How to Collimate a Telescope

How to Collimate a Telescope | Wondering the Steps.

At one night, the telescope party plan on the beach is finalized. I invited all my friends. All food arrangements are made. Suddenly you receive a call from your best buddy that his telescope has got misaligned. You can not imagine the party without him, and he cannot imagine the party without his telescope. So how haunting and tedious it may seem, but you have to manage it. What options you have?

You have to go through the following details that will serve as a base for your telescope collimation. The more you gather information on the topic better it is.

Make one thing clear, any machine, big or small, has mechanical parts that require proper maintenance and alignment for adequate working and prolonged performance. Be it your car, guitar, or any treadmill. So your efforts for the alignment of your machine are not in vain. They result in the added life of your star-watching device.

Before we proceed further, let’s take a comprehensive look at the different telescope types and basic design differences of the optical types.

Types of Telescope:

The telescope can be classified under lots of headings. A few of them are;

⇒ Optical types

  • Refractors
  • Reflectors
  • Catadioptric

⇒ Users needs

  • Professional
  • Amateur
  • Commercial

⇒ Site of use

  • Terrestrial
  • Sky
  • Space

⇒ Functional basis

Solar, they observe Sun.

Dobsonians are low-cost and portable.

Aerial works in space.

⇒ The wavelength of detected light

  • X-ray telescope
  • Ultraviolet
  • Optical
  • Infrared
  • Submillimeter
  • Radio

Since optical types are more common and related to collimation so we will discuss them categorically:

⇒ Refractors(dioptres):

It uses lenses for collection, focus, and perception of the image. They were the basic orthodox telescopes of old times. The work of Hans Lippershey and Galileo Galilee laid the foundation of its actual position and principles.

They work on the principle of light refraction. Some of the largest telescopes in the world are the Yerkes observatory, Lick observatory.

They are used in astronomy, land viewing, bird watching, spy glasses, photography, spectroscopy, etc.

They are associated with chromatic aberrations. These can be handled by using different configurations.

It has advantages over reflector by being user-friendly, better results in the photography fields, and spy wares.

Examples of refractors: Galileo scope, monocular, binocular, achromatic, apochromatic, non- achromatic, super achromatic, varicose gas-lens, etc.

⇒ Reflector (catoptrics) :

It uses mirrors for the reflection of light and the projection of images. Isaac Newton was the first person to design a reflecting telescope. As time, age, and technology advanced, it’s working also improved. They are cheaper than other counter products. Gran Telescopio Canaria in Spain is the largest reflector.

It is better than refractors in that it has minimum chromatic aberrations, gives more optics.

Examples of reflector-based telescopes are Newtonian, Dobsonian, Cassegrain, large liquid mirror, toroidal, Gregorian, herring, etc.

⇒ Catadioptric:

They are made by combining specially designed mirrors and lenses to project an image.

They can be designed using spherical surfaces. This helps develop a folded optical path, reducing the mass, so they become easier to carry. Their use is extensive in the photography field.

Examples include Schmidt camera, Kravtsov-Cassegrain, catadioptric dialytes, etc.

Operational facts and principles

The objective is the primary mirror or lens. It collects and culminates light rays on a Focal Point. The larger aperture of the objective, the more significant is the amount of light it collects, and finer details can be appreciated.

Uses of the telescope:

  • Observational astronomy
  • Study of birds, their habitats, and lifestyles.
  • Navigation of the aircraft or ships in water by observing ground features, maps, and charts.
  • Military activities like locating an enemy in a region or making terrestrial attacking plans.

Components of the telescope


It is closer to the eye. It is a device that presents an image to the eye after it is projected from the object. It measures the field of view and magnification of the telescope. They are interchangeable and adjustable to focus the object more clearly.


  • It is responsible for providing the magnification of the telescope and which in many cases is interchangeable. Magnification =telescope focal length (mm)/eyepiece focal length (mm)
  • It gives well defined crisper image.
  • It gives convenient eye relief. Wondering what it is? It is the total distance between the eyepiece and your eye with a focused image. People need to wear glasses.
  • It also assesses the apparent and actual field of view.

Objective piece:

It is closer to the object. It is a device whose aperture is responsible for collecting light from the thing. This projected light comes from far away to pass from the mirror or lens as parallel lines and focus. The eye perceives this as the image.


The greater aperture collects more light and reveals better and detailed information of the image at a distance.

Objective image is not magnified; it is magnified with the help of an eyepiece.

It refers to the resolution of the telescope.

Focal length

It is the distance between the objective lens and the focal point.


  • Magnification depends upon it. A larger focal length gives greater magnification.
  • For larger apertures, a smaller focal length is chosen to have a reasonable size of the telescope—ancillary optics help reach an effective focal length where the desired image is received.

The basic knowledge about these basic optics will help us in troubleshooting different problems during collimation.

After reviewing the standard basic optics, we will review the basic reasons for misalignment to proceed to alignment in an organized manner.

Reasons for misalignment:

Misalignment due to defects in optical surfaces ;

These occur whenever there is any deviation from the normal position. These effects can: tilt, decenter, or despace.

  • Tilted; it is angular and causes coma aberrations.
  • Decentre; is longitudinal and causes coma aberration by making irregular waveforms.
  • Despace; it is linear too. It results in conversion or diversion at a point different from the focal point.

As a general rule, misalignment sensitivity is directly associated with the number of optical surfaces and the greater extent of their surfaces. It also varies with the design and properties of the product telescope.

Misalignment due to external factors

  • Extremes of temperature; occur as a result of temperature differences towards extreme levels. Usually, it occurs due to an equilibrium in the optical elements and their surrounding atmosphere. Mirrors are affected more, so it’s common in reflectors.

Thermal cooling results in contraction and shorter focal length. Sometimes two surfaces of a mirror expand or contract differently and hence resulting in more misaligned images. They can be dealt with by using optics thermals following the surrounding temperature.

  • Mounts; their pinching pressure is reflected as a diffraction pattern. These diffractions are manifested as astigmatic problems since mount pinching can occur due to thermal expansion or contraction so they are held a bit loose in their holdings.
  • Force of gravity; causes deformation of glasses with thin ends and larger apertures. You can minimize this by using proper supports.

Read More: About the sighting in thermal optics

Intrinsic telescope aberrations;

These are due to inherent aberration limitations or manufacturing defects. These are important to know since they are usually not corrected by collimation. They are spherical, chromatic, coma, distortion defects, or defects due to aperture. Since they are due to the properties of their fundamental optics, you may reduce them to some extent by using complex and combination optics.

Indications for collimation:

1.Star test:

2. Prerequisite;

  • It should be a clear good seeing night with little or no turbulence.
  • Observe a bright non-twinkling fixed star.
  • A mount for tracking the star.
  • A high magnification eyepiece, I.e., At least 50-100 power optics.
  • An optional webcam to help you take immediate photographs. This can be of particular help in cases of cloudy weather.
  • You can analyze these images with the help of digital effects. You can enhance the webcam working by using a wide range of software programs, such as the K3CCD tool.
  • The telescope should be cooled down to the surrounding air temperature. This will not only avoid temperature artifacts but also helps in better alignment results. Though temperature adjustment may take an hour or longer, it is worth waiting.

3. Steps of star test

  • The telescope is focused on a bright star with all mounts fixed, and temperature ambiance achieved.
  • After the telescope has focused the star well, adjust the eyepiece to high magnification.
  • Observe the concentric rings and their center.

When you try to focus a bright star with high magnification,

4. Results:

  • The star looks like a doughnut shape which is dark in the center.
  • Streaks are moving away from the out-of-focus star view. These bleeding stars indicate temperature cooling down.
  • The concentric rings may or may not be grouped.
  • Spherical aberrations are expected if the concentric rings look more elliptical, and as you move along the focal point, its axis along the linear axis will move through 90 degrees.
  • The star view is shifted to one side.
  • The concentric rings are distorted or misty.

The above results indicate a star test positive for collimation.

What is collimation?

Definition: it is the process of aligning the optical components of a telescope to appreciate the minor details vividly.

Mechanism of collimation & Types of collimation:

Optical; refers to the accurate alignment of the optical surfaces for bringing the image in the focal plane.

Mechanical; this refers to the proper fixing of the physical parts of the telescope. It deals with focuser or secondary mirror being decentralized.

Pre requisites for collimation

Alignment revolves around two main components, which are the eyepiece and the primary mirror. The secondary or diagonal mirror is a small flat mirror meant just for reflection. It is located near the tube top.

Get the collimation cap ready.

Mark the center of your primary mirror. In most cases, it is pre-marked. Otherwise, you can do it yourself manually.

Most of the refractors, such as Newtonian and Schmidt-Cassegrain, require collimation.

Focuser should not be tilted by using the thumbscrew.

Refractors are aligned during manufacturing and hardly need any collimation afterward.

Whereas refractors almost always need

How to Collimate a Telescope?

Step 1: preliminary collimation,

The rough alignment of the mirrors is estimated. Remove the eyepiece and look down at the focuser. There should be retaining clips for the primary mirror at 12,4 and 8 o’clock positions.

If they are well placed, we can proceed further.

If not, then the screws are loosened by the screwdriver unless the secondary mirror slope faces the focusing tube.

Step 2:secondary mirror is streamlined with the primary;

Laser collimation is fitted in the tube where the eyepiece is held. The target on the collimate should face the primary mirror. The collimator should be adequately fixed to avoid any inconvenience during alignment.

After this laser is switched on to locate the center point of the primary mirror.

Once the laser dot locates the center with minor fine adjustment, the screws are tightened back with the help of a screwdriver.

Step 3:the primary mirror is rechecked,

At the bottom of the telescope, there are adjusting screws for the primary mirror. Try by loosening a single screw at a time; while you are doing it, keep an eye on the laser dot. If it moves towards the target center, you are in the right direction. But if it doesn’t, try by moving to the next screw and so on .once you have focused the laser dot in the center, fix back the screws, and proceed to the next final step.

Final re-checking by fine readjustment :

Suppose the laser dot gets decentralized while tightening the screws. Don’t panic .you need some fine adjustment by adjusting the screws. One dot is centralized. It’s all done. Enjoy the starry night with a high sense of achievement. You can even focus on yourself as a collimation star in your telescope.

Tool-free collimation:

Tools required to move it before steps:

  • A Cheshire eyepiece such as astromania. It is the real working unit of alignment and also a tube with a peephole. It has a shiny face, which is set at 45 degrees to the barrel. There’s another at the side. You can illuminate the reflective face by using the light into the opening.
  • Collimate cap or sight tube. It has a minute peephole in its center and a cap at one end to insert into the focuser. The lined-up optics are watched through it.
  • Laser collimates; it is a solid-state type. It is mounted on the focuser. It works by giving out a very narrow light beam along the axis of the focuser. This helps to locate where the focuser is pointing.
  • Screwdriver for loosening the screws.

Technical literature sources:

You can get a lot of literature and know-how on the topic by talking to someone experienced in collimation. A lot of guidance can be sought by reading the manuals and the telescope, some excellent easy steps for the beginners online, both in articles and videos.

Troubleshooting the collimation process:

  1. Why my scope needs collimation?

If you are looking for crisper images without missing any details such as tiny craters on the moon, avoiding star merging, Jupiter’s belt, etc., then, of course, your aesthetic sense owes you an alignment.

2. What is coma aberration?

When the image deviates from the focal point center, the result is coma aberration where the blurriness misses some fine details.

3. What could be the effects if the secondary mirror is off-set?

It affects to the extent that the focal plane illumination is diminished. This doesn’t interfere with the collimation much.

4. What happens when the focuser is tilted?

When it is tilted, it does not affect the image quality.

5. How often should it be done?

Most of the refractors don’t require collimation as they are factory-made.

Reflectors almost always require collimation especially Newtonian ones.


Collimation requires simple basic knowledge of the telescope components, optical elements related to alignment, and following rules in a step-wise approach. It only needs a bit of brain and bones to become a telescope collimation star!

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