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DPChallenge Forums >> Tips, Tricks, and Q&A >> astro / night sky photography
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09/29/2012 08:11:12 AM · #1
off to majorca in two weeks and will be taking advantage of rural location and no light polution to try my hand at some night sky photography

anyone geet me started on this

whats the best lens, aperture etc, trials or no trails, filters etc

taking my carbon fibre manfroto and gitso ball head. cable release and 5dmk2, L's 16-35,28-70,70-200 and probably 50 1.4 as well

after shots like this

//500px.com/photo/3588436

//500px.com/photo/10197685
09/29/2012 11:12:05 AM · #2
I was taking photos of the Milky Way a couple weeks ago with my D40. Some of the problems I encountered:

- not wide enough, even at 18mm.
- too much motion, even at 18mm.
- focus shift with AF lens set to MF

With your full-frame camera, wide-angle shouldn't be a problem. Your camera + 16mm is 96x73 degrees field of view. My camera + 18mm is 66x46 degrees.

You have two advantages and one disadvantage with your setup when it comes to motion:
- you'll be able to use a higher ISO than I could; thus, faster shutter
- the wide-angle lens/full-frame setup will also help, since there is a higher area of the sky recorded so each pixel sees "less"
- the higher resolution may make some motion more apparent, though scaled down to a more viewable size like HD resolution (or printed on something smaller than, say, 18x24"), this should disappear.

If you use a high ISO and want cleaner pictures, see if your camera has built-in long-exposure noise reduction; this captures your frame, then a frame with the shutter closed to get a "map" of the sensor noise at (almost) the same time as the actual shot, then subtracts that noise from the original image. This may or may not be available on Canons, and it may only work for JPEGs.

Another thing you might want to look into is focus accuracy at infinity. With many MF lenses, this is as simple as turning the focus ring to infinity, but with new, AF lenses, turning it all the way will most likely focus "past infinity" and give you blurry images. If the AF system on your camera won't find something to focus on in your shot, it might be a problem. Keep in mind that focusing on something, then recomposing, then switching it to manual focus and firing off shot after shot may cause the focus ring to shift and cause you to only get a few in-focus shots. This can be bypassed by a "switchable" AF-lock on the camera (something like "press AF lock to permanently lock focus, then fire off shot after shot with the focus fixed at whatever point).

If you're trying to shoot nebulae or other small features (e.g. Jupiter and its moons), you'll want a very fast shutter speed, meaning you'll want a high ISO. I doubt the 70-200 would be able to zoom in close enough, but with your 21 MP, you might be able to get a shot decent enough for you to brag about, but not clear enough for even a desktop wallpaper.

Finally, a note on sharpness, if you're really looking for the "sweet spot":
You'll want to balance the focal length, ISO, shutter speed, and aperture. All four can cause lack of clarity. Unless you're a statistician, it will be very hard to find the "sweet spot" for the four of them, while still getting enough light for a decent shot, by any other technique than trial-and-error. Remember, you'll probably prefer edge-to-edge good sharpness rather than excellent center/poor edge sharpness. Check out SLRGear's test for a visual representation for all focal lengths and apertures.

WARNING - THE FOLLOWING SECTION IS TECHNICAL AND NOT FUN:
If you really care about resolution that much and want to calculate the very best combination of settings, you have to first choose a focal length, then find out one set of camera settings that will give you a good exposure, then find the values for the following variables:
- maximum motion (lines/picture height) of the elements in the sky, at your focal length, for each shutter speed; this can be calculated as follows: 15*S/SV, where S is your shutter speed in seconds and SV is the "Sampling" value (arcseconds per pixel) for your camera and focal length, from this calculator: //www.howardedin.com/articles/fov.html. This is the maximum number of pixels that will be covered by an object moving in the sky at that shutter speed.
- resolution drop because of noise (lines/picture height) at each ISO for your camera - you can estimate this by looking at the images under Canon EOS 5D Mark II Camera Test - "Multi Target". Take the resolution value in height for your sensor (3744), and divide it by the "LPH" value where the curved lines become a jumbled mess at each ISO. This is the number of pixels that should be indistinguishable from each other.
- resolution (MTF; lines/picture height) at each aperture for your lens at the preferred focal length - 16-35mm L II. Also divide your sensor height (3744) by this, and again you should get the number of pixels indistinguishable from each other for the focal length.
Then, make a chart of sets of values that would give you the correct exposure (i.e. if your preferred exposure is 1 sec, f/5.6, ISO 1600, then include 1/2 sec, f/5.6, ISO 3200 and all other equivalents), highlight the highest value for each setting, and choose the set of settings with the lowest highlighted value.

Message edited by author 2012-09-29 11:16:43.
09/29/2012 11:59:02 AM · #3
This guy has some books (on CD) on astrophotography, including one for beginners, with some sample sections posted. It might not be suitable for your travel situation, but he has instructions for building a low-cost "barn-door tracker" for making long exposures without star trails.

There is also An Astrophotography Primer right here at DPC.
09/29/2012 12:27:19 PM · #4
16mm 30 second shots seem to be a sweet spot for exposure versus motion blur. The higher the focal length, the shorter the acceptable exposure. Use ISO to taste. Stacking shots is an advanced techniqe that works, but don't think ou can stack them manually. You can't. Find an app that will tell you when and where the Milky Way will be. If you can do it while the moon hasn't risen or is thin, that's good. Look for an interesting foreground object and paint with a flashlight.

There you go. Stream of consciousness advice. I really enjoy shooting wide field astrophotography.



Message edited by author 2012-09-29 12:29:41.
09/29/2012 12:32:28 PM · #5
yeah it has noise reduction and i think with a few bottles of wine and the warm air i wont mind waiting on them if memory serves me right it only kicks in on exposures over 2mins but i might be wrong.

yeah flash light and 550 ex will be going with as well :)

09/29/2012 12:32:57 PM · #6
For no-trails, you really need tracking for best results, but George's post gives you a good basis for getting the best results you can without tracking. I would only add that you can improve your results by taking multiple shots and doing image stacking. The stacking is best managed by software designed specifically for that, but can be handled manually if need be. Each shot needs to be rotated and/or translated depending on where in the sky you are aiming.

ETA: Focus is critical. Set manual focus and lens wide open. Get a bright star near center of frame and use live view at maximum magnification, or shoot a 10-second exposure and tweak focus until you have a sharp point. Now set the aperture you want, normally a stop down from maximum to get better corner performance, and shoot some test shots at your desired exposure. Review center frame and corners. Tweak settings if needed. Lather, rinse repeat.

Message edited by author 2012-09-29 12:37:11.
09/29/2012 12:35:27 PM · #7
my uncle who got me into photography had a home made tracking head for his tripod but cant really lug it to majorca, i'll have a play and see what i can get.
09/29/2012 12:47:50 PM · #8
Originally posted by DrAchoo:

Stacking shots is an advanced techniqe that works, but don't think you can stack them manually. You can't.

Registax for stacking images.

The home-made tracker in the earlier link I posted looks like it could fit in a small suitcase without taking up too much room or weight.
09/29/2012 01:24:23 PM · #9
Eventually I think I'll invest in one of these. The original had a good reputation, and supposedly the current model is even better. Small enough to pack in a suitcase.
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