A wide field Astrophotography Question
Q. If you take an untracked picture of the stars(say in the Orion region) and the stars start trailing, are you in effect going to register no new stars irrespective of your exposure time because the light that comes in after some time starts falling on different pixels on your CCD(which is why the stars appear as trails) but the sky appears brighter as there is constant skyglow. So, in theory, a 2 minute exposure under Delhi skies should show more stars(trails) than a 20 minute exposure with the same settings. Is this true? --- Posted by Avnish Anand of Amateur Astronomers Association, Delhi
Discussed by Akarsh Simha of Bangalore Astronomical Society and Guntupalli Karunakar of Pune who is also a member of the Amateur Astronomers Association, Delhi
Hi Avnish,
I believe this is correct. And yes, with longer exposure, your contrast of a
startrail should reduce, as you say.
> So, in theory, a 2 minute exposure under Delhi skies should show more
stars(trails) than a 20 minute exposure with the same settings. Is this true?
Yes. I think this logic is correct. Although more light is collected in the 20
minute exposure, it is spread over a larger area, so you can completely wash out
the faint stars.
I haven't experimented with this, though.
That's why, the better idea is to shoot at large ISOs.]
--- Akarsh Simha
Typically what ISO settings are being used, I used to do 400-800 , but one
thing I found , over long exposure (10s or more) & high ISO (1600 & above), the
noise creates lot of fake stars! (this shooting from skies above sinhgad near
pune - eastern sky has a city glow though).
--- Guntupalli Karunakar
I think stacking should eliminate the fake stars and keep only the real stars. So high ISO + Stacking works best, I think. But you can't take stacked startrails! Or can you?
--- Akarsh Simha
> I think stacking should eliminate the fake stars and keep
only the
> real stars.
ok, but I am shooting with DSLR at 10mp!!
> So high ISO + Stacking works best, I think. But you can't take
> stacked startrails! Or can you?
I am not using any tracking.. right now limited to 1-2s exposures on telescope &
mostly direct sky shots with camera on tripod with a wide/tele lens. I have to
figure out how to stack in GIMP.
--- Guntupalli Karunakar
(The discussion is ongoing, please join the group http://groups.yahoo.com/groups/Astronomy_Activities_2009/ and give your inputs for this topic.)