Testing the 300mm Nikkor w/ ASI183

Note: I discovered weeks after this test that the filter drawer of this lens actually contained a Nikon UV filter. That might have influenced the result, so further testing required.

And these are the results of some testing with this setup are a bit disappointing! actually it seems that the sharpness of this particular lens cannot compare with ‘real’ telescopes, and that’s maybe not a great surprise. To be comparing apples with apples: the APO image was taken with a F/7 Esprit 120mm with a focal reducer/flattener to F/5,5 resulting focal length 663mm. Second point: the total imaging time of the reference image was 150 minutes, compared to only 7,5 minutes for the F2.8 image, and 50 minutes for the F4 image. .

  • the images from the Nikkor are blown up to match the size of the reference image
  • the Nikkor images are clearly less sharp, with notable halos on the brighter stars in the F2.8 image
  • the star shape seems to be a bit irregular, that is not necessary a lesn issue it might be related to guiding issues.

The real strength of this lens is probably more in the area of wide-filed DSLR imaging?

Reference image: M45 30x300S ISO800 Nikon D750 Esprit120 F5 Meldert SEP2019 SQM2050, cropped to match the size of the ASI test images

And these are the images taken with the Nikkor 300mm F2.8 and the ASI183mm, using a Baader contrast booster 1 1/4″ filter to avoid blue/purple fringes and IR. The F/4 image can be recognised easily due to the 18-pointed stars, caused by the 9 iris blades in the lens. The F/4 image is clearly sharper, however the F2.8 image obviously got the image faster but with less sharpness.

M45 ASI183 Nikkor 300mm F 2.8 Baader Contrast booster 15x30s Gain111 crop
M45 ASI183 Nikkor 300mm F 4 Baader Contrast booster 25x120s Gain111 crop

300mm F2.8 Nikkor

This lens is a beauty and has been since long on my wishlist. The initial tests indicate some slight defocus on the left side of the image ( at F2.8). But sharp enough overall and especially in the centre. The intended use of this lens is to exploit it’s fast F/Ratio: fast comet imaging with the Nikon D750 / Large Milky Way or nebula complexes with the Nikon D750 and narrow band imaging with the ASI183mm.

Some test shots (unguided)

Crop of the rising Moon 300mm F2.8 Nikkor @ F4
M57 6 x 30″ on ISO400 @ F2.8, slightly cropped ; no guiding on an EQ8