Newt-Web Help: Telescope

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This help page is devoted to topics about Designing a Newtonian Telescope. For help with running the application, topics related to computers, web browsers and the internet see the help page about the Newt-Web Application.

Topics in this Document

Notes on Telescope Design

Telescope design is part science and part art. Many of Newt's calculations were taken from standard formulas in books.  Some were invented out of necessity by the author.  In some cases, more than one formula was available, giving somewhat different results.  There are differing optimizations based on the intended use of the scope (like planetary vs. deep sky work). And of course, every telescope builder has his or her opinion of the "correct" way to do something. So take this advice or ignore it as you see fit.

The purpose of a telescope is to collect light from a distant object, bring it to a clear and undistorted focus, magnify it, and deliver it to a detector (your eye, a camera, etc.).

Many factors can interrupt or distort the flow of light in a telescope, degrading the image quality.  That is the reason for Newt - to quickly find the problem areas in a design, adjust variables, and check the results.

Design Issues


Vignetting: A reduction of the amount of light reaching the focal plane by objects intruding into the light path.

Vignetting occurs when the cone of light strikes an object in it's path before it reaches the eyepiece.  The most common problem area is the inside diameter of the focuser. To see where the light cone might strike the focuser, look at the Ray Trace tab.

A tall focuser increases the probability of vignetting. A narrow focuser does the same thing.  Many telescopes are built with the standard 1¼ inch by 3½ inch focuser. At any focal ratio less than about f/10, this can cut off a fair amount of the light cone.  Using a low profile, wide focuser is one of the easiest ways to improve a telescope.

Even if the 75% zone light cone just passes through the focuser, it can help to alter the focuser.  What the eyepiece sees is all of the 100% zone, all of the 75% zone, and then a very sharp falloff of light to zero.  No 50% zone is available at all.  This will effect a low power, wide angle eyepiece the most, because this eyepiece has a very large field lens.  If the lens is wider than the 75% zone, it will get very little light near the edges of the field of view.

Another place vignetting occurs is at the front end of the tube.  Notice in the WHITE example that the 75% ray strikes the tube wall near the front.  This means that the 75% zone is reduced in size, and there is a sharp cutoff of light at it's edge.  To solve this problem, increase the diameter of the tube.  See the sample WHITE telescope for a 6 inch f/8 telescope (this is the default telescope which is displayed when Newt is started).


Glare: Stray light reflected in the telescope which interferes with the light from the object of interest.

A very common problem in Newtonian telescopes is glare. An easy way to check a telescope for strong glare is to view a star near the moon, but with the moon just outside of the field of view.  The light from the moon, shining on the inside of the tube wall, bounces around trying to get into the observers eye.  This stray light brightens the whole field, reducing contrast.  The same thing happens even when the telescope is pointed away from the moon - city lights, bright planets, and even fairly faint sky glow can get into places it shouldn't and reduce contrast.

One way to reduce glare is to install baffles in the tube. If these baffles closely match the desired cone of incoming light, and are spaced properly, then most of the stray light coming from other directions will hit a baffle instead of finding it's way to the eyepiece.  A rough, very flat black surface inside the tube walls will also help to soak up stray light.

Note: Baffles which closely match the light cone can cause another problem - tube current eddies.  See the Atmospheric Distortion section of this help file.

Adding baffles to a telescope can sometimes improve image contrast enough to allow the observer to detect very faint details which could not be seen before.

There are many ways to make baffles.  Thin wooden or metal ones are the most accurate and professional. One simple suggestion is to use foam weather stripping for sealing doors - available at any hardware store.  The foam should have a rough surface to soak up light. Fortunately, this type of foam is less expensive than the higher density, slick type. The tape backing will not hold up under the temperature range a telescope is used in, so it should be glued in place.  A couple coats of very flat black paint should be applied.

The algorithm used by Newt to calculate the position and diameter of the baffles only works for the lower end of the telescope.  Newt adds one baffle at the top, and one baffle on either side of the focuser.  In actual practice, several baffles should be added above and below the focuser to create as much shadow as possible on the inside tube wall opposite the focuser.

Improper Design

Improper Design: Mismatched optics which do not allow the full light cone to reach the focal plane unimpeded.

To deliver as much light as possible to the focal plane, the primary mirror, diagonal mirror, and all other components must match.  Too small a diagonal will produce a very small or non-existent zone of 100% illumination.  Too large a diagonal will block some of light from reaching the primary mirror and reduce contrast.  A compromise must be reached between 100% light zone size and contrast loss.

A telescope designed for planetary and high power use will usually use shorter focal length eyepieces.  These eyepieces have a fairly small field lens and don't need a large 100% zone.  Only the central area of the field needs to be fully illuminated, because that is where the object of interest is placed for viewing.  The focal ratio of the telescope is also fairly high, f/10 or more, so the light cone is very narrow.  Therefore a small diagonal mirror can be used which will not reduce the contrast very much.  High contrast is very desirable for planetary detail.  See the RED sample telescope for a 6" f/11 planetary telescope.

A telescope designed for deep sky, low and medium power use will most often use longer focal length eyepieces.  These eyepieces have a larger field lens, and need a correspondingly larger 100% illuminated zone.  The entire field should be illuminated as much as possible, so objects near the edge of the field of view are not dim.  This requires care to keep the 75% and 50% zones from being vignetted.  A larger diagonal mirror should be used. However, image contrast is still very important for seeing faint detail in extended objects. Using too large a diagonal can produce larger zones at the cost of degrading image quality.  See the BLUE sample  for a 10" f/5.6 deep-sky telescope

If at all possible, the minor axis of the diagonal should be kept under 20% of the diameter of the primary mirror.  This will keep the contrast high, which is so important for both planetary detail and detail in faint extended objects.

Some sizes of telescope are just hard to make work.  A small diameter primary mirror and a short focal ratio make a poor telescope.  The short focal ratio wants a larger diagonal and a low profile focuser, but the focuser does not shrink in proportion with the tube size.  There is also a limit on how much the clearance can be reduced between the edge of the primary mirror and the tube walls.  These two factors cause a large percent of the light cone to be used up reaching from the diagonal to the focal plane.  This forces the use of a larger diagonal.  See the BLACK example for a 4¼ inch f/4 telescope.

Atmospheric Distortion

Atmospheric Distortion Moving or variable-temperature air above or inside the telescope which distorts the image.

Much has been said in books and magazines on reducing air currents above and inside a telescope.  The only idea covered here regards the design of the light baffles. In an open tube design, there will always be tube currents. Some telescope makers use an oversize tube to allow the inevitable currents to stay near the walls, out of the light path.  Baffling a tube to reduce glare can interfere with this current, causing the air to flow into the light path as it moves past the baffle (tube current eddies).  This can cause severe image distortion, wavy images, double images, and other problems.

To solve this problem, the baffles can be made fairly shallow, and placed closer together.  This allows the air moving near the tube walls to flow closer and stay out of the light path.  Newt-Web has an option (on the Specifications tab) to use fixed diameter baffles.  When this option is on, all the baffles will be the same diameter as the front baffle. The front baffle diameter is designed to be the same diameter as the 75% zone cone of light as it passes through the front of the telescope.

Illumination Size

Illumination Size: The size of the area at the focal plane (the virtual image) illuminated by the primary optics.


The focal plane is generally fully illuminated in the center, and gradually tapers off in brightness toward the edge.  A common way of measuring the illuminated area is by defining the zone of full illumination (the 100% zone), and the area where the brightness has tapered off to 75%.

The 100% zone is the area at the focal plane which is fully illuminated by the primary mirror.  This area will have 100% of the brightness available from the primary mirror.  This is the area produced by the light cone from the primary, reflected from the diagonal, as long as there is no vignetting.  Changing the diagonal minor axis is the easiest way to change the size of this zone.

The 75% zone is the area at the focal plane which is ¾ illuminated by the primary mirror.  This area will be dimmer than the 100% area, tapering off in brightness from the edge of the 100% zone until only 75% of the brightness from the primary mirror is available at the edge of the 75% zone.

Visual Use:

An eyepiece will usually have approximately the same field lens diameter as its focal length.  So to fully illuminate the field of a 12 mm eyepiece, a 12 mm (½ inch) area of 100% illumination is required.  Full illumination is not absolutely required, and in fact usually drops off to around 75% near the edges of the eyepiece field.

The larger the eyepiece field lens, the larger areas of 100% and 75% illumination required.  This is also impacted by the diagonal mirror minor axis and any possible vignetting by other elements of the telescope, such as the focuser inside diameter.

Some practical limit must be reached, however, because increasing the diagonal size will also decrease contrast and light gathering ability.  One possible rule of thumb is to limit the size of the 100% zone to one half of the field lens size of the largest eyepiece you expect to use.

Contrast is very important in a telescope.  To see fine details in planetary images and faint nebulae alike, you need the maximum contrast possible.  In a Newtonian telescope, one of the biggest contrast killers is an oversized diagonal mirror.  If possible, the diagonal minor axis should be kept under 20% of the diameter of the primary mirror.  This is easy with high focal ratio telescopes but can be very difficult with shorter focal ratios.  See the Improper Design section.

Photographic Use:

Generally, to attain the brightest image (and utilize the full potential of the telescope's light gathering ability), the film in the camera should be as fully illuminated as possible.  This requires a substantially larger diagonal mirror than does visual work.

In a 35mm camera, the short dimension of the film is 24mm (about 1 inch).  The camera body requires the focal plane to be moved farther out from the focuser as well. Adding 2 inches of focal plane height for the camera body, and requiring a 1 inch area of 100% illumination will call for a fairly large diagonal mirror.

The other components of the telescope must be redesigned to accommodate photographic work.  The focuser inside diameter must be larger to prevent vignetting of the light cone, and the diagonal mirror spider mount must be strong enough to prevent the heavier mirror from vibrating or sagging.

A telescope which is optimized for photographic use does not usually perform well for visual work.

Spare Focuser Travel

Spare Focuser Travel: The extra amount of travel the focuser can move inward from the point where the image is in focus.

This is the distance from the top of the focuser tube (when racked all the way in) to the focal plane (where the light from the primary mirror comes to a focus).  Some eyepieces focus farther in than others.  Shorter focal lengths usually need to be racked farther in than longer ones.

Note: If you will be using the telescope for terrestrial viewing, you will need some extra "out" travel to focus on objects which are closer than the heavenly bodies.  You will usually have more spare "out" travel than "in" travel.

The amount of "Spare Focuser In Travel" should usually be about ½ inch.

One way to set up a telescope right on the first try (and to avoid drilling focuser holes and mirror mount holes all over the place) is to design the scope with about ½ inch spare travel.  Then, using the calculated measurements for component placement, mount the diagonal mirror and the focuser and place an eyepiece in the focuser.  Mount the primary mirror in it's mirror cell, and get an assistant to slide the mirror into the calculated position in the tube. Try to focus on a VERY distant object.  The assistant should slide the primary mirror in and out until the object comes into focus.  Mark this position, remove the optics, and then drill the holes for the primary mirror mount.  Using an object that is not far enough away to focus upon will give an improper result.  An object at least ½ mile distant would be the minimum.

Another way to get the proper Spare Travel is to measure where each eyepiece's focal plane lies in relation to it's seating position in the focuser.  The field stop in an eyepiece is usually at the eyepiece's focal plane.  This focal plane must coincide with the primary mirror's focal plane for an image to be in focus.  Measure the distance from the bottom of the eyepiece tube (the part that slides into the focuser) to the field stop.  Then measure the distance from the bottom of the eyepiece tube to where the eyepiece stops in the focuser (the top of the part that slides into the focuser).  If the field stop sits down in the focuser tube, you will have to rack the focuser out to reach focus.  If the field stop sits above the focuser tube, you MUST add some extra "in" travel to move the eyepiece in far enough to focus.  The amount to add is the distance of the field stop above the focus tube, plus any safety margin desired.

Using a Camera

Camera: a frustrating infernal contraption which captures your soul in a little box <grin>.

If you will only use the telescope visually, use Zero for the variable "Additional Height for Camera".  If you will be using a camera, enter the amount of additional height for the focal plane above the focal plane used for eyepieces. This is usually about 2 inches for 35 mm Single Lens Reflex cameras.  You should also add any height required for an off-axis guider.  This is usually three quarters to one inch.

This will increase the probability of vignetting the light cone, because a larger conic section will have to fit through the focuser and other parts of the telescope to reach the focal plane.  For this reason, a telescope designed for camera use should have the lowest and widest possible focuser.

The size of the diagonal mirror should also be taken into account.  A larger diagonal may be required to accommodate the 100% zone desired.  This will decrease contrast and block more light from the primary mirror.  A telescope should be designed for either visual or photographic work if possible, although a dual purpose scope can be built with some compromises.  See Illumination Size.


A few good books on telescope design are:

There are many others, mostly at a higher cost, that are very good.  There are some, also at a higher cost, that are a waste of money.  You'll have to discern for yourself.