Guide To Removing and Cleaning the SCT Corrector Plate

Under construction................

Introduction

A question often asked by the new owner of an SCT is - 'what's the best way of cleaning my corrector plate?'. The answer is not straightforward and depends upon the reason why it's necessary to clean it in the first place. It may be because you just can see a single greasy fingerprint on it, or it may be because your telescope has seen considerable service over the past year and the corrector plate is generally in need of a good cleaning all over. The key point here is, like any delicate optical surface, the corrector plate should NOT be cleaned unless absolutely necessary. A few specks of dust or the odd smudge will not significantly affect the image but doing a poor job of cleaning it most certainly will.

There are two possible ways of approaching this job. The first is to attempt to clean the corrector plate in-situ without removing it, and the second is to remove the plate from the telescope and clean it separately. If the corrector just has a discrete mark that you wish to remove (such as a fingerprint) but is otherwise clean then it's possible to tackle this small section whilst leaving the corrector in place. However, for a full aperture cleaning job it is preferable to remove the corrector plate for thorough cleaning on the bench, and in fact I would recommend this approach for all cleaning jobs.

Why should you remove the corrector?:

It's the only way to clean it edge-to-edge, the corners near the retaining ring being particularly problematic when it's left in place.
Cleaning in-situ does not allow you to use the large volume of solvents necessary to wash the crud off, the cleaning solution tends to creep around the retaining ring, picking up oils and depositing them elsewhere on the plate's surface. In this job it's the solvent that should do the work of dirt removal - not wiping with a cloth or tissue.
Grit gets trapped around the periphery of the plate, particularly in the corner under the retaining ring (there is always a small gap between this ring and the plate). If you attempt to wipe the plate with a cloth it's all too easy to pick some of this up onto your cloth and consequently there's a grave risk of scratching the corrector plate.
It's less obvious, but over an extended period of time (a year or two) fine dust and particulate material will collect on the inside surface of the plate. This may not be immediately visible on cursory inspection but it does contribute to an overall 'haze' which will damage image contrast. The plate has to be removed to clean this inner surface.
By cleaning the corrector plate maybe once or twice a year, it's removal gives you access to the interior of the OTA which you can clean at the same time. In addition, the focussing action will benefit by having the lube used on the mirror slider replaced. Both of these jobs can conveniently be done at the same time.

I'm Ham-fisted, can I safely remove my corrector plate without damaging it??

Corrector plate removal is a relatively simple DIY proposition for anyone who is not chronically accident-prone. You don't need to be an instrument technician, and if you carefully follow the instructions below then removal and replacement of the corrector plate is simple and safe. If you are still in doubt, or your DIY skills barely extend beyond wiring a plug, it may be safer in these circumstances to have your telescope serviced professionally - far better to do that than clean it in place because you are not confident about removing it. Remember that it should not be necessary to carry out this cleaning operation more frequently than once or twice a year.

A Note on Cleaning Solutions:

There are many 'secret' formulae for cleaning solutions around but the fact is that one is about as effective as any other. The critical thing here is not the chemical composition but the use of sufficiently large volumes of clean solvent to wash away the crud without leaving streaks, and secondly allowing ample time for the solvent to work. Personally I've tried several different solvents but now I only use 70% lab-grade Ethanol (ethyl alcohol) and 30% de-ionised water. Propanol (isopropyl alcohol) or methanol (methyl alcohol) are equally effective but the ethanol happens to be available to me for nothing. The solvent's job is to dissolve resinous deposits and release stuck particles of dust, and whilst there are stronger solvents available than the alcohols listed above (such as diethyl ether, acetone etc.,) there should be no need to use them. The stronger solvents merely work a little quicker, but also have the distinct disadvantage of being potentially damaging to plastics and painted surfaces. What is needed is time - time allowed for the solvent to work, to soften and eventually dissolve residues. After the solvent has done it's job it only remains to remove all traces of it - and the dissolved residues with it. This removal is best achieved using de-ionised water (or a weak solution of alcohol in de-ionised water) which will wash the dirty solvent away and then dry without leaving any marks.

Where to get these solvents from? Well, as mentioned, I work in a lab so getting hold of them is no problem for me. Had that not been the case then a visit to the local chemist would solve the problem (in the UK anyway), but in that case I would use either methanol or propanol as they would be cheaper than ethanol. Geting hold of the de-ionised water might be a problem, but I do know that de-ionised water is sold by the 1/2-litre bottle for re-filling lead-acid batteries (available from auto accessory stores - make sure there are no other addititves though, I've never seen any but it's worth checking). You will need around 500 mls of solvent and maybe a couple of litres of de-ionised water.

Steps in the Cleaning Process:

There are a few discrete steps in the cleaning process, each having a specific objective.

First: Removal of particulate material - dust and grit. These are potentially very harmful and must be carefully removed first.
Second: Removal of oily or resinous deposits. These require the solvent to get rid of, and their removal must be complete to avoid subsequent streaking.
Third: Solvent removal. If the solvent is pure and clean the final washes in step 2 are all that is needed, the solvent will evaporate without leaving any streaks. However, solvent tends to be relatively expensive and most users don't use a sufficiently large volume, therefore a final wash with copious amounts of de-ionised water (or weak solvent solution) will do the same job.

Cleaning Methods:

As noted in the introduction, it is possible to remove a small mark such as the odd fingerprint whilst leaving the corrector plate in place. It's not something I would attempt personally but if I outline the procedure and risks involved you can make your own choice. A single fingerprint is irritating to look at but it will not significantly affect the image, and it's mainly noticeable because it changes the reflective properties of the coatings.

Unlike cleaning with the corrector plate on the bench it will be necessary to wipe the marks away using either a lens cleaning tissue, a fine cloth, or a special 'Microweave' cleaning cloth. Others have reported success with a 'lens pen' for small marks although I've never tried one myself. When cleaning the plate in-situ you will be limited as to the amount of solvent you can use because you don't want it to accidentally creep under the edges of the retaining ring. Also, as you are actively rubbing the surface you must be quite certain there is no particulate material present that can scratch the coatings or the glass. I would suggest the following method:

Use a fine brush to gently loosen any dust on the surface of the plate, I've had good success in similar circumstances by using one of those photographic puffer-brushes. I removed the brush from it's squeezy bulb and thoroughly washed it in solvent, and then connected a rubber tube to the brush's hollow stem. The end of the tube was then placed in the hose of an ordinary vacuum cleaner, wadded with cloth to make a seal. Used this way the dust is actively removed by the vacuum after being loosened by the brush hairs. I suggest avoiding the canned air approach, it is just possible that high-velocity grit driven by the airflow may itself damage the surface, or that something nasty will be quirted onto the plate, but more importantly the dust is merely being re-distributed - blown into the gaps around the edge of the plate. Removal is better.

I have tried, for the purpose of this article, purposefully putting a fingerprint on the plate and then removing it again with different methods. I have a microweave cloth (originally obtained for cleaning a small LCD screen on my digital camera), and whilst this was eventually effective it took rather more rubbing than I was happy with. Further, it would not take long for the cloth to be saturated with greasy deposits (which would then be re-distributed onto the plate), and if a piece of grit got into the cloth it would be a disaster! I wouldn't use this method routinely. Secondly, I tried a commercial solution originally intended for use on computer monitors (called 'Optics Plus Cleaner' - mainly isopropanol with a little detergent). This I sprayed directly on the mark and polished off with a clean lens tissue. Again, a little more rubbing than I would have liked but very effective nevertheless. You end up cleaning a larger area than first intended with this method. Lastly, I tried using my usual 70%Ethanol/30%water mix in combination with lens tissues. Again, more rubbing was needed to remove final streaks simply because I was unable to wash the solvent away.

A note on tissues: Tissues sold specifically for the purpose of cleaning optics are guaranteed not to contain abrasive materials which may otherwise scratch the delicate surfaces. You might also get away with using ordinary soft tissues, but be aware they may also contain small amounts of tough fibrous material which might be a problem. It goes without saying that the tissues should be kept in sealed containers when not in use to avoid contamination with dust.

Corrector Plate Removal:

To properly clean the entire corrector plate it will have to be removed and the job completed on the bench. I know most new SCT owners will view such an operation with trepidation, but once you understand exactly what is involved, and the procedure is described step-by-step, I hope you will then feel more relaxed and confident about tackling it. After doing the job just once you'll consider removing and replacing the plate is really no big deal, and probably laugh at the detail these instructions go into. I will also outline a few simple precautions that will minimise any risks associated with handling the fragile glass plate.

First, you need to identify (in advance!) somewhere to work on the scope, and a convenient surface to support the plate needs be provided whilst it's being cleaned. For the latter, the draining area next to a sink (with a layer of clean cloth covering the hard enamel surface) would be ideal. Water and solvent can be liberally applied without it running all over the floor or table. If that isn't possible, I suggest covering a table top with a plastic sheet (bin liner) and a cloth placed on top of that to absorb the waste liquids.

There are a few tools you will need to make the job easier:

On the Meade LX series scopes the corrector plate is held in place with a plastic retaining ring, which in turn is secured using a circle of 8 small screws. These are round-headed hex socket screws, and the supplied wrench to loosen them is rather small compared to the size of the screw. This can be a problem if the screws have been in place for some time, it might require a lot of torque to break the seal and loosen them. Purchase instead a T-handle wrench, this offers more torque and the ability to press the hex wrench firmly into the hole. It is essential that the wrench be an exact fit in the screw's hex socket. Upon replacing the screws use no more pressure than can be applied with the original wrench when gripping the short end between thumb and forefinger, this will avoid over-tightening.
Again, upon first-time removal it's often found that the corrector plate is stuck to the series of cork pads against which it seats. The plate sits in a recess where it's edge is poorly accessible and thus it needs to be prised away from them with a little plastic tool. There is a gap of about 1/16" between the edge of the plate and it's recess, so it's possible to get the edge of a plastic screwdriver between the two (these screwdrivers are used in electronics). If you can't get a genuine plastic screwdriver them make your own by filing/grinding/sanding the handle end of an old toothbrush until it fits the gap. The problem of a 'stuck' plate may not be a problem on your own scope, it is most likely to occur on scopes that are several years old and have never had their corrector plate removed.
You will need a small plastic bag and rubber-band to cover the secondary mirror and housing whilst cleaning the corrector plate.

Procedure:

Tip the front end of the telescope upwards a little from horizontal position, the corrector plate should be facing forwards but tilted a little backwards from being absolutely vertical (i.e., at around 70-80 degrees to horizontal). Lock the DEC clutch firmly to prevent it moving. In this position the plate cannot fall out even if were loose with the retaining ring removed, yet it is relatively easy to tip it forwards from that postion to remove it.

Remove the ring of 8 screws, if the screws are stubborn press the wrench firmly into the socket and apply a steadily increasing pressure. I decided after first removal not to replace these factory screws, but instead used a cap-head design in stainless steel with the same thread (#8-32) and length. These have a somewhat larger hex socket and are easier to remove. If you do this remember that the larger wrench used is NOT an excuse to tighten the ring down more tightly!

With the screws removed you can lift away the retaining ring exposing the edge of the corrector plate (see Pics).

The front of a Meade LX200 with the corrector plate retaining ring removed.

The corrector plate alignment mark, in this case a pencil line on a patch of white paint.

Now, before removing the corrector plate, identify the mark which indicates the correct rotational alignment. There are various ways this can be done but on the Meade SCTs it's usually a pencil line on a patch of white paint (or Tippex). It's important when replacing the plate that it is re-aligned using this mark, so be careful when cleaning not polish it off! If it is indistinct go over the mark again again.

As noted earlier, it may be the case that if the corrector plate has not been removed for a number of years then it may have become stuck to the cork pads. If previous attempts to clean the plate without removing it resulted in solvent getting between the glass and cork pads it most certainly will be stuck. In either case, you will need to break the seal before the plate can be removed. Use your tooth-brush prybar (or plastic screwdriver) to gently lift an edge of the plate to break the seal. Once this is done in one position allow it to seat back down again and repeat the operation a little further around. Work you way all around the edge until satisfied the plate is entirely loose.

The next part is far more difficult to describe than to do in practice. It may sound awkward but it's really quite simple. You want to avoid touching the optical surface of the corrector when handling it (you definitely do when putting it back after cleaning!) so you'll need to protect it from direct contact with your hand. You can use a soft cloth, or perhaps even invest in a pair of white cotton gloves if you want to be professional about it. Latex gloves work fine too. Personally I just use a soft cloth.

If you are right-handed like me, drape the cloth over your left hand and then grip the plastic housing of the secondary mirror with it. There's not too much to get a hold of - but enough. Use the plastic prybar in your right hand and put it's edge into the gap between corrector and recess at the very top edge. Lever this edge of the corrector plate forwards until it's about 1/2" clear of the recess then just let go the prybar and let it drop on the floor. Now use the tips of your fingers (right hand) to pull the top edge further forwards - that's towards you and away from the recess - but leave the bottom edge of the plate supported by the recess. In one smooth motion continue pulling the top edge forwards until the plate is horizontal and it's weight is supported by your left hand gripping the secondary housing in the center of the plate. Swap your right hand over to the right-hand edge (as it came forwards your right hand will have crossed to your left) and use it to provide additional support. Move away from the scope carrying the corrector in the position and and then place it the same way up onto your cleaning platform.

You will not be cleaning the secondary mirror so slip the plastic bag over it (the secondary baffle that is) and use the rubber band to hold it in place. This will protect it from accidental splashes of solvent. If upon examination the convex secondary mirror really is dirty or the aluminised coating is corroded you have the choice having it cleaned professionally or getting it re-aluminised. I do NOT recommend attempting to clean this yourself unless you are very confident about what you are doing (in which case, you won't be reading this article!). Note that the convex secondary mirror does seem to appear less 'bright' than the primary usually does - I think this is an optical illusion.

Handling the Corrector Plate:

First - a note on the physical properties of the corrector plate. Your plate's surfaces are ground and polished to a very fine figure and then (usually) an anti-reflective coating is applied. Recent versions of such coatings (i.e., manufactured within the past 5 years or so) are sometimes described as 'multi-coatings' which simply means there are multiple layers applied on top of each other which together are more effective in reducing reflections than a single layer. Unlike the aluminised surface of your primary or secondary mirrors, the coated surface is reasonably tough and resistant to scratches. For example; it is possible to gently wipe the surface with a soft tissue without 'wiping the coating off' - a procedure that would immediately destroy an aluminised mirror. Nevertheless, the accurate figure demands the greatest respect if it is to remain in top condition so it should be treated akin to how you would treat the object glass of a fine refractor. On SCTs older than about 5 years (it's not possible to be exact) the coatings have been known to deteriorate spontaneously, and this may be visible as a fine white-ish deposit which is usually most notable on the inside surface as this recieves less regular attention. I don't know of any simple solution to this problem, there are certainly companies that specialise in applying these coatings so it's perhaps possible to have the old one stripped off and a new one put on, although the process involves a certain amount of heat so there is a danger (albeit slight) that the plate might be damaged. I suspect that if the loose powdery remains of the old coating are simply washed off during the normal cleaning process then that's all that is necessary - the result might not *look* wonderful but the optical performance will not suffer significantly. Corrector plates from around 10 years ago tended not to be coated anyway.

Cleaning the Corrector Plate:

Now you have the corrector plate on the bench you can press ahead with the main job of cleaning it. Probably the best method is to tackle it in quadrants, with the edge of the plate tipped downwards so that the solvent and rinse water can run off. For this job you'll need a pile of soft cotton balls, a beaker or similar to hold your solvent, and for the rinse de-ionised water a squeezy bottle is ideal. A second supply of solvent (only about 200 mls or so) in another squeezy bottle will be useful. In addition, you'll need either a pair of latex gloves or plastic forceps to handle the cotton balls.

The first job is to remove any loose dust, and for this the previously mentioned home-made vacuum brush is ideal. If you don't have one of these then use an ordinary soft brush that has been thoroughly washed in your cleaning solvent and then allowed to dry. It's best to keep a brush specifically for this job only, and keep it in a sealed plastic bag when it's not in use. Gently brush the plate all over the currently exposed face of the plate (the inner surface if that remains uppermost), start from the center near the secondary holder and use single radial strokes out to the edge.

With the plate tipped over so that it's edge touches the bench, use the squeezy bottle to squirt solvent over the lower quadrent of the plate, leave for about a minute for the solvent to loosen any stuck particles then squirt it again to wash them off. Dip a cotton ball in the beaker of solvent and gently swab the same area - don't press on it or rub hard, and use the radial strokes again. Discard the cotton ball immediately after use.

Give the surface you've just swabbed a good squirt of de-ionised water to rinse off all the solvent, then turn the plate around 90 degrees and repeat for the next quadrant. Continue all the way around - the whole job will take less than 10 minutes.

Allow the surface to drain as much as possible and allow it to dry. There should be no streaks left whatsoever provided the last rinse was done with clean de-ionised water (or diluted solvent in de-ionised water). If there are any marks remaining repeat from the point where you used the cotton balls.

It's now necessary to turn the plate over and clean the other side in exactly the same way. Be careful not to wash off the alignment mark!

Replacing the Corrector Plate:

The task of replacing the corrector is a little more tricky simply because you don't want to end up with more fingerprints on it than when you started. You can keep the latex gloves on for this job, or make use of the soft cloth again. The replacement technique is similar to the removal procedure in that initially you need to have the corrector plate horizontal and supported by your left hand from underneath (holding the front extension of the secondary holder) with your right hand steadying the edge. Your scope's OTA once again needs to be locked in a position with the front raised a little (remember to LOCK to DEC clutch), and then stand in front of it holding the plate. Note the position of the alignment mark on both plate and recess - these will need to match up eventually so it's as well to get them as close as possible to begin with.

The idea is to rest what will be the lowest edge into the recess (it won't fit all the way in of course until the plate is vertical), use your right hand (with cloth) to prevent that edge from slipping outwards, and then deftly lift the corrector plate up to vertical and push it fully into the recess in one fluid movement. Once again, this sounds more difficult than it is - support from your right hand will prevent any chance of the plate slipping out the recess, so you can't drop it, and once it's seated into the recess it cannot fall out. The plate will be free to rotate at this point so turn it a little if necessary until the two alignment marks line up.

All that remains now is to replace the platic retaining ring and the eight screws which hold it in place. Do NOT overtighten those screws!! Only tighten them finger tight - just use the short arm of the Allen wrench held between finger and thumb.

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	The front of a Meade LX200 with the corrector plate retaining ring removed.
	The corrector plate alignment mark, in this case a pencil line on a patch of white paint.