Do refractors need collimation?
Many do not need user collimation. Small and mid-range refractors are often aligned at the factory and the lens cell is fixed in place. Some larger or higher-end refractors have an objective cell with adjustable screws, and these can drift out of alignment after hard knocks or rough transport.
Check your manual or the maker’s website first. If the cell has no adjustment hardware, do not try to force it. If it does, the instructions below describe the general approach. For the wider picture, see our telescope collimation guide.
First, check whether the problem is collimation
Several problems can look like miscollimation in a refractor:
- Unfinished cooling, which distorts star images in a new setup.
- Poor seeing.
- A diagonal or eyepiece that is not seated squarely.
- Optics that are dirty or dewed.
- A focuser that tilts or sags under load.
Give the scope time to reach outdoor temperature and try a different eyepiece and diagonal before concluding the lens needs adjusting.
Star test for refractors
Use a bright star at fairly high magnification. Defocus a little inside and outside of focus. A well aligned refractor should show round, concentric rings that look similar on both sides of focus. If the ring pattern looks lopsided or the center looks shifted in the same direction on both sides, the objective may be slightly tilted. Refractors can also show some color fringing around bright stars as a normal property of the design, so do not mistake that for miscollimation.
What you need
- A Cheshire eyepiece, or the maker’s recommended tool, if the manual calls for one.
- The right size hex key or screwdriver for the cell screws.
- A flashlight.
- A stable setup and a steady star, or a distant target by day for first checks.
Adjusting the objective cell step by step
Makers differ, so treat this as a general outline and follow your manual if it says otherwise.
- Remove the dew shield if needed so you can reach the cell.
- Identify the push-pull screw pairs on the objective cell. Typically a larger screw pushes and a smaller one pulls, or the reverse, according to the design.
- Do the star test and note which direction the defocused pattern appears shifted.
- Loosen the locking or pulling screws slightly before pushing, so nothing is forced.
- Adjust one screw pair by a very small amount, typically a fraction of a turn.
- Snug the opposing screws so the cell is held firmly but not tightly.
- Repeat the star test and compare.
- Continue in small steps until the rings look even on both sides of focus.
Never loosen all screws at once, and never apply strong force. Glass objectives can be damaged by pinching.
Checking the diagonal and focuser
If the star test improves after you reseat the diagonal or swap eyepieces, the lens cell may be fine. Check that the diagonal clamp is tight and square, and that the focuser draw tube is not tilted. For a star diagonal, try the view with and without it if your setup allows.
When to stop and ask the maker
If the cell has no adjusters, the screws are sealed, or you are unsure which screws control tilt, stop. Contact the manufacturer or an authorized service shop. Poor adjustment can make the scope worse than before.
FAQ
How hard is it to collimate a telescope?
For a refractor with adjustable screws, it can take patience but little effort. For one without adjusters, it is usually not a user task.
How often do you need to collimate a refractor?
Often never. Check after an impact or if the star test shows a clear, repeatable problem.
What is the best telescope collimation tool?
For refractors, the tool is often the maker’s recommendation: a Cheshire eyepiece plus a star test. For reflectors, see our guide on collimating without a laser.
Why are refracting telescopes less used today?
Refractors are still popular for small to mid apertures and astrophotography. Large apertures are usually built as reflectors because big lenses are heavy, costly to make and can only be supported at the edge. Large professional telescopes are reflectors for this reason.