How to collimate a telescope Schmidt-Cassegrain

01Collimation, and why it changes everything

Visit collimation refers to the precise alignment of your telescope's optical elements. When the optics are perfectly coaxial, the light converges on a sharp point and the image reaches its full potential in terms of sharpness and contrast. As soon as this alignment is disturbed, even slightly, the stars lose their symmetry, planetary details become blurred and resolution drops.

On a Schmidt-Cassegrain (like the Celestron NexStar Evolution range), alignment is factory-set once the instrument is fully assembled. The only truly adjustable element, and the one that can move with handling, is the the inclination of the secondary mirror. It is on this mirror, and on it alone, that the collimation of a Schmidt-Cassegrain is played out.

The good news is that adjustment requires no exotic tools, is entirely reversible, and quickly becomes a routine that takes just a few minutes. Bad news for your images if you don't know: a decollimated instrument will never give the sharpness for which it was designed, no matter how good its optics.

02Is it really necessary to collimate your Schmidt-Cassegrain?

Unlike a Newton, which often needs to be checked at every output, a Schmidt-Cassegrain holds its collimation remarkably well. The tube is closed, the secondary mirror is securely held, and the factory alignment can be maintained for a long time. So there's no need to touch the screws if all goes well.

In practice, check collimation in these situations:

• After a rough ride, a long car journey or a plane trip.
• When you receive a new or used instrument, to get off to a sound start.
• When the stars seem soft, comma-shaped or comet-like, even in focus.
• Before a high-resolution imaging session (lunar, planetary or solar), where the slightest error can be seen.

The rest of the time, an occasional check is sufficient. If the star test described below is satisfactory, don't adjust anything: the best intervention is often the one you don't do.

03What you need

It all fits in a pocket. Here's what you need to gather before heading out under the stars.

• Phillips screwdriverSmall size. Screw type may vary by model: check your instructions.
• A shining starClose to the zenith, or the Pole Star, which moves very little in the sky.
• A medium-to-high-power eyepieceTo enlarge the image sufficiently and judge its symmetry finely.
• Patience and stable skiesAtmospheric turbulence makes adjustment difficult: opt for a calm night.

04Prepare the telescope before you start

Good collimation always starts with good preparation. Two points are essential.

Let the telescope warm up

A tube that is hotter than the ambient air generates internal convection currents that distort the image and completely distort judgment. Take out your instrument ahead of time and tune it for approximately 45 minutes to achieve thermal equilibrium, even more so if the temperature difference between inside and outside is significant. As long as the image is «boiling», there's no need to touch the screws.

Choose the right star

Aim for a bright star near the zenith This is where the layer of atmosphere through which the star passes is the thinnest, and therefore the most stable. Activate mount tracking to keep the star centered without intervention. If you prefer not to switch on the telescope, the’North Star is an excellent alternative: close to the celestial pole, it moves so little that manual tracking becomes unnecessary.

Set up your medium-to-high power eyepiece, then carefully center the star: good centering is essential to correctly assess collimation.

05Read the diffraction pattern (the star test)

Diagnosis is based entirely on the observation of a star slightly defocused. By gently turning the focus, the star is transformed into a disc with concentric rings, with a central dark spot which corresponds to the shadow of the secondary mirror.

Everything depends on the symmetry of this figure:

Proceed as follows: defocus on either side of the focus point (a little before, then a little after the focus point) and observe. If the dark spot remains well centered and concentric rings, your instrument is collimated. If the shadow is still moving in the same direction, move on to adjustment.

06Adjust secondary mirror screws, step by step

Adjustment involves tilting the secondary mirror very slightly using the collimation screw located on the support at the front of the tube. The golden rule can be summed up in one sentence: small gestures, with the star refocused between each one.

1. Defocus and find the offset

   On a bright, well-centered star, defocus until you see the rings and the central dark spot. Note precisely in which direction this shadow is shifted.

2. Create a finger shadow

   Bring a finger close to the edge of the front cell, without ever touching the corrector blade, pointing towards the screws. Its shadow becomes visible in the eyepiece. Move the finger along the edge until its shadow points towards the area where the rings are tightest, i.e. on the same side as the offset.

3. Identify the first screw

   Locate the collimation screw closest to your finger: this is the one you'll adjust first. If your finger falls exactly between two screws, adjust the opposite screw.

4. Shift the star towards the edge

   Using the racket, bring the defocused image of the star to the edge of the field, in the direction where the central shadow is offset.

5. Turn slowly, observing

   While looking through the eyepiece, turn the marked screw about a tenth of a turn with the screwdriver. If the star comes out of the field in the direction of the offset, you're turning in the wrong direction: reverse, so as to bring the star back towards the center.

6. Keep screws under tension

   If one screw becomes loose when turned, tighten the other two by the same amount. Conversely, if one screw forces too much, loosen the other two by the same amount. The mirror should be held firmly in place, but not excessively.

7. Check and repeat if necessary

   Once the star is recentered, check that the rings are concentric. If the shadow remains shifted in the same direction, continue with the same screw. If the shift has moved in another direction, simply repeat steps 2 to 6 for the new direction.

07What successful collimation looks like

Perfect collimation can be recognized by image of a highly symmetrical star, identical just before and just after focus, with a central spot perfectly in the middle of the rings. When in focus, the star shows a sharp spot surrounded by a thin ring of diffraction: a sign that the optics are delivering their full potential.

One final factor conditions everything: the atmospheric turbulence (seeing). If the air is unstable and the image flickers and bubbles, it becomes impossible to judge correctly. In this case, wait for a calmer night or aim for a more stable part of the sky, where the stars look still rather than flickering.

08Common mistakes and professional advice

• Set to cold without warming up. Until the tube is at equilibrium, air currents distort everything. Please wait.
• Turn too hard. The beginner's reflex. Stay on fractions of a turn and refocus after each gesture.
• Offset the star. A figure judged to be out of the center of the field lies: always keep the star well centered to evaluate.
• Wanting to collimate by bad seeing. A turbulent night is a lost cause for this setting. Choose your evenings.
• Confusing tools. A Bahtinov mask is used for focusing, not collimation: here, it's the diffraction rings that guide you.
• Forget to double-check on the evening of imaging. For high resolution, check collimation once the instrument has reached temperature, on site.

09Frequently asked questions