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?
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.
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.
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.
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.
I could add 116 shots of 120 seconds, in an attempt to bring out the outer spiral arms more clear. The total integration time is now over 8 hours and this is the LRGB combination below (all shot from Meldert, Belgium). Click on the picture to go to Astrobin.
These 4 nights were more or less used, there was some high-altitude clouds passing by which not always affect the photometer (click for a larger image)