How to Star Test a Telescope: Ultimate Guide

Photo of author
Written By: Zane Landers
Category: Learn

A well-functioning telescope not only broadens our perspectives but also fuels our curiosity about the cosmos. However, even with the right equipment, one might face difficulties in observing celestial bodies due to optical aberrations or manufacturing imperfections in the telescope. Some of these are caused by permanent flaws, but others can be compensated for or fixed if you know what you’re doing. Therefore, it is crucial to test your telescope’s optical performance to ensure that you are receiving the best and most accurate views of stars, nebulae, planets, and galaxies.

Star testing is a widely acknowledged and straightforward method for assessing the optical quality of a telescope. It leverages the power of observation and simple physics to discern potential issues in your telescope’s optics. Whether you are a novice skygazer with your first telescope or an experienced amateur astronomer, knowing how to star test your telescope can significantly enhance your astronomical observations. Unlike other methods of optical testing, it requires no additional tools or equipment besides your telescope and a good eyepiece, it can be done in minutes, and you can do it in the middle of a normal observing session. You might even find yourself star testing other astronomers’ telescopes if you’re offered a look through the eyepiece; it can be done with hardly any interruption to a night of stargazing.

In this guide, we will elucidate the process of star testing a telescope, from selecting the right star to understanding the common aberrations you may encounter. We will break down the complex methods into easy-to-follow steps, unraveling the technical jargon to present the information in an accessible and comprehensible manner. We’ll also provide you with tips and tricks to help troubleshoot common issues you might face in the process.

Regardless of the design, make, or model of your telescope, this guide aims to empower you with the knowledge to maintain its optical performance at its best. As you journey through these pages, you will find yourself equipped with the tools necessary to ensure your telescope delivers the optimum stargazing experience you desire.

What you’ll need for Star Testing

To star test your telescope, you either need to point it at a bright star that does not move much thanks to its position near the celestial pole (i.e., Polaris in the Northern Hemisphere, or Sigma Octantis in the Southern Hemisphere) or, if you have motorized tracking, any yellow, white, or bluish star of magnitude 3-4 or brighter will do the job. You can technically star test on any star without tracking, too, but you need to keep the star centered, as it can be distorted by coma, field curvature, or aberrations from your eyepiece if it is not.

Once you’ve got a suitable star centered in your eyepiece, you’ll want to insert an eyepiece that produces at least 35x magnification per inch of aperture but not more than 60x per inch (so 210-360x for a 6” telescope, for example). Do not use a Barlow lens, as this can alter the apparent correction and the appearance of some optical defects. Your telescope should be collimated and cooled down, and you should be using a high-quality eyepiece if possible.

Once this is all done, it’s time to defocus the star and get to work, but first we need to go into how star testing works and what you should expect to see.

Diffraction, or How Star Testing Works

When a beam of light—say, from a star—passes through a small opening (like the aperture of your telescope), it does not continue in a straight line. Instead, the light spreads out or diffracts. This diffraction is responsible for the formation of an interesting and important pattern: the Airy disk and its accompanying diffraction rings.

The Airy Disk is named after Sir George Biddell Airy, a 19th-century British Astronomer Royal who first explained the phenomenon. When starlight passes through your telescope, it interacts with the circular aperture, creating a central bright spot surrounded by concentric circles of light. The central bright spot is what we call the Airy Disk, and the surrounding concentric circles are the diffraction rings.

The Airy disk and its rings arise from the wave nature of light. As the light waves converge at the focal point, they interfere with each other, both constructively and destructively, based on their phases. This interference leads to the formation of a series of light and dark rings around the central bright spot—the Airy Disk.

The size of the Airy disk is inversely proportional to the aperture of the telescope—larger telescopes will have smaller Airy disks, and vice versa.

Airy Disks

In perfect focus, the Airy disk is the brightest point, surrounded by a series of fainter diffraction rings. With a good telescope under ideal conditions, the Airy disk is approximately the size of your telescope’s resolving limit according to the Rayleigh criterion. The resolving power of optical instruments, such as telescopes, hinges on two primary factors: the aperture size of the lens or mirror, and the wavelength of the observed light. It is not influenced by the focal length or magnification of the instrument. In the world of telescopes, we commonly refer to resolution in terms of angular value, typically reported in arc seconds.

The Airy pattern manifests as a vivid central disk, often referred to as the spurious disk, encircled by alternating dark and bright concentric rings. The physical distance, r = 1.22 λ F, to the dimmest point within the first dark ring, known as the minima, provides the radius. Alternatively, you can define the angular distance, A = 1.22 λ / D, which gives the angular radius of the Airy disk in radians.

Small telescopes are less frequently limited by atmospheric conditions or their own thermal issues, and thus the owners of small refractors and catadioptrics are used to “pinpoint” stars at most magnifications. Likewise, telescopes tend to put up “smaller” stars at lower magnifications; and stars often appear “fuzzy” when atmospheric turbulence, thermal issues, or the telescope’s own optics do not allow for the best performance.

However, it’s important to note that your eyeball is the limiting factor in your telescope’s ability to produce sharp stars. The Airy disk of a 6” telescope is a little under an arc second. A good eye has a resolution of a couple arc minutes, one of which is 60 arc seconds, meaning that magnifications below ~60x won’t even get close to resolving the Airy disk, and in most cases, 100x or more is needed to even begin to glimpse it. Many optically defective telescopes still produce “sharp” views at lower magnifications because the telescope itself is simply not the limiting factor, though bad optics will result in contrast loss that impairs detail in less obvious ways.

For resolving the Airy disk clearly, magnifications of 40-60x per inch of aperture are commonly used. With equal aperture, the telescope’s optical design doesn’t alter the Airy disk size, but it does influence how the light is distributed between the central visible disk and the first diffraction ring—the ring that lies just beyond the first diffraction interspace. For instance, a refracting telescope will channel more light into the disk and less into the first diffraction ring, while an obstructed telescope will redirect slightly less light into the disk and more into this initial ring. Telescopes with the secondary mirror on a spider have spikes, which are caused by light diffracting around the vanes; this is merely a cosmetic problem and not a defect per se.

On a good night, even an optically defective telescope should produce something resembling an Airy disk. You could theoretically star test while completely in focus if the atmosphere and air currents within your telescope didn’t matter. Aberrations such as astigmatism (producing a warped oval Airy disk and rings), pinched optics (distorting it into a triangle), and severe miscollimation are usually visible even amid turbulent air currents. 

However, those faint diffraction rings around the Airy disk, which are used to diagnose the actual figure and surface quality of an optic, are usually too faint and blurry to clearly resolve at the focal point of the telescope, and even if they were consistent, it would take a lot more effort. Thus, for diagnosing most optical issues with the telescope, we turn to the more common “star test” which involves putting the telescope slightly out of focus.

Defocusing Stars & Diagnosing Abberrations

When you slightly de-focus your telescope, the light spreads out over a larger area, and the diffraction pattern changes, too. You’re not going very far outside of focus, just enough to make the star appear a few times “bigger” and get some diffraction rings to show up. Too far, and you’ll miss them. The rings should be round and evenly illuminated, but in practice, this isn’t always the case.

There are essentially two types of aberrations in the star test. One is the optical defect, which is inherent in the telescope and not easily removed. The other are defects, which are usually caused by some exterior factor or mechanical problem and can be fixed.

First up are the incurable optical problems.

Spherical Aberration

The first thing you’ll notice in any telescope other than a refractor is that the defocused stars resemble a “donut” – this is from the shadow of the secondary mirror. The size of the central obstruction should appear exactly the same on either side of focus. If it doesn’t, this can be indicative of spherical aberration – though the moving-mirror focus design of many catadioptrics means that the obstruction may look a little different on each side of focus even with otherwise perfect optics, albeit not by much. 

The other way to look for spherical aberration (and the only option in refractors) is to examine the out of focus rings. A telescope with spherical aberration will appear to have some of the rings appear too bright with the focus racked inward, and some appear to brighten when outside focus. The worse this is, the worse the aberration. Spherical aberration means that the telescope’s optics are uniformly deviating from the desired curvature; in the case of a Newtonian reflector, it means the primary mirror is either ellipsoidal (undercorrected) or hyperbolic (overcorrected); over- and under-correction occur in other optical systems but are often caused by issues with optical elements not “matching”, since even a refractor has more than one optical surface that can be messed up. An undercorrected optical system will have the outside rings focus (i.e., brighten) too “soon” and the inside reach focus past the focal plane, while an overcorrected one will have the inner parts focus first, and the outer edges focus outside the actual “focal plane” of the instrument.

Spherical aberration is the bane of many cheap “hobby killer” telescopes. Spherical mirrors are easy and cheap to test and manufacture, so many cheap reflectors use them. A long focal length sphere deviates so little from a parabola that it will work just fine, but to keep shipping boxes small, many manufacturers ship fast f/ratio telescopes with shoddy spherical mirrors or with a “Bird Jones” type corrector inside the focuser in a vain attempt to fix the problem. Other than repolishing and recoating the mirror or lens of a telescope, there is no actual cure for spherical aberration.

Spherical aberration can also be caused by a telescope not cooling down enough and having an optical surface distorted by expansion/contraction with temperature, but you’ll know if this is the case if it coincides with an obvious thermal problem, which we’ll get into shortly.

Zonal Errors

Zonal errors are imperfections in the telescope’s mirror, lens, corrector, etc. that affect only a specific “zone” or area of the optics, not the entire surface. This is usually caused by a defect in a polishing tool that creates a “divot” in one radial area of the optic, producing a “hole” or “hill” on the otherwise decent curvature. Most zonal errors are radially symmetrical, but not always.

When a star is defocused, zonal errors can create irregularities in the diffraction rings. Instead of smooth, concentric circles, you’ll see a brightening, darkening, or fuzziness to some of the rings. With a non-concentric, irregular zone, you might see a distortion or lumpiness in one section of the rings, or the rings may appear brighter in one area and dimmer in another.

The most common non-concentric zonal error in telescopes is a defect in the Schmidt corrector plate in a Schmidt-Cassegrain or Schmidt-Newtonian telescope. These correctors are produced by pressing the thin glass plate against a “mold” while being polished. If the contact is not smooth, areas where the corrector is “dented” or springs up away from the mold can cause zonal errors. The video below is of a star test of a Criterion Dynamax 6 Schmidt-Cassegrain, which has some of these clearly visible in the star test. The Criterion Dynamax telescopes frequently suffered from this issue due to quality control and patent problems; interestingly, this sample still performs surprisingly well.

Similarly to an undercorrected or overcorrected surface, the only way to fix zonal errors is by repolishing the optic with the defects.

Turned Down Edge

Turned down edge is essentially an extreme zonal error where the curvature is too deep at the edge of the mirror or lens, essentially being over polished. Many cheaper refractors stop down the outer edges of their objective lenses because they tend to have them from the factory. Turned down edge, or TDE for short, is caused by the edges of the polishing tool for a mirror or lens getting “caught” as it drags off the surface, itself commonly caused by the tooling being too squishy and easily deformed. TDE commonly affects many amateur telescope makers, and many older homemade mirrors have TDE, or a worse form called rolled edge, which affects a larger area.

TDE is extremely obvious in the star test, as the edges of the outermost diffraction ring look “hairy” on one side of focus and too bright on the other. TDE is extremely difficult to remove by repolishing as it essentially requires bringing the rest of the optical surface down to match the overpolished edge, and as such, in a finished telescope, there is little you can do besides stop down the outer edges of the mirror or lens with a piece of paper or cardboard cut into a ring. While it might seem like you’re losing aperture if you do this, the aperture affected by TDE is useless anyway, and masking it off will significantly improve performance.

Mechanical Problems in the Star Test

Not all issues in the star test are incurable and permanent. Many are caused by thermal or mechanical factors. Astigmatism can be caused by your optics but is more often due to fixable mechanical factors, so we’ve put it in this section.


If the diffraction rings appear to be oval or elongated in one direction and then flip at a 90-degree angle on the opposite side of focus, this could indicate astigmatism. Astigmatism is caused when one axis reaches focus at a different point than the other, or essentially by the focal plane and/or an optic being shaped like a potato chip. Astigmatism caused by your eyeball will rotate with your eye. Astigmatism usually only affects Newtonian reflectors.

Astigmatism in the telescope can be caused by three things:

  • A bad secondary mirror – easily solved by getting a new one from a reputable optical manufacturer
  • A bad primary mirror, either the figure or the glass itself, having poor anneal
  • Bad support of the primary mirror
  • Bad thermal management of the primary mirror

Astigmatism caused by the primary mirror itself being defective is easy to diagnose; simply rotate the primary mirror in its cell and have a look again. If it moves, it’s your primary mirror. This could be because the primary itself has astigmatism, or because of a bad anneal of the glass itself. A bad anneal will cause different amounts of astigmatism at different temperatures, as it means that the glass cools and expands/contracts differently in different areas. A bad mirror will be consistently astigmatic all the time. Bad annealing is essentially incurable without re-melting the glass and grinding a new mirror; astigmatism in the mirror can be removed by repolishing. Smaller scopes with primary mirror astigmatism tend to be caused by bad anneal; larger mirrors with astigmatism are usually caused by inadequate support and care during polishing.

The good news is that the majority of cases of astigmatism are not caused by anything to do with the primary mirror itself, but rather by other elements of your telescope. One issue that is easy to diagnose is the primary mirror support. Many telescopes with thin primary mirrors don’t adequately support the edge. When the telescope is pointed lower in the sky, the mirror will “fold over” and suffer from astigmatism. If your scope’s astigmatism goes away when aimed near straight up and appears worse nearer to the horizon, then you’ve got a primary mirror support issue. Mike Lockwood’s guide on mirror support explains more; for many smaller consumer-grade scopes, even a simple improvement like properly shimming the mirror in its cell with paper or cork pads is enough to fix the issue. In some cases, uneven cooling of the mirror can also cause astigmatism, but like spherical aberration caused by cooldown, it is easier to notice the overarching thermal issue than the astigmatism itself.

The most common cause of astigmatism in telescopes, and the one that is likely to be the case if you’ve ruled out the above, is your secondary or diagonal mirror being out-of-flat. This can also happen with bad star diagonals in catadioptrics and refractors. A convex or concave surface at a 45-degree angle will produce an astigmatic reflection. In most cases, it is cheaper to just replace the secondary or diagonal than to actually repolish it to be flatter.

Pinched Optics

Pinched optics refers to the physical deformation of the optics due to mechanical stress, often because the telescope’s optical components are clamped too tightly to their oholders. This pinch can cause the starlight to scatter irregularly. In a defocused diffraction pattern, pinched optics might present as an asymmetrical or skewed pattern of the diffraction rings, with one part of the pattern looking different from the rest. This asymmetry often changes or even disappears as the telescope cools down and acclimates to outside temperatures. In many cases, an overtightened lens cell or mirror clips are to blame. Since either contacts the optic at 3 points, you’ll see a “trefoil” deformation that is unmistakable in appearance. Simply loosen your lens cell or mirror clips – not enough to allow the optic to rattle, but enough that you can rotate it in the holder with a strong nudge with two fingers. This should make the problem go away.


Miscollimation in the star test is pretty obvious; with significantly miscollimated optics, even the in-focus Airy disk will appear to have a “tail” pointing in one direction, and going slightly out of focus makes it even plainer to see. The secondary mirror will also, of course, appear out-of-center if your telescope is a reflector or catadioptric. Collimate your telescope before attempting to diagnose any other optical issues. You can actually use the star test to collimate extremely accurately without additional tools, too – check out our collimation guide for more information.

Thermal Problems

To avoid misdiagnosing thermal effects as optical defects, it’s essential to give your telescope ample time to acclimate to the outdoor temperature before conducting a star test. A general rule of thumb is to allow at least half an hour of acclimatization for every inch of aperture, but larger telescopes or significant temperature differences may require longer. Fans can be used to hasten the process. Here are some common thermal problems and what they will look like.

Tube Currents

Air currents inside the telescope, also known as tube currents, are caused by temperature differences within the tube of the telescope, usually because the instrument has not had enough time to reach thermal equilibrium with its surroundings. These currents can distort the path of the starlight as it travels through the telescope. Often, warm air can be trapped between the primary mirror and the end of the tube – or inside a thick corrector plate – and setting the telescope outside for long periods, wrapping it in insulation, or putting together a forced air system may be required to mitigate the worst examples.

In a star test, air currents will often manifest as a shimmering or ‘boiling’ effect, causing the diffraction rings to appear to move or ripple. The rings may look fuzzy, and their shape and brightness may fluctuate rapidly. If the seeing conditions are otherwise good, and the stars outside of the field of view appear steady, you can attribute the issue to internal tube currents rather than atmospheric turbulence.

Primary Mirror Cooldown

If the primary mirror of the telescope has not cooled sufficiently to match the surrounding temperature, it can also cause air turbulence immediately above the mirror surface. This issue, often referred to as the boundary layer, is especially common with larger mirrors, which can take longer to cool down. The fans at the back of many telescopes may cool the glass itself, but they may not properly mitigate the boundary layer or tube currents, which can require additional sideways-mounted fans attached to the tube or mirror box. In a catadioptric, insulation or a forced air system can be a requirement at larger apertures.

In a star test, an inadequately cooled primary mirror may cause similar distortions as tube currents. The diffraction rings may appear to ripple or waver and may look blurred or distorted. Additionally, the entire field of view can seem to ‘breathe’ or pulsate, which is another indication of thermal issues. You’ll also probably see some form of spherical aberration or astigmatism, caused by the optic cooling unevenly between the shimmers.

Large lenses can have similar cooldown issues to mirrors, but their smaller sizes mean the glass itself doesn’t usually hold as much heat, and cooldown is rarely more than a half hour even with fairly large refractors. And thankfully, tube currents are not as much of a factor since the optic is at the front of the tube and warm air will rise off of it and outwards, as opposed to a primary mirror where air has to rise all the way out of the tube. The boundary layer is also usually not a problem with refractors. Ironically, dew shields or heaters are often needed with refractors and catadioptrics (especially Schmidt-Cassegrains) since the front lens will radiate heat away so quickly and attract condensation from the air, though with catadioptrics the innards may still struggle to cool down properly.

Bad Seeing

Finally, we come to seeing, which can often mask the effects of bad thermals in your telescope. Seeing is a form of bad thermals too – just affecting your entire local area, not just the telescope.

“Atmospheric seeing” refers to the distortion or blurring of astronomical observations due to the Earth’s atmospheric turbulence. Even though we often think of our atmosphere as clear and calm, it’s actually filled with constantly moving air masses of different temperatures and densities. When starlight passes through these turbulent layers, it can become scattered and distorted, affecting the clarity of astronomical observations.

Bad atmospheric seeing can be caused by several factors, including high-altitude jet streams, local wind patterns, heat radiating from nearby structures and appliances, and even (in some cases) a warm body near the telescope. The effects can be particularly pronounced when viewing objects that are low on the horizon, as their light has to pass through more of Earth’s atmosphere. This is one of the main reasons telescopes are often placed high atop mountains or in space.

In a star test, bad atmospheric conditions can significantly distort the diffraction pattern. The Airy disk and diffraction rings may appear to boil, shimmer, ripple, or rapidly fluctuate in brightness and shape. This effect can resemble that of tube currents within the telescope, but there’s a key difference: bad seeing affects the entire sky, not just the field of view through your telescope. If you see that stars with your naked eye or through another instrument are also twinkling rapidly, that’s a good indication that you’re dealing with bad seeing conditions.

It’s worth noting that even a perfectly manufactured and aligned telescope can’t overcome the effects of bad seeing. When faced with poor atmospheric conditions, the best course of action is usually to wait for better seeing or try observing objects that are higher in the sky, where the effects of the atmosphere are less pronounced. High-magnification views are more impacted by seeing conditions, so dropping to lower magnifications can also sometimes help. Good seeing is often hard to come by, and even in locales where good seeing is common, it’s likely that the seeing and other local sky conditions will at least somewhat influence what you choose to observe.

Zane Landers

An amateur astronomer and telescope maker from Connecticut who has been featured on TIME magazineNational GeographicLa Vanguardia, and Clarin, The Guardian, The Arizona Daily Star, and Astronomy Technology Today and had won the Stellafane 1st and 3rd place Junior Awards in the 2018 Convention. Zane has owned over 425 telescopes, of which around 400 he has actually gotten to take out under the stars. These range from the stuff we review on TelescopicWatch to homemade or antique telescopes; the oldest he has owned or worked on so far was an Emil Busch refractor made shortly before the outbreak of World War I. Many of these are telescopes that he repaired or built.

Leave a Comment