azure1961p
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How many pixels to arc second?
#5625001  01/15/13 07:19 PM

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For planetary imaging with a Dbk21au04 how many pixels to arc seconds do I shoot for with an 1821mm (f/9) system barlowed for extra focal length? 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.
Pete

zAmbonii
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Re: How many pixels to arc second?
[Re: azure1961p]
#5625034  01/15/13 07:32 PM

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I just let ccdcalc do the math for me

RedLionNJ
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Re: How many pixels to arc second?
[Re: zAmbonii]
#5625058  01/15/13 07:45 PM

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Quote:
I just let ccdcalc do the math for me
Ditto here. And I'm pretty sure the optimal fratio 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 0.10.3 arcsec per pixel.
Grant

azure1961p
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Re: How many pixels to arc second?
[Re: RedLionNJ]
#5625130  01/15/13 08:28 PM

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Thanks guys. It truly is important for me to have true knowledge of the pixel image scale. Ill do what you suggested and again, gratis.
Pete

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Re: How many pixels to arc second?
[Re: azure1961p]
#5625135  01/15/13 08:30 PM

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Pete, To calculate scale factor you can use the formula:
S = (206.265 / EFL) * C
where 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.
Glenn

azure1961p
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Re: How many pixels to arc second?
[Re: DesertRat]
#5625152  01/15/13 08:37 PM

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Glenn!!
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" ?
Pere
Edited by azure1961p (01/15/13 08:38 PM)

DesertRat
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Re: How many pixels to arc second?
[Re: azure1961p]
#5625184  01/15/13 08:55 PM

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Hi Pete, the 2.5X barlow would be fine I think, however with any extension it might be working past 2.5X. Which is ok, just thought I'd mention it.
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...
Glenn

azure1961p
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Re: How many pixels to arc second?
[Re: DesertRat]
#5625371  01/15/13 10:45 PM

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Aaahhh so I measure the bottom of the barlow to where the DBK begins and add that length to the 1821mm?? Again greatly appreciated!!
Pete
Edited by azure1961p (01/15/13 10:51 PM)

KpS
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Re: How many pixels to arc second?
[Re: azure1961p]
#5625939  01/16/13 10:27 AM

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Multiplication factor written on Barlow is only a nominal value. The actual number depends on the distance of the chip. The only way to reliably find EFL is a measurement of the image. For example WinJupos provide image resolution in arcsec/pixel. Then, using Glenn's formula we obtain EFL.

DesertRat
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Re: How many pixels to arc second?
[Re: KpS]
#5626150  01/16/13 12:54 PM

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Yes, Karel is correct. To calculate the true magnification a barlow provides, you would need its negative focal length, which the manufacturer seldom provides (it can be measured however). From that and the distance between the image plane and the barlow you could calculate the effective focal length using the barlow formulas.
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
hence
EFL = (206.265*C*S) / D
where FR  focal ratio EFL focal length in millimeters C  size of ccd pixels in microns D  size of object in arcseconds S  size of object in pixels A  aperture in millimeters
Note that the parentheses in the formulas above are important.
Glenn
