Testing a refurbished 1985 Takahashi FC-100

[davidc135]

As a result of storage in damp conditions the flint lens which was matched with fluorite had become etched and cloudy. A decision was made to regrind, polish and figure the affected second surface. The front surface hadn't been noticeably affected whilst the rear fluorite lens was pristine.

The finished objective was Ronchi tested using auto-collimation and a 12'' professionally made flat.

Hover over the images for inside/outside focus info. EDIT The hovering doesn't work but inside or outside of focus in order they are: out, out, out, in, in, out, in and in.

As the test is double pass, actual errors are doubled. The edge is turned down somewhat but otherwise the correction is pretty good I think. Spherochromatism is modest with slight under-correction in red and over-correction in blue as you'd expect.

I'll be able to test it out on the heavens when some M8 mounting screws arrive in a few days.

David

Edited November 6, 2023 by davidc135

[rrb24]

Fantastic results, congratulations!

Roberto

[Adam J]

On 06/11/2023 at 09:24, davidc135 said:

As a result of storage in damp conditions the flint lens which was matched with fluorite had become etched and cloudy. A decision was made to regrind, polish and figure the affected second surface. The front surface hadn't been noticeably affected whilst the rear fluorite lens was pristine.

The finished objective was Ronchi tested using auto-collimation and a 12'' professionally made flat.

Hover over the images for inside/outside focus info. EDIT The hovering doesn't work but inside or outside of focus in order they are: out, out, out, in, in, out, in and in.

As the test is double pass, actual errors are doubled. The edge is turned down somewhat but otherwise the correction is pretty good I think. Spherochromatism is modest with slight under-correction in red and over-correction in blue as you'd expect.

I'll be able to test it out on the heavens when some M8 mounting screws arrive in a few days.

David

very nice, have been wanting to get into this for a while who did you get the flat from?

[John]

Interesting that the one surface that was damaged by the damp was on the flint element. There have been rumours for years that the fluorite element was the more fragile.

Looks like the work in the damaged surface has been successful 👍

[davidc135]

On 07/11/2023 at 21:02, Adam J said:

very nice, have been wanting to get into this for a while who did you get the flat from?

I bought this flat a couple of years ago off Astrobuysell. Oddly, 2 12 inch flats came up in the same week priced at around £1000 each. I offered half that to a local seller who accepted.

The flat was said by the seller to have been made by British Aerospace in the 80s and used by her and her husband in the making of the 13'' triplet and other objectives for their observatory and astronomical centre near Knighton, Powys.

I think decent ex Soviet flats up to 5 or 6'' can be had reasonably off ebay.

David

Edited November 8, 2023 by davidc135

[davidc135]

23 hours ago, John said:

Interesting that the one surface that was damaged by the damp was on the flint element. There have been rumours for years that the fluorite element was the more fragile.

Looks like the work in the damaged surface has been successful 👍

I believe the fluorite is very corrosion resistant but relatively soft and so, in these non AR coated objectives a Steinheil design was used placing the fluorite safely at the rear.

It seems odd that the visible damage was confined to R2 and not noticeable at the front surface. It may be that as the fluorite is slightly hygroscopic condensed moisture tended to stay put longer between the two lenses. It may also be true that the minute quantity of fluorite ions in the condensate acted as catalysts to speed up corrosion, but that's just my guess.

When Takahashi discovered its weakness a few years down the line this flint glass was replaced by a more resistant variety.

It could also be the case that the front surface has been affected slightly as the knife edge test shows some roughness but hopefully not enough to affect images. We'll see.

David

Edited November 8, 2023 by davidc135