Testing new glass

The skies cleared just in time to test my latest new-to-me lenses. I've been stepping outside my solar system comfort zone, to try out deep sky astrophotography with low cost vintage lenses. Mirrorless cameras like my Sony a6300 can use almost any DSLR or vintage SLR lens in manual mode. The short flange to sensor distance allows these lenses to be mounted and focus at infinity with only an inexpensive mechanical adapter. Astrophotography requires manual settings, so pairing mirrorless cameras and inexpensive vintage SLR lenses has great potential.

I first tried this with a Vivitar 135mm lens that I found on eBay for only $35 and was able to make some very nice images of Omega Centauri with it. Subsequently I tried a $50 Vivitar 200mm lens with a 2X teleconverter. This worked fine for Moon and solar eclipse images, but the chromatic and geometric distortion in image corners was poor.

Looking for a better alternative, I discovered that other astrophotographers were using vintage Nikkor manual focus ED lenses to produce good images. There are many of these on the second hand market in nice condition, especially from Japan. Prices run around $200 give or take a hundred. Quite a bit more than my bargain Vivitars, but only about 20% of the cost of new glass.

I decided on the following kit, which gives me four different fields-of-view from 3.2 to 7.5 degrees.

  • Nikon Nikkor 180mm f/2.8 AI-S ED manual focus telephoto lens
  • Nikon Nikkor 300mm f/4.5 AI-S ED manual focus telephoto lens
  • Nikon AI-S 1.4X Teleconverter

I also have a 72 mm UV/IR cut filter on order. Light outside the visual range suffers the worst chromatic aberration, and using a cut filter, than only passes the better focused visual spectrum, will make a noticeable improvement in images.

Daylight testing of both telephoto lenses showed a substantial increase in sharpness one stop down from the maximum, with a bit further improvement two stops down. These images were made at one stop less than maximum aperture. These exposures are made with a 1.5x crop sensor camera; their corners are not as extreme as they would be with a full frame body.

My test images below were made without a cut filter. I shot 30" images tracked using a Vixen Polarie mount. Sky conditions were good except for the first quarter Moon and Austin's sky glow. No darks or flats were used and processing was limited to a simple stack and stretch. The Sony a6300 produces 24 MP 6000 x 4000 images. For each lens combination the entire field of view is shown, with 100% crops from near the center of the image and from a corner.

180 mm

Here are the Nikkor 180mm f/2.8 AI-S ED lens results. Total exposure was 25.5 minutes.

Crop near the center

Crop at a corner

The lens

300 mm

Next the Nikkor 300mm f/4.5 ED was used to shoot 56 30" subs at f/5.6.

Crop near the center

Crop at a corner

The lens

420 mm

Last I added the Nikon 1.4X Teleconverter to the Nikkor 300mm f/4.5 ED for an equivalent focal length of 420 mm shot only 19 30" subs at an equivalent f/8.

Crop near the center

Crop at a corner

The teleconverter

Summary

I'm pleased with these first tests. After I get the cut filter I expect to further improve images with these lenses. I plan to head for some dark skies at Terlingua for some real imaging with these and will post my results then.

Content created: 2019-02-13

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