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How Focal Ratio Affects Your Astro Images – Sky & Telescope

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The important thing to astrophotography is mild — the extra, the higher. And one of many keys to understanding mild, and find out how to accumulate extra of it, is the focal ratio: the aperture of your optic divided by its focal size.

Seize Extra Mild, With out Spreading It Out

One of the vital elementary ideas of pictures is that the extra mild you accumulate, the much less noise you’ll have — shot noise specifically. Whereas each noise and sign improve with publicity time, noise lags behind on the sq. root of the sign. We usually common our particular person sub-frames relatively than sum them, so the web impact is that noise decreases with elevated sign or publicity time.

spreading the light
Rising the focal size of an optic “zooms” in on a goal but in addition spreads out there mild over a bigger space, “thinning” the ensuing image.
Richard S. Wright Jr.

There are a number of methods to place extra mild on our picture sensors. The obvious is to easily expose for longer, both by growing the size of particular person exposures or by taking many shorter exposures and stacking them together in post-processing software program.

You can even accumulate extra mild by growing the aperture of your optic. The trick with this, although, is that at the same time as you improve the aperture, you must make certain the focal size would not change. Usually, a bigger telescope can have a correspondingly longer focal size, which impacts picture scale, so the picture turns into bigger. This sounds nice till you notice that you simply’re spreading the identical quantity of sunshine over a bigger space (your digital camera’s sensor), and the extra you zoom in on the goal, the fainter it turns into.

Focal Reducer Results
Going the wrong way, a focal reducer will increase brightness, however on the tradeoff of picture scale. The sector of view widens, bringing in mild that was beforehand wasn’t falling in your sensor.
Richard S. Wright Jr.

Notice: Rising your digital camera’s achieve, or ISO, will not seize extra mild. Don’t cross Go, don’t accumulate $200. At a sure level, achieve turns into nothing greater than a scaling worth. Rising your ISO can definitely make issues look vivid on the display, however so does that little slider beneath the histogram. If you need much less shot noise, you want extra mild.

Why Focal Ratio Issues

The tradeoff between aperture and focal size is mirrored within the devices focal ratio. A 200-mm (Eight-inch) telescope with a 2,000-mm focal size has a focal ratio of f/10. In contrast a 200-mm astrograph (a telescope designed for astrophotography) that has a focal size of 400 mm can have a focal ratio of f/2. Each telescopes accumulate the identical quantity of sunshine, however the latter produces a a lot brighter picture of your goal.

Fast optics photo
Optics designed for quick focal ratios similar to this f/three Riccardi-Honders are characterised by giant extensive apertures, however very quick focal lengths.
Richard S. Wright Jr.

How is that this potential? Merely put, you might be swapping picture scales, buying and selling a bigger, fainter picture for a smaller one which concentrates mild extra and is thus brighter. The online result’s much less shot noise as nicely.

This may be complicated as a result of each Eight-inch telescopes are amassing the identical quantity of sunshine, so how can one be delivering “extra”? There are two methods to grasp this. The primary is that a quicker telescopes delivers extra mild “per pixel”.

Secondly, we aren’t involved with the signal-to-noise (S/N) ratio of the entire system, solely what’s delivered to our digital camera’s sensor. The f/10 optic would not make use of a substantial amount of its light-gathering capability. The f/2 system delivers extra mild proper to your digital camera.

Why (and Why Not) to Use a Focal Reducer

So we all know that a brighter picture has extra sign and fewer shot noise, which is very fascinating for our objectives. Due to this, a well-liked accent for astrophotography is the focal reducer. A focal reducer takes the sunshine cone from a slower optical system, f/10 for instance, and focuses it to a smaller space, successfully decreasing the focal size and dashing up the focal ratio.

Image sensor overlay
A telescope’s native picture circle represents the boundaries of the info the instrument can produce.
In case your digital camera sensor (represented with the purple rectangle) already covers many of the picture circle, there is no such thing as a mild left to herald from the surface. Too sturdy a focal reducer will simply produce a smaller however brighter model of this circle in your sensor like the complete subject proven above.
Richard S. Wright Jr.

There are limits to this seemingly magical system, although!

Even within the 21st century, designing optics is difficult. A focal reducer can pace up an f/10 telescope to f/7 or f/6.three. However there’s nothing available on the market that may pace up an f/10 optic to f/2. (Notice: A HyperStar system would not depend, it’s not technically a focal reducer, however as a substitute it replaces the magnifying secondary mirror in a Schmidt-Cassegrain telescope to benefit from the f/2 focal ratio of the first mirror).

Suffice it to say, actually quick optics should be designed from the begin to be actually quick optics, and altering them after the very fact just isn’t all the time easy.

One other limiting issue has to do together with your telescope’s native picture circle. The picture circle is the fully-illuminated space an optic produces. Your digital camera sensor should match inside this picture circle so as to seize the entire mild your optic can ship.

However whereas a focal reducer can get better misplaced mild falling outdoors your sensor, if it makes the picture circle too small, you’re going to finish up cropping your picture to do away with the darkish corners. This isn’t vignetting, which could be corrected with flat fields or post-processing tips; there may be merely no mild falling outdoors of the picture circle.

Imaging with quick optics has its personal tradeoffs, however there’s no query that quick focal ratios ship extra mild to a smaller space, and extra mild all the time means much less shot noise.


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