Prime focus photography experiments

One sunny Saturday in October 2007 I decided to do some simple comparisons of my three refractor telescopes when used for “prime focus” photography with my Canon EOS 350d DSLR camera. I pointed each telescope out of an upstairs window and photographed a chimney and TV aerials on a house about 200 to 250 feet away. This web page shows the results.

All these pictures were taken at ISO 100 and with “daylight” white balance. Each image is presented twice, a full frame and a crop of the centre portion. Each of the pictures on this page links to a larger version.

6″ f/5 achromatic refractor

750mm focal length, 150mm aperture, f/5, 1/2500 sec.

750mm focal length, 150mm aperture, f/5, 1/2500 sec. With Baader “fringe killer” filter.

This picture shows the colour cast from a Baader “fringe killer” filter. This cast is not objectionable when viewing astronomical objects, but is obviously a problem in terrestrial use.

750mm focal length, 150mm aperture, f/5, 1/2500 sec. With Baader “fringe killer” filter. Post-processed to restore colour balance.

Here is the same picture, but with some post-processing (using Canon “Digital Photo Professional” software) to restore the subjective colour balance, by boosting the blue gain by about 50%. I think this picture is sharper than the first one, but that might just be better focusing.

~520mm focal length, 150mm aperture, f/~3.5, 1/4000 sec. With Antares f/6.3 focal reducer.

A daytime focal reducer test fails to answer the two most important questions — “will it focus at infinity?” and “how flat is the field?”. However, it does show that the focal reducer is working(!) and not introducing too much vignetting. This picture is rather soft, maybe because it was out of focus. The depth of field at f/3.5 will be quite small.

Measuring the pictures suggests a focal reduction ratio of 0.70, rather than the specified 0.63. This suggests the focal reducer needs to be about 15mm further from the camera, so next time I’ll try it with an extension tube.

80mm f/7.5 apochromatic refractor

600mm focal length, 80mm aperture, f/7.5, 1/1000 sec.

~420mm focal length, 80mm aperture, f/~5.25, 1/2000 sec. With Antares f/6.3 focal reducer.

Unlike the earlier attempt to use the focal reducer, this image is still quite sharp. Perhaps using a reducer designed for an f/10 SCT on an f/5 refractor is just too extreme.

66mm f/5.88 apochromatic refractor

388mm focal length, 66mm aperture, f/5.88, 1/1600 sec.

~272mm focal length, 66mm aperture, f/~4.11, 1/2500 sec. With Antares f/6.3 focal reducer.

Once again, the focal reducer has made the image a bit soft.