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Back focus--a definition

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#1 Lem Jettison

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Posted 23 April 2017 - 03:39 PM

Greetings! 

 

I have not been able to come across a definition for "back focus" that is not replete with other jargon I do not yet understand. 

 

Can someone please offer a definition of back focus for a relative beginner? I believe I have an idea based on other resources but would be grateful if I could benefit from your definitions--in layman's terms. 

 

Many thanks in advance! 

~Lem



#2 DLuders

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Posted 23 April 2017 - 03:49 PM

Celestron explains "Back Focus" on their webpage http://www.celestron...ed-focus-issues as:

 

"Back focus is the distance from the end of your eyepiece drawtube to the focal plane of your telescope. It varies greatly depending on the type of telescope:

 

•Schmidt-Cassegrain telescopes (SCTs) typically have generous back focus distances of approximately 5 in. Similarly, Maksutov-Cassegrains (Maks) also have large back focus distances.

 

•Newtonian reflectors have much smaller back focus distances, sometimes as little as 1-2 in.

 

•Refractors usually have large back focus distances; their long drawtubes can travel well inside focus, too.

 

"Back focus directly impacts your ability to use accessories with your telescope. With limited back focus, you may not be able to use Barlows, focal reducers, binoviewers, SLRs, and 2 in eyepieces. The design most heavily impacted is the Newtonian due to its short back focus; the only two possible fixes are to move the mirror and its cell up in the tube and/or get a low-profile eyepiece focuser.

 

"Combining accessories can also cause back focus problems. Focal reducers also reduce back focus and, when used for prime-focus photography with an SLR, there may not be enough back focus left to accommodate the approximately 50 mm needed for the camera body. This is true even with an SCT's long back focus distance.

 

"For refractors, a related issue is back travel. Many refractors won’t work straight-through with eyepieces or small cameras at infinity because the drawtube won’t extend that far. A solution that works is to use extension tubes, such as a star diagonal--it acts as a drawtube extension.

 

"Another situation you may encounter is focal plane access when combining various accessories. As one example, some 2 in diagonals won’t achieve focus when used with short-focal length 1.25 in eyepieces because the focal plane is buried too far down in the big diagonal, out of reach of the small eyepiece. Here you’ll have to try another combination - switch from a 2 in to a smaller 1.25 in diagonal to get a focus."


Edited by DLuders, 23 April 2017 - 03:51 PM.

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#3 xiando

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Posted 23 April 2017 - 03:55 PM

It's the distance from the optical element's back lens surface to the focused image plane.

 

So, for instance, you have a camera with a back focus of 22mm and a Newtonian telescope. The telescope focuser will need to provide at least 22 mm of travel to achieve focus in the camera image.

 

Functionally, it's also the effective focal length of the camera in the previous case. The magnification of the Newtonian can be calculated using the focal length of the telescope divided by the back focus distance for the camera.

 

For instance, suppose your telescope has a focal length of 610 mm, and the back focus of the camera is again 22mm. the magnification of the system is

 

610mm/22mm = 27.73


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#4 Jon Isaacs

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Posted 23 April 2017 - 03:57 PM

Greetings! 

 

I have not been able to come across a definition for "back focus" that is not replete with other jargon I do not yet understand. 

 

Can someone please offer a definition of back focus for a relative beginner? I believe I have an idea based on other resources but would be grateful if I could benefit from your definitions--in layman's terms. 

 

Many thanks in advance! 

~Lem

 

Lem:

 

Back focus is inward focuser travel.  If you focus on a star with an eyepiece, it is how much inward focuser travel is left before you have no more.

 

It's confusing but it's a term from photography, back focus means focusing behind the subject.

 

Jon


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#5 Lem Jettison

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Posted 23 April 2017 - 04:25 PM

 

Greetings! 

 

I have not been able to come across a definition for "back focus" that is not replete with other jargon I do not yet understand. 

 

Can someone please offer a definition of back focus for a relative beginner? I believe I have an idea based on other resources but would be grateful if I could benefit from your definitions--in layman's terms. 

 

Many thanks in advance! 

~Lem

 

Lem:

 

Back focus is inward focuser travel.  If you focus on a star with an eyepiece, it is how much inward focuser travel is left before you have no more.

 

It's confusing but it's a term from photography, back focus means focusing behind the subject.

 

Jon

 

Hi Jon,

 

Thanks! Hope all is well with you.

Does it have any relevance for visual?

Does it mean the amount you can use a focuser before the focuser runs out of turns (and it is still out of focus), for example?

Or is it where the focused image resides as it travels along the incoming light path through the optics? 

 

Thanks again!



#6 Jon Isaacs

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Posted 23 April 2017 - 04:56 PM

Lem:

 

It does have relevance for visual. The focal plane of a refractor or reflector is fixed but different eyepieces have different focal planes so the focus at different points.  Adding a Barlow or Coma corrector changes things, binoviewers need inward  focuser travel.

 

Jon



#7 Lem Jettison

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Posted 23 April 2017 - 06:06 PM

Lem:

 

It does have relevance for visual. The focal plane of a refractor or reflector is fixed but different eyepieces have different focal planes so the focus at different points.  Adding a Barlow or Coma corrector changes things, binoviewers need inward  focuser travel.

 

Jon

Jon, can you expand on this "..it is how much inward focuser travel is left before you have no more." I'm not quite sure what you mean...

Many thanks!



#8 Jon Isaacs

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Posted 24 April 2017 - 04:30 AM

 

Lem:

 

It does have relevance for visual. The focal plane of a refractor or reflector is fixed but different eyepieces have different focal planes so the focus at different points.  Adding a Barlow or Coma corrector changes things, binoviewers need inward  focuser travel.

 

Jon

Jon, can you expand on this "..it is how much inward focuser travel is left before you have no more." I'm not quite sure what you mean...

Many thanks!

 

Lem:

 

You focus an eyepiece.  Say it comes to focus with the drawtube flange 25 mm from the focuser.  That would mean you could move the focuser 1 inch inward, you would have 25 mm of inward focuser travel left.

 

Jon


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#9 SteveG

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Posted 24 April 2017 - 04:48 PM

Or if you have an eyepiece that cannot quite come to focus, and the closest you get is when the focuser is racked all the way in, you do not have enough back-focus.

 

I avoid the use of that term, and always use "inward focus" or "outward focus" instead.


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