How many pixels to arc second?
Posted 15 January 2013 - 07:19 PM
Is there a millimeter focal length formula to arrive at how much a pixel or arc second covers and how would I arrive at it?
Thanks in advance.
Posted 15 January 2013 - 07:45 PM
I just let ccdcalc do the math for me
Ditto here. And I'm pretty sure the optimal f-ratio has been expounded on ad infinitum in other threads, but the consensus seems to be somewhere between f20 and f28 for a monochrome camera.
So if you're starting with f9 on a scope where the primary doesn't move for focusing, you could apply a 2.5x or a 3x barlow. I'd download CCDCalc and plug in your instruments' numbers, but you'll likely find the image scale comes out to somewhere in the region of 0.1-0.3 arcsec per pixel.
Posted 15 January 2013 - 08:28 PM
Posted 15 January 2013 - 08:30 PM
To calculate scale factor you can use the formula:
S = (206.265 / EFL) * C
S - is the pixel sampling in arcsec/pixel
EFL - focal length in millimeters
C - pixel size in microns
So for the your example at f/9 and 5.6u pixels of the DBK21 we have:
S = (206.265 / 1821) * 5.6 --> approx 0.6 arcsec/pixel
Somewhere between f/18 and f/25 would be a good place for high resolution work and that camera. Some Mars imagers like to go higher.
Posted 15 January 2013 - 08:37 PM
Thanks you nailed it for me. So at this point I look to my Barlow at 2.5x to turn the 0.6" per pixel to make it 0.24" ?
Posted 15 January 2013 - 08:55 PM
I get this for a 2.5X:
(206.265 / (1821*2.5)) * 5.6 = 0.254 arcsec/pixel
and this for prime focus:
(206.265 / 1821) * 5.6 = 0.634 arcsec/pixel
Of course thats more precision than called for...
Posted 15 January 2013 - 10:45 PM
Posted 16 January 2013 - 10:27 AM
Posted 16 January 2013 - 12:54 PM
Since even the primary focal length may not be known accurately its easier to just calculate the effective focal length of the whole imaging system using the formulas below:
To calculate the focal ratio (FR) or effective focal length (EFL) from an image of an
object (or binary star) of known size and measured image size in pixels we have the formulas:
FR = (206.265 * C / (D / S)) / A = 206.265*(C*S)/(D*A) = EFL / A
EFL = (206.265*C*S) / D
FR - focal ratio
EFL- focal length in millimeters
C - size of ccd pixels in microns
D - size of object in arc-seconds
S - size of object in pixels
A - aperture in millimeters
Note that the parentheses in the formulas above are important.