<p>As has been noted, the camera, if truly fixed, will point at spot with fixed declination. The RA it points to will be a simple function (namely so many hours and minutes ahead/behind) of the <em>local</em> sidereal time.</p>

<p>Either your camera is not truly fixed (a bit of shake) or (possibly more likely), you have not correctly determined the local sidereal time.</p>

CC BY-SA 3.0
2016-06-09T12:10:31.410Z
2016-06-09T12:10:31.410Z
adrianmcmenamin
<p>first doubt here:</p>

<p>I have a fixed camera, whose coordinates I know, pointing to the sky. After the first image, I performed a astrometry calculation from nova.astrometry.net and I got, among other things, the Ra and Dec of the center of view. I would like to use the calibration information for other pictures without needing to calibrate again, e. g. copy the calibration info and just update Ra and Dec of the next pictures. </p>

<p>I thought I could do so transforming the Ra and Dec to Az and Alt using the Julian Date of the first event(the calibrated one), and then go back to Ra and Dec using the Julian Date of the second event. But I get wrong results. I tried between different pictures and sometimes the error is bigger some time smaller.</p>

<p>I am using the functions eq2hor and hor2eq of Astrolib (IDL), which takes in account precession and nutation, so I assume than there is a gap in my knowledge and procedure, rather than a programming mistake. </p>

<p>What am I doing wrong? </p>

<p>Thanks in advance</p>

CC BY-SA 3.0
2016-06-07T14:23:59.250Z
What calibration information do you want to use for the other pictures?
Dean
CC BY-SA 3.0
2016-06-07T14:27:03.797Z
If the camera position is always fixed, just the Right Ascension changes for subsequent pictures. (Unless they're years apart in which case precession begins to matter.) Is this what you need to know?
Andy
CC BY-SA 3.0
2016-06-07T14:40:23.527Z
Dean, I want to use the distortion coefficients mainly.  

@Andy, it should be like that, although I found small variations in the Dec. But even if it was completely constant, should I just assume that the Ra moves 360deg in 23h 54m and found the new Ra from there? I thought that would not be precise.. I will check
ImanolUr
CC BY-SA 3.0
2016-06-07T14:55:10.710Z
Pretty sure the declination remains constant and RA varies. Consider a camera pointing straight up - Through the day (23h 56min)  it will describe a circle along the DEC line that is dictated by your latitude.
Andy
CC BY-SA 3.0
2016-06-07T15:14:00.863Z
I am computing: Ra_second_event= Ra_first_event + (JD2-JD1)*360/23h54m

or 360/0.99726966237 (fraction of the day)

 and it is still wrong...
ImanolUr
CC BY-SA 3.0
2016-06-07T16:01:00.380Z
A rule of thumb: for any location RA should drift by _just under_ 15 degrees per hour... (360 degrees in just under 24 hr)
Andy
CC BY-SA 3.0
2016-06-08T09:41:01.400Z
I found the mistake. Apparently for IDL x = 5 + 46/60 + 50/3600 is 5 and not the float point.. so the julian dates where computed wrong..
ImanolUr
<coordinate>
Pointing direction movement of a fixed camera
ImanolUr
# Pointing direction movement of a fixed camera

(Asked by: username_0 on 2016-06-07)

first doubt here:

I have a fixed camera, whose coordinates I know, pointing to the sky. After the first image, I performed a astrometry calculation from nova.astrometry.net and I got, among other things, the Ra and Dec of the center of view. I would like to use the calibration information for other pictures without needing to calibrate again, e. g. copy the calibration info and just update Ra and Dec of the next pictures.

I thought I could do so transforming the Ra and Dec to Az and Alt using the Julian Date of the first event(the calibrated one), and then go back to Ra and Dec using the Julian Date of the second event. But I get wrong results. I tried between different pictures and sometimes the error is bigger some time smaller.

I am using the functions eq2hor and hor2eq of Astrolib (IDL), which takes in account precession and nutation, so I assume than there is a gap in my knowledge and procedure, rather than a programming mistake.

What am I doing wrong?

Thanks in advance

**Tags:** coordinate

> **Comments:**
> **username_1**: What calibration information do you want to use for the other pictures?  
> **username_2**: If the camera position is always fixed, just the Right Ascension changes for subsequent pictures. (Unless they're years apart in which case precession begins to matter.) Is this what you need to know?  
> **username_0**: username_1, I want to use the distortion coefficients mainly.    @username_2, it should be like that, although I found small variations in the Dec. But even if it was completely constant, should I just assume that the Ra moves 360deg in 23h 54m and found the new Ra from there? I thought that would not be precise.. I will check  
> **username_2**: Pretty sure the declination remains constant and RA varies. Consider a camera pointing straight up - Through the day (23h 56min)  it will describe a circle along the DEC line that is dictated by your latitude.  
> **username_0**: I am computing: Ra_second_event= Ra_first_event + (JD2-JD1)*360/23h54m  or 360/0.99726966237 (fraction of the day)   and it is still wrong...  
> **username_2**: A rule of thumb: for any location RA should drift by _just under_ 15 degrees per hour... (360 degrees in just under 24 hr)  
> **username_0**: I found the mistake. Apparently for IDL x = 5 + 46/60 + 50/3600 is 5 and not the float point.. so the julian dates where computed wrong..  

---

## 1 answers

### Answer by username_3 on 2016-06-09. Score: 1 | Accepted answer

As has been noted, the camera, if truly fixed, will point at spot with fixed declination. The RA it points to will be a simple function (namely so many hours and minutes ahead/behind) of the *local* sidereal time.

Either your camera is not truly fixed (a bit of shake) or (possibly more likely), you have not correctly determined the local sidereal time.


---

