47 replies to this topic

[Scott in NC]

A fellow CN'er brought me this Takahashi FS-60C to test this evening. It's an 80th Anniversary model, produced in 2012. The 80th Anniversary models were made with blue and silver colored OTAs, and I don't believe that many were produced. I have a Q version (i.e., an FS-60 with 1.7x Q module) in blue, and this is the first one that I've seen in silver. As with the other FS-60s, this is a fast f/5.9 fluorite doublet with 355mm focal length. Outwardly, this FS-60C appears to be in very nice cosmetic shape.  Unfortunately, despite the lens coatings also being in very nice shape, there's significant fungal growth between the two elements of the objective. I've not had a chance to view through this scope, but I was very interested to see if the fungal growth had produced any detrimental effects upon the scope's spherical or chromatic correction. I suspected not, but you never know...

 

As always, first I'll present the scope set up on the optical bench, ready for testing.

 

[Scott in NC]

And on to the test results...

 

Green inside / outside focus:

 

 

 

Red inside / outside focus:

 

 

 

Blue inside / outside focus:

 

 

 

White inside / at / outside focus:

 

[Scott in NC]

Spherical aberration looks to be very well controlled, and is quite minimal in both green and red, and not very far off in blue. Chromatic aberration is also quite well controlled for a fast doublet, and the low dispersion characteristic of fluorite in combination with a well matched mating element appears to be quite effective here. 

 

On the Ronchigram images, ignore the streak at the 9:00 position, as that's an artifact from my optical flat, and also ignore the bright spot in the center, as that's a reflection from the LED light used for the test. In the "at focus" white light image you can see some smearing from the fungal hyphae, but I can't see any other detrimental effects.

[Scott in NC]

Here's the scope set up on a UA aluminum tripod and Stellarvue M1 alt-az mount. It's quite attractive, and I really like the Tak 80th Anniversary model colors (silver and blue) much better than the standard white. 

 

[Scott in NC]

Now for the ugliest shot, but the flash photography makes the fungal contamination look a lot worse than it really looks to the eye. Nevertheless, it's definitely there.

 

[Scott in NC]

This shot represents a closer approximation of what the lens looks like to the naked eye.

 

[Scott in NC]

It's getting late, so I need to sign off for now. But tomorrow I plan to do some reading to see what non-invasive steps I can take to remedy this scope's condition. It's not my scope, so I have no intention of trying to disassemble the objective lens, but I may try UV light when the sun finally comes out again. We had nice weather during yesterday's solar eclipse, but nothing but clouds and rain all day today.

[Lensman]

Nice wavefront. I would think the fungus will cause some scatter at least. Since it doesn't appear to be crinkling the wavefront, like it hasn't eaten into the coatings, it may just clean off...

[RichA]

Nice wavefront. I would think the fungus will cause some scatter at least. Since it doesn't appear to be crinkling the wavefront, like it hasn't eaten into the coatings, it may just clean off...

Just have to determine which element it's on an to use liquid on the fluorite for as short as time as possible.  Yes, the fluorite is coated, but coatings are sometimes slightly permeable. 

[Thomas_M44]

Very interesting.

 

Hopefully there is a game plan being explored to properly eradicate the mold.

 

Unfortunately, any further moist air contact, and the fungus will only continue to spread/proliferate.

[davidc135]

As the outer lens is fluorite and passes uv light the hyphae should be killed by exposure to sunlight or a lamp. But does too long in the hot sun pose a risk?

However, someone should undertake the probably simple job of disassembling and cleaning the doublet properly. Fluorite is more immune to water than, without a coating, it is to wear.

 

David

Edited by davidc135, 10 April 2024 - 05:46 PM.

[Lagrange]

As the outer lens is fluorite and passes uv light the hyphae should be killed by exposure to sunlight or a lamp. But does too long in the hot sun pose a risk?

However, someone should undertake the probably simple job of disassembling and cleaning the doublet properly. Fluorite is more immune to water than, without a coating, it is to wear.

 

David

Fluorite is pretty insensitive to heat. The main thing is avoiding rapid temperature changes that could cause thermal shock.

[Scott in NC]

Great news! With the approval of (and assistance from, as it's always nice to have a second pair of hands when undertaking a delicate project) the scope's owner, I was able to clean the optics and eradicate all traces of the mold.  Fortunately it appeared to have only had a cosmetic effect upon the coatings and must not have been present long enough to permanently damage the coatings, or worse eat through the coatings and etch the glass.

[CHASLX200]

Looks good and 300x easy with them optics.

[Scott in NC]

The first step was to purchase a spanner wrench off Amazon, and for the princely sum of $14 I found this and it arrived 24 hours after ordering it.

 

[SandyHouTex]

It's nice to know I bought a great scope.

[Scott in NC]

Next, I carefully removed the retaining ring at the back of the lens cell and removed the two elements. After removing the elements I made a couple of pencil marks ^ on the lens edges so that I'd know how they went together and in what orientation. Surprisingly there were no spacers between the two elements, so I'm guessing that the very small air gap is just maintained by the difference in curvature on the inside surfaces of the two elements.

 

 

 

[Scott in NC]

Next I carefully cleaned the inside of the lens elements where the mold was present, first using my usual cleaning technique of 91% isopropyl alcohol, Q-tips, and white unscented Kleenex. Then I took the extra step of going over the previously mold-infested areas with a Q-tip and hydrogen peroxide in the hopes of eradicating any residual fungal spores. I used an air blower bulb before and after cleaning to remove as much lint as possible. The lens looked nearly pristine when I was finished.

 

 

 

[Scott in NC]

It's nice to know I bought a great scope.

Sandy, what version of the FS-60 do you have?

[Scott in NC]

The scope was then reassembled, and here are some of the “before” and "after" photos. Some cleaning smudges show up on one of the “after” photos, but they're only visible when using a camera flash and photographing the objective with the black interior of the scope as the background. They are not residual fungus that wouldn’t clean off. They weren't visible without use of the camera flash, nor when the lens was held up to a bright light prior to installation back in the scope.

 

And apparently we introduced a tiny piece of lint in between the two elements when reassembling, but I'm not taking the scope apart again to try to blow it out. I can pretty much guarantee that if I were able to get that speck of lint out then two or three more pieces would take its place.

 

Before:

 

After:

​

 

 

Edited by Scott in NC, 14 April 2024 - 07:22 AM.

[Scott in NC]

And finally, just to assure myself that I reassembled the lens elements correctly in the lens cell, I redid the DPAC testing to make sure that I hadn't introduced any excess spherical aberration (which would have been expected if the lens orientation or spacing wasn't correct):

 

Green inside / outside focus:

 

 

Red inside / outside:

 

 

Blue inside / outside:

 

 

White inside / at / outside:

 

 

Perfection! I'm always thrilled when everything comes together and a task works the way I expect it to.

[ken30809]

Nice job Scott!

[davidc135]

A possibility with there being no spacers is that lint etc in the wrong place causes some wedge in the air space but the objective tests fine. Newton's rings would confirm. Maybe lint or dust is too fine to make any difference.

 

David

[chris charen]

Everytime I would look at the lens, that tiny piece of lint would annoy me ++. I would disassemble and clean again but that is my OCD talking.

 

Chris

[Scott in NC]

Everytime I would look at the lens, that tiny piece of lint would annoy me ++. I would disassemble and clean again but that is my OCD talking.

 

Chris

It’s really not as noticeable as the photograph makes it look. But you’re right. If the scope were mine then I’d consider doing just that someday. But not today, or not anytime soon. Fortunately for me, it’s not my scope so I don’t have to worry about it!