Sculptor Galaxy, first reasonable shot with the new setup

[andyc (Andy)]

This is about the first decent result from my new set-up, having finally got the autoguider to speak nicely to the mount. But I was sufficiently drunk to not notice the focus being a little off, always something to work on, quite apart from polar alignment, darker skies, collimation and exposure time . A stack of 5 x 4-minute exposures, stacked in DSS, including flats. EOS 60D at ISO 640, Skywatcher 150mm reflector on HEQ5pro, guided with Orion mini autoguider. Processed in Photoshop CS2.

A slightly larger version available here.

[Rex]

Good start Andy, more data will make it even better.

[raymo]

Good first try; 800 or 1000 ISO would have produced more detail
without too much noise.
raymo

[SimmoW (SIMON)]

Isn't it so nice when guiding goes well - which it has done here.

[andyc (Andy)]

Thanks guys! I have since seen what better focusing does, so should have better shots soon enough.

Raymo, I'll have to disagree on the ISO. Everything I've read (best exemplified in Craig Starks article ) says you lose dynamic range without getting any more photons beyond abut ISO400 - basically I should reduce my ISO!

""The camera’s internal gain (e-/ADU) for each ISO value points toward limiting the ISO to 400 and not using higher values. Both in theory and in practice, using higher values limits the dynamic range and does not let you pull out fainter details from the noise (even if they look brighter). The exact optimal ISO value will likely vary from model to model, but it's unlikely to be the high ISO settings.""

I think I understand the reason high iso works for you... I think because you (if I recall) don't use RAW and use the JPEGS. Because you're losing the bit depth, the image needs stretching before it leaves the camera (or the detail is lost forever in bit compression), which is exactly what high iso does above 400. It's at a high price of lost dynamic range and greatly increased noise, but that's less important if you only have an 8-bit image, and you can reduce the noise by stacking a lot of images (which you do).

I want to actually run test images to show these effects, but not quite had the time yet.

Having said all that, you do great things with your 8 bit shots Raymo! Go with whatever you're happy with and enjoy

[raymo]

Everything I have seen on IIS from highly experienced imagers seems
to suggest that the sweet spot [read best compromise] for astro work with most Canon DSLRS is around 850-1000. I don't see many posted images in this or the main section below 800. Of course, a lot of the images in that section are not taken with DSLRs.
raymo

[rustigsmed (Russell)]

some are even taken at iso 6400!
http://www.iceinspace.com.au/forum/s...=126321&page=2

I think craig stark's article was done quite some time ago - with older cameras that maxed out at iso 1600. I would experiment to see what works for you with your sky conditions and ambient temp.

great start to the photo!

[andyc (Andy)]

Yeah, Justin's image is pretty impressive! I think this is an issue I really want to spend some time testing thoroughly. The other issue I didn't mention was that dynamic range increases with stacking, but that can't compensate for saturated pixels because of reduced dynamic range in a sub. I also can't see any logic that says the signal will come out of the noise as you increase the gain (ISO) which stretches both signal and noise. The 'sweet spot' will vary with camera, but I really doubt I'd actually get more signal-to-noise from ISO1600 over ISO800 (or 400-640), as it logically doesn't make much sense, once there are fewer electrons to ADU points. But one day I'll prove to myself whether I'm right or totally wrong!

Craig Stark's article is only two years old, but the camera he used most in the test was a bit older (an EOS 450D), which as you say only pushed ISO to 1600. The question about Canon's treatment of darks is another one for testing too...