Bortle Scale

The Bortle Scale is a way to measure the quality of the night sky. It ranges from 1 to 9, with 1 being the darkest sky and 9 being the most light polluted.

To plan my dark sky trips, I look at the light pollution map to find a rental property with a low number on the Bortle Scale (Bortle 4 and under is ideal).

Technical Terms

Aperture: The diameter of the lens or mirror that collects light. The larger the aperture, the more light the telescope can gather, and the fainter the objects it can see. For Astrophotography, a larger aperture is generally better.

Focal Length: The distance between the lens or mirror and the image sensor. The focal length determines the field of view and magnification of the telescope. For Astrophotography, a longer focal length is generally better.

Focal Ratio: The ratio of the focal length to the aperture. It determines how bright the image will be and how long the exposure needs to be. For Astrophotography, a lower focal ratio is generally better.

Rig Setup

Camera

I am using a Canon EOS Rebel T7b DSLR camera. It is one of the most popularbeginner cameras for astrophotography.

NB. DSLR stands for Digital Single Lens Reflex. Earlier cameras used film instead of digital sensors, hence called SLR. Single lens refers to the fact that the camera uses a single lens for both viewing and taking pictures. Earlier cameras had separate lenses for viewing and taking pictures. Reflex refers to the mirror that reflects the light from the lens to the viewfinder.

Mount/Tracker

More than the telescope, the mount is the most important piece of equipment for astrophotography. I am using the Sky-Watcher Star Adventurer Pro. It is a portable equatorial mount that tracks the stars as they move across the sky. This is important for long exposure photography, as the Earth’s rotation will cause the stars to move across the frame if the camera is not tracking them.

Equipment Overview:

  1. Camera: Canon DSLR with a maximum shutter speed of 30 seconds and support for manual settings (M mode) and RAW format.
  2. Lenses:
    • 75-300mm Telephoto Zoom Lens:
      • Focal length range: 75-300mm.
      • Aperture: Variable, f/4 at 75mm and f/5.6 at 300mm.
      • Best suited for zoomed-in astrophotography of deep-sky objects (DSOs) like nebulae and galaxies.
    • 18-55mm Kit Lens:
      • Focal length range: 18-55mm.
      • Aperture: Variable, f/3.5 at 18mm and f/5.6 at 55mm.
      • Best for wide-field astrophotography, such as capturing constellations, the Milky Way, and star trails.

Key Concepts:

  1. Aperture (f/number):

    • Aperture controls the size of the lens opening and is expressed as f/number (e.g., f/3.5 or f/5.6).
    • Lower f-numbers (e.g., f/3.5) correspond to wider apertures, allowing more light to reach the sensor, essential for astrophotography.
    • Wide apertures (like f/4 or f/3.5) are ideal for capturing faint celestial objects like nebulae.

  2. Shutter Speed:

    • Shutter speed determines how long the camera sensor is exposed to light. For astrophotography:
      • Without a star tracker: Use the 500 Rule to calculate the maximum exposure time before star trails appear.
      • With a star tracker: Use exposures of 30 seconds to 2 minutes or more to collect sufficient light.
    • Use the Bulb mode on the camera for exposures longer than 30 seconds with a remote shutter release or intervalometer.

  3. ISO:

    • ISO controls the sensor’s sensitivity to light.
    • For astrophotography, use ISO 800–1600 as a starting point, adjusting based on light pollution and noise performance.

  4. Focus:

    • Switch to manual focus (MF) and use live view to focus on a bright star. Zoom in and adjust until the star is sharp.

  5. RAW Format:

    • Always shoot in RAW to preserve maximum image detail for post-processing.

Lens-Specific Recommendations:

  1. 75-300mm Lens:

    • Use the 300mm focal length for DSOs like:
      • Orion Nebula (M42).
      • Pleiades (M45).
      • Andromeda Galaxy (M31).
    • Aperture: Use f/5.6 at 300mm (widest for this focal length).
    • Pair with a star tracker to extend exposure times (1-2 minutes or more) and capture detailed images of smaller objects.

  2. 18-55mm Lens:

    • Use the 18mm focal length for wide-field shots:
      • Milky Way (if in a dark sky area).
      • Constellations (e.g., Orion, Cassiopeia).
      • Star trails (fixed tripod, long exposure).
    • Aperture: Use f/3.5 at 18mm for maximum light collection.
    • Shutter Speed: Apply the 500 Rule for exposures without a tracker (e.g., ~17 seconds at 18mm on an APS-C sensor).

Additional Gear Recommendations:

  1. Star Tracker:
    • Essential for long exposures to prevent star trails, especially at longer focal lengths like 300mm.
    • Enables capturing more faint detail in nebulae and galaxies.
  2. Remote Shutter or Intervalometer:
    • Prevents vibrations during long exposures.
    • Allows programming longer exposures in Bulb mode.
  3. Light Pollution Filter:
    • Improves contrast and visibility of faint objects in urban or suburban skies.

Post-Processing:

  1. Capture Multiple Frames:
    • Take 20–50 shots of the same object to stack later for reducing noise and improving details.
  2. Calibration Frames:
    • Capture dark, flat, and bias frames for better image calibration.
  3. Stacking and Editing:
    • Use software like DeepSkyStacker, PixInsight, or Photoshop for stacking and enhancing images.
    • Adjust colors, contrast, and noise to bring out nebula details.

Specific Targets for January in Seattle:

  1. Deep-Sky Objects (DSOs):
    • Orion Nebula (M42): Bright, easy to capture.
    • Pleiades (M45): Open cluster with faint nebulosity.
    • Andromeda Galaxy (M31): A large galaxy visible to the naked eye.
    • Rosette Nebula: Wide-field emission nebula (best with 18-55mm at 55mm).
  2. Wide-Field Shots:
    • Milky Way (if skies are dark enough).
    • Large constellations like Orion or Cassiopeia.

Practical Shooting Workflow:

  1. Set up the camera on a stable tripod and star tracker (if available).
  2. Frame the target using live view and manual focus.
  3. Configure the settings:
    • Shutter Speed: 30 seconds (or longer with Bulb mode).
    • Aperture: Widest available (e.g., f/3.5 or f/5.6).
    • ISO: 800–1600.
  4. Take multiple exposures of the target and calibration frames.
  5. Process the images using astrophotography software.

Software Tools

Siril

https://free-astro.org/index.php?title=Siril:scripts

Basic Steps: https://sathvikacharyaa.github.io/sirilastro/

Remove Stars from Image

https://www.starnetastro.com/experimental/ starnet2 -i ../Pleiades/result2.tif -o ../Pleiades/starless2.tif -m ../Pleiades/mask2.tif

Sources

Discussion forum: https://www.cloudynights.com/ https://astrobackyard.com/introduction_to_deep_sky_astrophotography/