long exposure with 10D

[perfectpixel]

Hi All,
this picture doesn't show it very well, but I made my first attempt at getting star tracks with a long exposure and got all sorts of noise on the picture.
Is this to be expected, or should I do something different?
Was it caused by too much ambient light?
(I did remember to cover the eyepiece)

shot was taken as:
ISO100
F/22
90minutes
24-40f/2.8 @ 24mm

I'd appreciate any comments.
thanks

http://www.photo.net/photodb/image-display?photo_id=1928246&size=md

[RichardtheSane]

That is a very good start for long exposures, nice trails. Do you know what the strak going diagonal to the trails is?
On long exposures you will get digital noise anyway, and there does seem to be a lot of ambient light - have you got time to go out waaaay into the countryside? How long was your exposure?

[defordphoto]

Richard:

unstuck wrote:
shot was taken as:
ISO100
F/22
90minutes
24-40f/2.8 @ 24mm

[RichardtheSane]

See, highlighted in bold so I ignored it.
The obvious is just so uninteresting for me :D :D :D
(Thanks RFM :) )

The noise is pretty well controlled by the 10D for a 90min exposure, but there is a technique which may help - I am sure someone will be able to advise better - it involves taking an exposure with the lens cap on or no lens at all and just the body cap on then using that exposure to cancel the digital noise out in PS. I wouldn't know where to start with it though!

[thomascanty]

RichardtheSane wrote:
The noise is pretty well controlled by the 10D for a 90min exposure, but there is a technique which may help - I am sure someone will be able to advise better - it involves taking an exposure with the lens cap on or no lens at all and just the body cap on then using that exposure to cancel the digital noise out in PS. I wouldn't know where to start with it though!

It's a program called Black Frame, and it can be found here: http://www.mediachance.com/digicam/blackframe.htm .

Another method I've seen people use before is to take a series of shorter shots (say, a minute or so each), with a slight pause between shots. Then, merge them all together into one image. I have the program that does this merging too, but can't recall the name of it right now. I'd check, except I'm at work at the moment.

[mattchase]

Wow...90 minutes is pretty crazy!

To answer your questions, yes it is sky glow (city lights), and yes you can get better results! I would recommend the same as others, try shorter exposures with a wider aperture, perhaps even with a higher ISO. You definitely want to open up your aperture, because you are blocking out the dimmer stars. Unless you are tracking them, it won't matter how long your exposure is if your aperture is too small, those faint stars are not going to show up. Something else I recently learned is not to shoot at your lenses widest aperture, to shoot at one or two stops down. So if you have a 1.8 lens, shoot at 2.8 or 3.5. This has to do with refraction or vignetting or...I don't remember exactly what the reason was, but it made sense when I read it! ;)

And most importantly, get as far away from city lights as you can!

Here is a shot I did about 40 miles northwest of Las Vegas, and at about 9500 feet altitude. This was taken at ISO 100, f2.8, and about 9 minutes 21 seconds. This was actually done when I had a D60, which had a bit more noise than the 10D shows with long exposures. This image is straight out of camera, besides resizing.
http://www.matthewchase.com/digitaldingus/stars/img_5289.jpg
The star trails aren't as long, but the sky doesn't have much noise, and there is no sky glow from the city. As was also mentioned, I could have done a dark frame to remove even more of the noise.

Here is another shot, done about 90 miles north of Las Vegas. This was done at f2.8, ISO 400, and 8 minutes 25 seconds, with camera tracking the stars. I did do some post processing to this image, because even though I was 90 miles away from Las Vegas, the sky glow from the lights was still visible. The red cast in the bottom left of the image is from Las Vegas, and it was very evident before processing, and is still very noticable in some of the other shots from this trip. I also did a dark frame on this image to help remove some noise.
http://www.matthewchase.com/digitaldingus/stars/img_4563.jpg
(others from this trip)
http://www.matthewchase.com/astrophotos/2_22_03/index.htm

The other method that was suggested, to shoot shorter exposures and combine them, is called stacking. You would take a series of shots, could be 5 or 10 or 50, at the same or similar exposure, and then combine them using a program such as AstroStack. This will help bring out some of the darker faint details while still retaining the brighter details (the programs know to not keep combining the brightest areas, so that they won't get blown out). I have tried this method a few times, had some luck with it, but still kind of prefer to just do a shot or two and tweak it myself in Photoshop.

[Jim_T]

Wow.. 90 minutes.. The longest I've done is 18 minutes..

The little dots are 'saturated' pixels. Some of the sensor sites can't get rid of their charge fast enough over time and become saturated.

They start to show up after about 90 seconds and get worse as the exposure time gets longer. It's one of the current technical problems with CMOS sensors.

Your image is pretty clean for 90 minutes..

One thing you can do is use a despeckle plug in.. I'm a Linux user, so I'm forced to use the GIMP image program. It has a despeckle plug in.

You can select the radius of specks you want to remove.. The saturated pixels are generally small, so you can set a small radius which will remove them and leave the rest of the detail in the image.

I don't know if they make such a plug in for Photoshop or the others, but I'm sure they do.

Here's a 16 minute shot of the north star and the rest of the stars rotating around it.. It was despeckled... If you click on original, you'll see the sky area is pretty clean:

http://www.pbase.com/image/20828208

[Jesper]

One weakness of digital cameras is that they seem to be not so well suited for very long exposures, because when you do that, the small defects in your sensor, such as stuck pixels, show up!

I've tried some test shots for stuck pixels with my 10D. I made test shots with the lens cap on, camera on M at f/22, 30 seconds and a range of different ISO settings. At ISO 100 to 400, no stuck pixels are visible, but at higher ISO modes I can see that the sensor has some pixels that are stuck on either red, green or blue.

If you make a 90 minute exposure I can imagine that even at a lower ISO setting the stuck pixels will show up. I think that explains those dots you see in your photo.

[perfectpixel]

thanks for all the responses.
I guess the bottom line is that this shot is encouraging???

I'll try wider aperture. I hadn't considered the dimmer stars, but I'm afraid this will make the hot pixel problem worse.

As for the diagonal line of light, I saw a streak in the general vicinity as I was setting up the shot, I thought "wow I just saw a meteor", but afer seeing the line on the exposure I have to assume it's a satelite ??? faint, but multiplr passes? I'm pretty sure it's not aircraft lights. Don't know really, just a guess.

There is a LOT of ambient light around here, as you can wee by the (almost) well illuminated bushes and neighbor's awning.
Maybe using the lens hood would reduce this problem?

To think, I had actuially thought of firing a flash before the end of the exposure to make sure the rose bush showed up!

As for the long exposure, I had hoped to make others on the order of 2 to 4 hours trying to get a nice arc of star trails from horizon to horizon. Maybe that kind of exposure is not possible with digital cameras (not even the 10D :( )

I'm not sure if Black Frame is intended for this type of noise. Unless you think I should take a black shot (lens covered) for 90 minutes as well? And in that case would the hot pixels showup?

Finally, I'd appreciate a link to the despeckle plugin (I have PS6) and the astrostack that was mentioned (hope they are shareware :D ).
The idea of doing multiple short exposures and stacking them sound viable, although it's more work for a lazy guy like me.

I'll repeat the long exposure if I'm away from city lights. As it is, I'm happy to be experimenting in the backyard (just have to remember to turn off the sprinklers next time). FWIW, I never had the curage to try this with film before. Too much at the mercy of the print lab.

I'd appreciate any more thoughts/comments/advice. I've always wanted to do type of shot.

cheers!

[vvizard]

Jim_t wrote:



One thing you can do is use a despeckle plug in.. I'm a Linux user, so I'm forced to use the GIMP image program. It has a despeckle plug in.





Good to see other Linux-users in here :) I'm a bit of a cheat though. Although I've run Linux primarly for 4-5 years, and Linux-only for about 2, I have now installed vmware, so that I can emulate windows2k and Photoshop-CS :) I also own the Crossover-office from codeweavers.com, so I can emulate Photoshop through wine if I wan't. I just don't like gimp :/

[thomascanty]

unstuck wrote:
I'm not sure if Black Frame is intended for this type of noise. Unless you think I should take a black shot (lens covered) for 90 minutes as well? And in that case would the hot pixels showup?


You're right, Black Frame wasn't really written with exposures of this length in mind. You could run it with a shorter black frame exposure to get rid of the more obvious and persistent hot pixels, though. It probably doesn't take 90 minutes for those to show up.

[James01]

WOW Matt those are some seriously cool star pics. What did you use to track the stars with the camera?

Good Job

[mattchase]

I have two devices, one a dedicated portable tracking device, the other a full blown telescope. For the shots on my website, I used the telescope and had the camera mounted on top of it (piggyback). I am in the process of getting a dovetail mount for my camera, so that I can take the telescope off but still use the full size tripod and tracking controls. This setup was moderately priced at about $800.

The portable kit has similar components, only they are all smaller and it lacks a few features. It has a smaller head, less counterweights, no polar alignment scope, is designed to support less weight, only tracks on 1 axis, and is mainly meant for using with a camera (though will work with a small telescope). It comes only with short legs (about 8" long), but as luck would have it, the hole on the bottom is the exact size used on many tripods (such as Bogen - I think 3/8" ). So I mounted it to my ultra light Velbon and have a full size tripod underneath, but still a compact and light kit for travel. This item is a steal too, it was only about $100 (for the mount and motor drive), and if you want to dabble in doing shots like above then this is a great way to start. http://www.telescope.com/shopping/product/detailmain.jsp?itemID=296&itemType=PRODUCT&iMainCat=6&iSubCat=24&iProductID=296

[defordphoto]

Matt! Awesome shots! I'm impressed.