When shall I clean my mirrors?

Only when it's mandatory! 

Yes, thanks, but when is it mandatory?

You'd be surprised how dirty professional telescope usually are! The reason is simple: Mirror cleaning on a large telescope is a risky intervention that costs time and money. Surprising fact: Even a lot of dust doesn't affect the reflection ratio that much. A very dusty mirror may have less than 1% light loss. This explains that even at professional observatories, mirrors are only cleaned when they really have to be. 

Really? Why shall we clean even clean it then?

Because dust creates light scattering. If you prefer, the small grains of dust create multiple reflections in almost all directions. Those small reflections illuminate the secondary mirror uniformly and tend to reduce the contrast. 

Which mirror shall I clean?

Ideally, both mirrors should be cleaned, but the secondary mirror can usually be cleaned without being dismounted. I can't repeat it more: Collimating a primary mirror is easy. Collimating the secondary mirror isn't. So before you start cleaning the secondary, ensure that all screws are tightened properly and the mirror can't move. 

What do we need?

Here is a checklist of the equipment we use:

• Wipes
• Demineralized water (1 liter)
• Detergent
• Isopropyl alcohol
• Oil-free compressed air 
• Sprayer for alcohol
• Sprayer for detergent
• Towel
• Marbles 

If you have no primary mirror baffle ring, you may also need a black permanent marker and marbles (you will understand why...)

Before your start dismounting anything:

• ensure the telescope won’t flip when 1 kg of mirror is missing!
• Measure the position of each mirror adjustment screw. You never know what will happen...
• Mark the position of each of the item you will disassemble. 

Dismounting procedure:

• Unscrew the mounting screws of the mirror holder
• Take a deep breath
• Remove the mirror and its mirror carrier
• Mark the mirror position on its carrier
• Estimate the gap between the mirror and the 3 holders
• Dismount the mirror

Mirror cleaning. Finally!

Preparation:

• Spray compressed air and remove as much dust and solid particles as possible 
• Put a towel in a tub 
• Place the mirror on it
• Put 1 drop of detergent and distilled water in a sprayer 

First cleaning phase:

• Spray the mirror with diluted detergent

• Cover the mirror with wipes (no rubbing!) 

• Spray the wipes

• Wait 3-4 minutes

• Remove the wipes without rubbing

• Flip the mirror and rinse with the detergent sprayer 
• Repeat 2 or 3 times

Second cleaning phase

• Place 1 wipe on the mirror
• Spray
• Drag the wipe away
• Drag it again on the other side
• Repeat until the entire mirror is cleaned (10 or 15 times?)
• Place the mirror in the tub, tilt it and rinse it with distilled water only
• Spray the isopropyl alcohol when the mirror is still wet from the distilled water

WARNING: if the mirror's edge is painted in black, the ink will dissolve and flow

Check if dirt remains and if yes, repeat the second cleaning phase until it’s clean

If necessary, repaint the edge with black ink. Here is the trick; place the mirror on marbles and turn it while the permanent marker is in a fixed position.

Mounting:

• Place the mirror again in the mirror cell and tap it for a correct positioning
• Remount the mirror holders (Hint: a paper can be inserted between the holders and the mirror, but not a credit card)
• Tape the screws (better than red Loctite...)
• Re-install it inside the telescope tube. 
• Tighten all screws
• I said all screws! 

Important note: Do not tighten the mirror clips! Those elements shouldn't press down the mirror, otherwise it will bend the mirror and create very strange optical aberrations. Believe me or not: your mirror rarely look down and will stay in position even if the clamps don't touch it :-)

YOU DID IT. And now it's time to collimate!

Usefull links!

An excellent Youtube video of Exo Photography

https://youtu.be/4hFShgpf77s?si=5q_nAWBXtQOcvljk