The Contrast Controversy

A few telescope owners maintain that premium optics produce higher contrast deep sky views in the eyepiece. Is there any truth to this? I set out to see for myself.

Suiter in his Star Testing Astronomical Telescopes, Chapter 13, titled ‘Roughness’ states that “…microripple would be indistinguishable from a perfect aperture in most dark-field observing situations.” “Microripple is one of those myths of telescope making that feeds on folk wisdom and hearsay.” Hmm, authority suggests otherwise, perhaps we can reason for ourselves.

Just what distinguishes a premium optic from an ordinary mirror? As a mirror maker and demanding visual observer for many decades, I certainly have my opinion, but from what I understand, what’s in vogue currently is the idea of micro-ripple or very fine surface roughness.

What is a good way to understand potential veiling from scattered light? While focusing a star, I don't know which way to turn the focuser. Half the time, the star's light concentrates into a brightening disc but half the time the star's light diffuses into an expanding disc, eventually disappearing from sight. In fact, for most stars in the field, they can't be seen until very near focus. If a star's de-focused disc of light is defocused so much that it fills the entire field, then that's the same as scattering all of its light across the field - that's as extreme as we can get.

It occurred to me that a simple experiment might settle the issue. I set up a telescope of moderate aperture, waited a half hour to become dark adapted, aimed the scope at a bright patch of stars in the Milky Way and attempted to de-focus the field. The focuser would not go out far enough so I had to slide the eyepiece partway out of the focuser to reach what I call, "ultimate defocus". Surprisingly, all the stars defocused into invisibility, leaving the field of view as bright or dim as when all the stars are focused!

Conclusion: even in the most extreme case where all the starlight is scattered, veiling glare is not observed.

Some say that a turned edge, present in less than premium optics, scatters light into a veil across the field of view. This indeed would be an extreme case, most turned edges being fractional wavelength in deviation, and even if a wave or two in error, will only scatter light a few arc-seconds away from the star. But let’s calculate the amount of scatter if a turned edge spread all of its light evenly across the field of view. A badly turned edge in an eight inch [20cm] mirror averages about an eighth of an inch [3mm]. That amount of light is about three percent. But this is far less than the star’s total light output, which we’ve already seen, if spread across the entire field of view, is thousands of times fainter than a human eye can detect. Put another way, if the entire mirror was badly turned, then this is no worse than scattering all the starlight across the field, which we’ve seen, is undetectable.

Here’s another convincing observation that you can make: take an open frame telescope, securely attaching (you will instantly permanently blind yourself if the filter is knocked off) a white light solar filter meant for full aperture use over the front of the upper end of the tubed assembly. Astonishingly, the Sun can be seen with lots of good detail despite the telescope’s optics exposed to broad daylight. Knowing that the filter reduces the Sun’s light by almost a million times, if veiling glare occurs with ordinary quality optics then the view should be overwhelmed with scattered daylight – clearly not the case.

Our observations confirm Suiter’s mathematical analysis that there is no physical basis for scattered light creating a contrast lowering veil across the field of view.

Of course, large scale surface roughness can scatter some light small distances from bright stars, which I find annoying and objectionable. Here are two test objects: NGC 404, a galaxy right next to the bright star Mirach and the Leo Dwarf Galaxy next to Regulus. Further, it hurts planetary and lunar contrast, a different kind of contrast that we’ve been discussing. Worse of all is spherical aberration - the dominate error in optics.

A premium optic is one where firstly the spherical aberration is unusually small such that the parabolization is uncommonly accurate and secondly the surface is smooth with no roughness.

Yet a very few owners report better views. Let’s take them at their word, that they did see a better view. The simplest and most direct explanation lies in our human nature. Every single one of us is subject to confirmation bias and selection bias. That means that when we look through the premium scope, every single one of us expects a better view. Consequently we tend to use the scope on a darker night and we try harder and remember aspects of the deep sky views that are better. We can ‘see’, subjectively, that the view is better. We are all subject to biases, there’s no escape for any of us other than to be mindful and aware of our biases.

What to do about this? Should we do anything at all? People ‘know’ and believe what they see, after all. Would a double blind experiment change peoples’ minds? Such an experiment consists of two telescopes, identical except for one with a premium mirror and one without. Neither the operator nor the observers would know which one is premium. Would the results settle the issue? I doubt it: this test has been conducted before yet the controversy exists. The power of belief in what we see is very strong.

Once I conducted a blind experiment where I casually mentioned the mirror’s quality to guest observers. Asking what they saw, a few praised the view when told the mirror was ‘premium’ and some criticized the view when told the mirror was ‘problematic’. Tellingly, most observers gave rather uniform accounts, particularly if they were experienced.

Worse of all, this attention on veiling glare turns us away from what does cause glare in the field of view: inadequate baffling. At a minimum, in a Newtonian telescope, the diagonal needs to be baffled. Additionally the focuser should be baffled along with the primary mirror. The test is to shine a flashlight aimed at the telescope. While looking through the eyepiece with your hands tightly cupped around the eyepiece, you should not be able to tell if the flashlight is aimed at the telescope, if the flashlight is turned on or off. Blocking extraneous external light thousands of times brighter than the stars is critically important to obtain the darkest background, as is using the scope in dark skies.

One last thought: there are two factors that truly matter: experience and dark skies. An experienced observer under dark skies can extract so much more from a merely decent scope than a beginner can from a top shelf scope. If you truly wish to see better then put in the hours under the stars to become an experienced observer. For starters, work your way through several of the Astronomical League’s observing programs. Along the way I guarantee you unforgettable views regardless of the ‘premium’ nature of your optics.

Mel Bartels