Old Skywatcher 120mm refractor.

[GriffinUcos]

Hello all. I have this very old Skywatcher in need of some TLC. The two lens elements look really poor. Fungus attack that seems to have had a go at the coating but not much in evidence on the glass. Washing them in Ammonia at the moment.

I know it's an old model as the elements are separated with three small pieces of foil.

I'm wondering if anyone has dismantled one of these before. My concern is that the rear element sits directly onto the ledge on the back part of the cell. Should there be something between the surfaces?

The locking ring and O ring are fine but when I split the two sections there seems to be an O ring channel but no O ring. Should there be one? 

Sorry I can't post an image yet, the paint is still wet!

Many thanks.

[rl]

Send us some photos when you can.

A 2-lens achromat is not too difficult to clean and service. I've taken several to pieces and managed to realign them ok. 

Try and remove the 2 elements together over something soft....

The important thing is to mark with a pencil on the edge an alignment point for reassembly; ideally it would make no difference if one element rotated with respect to the other but sometimes there is an optimum angle that minimises astigmatism. Most people use a "V" that points towards the front surface. This also helps in not accidentally reversing one element. The curves are not generally the same on the convex crown glass (usually at the front)

Cleaning; Baader Wonderfluid and the correct cloth will work wonders. Well worth the few pounds invested to make sure you don't scratch the coatings. 

On reassembly I generally put 3 short lengths of tape around the edge (over the tinfoil bits). try and feel with your fingers all the way around the circumference to make sure the optical axis passes through the centre of both component lenses before you put the tape on. 

Don't overtighten on reassembly; just enough to stop the lenses rattling. Else you risk deforming the glass and ruining the diffraction patterns, by pinching down on the tinfoil spacers. If the cell is too tight, the metal will shrink around the glass in cold weather causing further deformation. Commercial manufacturers have been guilty of this problem. 

A small amount of coating degradation is not the end of the world provided the affected area is only a small percentage. 

Triplets are a different matter altogether...

Edited February 26 by rl
extra info

[GriffinUcos]

Thanks rl. I have the wonderfluid and cloth on order. I'll get some photos tomorrow.

[GriffinUcos]

The lenses don't look too bad. Waiting for the wonderfluid.

The rear element appears to sit on the bottom ledge. It's a very close fit on the sides. The foil spacers are in place. There is no mark on the side and I was unable to get both lenses out as one, so I'll have to investigate the alignment.

The front element has an arrow pointing forward. 

This was followed up by an O ring and the locking ring.

No O ring in the recess between the two cell pieces. Not sure if there should be one but I think probably should to stop humidity ingress.The alignment grub screws are in place but rather short if there was a seal.

[rl]

Those don't look too bad. I think a gentle clean with the Wonderfluid will do the trick. If there is a lot of gunge then try a wash under a running tap with tepid water before going to the Wonderfluid; This may stop you rubbing in any sharp dirt, but to be honest they look basically ok.

I don't remember anything like a waterproof seal on Skywatcher OGs before...I think it's all there.

There may be residual marks on the convex front element which will help you keep the correct orientation on reassembly.

[Franklin]

Did you sit the lens on a tube-stand, narrower than the cell and drop the cell down away from the doublet? That way you can remove them together and keep the elements aligned and put your own pencil mark on the edge. Just for future reference

[GriffinUcos]

Thanks rl. I'm happy about the front to back orientation, I'll just have to rotate them to minimise astigmatism.

Glad you think it's all there. I'm not keen on direct contact glass to metal but what's the alternative? Slivers of self-adhesive tape  on the ledge?

Franklin, thanks. Err no! Not sure why I didn't, I will have to put that down to advancing years. Once the scope is back together I'll rotate the front element and try to find the best alignment. 

Many thanks.

 

[Franklin]

To find the best alignment of the objective elements can take time and lots of patience. When you first put the cleaned elements together, mark the sides with a pencil to establish a reference point, get it back in the cell and don't over tighten the retaining ring, just snug is all it needs. Get the scope back together and star-test, if it's not good or you think it could be better, then back to the drawing board. Break down again and rotate the elements 180 deg from your mark, reassemble and star test again. If improvement is still needed just repeat at 90 deg, in both directions, reassemble and star-test each time. This way you will always have a return reference and from the star-test be able to see the differences alignment actually makes. When doing this if you notice astigmatism increasing then you know to go in the other direction etc. A total ball-ache but rewarding to get it spot on. Good luck.

Also, don't go putting any padding or shims in there, just sit the glass on the metal rim, it's meant to be like that.

Edited February 28 by Franklin

[GriffinUcos]

Thanks, Franklin. The lenses are back in the cell and ready to adjust. I'm going to do it for the experience. Sadly there is some marking on the lenses, probably fungal attack. Hard to tell if it's just the coatings or whether it's in to the glass. 

[Moonlit Night]

Very much looking forward to seeing the finished project 

[GriffinUcos]

Back together. Lenses are marked and the image looks like it's smeared with vaseline. Some rearranging to do. I'm tempted to try and find replacements as the scope itself is fine.

Edited March 11 by GriffinUcos

[bosun21]

Are you sure you have replaced the rear element the correct way round? As you said that the front lens has an arrow on its side so that is probably fitted correctly. If the image appears as if it's smeared with vaseline throughout its range of focus then my money is on the lens orientation being wrong.

[RT65CB-SWL]

Same happened to me when I gave the rear element of my Photax/Paragon 400mm telephoto camera lens a clean. Quick unscrew of the retaining ring and flip the lens over, reassemble and hey presto! 

[Ratlet]

I'm with @bosun21.  I had a similar experience with a Tair 3 used for imaging here:

It was pretty wild with fungus, but cleaned up nicely.  Had one of the elements in the wrong way round and you can see the impact on page 2.

[GriffinUcos]

Hello! As suggested flipped the first element and now focuses OK in daylight. I'll try a star test this weekend if I can. 

Tried cleaning again with Baader wonderfluid but still have white-ish marks on the lenses. It's an old scope and is what it is.

Many thanks for the advice.

[GriffinUcos]

There is a lot of truth in the adage, "Don't look at the lens, look through it"

Good view of the moon, Jupiter and M42 last night so I'm happy enough. I eyeballed Polaris through the hole where the polarscope should be and got a good enough alignment for visual use.

Thanks all.