Please help me rebuild my ed80

[assouptro]

Hi all.

I had condensation in between the front tenses in my ed80 doublet and decided to strip it down for a service.

during the strip down I was distracted,(that's my excuse anyway) basically I wasn't careful enough and I lost track of which way round the lenses go!

it has been in storage now for a couple of years but I want to reconfigure the lenses correctly so I can get some use from it. 

It was a cracking little scope!

there are 2 elements. One is thicker and has a concave side and a convex side.

the other thinner element is convex on both sides but one side is more pronounced.

there is a really thin gasket that I think separated the lenses, or it may have just been at the front to protect the first lens in the housing or at the back to protect the rear cell when screwing in the retaining piece.

ill include some photos.

i have learned my lesson. Don't mess unless you are recording the process properly for reassembly 

i have scoured the internet for help but can't seem to find anything.

can anyone help me ? 

Cheers 

bryan 

 

[Ben the Ignorant]

The biconvex lens is at the front (toward the stars). The concave/convex lens (named a meniscus) is at the rear (toward the focuser). The thin gasket goes in-between. One of the front lens' bellies matches the concave part of the rear lens, thus there is only one way of placing the rear lens inside the cell.

Which side of the front lens will match the rear lens must be decided by trial and error. When mating the glasses, be very careful that the protruding center of the biconvex lens does not rub on the other, they would scratch each other. The side that goes into the rear lens is very probably the one with the most curvature.

Seeing you wear gloves, you're probably careful enough. Tighten the ring gently, with only the force of two relaxed fingers, or you will deform the optics. Each lens should have a little mark on the side, the marks must be aligned so the lenses will compensate each other's imperfections. I hope the marks have not been covered by the black paint if it's an aftermarket job.

In case the marks are absent, trial and error by 45° increments will get you close. Then use 15° increments to get closer, then 5° for optimum clarity. If the lenses play too much inside their cell, tighten it with little strips of tape (6 strips at 60° spacing is best). The lenses must enter the cell by their own weight, with an ever-so-slight friction, with no need to push them. If there is excess friction between the lenses and the cell, the lenses will be compressed and deformed, especially after the retaining ring is tightened.

Do not screw the cell too hard on the tube or the tension will deform the cell, which in turn will deform the lenses. Make sure the dewshield is not too tight on the cell, or same will happen. Lots of things to check for only two lenses but it's worth it. In case you wonder, I know this because I've done it on 6 or 7 objectives, not merely because I've read about it.

[Oddsocks]

Hi Bryan.

There are two common lens layouts for doublets, Fraunhofer or Steinheil.

Fraunhofer is used for medium-long focal lengths and Steinheil for shorter focal length instruments, the change over point for deciding which layout to use depends on the glass composition used for the flint and crown glass elements and there is a small overlap between them. I can't say for sure which the ED80 uses and unless another SGL member knows for certain then I would suggest assembling the elements in the cell as if it were a Fraunhofer layout and then star test to prove it.

If it is a Steinheil and not Fraunhofer then the star test will quickly show that the Fraunhofer is the wrong orientation and the two elements in the cell remain with the mating surfaces still facing each other but the end facing the sky is swapped over to face the focuser and vice versa.

The images in the linked document below show the common Fraunhofer and Steinheil layouts and you can see they are assembled in the same way, just that the sky facing and focuser facing ends swap over.

For the common Fraunhofer layout:

The thinner, convex-both-sides element is at the front, facing the sky, with the flatter convex surface facing out skywards and the more pronounced convex curve facing inwards towards the focuser end. Next is the spacing ring (gasket) and finally the thicker element, the concave side faces out towards the sky so that the front elements' convex surface matches its concave surface and the two elements nestle together. The convex surface of the thicker element therefore points towards the focuser.

The top left image in the link posted below shows the common Fraunhofer doublet layout. The bottom left image shows the Steinheil layout.

When you reassemble the lens cell check that the surfaces of the front and rear elements are not in contact with each other at any point.

If all is well tighten the retainer ring just enough to stop the lens elements rattling around and moving if you tap the lens cell, too tight and you induce optical aberations due to pinched optics and will then have trouble deciding whether you really have assembled the lens elements in the correct order!

http://www.myoptics.at/jodas/doublet.html

 

[Rick_It]

Hello, have a look at this website:

 

http://telescope-optics.net/achromats.htm

Here you have the optical configurations of Fraunhofer and Steinheil. Disregard all the others in that pages.

80mm ED uses FPL-53 (flint) and Schott BK-7 (crown): http://www.skywatcherusa.com/product/proed-80mm-doublet-apo-refractor/

Here you have also a page with many ED and APO optical configurations:

http://telescope-optics.net/semiapo_and_apo_examples.htm

But unfortunately there is not the 80mm ED, nor any FPL-53/BK-7 doublets. Another thing I know is that the project of the 100mm f9 ED should be a scaled-up version of the 80mm, whith - of course - less pronounced curvature of the lenses, being it an f9. So, if you can find on internet a picture of the disassembly of a 100mm f9 ED, use that.

 

[Rick_It]

EDIT: ok, that solves your problem. From the skywatcher USA page posted above:

The P80ED will benefit from the inclusion of Schott glass being used for the positive front Crown element in the objective

So, it is a fraunhofer. Positive (crown) element must be placed forward, facing the sky. Look at telescope optics page to see how a fraunhofer looks like

[assouptro]

Thanks guys! 

I now have a decent shot at getting the right configuration, I can finally dust it off and start enjoying it again ?

[paulastro]

Of course, if you follow the paulastro number 1 rule of telescope ownership: -

NEVER take any telescope apart if you're not confident you can put it back together properly afterwards - or you have a good friend who can do it for you. 

- you'll never have this problem in the first place  .

[mikeDnight]

[assouptro]

6 hours ago, paulastro said:

Of course, if you follow the paulastro number 1 rule of telescope ownership: -

NEVER take any telescope apart if you're not confident you can put it back together properly afterwards - or you have a good friend who can do it for you. 

- you'll never have this problem in the first place  .

I have a condition. I have suffered from it since I was able to hold a screwdriver.

I have always pulled things to bits to see how they work, sometimes I learn something, sometimes I wish I'd never started.

i have stripped and cleaned this scope several times, carefully placing each component in such a way that it's a simple reverse process putting it back together.

this time, however, I was distracted by family stuff and it was a while before I could pick up what I was doing.

I didn't build a contingency in my plan to take a break from it and then I wasn't sure which way round they went? 

It's a mixture of over confidence with regards my practical abilities and believing I can get around any problem thrown up. 

I could figure it out via trial and error, give it to a telescope shop to service or ask the lovely helpful non judgmental people on this forum.

I chose the latter, and haven't been dissapointed. There are some very helpful astronomers here and they have reaffirmed my faith in humanity! 

Thanks for the advice 

Bryan 

[assouptro]

5 hours ago, mikeDnight said:

Really what? 

I don't get it? 

[Tozza Hen]

21 hours ago, Ben the Ignorant said:

Do not screw the cell too hard on the tube or the tension will deform the cell, which in turn will deform the lenses.

Is this what is known as pinched optics? I have never owned a refractor and been looking into getting one - I've come across the term 'pinched optics' and wondered exactly what this meant?

Tony

[assouptro]

7 minutes ago, Tozza Hen said:

Is this what is known as pinched optics? I have never owned a refractor and been looking into getting one - I've come across the term 'pinched optics' and wondered exactly what this meant?

Tony

I believe it is, Tony.

every day is a school day! 

[paulastro]

15 hours ago, assouptro said:

I have a condition. I have suffered from it since I was able to hold a screwdriver.

I have always pulled things to bits to see how they work, sometimes I learn something, sometimes I wish I'd never started.

i have stripped and cleaned this scope several times, carefully placing each component in such a way that it's a simple reverse process putting it back together.

this time, however, I was distracted by family stuff and it was a while before I could pick up what I was doing.

I didn't build a contingency in my plan to take a break from it and then I wasn't sure which way round they went? 

It's a mixture of over confidence with regards my practical abilities and believing I can get around any problem thrown up. 

I could figure it out via trial and error, give it to a telescope shop to service or ask the lovely helpful non judgmental people on this forum.

I chose the latter, and haven't been dissapointed. There are some very helpful astronomers here and they have reaffirmed my faith in humanity! 

Thanks for the advice 

Bryan 

Thanks Bryan.  I have to say, in my case another well known quote from Clint Eastwood applies, 'A man's got to know his limitations'  - at least as far as taking objectives apart is concerned

[assouptro]

5 hours ago, paulastro said:

Thanks Bryan.  I have to say, in my case another well known quote from Clint Eastwood applies, 'A man's got to know his limitations'  - at least as far as taking objectives apart is concerned

Yeah I got to agree, Clint Eastwood had some good ones

like:

 

I don't believe in pessimism. If something doesn't come up the way you want, forge ahead. If you think it's going to rain, it will

 

God gave you a brain. Do the best you can with it. And you don't have to be Einstein, but Einstein was mentally tough. He believed what he believed. And he worked out things. And he argued with people who disagreed with him. But I'm sure he didn't call everybody jerks.

 

Sometimes if you want to see a change for the better, you have to take things into your own hands.

 

If you want a guarantee, buy a toaster

?

Cheers 

Bryan

[assouptro]

On 24/05/2017 at 03:09, Ben the Ignorant said:

The biconvex lens is at the front (toward the stars). The concave/convex lens (named a meniscus) is at the rear (toward the focuser). The thin gasket goes in-between. One of the front lens' bellies matches the concave part of the rear lens, thus there is only one way of placing the rear lens inside the cell.

Which side of the front lens will match the rear lens must be decided by trial and error. When mating the glasses, be very careful that the protruding center of the biconvex lens does not rub on the other, they would scratch each other. The side that goes into the rear lens is very probably the one with the most curvature.

Seeing you wear gloves, you're probably careful enough. Tighten the ring gently, with only the force of two relaxed fingers, or you will deform the optics. Each lens should have a little mark on the side, the marks must be aligned so the lenses will compensate each other's imperfections. I hope the marks have not been covered by the black paint if it's an aftermarket job.

In case the marks are absent, trial and error by 45° increments will get you close. Then use 15° increments to get closer, then 5° for optimum clarity. If the lenses play too much inside their cell, tighten it with little strips of tape (6 strips at 60° spacing is best). The lenses must enter the cell by their own weight, with an ever-so-slight friction, with no need to push them. If there is excess friction between the lenses and the cell, the lenses will be compressed and deformed, especially after the retaining ring is tightened.

Do not screw the cell too hard on the tube or the tension will deform the cell, which in turn will deform the lenses. Make sure the dewshield is not too tight on the cell, or same will happen. Lots of things to check for only two lenses but it's worth it. In case you wonder, I know this because I've done it on 6 or 7 objectives, not merely because I've read about it.

Thanks Ben

that was really helpful, I have reassembled as you described and am getting the lovely views I am used to.

the lenses have been painted black around the edges and I couldn't find any markings, I'm not sure if I'll be able to reorient them to the perfect position? 

How would I test if they need reorientation? Would it make stars "flare"? 

Could i test reorientation in day light? If so how? 

Or would it be colour correction? 

I could see good contrast of jupiters cloud belts with a 12.5mm lens when focused. However when traveling in and out of focus I would get blue halo to the top left and then as I travelled through focus I got a red halo 180degrees appearing at bottom right. Is this how it should be? 

i have always noticed the blue and red traveling in and out of focus and I'm sure (from memory) they were either side of the object I am focusing on but I may be wrong!?

anyway if you find the time to reply, I would be greatful as your advice so far has been excellent! 

Thanks again 

Bryan 

[Ben the Ignorant]

How would I test if they need reorientation?

Make a little mark without thickness on each lens, because the tolerance in centering is around 1/50th mm, so paint dots would push the lenses. A little dent is best, do it with something as soft as possible, like the tip of a beveled copper wire, for example, it's less likely to scratch the lens deeply if your hand slips.

Poorly rotated lenses make the overall image soft, reduce contrast, and cause chromatic blur. The fact that your chromatic pattern is not round and symmetric indicates the lenses are misrotated, unless atmospheric spread intervenes. Compare with another scope to check that.

Then it's a tedious job of trial and error: try an alignment, look at a very bright, fully illuminated scene for global quality. And also an artificial star (intra/extra), cause a moving one will annoy you, note or draw the result, rotate one lens 45°, and so on until you've covered the 360°. Then refine the setting with 15° rotations, and optimize with 5° rotations if you wish, but the 15° range will get you very close to the ideal image, so son't try too hard, seeking perfection often ruins a perfectly acceptable adjustment.