From: "Tim" <tjp314@pacbell.net>

Date: January 9, 2006 6:19:11 PM MST

To: Astro_IIDC@yahoogroups.com

Subject: Re: Image scale comparison for different pixel sizes

Milton:

Where this will become really interesting to me would be if you did a comparison of DSO 

images taken with the two cameras.  For example, I've cogitated about purchasing a 

wonderfully compact Scorpion 1600x1200 camera (4.4 micron pixels and is uncooled) at 

some point to compare with the sensitivity of my awesome but big, heavy, and bulky 

ST2000XM, which has 7.4 micron pixels and is cooled.  But the Scorpion is an expensive 

camera, so I'm afraid to just buy one to find that out.  ...so it'd be cool if you get a chance 

to do DSO comparisons betwixt the Flea and the Dragonfly2.

For example, if you were to match the image scales for the two cameras by changing the 

effective f/ratio, such that the same numbers of photons are hitting a single pixel with 

both cameras, is there a difference in the resulting images?  ...I suppose this opens a can 

of worms with respect to the differences/similarities between film and ccds, and aperture 

and focal ratios (like I've seen recently on other newsgroups).  I would think that quantum 

efficiency and well depth differences between the two cameras would be more important, 

given the same image scales.  The smaller pixels might take longer to pull in the same 

amount of light, but the greater number of pixels would make for a more aesthetically-

pleasing image, perhaps.

-Tim.

--- In Astro_IIDC@yahoogroups.com, "Milton Aupperle" <milton@o...> wrote:

Hi Folks;

In the "Other" folder of the Files section, is a jpeg image entitled

"FleaVersusDragonFly2.jpg". I took the images on the same night

(seeing was really poor, about 2 to 3 out of 10) with the same scope

and focal length but using two different color cameras, a 640x480 Flea

and a 1024x768 DragonFly2 camera. The 1024x768 DragonFly2 camera can

"crop" out portions of the CCD and delivers 640x480, 800x600 or

1024x768 video, so for this test I ran it in 640x480 mode so that it

can be directly compared to a 640x480 Flea camera.

The images show what difference pixel size makes with respect to image

scale, which is a factor of 1.59 times. The Flea CCD has 7.40 micron

sized pixels where as the Dragon Fly CCD has 4.65 micron pixels. So

the larger the pixel, the less magnification you get for the same

pixel area. To achive the same scale would require increasing the

focal length by 1.59 times.

The flip side of this is the light collecting capability of the CCD

for pixel size. Even though both cameras have 1/3" CCD, the Flea CCD

captures more light (2.5 times) than the DragonFly2 CCD did. The Flea

camera used less gain (36% verusus 48%) and less exposure time

(19.35ms versus 33.33 ms) to achieve the same brightness level.

So now you have some idea as to what difference pixel size makes with

respect to imaging when selecting a CCD for a particular camera.

TTYL...

PS: I uploaded a new lunar quarter image that I took January 4th to

the Lunar section.

Milton Aupperle