Spring has Sprung!

It's galaxy season!

Spring is here, and dozens upon dozens of galaxies are within reach of small scopes. Time to get out under mild spring skies and observe!

http://www.astromart.com/common/imag...7.jpg&caption=

Happy galaxy hunting.

Uncagalaxy

On Tuesday, 14 April 2015 22:22:47 UTC+2, Uncarollo2 wrote:
It's galaxy season!

Spring is here, and dozens upon dozens of galaxies are within reach of small scopes. Time to get out under mild spring skies and observe!

http://www.astromart.com/common/imag...7.jpg&caption=

Happy galaxy hunting.

Uncagalaxy

Images nearly always seem to produce an artificiality not seen by the human eye. If a bright star is "inflated" by the exposure how can one be certain that a galaxy is not similarly afflicted to some lesser degree? Is the "surface brightness" really so low as to avoid all danger of this occurring?

Is it not possible to reproduce images where brighter objects [usually stars] are not "inflated" but merely appear more intense? i.e. Appearing more as they would to the optically aided human eye. I find pinpoint stars, of varying intensity, far more satisfying than the typical "bloated overexposure" seen in many astro images.

On 15/04/2015 06:47, Chris.B wrote:
On Tuesday, 14 April 2015 22:22:47 UTC+2, Uncarollo2 wrote:
It's galaxy season!

Spring is here, and dozens upon dozens of galaxies are within reach of small scopes. Time to get out under mild spring skies and observe!

http://www.astromart.com/common/imag...7.jpg&caption=

Happy galaxy hunting.

Uncagalaxy

Images nearly always seem to produce an artificiality not seen by the human eye.

It is only an artificiality not seen by the human eye because the human
eye struggles to see the extensive faint diffraction ring pattern around
a strong point source whereas a CCD will with a long exposure. We see
the inner couple of rings if the seeing is very good. The CCD sees
however much of it is above the threshold of detection at a given
exposure time.

In the old days of film you also got halation reflections off the back
of the film and in CCDs you can get scatter along some preferred
directions. There were coatings supposed to prevent it.

The human eye is not as good as you think either. Very bright stars with
the naked eye appear to have transient radial spikes (and have been
drawn that way since the beginning of representational art).

If a bright star is "inflated" by the exposure how can one be certain
that a galaxy is not
similarly afflicted to some lesser degree? Is the "surface
brightness" really so low
as to avoid all danger of this occurring?

Yes. The effect is there but it makes very little difference to an
object that is barely above the detection threshold to begin with.

Is it not possible to reproduce images where brighter objects [usually stars] are not
"inflated" but merely appear more intense? i.e. Appearing more as
they would to the optically aided human eye.
I find pinpoint stars, of varying intensity, far more satisfying than
the typical "bloated overexposure" seen in many astro images.

Deconvolution techniques will do it provided that the point spread
function is well behaved and uniform across the field of view. You gain
resolution proportional to local signal to noise though so the effect
isn't as pleasing as you might hope. And the ringing around point
sources in regions extended emission can look annoying. Basically the
code can't tell the difference between that being real and it being an
artefact of the bright star (which it almost certainly is).

An isolated bright star against a mostly black background contains
strong hints in the deconvolution since the sky brightness can never be
negative. This is a power constraint in astronomical imaging.

--
Regards,
Martin Brown

On Wednesday, April 15, 2015 at 12:47:34 AM UTC-5, Chris.B wrote:
On Tuesday, 14 April 2015 22:22:47 UTC+2, Uncarollo2 wrote:
It's galaxy season!

Spring is here, and dozens upon dozens of galaxies are within reach of small scopes. Time to get out under mild spring skies and observe!

http://www.astromart.com/common/imag...7.jpg&caption=

Happy galaxy hunting.

Uncagalaxy

Images nearly always seem to produce an artificiality not seen by the human eye. If a bright star is "inflated" by the exposure how can one be certain that a galaxy is not similarly afflicted to some lesser degree? Is the "surface brightness" really so low as to avoid all danger of this occurring?

Is it not possible to reproduce images where brighter objects [usually stars] are not "inflated" but merely appear more intense? i.e. Appearing more as they would to the optically aided human eye. I find pinpoint stars, of varying intensity, far more satisfying than the typical "bloated overexposure" seen in many astro images.

The data was taken with 16 bit resolution, therefore the brightness ranges over a 64,000 to 1 level. The galaxies range from about 36,000 for the bright core to around 50 for the faintest parts in the spiral arms. That is a range of 720:1. These ranges all exceed what can be printed on film or reproduced adequately on a computer screen, by a factor of 10 to 100.

The only way to show the entire detail captured in the data is to logarithmically stretch the data (basically bring the faint detail up) until the faintest detail and brightest detail can be shown on a screen in approximately 64 discrete steps from totally black to totally white. Film is a little more forgiving in that it has non-linearities on both ends.

The stars, of course, we don't really care about since they contain no detail, and we let the brightest ones saturate. The glow around the brightest stars is a combination of scattered light in the atmosphere and on the optical surfaces of the imaging train. There can also be internal reflections from the filters.

The reason you never see classic diffraction rings around any of the stars is because the images are all long time exposures, and the motion of the atmosphere blurs them out. The only way to capture star diffraction rings is to take very short exposures of less than 1/100 sec on a very still night. Of course that will not show any of the faint nebulae or galactic detail, all you will capture is a few bright stars.