From: Mark Gaffney <markgaffney@mac.com>
Date: September 25, 2008 3:27:12 PM MDT
To: Astro_IIDC@yahoogroups.com
Subject: Re: [Astro_IIDC] calculating in arc seconds etc....
Hi Milton, Thanks for all the detailed info! That must have taken you a while to compile! I basically found the widget & input the info from the site about my current camera (which I think you sent me, TIS having updated their site since mine was bought) as to chip description (there`s a link there)-pixel width & height/chip width & height (for which I`ve input the number of effective pixels) & the new scope`s FL, got a reading & gone to bed! I had no idea you`d be working at it for ages afterward. I hope the info you created will be of use to others as well. I thought the focus would change FOV but wasn`t sure how to calculate for it. Today I was going to try the same for the PGR camera I`m looking at. Thanks! Mark.
On 26/09/2008, at 4:01 AM, Milton Aupperle wrote:
Hi Mark;
The usual way is this:
Field of View Width in Arc Minutes = (CCD Width in mm / Focal
lenght in mm) * (57.3 * 60.00);
Field of View Height in Arc Minutes = (CCD Height in mm / Focal
lenght in mm) * (57.3 * 60.00);
Calculating the CCD Width / height is sort of tricky because the
width of the CCD on the CCD Spec sheet usually includes extra readout
pixels around the edges and also includes the gaps between CCD pixel
elements. We don't know what the actual Gap between pixel cells is
and it's usually assumed that the gap is include din the pixel cell
size.
What I use to calculate the mm width and height of the CCD is:
CCD Width in mm = (Number of Horizontal Imaging Pixels) * (Pixel Size
in Microns / 1000);
CCD Height in mm = (Number of Vertical Imaging Pixels) * (Pixel Size
in Microns / 1000);
So for a TIS 640x480 Mono or Color Camera 1/4" diagonal CCD with 5.6
micron pixel size:
CCD Width in mm = (640) * (5.6 / 1000) = 3.584 mm wide
CCD Height in mm = (480) * (5.6 / 1000) = 2.69 mm high
With a 1000 mm focal length, the field of view is:
Field of View Width in Arc Minutes = (3.584 / 1000) * 3438.0 =
12.32 arc minutes Wide
Field of View Height in Arc Minutes = (2.69 / 1000) * 3438.0 = 9.25
arc minutes High
For a PGR Flea 640x480 1/3" diagonal Color Mono CCD camera and the
same 1000 mm focal length , the pixel size is 7.4 microns:
CCD Width in mm = (640) * (7.4 / 1000) = 4.736 mm wide
CCD Height in mm = (480) * (5=7.4 / 1000) = 3.55 mm high
Field of View Width in Arc Minutes = (4.736 / 1000) * 3438.0 =
16.28 arc minutes Wide
Field of View Height in Arc Minutes = (3.55 / 1000) * 3438.0 =
12.21 arc minutes High
For my PGR Grashopper which is 1384 x 1036 2/3" Diagonal Color Mono
CCD Camera and the same 1000 mm focal length, the pixel size is 6.45
microns:
CCD Width in mm = (1384) * (6.45 / 1000) = 8.927 mm wide
CCD Height in mm = (1036) * (6.45 / 1000) = 6.682 mm high
Field of View Width in Arc Minutes = (8.927 / 1000) * 3438.0 =
30.69 arc minutes Wide
Field of View Height in Arc Minutes = (6.682 / 1000) * 3438.0 =
22.97 arc minutes High
However there are some other complications too:
1) If your using a Focal Reducer or Barlow, the distance to the
camera's CCD will change the expected focal length and you don't know
what the true focal length is.
2) If your using an SCT / MAK scope with built in focuser, the field
of view will change as you change focus, because focussing moves the
mirror and that changes the focal length.
In Case 1, the only way to calculate the true pixel size in arc
seconds is to measure the distance of two known object on screen. For
northern hemisphere, I like widely spaced double stars like Mizar in
the Big Dipper, which has a distance of 708 Arc seconds between the
two bright stars components. If my on screen pixel distance was say
900 pixels between them, then each pixel is 1.27 arc seconds (900 /
708) and a TIS 640x480 CCD's width of view is 1.27* 640 = 812.8 arc
seconds or 13.54 minutes of arc (812.8 / 60.00). Since you know the
arc seconds per pixel, you can work the calculations above backwards
and calculate out what your true focal length is. I do this all the
time with my Focal Reducer to determine what my "real" focal length was.
Case 2 bit me in the butt when I was imaging this summer with
separate LRGB images, because my RGB filters focus at a different
point than my unfiltered Luma was. Re-Focussing for the thickness of
the filter with an SCT caused the focal length to change, and the
Luma pixels are not at the same scale as the R G B images are and the
stars were 3 pixels off at the bottom corner than the RGB images are.
so the image has to be re-scaled.
HTH..
Milton Aupperle
On 25-Sep-08, at 11:15 AM, Mark Gaffney wrote:
Hi, does anyone know of a link to an online calculator for details
involving given scope &
CCD...image scale, FOV,resolution etc. Preferably one that lists
TIS & PGR cameras? Mark.
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