Reading your first astronomical image

Your first set of files can be a surprise. Here is what you are looking at.

Raw frames look terrible, and that is correct

A single raw frame is dim, grainy, and often shows almost nothing of the object. This is normal. Each frame caught only a few seconds of light, and the sensor's own electronic noise is a large fraction of what you see.

Raw frames are the actual measurements. They are the most scientifically valuable thing you have, which is why we keep them.

Stacking is arithmetic, not decoration

Stacking aligns your frames on the stars and averages them. The object's light appears in the same place every time, so it survives averaging. The noise is different in every frame, so averaging cancels much of it out.

Nothing is added to your image by stacking. It only removes what was never real.

Stretching is a change of display, not of data

Astronomical images have an enormous range of brightness — a galaxy's core can be thousands of times brighter than its outer arms. A screen cannot show that range at once, so the image is stretched: the faint end is brightened much more than the bright end.

Stretching changes how the data is shown, not what was measured. This is why we keep both: a stretched version to look at, and the original measurements to work with. If you want to measure something, use the originals.

What each file is for

• Raw frames — the individual measurements. Use these for anything scientific.
• Stacked image — all your frames combined. This is usually the good one.
• Preview — a stretched picture for looking at and sharing.
• Logs — what the telescope actually did, including anything that went wrong.