22 replies to this topic

[Scott in NC]

A fellow CN'er brought me this Takahashi FC-50 to test yesterday. Based upon the "battleship gray" paint scheme, I estimate that this scope was produced sometime in the 1980s to early 1990s. The serial number appears to be from before the time period when the last two digits signified the year of manufacture.  This refractor is an f/8 fluorite doublet, and sadly, it doesn't appear to have been well cared for over the years. The external cosmetics show some wear, but are still in the realm of what I consider acceptable for a 30-40 year old scope. The optics however, as you'll see, are atrocious. They've been affected by the fogging issue that has been reported before with some of these older FC-series objectives. One of the spacers has some gunk around it, which leads me to believe that a prior owner may have disassembled the optics in order to try to clean the lenses, and then possibly attempted to glue that spacer back in place.

 

So here's the scope set up on the optical bench for testing.

 

[Scott in NC]

Ronchigrams:

 

[kasprowy]

For someone (me) with no understanding of this, how do you decipher it? I notice the lines are straight which I assume is good. But the fogging is the graininess? Thx

[RichA]

Ronchigrams:

 

FC-50.png

Good job.  Bands look good, CA control looks fantastic.  Some irregularities in the surface, but I presume that's the flat? Or not. 

[Scott in NC]

Interestingly, this scope appears to have had quite good control of spherical aberration prior to succumbing to mishandling.  Even blue appears to be just mildly overcorrected, with best correction somewhere between green and red. The dark "notch" at the 1:00 position is due to opacification of the lens in the region of a spacer that to me looks like it may have been glued into place. If you enlarge the images, you'll see a multitude of streaks and haziness all over the lens.

 

I wish I had a way to star test this scope, but it didn't come with a proper visual back at the end of the focuser, and I don't have the right adapters to allow attachment of a diagonal or eyepiece. It's probably just as well, I guess. I told the owner that if it were my scope, unless I got it for such a fantastic price that it was worth keeping just for an office decoration that I'd try to return it if I could. I don't often say this about a scope, but there you have it.  And it's really quite a shame, as this was a very nice (and expensive) scope back in its day.

 

I usually try to show off nice examples of scopes, but I figured it was time to show everyone something less than optimal. Thanks for reading!

[Scott in NC]

For someone (me) with no understanding of this, how do you decipher it? I notice the lines are straight which I assume is good. But the fogging is the graininess? Thx

Read post #5, and this should explain it better. And yes, a lot of the "scarring" that you'll see in the Ronchigram images is due to opacifications in the lens.

[AstroApe]

Poor little scope  Pretty rough lookin' surface for a small Tak. Is the center spot reflections from the light? I notice it has a horizontal bar going through it.

 

I bet it still puts up a decent image in the dark

[Scott in NC]

Good job.  Bands look good, CA control looks fantastic.  Some irregularities in the surface, but I presume that's the flat? Or not. 

Yes, I forgot to mention one good thing about this lens. There is very little chromatic aberration, as would be expected of a slow-ish (f/8) fluorite doublet. And as I mentioned above, the irregularities seen are not in the flat, but in the objective. I used my essentially perfect 6" Zygo flat.

[Scott in NC]

Is the center spot reflections from the light? I notice it has a horizontal bar going through it.

Yes, the bright center spot is just a reflection from the LED off the optical flat.

[RichA]

Yes, I forgot to mention one good thing about this lens. There is very little chromatic aberration, as would be expected of a slow-ish (f/8) fluorite doublet. And as I mentioned above, the irregularities seen are not in the flat, but in the objective. I used my essentially perfect 6" Zygo flat.

That is interesting.  Normally, you'd see a pretty uniform surface on such a small optic.  You might expect to see symmetric high or low areas, but these are small areas, scattered about, almost "micro-zones."  I can't remember ever seeing such a thing, even on a large (where you would expect inperfections) optic except some mirrors made by amateurs and not polished enough.

Edited by RichA, 28 July 2024 - 07:39 PM.

[Scott in NC]

Yes, the surface was likely very uniform to start with. These pictures don't show it as well as I'd like, but there's lots of gunk over the surface of the lenses, and likely in between the lenses too. All of that gunk is distorting the DPAC images. This has nothing to do with the original lens polishing and figuring.

 

[Tamiji Homma]

Hi Scott,

 

The particular FC-50 is probably pre-Multicoated one.  It is known to have the center fuzzy circle due to lack of coating, if I remembered it correctly.

 

My FC-50 is 1992, MC version.  It is very clean and it performs excellently. It is one of those small but performs big scope

 

I remembered that Takahashi offered refiguring/recoating service for old FC lens before.

 

Here is lens shot of mine.

 

 

 

Tammy

Edited by Tamiji Homma, 29 July 2024 - 10:47 AM.

[Scott in NC]

Yours looks beautiful, Tammy—thanks for sharing!

[RichA]

Yes, the surface was likely very uniform to start with. These pictures don't show it as well as I'd like, but there's lots of gunk over the surface of the lenses, and likely in between the lenses too. All of that gunk is distorting the DPAC images. This has nothing to do with the original lens polishing and figuring.

 

IMG_6059 resized.JPG

IMG_6060 resized.JPG

IMG_6061 resized.JPG

I wonder if Tak could clean it, or if it's permanent?

[SandyHouTex]

Ronchigrams:

 

FC-50.png

Can't get more perfect than that.

[CHASLX200]

The gray stuff was the best.

[Terra Nova]

The gray stuff was the best.

I beg to disagree. I’ll put my green F8 FC76 up to any grey one, and they solved the fogging/deterioration issue by then.

[Jeff B]

Hi Scott,

 

The particular FC-50 is probably pre-Multicoated one.  It is known to have the center fuzzy circle due to lack of coating, if I remembered it correctly.

 

My FC-50 is 1992, MC version.  It is very clean and it performs excellently. It is one of those small but performs big scope

 

I remembered that Takahashi offered refiguring/recoating service for old FC lens before.

 

Here is lens shot of mine.

 

 

 

Tammy

That's an interesting and valuable bit of history and information.

 

Thanks Tammy!

[davidc135]

Yes, the surface was likely very uniform to start with. These pictures don't show it as well as I'd like, but there's lots of gunk over the surface of the lenses, and likely in between the lenses too. All of that gunk is distorting the DPAC images. This has nothing to do with the original lens polishing and figuring.

 

IMG_6059 resized.JPG

IMG_6060 resized.JPG

IMG_6061 resized.JPG

 

If the scope belongs to someone else, I expect that you don't feel you're in a position to dismantle it and see what the problem is; whether it's gunk that could be cleaned off or corroded pitting.

There's no point in sending it off for cleaning as it's easy to do whilst I'm sure Takahashi have stopped repolishing these early objectives, if that's what's needed.

 

David

[CHASLX200]

I beg to disagree. I’ll put my green F8 FC76 up to any grey one, and they solved the fogging/deterioration issue by then.

I was talking looks.  I loved them gray mounts and OTA's.

[Weisswurst Josef]

In the 80s Takahashi used serial numbers with four digits. The first one telling the year.

So the "6" at mine stands for 1986. Took me some time to get this.

Mine is without "MC" and I like this. Performs pretty well. A fine little scope.

 

 

 

 

Edited by Weisswurst Josef, 11 November 2024 - 10:36 AM.

[Erik Bakker]

In the 80s Takahashi used serial numbers with four digits. The first one telling the year.

So the "6" at mine stands for 1986. Took me some time to get this.

Mine is without "MC" and I like this. Performs pretty well. A fine little scope.

 

 

P1170576.JPG

 

 

P1170824.JPG

Unless these older 50 or 60mm f/8 FC’s were mistreated or have another issue, these are very fine scopes. Spherical correction and color abberations are generally extremely well controlled. Think close to 100mm DL/DZ level because of their small apertures and relatively slow optics. For many years I’ve had a Vixen FL 55S f/8 from that era with absolutely stunning optics. So small, f/8 and well made fluorite optics work together create extraordinary performers. The small aperture being both their benefit and limitation.

 

It is good to remember that in these days, the little fluorites were crafted to perfection, not to a low price point.

[Bomber Bob]

I sold my 1988 FC-50, but kept its DPAC results as a reference:

 

 

FYI:  My 1975 TS-50 and 1960s Swift 838 (also 50mm F14) have similar patterns.  Outstanding small optics!