Kowa 82 series prisms query (2 Viewers)

[dipped]

Following another scope mishap the prism housing on my old 824 has cracked and now been re-glued/epoxied.

Whilst cleaning the prisms I noticed the coating has been impaired and they now need to be replaced.

Does anyone know if the two porro prisms in the angled bodied 821/823 are identical to the two porro prisms in the 824. I am thinking about options for sourcing these.

Also same question for the current angled Kowa 82 SV scope which is basically the same scope as the 821/823 but with a different eyepiece mount.

 

[henry link]

It seems very likely that the Porro prisms are identical in both versions since that's the simplest way to do it. The straight version just needs empty space between the Porro and the eyepiece field stop equal to the length that light travels through glass in the semi-pentaprism of the angled version.

How is the prism coating impaired?

 

[dipped]

It seems very likely that the Porro prisms are identical in both versions since that's the simplest way to do it. The straight version just needs empty space between the Porro and the eyepiece field stop equal to the length that light travels through glass in the semi-pentaprism of the angled version.

How is the prism coating impaired?

Thanks for that Henry.
Before the second break the prisms seemed misty. Maybe because the nitrogen escaped. The top one nearest the eyepiece was "cleaned", I thought carefully after the second break but it seems to have made things worse. The prism nearest the objective, the focusing one seems to have retained its coating as far as I can tell.
The prisms seemed clean and clear but on reassembly you can see misty patches.

 

[Binastro]

Kingsview Optical Ltd. Hythe can probably recoat or replace the prism.

So could Kowa.

Regards,
B.

 

[henry link]

Misting on the prisms could be off-gassing from lubricant or maybe the glue used for the repair, possibly on the uncoated light reflecting surfaces rather than the coated light transmitting surfaces.

 

[dipped]

Kingsview Optical Ltd. Hythe can probably recoat or replace the prism.

So could Kowa.

Regards,
B.

Thanks, that's interesting. I'm waiting to hear back from the official UK Kowa repairer.

Misting on the prisms could be off-gassing from lubricant or maybe the glue used for the repair, possibly on the uncoated light reflecting surfaces rather than the coated light transmitting surfaces.

I think you hit the nail on the head there. I think I used Araldite epoxy internally and re-sealed the scope up prematurely on the original break. I'm pretty sure the prisms were ok prior to the first repair.
I wonder how much new prisms might cost or whether I should take a punt on a cheap used 821/822.

 

[Binastro]

Telephone Kingsview and ask or perhaps a repairer has one in stock.

Perhaps the prism can be repaired.

Regards,
B.

 

[dipped]

I've heard back from the Kowa repairers and they have heard back from Kowa Germany who say the parts from the Kowa 82sv don't fit the 824.

Kingsview said they don't really do the repair I want.

A BF member may be able to help me though so that sounds promising.

An ebay search has proved fruitless apart from complete scopes.

 

[jring]

Hi,

I am still trying to understand how the the prism coatings were damaged by cleaning them... MgF2 based multicoatings are notoriously inert to chemicals and also not really easy to damage mechanically (both unlike lotus effect based anti rain coatings on objective lenses).

If we assume something inside the glued back together body is gassing out and causing the prisms to mist up again, cleaning or replacing the prisms is not really going to help unless the root cause is identified and eliminated before...

Maybe some images of the misted prisms might help... or just try different solvents, a soft cloth or brush, rubber gloves, eye protection and possibly a gas mask... all of the above preferably outside...

Joachim, who didn't find a cheap 820 series donor on ebay or german classifieds sites...

 

[dipped]

Joachim, I am indebted to you. The prisms have indeed cleaned up albeit with a lot of elbow grease. I am so grateful. I took pictures of them before cleaning but there's no need to post them now.

Many thanks to Henry and Binastro too for their advice.

I think the araldite may have caused the original misting. I used JB weld this time.

 

[jring]

Hi dipped,

great to hear this!

Enjoy the view, which is probably as spectacular as my cherry TSN-3...

Joachim, now back on the balcony to peep into a mute swan nest across the river...

 

[dipped]

Would silicone grease be better than lithium for the focusing thread?

 

[jring]

Hi,

Ideally, I would look for a hydrocarbon based vacuum grease for the expected temperature range. Vacuum due to their very low vapor pressures which leads to no or very little gassing out and thus no or very little residue.

Silicone grease tends to creep where it is not wanted... like optical surfaces...

But since price information for dedicated vacuum greases tends to require a stable chair, just compare vapor pressures for "normal" greases in the desired operating temperature range and take sth on the lower end... after all we don't really operate in a vacuum.

Joachim