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View Full Version : Photography with static tripod: ISO -vs- shutter speed -vs- aperture?



starsRgreat
2010-Aug-11, 06:51 PM
Is there a general recommendation here? I've got a Tamron 400/4 lens mounted on an Olympus E-3 (so 800mm effective, or 3 degrees field of view), which works fine up to a point, but I can still see objects moving through the frame as I try and line things up.

I'm using a normal (not-moving) tripod, so I can't track objects; are there any best practices for the way to get the best results under these conditions?

If I increase shutter speed, there's less motion, but the image is darker. So I brighten it up after the fact, but then it's noisier. So I reduce ISO, but that then slows the shutter speed down, and I get more blur from motion.

And all of this is ignoring possible changes in aperture; presumably for that I want the widest aperture which'll retain sharpness, because depth of field isn't really an issue?

Sorry, this is a fairly general question, and there may not be a simple answer, but any tips would be much appreciated.

RickJ
2010-Aug-12, 12:49 AM
Now you know why astro imagers don't use that type of setup. It doesn't work except to photograph the moon. Everything else is far too dim. The longer the focal length of the lens the more it magnifies the earth's motion. A 50mm lens equivalent is about the longest lens you can use on a static tripod for a single exposure of a starfield. With proper stacking software you can use a short focal length lens and take many short images (15 seconds or so and then let the software align the stars when they are all stacked. This will increase signal faster than the noise so you get a much better result. Still DSLR cameras generate a lot of noise so there's a limit here but it will help.

Better would be to build a barn door tracker. Cost is nil. Google the term for lots of ideas here. But again this is limited to rather short focal length lenses. 800mm really requires an equatorial mount and guiding telescope as only the very expensive ones (many thousands of dollars) can track 800mm for more than a minute or two without guiding.

Rick

starsRgreat
2010-Aug-12, 03:12 PM
So fundamentally it's the lack of tracking that's causing the problems? A barn door tracker looks like it should be easy enough to make, and I have hinges and threaded rod and stuff sitting around to try that with -- and I'll look into stacking software, too.

Thanks!

RickJ
2010-Aug-13, 02:36 AM
It is good for a 50mm lens equivalent, maybe 135mm with a two hinge model but certainly not an 800mm equivalent. That takes guiding. Back about 1960 I built a two hinge (called double door back then) model driven by a wind up alarm clock I tore apart (1920's model likely worth something as an antique if I hadn't destroyed it) that was set up with adjustments that allowed crude guiding. I got it to work with a 400mm lens for one image after a year of hair pulling. Never worked again however. I moved to an equatorial mount.

Another solution is a Poncet platform (table). Two axis ones are available commercially for Dobsonian telescopes for deep sky photography. You could set a small tripod on one that would allow you to point anyplace in the sky, add a small scope parallel to the camera for guiding.

The earth turns such that at the celestial equator stars move 15 seconds of arc per second of time. An 800 mm lens can easily resolve 3" of arc so you would be limited to a 1/5th second exposure before stars streaked!

Whatever your tracking platform it must be polar aligned or stars will streak. The more accurate the longer it will track. I found that with a 50mm lens I could just "eyeball it". Look at the hinge and judge it was within a degree or two of pointing at the pole. This would be fine for 10 minutes with a 50mm lens. Polar aligning for that 400mm lens was nearly impossible and one reason I failed so much with it.

Don't forget you can also take 15 second images with a 50mm lens that is stationary and then align and stack them into the equivalent of a much longer image. Not using a DSLR I'm not familiar with what software is out there for RAW format (don't use a lossy compression like JPG for this!!!). I believe Deep Sky Stacker would work and is free last I knew. Google the term. If using a 150mm lens then you'd have to use 5 second exposures for the same ending resolution but since that lens resolves finer detail even shorter would be needed making 50mm equivalent about the max focal length for this mode.

Rick

Arnold Rimmer
2010-Aug-14, 11:52 AM
I was wondering about the exact same thing as starsRgreat, so thanks for your answers RickJ. After some searching on how to build a barn door tracker I found a couple useful guides among very many.

There are many guides out there that makes things much too complex for my use (Digital SLR and max 80mm lens). The simpler build the better and the more acurate polar alignment the better seem to be the way to go.

This guide is what I settled on:
http://www.eskimo.com/~rachford/widefield/barndoor.html

I'm sure it will be a fun and rewarding project! Gonna go buy the materials on monday! :)

starsRgreat
2010-Aug-16, 06:21 PM
This guide is what I settled on:
http://www.eskimo.com/~rachford/widefield/barndoor.html


Thanks for the link! I'd done a lot of searching around as well, but haven't seen that link before -- it looks like there's some ingenious solutions to the most common problems, which still remain actually possible for the average tinkerer..