the apparent motions in the sky." I couldn't find anything useful at https://www.minorplanetcenter.net/. It may be there but I couldn't find it. I use JPL.

[Patrick Chevalley]

The more simple is probably to follow the PHD2 documentation, here the detail.

When you are at https://www.minorplanetcenter.net , click Observer  then  Ephemeris Service.

Type the object name in the big box on top.

Scroll down to the middle of the page, there is the checkbox "Separate R.A. and Decl coordinate motions".

Click the button "Get ephemerides/HTML page".

What you need is the in the last two column.

Patrick

[Tony Cooper]

Thanks Patrick. Useful to know. Still one question left which I will biol down to: does PHD2 multiply the RA values by cos(D)? I guess it does. That's what " PHD2 will automatically adjust the rates to compute the apparent motions in the sky" means.

[Patrick Chevalley]

Yes PHD2 do the multiplication by cos(Dec), that's why the "coordinate motions" is need, not "sky motion".

Patrick

[Bruce Waddington]

Patrick’s advice will be your best bet.  However, you’ve got your math reversed here in the first part of the message.  1 hour of RA (time) equates to 15 degrees of movement.  So 1 second of RA (time) means 15 arc-sec of movement.

 

Bruce

--
You received this message because you are subscribed to the Google Groups "Open PHD Guiding" group.
To unsubscribe from this group and stop receiving emails from it, send an email to open-phd-guidi...@googlegroups.com.
To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/b8a30ea7-d70f-4a55-90fd-108906b66a79n%40googlegroups.com.