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 (Steve H)]

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.

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..

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
• 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

• The focal point of the objective mirror or lens.
  www.opticalvision.co.uk/content.asp
• 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/
  
  
  
  

[janoskiss (Steve H)]

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 (Jeanette)]

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 (David)]

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.