DeSo: @StarGeezer
Twitter: @UKStarGeezer

Centre-spotting the primary mirror

Applying a centre spot to your primary mirror makes aligning and collimating your telescope optics much easier, quicker and more accurate. Follow this simple guide to apply a centre spot if one does not come preinstalled with your telescope.

This guide is based on the Celestron AstroMaster 130EQ Newtonian Reflector telescope. However, these steps are essentially the same for other similar telescopes.

If you have not removed the primary or secondary mirrors before, please follow the mirror removal and mirror cleaning guides.

Time needed: 10 minutes

Applying a centre spot (or center dot) to the primary mirror is actually quite straight-forward – just follow these simple steps. Prepare materials in advance – as you will need to obtain or print a transparent centre spotting template and will need a suitable centre spot sticker before you can proceed.

  1. Preparation

    You will need a centre spot template in order to carry out this task. I found the free version by joberreiter on GitHub to be perfect – do consider making a donation as a thank you for their work. You will need access to a printer (or a kind friend or workplace) and OHP/Transparent sheets in order to print and prepare the template.
    You will require a centre spot/dot donut sticker – blackened with a sharpie/indelible marker. A hole reinforcer works perfectly for this and is cheap to buy.
    Ensure you have a suitable space to work comfortably – with enough space to manoeuvre the mirror, position the template etc.

  2. Prepare centre spot sticker

    A hole-punch reinforcer is ideal for this – blacken a white or lightly coloured sticker using a sharpie/permanent marker.

  3. Prepare roll of tape to hold the centre spot to the template

    Roll a short length of Sellotape over onto itself to form a roll (scotch-tape would be even better). Trim the roll shorter than the width of the centre spot sticker – to avoid any adhesive catching the mirror surface. Then roll between fingers to weaken the adhesive – this is so that it will release the centre spot onto the mirror once positioned.

  4. Stick tape roll to the centre spot on the reverse of the template

  5. Attach centre spot sticker to the tape

    Position the centre spot sticker blackened side down – adhering to the tape. Ensure the centre spot sticker is central to the template – you can make minor adjustments.

  6. Gently peel centre spot sticker from its’ backing tape

    Gently peel centre spot sticker from its’ backing tape to release it. You now have a prepared centre-spotting template.

  7. Position centre spot template over the primary mirror

    Carefully position the readied template over the primary mirror – take time to ensure it is central and you can hold the template in position securely.

  8. Gently push the centre of the template down to apply the centre spot

    Press very lightly down on the centre spot while keeping the template central – as the spot contacts the mirror it will adhere. Carefully withdraw the template up/away from the mirror and the dot should be left behind.

  9. Check spot is central and adhere securely to mirror

    Check the spot is central before adhering it securely to the mirror.
    If you have made a mistake in positioning the centre spot, it is best to easier to remove the sticker before you have firmly adhered it – which is much harder.
    If you are happy with the position, then gently press with a fingernail/implement around the sticker to adhere it securely – ensuring no contact is made with the mirror surface.
    Congratulations – you have now centre-spotted your mirror ready for accurate collimation.

What next?

Now that you have cleaned your primary and secondary mirrors you may wish to: