Telescope collimation guide India — step-by-step
You've set up your reflector telescope, pointed it at a star, and the image looks soft and asymmetrical — even when you rack the focuser to its sharpest point. Stars look like comets or ovals rather than tight points. This is almost certainly a collimation problem, not a focusing problem.
Collimation is the process of aligning the optical elements of a Newtonian or Dobsonian reflector — the secondary mirror, the primary mirror, and the focuser — so that all three are precisely co-axial. A properly collimated telescope shows tight, round stars. A badly collimated one shows asymmetrical blurs.
The good news: collimation takes 5–10 minutes once you've done it twice. This guide walks through the complete process for Dobsonian and Newtonian reflectors.
Collimation essentials — quick facts
5 min
Time to collimate once you know the process
3 bolts
Secondary mirror adjustment screws
3 bolts
Primary mirror adjustment screws (plus 3 locking)
Cheshire
The collimation eyepiece — most accurate tool
India-specific note: India's temperature fluctuations — especially between the hot afternoon and cool night, and between monsoon humidity and dry winter air — cause telescope components to expand and contract slightly. This means reflectors may need re-collimating more frequently than in stable European climates. Checking collimation before each observing session takes 60 seconds with a Cheshire eyepiece.
What you need — collimation tools
Three approaches — from simplest to most precise
Collimation cap
Free — a cap with a hole. Good for rough alignment. Every reflector should have one.
Cheshire eyepiece
Best tool for precise collimation. ~₹1,500–₹2,500. Highly recommended.
Laser collimator
Fast, easy. ~₹2,000–₹4,000. Less accurate than Cheshire for primary. Good for secondary.
For most Indian Dobsonian owners, the Cheshire eyepiece is the most accurate and reliable tool. A laser is faster but can be misleading if the laser itself isn't perfectly centred in its housing. EDISLA can advise on which tool works best for your specific telescope — WhatsApp us.
Step-by-step collimation — Dobsonian / Newtonian reflector
1
Remove the eyepiece and insert your collimation cap or Cheshire
Do this in daylight or under a bright room light — not in the dark. Point the telescope at a bright, evenly lit surface (white wall or overcast sky). You should be able to see the secondary mirror reflected in the primary, and the primary reflected in the secondary.
2
Check secondary mirror centration under focuser
Looking through the collimation cap, the secondary mirror (small elliptical mirror) should appear centred in the focuser tube. If it's offset to one side, loosen the central bolt on the secondary holder and rotate the entire secondary spider assembly until the secondary is centred. Don't touch the three tilt bolts yet.
The secondary spider has one central bolt (holds it to the spider) and three or four tilt bolts (adjusts the angle). These are two separate adjustments.
3
Tilt the secondary to centre the primary mirror reflection
Using the three tilt bolts on the secondary holder, adjust the secondary mirror's angle until the reflection of the primary mirror is centred in the secondary. The primary mirror should appear as a circular reflection, centred within the elliptical secondary mirror. Use small turns — a quarter turn at a time. Loosen one bolt, tighten the opposite bolt to tilt. This is the hardest step on the first attempt — it becomes intuitive quickly.
4
Centre the primary mirror reflection using primary collimation bolts
Now the primary mirror's three collimation bolts come into play. The collimation hole (a circular mark or spot at the centre of the primary mirror) should appear centred in the reflection you see. Adjust the three primary bolts — loosen the locking bolt, turn the collimation bolt, re-tighten the locking bolt — until the centre spot is perfectly centred. On Bresser Dobsonians, the primary bolts are at the back of the mirror cell.
Always re-check step 2 and 3 after adjusting the primary — adjustments interact with each other.
5
Star test — final verification
Wait for a suitable night and find a bright star (Vega, Sirius, Canopus, Arcturus). Centre it and defocus slightly. The defocused star should show perfectly concentric rings (like a bullseye) both inside and outside focus. If the rings are brighter on one side, the secondary still needs adjustment in that direction. Perfect symmetry = perfect collimation.
The star test only works in still air (good seeing). On turbulent Indian nights, star rings look asymmetric even in a perfectly collimated telescope. Try on a still winter night for best results.
When should I collimate? — India schedule
| Situation | Collimation needed? | Why |
|---|---|---|
| Before first use | Yes — always check | Transport can shift mirrors |
| After transport in a car | Yes — check and adjust | Road vibration shifts secondary |
| Temperature change >10°C | Check — may need adjustment | Thermal expansion of tube |
| Season change (summer to winter) | Check | Significant temperature differential |
| After monsoon season | Check — often needed | Humidity cycles cause movement |
| Every session (30 sec check) | Quick visual check only | Catches small drift before it affects views |
Frequently asked questions
How do I know if my telescope needs collimation?
Signs that a reflector needs collimation: stars appear asymmetrical or comet-shaped at high magnification, the defocused star ring pattern is not concentric (one side brighter than the other), the image never appears completely sharp even at perfect focus, or the Cheshire eyepiece shows the primary reflection off-centre in the secondary. If you see any of these signs, collimation is needed.
Do Bresser and EDISLA Dobsonians need collimation?
Yes. All Newtonian and Dobsonian reflectors require periodic collimation, including Bresser Messier Dobsonians and the EDISLA Astra 114. However, quality telescopes like these ship well-collimated and maintain collimation longer than budget telescopes — typically needing adjustment only after transport or significant temperature changes. EDISLA provides WhatsApp collimation support to all customers.
EDISLA provides full WhatsApp collimation support for all telescope purchases
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