Finderscope Experiment

Too Many Aberrations!

When I purchased my 130 mm Stellarvue refractor years ago it came with an 80 mm f/3.75 finderscope. I was curious to see if the finderscope could function as a 300 mm focal length telephoto lens when attached to my new Nikon Z6 2 camera. On February 13th I mounted the combination on a fixed tripod as shown in the picture below and hurried outside to take advantage of some rare clear sky.

Exposure times were limited to two seconds to avoid star trails. I compensated for short exposures by setting ISO at 51200. Focusing was extremely difficult because the finderscope has a helical focus mechanism. Since the camera was fixed to the tripod, I had to turn the entire finderscope to focus. This somewhat stiff rotation caused so much vibration on the unsteady tripod that it was hard to tell if stars were focused. Consequently, star images are a little bloated in the images that follow.

Lens aberrations are immediately apparent away from center in this full frame image of the Pleiades star cluster made by stacking seven 2-second unguided exposures.

The finderscope lens is probably a simple single spherical lens. Only the very center of such a lens is relatively free from aberration. There's a reason good telephoto lenses are so expensive! Unlike the finderscope, quality telephoto lenses contain multiple lens elements which, in combination, correct for the spherical and chromatic aberration seen here. I cropped out the majority of edge distortion and ended up with the next Pleiades image.

In the cropped picture above you can see how individual star images are slightly out of focus and bloated. You can also see a blue fringe on stars from chromatic aberration. On the positive side, the seven-image stack actually captured some silvery reflection nebula around two of the stars! The image scale also nicely includes the entire cluster!

I next tried capturing stars of Orion's belt with a stack of ten 2-second unguided exposures. The same edge aberrations were present in these full frame images, so I cropped out the edges to leave the following central region, slightly more than three degrees across.

The Flame Nebula and Horsehead Nebula are slightly visible on the left near the left belt star, Alnitak.

It's hard to point a camera at Orion without trying at least one image of the Orion Nebula, M42. The final cropped image below is just a single 2-second exposure. Notice blue fringes on stars due to chromatic aberration.

I really like the image scale of these pictures, but the finderscope/camera combination is really impractical. I don't think I'll try this again. Perhaps I'll buy a real 300 mm or 400 mm telephoto lens sometime in the future. For now, I'll look forward to using the camera with my 130 mm refractor for some guided exposures.