reverse equations ALT-AZ to RA-DEC

[G. Warren Coleman]

Some time ago Tom Crowley furnished equations to determine ALTitude angle
and AZimuth for known RA (right ascension), DEClination, and LST (local
sidereal time), LONgitude, and LATitude. These equations may be found
at http://en.wikipedia.org/wiki/Altitude_(astronomy)#Elevation_angle
and are based on a spherical triangle. The same spherical triangle
may be solved for RA and DEC when ALT and AZ are known instead of vice-versa. I do not have Greek letters readily available so I will use all cap multiple English letters instead, except the single letter, H, will
be hour angle.

Declination, DEC, must be solved first because it is used to find hour angle, H.

sin DEC = sin LAT sin ALT + cos LAT cos ALT cos AZ

cos H = (sin ALT - sin DEC sin LAT)/(cos DEC cos LAT)

The arc cos function may be used to find H.

H = LST - RA therefore RA = LST - H

The U. S. Naval Observatory website Astronomy Applications department
has an on-line calculator for LST and formulae that can be used with a scientific calculator if Web access is not available.
Best wishes, Warren

[James Brown]

You could always use my Clock which has a Sky Map as well. Here:

http://www.seti.net/html/SETINet/Engineering/Software/Clock/clock.htm

J...@SETI.Net
www.SETI.Net

--------------------------------------------------
From: "G. Warren Coleman" <wd4...@yahoo.com>
Sent: Friday, November 21, 2008 6:14 PM
To: "SARA-radio-astronomers List" <sara...@googlegroups.com>
Subject: [SARA] reverse equations ALT-AZ to RA-DEC

[Peter King]

Thanks for posting these equations - it was easy to find the method for RA-DEC conversion, but yours was the only one with the reverse equations.
I tried it and it appears that there is a problem if the azimuth is <180 degrees.
In that case, RA = LST + H seems to work instead of RA = LST - H.

[J...@seti.net]

My Astronomical Clock can be used for these conversions.
You can download it here:
http://www.seti.net/operation/operation.php#clock

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[ARNO...@aol.com]

Hello ,I can not download our clock ,   i get to your page but than dont find the download link can you please help regards  wolfgang , dj3q

,

 

 

n

 download your clock I grt to xo

 

In a message dated 29.11.2014 01:14:12 Mitteleuropäische Zeit, J...@SETI.Net writes:

http://www.seti.net/operation/operation.php#clock

[J...@seti.net]

Arnold – Look for the word “Down Load” top left of the section on the clock.

--

[ARNO...@aol.com]

I stillcant find the link i only get to this page but cant find tere any link to downlod sorry

wolfgang

 

 

[Jan Lustrup]

I found it....click on the picture of the clock and the on the next image click on Down load.

Jan

 

From: ARNOLDWO via Society of Amateur Radio Astronomers

Sent: Sunday, November 30, 2014 7:02 PM

To: sara...@googlegroups.com

Subject: Re: [SARA] Re: reverse equations ALT-AZ to RA-DEC

 

--

[J...@seti.net]

I see.

Click on the Astronomical Clock icon in the center and it will take you to the download location down further on the page.

 

From: ARNOLDWO via Society of Amateur Radio Astronomers

Sent: Sunday, November 30, 2014 10:02

To: sara...@googlegroups.com

Subject: Re: [SARA] Re: reverse equations ALT-AZ to RA-DEC

 

--

[ARNO...@aol.com]

I still cannot get it to work,could You send the program as a file attached to an email to me  ?arno...@aol.com

 

In a message dated 30.11.2014 15:40:55 Mitteleuropäische Zeit, J...@SETI.Net writes:

Arnold – Look for the word “Down Load” top left of the section on the clock.

 

From: ARNOLDWO via Society of Amateur Radio Astronomers

Sent: Sunday, November 30, 2014 05:59

To: sara...@googlegroups.com

Subject: Re: [SARA] Re: reverse equations ALT-AZ to RA-DEC

 

Hello ,I can not download our clock ,   i get to your page but than dont find the download link can you please help regards  wolfgang , dj3q

,

 

 

n

download your clock I grt to xo

 

[Steve Olney]

Downloaded SET Net Astronomical Clock.   I cannot set coords in Star Finder.   Up/down control only goes in steps of whole degrees and hours.  Trying to enter decimal values are overwritten when app is in run state.  Entering after stopping is possible, but is overwritten when started again... ?????

W7 Pro

Steve Olney

[J...@seti.net]

Yes – I see that Steve. You can run the up/down select to change the decimal inputs for DEC/Ra but you cannot enter the values directly. A bug for sure.

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[Steve Olney]

Unfortunately the U/D control has only integer increments and so, for example, there is no way at all I can find to enter 8.58 for RA, nor -45.17 for DEC...

[J...@seti.net]

I understand that Steve. No quick fix on my end unfortunately.

I use the clock for positioning a 3 meter dish at 1.4GHz so fractional DEC/Ra are beside the point for me. It’s a 3 1/2 degree antenna.
How are you using it. Maybe there is some other way to skin this cat.

--

[Steve Olney]

G'day James,

On Monday, December 8, 2014 12:04:23 PM UTC+11, James Brown wrote:

I use the clock for positioning a 3 meter dish at 1.4GHz so fractional DEC/Ra are beside the point for me. It’s a 3 1/2 degree antenna.

Not sure if you are pulling my leg or not - but only having integer increments of hours in RA causes a step of about 10 degrees in AZ for a 8.58 RA, -45.17 DEC object...

How are you using it. Maybe there is some other way to skin this cat.

Actually - I have found a way to skin the cat - use the AZ/EL U/D controls (integer degree increments) to make the fractional parts of the RA and DEC textbox close to the object's fractional part values.   THEN use the RA and DEC U/Ds to get the integer part right.... Quite a circus as changes in AZ/EL affect both RA and DEC...LOL... 

[Michiel Klaassen]

Hi All,

Perhaps it is not what you need exactly, but on http://parac.eu/projectmk8.htm you can find two scripts to do AZEL to RADEC conversion.

One is static and the second one updates every second. These scripts can be usefull for drift scans.

See also http://rhodesmill.org/pyephem/index.html for more inspiration.

Michiel

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[J...@seti.net]

Steve – Try this version

http://www.seti.net/operation/operation.php#clock

 

Regards........ Jim

 

From: Steve Olney

Sent: Monday, December 08, 2014 21:48

To: sara...@googlegroups.com

Subject: Re: [SARA] Re: reverse equations ALT-AZ to RA-DEC

 

[J...@seti.net]

Steve (and all).

There are several ways to use the clock.

From the main screen you can:

Change the Az/El and the DEC/Ra is calculated

Change the DEC/Ra and the Az/El is calculated

 

From the Mount you can:

Pull the Az or El pointers to your current antenna position and the DEC/RA is calculated and shown on the Sky Map

 

From the Sky Map you can:

Left click to set your antenna position

Right click to set the star position

Move your mouse around and see the DEC/Ra, Az/El and hour angle calculated as you move

 

 

From: Steve Olney

Sent: Monday, December 08, 2014 21:48

To: sara...@googlegroups.com

Subject: Re: [SARA] Re: reverse equations ALT-AZ to RA-DEC

[Steve Olney]

Jim,

Uninstalled previous version - installed linked version.

Behaviour unchanged.

Still cannot do this as you describe...

"Change the DEC/Ra and the Az/El is calculated"

BTW - To make any alteration you must "Stop" the updating otherwise the textbox you are trying to alter goes bananas as it tries to auto-correct or something.  Here I have entered RA 8.58 and DEC -45.17...

...and now I press the "Start" button....

... you can see the RA and DEC decimal values have been trashed and replaced with the nearest integer value of the RA and DEC read-only textboxes.

I don't understand why you are not seeing this...

[Steve Olney]

Jim (and all),

Correction: there IS a change from the previous version (RA and DEC increments are now 0.01 instead of 1.0 - a heads-up would have been a time-saver).

This change allows a workaround the bug where you cannot type RA or DEC decimal values in directly as stated.

1. Go to MAP view and right click on the approximate position of the object of interest.    This places decimal values in the RA and DEC textboxes on the Main Screen.

2. Do NOT try and alter these value to their correct accurate values by typing directly into the textboxes (and adjustments made by doing this are overwritten).   Instead use the U/D controls to arrive at the correct RA and DEC values.

[J...@seti.net]

Steve – Try setting the DEC/Ra by right clicking on the Sky Map. That sets where the Star is. I’m looking into why you cannot change the edit box for DEC and Ra directly. The spinners work but direct change does not.

Humm.

From: Steve Olney

Sent: Tuesday, December 09, 2014 12:54

To: sara...@googlegroups.com

Subject: Re: [SARA] Re: reverse equations ALT-AZ to RA-DEC

--

[Steve Olney]

Jim,

My head is spinning :-)

Did you read my last post which described the workaround ?

I think I'll wait for the solution before posting again.

Thanks for the app.

Cheers

Steve

[Steve Olney]

G'day Jim,

Another odd behaviour I found is that the left-click on the map for positioning the antenna is flakey in some parts of the map...

The red circle shows the position of the left-click antenna pointing and you can see the representation of the beam is way-off.    The green circle is the right-click position for an object.

Steve

[J...@seti.net]

That is an odd looking figure.  What Lat/Lon did you enter.

 

From: Steve Olney

Sent: Tuesday, December 09, 2014 14:52

To: sara...@googlegroups.com

Subject: Re: [SARA] Re: reverse equations ALT-AZ to RA-DEC

 

--

[Steve Olney]

Lat: -33.6, Lon: 150.7

[padam ghimire]