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

Imaging with ASI and Nikkor lenses

It would be nice to image through some of the Nikkor lenses with the ASI183mm Pro. In some cases you reach telescope fields and resolution.

As an example: the 300mm F2.8 Nikkor, attached to the ASI 183mm, delivers a field of 151′ on 100′ with 1,65″ pixelsize.

The Nikon FF D750, combined with the Esprit 120mm, delivers a field of 146′ on 98′ with 1,46″ pixelsize.

Comparing the combinations Esprit/D750 and ASI/300mm: the fields are almost overlapping

Actually the Nikkor 300mm can be seen as an 110mm F 2.8 Apo? The ASI will also use only the center field of the image circle, which avoids all image issues that might occur towards the edges of the field.

The next issue is connectivity! how to connect an ASI body to the Nikkor lenses? The Nikon F-mount backfocus is 46,5mm. The setup that works for me is the following:

  • Nikkor lens with F-mount
  • SI Nikon lens adapter. But ONLY the first ring that attaches to the lens. That ring has 11mm optical length and uses M57x0,75mm male thread. Link to TS.
  • TS Optics M57 to T2 adapter Link
  • Filter manual wheel ATIK Optical length 20mm (could be replaced by a motorized ZWO ASI FW)
  • ASI 183mm camera without the T2 F/F/ connector.

The total optical length of this setup is close to 46,5mm and gets me sharp images. The 1 “1/4 filters that are used might still stretch the focus further out.

Further possibilities to explore:

  • autofocus using a belt and a Pegasus FocusCube 2 or a ZWO MotoFocus
  • simultaneously imaging with ASI 300mm H-Alfa on F 2.8 and Esprit/Nikon RGB: given the faster optics of the Nikkor, all H-Alfa and O-III imaging can be done in the same timeframe as the Esprit/Nikon RGB.

Waiting for a clear night!!!!