Focal Plane The plane that the telescope (or eyepiece) focuses on. When you turn the focus knob on the telescope, you are moving the eyepiece back and forth until you make the two focal plane s coincide. [>>>] Focal plane: The imaginary surface on which a lens projects a focused image.
What is the focal length of a telescope?
The focal length is, however, an important element for two major telescope formula. The first thing you can calculate is the magnification (or power) of your telescope. The second thing you can calculate is the focal ratio (brightness and field of view).
What is a focal plane?
Definition of focal plane : a plane that is perpendicular to the axis of a lens or mirror and passes through the focus : a plane that is perpendicular to the axis of a lens or mirror and passes through the focus
What is a first focal plane scope?
A first focal plane scope means that the reticle is placed in front of the magnification lens. As a result, the scope’s reticle size changes as you shift through magnification. The biggest benefit of using a FFP rifle scope is that the holdover points remain the same throughout all magnifications.
Where is the reticle on a second focal plane scope?
With a second focal plane scope, the reticle is located behind the magnification lens, or closer to your eyeball. Since whatever the scope is viewing has already been magnified, the reticle appearance won’t change as you adjust magnification power up and down.
What does a focal plane mean?
Definition of focal plane : a plane that is perpendicular to the axis of a lens or mirror and passes through the focus.
What is the focal ratio of a telescope?
The focal ratio is a unitless number, and can be found by dividing the focal length of the telescope by the aperture. For example, if a telescope had 80mm of aperture and a focal length of 500mm, then the focal ratio of the scope would be 500mm/80mm = f/6.25.
What is the function of a focal plane?
Just like a human eye focuses and refocuses on points of interest, the focal plane directs the attention of the viewer to the parts of the image that are most important, and helps blur out any background or foreground distractions.
What is a good focal For telescope?
Focal ratios between f/5 and f/8 are a nice compromise, easy on eyepieces yet providing decent wide-field capability. One way to improve a telescope's wide-field performance is to choose one fitted to accept bigger eyepieces.
What can I see with a 700mm focal length telescope?
With a 70mm telescope, you will easily be able to see every planet in the Solar System. You will also be able to take a great look at the Moon and clearly distinguish most of its recognizable features and craters.
Is 400mm focal length good for telescope?
* 400mm (f/5.7) focal length and 70mm aperture, fully coated optical glass lens, with high transmission coating, can create stunning images and protect your eyes. The perfect telescope for astronomers to explore the stars and moon.
Where is focal plane located?
The rays are being focused to a focal point at the right on the lens axis. The distance from the center of the lens to the focal point is the lenses focal length. A plane drawn perpendicular to the lens axis at the focal point is the focal plane.
What is first focal plane?
A first focal plane (FFP) scope has a reticle that is placed towards the front of your riflescope's erector tube assembly and your magnification lenses. Your reticle will appear to be the same size regardless of magnification power because it is placed in front of the magnification lens.
What is the focal plane and image plane?
The focal plane is where parallel lines converge and the image plane is where the image comes to a focus, and you would absolutely not place your sensor at the focal point where those diagrams I found on a college website depict.
Is a 900mm telescope good?
The focal length of the telescope is 900mm, so to achieve the maximum useful magnification, then a 4.5mm eyepiece would be ideal. One of the best parts about planetary viewing or imaging is that since the objects are so bright, you can do it just about anywhere regardless of light pollution.
What kind of telescope is best for viewing planets?
If you're wanting to view faint deep-sky objects like nebulas and galaxies then you'll want a reflector telescope whereas a refractor telescope is better suited for views within our own galaxy such as the moon and other planets.
Is higher focal length better for telescope?
A longer focal length will shorten the field of view but increase magnification, which is ideal for observing planets and the moon. A shorter focal length offers a larger field of view which is better for astrophotography and observing galaxies, nebulas, and other deep sky objects as they are larger but dimmer targets.
What is a good focal ratio?
Focal Ratio – Faster, Brighter, Smaller A long focal ratio implies higher magnification and narrower field of view with a given eyepiece, which is great for observing the moon and planets and double stars. For such objects, a focal ratio of f/10 or more is ideal.
What focal ratio is good for planets?
Slow f/11 to f/15 focal ratios are usually better suited to higher power lunar, planetary, and binary star observing and high power photography.
What is focal length and ratio?
The focal length divided by the aperture of a mirror or lens gives you the number known as the focal ratio. It is usually written, for example, as f/6, where the focal length is six times longer than the aperture, and for this reason is sometimes referred to as the f-number.
What does focal length mean on telescope?
Generally expressed, focal length is the distance (given in millimeters) between the telescope's primary lens or mirror and the point where the light rays come together in focus.
What is a good focal length for a telescope?
It all depends on what you want to see in the night sky, for planets and the moon, a longer focal length is helpful because it increases the magnif...
How does the focal length affect telescopes?
The focal length directly impacts the magnifying power of your telescope as well as the field of view: two very important factors for stargazing an...
How to find my telescope focal length?
You can generally find the focal length on a label placed on your telescope’s tube or inside the instruction manual.
What is the focal ratio of a telescope?
The focal ratio is used to determine the “speed” of a telescope. The closer the focal ratio number is to 1 the “faster” the telescope is. For example, if you are looking at a distant nebula through a telescope with a focal ratio of 4, you will see it much faster than through a telescope with a focal ratio of 8 but the image will be smaller ...
What is focal ratio?
At its core, the focal ratio is a simple relationship between the focal length of a telescope and its primary aperture.
Why is a lower focal ratio better?
A lower focal ratio is going to display more objects in a single shot due to the field of view but it will not focus in on singular objects as well. A higher focal ratio is going to produce cleaner more defined images at the expense of a wide field of view.
Why is a higher focal ratio important?
If there is a distant mountain peak that is particularly beautiful to look at, a higher focal ratio is going to allow you to zoom in on that specific part of the scene and display its finer details in higher contrast.
Is focal ratio important for astrophotography?
While not as important for casual viewing, the focal ratio can have a profound impact on astrophotography. Getting good shots of different objects is going to require different focal ratios. If you want to take a picture of a binary star system in high detail, you are going to need a high focal ratio. Trying to take the same picture with a low focal ratio just won’t work for photography.
What is the focal ratio of a telescope?
The third key specification of a telescope is the focal ratio, which is the focal length divided by the objective diameter. A long focal ratio implies higher magnification and narrower field of view with a given eyepiece, which is great for observing the moon and planets and double stars.
Which is better: a lower focal ratio or a wide field telescope?
But if you want to see wide views of star clusters, galaxies, and the Milky Way, a lower focal ratio is better. You get less magnification, but you see more of the sky. Wide field telescopes have a focal ratio of f/7 or less. Focal ratio also influences the brightness of extended objects like a nebula or galaxy.
What is the aperture of the objective lens of a telescope?
The aperture of the objective lens of this simple telescope is D. The focal length of the objective lens if F. The focal length of the eyepiece is f. So the magnification is F/f. The focal ratio is F/D.
How does a telescope work?
To get an image suitable for observing with our eyes, a telescope uses a second lens, or collection of lenses, called an eyepiece at the focal plane. The eyepiece magnifies the image from the objective. The eyepiece also has a focal length. The magnification of a telescope and eyepiece is very simple to calculate.
What is the magnification of a telescope?
The eyepiece also has a focal length. The magnification of a telescope and eyepiece is very simple to calculate. If the focal length of the objective is “F” and the focal length of the eyepiece is “f”, then the magnification of the telescope/eyepiece combination is F/f.
What is the aperture of a telescope?
Aperture also influences most of the other key specifications of a telescope, including practical (but non-optical) specs like cost and weight. A good backyard telescope for us amateur stargazers has an aperture of 80 mm to 300 mm (3.15” to 12”) or more.
How much magnification does a telescope need?
Another rule of thumb… the maximum useful magnification of a telescope is about 50x the aperture in inches. Any higher and the image gets too dim and fuzzy to be useful. So a 4-inch scope can get you about 200x before the image gets too fuzzy and dim, a 6-inch scope gets you 300x, and so on. This is not a hard-and-fast rule.
What is a first focal plane scope?
A first focal plane scope means that the reticle is placed in front of the magnification lens. As a result, the scope’s reticle size changes as you shift through magnification. Here’s an example:
What is the benefit of using a FFP scope?
The biggest benefit of using a FFP rifle scope is that the holdover points remain the same throughout all magnifications.
Is an FFP scope good for AR?
This makes FFP rifle scopes — like the best scope for AR-10 — great for long-range shooting. However, there are two draw backs: First, FFP reticles could appear large and thick at higher magnifications, while small and thin at lower magnifications. This could make it a bit harder to see your target.
What is a first focal plane scope?
In a first focal plane scope, the reticle is placed in front of the magnification lenses. Think about this for a second. Just like the target and anything else you look at, whatever is in front of the magnification lenses is going to get… magnified. That means that the reticle itself will grow and shrink in size if you change the magnification on a variable power scope. Of course, on a fixed [read non-adjustable] magnification scope, the reticle will always look the same, even if it’s in the first focal plane, because there’s no magnification setting to change. Generally speaking, first focal plane scopes are more of a pain to manufacture, so you’ll often seem them priced higher than a second focal plane scope with comparable features. However, technology benefits march on and over the past couple of years, first focal plane scopes have become much more affordable.
Where is the reticle on a second focal plane scope?
With a second focal plane scope, the reticle is located behind the magnification lens, or closer to your eyeball. Since whatever the scope is viewing has already been magnified, the reticle appearance won’t change as you adjust magnification power up and down. The reticle appears the same at any power setting.
How much MOA does a first focal plane scope hold?
In a first focal plane scope, since the reticle grows in size proportionally with the target as magnification is increased, that same 10 MOA holdover works at any power level.
Why is the reticle always the same?
Of course, on a fixed [read non-adjustable] magnification scope, the reticle will always look the same, even if it’s in the first focal plane, because there’s no magnification setting to change. Generally speaking, first focal plane scopes are more of a pain to manufacture, so you’ll often seem them priced higher than a second focal plane scope ...
What is a reticle on a scope?
The reticle that you see through a scope is a physical thing . Back in the day, reticles were made from actual Wooly Mammoth hairs Crazy Glued onto the glass lenses. In more modern scopes, the reticle is usually a pattern etched onto glass.
What focal plane do you use for holdover?
So, if you’re using a second focal plane 3-9x or 5-20x scope (or any other), before you use the reticle to adjust for holdover, you’ll need to quickly zoom up the power level to the maximum setting and then get to work.
What happens when the reticle is fixed?
With a reticle that is “fixed” in size is paired with a change in magnification, one thing (the reticle) remains constant while the other (the target) changes in perceived size. That throws off the predictable relationship between reticle and target.
What is the simplest optical design of telescope?
The simplest optical design of telescope to start out with is the refracting type ; it uses a lens (like the eye does) to gather and focus the light (to refract means to bend the light). The other main type of telescope is the reflector; it uses a mirror or mirrors to gather and focus the light; we'll look at those later.
How does a telescope reflect light?
As with the Refractor diagram above, the starlight is entering from the right. Since the light gathering and focusing is done by the primary mirror, the light heads back up the tube, rather than coming out the rear. The flat Diagonal mirror (also called the "secondary") reflects the focused light out the side of the telescope to the eyepiece. Otherwise, the optics are performing the same jobs as those in the Refractor. Using a mirror rather than lenses has some benefits; only one face of the objective needs to be optically figured (as opposed to both faces of each of the multiple lenses in a Refractor); and the light doesn't pass through the mirror (its reflective coating is put on its front surface), so the glass can be much poorer quality. Thus, large reflector objectives can be made much less expensively than similar sized refractor objectives. Larger Newtonian reflectors may be mounted in collapsible openwork assemblies rather than solid tubes for increased portability. Mirror systems also don't face the color-correction problems that lens systems do (all colors of light reflect from mirrors the same way, but they refract through lenses by varying amounts).
How does the objective lens work?
In the telescope, the Objective lens or mirror also forms an image of the stars, at a point in front of the Eyepiece called the Focal Plane. The Eyepiece is actually just a magnifying glass which is the observer uses to look at the Focal Plane image. The two bundles of light rays (one from each star) emerge from the Eyepiece (and enter the eye) separated by a wider angle than the one at which they entered the telescope; their images then focus farther apart on the retina, and the brain sees them more widely separated, or in effect "closer up" than without the telescope. The longer the focal length of the telescope objective is, the farther apart the star images will be on the Focal Plane, and the higher the power of the Eyepiece, the more the Focal Plane will be magnified; these two properties together (focal length of telescope and power/focal length of eyepiece) determine the total magnification provided by the system (see the 'Scope Computer page for more information on calculating magnification).
How does a telescope work?
The main function of the telescope is to gather a large bundle of incoming light rays, condense it down, and re-focus it to send it into the eye as a smaller (brighter) beam of once-again parallel rays. That beam is focused by the eye's lens into a point on the retina, just like the starlight was in the unaided eye in the first diagram; the image is notably brighter, though, since the telescope gathered light over a much wider area. Obviously, the larger the diameter of the telescope's Objective (main lens or mirror) is, the brighter the star image will be; also, larger diameters allow fainter stars to be viewed (by gathering enough of their sparse rays for the eye to detect them).
What are the targets of astronomy?
One of our standard targets in astronomy is the stars. It just so happens that they are an "ideal" optical target; they are so distant from us (excepting, of course, our nearby Sun), that they appear as points of light to us, even when they are notably magnified. Indeed, all the light rays reaching us from a distant star can be considered to be parallel; this makes for easy modeling. In the following examples, we will use the light from one distant, point-like star to show how a telescope gathers and focuses light. (Later on we'll expand the model to explain magnification and the viewing of objects which appear larger than "points".)
What is the goal of apochromatic lenses?
At a higher cost, you can make one of the lenses of a more exotic material, and even add in a third element, to achieve an "Apochromatic" design; the goal with these is to reduce or eliminate the color dispersion that a simple Achromat still has, and to improve contrast and sharpness.
What does a larger diameter telescope do?
Obviously, the larger the diameter of the telescope's Objective (main lens or mirror) is, the brighter the star image will be; also, larger diameters allow fainter stars to be viewed (by gathering enough of their sparse rays for the eye to detect them).
What is the back focus of a telescope?
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.
Why won't 2 in diagonals achieve focus?
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.
How far can a Newtonian reflector travel?
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.
Can a M42 telescope work with a SCT?
The M42 Spacer Kit can adjust back-focus between your camera and telescope for the best astroimaging results. 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. ...
