KStars gravitational lensing

[Giovanni Ceribella]

Dear Akarsh, dear Jasem,

As a simple user, I can add that the feature still exists and works in 
my installation KStars 3.7.5. The corresponding option is in the 
settings window, under the "advanced" menu and in the "general" tab 
(just below the "Correct for atmospheric refraction" option). Turning it 
on visibly applies a radial distortion to the stars next to the Sun, 
which is the expected effect.

However, If I take the star HD207520, which Kstars shows as grazing the 
limb of the sun at 7UT of today, its distance (as measured by Kstars) 
from the center of the sun is 15'54'' without the GR effects and 15'58'' 
with it. But:

    In [1]: import astropy.units as u
    In [2]: from astropy.constants import M_sun,c,G

    In [3]: impact_parameter = (16. /60 /180*np.pi) * 1 * u.AU
    In [4]: impact_parameter = impact_parameter.to(u.m)

    In [5]: schwarzschild = 2*G*M_sun/c**2

    In [6]: angular_delta = 2*schwarzschild/impact_parameter

    In [7]: angular_delta.value
    Out[7]: 8.483180496594135e-06

    In [8]: angular_delta.value*180/np.pi*3600
    Out[8]: 1.749781581489136

So the deflection as measured by Kstars is ~twice as large than the 
actual one. I could not check if this is an issue of the distance 
measuring algorithm and the presented view is correct, or if the view 
itself is off by that amount.

Best regards,

Giovanni

Il 14/02/25 07:07, Akarsh Simha ha scritto:
> I implemented this feature, but I did not verify it, nor is it 
> maintained properly at present. The main source of computational 
> errors isn't so much calculating the lensing itself (the formula is 
> fairly straightforward), but in ensuring we have the correct position 
> of the sun, if I recall correctly. I wouldn't have the time to look 
> into it, but I would assume that it is unmaintained and therefore not 
> functioning as intended.
>
> This correction is explained in the Explanatory Supplement to the 
> Astronomical Almanac, and one could take up re-implementing it 
> according to that book.
>
> Regards
> Akarsh
>
>
> On Thu, Feb 13, 2025 at 8:39 PM Jasem Mutlaq <mutlaqja at ikarustech.com> 
> wrote:
>
>     Hello everyone,
>
>     I'm forwarding an email I received from Dr. Lorenzo regarding
>     the "Gravitational lensing effects around the Sun" feature in
>     KStars. I actually never used this at all, anyone has any ideas?
>     --
>     Best Regards,
>     Jasem Mutlaq
>
>
>
>     ---------- Forwarded message ---------
>     From: *Lorenzo* <bzzlnz at unife.it>
>     Date: Fri, Feb 14, 2025 at 12:24 AM
>     Subject: KStars gravitational lensing
>     To: <mutlaqja at ikarustech.com>
>
>
>     Dear Jasem,
>
>     I am Lorenzo, a post-doc in astrophysics at University of Ferrara,
>     Italy. I'm preparing a lecture for high school students on
>     gravitational
>     lensing, and I just found out KStars. In particular, the website
>     mentions: "Switch on and switch off gravitational lensing effects
>     around
>     the sun to understand Eddington’s experiment". I am trying to
>     visualise
>     the deflection of starlight caused by the Sun on May 29, 1919, from
>     Principe Island (i.e., Eddington’s experiment), however, I am
>     unable to
>     observe the expected deflection of nearby stars. Could you please
>     clarify if this feature is functioning as intended, or if there are
>     specific settings I need to adjust? I've already downloaded some
>     extra
>     stars catalogues. Do you confirm that I just have to press the "R"
>     key
>     to activate the lensing effects?
>
>     Thank you for your time and assistance.
>
>     Best,
>
>     Lorenzo
>
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