Choosing the Right Imager for the Right Scope

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  • By Minos Kritikos

Choosing the right camera for the right scope - Sampling Rate/Resolution

  • The pixel pitch/size in a camera sensor, combined with the focal length of a telescope, determines the sampling resolution of the imaging system.
  • The maximum resolution a telescope can achieve, is defined by its aperture and is reported as the Dawe’s Limit. The smaller the value, the higher the resolution.
  • When a camera is perfectly matched to its telescope, this is known as Perfect Sampling (PS).
  • When the pixels are too big, causing the imaging resolution to be below the Dawe’s Limit, the system is Under Sampling (US), which leads to square stars but easier tracking/guiding.
  • Vice versa, when the pixels are too small and the imaging resolution is too high, the system is Over Sampling (OS), which leads to bloated stars and more demanding tracking/guiding.
  • It does not always happen that there is a camera that can perfectly matched a telescope, but when there is a mismatch, make sure it’s as close to your scopes’ Dawe’s Limit for Decent Sampling (DS)
  • It must also be noted, that imaging resolution can only ever be as good as the atmospheric seeing can support. Therefore, even if you are sampling at s higher resolution than your local seeing supports, your system will only resolve as good as the seeing.
  • For example, a long exposure of 60 seconds, will have many seeing variations during that exposure. The acquired image resolution will only ever be as high as the worst seeing that occurred during those 60 seconds. Therefore, when choosing your imaging system, ask how good is your local seeing?
  • In general, aiming for your imaging system to achieve at least a sampling resolution of 1 - 2”/pixel, is sufficient for orb-like stars with plenty of target detail.

Below are some sampling examples with some popular Deep Sky Astrophotography scopes and astronomical camera combinations. A checkmark has been placed next to the most appropriate cameras for those scopes. This list only takes sampling into account and not FoV, camera sensitivities, etc.. For more combinations, visit: https://astronomy.tools/calculators/field_of_view/

  • Explore Scientific ⌀80mm, f= 480mm, f/6, Dawe’s Limit = 1.45”/pixel
    • ASI1600 = 1.63”/Pixel (DS) ✔
    • ASI183 = 1.03”/Pixel (OS) ✔
    • ASI294 = 1.99”/Pixel (US)

 

  • Explore Scientific ⌀102mm, f= 714mm, f/7.5, Dawe’s Limit = 1.14”/pixel
    • ASI1600 = 1.1”/Pixel (PS) ✔
    • ASI183 = 0.69”/Pixel (OS)
    • ASI294 = 1.34”/Pixel (DS) ✔

 

  • Explore Scientific ⌀127mm, f= 952mm, f/7.5 Dawe’s Limit = 0.91”/pixel
    • ASI1600 = 0.82”/Pixel (DS) ✔
    • ASI183 = 0.52”/Pixel (OS)
    • ASI294 = 1.1”/Pixel (DS) ✔

 

  • Sky-Watcher Esprit ⌀80mm, f=400mm, f/5 Dawe’s Limit = 1.45”/pixel
    • ASI1600 = 1.96”/Pixel (US)
    • ASI183 = 1.24”/Pixel (OS) ✔
    • ASI294 = 2.38”/Pixel (US)

 

  • Sky-Watcher Esprit ⌀100mm, f=550mm, f/5.5 Dawe’s Limit = 1.16”/pixel
    • ASI1600 = 1.42”/Pixel (DS) ✔
    • ASI183 = 0.9”/Pixel (DS) ✔
    • ASI294 = 1.73”/Pixel (US)

 

  • Celestron RASA8 ⌀203mm, f=400mm, f/2 Dawe’s Limit = 0.57”/pixel
    • ASI1600 = 1.96”/Pixel (US)
    • ASI183 = 1.24”/Pixel (US) ✔
    • ASI294 = 2.38”/Pixel (US)

 

  • C8 EDGE w/ 0.7x ⌀203mm, f=1422mm, f/7 Dawe’s Limit = 0.57”/pixel
    • ASI1600 = 0.55”/Pixel (PS) ✔
    • ASI183 = 0.35”/Pixel (OS)
    • ASI294 = 0.67”/Pixel (DS) ✔

 

  • 25 EDGE w/ 0.7x ⌀235mm, f=1645mm, f/7 Dawe’s Limit = 0.49”/pixel
    • ASI1600 = 0.48”/Pixel (PS) ✔
    • ASI183 = 0.30”/Pixel (OS)
    • ASI294 = 0.58”/Pixel (DS) ✔

 

  • C11 EDGE w/ 0.7x ⌀279mm, f=1960mm, f/7 Dawe’s Limit = 0.42”/pixel
    • ASI1600 = 0.40”/Pixel (PS) ✔
    • ASI183 = 0.25”/Pixel (OS)
    • ASI294 = 0.49”/Pixel (DS) ✔

 

  • C14 EDGE w/ 0.7x ⌀356mm, f=2737mm, f/7 Dawe’s Limit = 0.33”/pixel
    • ASI1600 = 0.29”/Pixel (PS) ✔
    • ASI183 = 0.18”/Pixel (OS)
    • ASI294 = 0.35”/Pixel (PS) ✔

 

  • WO REDCAT ⌀51mm, f=250mm, f/4.9 Dawe’s Limit = 2.27”/pixel
    • ASI1600 = 3.13”/Pixel (US)
    • ASI183 = 1.98”/Pixel (OS) ✔
    • ASI294 = 3.82”/Pixel (US)

 

  • WO GT71 w/ 0.8x ⌀71mm, f=336mm, f/4.7 Dawe’s Limit = 1.63”/pixel
    • ASI1600 = 2.33”/Pixel (US)
    • ASI183 = 1.47”/Pixel (DS) ✔
    • ASI294 = 2.84”/Pixel (US)

 

  • WO GT81 w/ 0.8x ⌀81mm, f=382mm, f/4.7 Dawe’s Limit = 1.43”/pixel
    • ASI1600 = 2.05”/Pixel (US)
    • ASI183 = 1.29”/Pixel (DS) ✔
    • ASI294 = 2.49”/Pixel (US)

 

  • WO GT102 w/ 0.8x ⌀102mm, f=562mm, f/5.5 Dawe’s Limit = 1.14”/pixel
    • ASI1600 = 1.39”/Pixel (DS) ✔
    • ASI183 = 0.88”/Pixel (OS)
    • ASI294 = 1.7”/Pixel (US)