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View Full Version : ASTRO: another comparision of AP155/ST10XME with C14/IMG1024S


Richard Crisp
February 16th 04, 02:08 PM
Here's another comparison image. This compares Thor's Helmet Halpha taken
with an Astro-Physics AP155EDF and ST10XME camera (binned 1x1) with one
taken with a C14 and IMG1024S camera. In this comparision, the plate scales
were close: AP155/ST10XME: 1.29 arc-sec/pixel, C14/IMG1024S: 1.12
arc-sec/pixel.

In this comparison, all sub exposures were 20 minutes. There were four such
subs with the AP/ST10, while there were three subs with the C14/IMG1024S.

http://www.rdcrisp.darkhorizons.org/comparison_thor_page.htm

Even though the AP155 has a focal ratio of f/7 and the C14 is at f/12.46,
the C14 goes deeper with the same sub exposures.

Richard Crisp
February 16th 04, 05:58 PM
"Bill Meyers" > wrote in message
...
> Hello, Richard,
> Two wonderful sets of comparison photos. It seems to me that the AP
images
> show less but are more contrasty, the dark areas look darker and the
bright
> areas look brighter. But there is definitely more information in the C14
> images, it seems to me..
> What inferences do you yourself draw from the comparisons?
> Ciao and thanks for the images,
> Bill Meyers

I should probably reprocess both sets of images. The contrast is a function
of the processing pretty much.

I was surprised that the C14 exposed faster with the same plate scale.
Someone had told me that exposure time is set by focal ratio solely for
extended objects, that aperture did not factor in. There is a difference in
the cameras but not a 4x difference.

The f./7 of the '155/st10 system is a lot faster than the f/12.46 of the
c14/DM system and going by the standard ratio of the squares, it is roughly
a 3.16x or so faster. There has to be more to it though.



>
> Richard Crisp wrote:
>
> > Here's another comparison image. This compares Thor's Helmet Halpha
taken
> > with an Astro-Physics AP155EDF and ST10XME camera (binned 1x1) with one
> > taken with a C14 and IMG1024S camera. In this comparision, the plate
scales
> > were close: AP155/ST10XME: 1.29 arc-sec/pixel, C14/IMG1024S: 1.12
> > arc-sec/pixel.
> >
> > In this comparison, all sub exposures were 20 minutes. There were four
such
> > subs with the AP/ST10, while there were three subs with the
C14/IMG1024S.
> >
> > http://www.rdcrisp.darkhorizons.org/comparison_thor_page.htm
> >
> > Even though the AP155 has a focal ratio of f/7 and the C14 is at
f/12.46,
> > the C14 goes deeper with the same sub exposures.
>

Bill Meyers
February 16th 04, 06:03 PM
Hello, Richard,
Two wonderful sets of comparison photos. It seems to me that the AP images
show less but are more contrasty, the dark areas look darker and the bright
areas look brighter. But there is definitely more information in the C14
images, it seems to me..
What inferences do you yourself draw from the comparisons?
Ciao and thanks for the images,
Bill Meyers

Richard Crisp wrote:

> Here's another comparison image. This compares Thor's Helmet Halpha taken
> with an Astro-Physics AP155EDF and ST10XME camera (binned 1x1) with one
> taken with a C14 and IMG1024S camera. In this comparision, the plate scales
> were close: AP155/ST10XME: 1.29 arc-sec/pixel, C14/IMG1024S: 1.12
> arc-sec/pixel.
>
> In this comparison, all sub exposures were 20 minutes. There were four such
> subs with the AP/ST10, while there were three subs with the C14/IMG1024S.
>
> http://www.rdcrisp.darkhorizons.org/comparison_thor_page.htm
>
> Even though the AP155 has a focal ratio of f/7 and the C14 is at f/12.46,
> the C14 goes deeper with the same sub exposures.

Stefan Lilge
February 16th 04, 08:09 PM
Richard,

interesting comparison. The C14 is a clear winner here. Probably the old
saying that larger optics have better contrast (everything else being equal
of course) is not so wrong :-)

Stefan

"Richard Crisp" > schrieb im Newsbeitrag
. com...
> Here's another comparison image. This compares Thor's Helmet Halpha taken
> with an Astro-Physics AP155EDF and ST10XME camera (binned 1x1) with one
> taken with a C14 and IMG1024S camera. In this comparision, the plate
scales
> were close: AP155/ST10XME: 1.29 arc-sec/pixel, C14/IMG1024S: 1.12
> arc-sec/pixel.
>
> In this comparison, all sub exposures were 20 minutes. There were four
such
> subs with the AP/ST10, while there were three subs with the C14/IMG1024S.
>
> http://www.rdcrisp.darkhorizons.org/comparison_thor_page.htm
>
> Even though the AP155 has a focal ratio of f/7 and the C14 is at f/12.46,
> the C14 goes deeper with the same sub exposures.
>
>

jerry warner
February 17th 04, 07:07 AM
both stunning hi-qaulity images each with its own information bank. Superb
craftsmanship as ususal. Congrats!
jerry




Richard Crisp wrote:

> Here's another comparison image. This compares Thor's Helmet Halpha taken
> with an Astro-Physics AP155EDF and ST10XME camera (binned 1x1) with one
> taken with a C14 and IMG1024S camera. In this comparision, the plate scales
> were close: AP155/ST10XME: 1.29 arc-sec/pixel, C14/IMG1024S: 1.12
> arc-sec/pixel.
>
> In this comparison, all sub exposures were 20 minutes. There were four such
> subs with the AP/ST10, while there were three subs with the C14/IMG1024S.
>
> http://www.rdcrisp.darkhorizons.org/comparison_thor_page.htm
>
> Even though the AP155 has a focal ratio of f/7 and the C14 is at f/12.46,
> the C14 goes deeper with the same sub exposures.

David Whysong
February 20th 04, 03:03 AM
In sci.astro.amateur, Richard Crisp > wrote:

> I was surprised that the C14 exposed faster with the same plate scale.
> Someone had told me that exposure time is set by focal ratio solely for
> extended objects, that aperture did not factor in. There is a difference in
> the cameras but not a 4x difference.

Focal ratio is not the whole story -- you really want to compare the
pixel size divided by the focal ratio, squared. And (24/12.42)^2 is
much greater than (6.8/7)^2. That's ignoring QE, throughput, etc. but
it's a good first approximation.

> The f./7 of the '155/st10 system is a lot faster than the f/12.46 of the
> c14/DM system and going by the standard ratio of the squares, it is roughly
> a 3.16x or so faster. There has to be more to it though.

Yes, QE and pixel size are important. The QE of the SITE back-illuminated
chip is a little higher than the Kodak, though XME chips are pretty decent.

But the C14/SITE setup has a big advantage because the pixels are huge -
24 microns, as opposed to 6.8 micron pixels on the ST-10XME. That's a
factor of 12.5 more area! The C14 gathers a lot more photons (a factor
of 5 if you just consider the ratio of apertures squared, but the actual
advantage in light gathering is probably more like a factor of ~ 3 due to
losses from the central obstruction, reflection, etc.) but the longer focal
length means the scale of the focal plane is larger. Sure, the refractor is
faster and that works to it's benefit (by a factor of ~ 3), but the larger
pixels of the SITE chip, combined with the large aperture of the C14, more
than compensate.

You haven't perfectly matched the pixel scale. If you did (or if you
re-sample the image to the same scale) then for this kind of comparison
the focal lengths and pixel sizes are no longer important; you just care
about QE, aperture, and light throughput. In that case, the C14 aperture
gives it a clear advantage. Roughly guessing at the light throughput for
a C14, I'd guess that it's about 3x more sensitive than the AP155/ST-10
combination.

Of course, the AP155 has other advantages over a C14, notably the ability
to do extremely wide-field imaging.

Dave

David Whysong
DWhysong (at) physics (dot) ucsb (dot) edu