View Full Version : Still not quite understanding, info appreciated.
26th of February 2009 (Thu), 10:03
I've done the searches, thanks in advance though :)
Can you guys help me get a better understanding of exactly what I need to get shots of the Milky Way rising up over the horizon line? I am talking about the wide angle stuff here with only my camera guys, no scopes. I have done plenty of star trail photos so far and it's really fun, but now I would like to cancel out the rotation and track. I've read about GEMs, motor drives, etc and it all has me a bit confused.
If you could provide a link to any hardware mentioned that would be great too, thank you!
This is exactly the type of shot I am talking about...
26th of February 2009 (Thu), 11:45
In short you need something that will follow the movement of the stars across the sky. This can be a home made barn door tracker and motor drive right up to a german equatorial mount costing many $. Either way the mount needs to be set so that it moves in near perfect sync to the rotation of the stars, for us northern hemisphere folk, normally by aligning on a spot near Polaris, the celestial pole, the point at which the rotation of the starfield takes place. Download a copy of Stellarium (Freeware) and you will see how it all works.
How accurately you need to track will be dictated by how long an exposure you want and how big a portion of the sky you want to image. For widefield exposures you wont need as good a tracking as say imaging a part of the sky only a few arc seconds across.
The best device I have come across for a camera/lens combination is an Astrotrac. This will give you perfectly good tracking for up to about 20 minutes for widefield exposures. Note that unless you have good dark skies you will hit the sky glow limit (the point at which the background light from the sky starts to have an impact) long before that. Most of us therefore stack a number of much shorter exposures on top of each other.
26th of February 2009 (Thu), 14:00
In short you need something that will follow the Note that unless you have good dark skies you will hit the sky glow limit (the point at which the background light from the sky starts to have an impact) long before that. Most of us therefore stack a number of much shorter exposures on top of each other.
I already do that exact process for my star trail shots. Sometimes up to 200-250 shots of about 30 seconds each, stacked on top of each other. Seems to really help fight the light pollution as I live in a very suburban area. I'm hoping to drive out to the local desert once I get my tracking setup finalized.
And thank you for the info on Astrotrac, I will check it out and see if it's budget friendly or not.
26th of February 2009 (Thu), 15:29
Thanks for asking this. I was just coming here to search for the exact same thing. Hopefully there are a bunch of replies letting us know pro/cons of certain devices.
In one of my other posts Adrena1in suggested a Barn Door tracker or an EQ1. I found these at Amazon which will allow a camera to be able to track.
Orion EQ-1 Equatorial Mount
Orion EQ-1M Electronic Drive System (to auto track)
Orion 1/4-20 Adapter for EQ1 Mount (for camera)
I also bookmarked a page at home last night that had a link to a small and less costly device.
Here it is, thanks to Umbra:
Taken from this page:
Counter Earth Movement (http://photography-on-the.net/forum/showthread.php?t=649519)
27th of February 2009 (Fri), 02:17
I already do that exact process for my star trail shots. Sometimes up to 200-250 shots of about 30 seconds each, stacked on top of each other.
It's not quite exactly the same as you do for your star trails. For star trails you'll stack with the foreground staying constant and add the bright part of the star trail from each frame so that you build up a line (curve) from 200-250 star dots.
If you are stacking to get a milky way shot, all of the frames are aligned with the stars from each frame on top of each other so they become a brighter dot rather than a line. The foreground gets blurred but you can mask that out later.
If you took one of your star trail exposures and zoom right in on a bright star, you'll see how elongated it is compared to its width. Use that to determine the length of each sub-exposure for the tracking setup you have (if any). For example, if you took a single 30 second exposure and the brightest star showed as being three times as long as it is wide, you could use multiple 10 second exposures and get good round stars in your image.
11th of March 2009 (Wed), 22:30
Great info here guys, thank you! I bought the system Mac linked to. Just got it tonight so I will try it out and report back.
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