How to tell if telescope needs collimation

telescope collimation

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Astronomy is one of the most fascinating fields in the world! When using a telescope, it is vital to understand the concept of ‘collimation.’ Lenses and mirrors are optical elements in telescopes that require precise alignment. By aligning your telescope, you ensure that light travels straight through, hitting each mirror or lens at the right angle and providing clear, detailed images. 

Observing celestial objects can be difficult due to misalignment. Therefore, it is imperative to perform proper collimation when stargazing to get the most out of your observations. Your telescope is your instrument, and the universe is your music. It’s like fine-tuning a musical instrument.

how to tell if telescope needs collimation

Telescope Needs Collimation?

Recognizing Signs of Poor Collimation 

Persistent blurriness or double images are the most common signs indicating your telescope needs collimation. For instance, if you observe Venus and see more than one crescent, it suggests that your telescope’s collimation is off. Similarly, if an ordinarily bright star appears less sharp than usual, it may be time to check your telescope’s alignment. 

Impact of Poor Collimation on Image Quality 

Poor collimation can adversely affect your stargazing experience. Only if your telescope’s mirrors or lenses are perfectly aligned will light scatter rather than focus in one direction. The resulting images are blurry, distorted, or lack detail. Poor collimation may prevent you from observing Saturn’s rings or Jupiter’s moons. You need good collimation if you want your observations to be precise and your stargazing experience to be enjoyable.

telescope needs collimation

The Star Test Method

Necessary Equipment and Ideal Conditions 

Conducting a Star Test requires a telescope and a bright star. Atmospheric conditions may affect your results if you choose a star high in the sky. The test should occur on a clear night without moonlight, which may cause light pollution. 

How to Conduct the Star Test 

  • Step 1: Acclimate Your Telescope Allow your telescope to adjust to outdoor temperature, as temperature differences can influence the test results. 
  • Step 2: Select a Star. Opt for a bright star in the sky. 
  • Step 3: Focus on the Star Employ a high-power eyepiece to enlarge the star’s image and center it in your view. 
  • Step 4: Defocus the Star. Gradually defocus until a light ring appears around it. This ring comprises a series of concentric circles. 
  • Step 5: Examine the Patterns: It’s time to observe these patterns closely on both sides of the focus. 

Analyzing the Results 

Ideally, the star test would yield perfectly round, evenly lit concentric circles. These circles suggest your telescope’s optics are correctly aligned or ‘collimated.’ 

Your telescope’s optics may need to be adjusted if the circles appear distorted, asymmetrical, or unevenly illuminated. 

Potential Aberrations 

Several aberrations or variations might be noticeable in your star test. It may indicate a tilt in your telescope’s optics if the rings are brighter on one side. Astigmatism could be characterized by rings that appear more elliptical than circular. 

Do not be alarmed if you notice such problems! Depending on your telescope’s optics, you may need a collimator. It may be necessary to seek professional assistance or follow the manufacturer’s instructions.

how to tell if telescope needs collimation

The Collimation Cap Method

A collimation cap is a simple but effective tool used to align telescope optics, a process known as collimation. This cap, often supplied with the telescope or available for purchase separately, is a dust cap with a small hole in the center. It works by narrowing the view to a single light path, making it easier to see whether the mirrors of your telescope are aligned. 

How to Use a Collimation Cap 

  • Step 1: Insert the Cap Start by inserting the collimation cap into the focuser instead of an eyepiece. 
  • Step 2: Make sure your telescope is pointed at a neutral and non-reflective surface, preferably in dim lighting. 
  • Step 3: Look through the hole at the collimation cap. At the back of the telescope, you can see the reflection of the primary mirror on the secondary mirror. 
  • Step 4: It is essential that the reflections of the primary and secondary mirrors are aligned. It is also necessary to align primary mirrors (and the collimation capsule) with secondary mirror reflections. 
  • Step 5: Adjust if Necessary. If the mirrors are not aligned, you must adjust them. This typically involves turning the adjustment screws on the primary mirror until the reflections are centered. 

Adjustments should always be made in small increments according to the telescope’s instructions. 

Interpreting the Results 

Perfect collimation is indicated by the concentric alignment of all components when viewed through the cap. If everything is correctly aligned, The primary mirror has a series of concentric circles around the outside, the secondary mirror, and the hole in the collimation cap. 

Using a collimation cap, you can ensure that your telescope’s optics are well-aligned, leading to sharper and more detailed views. Collimating your telescope may seem daunting at first, but it will become second nature with practice.

collimation cap picture

Can You Make Your Own Collimation Cap?

Collimation caps are invaluable tools for fine-tuning the optics of your telescope. However, you don’t always have to buy one. You can create your efficient collimation cap with ingenuity and some everyday household items. 

Materials Needed 

  • Telescope’s eyepiece dust cap or a film canister cap. 
  • Black marker pen. 
  • Aluminum foil. 
  • Pair of scissors. 
  • Sewing needle or pin. 
  • Adhesive tape. 


  • Step 1: Make sure your dust cap is centered using the black marker. You will use this to create the hole. 
  • Step 2: To puncture the cap, use a needle or pin to make a small, precise hole at the center mark. The diameter of this hole should be 1-2 mm. 
  • Step 3: Attach the aluminum foil to your dust cap, making sure it is slightly larger than the dust cap. The foil’s corresponding hole must be aligned perfectly with the dust cap’s inner side using adhesive tape. 
  • Step 4: If you lift the cap toward a light source, you can see through the hole. It doesn’t matter if there are obstructions as long as the light shines through. 

Tips for Optimal Usage 

  • Ambient Lighting: Reduce glare and facilitate easier alignment by collimating in dim lighting. 
  • Alignment Check: Insert the collimation cap into the eyepiece holder and peer through the hole. The primary mirror’s reflection must be perfectly centered in the secondary mirror. 
  • Adjustments: Adjust your telescope’s mirrors by referring to its manual in case of misalignment.
collimation technique

Observing Venus

We can learn a great deal about the alignment of your telescope’s optics from Venus, our nearest neighbor and second planet from the sun. Due to its bright luminosity and distinct phases, Venus is a superior celestial body to our moon for this purpose. 

Indications for Collimation: When your telescope is collimated correctly, the image you see should be sharp and well-defined. If Venus appears blurry or distorted, it could indicate that your telescope’s mirrors are misaligned, and collimation is necessary. 

Observing Multiple Crescents: Venus goes through a complete set of phases, just like our moon. At times, it presents as a crescent, similar to a crescent moon. Your telescope’s optics may need to be aligned if Venus appears with more than one crescent. The need for collimation is evident from this. 

What to Look For: It would help if you looked for a single, sharp, well-defined image when observing Venus. Depending on its current phase, the planet should appear as a small disc or a crescent. You should not see multiple images or crescents. If you do, it indicates that your telescope needs collimation. 

How to Collimate Your Telescope

A telescope’s optimal performance depends on the process of collimation. It is necessary to adjust a telescope’s optics to align perfectly. The lens must be aligned to focus light accurately to deliver sharp and clear images. Regardless of their power or sophistication, collimated telescopes provide the best viewing experience. 

The following guide will walk you through the process of collimating different types of telescopes, focusing primarily on Newtonian and refractor telescopes. 

Newtonian Telescopes 
  • Center Spot Check: Check the center spot on your primary mirror. It should be perfectly centered. You can do this visually by removing the eyepiece and looking down the focuser. 
  • Rough Alignment: Using a collimation cap, align the secondary mirror to see the entire primary mirror reflected. 
  • Fine-tune Alignment: Adjust the tilt of the secondary mirror using its adjustment screws until the center spot is centered in the eyepiece’s field of view. 
  • Primary Mirror Adjustment: Then, align the secondary mirror with the eyepiece’s field of view by adjusting the primary mirror’s collimation screws. 
  • Verification Finally, verify the collimation by observing a bright star. Defocus the image slightly. Your telescope is well-collimated if the resulting “donut” of light is symmetrical. 
Refractor Telescopes 

Compared with Newtonian telescopes, collimating refractor telescopes are less prone to misalignment due to their construction. However, if necessary, here are the steps 

  • Visual Inspection: Visual Inspection Start with a visual inspection. Look through the focuser without an eyepiece. There should be a perfect circle between the objective lens and the focuser. 
  • Star Test: You can perform a star test by observing a bright star. It may be necessary to collimate the star if it appears distorted or elongated. 
  • Adjustment: Adjust the objective lens cell screws until the star appears as a perfect point of light with a diffraction ring around it when slightly defocused. 

Tools Needed for Collimation

As a result of collimation, telescope optics are aligned, resulting in more precise and sharper images. A collimation cap, Cheshire eyepiece, and laser collimator can aid this process. 

Collimation Caps: Collimation caps are simple devices that fit onto the focus of a telescope. They feature a small hole in the center that allows users to align the primary and secondary mirrors of the telescope manually. 

Cheshire Eyepieces: Cheshire eyepieces are more advanced tools used for collimation. These devices use a bright ring at the bottom and a reflective surface at an angle to illuminate the telescope’s optics, facilitating easier alignment. 

Laser Collimators: Laser collimators project a laser beam onto the mirrors of a telescope. Users can accurately align their optics by adjusting the mirrors so that the beam hits specific targets. 

Purchasing Collimation Tools: Before buying collimation tools, you should consider the type of telescope you own, your budget, and your experience level. Choosing the right tool for your telescope depends on its size and design. 

Online retailers and astronomy shops offering collimation tools include Astronomics, OPTCorp, High Point Scientific, and Skies Unlimited. Several options are available on these platforms to choose the right tool. 

SVbony collimating eyepieces, suitable for telescopes with F-6 apertures, are also available on Amazon. 

Choosing the Right Collimation Tool 

Collimating tools are selected based on your telescope type and specific needs. It may be sufficient for beginners to use a few collimation caps or Cheshire eyepieces. A laser collimator may be helpful for experienced users or those with more complex telescope setups. 

How often should you collimate a telescope?

Newtonian reflectors must be checked for collimation every third time you use them. Collating your telescope a few times a year may only be necessary if you observe Deep Sky Objects (DSOs). 

Many Dobsonian telescope owners collimate their telescopes every time they set up, which can become an efficient process with practice. For the best results, collating on the side of the sky where you’ll be observing is recommended. If you pass the meridian, you should recheck the collimation. 

Some types of telescopes, like refractors and Cassegrain telescopes, probably won’t have to collimate them since they’re permanently collimated when they’re manufactured. However, the collimation might need more frequent adjustments if your telescope travels frequently, especially over bumpy roads. 

Collating your telescope once a month is recommended if you use it regularly. Regardless of its type, any telescope will usually require collimation adjustments before each observing session. 

How important is it to collimate a telescope

Collimating a telescope is of paramount importance for several reasons. At its core, collimation refers to aligning the telescope’s optical components, such as the mirrors or lenses. This alignment determines a telescope’s optimal performance. 

When a telescope is not well-collimated, it suffers greatly in image quality. Often, the image’s contrast is compromised. A slightly off alignment can still affect the image quality you see through the telescope. 

The primary purpose of collimation is to align two axes to form a single axis. Secondary mirrors are typically tilted and positioned to achieve this. A focuser tube’s center axis is aligned with the optical paths of mirrors during collimation. 

Well-collimated telescopes, particularly Newtonians, must be nurtured, cared for, and understood to perform at their best even when costly and advanced; collimated telescopes can provide clear and sharp images of celestial objects. 

Last words

Adjusting your telescope’s parts to line up right or collimate is essential. To see if your telescope needs collimation, you have to look into the part where you usually put your eye, without any eyepiece. If the mirrors inside don’t line up and you see them off-center, your telescope needs collimation. 

Even though this might seem hard at first, with time, it gets easier. Checking and fixing your telescope often ensures you get the best view of the stars. So, keep doing this and enjoy looking at the night sky in the best way possible!

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