5 replies to this topic

[BYoesle]

I recently bought a pristine and highly-regarded Takahashi FC-60 f8.3 (60/500) fluorite doublet refractor:
 

 
This telescope reportedly has a Steinheil objective design with the fluorite component as the rear element. Both FC and FS series are now being made in various apertures and focal lengths, with the FS series placing the fluorite at the front.
 
I would like to consider using my FC-60 for imaging, and know it at one time had a dedicated focal reducer, but this apparently hasn't been available since the telescope's discontinuation many years ago.
 
I contacted Takahashi America for guidance as to which of the currently produced focal reducers might be best to use. They sent the now outdated system chart, and advised to just use it as a "portable visual telescope" since none of the original optical components are now available.
 
I then wrote to Takahashi Japan "Support" hoping the Takahashi optical designers might be of more help, but was contacted by "sales" and received essentially the same response, along with an admonition not to contact Takahashi Japan again for such inquires if I didn't live in Japan.
 

 
I then found an Astrobin webpage with many FC-60 images, and lo and behold, two imagers used the Takahashi C0.72X reducer for some pretty nice images with the "incompatable" FC-60. See examples here and here. This is despite its intended use for the FS series: "The RD C0.72X reducer is dedicated to Takahashi FS-60 refractors: FS-60CB, FS-60Q and old FS-60 models." Takahashi Europe (appears to be the most informative TAK website).
 
On the other hand, Takahashi Europe states "The 76D RD reducer is dedicated to Takahashi FC refractors: FC-76DC/FC-76DS, FC-100DCU/FC-100DF and FC-100DZ. It is particularly suitable for telescopes equipped with a small-format focusing system similar to that of the FS-60CB." Emphasis added.
 
I have little technical optics understanding, but it seems the two parameters that might be most important for matching a focal reducer to an objective would be the focal length and the focal ratio. The 76D RD appears to be listed by Takahashi as having a wide variety of compatibility with telescopes with various focal lengths (570 mm to 800 mm) and focal ratios from 5.6 to 8.0, and apertures from 60 to 100 mm.
 

Takahashi Japan catalog & Takahashi Europe website.
 
Both reducers are identically priced, and appear very similar mechanically with identical imaging plane distances. Least suitability based on FL in light red (but the only real imaging examples known), best in light green. Highlighted focal ratios vary from 7.5 to 8.0. Click for larger.
 
So my questions for the optics experts are:
 
1) despite the lack of imaging evidence, would the 76D RD - which is used for telescopes with longer FLs and FRs - be the better reducer for use with the FC-60?
 
2) despite being recommended for the telescope with a much shorter FL and faster FR, should I go with the known successfully used RD C0.72X?
 
Thanks for any help you can provide, as Takahashi "support" in this case was lacking.

 

ADDENDUM: There is also the apparently still available reducer/flattner for the Sky 90 which physically appears very similar to the OEM reducer/flattener for the FC-60, and has the same focal length of 500 mm as the FC-60 (f8.3), but which is f5.6, and which is close to the FS-60CB. This reducer is also stated to be usable with the FC-60C and FS-78.

Edited by BYoesle, 10 April 2025 - 09:39 AM.

[BYoesle]

UPDATE

 

The Takahashi statements that there are no compatible focal reducers for use with the legacy FC-60 didn't sound very credible.

 

I was pointed to a Japanese website that shows the FC-60 spot diagrams, and the effects of current reducers and flatteners. The 76D RD reducer appears to do a pretty good job:

 

 

These also appear to be identical to Takahashi spot diagrams.

 

If they are, I'm chagrined at the statements from Takahashi that there are no compatible focal reducer(s) available to use with the FC-60.

[Sean Cunneen]

Moving this to Refractors for a better fit/different eyes. Hopefully someone there can offer some personal experience...

 

Sean

[Traveler]

Too bad Takahashi Japan isn't helping...
Ultimately, purchasing an FS-60CB (F5.9) could be easier and maybe even cheaper (when bought used). There is also an excellent multi flattener for sale for the CB.

[BYoesle]

Thanks Traveler. But I'm going to stick with the FC-60 as it seems to be better regarded for visual use than the FS-60CB, and it will be a daytime solar outreach telescope as well.

 

 

My limited knowledge is that a focal reducer/flatteners are best designed based on the focal length of the objective, and in that case the Sky 90 (identical 500 mm FL) 0.8x reducer might be the better choice over the 76RD.

 

But then again, why would Takahashi apparently produce a spot diagram for using the FC-60 with the 76RD if it wasn't considered a usable device, and seemingly not one for the Sky 90 reducer?

 

Both would need an adapter ring, which is no longer made by Tak, but not difficult to have made. Perhaps that's the reason it's not recommended.

 

For imaging, the 76RD has a specific image plane distance. I can find no documentation or specification for the Sky 90 reducer, and would have to find it empirically, and therefore a strike against it, even though it is a little less expensive.

[Traveler]

I know the sky 90 and its reducer. It came only in focus with a camera angle adjuster:

 

https://takahashi-eu...justers?lang=en

 

Maybe your very nice fs-60 need one too. I know my fs-60cb did...