Some notes about SLR lens performance in astronomy imaging

When it comes to astrophotography with a digital SLR and on-the-shelves lenses, few questions arise : what exposure time, what f/stop, will it focus as sharp as my good old analogic (film) equipment, what ISO setting, aso... The story that follows is the story of the tests I did when trying to answer these questions.

Each design is unique. Each lens is unique.

First, we will highlight how, on many lenses, real sharp infinity focus is not achieved by setting the focusing ring at its end of course or on the infinity mark. Instead, it is somewhere else. Thus the long known knife edge focusing technique used by prime focus photographers. For lens photography an autofocus on a bright star (mag 0 or brighter) gives decent results. Whether the autofocus gives the sharpest focus is not covered here.

Then we will show the impact of the f/stop setting on image quality. For each diaphragm setting, image quality at the center of the field and in the corners will be shown.

On this lens, the focusing ring end of course is on the infinity mark. Although setting the focus there was giving good results on film (with a Nikon FM2), it does not any more with a D200.

The two images below show the focusing difference between the end of course and the auto-focus setting.

Images are full resolution crops of the 10 MPix Nikon D200. Camera settings : ISO400, f/2.8, 2min, no substraction of any dark signal. The bright star is alpha centauri on which auto-focus was performed.

The 8 images below show the impact of f/stop on the photograph, both in the corner and the center of the frame.

All images ISO 400. Exposure times : 30sec, 1min, 2min, 4min.

The full resolution crops above are located as shown in the image. This a non processed image : the pinkish zones on the outer region of the image are thermal noise induced by detector electronics.

All images above taken during a trip to the French island Réunion in the Indian ocean in June 2006. The camera body is a Nikon D200.

On this lens, the focusing ring end of course is beyond the infinity mark. Oddly, the D200 needs a few seconds to perform the auto-focus operation on this lens, thus enforcing the need for the camera to be firmly mounted.

The three pictures below show the focusing difference between the end of course, the infinity mark, and the auto-focus setting.

Images are full resolution crops of the 10 MPix Nikon D200. Camera settings : ISO400, f/2.8, 2min, no substraction of any dark signal. The bright star is alpha centauri on which auto-focus was performed.

The 8 images below show the impact of f/stop on the photograph, both in the corner and the center of the frame.

All images ISO 400. Exposure times : 30sec, 1min, 2min, 4min.

The full resolution crops above are located as shown in the image. This a non processed image : the pinkish zones on the outer region of the image are thermal noise induced by detector electronics.

All images above taken during a trip to the French island Réunion in the Indian ocean in June 2006. The camera body is a Nikon D200.

Astro-photography is maybe the most discriminating application for lens quality. Distant stars, as seen from Earth due to their distance, are perfect dots, perfect two-dimensional diracs. We expect to have them represented as such on any night sky picture. As you may have noticed in the above images, this expectation is far from being fulfilled.

The bird-like or potatoe-like aberrations are the drawbacks of luminous lens designs. They are clearly revealed on starfield images, and wll also impact daylight images taken with the same lens/settings.

For autofocus operation, best results where reached with the camera firmly mounted on a tripod (or equatorial mount) so that the star on which the focus is performed is steady on the collimator. Obviously having luminous lenses such as f/1.4 helps a lot.