Piggyback Astrophotography Lenses

Most any lens will work for Piggyback astro-imaging. Single focal length lenses are better than zoom. However, some single focal length lenses are better than others. How good your images look will depend on your lens choice and your choice of f-stop. Imaging pinpoint stars across the full field of a 35mm negative is a very difficult thing to ask of a lens. Typically, faster lenses will have a tendency to show optical defects unless stopped down a little. Here are some examples of these defects and how to correct them:

• Vignetting:

  This defect shows itself on an astrophoto as a brighter area at the center of the image with darker corners. This defect is typically found when lenses are used wide open. The solution is stop down the lens a stop or two (or three for a really fast lens). This makes for a longer exposure, but imaging faster is no bargain if the image is poor. Here is an example of my Olympus 50mm f/1.4 lens at various f/stops. Notice the vignetting reduction as the lens is stopped down.

  Click Here for image

   

• Corner Optical Defects:

  This defect shows itself often in very fast lenses. Typically, a person will want to image with a lens wide open to build up an exposure as fast as possible. Lenses vary in this regard, but often this is the wrong choice. Here is an example of my Olympus 50mm f/1.4 lens at various f/stops. I have zoomed in to the star images at the corner of the negative. Notice that the star images sharpen up as the lens is stopped down.

  Click here for image

   

• Chromatic Aberration:

  This defect causes different colors of light to focus at different distances from the lens. Typically, lenses have problems focusing blue light at the same place as the rest of the colors. This affects longer focal length telephoto lenses more than shorter ones. Expensive "ED" lenses are better at correcting this. Typically, this defect will show itself worst when the lens is wide open. Here is an image taken with my Olympus 180mm f/2.8 telephoto lens at f/2.8 and at f/4 . Notice that the blue halo around a bright star is much reduced when the lens is stopped down.

  Click here for image

   

The latest crop of 400 and 800 speed color negative films (as of 1/02) is a mixed bag for astrophotography. There is a disturbing trend emerging in some of the current reformulated film emulsions. Fuji is migrating to a "4th color layer" technology. Their Superia 400 and 800 speed films already have it. A comparison of spectral sensitivity curves shows that the layer that produced red color has had its peak sensitivity shifted from about 650 nanometers to about 625 nanometers. This apparently better mimics the sensitivity of the human eye. But, it is a disaster for astro-imaging. The all important hydrogen alpha emission line is at about 656 nanometers. This new breed of film does not record it at all. The effect is weird. An image with the new films show all the stars the same as before, but red nebulosity has vanished!

Kodak has also introduced some films that have downshifted the red spectral sensitivity peak away from the h-alpha line. The MAX 400 is particularly bad. Here are links to spectral sensitivity curves from Kodak's web page that show a good h-alpha recording film (LE 400) vs. a bad one (MAX 400). Note the peak of the "cyan forming layer" that records red color is much lower on the MAX 400.

• LE 400 Spectral Sensitity Curve
• MAX 400 Sensitivity Curve

Agfa Vista and Futura films were not tested, because their spectral sensitivity curves indicate they would be poor choices. Red sensitivity has been downshifted away from the h-alpha line. In fact, Agfa makes a big deal about this shift (as an improvement). It probably is an improvement for regular photographs...but not astrophotographs!

Poloroid high-definition 400 speed film (made by Agfa) is, by far, the worst film tested. It not only records red and blue nebulosity poorly, but images stars much less deeply than all the others. And to boot, the bright stars that are imaged have a bright blue halo around them not visible in the other films tested. Yuck!

Here are the current 400 and 800 speed films that have a good h-alpha (red nebula) response They are listed in order of preference:

• Kodak LE 400
• Kodak Supra 400
• Kodak Royal Gold 400

Here are the films that have a moderate to marginal response to h-alpha (in order best to worst):

• Fuji NGH II 800
• Fuji NPH 400
• Fuji Superia XTRA 800 (old - no 4th color layer)
• Kodak Portra 800
• Kodak MAX 800
• Kodak Supra 800

Here are the films with essentially no h-alpha response:

• Kodak Portra 400 NC
• Kodak MAX 400
• Fuji Superia XTRA 800 (new - with 4th color layer)
• Fuji Superia XTRA 400 (new - with 4th color layer)
• Poloroid High Definition 400

The blue nebula response differences are more subtle. There are some differences, and the films with the best red response are not the best for blue.