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Go Back   IceInSpace > Equipment > ATM and DIY Projects

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  #1  
Old 01-12-2016, 10:09 AM
Stefan Buda
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Making a Busack-Riccardi-Honders Astrograph

One of my current ATM projects is the making of a small RH astrograph.
I was wondering that, if there is enough interest, I could make my progress public on this forum. It would allow people to see what goes into making a precision instrument and why they cost so much.
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Last edited by Stefan Buda; 01-12-2016 at 10:24 AM.
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  #2  
Old 01-12-2016, 10:39 AM
Kunama (Matt)
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I doubt I would ever build one but it would be great if you could post a progressive build thread !!!! and Lots of Pictures !!!! My favourite part of the forum is the ATM/DIY section..... (Yes, I know I also frequent the classifieds Col !!!)
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  #3  
Old 01-12-2016, 11:27 AM
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ZeroID (Brent)
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Me too, we can all drool together ..
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  #4  
Old 01-12-2016, 02:09 PM
Rod
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That would be fascinating Stefan. Please keep us up to date on your project.

Rod.
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  #5  
Old 01-12-2016, 02:12 PM
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Shiraz (Ray)
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yes please - we would all benefit from hearing what is involved.

sounds exciting.
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  #6  
Old 01-12-2016, 04:08 PM
Stefan Buda
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Thanks guys,
Four followers is more than enough for me :-)
Ok, but let me first explain what the Korean War has to do with it.
Back in the early 50s the Australian government purchased a stockpile of optical glass for military purposes. Some 20 odd years later they concluded that WW3 was not going to happen for a while, or if it did, it would not last long enough to make any optics out of it and so they sent the glass to land fill. Fortunately some people managed to save some (unknown proportion) of it.
Some of this glass started circulating through various markets and by various individuals and a large porion of it was stored in ammo boxes on a farm near Warrnambool. I started collecting optical glass back in the 80s and came to poses about 40kg of various size slabs and discs, mostly purchased for around $5 per Kg.
One of my ambitions is to turn as much of my glass into optics as I can and this is where the Korean War connects with my BRH astrograph project.
As a first step I selected the largest slabs of suitable glass from my collection and that is what determined the size of the clear apperture of the instrument, which will be 150mm.
Busach's prescrption calls for all elements to be made from the same type of borosilicate crown, but my largest suitable slabs were of a Schott glass called PSK3 and after much raytracing I decided to use BK7 for the small corrector lenses - mainly because I had a couple of old laser mirrors with Ohara BK7 substrates.
There is a neat raytracer on Busack's website called PointSpread, http://www.busack-medial.de/download.htm that allows you to play with your own version, although I used Zemax to optimize mine.
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Last edited by Stefan Buda; 01-12-2016 at 08:44 PM.
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  #7  
Old 01-12-2016, 04:54 PM
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Satchmo (Mark)
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HI Stefan

I'm afraid without the provenance of the 'melt data' on the glass, giving a designer the figures of the actual measured dispersion and refractive index of the individual pieces , this glass is really just `land fill ' .

If you were going to actually manufacture such an instrument you would just buy new glass with all the required melt data - BK7 is not very expensive compared to the value of such a finished instrument . I'm not trying to be picky - its just one of those cold hard realities of using optical glass. You may be able to fudge the result by sticking to the design and aspherising one surface via null test in the end result ?

I've thought about these all spherical designs , but to be honest polishing and figuring 8 optical surfaces just doesn't seem to make any sense compared to one paraboloid and an off the shelf Wynne corrector ( at F3 ) . Maybe I have missed something !
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  #8  
Old 01-12-2016, 07:12 PM
Stefan Buda
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Quote:
Originally Posted by Satchmo View Post
HI Stefan

I'm afraid without the provenance of the 'melt data' on the glass, giving a designer the figures of the actual measured dispersion and refractive index of the individual pieces , this glass is really just `land fill ' .

If you were going to actually manufacture such an instrument you would just buy new glass with all the required melt data - BK7 is not very expensive compared to the value of such a finished instrument . I'm not trying to be picky - its just one of those cold hard realities of using optical glass. You may be able to fudge the result by sticking to the design and aspherising one surface via null test in the end result ?

I've thought about these all spherical designs , but to be honest polishing and figuring 8 optical surfaces just doesn't seem to make any sense compared to one paraboloid and an off the shelf Wynne corrector ( at F3 ) . Maybe I have missed something !
Thanks Mark for the comments but I don't think the accuracy of the glass dispersion data for this instrument is as critical as it would be for an apochromat, for instance, where all the power comes from refraction. Here it is important that the two main lenses come from the same melt and I don't think Schott or Ohara glasses deviate too much from their intended specs. Anyway, a few years ago I made a small Companar using "landfill glass" and it works very well. The worse that can happen is that I will be required to make a couple of precision prisms from the offcuts and measure the glass myself and then adjust the curvatures accordingly. And yes, I will null test it but I don't know at this stage how useful such a test will be as the design is not meant to be diffraction limited.
Please don't try to understand why I'm making it because probably has something to do with insanity.
Regarding the corrected Newtonian: I count 8 optical surfaces and at least one of them is not an easy one. Besides, it is such a 20th century thing.

Last edited by Stefan Buda; 01-12-2016 at 08:46 PM.
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  #9  
Old 01-12-2016, 07:44 PM
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Quote:
Originally Posted by Stefan Buda View Post
a few years ago I made a small Companar using "landfill glass" and it works very well.
You made a companar?

Dude,
respect!
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  #10  
Old 01-12-2016, 08:34 PM
Stefan Buda
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Thanks Clive.
I believe that it may be the only one ever made or at least the only successful one made by an amateur, as I have never seen any proof for the existence of other ones.
http://www.parkes.atnf.csiro.au/news...iches_Head.jpg

And of course, it should make the BRH project feel like a walk in the park. :-)

Last edited by Stefan Buda; 01-12-2016 at 08:50 PM.
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  #11  
Old 01-12-2016, 08:52 PM
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Stefan.. I always loved that design on an intellectual level. practicality aside, so much win!

That may be unfair,
it is a practicable design even with common glass. Coatings probably being the biggest obstacle.

Love your work.

~c
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  #12  
Old 01-12-2016, 09:20 PM
Wavytone (Nick)
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Clive if you liked that on an intellectual level, try this

https://www.google.com/patents/US3926505

Esp. Figure 3. Now if Stefan can make a 150mm BRH, maybe one of these could be the next challenge.

Or a Baker-Nunn...
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  #13  
Old 01-12-2016, 09:33 PM
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You would think at some point the designer would say... yeah... nahh.. a bit much glass there.
But yes... brilliant none the less.
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  #14  
Old 02-12-2016, 11:05 AM
Stefan Buda
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Quote:
Originally Posted by Wavytone View Post
Clive if you liked that on an intellectual level, try this

https://www.google.com/patents/US3926505

Esp. Figure 3. Now if Stefan can make a 150mm BRH, maybe one of these could be the next challenge.

Or a Baker-Nunn...
Thanks for the link. I bookmarked it and will have a closer look when time permits.
I've been thinking about making a small solid Schmidt of about f/1 for years but other projects keep getting in the way.
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  #15  
Old 02-12-2016, 11:21 AM
Stefan Buda
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So back to the BRH.
In fact I started working on it early last year and then had the project on hold for about a year.
I started by slicing off the corners of the PSK slabs and two other slabs of optical glass, one a barium flint and the other a dense barium crown, to be used as grinding tools. I already had two glass disks that were suitable for making up the 4 grinding discs needed for the PSK lenses. The cutting was done on my milling machine and the mess was confined with the use of an ice cream container. The diamond plated tool used was one that I made many years ago and it still cuts well.
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  #16  
Old 02-12-2016, 12:33 PM
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Stefan... have you ever considered moving to a cooler climate?
https://www.kiwistaroptics.com/careers

Incidentally... you might enjoy some of the designs, here:
http://www.cloudynights.com/topic/48...ew-astrograph/

I like the idea of the prescription detailed in post#14 (uses a trepanned C11 primary) 12" clear aperture @ f2.5 and 6 microns in the extreme corners of a 16803 format chip... lots of complimentary radii and only 1 full aperture convex surface. I'd be curious to know what BK7 costs in blanks that size?
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  #17  
Old 02-12-2016, 12:59 PM
bratislav (Bratislav)
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Quote:
Originally Posted by Stefan Buda View Post
Thanks Clive.
I believe that it may be the only one ever made or at least the only successful one made by an amateur, as I have never seen any proof for the existence of other ones.
Harrie Rutten told me that he is aware of several Companars successfully built, the largest one being 12 inches. It was also commercially offered (at least at some stage) by Oldham Optical in UK. I don't know how many of those mentioned by Harrie were amateur creations though.

Bratislav

PS most of that old "landfill" glass was made by Chance Pilkington and on larger pieces it has melt data numbers scribed on the glass
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  #18  
Old 02-12-2016, 02:10 PM
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Originally Posted by bratislav View Post
Harrie Rutten told me that he is aware of several Companars successfully built, the largest one being 12 inches. It was also commercially offered (at least at some stage) by Oldham Optical in UK. I don't know how many of those mentioned by Harrie were amateur creations though.

Bratislav

PS most of that old "landfill" glass was made by Chance Pilkington and on larger pieces it has melt data numbers scribed on the glass
And as Stefan has intimated with respect to the huff and puff about melt data... it really isn't that important on a two element, zero power corrector (of the same glass). It has very little bearing on over all system performance other than to shift the wavelength at which spherical aberration is at a minima.

It's a bit like agonising over the spherochromatism of a Schmidt plate.... you don't need anything more exotic than white water plate (as long as it is homogeneous) and have a rough idea of the refractive index.

~2c
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  #19  
Old 03-12-2016, 08:19 AM
Stefan Buda
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Quote:
Originally Posted by bratislav View Post
Harrie Rutten told me that he is aware of several Companars successfully built, the largest one being 12 inches. It was also commercially offered (at least at some stage) by Oldham Optical in UK. I don't know how many of those mentioned by Harrie were amateur creations though.

Bratislav

PS most of that old "landfill" glass was made by Chance Pilkington and on larger pieces it has melt data numbers scribed on the glass
Bratislav,
It would be nice to actually see some proof as I don't think that Harrie has actually seen any of those instruments. Why would professional astronomers waste money on such a large and expensive instrument?
Remember that the Companar is huge relative to its focal length. And if those were amateur instruments then we should have seen some results.

Anyway, back to the topic:
In the late 90s I cobbled together a curve generating machine for my 16" Dall-Kirkham project and I've been using it ever since for making all sorts of lenses and mirrors. It is this machine that I used for shaping the glass for the BRH.
Here you can see the shaping of one of the 6" grinding tools.
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  #20  
Old 03-12-2016, 08:27 AM
Stefan Buda
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Originally Posted by clive milne View Post
Stefan... have you ever considered moving to a cooler climate?
https://www.kiwistaroptics.com/careers

Incidentally... you might enjoy some of the designs, here:
http://www.cloudynights.com/topic/48...ew-astrograph/

I like the idea of the prescription detailed in post#14 (uses a trepanned C11 primary) 12" clear aperture @ f2.5 and 6 microns in the extreme corners of a 16803 format chip... lots of complimentary radii and only 1 full aperture convex surface. I'd be curious to know what BK7 costs in blanks that size?
Clive,
Even Melbourne too cold for me..
Yes I have seen that CN thread. I prefer the ultra compact formula of the BRH. Especially from a commercial/manufacturing point of view.
Let me know if you find out the cost of BK7 blanks.
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