Advice for successful star trails.

[pthornto]

I would like to take some long exposure/star trails photos during the next new moon. I was planning on using Tri-X. I'm wondering:

1. What settings for exposure would give me the best chance of success. I'm thinking of stopping down to f5.6 or 8 and using a bulb exposure for a few hours.

2. Should I plan on giving the film more development time (say +20%), since any extra contrast can only help.

3. Also what sort of reciprocity failure should I expect out of Tri-X? Will it severely lose speed after a few minutes?

Any thoughts or experiences with your star-trail exposures are greatly appreciated!!

Paul

 

[mts]

The f-number is effective for extended objects, but not for point sources (stars). To record nebulosity for example the Milky Way you need a fast lens. To record star images the f-ratio isn't important, but the clear aperture defines the faintest stars that you will record because larger lens diameter collects more photons. Star trails are sort of in-between the two because the point source that is being recorded moves owing to Earth rotation. For a standard ~50mm lens on a 35mm camera the motion is discernable for exposures longer than about 30 seconds.

If you stop the lens down too far you will reduce the light gathering power to the extent that you will record only the brightest objects. If you are planning a few hours exposure then you have no choice but to stop down to ~f/5.6 or f/8 to reduce fogging by sky brightness which is an "extended" object. Pushing the film in processing will indeed increase its speed and increase gamma (contrast) but at the expense of chemical fogging. You will probably find better results using a high-contrast developer (D-19 is the old standby) instead of one optimized for pictorial work like D-76, Xtol, or TMax developer. Typical processing time for D-19 is 4-6 minutes at 20C for pretty much any emulsion you choose to use.

Owing to reciprocity failure you won't gain much if anything by using faster film than Tri-X. I have considerable experience using P3200 for photographing image tubes but the subject matter (faintly glowing green phosphor) falls into the category of extended objects. Tri-X is a good choice for star trail photography. Remember that reciprocity effects are in proportion to the exposure time to light, and consider that owing to Earth rotation that your effective exposure (neglecting sky background) is perhaps 30 seconds because the star image moves to new previously unexposed emulsion in about that time.

Reciprocity effects are important for long exposures made with continuous light exposure as for example photographing a faint extended objects (nebulae) or faint stars when using a tracking mount. You will obviously suffer some reciprocity failure owing to sky background, but it is not particularly noticeable in star trail images. If it were an important effect the star trails would become fainter as the night evolves!

Have fun! Open the shutter and go off for tea or a snooze and return to close up later (well before twilight). Making star trail images is easier with LF because the lenses are typically f/4.7 or thereabouts and the format lends itself to making and processing single exposures. The advantage of 35mm format is however evident if you have available to you a fisheye lens.

 

[pthornto]

Thanks for such a detailed response. I'm glad that Tri-X will work okay as this is what I'm familiar with, although I've never thought of using D-19 as a developer. Now I just need clear skies to coincide with the new moon!

 

[mts]

Sample images

Here are two examples of untracked (tripod) images, both processed in TMax-clone developer. One image was made using P3200 film as a 10 sec. exposure using an 85 mm f/1.2.

The other is a 30 sec. exposure made on Tri-X with a standard 50 mm f/2 lens. This 30 sec. exposure shows slight elongation of the star images owing to diurnal rotation. The camera is a Minolta-7000.

As the comet (Hyukatake) is an extended object you can see the performance of the faster lens records considerably more. Likewise the faster film improves the image but at the expense of considerably more grain.

At the time I made these images in 1996 I had no D-19 mixed and therefore used what I keep on hand, viz. TMax-clone (formula posted elsewhere in a recent TMax developer thread).

 

[Poisson Du Jour]

The deeper the aperture, the more star trails you will see. My stock choice is Bulb/f5.6 (reversal film set to —0.6 stop to enhance contrast) with a 24mm ultra-wide lens) for 1hr 45 to 2 hours, then a pause before the camera does the next set (using intervalometer): I "set and forget" and go to bed. New Moon is ideal, or a waxing crescent no brighter than 2%: a waning/waxing moon will turn night into day over a long exposure. Atmospheric haze, bushfires or smog from cities can stuff up the exposure. Drifting clouds similarly will record as blurs, so you quite literally need to have the stars and moon align for a perfect, clear, cool (or cold, as here!) night.

I wouldn't be too concerned about Reciprocity; all films will show it at extended exposures, reversal film manifesting as a magenta or green cast (tungsten film will be stable). You're only doing it for fun, not critical to a point: people shouldn't be concerned if the image comes out black or blue or pink or green. You are allowed to have fun! Personally I like the magenta effect of Provia 100F but also the cool blue of tungsten reversal film or the eerie green of Velvia. Whatever film you run through though is a very personal choice.

Ensure you and the camera are safe wherever you go and that the camera, on the tripod, is not in the way of places where people might go wandering at night in popular areas (as is human nature), or animals. In Australia, kangaroos are the major menace, bumping into the tripod and ruining exposures. So I look for elevated (granite) plateaux where roos don't normally go (they can't see the edge at night and might tumble off). Wherever you go, take a chair, a book or an iPod (keep any light back from the camera) and while away the time as the camera goes through the exposure. Have fun!