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c8 repair – removing the corrector plate February 29, 2012

Posted by aquillam in Astronomy, telescope maintanance.
Tags: , , ,

This post is one in a series about C8 repair and maintenance.  You should see the introduction and disclaimers before trying anything here.

A Cassegrain telescope has a large mirror at the bottom with a hole for the eyepiece and a small secondary at the top of the tube. They can have expensive, hyperbolic mirrors, or they can have some sort of corrective lens. If the corrective lens is on the front, it is a Schmidt-Cassegrain. That is what the old orange-tube Celestron 8 telescopes are, and you can tell because it says it is a Schmidt telescope right on the plate with the serial number.

That corrector plate is NOT just a plane piece of glass. It is an integral part of the optics of your telescope, and was ground to match your primary. If you damage the corrector plate, it has to be replaced by Celestron, and it’s going to cost you as much as a new telescope. So, first,  a really really really important warning.

Really. Very. Important.

You must be extremely careful doing anything with the optics.

In addition to the obvious problem if you break the optics, there is also the problem of keeping them aligned. If you mis-align your optics, your telescope will never be the same. You may still be able to see things, but your $800 classic telescope might be no better than a $40 department store special.

There are very few reasons to open the ‘scope, so make sure your problem warrants it. A little dirt inside is fine, and won’t do much of anything to the view. Even a few pits won’t be noticeable. Dew inside can be dried with a hair dryer pointed in the eyepiece opening, and a few light water spots also won’t affect the view. Of course if the ‘scope has seen 30 years of heavy use from people who only vaguely know what they’re doing, or if you get caught in the rain, it might be worth opening things up. Also, on the older C8s the only way to get to the focuser is to take off the corrector and remove the primary. I won’t be covering that here anytime soon (I have 7 hard-used, 30+ year old C8s, and ONE of them needed some help with the focuser about 10 years ago.)

If you decide that you really do want to open up your scope, you will need a small allen wrench and a pencil. Tweezers are good if you don’t have slightly long nails. Also, whatever equipment you need for whatever maintenance you want to do.

Before you begin, make sure the dec lock will lock easily and tightly (see adjusting the dec lock.)

Mark the secondary and outer tube

The very first thing to do is mark the telescope in several places. Note I placed marks on the secondary cell, the ring that holds the corrector in place, and the outside edge (the ring orientation doesn’t matter much, but being over-cautious is valuable when dealing with the optics.)

Rmove the screws from the retaining ring

Remove the screws from the retaining ring. It helps to tilt the 'scope so the screws can't roll away onto the corrector plate.

Remove the screws that hold the retaining ring in place. Be careful not to drop the screws as they could scratch the corrector plate. You may want to adjust the tilt of the telescope while you do this so the screws are less likely to fall onto the corrector plate.

Remove the ring. I do this by hooking my thumb nails on either side and pulling outward while lifting. If you don’t have long nails, I would suggest one of those flat plastic toothpick/flossers to lift the ring until you can get hold of it without touching the part of the corrector plate that you want light to go through. Make sure whatever you use is though enough to lift the ring but won’t scratch.

Corrector plate serial number

Mark the position of the serial number on the corrector plate on the outside support ring. Also note the cardboard spacer

With the ring off, you can see the serial number on the corrector plate. It probably won’t be the same as the ‘scope’s serial number. Note the position of the serial number. Mark the outer support ring so you can realign this latter. I placed 2 marks so I can’t get confused about which part of the serial number I marked.

There are some boxboard or cork spacers holding the corrector plate in the correct position. Try to remove these first. A jeweler’s screwdriver or fine tweezers may be helpful, but again be careful, but again, be careful not to scratch anything.

The ‘scope will be very back-end heavy once the corrector is removed, so the closer to vertical it is, the better. However, I usually find that it’s a little easier to see what I’m doing if I have the ‘scope tipped just a little bit. Make sure the dec lock in engaged and firm.

Lift the corrector

Lift the corrector using the secondary. Be very careful since this is the place you are most likely to do damage to it. If it sticks, it may come free suddenly, so be prepared for that.

Hold the secondary cell and lift gently. If it won’t move, try to shift the corrector plate a little to one side. DO NOT FORCE ANYTHING! This is the part where you are most likely to damage the corrector plate accidentally.

If your ‘scope has been exposed to moisture (left in the rain, used in very dewy conditions, etc.), the corrector plate may be stuck to the interior mounting ring. You’ll have to weigh  the risks of damaging the ‘scope with the benefits of getting the corrector plate off. Patience is key. You might try something like one of the plastic toothpicks around the edge. Use something that has a little give (NOT a jeweler’s screwdriver) so you don’t chip or crack the corrector. It may help to warm it a little, like with a hair dryer. Additionally, be careful not to damage the supporting ring underneath, which has a soft cover on the rim. If the corrector plate is really stuck, be warned that when it does come free, it may come free suddenly. Don’t get so immersed in trying to force it free that you don’t have a grip when it does come free. Again, patience.

If you are opening it to clean it, be careful what you use. Check your manual if you still have it. Mine recommends a solution of one drop mild dish soap in a gallon of distilled water. I use a cleaner from Orion, but I usually find I have to follow that up by “huffing” on the optics to remove the final bit of film. Some commercial optics cleaners are available, which may also contain rubbing alcohol. This helps the cleaning solution dry faster and with fewer streaks or spots, but isn’t safe for all optical coatings.

A note about canned air:  if you get the nozzle too close to your optics, if you shake the can, or the can is not perfectly upright, you might not be pleased with the results. Condensation and propellant can come out in a glob and damage the optical coatings. You can find bulb dusters and fine brushes at camera shops 9assuming you can find a camera shop.) High quality lens paper does a very nice job too. When cleaning the inside, be even more careful than when cleaning the outside. Use a very light touch (almost not touching), and expect to go through at least half a bag of cotton balls.

When reassembling, you basically do everything in reverse.

Marks to aligh serial  number with

Set the plate into place with the serial number aligned between these marks.

Identify the location of your marks so you can replace the corrector plate.

You may want to clean off the support ring the corrector plate sits on, especially if the corrector was stuck. Be sure not clean off your marks! Clean it with the same solution as the optics and give it time to dry before putting the corrector plate back in. Do NOT put anything on it, no matter how hard it was to get the corrector out. Oils and solvents can damage the soft covering or generate a film that will cover the inside of your corrector plate.

replacing the spacers

Replace the spacers. Tweezers may help.

Set the corrector in with the serial number aligned with your marks.

Slip the spacers back in around the corrector plate. There should be 3 – 4, depending on how many are needed to keep your corrector in place. If the cork spacers are damaged, you can replace them with boxboard (like a cereal box) or folded paper. Use the cork as a template for the replacement pieces, and make sure they are shorter than the corrector plate so they don’t interfere with the outer ring. It doesn’t matter where they go, as long as they are roughly evenly spaced. Tweezers may be helpful for getting them into place. You may also need to slide the corrector plate to the side to open the gap. Push sideways on the secondary cell to move the corrector.

Replace the ring

Carefully set the ring in place...

Once the corrector and spacers are settled in place, set the ring on.

Align the ring

...rotate the ring to align it with the holes in the support ring below, and push it into place.

Carefully rotate it so the holes align with the holes in the support ring below, then push it down into place.

Replace the screws, tightening firmly but not too hard.

Chances are good you’ll have to collumate your telescope after that. But that’s a post for another day. One that follows a clear night without students.

Much gratitude to Bob, who taught me  everything I know about repairing and maintaining telescopes. Miss you man.



1. James Saxon - October 14, 2014

Thank you for the information! My corrector plate was nearly glued in place and I was able to free it by inserting strips of very thin brass shim stock. I made a sandwich with two flat pieces on the outside and then wiggled two pieces that had a fold on the end in between those. By manipulating the two middle pieces I was able to insert one of the folded ends into the other making a lump in the sandwich on the inside of the scope. The lump pressed against the plate as I tried to pull the sandwich back out thus giving me a handle. YMMV

2. Anonymous - February 29, 2012

Thanks for the great tutorial.

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