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SK5 blanks, what to make?

Started by dan_h , Jan 09 2013 09:18 AM
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20 replies to this topic

#1 dan_h

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Posted 09 January 2013 - 09:18 AM

I found a couple of SK5 lens blanks that I had bought and forgot I had. Plano concave, about 110mm dia, 18mm thick. Other than the fact that they are a heavy crown glass with a higher index of refraction, they don't seem to be anything special. I am thinking about using them for grinding tools as I don't have a lot of money tied up here.

Anybody out there who can suggest a better use for this glass? Thanks for any ideas.


dan

#2 Norm Meyer

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Posted 09 January 2013 - 10:58 AM

I don't know if they are suitable for a "Houghton" but it
may be worth checking it out. I came across a bunch of BSC2
crown glass a few years ago and have plans for a 6" Lurie-Houghton f5 in the near future.

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#3 ed_turco

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Posted 09 January 2013 - 12:59 PM

Please don't use these for tools. These are
optical glass, and the way prices are now, they are worth an awful lot of money.

#4 MKV

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Posted 09 January 2013 - 01:35 PM

I found a couple of SK5 lens blanks that I had bought and forgot I had. Plano concave, about 110mm dia, 18mm thick. Other than the fact that they are a heavy crown glass with a higher index of refraction, they don't seem to be anything special. I am thinking about using them for grinding tools as I don't have a lot of money tied up here.

Anybody out there who can suggest a better use for this glass? Thanks for any ideas.

Dan, sell them on eBay or CN classifieds and buy yourself something you can use, but don't use them as tools - Ed's absolutely right.

Mladen

#5 MKV

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Posted 09 January 2013 - 01:42 PM

I don't know if they are suitable for a "Houghton" but it may be worth checking it out. I came across a bunch of BSC2 crown glass a few years ago and have plans for a 6" Lurie-Houghton f5 in the near future.

Norm, Dan't blanks are only 4.3" in diameter - making a 4.3" Houghton is not really an option..

As for you, why an f/5? Why not sell the crown blanks and buy yourself a Paracorr2?

Mladen

#6 dan_h

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Posted 10 January 2013 - 09:06 AM

Thanks or the comments.

I had at one time considered a small Houghton but this stuff would make a rather heavy little scope.

I never considered selling them as blanks since I got them dirt cheap and they are a little small. Maybe I'll trade for a glass I would be able to utilize better.

dan

#7 bremms

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Posted 10 January 2013 - 07:07 PM

I have a 130mm dia piece of very dense glass. Don't know the type of glass, it was for a DOD optic. It's a coated very thick positive meniscus. It doesn't feel or look like a standard flint or crown. Very dense, feels like about 4-5 gm/cm3. Like to find out the type of glass and get it to someone that can use the glass. It was sold to me as an "Achromat" and it has what looks like a seam, but it's not. it's a groove ground in the edge.
If anyone is interested I'll try and get a spherometer together and measure the lens. It's in a protective plastic container. I'm guessing its an exotic type of glass.

#8 Crayfordjon

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Posted 11 January 2013 - 04:05 AM

Make a mirror, dont waste such nice glass for tools.

#9 ed_turco

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Posted 11 January 2013 - 11:57 AM

Those SK5 blanks -- I looked SK5 up in a glass catalog and they are not ordinary crown, but Special Crown, and not in the ordinary run of crown glasses. I'd advertize these and some sharp cookie will be glad to have them. I think that a very fair price for each of them, but others may share their thoughts.

And the sharp cookies are all over the place in this forum!

#10 dan_h

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Posted 14 January 2013 - 09:40 AM

Those SK5 blanks -- I looked SK5 up in a glass catalog and they are not ordinary crown, but Special Crown, and not in the ordinary run of crown glasses. I'd advertize these and some sharp cookie will be glad to have them. I think that a very fair price for each of them, but others may share their thoughts.

And the sharp cookies are all over the place in this forum!


Thanks for the comments Ed. I will have to rethink what I can do. Still, I'd love to hear some suggestions on what can be done beyond a normal achromat.

dan

#11 Mike I. Jones

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Posted 14 January 2013 - 10:50 PM

Well, you could pop for a blank of Ohara S-FPL53 and make this apochromatic lens. 100mm aperture, 1000mm EFL, corrected over the 0.4358-0.6563 µm spectrum and 20mm diameter FOV.

Attached Thumbnails

  • 5623595-100 x 1000mm S-FPL53 SK5 apo data sheet.jpg

#12 Mike I. Jones

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Posted 14 January 2013 - 10:51 PM

Here's the OSLO file.
Mike

Attached Files


#13 MKV

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Posted 15 January 2013 - 05:34 AM

Well, you could pop for a blank of Ohara S-FPL53 and make this apochromatic lens. 100mm aperture, 1000mm EFL, corrected over the 0.4358-0.6563 µm spectrum and 20mm diameter FOV.

Nice, Mike, really, beautiful.

For those interested, OSLO lists S-FPL53 as being 15.5 time more expensive than BK7. Being conservative and assuming a 4-inch BK7 would cost you about $100 the S-FPL53 disk would cost "only" $1,550. On the other hand, for "merely" $445 more you can get Meade 115 mm apo OTA, eyepieces, rings, star diagonal, dew shield, and a bunch of other accessories, and all you have to do is plop it on a mount and start observing.

Then, you can find Orion 100 mm ED apochormatc on eBay, or on Amazon for $$769.00 for a new and $499.00 for a used one!

If you sell the SK5 blanks, that would pretty much cover the cost of a complete 100 mm apo OTA (SK5 glass is 2.5 times the cost of BK7).

But there is something to making your own lenses which keeps all of us coming back for more. :)

Mladen

#14 Mike I. Jones

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Posted 15 January 2013 - 06:21 AM

Your estimate on the BK7 cost might be a little high. I was thinking more like about $600-$700 for a 110x16mm S-FPL53 blank. Should get a real quote from Naked Optics or Newport on it before scaring the poor guy with sticker shock.
Mike

#15 MKV

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Posted 15 January 2013 - 07:14 AM

Your estimate on the BK7 cost might be a little high. I was thinking more like about $600-$700 for a 110x16mm S-FPL53 blank. Should get a real quote from Naked Optics or Newport on it before scaring the poor guy with sticker.

Mike, I was merely quoting OSLO's relative price estimate (because it's quick and I didn't relaize the price difference was so huge), and what 100 mm apo OTAs go for. It's good to know there are cheaper options. Never heard of Naked Optics. I don't know about Newport. I was trying to get a quote over the phone and they told me to send an e-mail! But I never got a reply form them. It's like pulling teeth.

Mladen

#16 ed_turco

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Posted 15 January 2013 - 02:23 PM

Always better to build your own; you get a lot smarter that way and even impress experts on here.

And then, this telescope is yours! Not somebody else's product.

#17 dan_h

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Posted 16 January 2013 - 09:44 PM

Mike,

Thanks for taking the time to post the design. It took me a day or two to think about this but I don't see me risking $500+ on a block of glass with my skill set.

At first when I looked at your post, I thought that the SK5 was the positive element. Now I realize it is not and the design is a much better fit to the blanks I have. Those are really nice spots and the curves don't look too unmanageable. I need to ponder this but the FPL 53 is outta my league right now.

dan

#18 ed_turco

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Posted 16 January 2013 - 09:47 PM

But Dan,

Someday, someday . . . :)

#19 ccaissie

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Posted 17 January 2013 - 11:55 PM

I no longer have, and maybe Schott no longer issues, the great partial dispersion charts that would give a guideline as to which common glass might give better color correction than the old BK7/F2 combo.

I found through these charts that there are some nice combos like Kzfsn4 /SSk2, BK7 / Kzfs2. Someone with a better handle on data could find a mate for the SK5 without just saying "Fluorite..cough up the bucks"?

Attached Files


#20 ccaissie

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Posted 18 January 2013 - 12:16 AM

Just shoved sk5 into a Kzfsn4 combo (ATTACHED)and it is much better than the standard doublet combo. Not optimized, as in the OSLO attachment in the previous post. Some combo with SK5 must be worth pursuing...

Attached Files


#21 dan_h

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Posted 18 January 2013 - 06:43 AM

Just shoved sk5 into a Kzfsn4 combo (ATTACHED)and it is much better than the standard doublet combo. Not optimized, as in the OSLO attachment in the previous post. Some combo with SK5 must be worth pursuing...


This was my original thinking, that there must be something this glass can be used for. However, its dispersion characteristics are really nothing special and it does not fall off of the line for normal glasses. It seems this glass works in some designs because it has a higher than usual index of refraction which provides for less curved surfaces than BK7 but beyond that, I don't have the knowledge/intuition to envision anything else. Hence I asked the question.

dan
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