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THE TELESCOPE MAN
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Tools · an experiment

The living glossary

Astrophotography words, shown on my own data

16 terms, each with a one-line meaning and a demo you can play with, built from my real captures and measurements. Every one links to where it matters in my reviews.

By Damon Scotting16 termsCaptures from Bortle 6.8 (in my garden)

01Optics#

Focal length

The distance over which a telescope brings light to a focus. A longer focal length magnifies more and shows a smaller patch of sky.

02Optics#

Focal ratio

Focal length divided by aperture. A lower number, a faster telescope, gathers the light for each patch of sky in less time.

04Optics#

Pixel scale

How much sky each pixel covers, in arcseconds. It decides whether a star is drawn by many pixels or squeezed into a blocky few.

05Optics#

Plate solving

Software matching the stars in a picture to a star catalogue to work out exactly where the telescope is pointing, the image scale and its rotation.

06Sharpness#

FWHM (star size)

Full width at half maximum: how wide a star is at half its peak brightness. Smaller means better focus, steadier air and better optics.

07Sharpness#

Guiding RMS

The typical size of a mount's tracking error while guiding, in arcseconds. Lower means rounder stars.

08Sky#

Bortle scale

A nine-step scale of how dark the night sky is, from 1 for a pristine site to 9 for an inner city.

09Processing#

Gradient removal

Modelling the uneven glow from light pollution, the Moon and the sky across a frame, and subtracting it.

It is always my first processing step, and the one that matters most from a suburban garden.

Where this matters in my reviewsGraXpert99PixInsight78

10Capture#

Integration time

The total exposure time of every frame stacked into one image. Noise falls with its square root, so four times the time halves the noise.

11Processing#

Stretching

Brightening linear data, which looks almost black straight from the camera, so the faint detail shows without blowing out the stars.

Every raw stack on my review pages has one plain automatic stretch, so you can see what the telescope really caught.

Where this matters in my reviewsPixInsight78Adobe Photoshop67

12Processing#

Histogram

A graph of how many pixels sit at each brightness, from black on the left to white on the right.

13Processing#

Deconvolution

Undoing some of the blur from the optics and the air with maths, so stars shrink and fine structure sharpens.

14Processing#

Noise reduction

Smoothing the random grain in an image while keeping the real detail.

15Processing#

Star removal

Separating the stars from the nebula so each can be processed on its own, then putting the stars back.