Spherical glass vs parabolic acrylic

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Hello,

I am trying to buy the parts to perform a Schlieren experiment (see an example here: https://bit.ly/2mwbzkl)

It is suggested to use a Spherical Primary Telescope Mirror (glass), however when i look into getting larger than 160 mm versions, they start to get extremely expensive.

So, as an alternative, I found this parabolic mirror (acrylic) which come in various larger sizes:
https://ebay.to/2mwYKq0

Do you think that the acrylic ones could be good enough to produce some interesting Schlieren images? I am not really looking at lab grade stuff since it's too expensive (if anyone knows a good deal i would be interested), but I would rather have it able to produce "pretty" good images.

Thanks a lot!
 

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  • #2
OmCheeto
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Hello,

I am trying to buy the parts to perform a Schlieren experiment (see an example here: https://bit.ly/2mwbzkl)

It is suggested to use a Spherical Primary Telescope Mirror (glass), however when i look into getting larger than 160 mm versions, they start to get extremely expensive.

So, as an alternative, I found this parabolic mirror (acrylic) which come in various larger sizes:
https://ebay.to/2mwYKq0

Do you think that the acrylic ones could be good enough to produce some interesting Schlieren images? I am not really looking at lab grade stuff since it's too expensive (if anyone knows a good deal i would be interested), but I would rather have it able to produce "pretty" good images.

Thanks a lot!
Can't help you with your specific problem of finding a cheap "concave" lens, but you might want to experiment with a "convex" lens.
I discovered last Halloween that a vehicle's windshield produced almost the same effect, when shining into my living room.

Here's what I witnessed when I put an oil lamp in line with the light, and took an image on my living room wall:

pseudo.schlieren.photography.png


Kind of "dollar store-ish". But still, kind of fun.
 

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  • #3
ChemAir
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May want to check the focal lengths. The video/demo has a good deal longer focal length (1300mm) than the plastic version (460mm). This shouldn't be an issue from a physics/optics standpoint, but if apparatus crowding is a problem.........
 
  • #4
Tom.G
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That acrylic mirror you referenced would be a poor fit for what you want. It has spot size at its focal point of 15mm, 0.6 inches; not good for getting clear Schlieren images, the smaller the spot size the better image you will get.

Here are a couple criteria you can use to help evaluate mirrors:

Look at the f-number, that is the focal length divided by the diameter. That is a limiting factor in the resolution of a lens or mirror. Anything from about 7 or 8 upward for f-number is rather good and above 10 it doesn't matter. The f-number for the acrylic mirror is 2.8, for the telescope mirror it is 8.1.

You also must have a 'good' surface, that is smooth without ripples or un-intended changes in curvature. To see what this can do to spot size, look at the images at the bottom of http://av.jpn.support.panasonic.com/support/global/cs/dsc/knowhow/knowhow15.html
Also see the last image on the instructables.com page you referenced. That mirror would be fine for your usage, astronomy people would consider it 'not very good.'

Cheers,
Tom

p.s. The above holds for both mirrors and lenses, a concave mirror does the same thing as a convex lens. The difference is a mirror has the image in 'front' of it and the lense has the image in 'back' of it. Also large mirrors are lower cost than large lenses (only one surface to shape and polish).
 
  • #5
OmCheeto
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...
Kind of "dollar store-ish". But still, kind of fun.
It appears that I stumbled across something called "Shadowgraphy".

PRINCIPLES AND TECHNIQUES OF SCHLIEREN IMAGING SYSTEMS
Amrita Mazumdar
Columbia University, New York, NY
Originally released July 2011

1 Introduction
Exploration within the field of schlieren imaging stagnated in the 1900s, largely due to the precise nature
of constructing a Schlieren system, the exorbitant cost of creating a system large enough to study everyday
objects, and the stationary nature of the system.
Related Systems
4.1 Shadowgraphy


Shadowgraph [wiki]
Shadowgraph is an optical method that reveals non-uniformities in transparent media like air, water, or glass. It is related to, but simpler than, the schlieren and schlieren photography methods that perform a similar function.


Schlieren_photography [wiki]
If a knife edge is not used, the system is generally referred to as a shadowgraph system...

Guessing that it's the "exorbitant cost" that has kept me from studying this further.

But I find it fascinating that REALLY OLD people could do this:

Schlieren [wiki]
History
Schlieren were first observed by Robert Hooke in 1665 using a large concave lens and two candles.

And I'm scratching my head, 353 years later; "How did he do that!"
 

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