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[Chicago Astronomer - Astro Joe]

Replacing the 90mm Finder Scope Bases on the C11

04 April 2013

(Simple Corrector Plate removal becomes a complicated Mod project)

This is really two stories in one - one project that should have been a straight forward job and a couple of hours tops. The other, something I encountered that stopped me in my tracks in starting that simple job. I always anticipate complications, but not like this.

From day one, these generic bases that holds up the 90mm Meade finder scope and rides on top on the C11 - have been substandard....

Full story here:
astronomer.proboards.com/index.cgi?board=Equipment&action=display&thread=2963&page=1

Wobbly and sloppy, the refractor once fell out of the base, crashing to the ground and causing minor damage to the Dew Shield...

Master Crafter and fellow Astronomer Rick came to my rescue and made me neat bases for the brackets he made me earlier. Now, a matched set awaited to be mated, but this needed the C11 to be taken apart to install. 2012 was a tragic year for the Chicago Astronomer...and I needed the C11 whole for the Chicago Astronomer and Chicago Park District Star parties we worked. By the time the season was over, it was just too cold - and the workshop is not heated...

April is a good time to install this bases....and besides, the C11 needed a good cleaning after serving our fans last year...

Yep, the corrector needs some heavy cleaning.

Chicago Astronomer Forum member Mo, (Andy) suggests that the secondary housing be checked for eccentricity...

In actuality, the housing is ever so slightly off. I'll correct it when I re-assemble.

We have gone through this process of dis-assembly before without a hitch..

The Corrector Plate retaining ring gets removed quickly, but....when I attempt to remove the corrector plate...it does not budge. I apply some force....nothing. I tug on the secondary housing some more - stuck on tight.

What the hell!

During the previous dis-assemblies, the corrector plate came off without struggle. Nothing has changed since. Too clean of a plate....heat/cold...new paint vapors???

I examine the circumference of the plate...and all looks ok...

I can easily move the spacers around the edge of the plate...but the hunk of glass just won't move! I use popsicle sticks to wedge and pry around the edge of the plate...

I tap all around the edge with the sticks...and nothing.

Damn.

I resort to giving the secondary one more strong tug....

...and the secondary came apart!

The cork laying in between the plate and the front cell has bonded tight. Well, at least now, I can use the opening as a place to grab and pull.

Nothing.

I use my closed hand and rap from the inside out...got to loosen now.

Nothing.

?

I need to break this Satan seal somehow...so I grab my 91% Alcohol and apply two cap fulls via a Q-tip around the edge...

I can see the alcohol work its way under the plate and saturating the cork. I tug and nothing. I apply another cap full and wait a bit. I gently tug at the plate and I can see the vacuum created, sucking in the alcohol where it's needed. Much more play now, but still won't release.

After about 10 minutes, I give the plate small repeated tugs....and I hear it before I see movement. What ever held this plate tight - got beat down and finally released...

The back side of the cork was attached to the cell housing with double sided tape...and this is what finally let go...not the bond to the plate!

Daang!

I do not know what happened here and why it grabbed on tight, but I will have to remove the stuck on cork and replace it with new. This took much longer than I anticipated and it was already 11:00 pm. As I need to awake by 5:30 am...I was beat. I'll pick up tomorrow evening and left the tube on the workbench.

But wait....


But, with all that shaking, tapping and alcohol - the primary got dirty...a lot.

It just doesn't stop...

The alcohol left some splatter on the primary and I needed to get it off quick before it sets. Using the cleaning solution I made and have been using, I use a great many cotton puffs and lift the crud off...leaving a pretty decently clean mirror.

(Yea yea...we all hear about how "you got to leave the mirror alone for Christ sakes!". But if done carefully and smartly, all is good - but we will see.)

Setting the primary and corrector plate in clean new pizza boxes, I decide I might as well install a bracket base and have some sense of accomplishment this evening...

Out come the old raggedly bases....and in with the new...

Master Crafter/Astronomer Rick milled these bases and dovetails to tight tolerances - with with the circumference bend of the OTA too. With the Metal cleaner/conditioner to remove oxidation and give the parts a great luster...the two pieces slide against each other like buttered teflon... ;D

Back to this project tomorrow for part two.....

[-=CROMAN=-]

WOW! That's some ordeal you have going on there. I thought I had issues with my C8. Those corrector plates are scary things.

[Chicago Astronomer - Astro Joe]

I didn't have time last night to attend to this, but did check the plate over.

The cork is so solidly bonded to the plate, that I cannot even pull it off. I soaked it with more alcohol and left it be for a clean up session tonight. Probably will need to be razor bladed off.

I could not find small rolls of thin cork, and the Einsteins at Home Depot and Ace have no idea what I'm asking for...thinking I mean "Caulk". Whuuuut....?

Simpleton Morons.

So, I will use foam weatherstrip that will work nicely. The secondary looks decent with just a few specks that I will lift off.

I have read where other Astronomers have also encountered stuck Corrector Plates and the alcohol is the solution to removal. Apparently, there is a "Blackening agent" used on the cork, applied by Celestron for aesthetic reasons. The ink reacts with moisture, solvents, vapors etc....and becomes an adhesive - locking in the Corrector Plate, such in my case and many others. This Devils spit needs to be removed and placed with a more inert material.

Let's see what tonight brings.

[patrickm]

Wow Joe. What a PITA.

Look forward you you out-smarting it all.

For cork I'd recommend an art supply store preceded by a phone call.

[skyluvr]

Hi Joe,

Quite the ordeal with the corrector.

I get cork from the auto parts store. Comes in a roll used to make gaskets.

Rick

[Chicago Astronomer - Astro Joe]

Corrector Plate Saga

Part 2 -

I came back two days after I encountered the complication of a severely stuck on Corrector Plate.

Looking it over, the plate was in pretty decent condition and the cork seemed soft and pliable now.

Having doused the cork with ample amount of 91% alcohol...the Devil's spit met it's match and meekly let go as I easily pulled off the offending cork...

It was sticky and the blackened side was a bit brittle. Pulling both pieces off, left some residue along the edge...

The residue would not come off easily, using the cotton puffs and cleaning formula. I tried out a spot with turpentine, but that also failed. The gunk needs to be scrapped off...

Wetting down the plate and using a brand new razor, the hard bits came off easily and in no time, all evidence of the cork was now gone.

(Listen: Corrector Plates are robust and tough. With proper and careful handling, it will take a decent amount of cleaning without advise effects....including a sharp razor. Get over it.)

A preliminary cleaning of both sides now...

...using numerous cotton puffs and the formulated cleaner, (Filtered Windex, Alcohol and Photo-flo), bringing it back to a clean and clear state.

Now...to the Corrector Plate cell holder....

The residue left behind was still pretty sticky and could be useful in what ever I line the edge with. I had forgone the thought of replacing the old cork with new and had some ideas on what else I could use. Perhaps some felt, inner tube rubber, weatherstrip.....

I grabbed what was close to me and evaluated...

I reached for the sticky sided weatherstripping first. It needed to be cut down to size and could work. But, after applying it, it didn't like the way it compressed after placing the plate on it. It was just too weak and thinly feeble to support the plate. And, it was already starting to shed. Out it came and tossed aside.

I read on another astro site about a dude who used the soft side of Velcro for padding with good success. I think I will try it out as well...

Matching it up next to the old cork, it's pretty close in width and thickness.

It's narrow and a single slice down the middle was just wide enough to fit the support edge...

Placing the plate on, it's a good fit...

Placing the locking plate ring on, it looks good too...

I think the Velcro will serve well and will never stick to the plate. I'm thinking about using a strip for the locking ring as well, but we will see.

We will give the plate another cleaning before re-installing the secondary, as well as going over the now empty tube. There is also an alignment procedure I would like to conduct with the empty tube - observing if the baffle tube is in concert with the light path.

More on Part 3....

[Chicago Astronomer - Astro Joe]

Thank you Patrick and Rick.

Both would have the cork, but I think I'm more comfortable with the Velcro solution.

If it doesn't work out...I'll yank it out and get the cork.

[skyluvr]

Nice,

The Velcro looks like a good solution. The loose nap of the Velcro breaks up the surface area. That should help keep the corrector from sticking. As long as the adhesive does not get to the surface.

I am curious though, the C 14 has nothing under the corrector. Just shims along the outside circumference. Did the C 11 come with material under the corrector? Are you using the Velcro instead of the shims? I can see where the Velcro would help hold the corrector during the centering operation.

Rick

[Chicago Astronomer - Astro Joe]

Rick,

The C11 was originally padded with the cork underneath the plate and cork spacers around the edge.

See the original tear down project:

astronomer.proboards.com/index.cgi?board=ATM&action=display&thread=3208

It shows the factory build, including the cork.

I attempted to reuse the original cork shims around the edge when putting it back together three years ago, but they looked tired and lame, so I used strips of vinyl instead.

Worked just as good.

This time around, I'm thinking about solid rubber pads.

We will see.

The C14 is a hefty hunk of glass resting on that bare metal edge. Can all C14's be like that? I wouldn't think so.

Are you the first owner of the scope?

[skyluvr]

Yep first owner

Maybe I am not remembering it correctly, its been some time. If there was anything under the corrector it was small squares of cork. Nothing along the whole perimeter of the flange.

I do recommend that you follow the steps list here www.wilmslowastro.com/tips/c14_optics_alignment.html

After I performed these steps on the C14 the alignment was much better. I found that it was easier to collimate and the stars were much sharper.

Great writeup so far. I look forward to following along.

Rick

[Chicago Astronomer - Astro Joe]

Part 3 -

Putting things back together...

On this longer than necessary project, we prep the tube for the primary and plate. Using Gorilla Duct Tape, we lift off the debris from the flocking and swab down the rear of the cell with alcohol. There was still a nice coating of lubricant on the baffle, but it's wise to clean it off..

The Dow Corning High Vacuum lubricant is doing it's job well. The small globs at the rear of the baffle that was pushed back - has not liquified and oozed down and remained viscous. Wiping down the baffle with a paper towel dampened with 91% alcohol, it displays crud.

I re-apply the recommended grease to the now pristine baffle...

Not a lot, just enough to coat the surface and we take a look at the primary.

Everything in the back the glass looks good. Nothing rattling, chipped or cracked. I apply a thin layer of the Dow Corning to the baffle sleeve and examine the business end...

Some dust but the majority of the debris and alcohol stains have been removed earlier. Breath and many cotton balls is the best finishing technique...

Applying practically no pressure, I drag numerous cotton puffs on stubborn bits and streaks...leaving behind a near pristine surface. (These are not the best cotton balls, leaving bits of material, but it was all that was handy. Blowing the residue off was all that was needed.) Giving it a final Giotto torpedo blasts of air, it's ready to go back home...

There is some minor scuff on where the secondary housing made contact with the mirror, but it's no big and I leave it alone. There is quite a firm feel to the mirror going back over the baffle and it take some effort to push it back. This is good. No wobble, tilt or slop can be felt as I test for fit. re-installing the focusing mechanisms at the rear, I secure the primary to the tube and we are ready for the corrector plate.

Examing the secondary, there are just some minor specks about and I use the blower and some cotton to remove the stubborn bits. Setting it in the plate opening, we get the 1/8th" gap around the housing...

I wanted to use the Velcro here to pad the housing, but the Velcro is too thick here. I re-use the vinyl pads that I incorporated during the last take-apart project and it works well...

No, wobble or shift.

I still needed to figure out what I was going to use to pad the corrector plate along the edge against the cell collar. I tried some scrap Velcro and laying them dry, they work beautifully!...

With just enough thickness, they pad the plate rim firmly and give when needed. I used the sticky side against the rim - folding it over the top to pad the lock-in ring as well...

Installing the Velcro pads centered in-between the screw holes, I evenly space them around the cell. Dropping the corrector plate in, it's a firm fit and I like how it feels.

Begone is the possibility of another stuck corrector plate...! ;D

But for now....it's getting late and I leave installing the plate till next time in part 4.

[mojocvh]

Hey Joe, welcome to the club!
The club of cold sweats!!

As you know I had a bit of a 'mare with my C8 as well but all problems are surmountable. I can't believe the secondary pulled out like that :

OK I see a really nice alignment marking on the primary, does this follow through on the secondary as well?
As you are going to be collimating from scratch this will be the ideal opportunity to check it out and have a clear set of alignment orientations noted. If you have these and you rebuild to them, it's one set of variables to discount come collimation.

Centring of secondary in corrector plate.
I was fortunate that my secondary went back into the corrector tickity boo with original washers intact.
However, I still measured the centring as, like above, it's another variable to be put to bed.

That velcro is a pretty novel solution, just be sure to have the clamping ring down and secured BEFORE the weight of the secondary/corrector has the chance to compress it in a "downwards" direction..

I've had problems with getting online recently so am behind with this, the latest story of the beast.

Good luck for the build Joe.

cheers

Mo.

[magic612]

Joe, if you ever decide you want to replace that Velcro with some cork, try looking in a hobby store that sells model railroad equipment. The track ballast often comes in cork strips; in the smaller track gauges you could probably just split the stuff in half and tack it around the outside edge.

Looks like the Velcro is a good solution though. Nice job on detailing this cleaning / replacing parts.

[mojocvh]

I wish they'd go back back to putting centering marks on the secondaries, make the job of optical set up 100% easier IMO .