What is focusing

[tickmatrix]

Could someone tell me what is happening when I turn my focusing knob and an image becomes clear (in focus) 

[cajen2]

There are many optical experts on here. I'm not an optical expert, but basically it goes like this:

the light enters the scope and passes through the lenses (refractor) or is reflected by the large mirror (reflector). In both cases, the light becomes a cone because of the curvature of the lens / mirror. (There are other lenses or mirrors involved but the principle stays the same). The point at which the rays of light converge is where you want your eye to be. Focusing moves the eyepiece to that point, delivering a sharp image.

Ok, experts, I know there's more to it than that, but I'm trying to keep it straightforward!

Edited February 23, 2022 by cajen2

[AstroMuni]

10 hours ago, tickmatrix said:

Could someone tell me what is happening when I turn my focusing knob and an image becomes clear (in focus) 

In a nutshell the lens or mirror is bringing the light gathered through the various portions of the lens/mirror into a small circle. This is what you see depicted in the various science papers. So think of it as a collector of light, hence the larger the lens the more the light gathering power. Here is the basic science as explained in BBC bitesize https://www.bbc.co.uk/bitesize/guides/zt7srwx/revision/1

If this is too basic then let me know

To make it more complex, various wavelengths of light bend at different angles so they will not end up in the same point in the small circle. And this is where comes the need to use multiple lenses to correct for the various errors you hear about (coma, chromatic aberration etc.)

[Ouroboros]

I find a diagram helps visualise what’s going on. The primary lens (or mirror) brings rays of light from each point on an object to sharp focus at the image plane. In a telescope the eyepiece magnifies the image collected by the primary lens or mirror. You can see from the diagram that away from the image plane, rays from different parts the object overlap in a jumbled way and the image appears increasingly fuzzy.  This image is from an educational site. https://www.physicsclassroom.com/class/refrn/Lesson-5/Image-Formation-Revisited

[tickmatrix]

so when you adjust the focus knob you're moving the eyepiece focal point closer to the telescope focal point, where they meet is the point where the image is clear.

 Do I have this right?

Edited February 24, 2022 by tickmatrix

[michael8554]

Yep

[AstroMuni]

55 minutes ago, tickmatrix said:

so when you adjust the focus knob you're moving the eyepiece focal point closer to the telescope focal point, where they meet is the point where the image is clear.

 Do I have this right?

In this there are 3 lenses in play - the large one at the far end of the scope (objective), the eyepiece near your eyes and the lens in your eye itself So together the focus needs to be at a point where your eyes see a clear image. Remember that the eye is capable of adjusting the focal length of its lens. You will hear the term called eye relief as well and thats to do with how close you need to bring your eyes to the eyepiece to get focus.

Edited February 24, 2022 by AstroMuni

[Captain Scarlet]

You can categorize “focusing” into two groups: 1. Those where the point of focus is fixed and you move the eyepiece forwards or backwards to meet it; and 2. those where the eyepiece stays fixed in position and you move an optical component of the scope to change the position of the scope’s focus.

examples:

A Newtonian has a fixed focus point, and you use a Crayford or rack and pinion mechanism to move the eyepiece’s focal point into the focus position.

A Mak has a focus knob which changes the separation of the two internal mirrors thereby moving the focus-point into the focus of the eyepiece.

Cheers, Magnus

[Pixies]

But bear this in mind:

The pictures above are of parallel rays of light coming from infinity and converging to the focal point of the scope, then the eyepiece being adjusted so that it's focal plane meets the telescopes focal point and magnifies the image for you to see. This applies only to objects at infinity (or at a distance to be equivalent to infinity). The scope's focal length defines the point that parallel light rays converge. However, the scopes optics are busy converging light rays from many distances, not just infinity!

Something closer - say the TV aerial next door - will have light rays diverging when they hit the scope first lens/mirror. The scope's optics will still converge these rays to a focal point, but one that is further away from  the objective. In order to view this in focus through the eyepiece, you rack the focuser out until the eyepiece's focal plane meets this other focal point. That's why you rack out, to see closer objects in focus.

Note - as per @Captain Scarlet's post above, this refers to newts and refractors

Edited February 24, 2022 by Pixies

[Pixies]

@vlaiv recently posted a really good diagram, recently.

 

[vlaiv]

1 hour ago, Pixies said:

@vlaiv recently posted a really good diagram, recently.

 

I do post a lot of stuff, so not really sure which one are you referring to.

In any case - I think original question has been answered quite well.

Focusing can be defined as aligning of two focal planes - one of the eyepiece and one of the telescope (however, keep in mind that there is no singular focal plane of the telescope - focal plane depends on how close or far away observed object is. Close objects shift focal plane further away. If we place object at focal length away from objective lens / mirror - we move focal plane to infinity! With scope with moving optical elements like Maks and SCTs - position of focal plane also depends on separation between optical elements.

Similarly - there is no single focal plane of the eyepiece.

In ideal case - object at infinity and observer with good eyesight (no need for glasses) - both focal planes will be situated at exact focal lengths of respective optics (OTA and eyepiece).