Andromeda - Latest and Longest

[nighthound]

This is the Andromeda Galaxy again but this time it's 24 minutes. More detail overall. I'm going to double the exposures next time we get a clear steady night but overall I'm happy with this one.

Sky 90 II • G-11 • 20D • IDAS Light Pollution Filter

http://i3.photobucket.com/albums/y67/Nighthd/Astrophotography/andromagain2.jpg

NH

[David Cramer]

NH, these are simply breathtaking. I have one question. With these long exposures, why are there no star trails? Thanks for sharing your talents with us.

[dmwphoto]

Very well done. I assume this is through a telescope?

[MGlennn]

Beautiful & fascinating....keep 'em coming !! :D :D :D

[Ann]

Terrific shot!

[Rudi]

NH, these are simply breathtaking. I have one question. With these long exposures, why are there no star trails? Thanks for sharing your talents with us.

I'm not NH, but will attempt to answer your question, David.

The Earth turns quite rapidly. In fact, with a normal perspective lens, on a full frame (film) camera you have only about 20 seconds before star trails become apparent (you get movement straight away, obviously, but it does not become apparent in normal-sized prints until about 15 seconds after you open the shutter).

To compensate for this, the camera (with scope attached in this case) has to be mounted on a driven equatorial mount. This is a special mount which, when properly aligned, compensates for Earth's motion, and keeps the stars perfectly still while the exposure takes place.

Make sense? :)

[nighthound]

Thanks everyone.

Well said Rudi. I expand on this and add that as the lens focal length increases, tracking accuracy becomes more critical. In this case at 407mm (refractor telescope), I must make a fairly accurate polar alignment before beginning in order to get about 2 minute exposures without visible drift or tracking error. With my large scope at 2000mm I must make a very accurate alignment. I won't bore you with the process used to achieve this.

The exposures I get are stacked in a software program designed specifically for astrophotography. The stacking process combines the exposures to give an image that is a total of the sub exposures taken. It's main benefit is to raise the signal to noise ratio. More of the good stuff and less of the noise that occurs at high ISO settings and long exposure. Processing this way helps eliminate the need for pin point alignment which would be mandatory at a single 30 minute exposure. Mechanical gear flaws in the mount also make this extremely difficult.

And lastly the camera I use(20D) is not designed for long exposure astrophotography. Cameras that are(CCD imagers) function with an internal cooling system, sometimes liquid cooled. This keeps the noise levels down during the long exposures. The chips or sensors are also much larger and more sensitive to light. A quality CCD camera would cost between 4 - 10 thousand dollars, and there are no lenses, that would be your scope.

Thanks again.

NH