How Focal Ratio Affects Your Astro Images

The key to astrophotography is mild — the extra, the higher. And one of many keys to understanding mild, and methods to gather extra of it, is the focal ratio: the focal size of your optic divided by its aperture.

Capture More Light, Without Spreading It Out

One of essentially the most basic ideas of images is that the extra mild you gather, the much less noise you’ll have — shot noise specifically. While each noise and sign improve with publicity time, noise lags behind on the sq. root of the sign. We sometimes common our particular person sub-frames slightly than sum them, so the online impact is that noise decreases with elevated sign or publicity time.

spreading the light
Increasing the focal size of an optic “zooms” in on a goal but in addition spreads obtainable 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 most evident is to easily expose for longer, both by rising the size of particular person exposures or by taking many shorter exposures and stacking them together in post-processing software program.

You can even gather extra mild by rising the aperture of your optic. The trick with this, although, is that whilst you improve the aperture, you need to make certain the focal size does not change. Often, a bigger telescope may 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 digicam’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 discipline of view widens, bringing in mild that was beforehand wasn’t falling in your sensor.
Richard S. Wright Jr.

Note: Increasing your digicam’s achieve, or ISO, will not seize extra mild. Do not cross Go, don’t gather $200. At a sure level, achieve turns into nothing greater than a scaling worth. Increasing your ISO can actually make issues look brilliant on the display screen, however so does that little slider below the histogram. If you need much less shot noise, you want extra mild.

Why Focal Ratio Matters

The tradeoff between aperture and focal size is mirrored within the devices focal ratio. A 200-mm (8-inch) telescope with a 2,000-mm focal size has a focal ratio of f/10. By distinction a 200-mm astrograph (a telescope designed for astrophotography) that has a focal size of 400 mm may have a focal ratio of f/2. Both telescopes gather 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 resembling this f/3 Riccardi-Honders are characterised by massive large apertures, however very quick focal lengths.
Richard S. Wright Jr.

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

This might be complicated as a result of each 8-inch telescopes are gathering the identical quantity of sunshine, so how can one be delivering “more”? There are two methods to know this. The first 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 digicam’s sensor. The f/10 optic does not make use of an excessive amount of its light-gathering capability. The f/2 system delivers extra mild proper to your digicam.

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 extremely fascinating for our objectives. Because of 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 lowering the focal size and dashing up the focal ratio.

Image sensor overlay
A telescope’s native picture circle represents the bounds of the information the instrument can produce.
If your digicam sensor (represented with the red rectangle) already covers many of the picture circle, there is no such thing as a mild left to usher in from the skin. Too sturdy a focal reducer will simply produce a smaller however brighter model of this circle in your sensor like the total discipline proven above.
Richard S. Wright Jr.

There are limits to this seemingly magical system, although!

Even within the twenty first century, designing optics is difficult. A focal reducer can speed up an f/10 telescope to f/7 or f/6.3. But there’s nothing in the marketplace that may speed up an f/10 optic to f/2. (Note: A HyperStar system does not rely, it’s not technically a focal reducer, however as an alternative it replaces the magnifying secondary mirror in a Schmidt-Cassegrain telescope to make the most of 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 will not be all the time easy.

Another 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 digicam sensor should match inside this picture circle with the intention to seize all the mild your optic can ship.

But whereas a focal reducer can recuperate misplaced mild falling outdoors your sensor, if it makes the picture circle too small, you’re going to finish up cropping your picture to eliminate the darkish corners. This will not be vignetting, which might be corrected with flat fields or post-processing tips; there’s 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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