Focal Length and Focus


Over the past two weeks, focal length and its relationship to focus has been often discussed between my brother and I, however I have yet to sit down and figure it out. In doing so I hope to answer the following:

Understanding Focal Length, Focus, and Objective Focus

As usual, I began with a Google search. 10 seconds in, I had my first epiphany, I have a Reflecting telescope not a refracting one. The diagram below is borrowed from here.

reflecting telescope

As explained in the article, my telescope’s Newtonian tube has a large concave mirror in the back. Light travels down this tube and hits the concave mirror, reflecting back up the tube and narrowing into a smaller, secondary mirror. This mirror is flat and sits at a 45 degree angle, reflecting the light up into the eyepiece. The focal point is the point where the light narrows and the image is focused.

Now, how can you adjust the focal point?

With my Newtonian’s body barrell fully extended, I am able to adjust the focus by increasing/decreasing the eyepiece’s distance from the secondary mirror.

normal focus view

How do eyepieces affect this?

For starters, eyepieces are simply means of exploiting the most out of a given telescope’s aperature, affecting field of view and mangnification:

“One of the most common misconceptions in amateur astronomy is that magnification is the most important aspect of a telescope. In reality, the diameter (aperture) of a telescope determines its power and different eyepieces are used to get the best view of a given object.” - Understanding Eyepieces

It turns out one of the primary differences between my eye and my brother’s camera is the following:

“But, placing your eye at the focus point of a telescope does not produce an image. Why not? Because your eye is also an optical system. Your eye focuses light just like a telescope does, and it cannot focus on a real image such as that created by a telescope. It requires a virtual image, which is what an eyepiece creates” - Understanding Eyepieces

Check out this borrowed image: eye focus

As magnification power increases (calculated by focal length (mm) divided by eyepiece height (mm)), exit pupil decreases (aperature divided by magnifcation). Too small or too large an exit pupil and you’ll be out of luck when trying to observe anything.

So, finally, what about the barlow?

A barlow basically 2x or 3x the focal length of a telescope through its diverging lens (see below, borrowed from here).


It turns out, a barlow’s amplification effect is not fixed: “the amplification factor of a Barlow is a function of its position in relation to the eyepiece and the objective lens (or primary mirror)” (How does a barlow work?).

Furthermore, as noted by Vondragonnoggin on the Cloudy Nights Forum in this post:

“You are most likely running out of outward focus travel. You can remedy this by getting some extension tubes. Like a 1” and a 2” extension to cover your bases. Dobs are notorious for their lack of outward focus travel. Barlows require extra outward focus travel and reducers require extra inward focus travel.”

However, Starman1 said the following which Vondragonnoginn acknowldeged might be the case:

“Almost all barlows require extra INWARD movement of the focuser. This is true because most are short focal length, short-tube, barlows.”

It seems if I attempt to bring the barlow into focus and the image is becoming sharper and smaller, but does not quite reach the point of focus it is an inward problem, if the opposite is true it is an outward problem, and if it goes in and out of focus, a barlow problem.


Besides the two goals stated in the overview, my real main goal is to avoid my current hack, unextending the barrell of my Newtonian a half inch. I am optimistic that, based on the advice from Cloudy Nights and a better understanding of how all the pieces fit together, I can bring my Barlow into focus tonight with my adapters/extenders (see below).

barlow and extender