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Cleaning the primary and secondary mirrors

Simple step-by-step guide for cleaning the primary and secondary mirrors of your telescope.

Guide based on the Celestron AstroMaster 130EQ telescope, but these steps will apply to other telescope mirrors.

If you have not removed the primary or secondary mirrors before, please follow the mirror removal guide here.

Time needed: 1 hour

Cleaning the telescope primary and secondary mirrors.

  1. Preparation and precautions

    You will need to source deionised water to rinse the mirrors and cotton-wool or soft microfibre cloths for the cleaning.
    If your sink is ceramic/hard then try to pad it with a soft material (plastic liner or similar) – this is to avoid damaging the primary mirror if it slips or is dropped while cleaning. Don’t underestimate how slippery the mirror will become when wet/soapy.
    Remove any jewellery to avoid scratching the mirror surfaces.
    Ensure your hands are clean

  2. Remove mirrors from the telescope ready for cleaning

    If you have not already removed the mirrors from the telescope, follow the mirror removal guide for assistance.

  3. Remove any loose debris with a hand blower

    Use a hand-pump blower, blow carefully or use a suitable mirror/optical brush (very soft and clean) to remove any loose debris from the surface of the mirrors.

  4. Soak mirror surfaces in warm water with a mild detergent

    Fully clean and rinse a sink to ensure it is free from dirt, grease or oils – or a plastic tub large enough to fully submerge the primary mirror. Fill this with fresh warm water and a dilute, mild dishwashing detergent.
    Primary mirror: submerge the primary mirror fully in the soapy water and agitate by moving the mirror around/through the water in circles, then allow to soak for 10 minutes.
    Secondary mirror: do not leave the secondary mirror assembly to soak, instead hold the mirror surface under the water and agitate repeatedly. This is to avoid the adhesive on the back of the mirror degrading and any potential for the mirror to come loose from the holder.
    After this initial soak and agitation it is advisable to drain and refill the sink, as before – to remove any dirt already released from the mirrors. Submerge the primary mirror again, as before.

  5. Very gently wipe the mirror surfaces

    Agitate the primary mirror again by moving around in the water. In the case of the secondary mirror, hold this under the water for these steps.

    Using fresh cotton wool or a clean, lint-free, microfibre cloth:
    Soak a piece of cotton wool/cloth in the water and position this above the submerged mirror surface.
    Do not apply any pressure – using only the weight of the wool/cloth itself, lower this onto the centre of the primary mirror/edge of the secondary and move the wool/cloth across the surface in one smooth movement in a straight line to the edge.
    Repeat with a clean piece of wool/cloth for each line across the mirror until the whole mirror surface has been cleaned.
    Inspect the mirror – you can soak again and repeat the above process for any remaining marks.

  6. Remove stubborn marks from mirror surfaces – only if strictly necessary

    Small marks will barely cause any noticeable detriment to the performance of the mirror, so you must weigh up the risk of scratching the surface before any further cleaning.
    If stubborn marks remain you can attempt a more aggressive clean: ensure hands are immaculately clean and fully saturated from the cleaning so far. Create a lather with concentrated dishwashing detergent on a fingertip and very, very, gently use this on the stubborn stain. your third/ring finger is likely to put the least pressure on the mirror.
    If doing this step, then repeat the cotton-wool/cloth cleaning of this area of the mirror before finishing up.

  7. Thoroughly rinse mirror surface with deionised water

    Hold the back of the mirror and ensure you have a secure grip. Remove mirror from the water and hold it at an angle so that any liquid can run off easily.
    Using deionised water, rinse the mirror thoroughly.
    Try to minimise any deionised water remaining on the surface by rinsing and tilting.

  8. Air-dry mirrors tilted at an angle to allow water to run off

    To dry, place the mirror(s) at an angle on a thick/padded towel with the mirror surface facing upwards and tilted – so that any remaining droplets of water can easily drain from the mirror.
    The deionised water should not leave any residue or marks as it dries.
    Only once fully dried, you can centre-spot or reassemble your telescope.

What next?

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