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Part 2: Breaking Down the Night Sky

Light Pollution


Too few people have seen a 'Bortle 1' night sky. (true dark sky) Urban areas are almost always drowning away their night sky with excessive amounts of artificial lighting. You see the fewest stars in Bortle 9 areas, and you'll see the most in Bortle 1 areas. This Bortle scale is easily referenced and visualized with awesome online resources like lightpollutionmap.info.


Light pollution is never beneficial, especially when it comes to astrophotography. To beat it: you can travel to rural areas, use Light Pollution filters, and/or even use monochrome (black and white imaging) cameras if you're stuck in the middle of the city glow.

Note - A full(er) moon also reflects a ton of sunlight into the night sky, making deep sky astrophotography more of a hassle without filters.


Additional obstacles arise when poor 'seeing' is present. The term 'seeing' is basically referring to atmospheric air clarity. If there is a lot of turbulence in the air, it will negatively impact the finest detail in your photos.


The yellow lighting in this image? That's not the sun setting, that's city glow/light pollution from a town behind the mountains. (This is a rare exception where I liked it in my photography)


Star Navigation


The single most important asterism (star pattern) to learn, is the one the helps you find your celestial pole. For those in the northern hemisphere, that's 'The Big Dipper,' which will point you to the North Star. (Polaris) 'The Southern Cross' will be your guide in the southern hemisphere. Once you are oriented, pay attention and watch out for constellations as everything rises in the east and sets in the west. I recommend first learning to find 'Scorpius' for Milky Way nightscape photography, and 'Orion' for it's deep space targets with a telescope.

Fun fact - The North Star is not the North Pole. (but it's always within 1 degree)


Constellations


The majority of constellations are seasonal, with only a few (circumpolar) exceptions. This means that every target you would want to photograph has a single/seasonal window of opportunity every year. Deep space targets come and go, and so does Milky Way nightscape season.


Coordination/Celestial Sphere


On Earth, every position has GPS coordinates, referencing latitude and longitude. We apply a very similar system to the night sky. The 'celestial sphere' contains a grid system referencing Declination and Right Ascension, respectively.

Note: Altitude/Azimuth coordinates vary based on the location of the observer and do not represent objective target locations like Dec/RA coordinates do.


Angular Sizing


The celestial sphere is 360 degrees. For perspective, the full moon in the sky takes up just half of a single degree. For accuracy, we can break down a degree much like an hour. Each full degree has 60 arc-minutes (60') and a single arc-Minute can be broken down into 60 arc-seconds. (60") The moon, half of one degree, is also equal to about 30 arc-minutes.

Note - the angular size/diameter of targets varies (especially for planets in our solar system) depending on how close/far they are from the Earth at that time. Venus for example, varies from just 9.7" to 66"! (66 arc seconds is equal to 1 arc-minute plus 6 arc-seconds. (1' 6")

Angular sizes of targets become very relevant when selecting telescopes.


So once you know the size of these objects, what telescope would fit best for you? Well, here's my favorite framing website: telescopius.com. There, you can input telescope and camera combinations for incredible previews of how it will look/do. (I do lots of testing there before buying anything)


Example angular sizes:

  • M42 Orion Nebula: 85' x 60' (arcmin)

  • International Space Station: 66" (arcsec)

  • Sun: 31'

  • Moon: 31'

  • Mercury: 4.5 - 13"

  • Venus: 9.7 - 66"

  • Mars: 3.5 - 25.1"

  • Jupiter: 29.8 - 46.9"

  • Saturn: 14.5 - 20.1" (33.8 - 46.9" including the rings)

  • Uranus: 3.3 - 4.1"

  • Neptune: 2.2 - 2.4"

Conclusion


I know this is a lot consolidated, but trust me, this is much better than needing to hunt it all down across the internet like I had to. Astrophotography is very complex, but it's rewards are all the reason.

Clear skies!

Hozzászólások


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