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Cover glass error vs tube length

 
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Pau
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Joined: 20 Jan 2010
Posts: 4000
Location: Valencia, Spain

PostPosted: Mon Nov 27, 2017 5:45 am    Post subject: Cover glass error vs tube length Reply with quote

With a microscope objective both cover glass and tube length mismatching with the objective design can produce spherical aberration. Both are related with the objective NA.
I'm aware that at the times of monocular straight tube microscopes some models had regulable length tubes and they can be used to compensate the cover glass error.

Does anyone know the formula(s) to compute both errors compensation?

How could it be applied to infinite corrected objectives? (changing the tube lens focus distance, I guess...)

It could be useful in situations where we need to shot through a glass or when we want to use a glass cover corrected objective without cover.
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Ichthyophthirius



Joined: 07 Mar 2013
Posts: 728

PostPosted: Mon Nov 27, 2017 10:06 am    Post subject: Reply with quote

Hi Pau,

This is the basic formula given by Michel (1957)*:

dtm = 0.4 x dd x M^2 (mm)

dtm: change in tube length required to compensate spherical aberration
dd: deviation of coverglass thickness (usually from 0.17 mm)
M: Magnification, I think

This formula doesn't use the NA for the calculations; I'm sure there is a more sophisticated formula for these calculations, this is just a rule of thumb.

Michel (and other authors) caution that this is not a good way of compensating cover glass thickness. The changes required are just too big for this to be of any practical value.

The reason for the extendable tubes was different: When using an eyepiece micrometer, you could match it to a stage micrometer with a convenient ratio by pulling the tube in or out. This saved efford at a time when calculations had to be made by hand.

Regards, Ichty

*Michel, K.: Die wissenschaftliche und angewandte Photographie. Bd. 10. Die Mikrophotographie, 1957.

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Ichthyophthirius



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PostPosted: Mon Nov 27, 2017 10:28 am    Post subject: Reply with quote

An example for a calculation taken from Henkel ( www.klaus-henkel.de/mikrofibel.pdf ):

Objektive 40:1
Coverglass deviates by 0.02 mm

12.8 = 0.4 x 0.02 x 1600 (mm)

That's 12.8 mm with a coverglass that's well within the range sold as standard (0.15 - 0.19 mm).
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Pau
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Joined: 20 Jan 2010
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Location: Valencia, Spain

PostPosted: Mon Nov 27, 2017 10:59 am    Post subject: Reply with quote

Ichthy, thank you!

Unfortunately this doesn't seem to be a practical way to deal with the problem. 10.8mm could be acceptable but...

0.4 x 0.17 x 1600 = 108.8 (mm) to compensate for no cover with a 40X corrected for 0.17.
Magnification and coverage will be too altered...and maybe other aberrations added
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