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title: 'Focal Reducer / Field Flattener"
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This device is like a little magic tube for your [Time Machine](/astropotamus/time-machines). Let's explore what it does and if you need one.
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## So what is an FF/FR?
Imagine having two telescopes in one - a long focal length instrument for lunar and planetary work and a short focal length scope for deep sky observing. That's what a Focal Reducer does. It converts your 2000mm/f10 telescope into a 1260mm/f6.3 telescope. While it may seem like you're reducing the power of your scope, you're actually widening its field of view, shortening the time it takes to get imaging data, and in this case you're also *flattening the field* so things are sharper out near the edges. Let's explore what this means a little more.
### What is a Focal Reducer?
FF/FRs reduces your telescope’s focal length and f/ ratio by 37%, turning your long focal length telescope into a faster, shorter focal length instrument. An f/10 focal ratio now achieves a f/6.3. For visual use, this means you get lower power with the same eyepiece and a wider field of view. For imaging, you get a wider field and much shorter exposures. It's like putting a throttle limiter on your Lamborghini so you can drive it at normal speeds on normal roads and actually enjoy it more.
### What is a Field Flattener?
Telescopes are not perfect. They create a circle of light that they send through the scope to the eyepiece or camera sensor. For an eyepiece, your eyes will focus on the things you see in the field of view and adapt pretty quickly if something is slighty out of focus. In the case of a camera sensor, that doesn't happen. The sensor is focused at one place only. And if that circle of light isn't evenly in focus throughout the entire field of view, then the center will look good but the edges will be wonky.
A field flattener does just that - flattens the field. The circle of light is *flattened* so that it looks the same at the edge at it does in the center. In essence, it woulde be like flattening a spoon; once you remove the curved surface and it's flat, the distortion goes away and you get a nice image. Or something like that. I'm not a poet. Now, this isn't perfect or 100%; there will still be some curvature. But it's signficantly reduced, which is a good thing when taking astro images!
### What Does This Mean for Astrophotography?
In short, it means you can take wider images, with less time to get each image, and they are all in focus from edge to edge. This reduces the amount of wasted, out of focus pixes in any single image, reduces the amount of time it takes to get the data, and makes it easier to capture wide field views so you don't have to stitch as many images together afterwards in a mosaic. A win-win-win!