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Usaf 1951 test targetpdf

Also, regarding the front/back surface point, if the back of the mirror is surfaced, you need to get the right shape on each surface, rather than just the front.

Learn more about our IT8 Calibration. Our high-quality Targets are available at an affordable price independent from our SilverFast scanning software. You can order the IT8 targets through our Online Store.

Look at the slopes of the lines from $400\ \mathrm{nm}$ to $700\ \mathrm{nm}$ - silver varies from $80\%$ to $95\%$ reflectance, while aluminum stays between $90\%$ and $93\%$. So yes, a primarily red image will be slightly dimmer in aluminum than in silver, but all images are slightly reddened (the blues are suppressed more heavily than the reds) in silver, whereas aluminum gives a more faithful representation of the colors. (Plus primarily blue images will actually be darker in a silver mirror than an aluminum mirror.) For everyday use, it's easy to compensate for a darker image (just light up the room more), but not so easy to compensate for an image that has the wrong colors.

USAF TargetThorlabs

More information about the Resolution Target, in-depth instructions as well as additional application and background data can be found in the Detailed Documentation.

I see that silver has a higher reflectance than aluminum. So why use aluminum instead of silver? If one wants the highest quality mirrors I assume that cost is secondary, so what is the physics that I am missing here?

Telescope mirrors and other mirrors used by scientists telescopes regularly do use a silver coating. See for instance here. However, aluminum coating are the norm (certainly for the large primary mirrors deployed in telescopes) because of durability reasons. I quote from the text linked to above:

The Resolution Target can also be used to make statements about the sharpness at different resolutions, in order to find the optimum resolution for scanning and to differentiate the sharpness at different points in the scanned area of a flatbed scanner.

"Like the family silver set," explains Tom Geballe, Gemini Senior Astronomer, "which slowly develops brown tarnish spots over time and must be regularly polished, the shiny silver on a telescope mirror also tarnishes rapidly reducing reflectivity and increasing emissivity. The observatory's engineers, however, can't just grab a cloth and some polish when the tarnish spots appear."

USAF 1951resolutiontargetdownload

Nobody has yet mentioned the one aspect of silver that the graph depicts. Silver absorbs strongly near 315 nm. If you were interested in UV light from the stars, you would have a hole in your data (between 310 and 320 nm) if you used silver. Not only that, any light gathered with a wavelength less than 310 nm would require longer acquisition times.

USAFresolutionTestchart

If anything, actual development has tended to go the opposite direction: if I'm not mistaken, the "Zerodur" ceramic used in the mirrors for the Keck telescopes is nearly (fully?) opaque. Although it's roughly double the diameter (and therefore quadruple the area) the primary mirror in each of the Keck telescopes totals only about 18 tons (but note the "totals"--each mirror is actually built of 36 segments, each of which is around half a ton).

It ain't necessarily so. Some large telescopes are using silver coatings. For example the Gemini telescopes (Boccas et al. 2006).

ResolutionTargetPDF

On your second question: a back surface reflective coating implies an additional reflective surface: the air-glass interface. This leads to increased light losses and the need for anti-reflective coatings.

The challenge with using silver as a coating material is that, unlike aluminum, it tarnishes with exposure to air, specifically to sulfur.

The structure and elements of the Resolution Target are defined to an American military standard. On the target there are smaller and smaller elements, each consisting of black bars of precisely defined width and spacing. These individual elements are numbered and organized into groups on the Resolution Target. The smallest of these elements for which the scanner can just differentiate between two bars, for which the intervening space is thus barely still detectable, can be indicative of the usable resolution of the scanner.

More information about the Resolution Target, in-depth instructions as well as additional application and background data can be found in the Detailed Documentation.

With the Resolution Target you can determine the best actual usable resolution of your scanner right away. Therefore, you can consult the Quick Manual please.

How to useUSAF 1951 test Target

An optical (also called physical) resolution has always been a requirement in high-end scanning for best quality. Unfortunately today with low priced desktop scanners the term “resolution” has become unclear.

Worse, glass used in most lenses (for example) is much softer and weaker than the Pyrex used in those telescopes, so it would almost certainly require a mirror that was still thicker and more massive and/or more elaborate support structure.

The "wave" of telescopes that (sort of1) culminated in the Hale 200-inch at Mount Palomar was based (in large part) on the invention of Pyrex. Earlier glass was enough weaker that it required a considerably thicker mirror and/or better support structure so the center didn't "sag" under its own weight. The Hale's 200-inch mirror uses a honeycomb design to reduce its mass (to around 14.5 tons), and still requires a fairly elaborate structure to support it without distorting the mirror.

From a viewpoint of the material used, a front surface mirror has fairly simple, mostly mechanical requirements, such as the ability to polish it to the required smoothness, and stability to maintain the intended shape across temperature changes. Even so, the tolerances involved are so fine that meeting requirements is fairly non-trivial.

USAFresolutionTargetCalculator

As a second important application, the target helps you find the optimum sharpness settings for a chosen resolution. You can also find the best compromise for your scanner between high resolution and sharpness and determine which parts of your flatbed scanner are scanned particularly sharply or blurry. The SilverFast Resolution Target is equally well suited for flatbed or film scanners.

It can be stated that the targets from LaserSoft Imaging have excellent quality. They are perfectly suited for scanner calibration and fulfill highest quality requirements.

The paper reports on durable coatings that reduces the tarnishing that favours an aluminium coating (as well as aluminium's superior performance shortward of 400nm). I guess that these are rather expensive, so aluminium is still the norm unless you want to be re-silvering the mirror very frequently.

USAF 1951 Targetdimensions

Johannes makes a good point about durability. As a footnote, I'll add that aluminum has another nice property over silver, at least as far as your plot shows: constant reflectance over the visible spectrum.

The SilverFast Resolution Target in combination with SilverFast software is a simple way to determine just what the maximum resolution of your particular scanner is! Obtain scans with the best possible crispness and sharpness.

Based on the USAF-1951 standard, LaserSoft Imaging developed the SilverFast Resolution Target to make the actually usable resolution of a scanner measurable. This is a transparent scan original, which is suitable for either flatbed or film scanners.

The reason for going with silver rather than aluminium is a slightly improved reflectivity in the near- and mid-infrared (99.1 per cent at 10 microns), but also that the emissivity of the silver coating is around 38% that of aluminium, which reduces background in infrared observations.

1. You could argue that it really "culminated" in the BTA-6. This is a 6 meter telescope located in Russia. Although its designers (apparently) believed they could successfully fabricate a mirror this size, its mirror cracked after a fairly short time, and had to be replaced. Although larger than the Hale telescope and therefore theoretically more sensitive and capable of higher resolution, its contributions have been comparatively minimal. In any case, its mirror design is enough like the Hale's that it doesn't change the overall situation to any significant degree.

With the Resolution Target you can determine the best actual usable resolution of your scanner right away. Therefore, you can consult the Quick Manual please.

If you wanted to transmit light through the mirror, you almost certainly could not use such a honeycomb design, which (in its case) would roughly double the mirror's mass.

Doing a quick back-of-the-envelope calculation, I'd guess the primary mirror in such a design would be somewhere around 100 tons.