Enviro-mentality disease hits Takahashi, the new scope

On Thursday, July 9, 2015 at 3:39:45 AM UTC-4, RichA wrote:
On Monday, 6 July 2015 09:03:41 UTC-4, wrote:


There was some one who was cutting up perfectly good 18-inch f/4 mirrors in order to make three or four smaller off-axis optics sets. Stupid.

Love to know how they did it without changing the figure of what was left.

Maybe the nerd who performed that horrifying atrocity will drop by SAA and explain it to us. Maybe it's a trade secret!

I imagine that the original blank was biscuit cut to within a few mm of the front surface before polishing and figuring.

On Thursday, 9 July 2015 13:00:15 UTC+2, wrote:

I imagine that the original blank was biscuit cut to within a few mm of the front surface before polishing and figuring.

I saw water jet cutting being offered for making elliptical flats out of round blanks.
I wonder how that would affect the figure of a high quality optical surface?
Presumably it would depend on the residual strain left from annealing the glass.

I once tried to work three, full thickness, Monax, low expansion 8.75" mirror blanks into optical flats.
Two of the blanks were given to me after they had been separated with brute force after sticking together during fine grinding.
The blank with the largest chip [on the *unworked* face] would never take a proper figure.
It consistently showed a massive twist in the lines under monochromatic light.
No doubt this also affected the pitch lamp while cold pressing.
Figuring repeatedly produced highly unpredictable results on test.

You see telescope ads occasionally after a clumsy owner has tilted and chipped an achromat during cleaning.
They always claim it hasn't affect the performance because they have painted the chip matt black!
I wouldn't touch such an "offer" with bargepole after my experience with the damaged Monax flats.
Besides, the painted chip would act as a small obscuration of the clear aperture.
Probably causing diffraction effects.

Whoops! I meant to say "lap" not "lamp" in the above.

On Thursday, July 9, 2015 at 9:36:04 AM UTC-7, Chris.B wrote:

I once tried to work three, full thickness, Monax, low expansion 8.75" mirror blanks into optical flats.
Two of the blanks were given to me after they had been separated with brute force after sticking together during fine grinding.
The blank with the largest chip [on the *unworked* face] would never take a proper figure.
It consistently showed a massive twist in the lines under monochromatic light.
No doubt this also affected the pitch lamp while cold pressing.
Figuring repeatedly produced highly unpredictable results on test.

You see telescope ads occasionally after a clumsy owner has tilted and chipped an achromat during cleaning.
They always claim it hasn't affect the performance because they have painted the chip matt black!
I wouldn't touch such an "offer" with bargepole after my experience with the damaged Monax flats.
Besides, the painted chip would act as a small obscuration of the clear aperture.
Probably causing diffraction effects.

Black-painted chips don't seem to affect this telescope very much...

http://astroanecdotes.com/2015/03/26...ting-incident/

big grin

\Paul A

On Thursday, 9 July 2015 12:36:04 UTC-4, Chris.B wrote:
On Thursday, 9 July 2015 13:00:15 UTC+2, wrote:

I imagine that the original blank was biscuit cut to within a few mm of the front surface before polishing and figuring.

I saw water jet cutting being offered for making elliptical flats out of round blanks.
I wonder how that would affect the figure of a high quality optical surface?
Presumably it would depend on the residual strain left from annealing the glass.

I once tried to work three, full thickness, Monax, low expansion 8.75" mirror blanks into optical flats.
Two of the blanks were given to me after they had been separated with brute force after sticking together during fine grinding.
The blank with the largest chip [on the *unworked* face] would never take a proper figure.
It consistently showed a massive twist in the lines under monochromatic light.
No doubt this also affected the pitch lamp while cold pressing.
Figuring repeatedly produced highly unpredictable results on test.

You see telescope ads occasionally after a clumsy owner has tilted and chipped an achromat during cleaning.
They always claim it hasn't affect the performance because they have painted the chip matt black!
I wouldn't touch such an "offer" with bargepole after my experience with the damaged Monax flats.
Besides, the painted chip would act as a small obscuration of the clear aperture.
Probably causing diffraction effects.

"Nothing" ever affects the performance when someone is trying to sell something.

On Thursday, 9 July 2015 07:00:15 UTC-4, wrote:
On Thursday, July 9, 2015 at 3:39:45 AM UTC-4, RichA wrote:
On Monday, 6 July 2015 09:03:41 UTC-4, wrote:


There was some one who was cutting up perfectly good 18-inch f/4 mirrors in order to make three or four smaller off-axis optics sets. Stupid.

Love to know how they did it without changing the figure of what was left.

Maybe the nerd who performed that horrifying atrocity will drop by SAA and explain it to us. Maybe it's a trade secret!

I imagine that the original blank was biscuit cut to within a few mm of the front surface before polishing and figuring.

Rather than cutting circles out, they should have just sectioned it with a diamond blade and lots of water cooling. But it probably still wouldn't have prevented strains in the glass from being released, screwing up the surface.

On Thursday, 9 July 2015 19:19:09 UTC+2, palsing wrote:

Black-painted chips don't seem to affect this telescope very much...

http://astroanecdotes.com/2015/03/26...ting-incident/

big grin

\Paul A

A fool and his gun are soon parted. [Usually by bars, or the grave.]

Meanwhile, back at the ranch, a telescope is not just for Christmas. With suitable care, it can enjoy many new Christmases. Gun totin', telescope-murdering fools, notwithstanding.

We may even be staring, in bemused disbelief, at the all-new, bullet-proof Pope-mobile. Pininfarina-designed, two great telescope mirrors will be mounted on ridiculously over-decorated, solid gold axles as wheels for His creaking, medieval chariot. Snake oil will be pumped copiously as a low-friction lubricant. Allowing the Chariot to be pulled by endless teams of carefully-groomed [and well-oiled] choir boys in fetching leotards, as far as the eye can see.

While the gathered throngs of open-mouthed droolers will gawp back at themselves, in total awe, as they snap greatly enlarged selfies on their thrice-blessed iPhonies. [For a small charge.] All, without ever having to turn their backs on their ancient, creaking Idol. As He trundles endlessly past on his self-promotion travails. © ;ø]