Return to the Cloudy Nights Telescope Reviews home pageAstronomics discounts for Cloudy Nights members
· Get a Cloudy Nights T-Shirt · Submit a Review / Article

Click here if you are having trouble logging into the forums

Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green Gu… uh, User

Astrophotography and Sketching >> DSLR & Digital Camera Astro Imaging & Processing

Pages: 1
jblaschke
super member


Reged: 03/07/08

Loc: New Braunfels, Texas
Focal length star trails formula?
      #4600556 - 05/24/11 04:11 PM

Folks, what's the formula for determining the length of exposure possible before getting start trails? I know I've seen it before, but I'm coming up empty on my searches. Thanks.

Post Extras: Print Post   Remind Me!   Notify Moderator  
pereloup
member


Reged: 02/10/11

Re: Focal length star trails formula? new [Re: jblaschke]
      #4601058 - 05/24/11 09:42 PM

If I remember correctly it's 600/(focal length) and that's in seconds of exposure time. So with a 50 mm lens, you should not see star trails under 12 seconds.

Post Extras: Print Post   Remind Me!   Notify Moderator  
jblaschke
super member


Reged: 03/07/08

Loc: New Braunfels, Texas
Re: Focal length star trails formula? new [Re: pereloup]
      #4601068 - 05/24/11 09:48 PM

Thanks! That sounds about right.

Post Extras: Print Post   Remind Me!   Notify Moderator  
luigis
sage


Reged: 07/27/10

Loc: Buenos Aires, Argentina
Re: Focal length star trails formula? new [Re: jblaschke]
      #4601091 - 05/24/11 09:57 PM

600/focal_length is wrong because it depends on the area of the sky you want to photograph. The poles are not the same as the equator.

The real formula is:

Trail length = F * E *(2Pi/T) * Cos(D)

F = Focal Lenght
E = Exposure time
T = Duration of sidereal time (same unit as E)
D = Declination of the star

Source:
http://www.wilmslowastro.com/software/formulae.htm#StarTrails

A reference:
http://www.astropix.com/GADC/SAMPLE7/SAMPLE7.HTM


Post Extras: Print Post   Remind Me!   Notify Moderator  
Samir Kharusi
Carpal Tunnel
*****

Reged: 06/14/05

Loc: Oman
Re: Focal length star trails formula? new [Re: luigis]
      #4601444 - 05/25/11 01:59 AM

All these formulae end up with subexposure times too short to be of much use, because they aim to have trails shorter than a pixel on your multi-megapixel sensor. For practical use one needs to determine what kind of display he wishes to end up with. e.g. for web display a 1200x800 pixel display is very adequate (one megapixel) so trying to have nil trailing on an 18 megapixel sensor is about 4x overkill (square root of 18). My suggestion is to start with 30sec subs using a 14mm lens and halve the subexposure for each doubling in focal length. Works quite satisfyingly for absolute beginners as demonstrated here, nil science required. Even a tripod is an optional, not an essential requirement Of course, if chasing large prints then one has to pay attention to the formulae.

Post Extras: Print Post   Remind Me!   Notify Moderator  
faltered
professor emeritus


Reged: 04/01/05

Re: Focal length star trails formula? new [Re: Samir Kharusi]
      #4602415 - 05/25/11 02:26 PM

I like this formula -

Press Button - If you like what you see - Keep - If not - Try Again.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pages: 1


Extra information
13 registered and 21 anonymous users are browsing this forum.

Moderator:  Dave M, fishonkevin, WOBentley, tecmage 

Print Thread

Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is disabled
      UBBCode is enabled


Thread views: 1606

Jump to

CN Forums Home


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics