M51 - First Light with ATIK-16HR

The only thing I can think of is that the DSLRs are more sensitive to the wavelengths that light polluition occupies, orange sodium and all that.
This is something I would not have expected expecially as many CCDs have excellent red sensitivity (which is what you'd need for h-alpha).
So maybe its just not sensitive at the sodium emission frequencies, smart design if so!

Any ideas why this might be? I'm at a loss to explain. Maybe when I get on to taking LRGB images it might become clearer. I always wondered why DaveMc never had problems imaging galaxies from light polluted celbridge. Obviously to do with it being monochrome CCD.

Two theories...

I suppose with the DSLR its red filtered sensors are going to pick up most of the LP, the green will also to a lesser extent. I cant find a response curve for the 350/300D but I would guess that the response is higher for say low pressure sodium (~590nm) than it is for blue.

It will be very interesting to see how the galaxy looks with an LRGB image. In particular whether using just LRGB images combined shows any hint of light pollution while also showing the redder parts of this galaxy (patricularly NGC 5195)

The simpler answer might just be that light pollution doesnt affect telescopes as much as we think. When you look up and can only see mag 4 stars its depressing. But a sensitive CCD, a good scope, narrow filed of view will get enough light from the target so that LP disappears into the background noise. It would be interesting to see how this performs in a really dark sky.

Guess you'll just have to go take more images

That image shows the great potential for the camera.

There seem to be trails of dots in places in the image. Stacked bad pixels?

Cheers,
~Al

Well, I've taken M42 images in very low level light pollution and areas of high light pollution using the 300D and there is just no comparison (I wont put them up as many are m42 fatigued :lol: ), the orange glow is just not there in the low pollution shots, while the polluted shots look like I had an orange filter on the scope. Thermal cooling is not a factor in this (except for random/thermal noise which are distinctly visually different). Frequeny response is the key here methinks, you learn something new everyday, yet another reason to get a proper CCD

The simpler answer might just be that light pollution doesnt affect telescopes as much as we think. When you look up and can only see mag 4 stars its depressing. But a sensitive CCD, a good scope, narrow filed of view will get enough light from the target so that LP disappears into the background noise. It would be interesting to see how this performs in a really dark sky.
~Al

I asked a very similar question to the big dobsonian yahoo group regarding whether its worth getting a big dob in a light polluted environment and they came back and said you'll end up with a brighter background and a brighter object, the object would be bright enough that you can put a good LP filter on it to darken the background.

Of course, the QE of the CCD is a measure of its respoinse over various wavelengths. All CCD's have a poor response at the blue end of the spectrum though some manufacturers are producing CCD's with enhanced coatings that improve the QE in the blue end (even into the UV portion of the spectrum).

A CCD will capture photons from a light pollution source equally as well as from your target. But to an extent, you can process that sky-brightening out. For narrow-field images, its easy enough becasue you just have a borghter overall background which is easy to remove using levels or curves. For wide-field images, light pollution will manifest itself as a gradient. Tools are available to reduce these gradients which are a little trickier than a uniform brightening.

Of course, as you say Dave L, the cooling ability of the imager affects the overall quality of the image (by reducing thermal noise) and is presumably linear over the spectrum. And so will have little or no effect on the light-pollution photons over and above your target photons.

As Dave G says, I've never really apparently suffered from light pollution in Celbridge. But I guess that's because I image narrow fields and I image in mono. Certain my colour images have been poor - light pollution? Probably...

Interesting that the CCD sensor spec sheet for the Atik doesn't give QE in % - just relative values. That's unusual.

Dave

Maybe you lads in Celbridge are luckier then you think when it comes to light pollution. Anything over 3 minutes unfiltered here and its wash out city.