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How to clean a corrector plate ?

The cleaned corrector plate of a Maksutov-Cassegrain telescope.

Trick, strip and pick

After some years of use, if you look at the corrector plate of your catadioptric telescope in full light, you will be surprized to discover a great amount of dusts, grease, maybe sap droplets and other deposits forming a thin film all over the surface. If kids play around your scope, sugar deposits and other food can be found... 

How to clean all that matter ? Here are some useful tricks to dismount and strip down the corrector plate of a scope in order to clean it and pick up all deposits.

First advice, do *never* try to blowing the dust with a compressed air or using a clean hair brush. Dust is often very resistant, made of sand grains that can damage the coating and the glass if you try to move them brutaly using an air flow. Worse, the compressor itself can also blow dusts to your glass and ruins your efforts. Thus forget these tools.

Then, if you never proceed to such a cleaning, do a first test with a small piece of glass to see how easy or difficult is for you the task of removing all deposits without leave any streak or mark on the glass.

Here are some hints : in all the process of cleaning, the rinsing is the key. Avoid also doing any wiping. Do not use cotton ball but only pure surgical cotton to purchase in a dispensary or you will scratched the coating on the first drag across. Heat your cleaning solution and soak/rinse it multiple times.

Basic material :

- A large table to support both the OTA and the plate side by side

- A marker to mark the exact position of the plate vs the tube

- Make a light box to protect the secondary mirror and some scotch to protect it

- A powerfil light source (> 100 watts) to lighten the plate

- A large soft cloth of 50 x 50 cm to avoid touching the plate once in hand

- A recipient for your cleaning solution

- Some very clean recipients containing each 20 pints of distilled water (ou renew regularly your bucket)

- Hundreds of surgery coton balls

- A thin-blade plastic screwdriver or a T-wrench of 9/64" (check the screw head)

- A small plastic screwdriver or a teeth brush to lift up the plate

Remove the corrector plate

If you are used and confident about this job, you have to remove the corrector plate from the scope for cleaning otherwise you will have difficulties to access and clean the few millimeters near the edge of the corrector and to remove the crud collected at that place.

This work has to be reserved to advanced handyman used to handle with care mirrors and corrector plates. Without this skill but if you absolutely want to learn how to proceed, contact your national representative or an experienced amateur in a near club to acquire this experience. But do not proceed without having at least observed once a friend doing the job. Like for recipes or in computing, a field experience is better than thousands words of theory.

Removing the plate is not a difficult task but you have to remember its exact positionning with the tube. With time, the low quality screws can be tight and being given the wrench is quite small, you will need a specific T-handled wrench of 9/64" to fit the screw sockets. Think in the same time to replace your screws by similar models in stainless steel cap-head to preserve them from moisture.

Place your scope in a clean area, under a bright light source, so you can see what you are doing, and point the scope slightly upwards. Lock the DEC clutch and remove the ring of screws. Lift away the plastic retaining collar and look at the corrector. You should see at the edge a pencil mark which indicates the correct rotational alignment. Note this position, go over the mark again with a marker if it is nearly indistinct. Avoid using liquid Tipp-Ex than can drop.

The first time out the corrector will most probably be stuck to the cork pads that support it. Use a thin-blade plastic screwdriver and work your way around the periphery judiciously prising the edge of the corrector upwards to loosen it. Let it drop back down into position after the seal is broken. Gently hold the outer edge of the secundary holder whilst doing this so you can feel what's happening and control its movement. DON'T use a metal screwdriver for this job otherwise there is a good chance of chipping the edge - grind or sand the handle end of an old toothbrush to suit it if you have nothing else.

When it is loosened all the way around it is ready to come out. It will not fall out of its own accord neither as you pointed your scope upwards slightly. Make sure you have made space nearby on a table to place the corrector plate, and there are no hazards between the scope and the table to trip over...

Using a soft cloth between hand and plate, get hold of the edge of the secondary holder with your left hand. There is not much to get hold of but enough. Use your plastic pry to lift the top edge out all the way out until you can get your finger on the edge of the plate there - the plate remains supported by its bottom edge at this stage. In one easy motion slowly tip the plate all the way horizontal so your left hand ends up the plate, supporting it from beneath with your left hand on the secondary holder and at the edge the paslm of your right hand. Transfer it like this to your cleaning table (a plastic sheet covered with absorbant cloth is good).

Chris has made particular mention of the technique for handling it because you are doing to put it back the same way thus avoiding putting any new finger prints on it.

While you are cleaning the inner surface of the plate keep the secondary covered with a plastic bag and use a rubber band to hold it - do not attempt to clean the secondary itself under any circumstances even if it appears a little hazy, you will only do more damage than good. That is a specialist job.

Now you are free to slop plenty of your cleaning fluid all over your corrector plate without fear of it getting into the OTA - and you do need plenty of fluid. But what fluid ? 

Cleaning the corrector

Your cleaning fluid can be next : 

- 1/3d of 91% to 99% pure isopropyl alcohol (see your dispensary or an university chemical lab)

- 2/3d of distilled water

- 2 drops of dishwashing soap (soft) per liter of solution.

NB. You can also use ethyl alcohol instead of isopropyl. Usually lens cleaner, windex and even alcohol alone leave streaks, so distilled water for the rinsing is the best solution. Detergent also left smears impossible to remove.

Clean the inside surface of the plate first, then the outside. It is important to do the complete job in once. If you only clean one section before doing to the rinsing, surrounding dust will end up getting ground in and scratching the coatings. The corrector should feel very smooth and slick by the time you get to rinsing.

If you want to use pure surgical cotton, know they are not very absorptive material, so it is hard to soak up the solution on the corrector lens. The trick is to keep the cotton loose and fluffy, just as it is when you tear it off the roll.

Put is slowly across the surface, with light pressure. It makes a difference. Then change the cotton ball almost every stroke, or you won't remove the dirt, just move it around... After you did it your room should look like a white tribble exploded around you !

Do a small section of your lens at a time, but go over the whole thing at least twice.

The real trick is volume and allowing plenty of time for the fluid to soak into the crud and lift it away before rinsing (again with copious volumes of distilled water). 

The coatings are actually fairly tough so providing there is absolutely no gritty residue remaining, and your cloth is clean, you can wipe it afterwards. You cannot get away with this on an aluminised surface.

While you have the plate off use a vacuum to clean out the inside of the OTA, get rid of all the dust that has collected in there but avoid touching the primary mirror. Also, if you have previously bought some silicone high-vacuum grease you can treat the mirror to a lube job - this will reduce mirror shift using the standard SCT focuser.


Re-assembly is the reverse of the above procedure ! The plate will be an easy fit going back in and it should be loose enough to turn slightly to exactly align the pencil mark you reinforced. Don't over-tighten the ring of screws, it is not necessary and might cause tube distortions.

Good luck !

I thank Chris Heapy  and Doug Hoy  for their advice.

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