Astrophotography: Estimated Exposure Times

The following are very rough estimates for exposure times in astrophography, asuming you are using ASA 400 film. Your results will vary depending on the film you are using (ASA or "speed"), the clarity of the sky, and the brightness of the sky background (moonlight and light pollution). To ensure a good result, you should bracket your exposure: take 3 exposures of each object, one with the recommended exposure time, one with 1/4 the recommended time, and one with 4x the recommended time. Chances are, one of the three will come out with the proper exposure level!

I. Tripod w/ 50mm lens on 35-mm camera:

Object	Exposure Time	Comments
Moon	1/250 sec	Longer for crescent, shorter for full.
Constellations	5-30 sec	Gives pin-point stars.
Star Trails	60-3600+ sec	Longer exposure needs larger f/stop# (22 rather than 2) to cut down sky background. Increase f/stop one # for every 2x in exposure time.

II. Piggyback w/ zoom lens on 35-mm camera:

Object	Exposure Time	Comments
Moon	1/60 sec	Longer for crescent, shorter for full.
Constellations	5-120 sec	Too long and the sky background fogs.

III. Prime Focus (35-mm camera behind 8-inch telescope):

Since our telescopes don't track very well, we can't do long exposure pictures (the images would "trail"). However, we can do short exposures on brighter objects. The field of view is much narrower than with the piggyback/zoom lens combo -- the telescope is like a super-zoom lens -- so this option is good for closer up pictures of the moon, planets, and bright stars. The moon is bright, so your exposure time will be short and you can even use the exposure meters visible as you look through the viewfinder (center the moon and adjust the exptime until the needle at far right of the viewfinder is horizontal). I don't have much experience with the exposure times below, so you may want to bracket with 5 exposures (1/16, 1/4, 1x, 4x, and 16x the suggested exposure time) to ensure that one is optimally exposed.

IV. Prime Focus (35-mm camera behind 0.5-m telescope, no lens):

Object	Exposure Time	Comments
Moon	1/250 sec	Longer for crescent, shorter for full.
Venus	1/1000 sec	Varies a lot with orbital position -- bracket!
Mars	1/500 sec	Varies a lot with orbital position -- bracket!
Jupiter	1/250 sec	Good for bands, 1/4 sec for moons.
Saturn	1/60 sec	Good for rings, 1 sec for moons.
Deep Sky Objects	60-600 sec	The longer the better, though eventually sky background will fog and telescope tracking errors will blur star images. Experiement and bracket!

A note about film speed (ASA #): if you are using a film with a speed other than ASA400, you can adjust the exposure times shown above:

• Example #1: If you are using ASA 100 film, your film is 400/100 = 4 times slower than ASA 400 film, so you should make up for this by increasing all the exposure times listed above by a factor of 4. If the table suggests you use a 1/60 sec exposure, increase it to 1/15 sec.
• Example #2: If you are using ASA 800 film, it is 400/800 = 1/2 times slower (or 2 times faster) than ASA 400 film, you should make up for this by decreasing all the exposure times listed above by a factor of 2. If the table suggests you use a 1/60 sec exposure, decrease it to 1/125 sec.