Problems with star trails.. help?

[jrm27]

Hi all:

So, last night I set about doing some star trail shots as the stars were beautiful out my back door. I live in a very dark area and set up my XSi on my tripod with the kit lens. I set it at f4, ISO400 and 30 second exposures. I tethered it to my netbook and told it to take 150 exposures. I live in a valley between two hills, so the picture was looking up the valley walls, over a ridge to a really nice night sky. There were a few houses about 3 miles away on top of the ridge with the lights on (quite low) that I thought would help the composition.

For the first 45 or so pictures, the detail was pretty good. In the first picture the stars are very clear and noticeable. However, as I got towards picture 50 sky detail was beginning to be lost... by that I mean that on my picture stars are much less visible than in the first shot. By the time I got to picture 100, it seems that there was no sky detail and the light given off by the houses was just a big bloom of noise.

Can you help me understand what would cause this loss of sky detail and how to avoid it on my next time out? Is that sensor heat building up and affecting the detail? I figured the XSi could handle at least an hours worth of pics... maybe not? I'd post pics, but we've just moved and don't have internet at our home yet (posting from work.... shhhh, don't tell anyone!).

Little help? Thanks!

-jon

[F-117HWK]

Maybe I'm confused, but if you are going for star trails, wouldn't you set the camera to Bulb and leave the shutter open for 15min or more? Not take a bunch of pictures? I guess I'm confused as to the point of multiple shots.

[VIGER]

You got condensation buidup on you're lens or the cold got the infinity point to change.

http://photography-on-the.net/forum/showthread.php?t=753272



: )

[jrm27]

hmmm... yeah, that might be the reason. I tried to keep a lookout for condensation on the lens and didn't see any. Perhaps I'll need to pay closer attention.

Can you help me understand how the infinity point would change due to cold?

Thanks for replying, much apprecaited. The shots you took in the link you provided are great!

[VIGER]

Cold expands metal therfore focal lenght thus focus is out of yank ... ;) That's why some telescope are made out of carbon fiber. The problem is with the lens. Many the focus moved on vibration. Crawls? Tape it pehaps.

Also F5,6 would possibly give you a sharper image.

What lens are you using?

[jrm27]

Thanks Viger, just the kit lens with the XSi (18-55). I had it 18mm (so roughly 28mm on a crop). Lens crawl is a good point. It was pointed at a pretty extreme angle. But since it wasn't zoomed our anything, I don't know how it could crawl any wider. Or perhaps I am missing the intent of the term?

Any other input anyone (viger included)? Thanks!

[jrm27]

anyone else have anymore thoughts?

[Adrena1in]

Any chance of posting some of the images here? Dew does sound like the most likely cause, (I've left the camera doing it's thing before and 50 minutes is all I can get in cold conditions before dew phugs up the lens. See this animation I did a while back (http://www.vimeo.com/2336508).

Focus crawl is something I suffer from very badly with my nifty fifty, so it does happen. Just the vibration of the shutter can cause the camera to lose focus in only 5 or 10 shots. (Very annoying the first time I left it to take lots of exposures, and came back to find the first few were focussed, but the rest weren't!) Didn't have the problem with my own 18-55mm lenses, but I guess they're all different. With AF off does the focus feel very loose?

[jrm27]

Actually, i think I might have deleted the whole set of pics... I'll see if I can recover them and post anything!

I'm planning on trying again this week when the clouds go away, so I'll keep an eye out for dew. I'll also keep an eye on focus creep. The focus doesn't feel very loose, but admittedly, I'm not wuite sure what it should/shouldn't feel like. Is there a way to combat focus creep if that is indeed the case?

[MintMark]

People suggest using a little sticky tape to hold it in place, but I've never tried it. I always thought it would be hard to apply the tape without moving the focus ring.

I'll have to check a set of images I took a while ago... I thought they were all out of focus, but now I'll look to see if they start in focus and get worse.

[Extinctionjet]

Hi all:

So, last night I set about doing some star trail shots as the stars were beautiful out my back door. I live in a very dark area and set up my XSi on my tripod with the kit lens. I set it at f4, ISO400 and 30 second exposures. I tethered it to my netbook and told it to take 150 exposures. I live in a valley between two hills, so the picture was looking up the valley walls, over a ridge to a really nice night sky. There were a few houses about 3 miles away on top of the ridge with the lights on (quite low) that I thought would help the composition.

For the first 45 or so pictures, the detail was pretty good. In the first picture the stars are very clear and noticeable. However, as I got towards picture 50 sky detail was beginning to be lost... by that I mean that on my picture stars are much less visible than in the first shot. By the time I got to picture 100, it seems that there was no sky detail and the light given off by the houses was just a big bloom of noise.

Can you help me understand what would cause this loss of sky detail and how to avoid it on my next time out? Is that sensor heat building up and affecting the detail? I figured the XSi could handle at least an hours worth of pics... maybe not? I'd post pics, but we've just moved and don't have internet at our home yet (posting from work.... shhhh, don't tell anyone!).

Little help? Thanks!

-jon


make sure the iso is as low as possible to eliminate noise and make sure its trigger release and set to it on the camera, maybe if cloud apear they can add noise or even fog, look closely, dont take your eyes off it. also a intervallometer may help.

[Adrena1in]

People suggest using a little sticky tape to hold it in place, but I've never tried it. I always thought it would be hard to apply the tape without moving the focus ring.

I need to do that with my nifty fifty the next time I want to take a lot of shots, as the focus is so sensitive.

Plus I disagree that it's bet to set the ISO as low as possible. Stars are dim enough as they are, and dropping the ISO will eliminate your chances of capturing the really dim ones. It's okay if you want to do constellation shots I guess, but for star trails I'd never go lower than ISO 800 myself. Noise can be removed by subtracting dark files later on.

[hollis_f]

Noise can be removed by subtracting dark files later on.

Even without dark files the stacking process will reduce the noise. The signal:noise ratio will increase as the square root of the number of shots stacked. So stacking 4 shots will give 2x better S:N, 100 shots will improve it 10-fold. If I ever see clear skies and no Moon again I'm going to go out and try my new 7D at ISO1600.

[MintMark]

Even without dark files the stacking process will reduce the noise. The signal:noise ratio will increase as the square root of the number of shots stacked. So stacking 4 shots will give 2x better S:N, 100 shots will improve it 10-fold. If I ever see clear skies and no Moon again I'm going to go out and try my new 7D at ISO1600.

Oooh I'd love to see a 100% extract from one of the light frames. I know what ISO 1600 looks like up close from my 1000D and I'd be interested to see how much improvement there is.

[hollis_f]

Oooh I'd love to see a 100% extract from one of the light frames. I know what ISO 1600 looks like up close from my 1000D and I'd be interested to see how much improvement there is.

Actually, having played with the camera a bit more I reckon ISO6400 may be worth trying! Weather looks like it may be OK on Saturday and the Moon don't rise until gone 22:00, so I may have a chance then.

[jrm27]

well, as a followup, we had a clear night last night so I tried again!

This time to prevent focus change due to the lens getting cold, i set up my tripod and camera and let it sit for about a half hour and then readjusted the focus before starting the process. That seemed to work very well. I'm still processing the pics, but I know I need to plan better. About 60 shots in, the moon came over the hill and lit up the scene! Yikes.... I'll post up the results later, but I know I'll be trying again!

As an aside, does anyone know of a stacking program for mac? I've been copying the frames one by one and compositing them in PS. No big deal, but just takes a while. Thanks!

[MikeMcGowan]

Let me get this straight, it's good to use high ISOs for long exposures? I thought that was a bad idea, long exposures already create noise, bumping the ISO up that high can't help that out.

[SteveInNZ]

Welcome to the world of astrophotography where all the usual rules go out the window.
Yes, high ISO results in more noise but we are looking at very faint signal and if we didn't crank the ISO up, there'd be nothing to see. By taking lots of (noisy) images we can stack them on top of each other and effectively reduce the noise because the image increases linearly with each exposure and the noise doesn't. The more you stack ,the wider the gap between noise and data.

Steve.

[ckckevin]

so i guess i'd have to look up how to stack pictures together now...

[Adrena1in]

so i guess i'd have to look up how to stack pictures together now...

For easy star trails just download the free software at www.startrails.de (http://www.startrails.de). Simples.

For stacking multiple shots of the same target, such as a deep sky object or a planet, use "Deep Sky Stacker" or "Registax", also both free.

[ckckevin]

bw!awesome! Thanks!

[MikeMcGowan]

Welcome to the world of astrophotography where all the usual rules go out the window.
Yes, high ISO results in more noise but we are looking at very faint signal and if we didn't crank the ISO up, there'd be nothing to see. By taking lots of (noisy) images we can stack them on top of each other and effectively reduce the noise because the image increases linearly with each exposure and the noise doesn't. The more you stack ,the wider the gap between noise and data.

Steve.

I guess that explains why all the stars didn't come out in this picture (http://farm4.static.flickr.com/3504/4006181440_320204aec3_o.jpg) I took, the sky was completely covered with them. The most I've ever seen, but the picture didn't come out with too many. I shot it at f/3.5 and iso 100. I also used a fisheye, any reasons why that would be a bad idea?

[SteveInNZ]

I think that once you've decided what technique you are going to use, it comes down to personal preference and artistic skill. I prefer yours with trees and less stars than mine here (http://photography-on-the.net/forum/showthread.php?t=753668) with no trees and lots of stars.

The wider view of the fisheye might bring in the area around the celestial equator. The stars north of the equator will curve towards the north pole and those south of it curve southwards. The ones in the middle are straight. It looks a bit odd. Some people really like it and some don't.

Steve.

[MikeMcGowan]

I think that once you've decided what technique you are going to use, it comes down to personal preference and artistic skill. I prefer yours with trees and less stars than mine here (http://photography-on-the.net/forum/showthread.php?t=753668) with no trees and lots of stars.

The wider view of the fisheye might bring in the area around the celestial equator. The stars north of the equator will curve towards the north pole and those south of it curve southwards. The ones in the middle are straight. It looks a bit odd. Some people really like it and some don't.

Steve.

I'll have to try both ways next time I get a chance to have all those stars and a clear sky. I'd much rather have the quantity of stars you have with my trees.