Re: Celestron NexImage Imager vs Ordinary Webcams

On May 2, 4:51 am, Pierre Vandevenne <pie...@xxxxxxxxxxxxxx> wrote:
On May 1, 1:08 am, Hughes <eugenhug...@xxxxxxxxx> wrote:





On May 1, 6:45 am, Hughes <eugenhug...@xxxxxxxxx> wrote:

On Apr 29, 1:23 pm, Chris L Peterson <c...@xxxxxxxxxxxxxxxxxx> wrote:

On Tue, 28 Apr 2009 15:12:47 -0700 (PDT), Hughes <eugenhug...@xxxxxxxxxx>
wrote:

CMOS sensors have only small portion in the pixel that
receives photons because half of the surface are the
support electronics that converts it to voltage compares
to CCD where the conversion is done outside of the
sensor (so the entire pixel is photon bucket). Why would
you use CMOS for astronomical use.

Many CMOS detectors have large pixels. Some even bury most of the
control logic underneath the pixels. And many CCD detectors have a large
area of each pixel covered with circuitry. The CCD detectors used in
webcams and video cameras lose fully half their surface area to the
interline transfer columns. And these days, most detectors that have
both pixels and circuitry on their surface use microlenses to get much
closer to a 100% fill factor. So you can't compare CCD and CMOS by this
criterion.

CMOS generally has somewhat worse noise characteristics than CCD, but
the difference between the two has grown less in recent years. Canon
DSLRs, with their CMOS detectors, have pretty much the best noise
performance of any DSLRs, including those that use CCDs.

CMOS has some advantages, in that parts of the chip can be read out
without reading the whole chip, and the chip can be read
non-destructively. I have an astronomical camera that I can check how
the exposure is going without interrupting the exposure in progress..

In picture I take with my Webcam. I can see that there
is a larger box holding out pixel arrays of 8 x 8 pixels
when I zoom in the picture. See the following where
a printed material half toned color is imaged that
covers 10mm at a distance of 3.6 meters from the
objective lens:

http://www.pbase.com/image/111769165/original

Also why does my program outputs in bmp
instead of jpeg. It means the CMOS circuitry
of it is outputting in raw format and not jpeg
or is it only the program saving in bmp what
is a jpeg image?

Don't you have any images taken with your NexImage, QC4000,
etc. that you can share in your website for comparisons?

I have some planetary images from a few years ago posted on my website,
all made with the QC4000. I use the NexImage mainly on a PST, and I
don't think I've posted any images.
_________________________________________________

Chris L Peterson
Cloudbait Observatoryhttp://www.cloudbait.com

I see. I'm still evaluating my old webcam to see if I need
to buy a NexImage. I'm stuck in a portion in the analysis
of my old webcam. I don't know it's pixel pitch as
the manufacturer didn't mention it anywhere in the
web site or manual. I'm just assuming it is 5.7 micron.

At distance of 3.6 meters. I can see 10mm field
of view when imaging a ruler.

Now 10mm/640 =  15.625 micron.

This means that for every pixel in my 640x480 webcam,
it can cover 15.625 micron. What relevance is this
is determining the actual pixel pitch of the webcam?
Can you determine it just by the data above with
the following given specs of the lens (Aperture:100mm,
focal length: 1000mm, F/10)??

You may say all webcam has 5.7 micron pixel
pitch. But suppose it's unknown and one wants
to determine it. How do you do that using the
above method of imaging field of view and
distance?

Second method of determining pixel pitch
may be this. My Cmos sensor has size
of 1/4". It may mean diagonal. So using
the formula sine theta = Y/R.

Sine 45 = Y/R
0.707 = Y/  1/4"
Y= 0.176mm

0.176mm/640 resolution = 0.00027 or 0.27 micron
instead of expected 5.7 micron pixel pitch. What's wrong??

Oh. I realised wrong units.

Y= 0.176" x 25.4mm/inch = 4.49mm
4.49mm / 640 resolution = 7 micron which is short of the
5.7 micron standard in Webcam. Why?

Because both sensors are type 1/4 and you are dividing the length of
the diagonal by the horizontal resolution.- Hide quoted text -

- Show quoted text -

How does the 5.7 micron pixel pitch value come from
type 1/4 640x480 spec? Hope you can show calculations
as I couldn't solve for it.

Are 1/4 type 640x480 webcam all 5.7 micron pixel pitch?

Also in my webcam image zoomed in the following:

http://www.pbase.com/image/112008989/original

I can see grid of square composing of 8x8 pixels,
what is the grid called in cmos sensor?? Is it
present in ccd too?

Well. Solving for the unknown pixel pitch of my
1/4 type 640x480 cmos a4tech sensor.

Here are calculations data using the moon as reference..

Airy_diskdiameter_arcsec = 206254 1.22 x lambda_mm /
D_aperture_mm = 1.384 arcsec
moon 3476 kilometers diameter at distance of 384,403 kilometers

= 2 * Atn (0.5(3476)/384,403)) * 180/pi = 0.518 degrees or 1865 arcsec

moon linear diameter in focal plane =
linear diameter_mm = focal_length_mm x theta_obj_arcseconds / 206265 =
9mm as image in focal plane
(or prime focus)

Image/Object (half toned printed brochure target) in the following is
size 10mm horizontal at distance of 3800mm

http://www.pbase.com/eugenehughes/image/111769165

angle of object subtended
theta (degrees) = 2 * Atan(0.5(10)/3800) * 180/pi = 0.15 degrees or
540 arcsec

10mm target subtends 540 arcsec at distance of 3800mm

linear diameter_mm = focal_length_mm x theta_obj_arcseconds / 206265
= 1000 x 540/206265
= 2.618 mm at focal plane or prime focus

Since 640x480 webcam can cover the entire 10mm original object, then

2.618mm/640= 4 micron

But 1/4 webcam should have 5.7 micron, calculations only produce 4
micron, why?

Hope others can solve. I've been solving for it for many
hours already. Thanks.

Hughes
.

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