C&C requested: Observed shape, size, magnitude, colour of stars

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Andy Lucy Forum Member • Posts: 55
C&C requested: Observed shape, size, magnitude, colour of stars

I’m in the early stages of astrophotography and would welcome comments on the image below and the methods I used to obtain it.

Currently, I’m trying to see what is possible in astrophotography with the camera and lens I use for maximum reach in wildlife photography. This is a Nikon 300mm PF f/4 lens mounted on a Nikon J5 camera (with an FT1 adaptor), which has a 20MP 1-inch sensor (pixel pitch 2.4 microns). In wildlife terms, this setup has an equivalent focal length of 810mm; in astrophotography terms it has a 75mm aperture, 300mm focal length and 1.6 arcseconds / pixel. The 300mm PF lens is (by manufacturer MTF figures) exceptionally sharp in the centre even wide open, and the J5 sensor only uses the centre part of the image circle, so there is potential for very sharp images.

I was aiming to see the faintest possible stars from a single exposure, so I used 30s exposure time (the maximum the camera allows when the FT1 adaptor is used), ISO 6400 and f/4. The camera was mounted on a static tripod and aimed at Polaris. Celestial North is in the upper right quadrant. Focus was perfect, according to a Bahtinov mask image. The location was a fairly dark rural site in East Yorkshire.

Post processing (Lightroom) was minimal: no lens corrections, no chromatic aberration reduction, no sharpening, blacks slider set at -93, luminance reduction at 19.

My observations are:

  • Magnitude 15 stars are easily visible, and a few of magnitude 16. [With only 1.6 arcseconds / pixel, it’s relatively easy to distinguish stars from noise since the stars cover a cluster of pixels]
  • Star trails are seen in all but the upper right quadrant, but in the centre of that quadrant stars do appear circular.
  • Polaris appears about 240 arcseconds across
  • Stars greater than magnitude 9 are blown out in their centre.
  • Bright stars on the edge of the image show some colour fringing about 2 pixels wide.
  • Faint stars appear in a very wide range of colours: red, blue, yellow, brown, green, pink, orange

Comments on the image and any part of what I have done would be helpful, and I am particularly interested in why the faint stars show such varied colour.

Regards,
Andy

 Andy Lucy's gear list:Andy Lucy's gear list
Canon PowerShot S95 Nikon 1 J5 Nikon AF-S DX Nikkor 18-135mm f/3.5-5.6G ED-IF Nikon AF Nikkor 50mm f/1.8D Nikon AF-S DX Nikkor 55-300mm f/4.5-5.6G ED VR +2 more
Nikon 1 J5 Panasonic Lumix DMC-TS1 (Lumix DMC-FT1)
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