Long Exposures for Star Trails

I have a Canon Rebel T2i. I have read that you can capture star trails by doing one long exposure or multiple 30 second exposures and stack them.

My question is how do I know how long of an exposure I can do before noise and "hot pixels" become an issue?

I'm assuming I'd need a better camera to do star trails with one long exposure but just curious how to find this info out.

Thanks!

cloudynights.com forum is a one stop shop for all your AP needs.
--
-C41vin Tirr311-

-"Technology, can't live with it and can't live without it."-Yours Truly-

-With KX max shutter speed @ 1/6000th a second I can't be told, "No time for pictures."

Jeffmqt said:

I have a Canon Rebel T2i. I have read that you can capture star trails by doing one long exposure or multiple 30 second exposures and stack them.

My question is how do I know how long of an exposure I can do before noise and "hot pixels" become an issue?

I'm assuming I'd need a better camera to do star trails with one long exposure but just curious how to find this info out.

Thanks!

Click to expand...

Actually your rebel will be fine. If you want one long exposure set long exposure noise reduction the camera then takes a black exposure the same length as the main exposure. This subtracts noise. I've done STs up to 1.5 hours without too much noise.
Like this one with a 40D at abt 1 hr.



If you want to stack then turn LENR off. use a locking cable release & go as long as you want. Stack in a prog shuc as Startrails.de
http://www.startrails.de/html/software.html
This was stacked with about 50, 30 sec exposures at ISO 200 f3.5 with a 7D



--
Brian Schneider

Thanks so much for the advice!! Hope we have some clear nights this week!

Jeffmqt said:

Thanks so much for the advice!! Hope we have some clear nights this week!

Click to expand...

Good luck I hope you'll post some.
--
Brian Schneider

Also keep in mind that star trail length depends on focal length (and pixel size). Rule of thumb: star trails get noticeable once your exposure is longer than 600/focal length( in mm).

You need to get as far away from the city lights as possible for best results. You would be supprised how much light polution there is even in your backyard if you are in the city. And I believe you get star trails at around the 25 second mark. But please don't quote me on that exact figure.

Terry Geig said:

You need to get as far away from the city lights as possible for best results. You would be supprised how much light polution there is even in your backyard if you are in the city. And I believe you get star trails at around the 25 second mark. But please don't quote me on that exact figure.

Click to expand...

Correct,trailing starts to be noticible 25-30 seconds. If you use the stacking method you can shoot closer to cities since the 30 sec exposures don't pick up a lot of background light. The software stacks & blends using "brighten" mode thereby keeping fairly dark sky & bright stars. See this picture , it was taken abt 15 km from a city of 200,000 facing the city.



--
Brian Schneider

When the camera is taking the black exposure do I have to leave the camera stilll? Or can I pack it up and go home?

Also, for long exposures, do most people have some kind of auxillary power source? Seems like battery power could be a limiting factor.

Thanks again for everyone's help!

Jeffmqt said:

When the camera is taking the black exposure do I have to leave the camera stilll? Or can I pack it up and go home?

Click to expand...

Leave the camera turned-on and pack-up and go home (or wait and then shoot another image when it's done). The camera need not be still while the dark-frame-subtraction is being imaged.
--
-Dave
http://pixseal.com

Jeffmqt said:

When the camera is taking the black exposure do I have to leave the camera stilll? Or can I pack it up and go home?

Also, for long exposures, do most people have some kind of auxillary power source? Seems like battery power could be a limiting factor.

Thanks again for everyone's help!

Click to expand...

I like to do non trailed shots at high ISO as well so I leave my trailed shot till last.
Date/Time 10-Oct-2010 21:09:26
Make Canon
Model Canon EOS 7D
Flash Used No
Focal Length 10 mm
Exposure Time 30.00 sec
Aperture f/3.5
ISO Equivalent 3200



I've packed up & by the time I get home the black frame is done. It should be done at about the same temperature as the main frame so dont turn the heater on (in practice I don't know how much difference it makes)

For power a battery grip will double the time you can shoot. With a single battery my 50D lasted 1.5hr + 1.5 dark frame & battery was nearly gone.

I bought batt grips for mt 7D & 60D so if we ever get some clear weather I'll be out there shooting away.
--
Brian Schneider

Here's my first try at the star trails. I stacked a bunch of images instead of doing the one long exposure method.

One problem I ran into is I did have a bunch of pink, white and purple pixels I had to clone out. I'm assuming this is because my sensor got too "hot" after being in use for 40 minutes?

Is there any way to use noise reduction with continuous shooting? Seems like if there was a 30 second pause after each shot my star trails would be dotted lines!

I like stacking the images because I can preview the image after one 30 second exposure to check lighting, focus etc...

Anyway, any tips or critiques would be greatly appreciated! Hopefully I'll be able to get out a few more times this week so I can put any advice into immediate use!