Prime Focus

[lost_in_space]

Can someone explain prime focus please? I see it mentioned a lot and know it is something I should know, but it is not registering in my grey matter.

[janoskiss]

Huh? Watchutawkinibowt?

If you google it you will find it's some sort of finance/investment company. I've never seen the phrase "prime focus" used on these forums before. :shrug:

Edit: After searching for "prime focus" I see there are quite a few messages saying "taken at prime focus". It seems it's the phrase to use when you cannot specify the exact conditions under which a shot was taken.. :rofl:

PS. Yes, I'm ignorant and loving it...

[lost_in_space]

OK, It's late and I guess I was being lazy. I should have known this.
Google is your friend.

define:"prime focus"
Definitions of "prime focus" on the Web:

The point in a reflecting telescope where the mirror focuses incoming light to a point.
astronomy.nju.edu.cn/astron/AT3/GLOSS_P.HTM (http://www.google.com.au/url?sa=X&start=0&oi=define&q=http://astronomy.nju.edu.cn/astron/AT3/GLOSS_P.HTM)
A reflecting telescope's primary mirror focal point. Astronomers place cameras at prime focus to record bright, wide field images.
www.ccastronomy.org/terminology.htm (http://www.google.com.au/url?sa=X&start=1&oi=define&q=http://www.ccastronomy.org/terminology.htm)


The focal point of the objective mirror or lens.
www.opticalvision.co.uk/content.asp (http://www.google.com.au/url?sa=X&start=2&oi=define&q=http://www.opticalvision.co.uk/content.asp%3FSEC_ID%3D11)
When the eyepiece is removed from a telescope and replaced with a camera body it is said to be at prime-focus. It is the point at which a telescope’s optics direct incoming light.
www.dirtyskies.com/index.php/glossary/ (http://www.google.com.au/url?sa=X&start=3&oi=define&q=http://www.dirtyskies.com/index.php/glossary/)

[janoskiss]

So when you simply place the film or CCD in the focal plane, it is referred to as "taking it at prime focus". You are using the telescope as a big telephoto lens.

[rumples riot]

Images taken at the focal plane of a camera or sensor without any other optical elements intervening from the secondary to the focal plane. Prime focus.

[jjjnettie]

Prime focus is when you photograph through the scope without an eyepiece present.
( as described above)
A-focal is when you hold the camera up to the eyepiece and take the photo through it.

[ving]

and since the question has been answered so well...

prime focus is also the name of the newsletter bought out by MacArthur Astro Soc. :)

[rumples riot]

Stop spamming Ving. No wonder you are on 4k posts. :P