What are Flat Field Optics?
Flat field optics are specially designed optics that bring all light wavelengths to focus at the same plane. In other words there will be no chromatic aberration or coma across the entire field-of-view. Telescopes with built-in flat fields are typically more expensive because they involve bringing additional lenses into the light path to ensure the flat field.
Before I talk about flat fields and why they are important, I need to talk to you about how a foundational piece of technology works and why it’s crucial for everything from capturing that perfect photograph to exploring the depths of space. Optical systems, particularly refractors like those found in telescopes and cameras, are designed to bend light so that it converges at a single point. This is the focal point, crucial for producing a sharp image.
However, there’s a catch that many newcomers to photography or stargazing might not anticipate. The light doesn’t always play fair across the entire image. Due to the inherent physics of lens shapes and light paths, the focal point is not uniform. While the center of the image might be razor-sharp, the edges can often tell a different story. They might appear blurred or distorted. This is particularly noticeable when observing stars, which can look like small dots in the center but turn into disappointing smears towards the edges. The stars at the edges can be described as little comets, a phenomenon known as coma.
I remember when I started getting serious about this hobby. Back in the 1990’s digital photography was just beginning to supplant film photography. CCD sensors were becoming more widespread, but their size was very small for an affordable camera. My first astrophotography camera had a chip size of only 768 x 512 pixels. It was so small that it was hard to locate objects and keep them on the chip. The concept of flat field optics wasn’t really an issue because the sensors were so small that you wouldn’t notice any difference in focus across the image. But as sensors got bigger, the unevenness of focus in most optical systems became much more apparent.
It’s frustrating when you’re trying to get a clear, comprehensive picture. Whether you’re an astrophotographer trying to capture the grandeur of the Milky Way, or a wildlife photographer aiming for a detailed landscape shot, that decline in sharpness can turn an awe-inspiring moment into a blurred disappointment.
And it happens more often than one might think. Why? Because as light is bent by the lens, the edges of the lens focus the light at a slightly different point than the center. It’s a common obstacle that photographers and astronomers must overcome to ensure the entire image is uniformly in focus. The good news is, there’s a solution to it, which brings us to field flatteners—our next topic.
The Role of Field Flatteners in Even Focus
Now that I’ve walked you through the challenges optical systems face in achieving consistent focus, I’ll introduce a pivotal component known as the field flattener. What is a field flattener? Think of it as a corrective lens. Its sole mission is to resolve the issue of uneven focus across an image, which is especially troublesome along the edges.
But that’s not all field flatteners can help. There is another phenomenon known as vignetting, which is unevenness in illumination across the entire field. It is common for optical systems to exhibit this in the form of light fall-off toward the edges. Field flatteners can help even this out.
Field flatteners are essentially supplementary optics inserted into the light path between the telescope or camera and the imaging device. They correct the field curvature that occurs as a result of the lens design, ensuring that the stars or points of focus are sharp and clear right to the corners of the image.
As a general rule, any telescope that is marketed as an astrograph has an inherently flat field. Most of the time, astrographs will not need field flatteners.
To appreciate their importance, imagine looking at an image where only the center is in crisp focus, while the periphery blurs into obscurity. A field flattener steps in to prevent this by evening out the focus plane, which is vital for high-resolution imaging in both astronomy and photography.
A direct comparison of images, with and without a field flattener, quickly reveals their value. See the images below. Those without often display a distortion known as ‘field curvature,’ where the image quality degrades toward the edges. When a field flattener is used, the entire image is consistently sharp, showcasing an unblemished expanse.
For astrophotographers or optic enthusiasts, the addition of a field flattener can be transformative. It is not just about the details, but the expansion of the usable field. Without it, the edges of the cosmos or a landscape could appear soft and undefined, with stars becoming elongated or objects losing their distinctness.
Moving forward, it’s essential to recognize that the effectiveness of a field flattener hinges on precise placement in the optical train. In the following section, I will explain why the correct positioning, known as back focus, is crucial, and what happens when this delicate balance is off.
Precision Placement: The Key to Effective Field Flattening
Mastering the intricate dance of field flattener placement is critical for sharp, uniform images. It all boils down to one crucial aspect: back focus. Back focus is the distance between the last element of the lens assembly and the flat field lens or sensor. Too close or too far, and the magic is lost; the image won’t be perfectly flat.
Imagine constructing a card house; precision is your ally, one slight misalignment and the structure wobbles. Similarly, a field flattener needs precise placement to perform optimally. This is no game of chance. Camera and telescope manufacturers often provide exact specifications for back focus to eliminate guesswork.
The meticulous task of setting the perfect distance tests patience but rewards with performance. For telescopes, this involves adjustments to the focuser drawtube. For cameras, spacers or adjustable adaptors finely tweak the gap. DIY setups may trial and error, but for astronomers and photographers alike, the pursuit of stellar image quality justifies the endeavor. The good news is that most field flatteners are designed for specific types or even models of telescopes. And the manufacturers will include precise back-focus distance values. Sometimes spacers in the light path will be necessary to achieve the precise distance.
Complications do arise. Not all optical systems are made equally and neither are their flatteners. Each lens carries its own requirements for the correct back focus distance. Some high-end systems even employ adjustable flatteners, granting the user the power of precision alignment for a range of lens configurations.
Perfection in back focus creates a seamless transition to the next topic: the elegant Petzval design. This innovative approach to optical engineering has reshaped expectations, promising a flat field without external interventions like field flatteners. The genius of Petzval design lies in its built-in solution, tackling the focus variance challenge at its core, revolutionizing optics for users who value both simplicity and performance.
The Petzval Solution: Seamless Focus Without Field Flatteners
I’ll now introduce you to an ingenious alternative in the world of optics: the Petzval design. Named after its inventor, Joseph Petzval, this design has the unique capability of creating a flat field focus naturally.
How does it stand out? Unlike typical lens systems that require the inclusion of an external accessory to correct field curvature, the Petzval design utilizes a sequence of lens elements specifically arranged to produce a flat image field. This innate correction leads to an even focus across the entire image without the need for field flatteners.
Let’s consider telescopes, where high precision is paramount. Petzval-design telescopes offer astronomers images that are sharp right to the edge of the field of view, a coveted feature when observing the night sky.
The advantage extends to photography as well. The even focus provided by the Petzval design gives photographers the confidence that their subject, no matter where it is placed in the frame, will be in sharp focus.
Adopting Petzval lenses does come with a higher upfront cost. However, the investment pays off with the elimination of additional components and effort required to achieve the desired flat-field performance.
As we wrap up, remember that while the Petzval design solves the flat field conundrum ingeniously, not all optical systems require this solution. The right choice depends on your specific use case and preferences.
Choosing the appropriate optical tools is a critical decision, so consider attributes like versatility, cost, and complexity. But knowing the impressive capability of the Petzval design, it might just offer the streamlined focus and convenience you need.