from Astrobin

[raymo]

I have been learning how to use Astrobin with the help of an IIS
member. I have reprocessed the M20 image that I posted recently,
and want to see if I can provide a link to the higher res version I
have in Astrobin http://astrob.in/105011/0/. Now to see if it
works. I wonder why it's not blue like the others I've seen.
I just left clicked, and it went blue. The wonders of technology.
raymo

[cometcatcher]

:thumbsup: Yep, works okay this end.

[LightningNZ]

Looks sweet :)

[raymo]

Thanks Kevin and Cam.
raymo

[Bassnut]

Apart from noise, this is very good. Why the hell is it in "beginners astrophotography"?.

[DJT]

+1 :thumbsup:

Very nice indeed

[raymo]

Hello Fred; I sincerely hope that I'm not annoying you again. I have
managed to produce one image that I am pretty happy with; but like
many other beginners I imagine, I am hesitant about taking that step,
especially as almost all of my other results certainly wouldn't qualify.
I guess it's sort of borderline I suppose; I have seen better in the beginners' forum, and the occasional less good one in the open forum.
raymo

Thankyou David.

[cometcatcher]

Fred, even though some of us are not beginners, the quality of images in the main deep sky forum makes it very intimidating to post anything there.

I've started posting some of my best there but my images have a long way to go yet.

[codemonkey]

That's a cracker, raymo!

[raymo]

Thanks Lee.
raymo

[jsmoraes]

Your photo is very nice. But it is with some saturation and noise. You use the same camera that I use.



With Canon, darks are very very important. And my experience says that the best with bright objects is with less ISO and more numbers of frames.

If I use ISO 400 or 200 I got more smoth stars and better colors. I normally use ISO 400 and if the object is faint, ISO 800.




I feel the same. Actually there are a very powerfull team there. With impressive and amazing quality of photos. But I thing that this forum is about astronomy, not photography. And there are two differents goals in astrophotography:

a) images with visual impact
b) images with contents

Some photos from NASA and ESO are not perfect and beautiful, but they have informations very interesting.
My photos aren't perfect or very beautitul, but they have some informations very interesting, too. Mainly because of the resolution of my set. And they are done with a stock Canon.

I think they can be published there without that virtual intimidation. Some times they bring some new information about the object.

[blink138]

that looks much more professional raymo, i have seen all of your pics but i am not sure if it is the astrobin that makes it better?
i have a bit to catch up on ha ha!.......... we just need the bloody weather to cooperate a little more
good cool nights for dslr sensors too!
pat

[raymo]

thankyou Jorge and Pat. I was going out tonight to get more subs of
M20 at lower ISO. Once again got set up and aligned, and then got clouded out yet again. It's now six weeks since I got an image.
It would look better in Astrobin, Pat, because it is a much higher resolution image.
raymo

[cometcatcher]

You an I need an observatory Ray. Save on setup time. I kind of have one, called the window lol. But it's very restricted.

Do you check the latest satellite image before you setup? Seeing cloud coming on radar or satellite would save a lot of work setting up for nothing.

[raymo]

Funny you should mention an observatory; just this evening my
daughter and son in law got SWMBO to agree to me having one, and they will help fund and erect it.:):):).
I do refer to all the weather data, but the weather here for most of
the year is like Melbourne, "if you don't like the weather, just wait 10 minutes. At this moment it is 4 degrees, 99% humidity, and I've just
brought my gear in absolutely soaking wet.
raymo

[cometcatcher]

That's great news on the observatory!

[raymo]

Yes it is, but they are in the process of moving into town, so will have to get settled first. Oops, hijacking my own thread, it's easily done isn't it.
raymo

[nebulosity.]

Very nice Ray!

Great news about the observatory build!

Cheers
Jo

[raymo]

Thanks Jo, my best effort so far, I think.
raymo

[andyc]

Hi Raymo, really lovely image! I'm very impressed. I have a couple of constructive comments, which, given my limited astrophotography experience you're free to totally disregard and don't come with a guarantee of correctness, but here goes:

1: I've been trying to read quite a lot about ISO settings, and so far as I can work out: you don't gain any more at all in sensitivity beyond about ISO800-1600, possibly even less; but you lose 50% in dynamic range per ISO doubling; and background noise covers a bigger fraction of that dynamic range. I'm still not perfectly sure about the details, but you may be better off shooting at ISO 800/1600 rather than 1600/3200 and then stretching with 'curves' in Photoshop? Perhaps that would make it a little less noisy? If you're using Photoshop CS2 or above, you can try experimenting with the "despeckle" and the "Reduce Noise" functions too.

2: Are you shooting in RAW format? It looks like each of your stars has a small dark halo around it, which in my images is caused by the Canon image processing software adding an unsharp mask to the image, which is most obvious in the stars. You can't get rid of it in the pre-processed JPEGs, but if you open the RAW image in the Canon Digital Photo Professional software (if that was supplied with your camera), you can set the unsharp mask strength to zero and instantly remove the halos (and also play around with the noise reduction settings there). Obviously that's only if those halos are from the unsharp mask!

Please don't take these as negative criticisms, it's a smashing looking picture with lots of good data, but I think you could get it smoother still! And I'm also trying to learn more about all this for my DSLR astro shots.

[codemonkey]

This is an interesting comment and one I'd like to see explored and clarified a bit more because it differs from my own understanding, which may well be wrong.

My understanding is that dynamic range is basically the range that constitutes the "intelligible" part of the signal and by increasing the ISO, a large part of the dynamic range loss actually comes from the increased noise drowning out the signal making it "unintelligible". The remainder would be highlights that get lost due to clipping that wouldn't otherwise take place. So unless I'm mistaken, you don't lose out with decreased dynamic range and increased noise, I think they're one and the same.

The flip side to that is that some cameras, (some?) Nikons at least, clip values within a certain range of 0, so by upping the ISO and pushing the signal outside of that range you'd actually be increasing your dynamic range.

I saw a really good article on this a while ago that explained it much better than I can, but I have no idea where it is now. It indicated that there's a certain sweet spot with dSLRs where you get the most dynamic range... ISO 400 was a good spot for one of the particular cameras.

[cometcatcher]

But does any of this, dynamic range etc take stacking into account? If as Ray is doing, takes many 8 bit jpegs and stacks them in DSS, the final image is 32 bit anyway.

With the testing I did on my Pentax K-x, I couldn't see any difference between 1 x 4 minute sub at ISO400, 2 x 2 minute subs stacked at ISO800 or 4 x 1 minute subs stacked at ISO1600. I never took the test over ISO 1600 but assume (perhaps incorrectly) that 8 x 30 seconds stacked at ISO3200 or 16 x 15 seconds at ISO 6400 would be much the same.

Now I have read that longer subs are better, but not sure if that applies to DSLR's that are not cooled. Long subs on DSLR's suffer from thermal noise.

Perhaps without realising it, Ray processes in 32 bit by doing the processing in house with DSS. That's an advantage to processing in 16 bit say with photoshop or worse in 8 bit with other software.

Two things come to mind that would help with this image though, more subs (the more the merrier lol) and taking flats to reduce the vignetting.

[raymo]

Thanks for your comments Andy and Lee. From what I have read, the ISO
sweet spot for most Canon DSLRs seems to be between 800 and 1000.
I am lowering my ISO as my tracking gets better. Now mostly using 1600. I know nothing about losing dynamic range at higher ISOs, but I
do know that images are much brighter, and more stars are visible, with
each ISO step up to 6400. {my camera's max} Beyond that figure I
wouldn't know. Obviously, the noise increases too.
I cannot see the haloes you mention, even at 400x{ when the pic is pixelated anyway}, but my eyes are not what they were. Assuming that they are there, maybe I've oversharpened a little. I'll have a look in DPP
at the unsharp mask setting. I use the ancient PS 7, so don't have the
features you mention. I don't use raw; too much stuffing around, and
humungous files. I don't use separate darks, flats, etc. I'm just
trying to produce the best results I can using basic methods.
raymo

[raymo]

Thanks for your contribution Kevin; you're spot on with the more subs comment, and I can say that I have taken two 30sec 6400 subs, and
four 30 sec ones, and couldn't see the slightest difference; even the noise looked pretty much the same. I can see differences in star densities in
the four corners of the image, but can't see any vignetting. It would
surprise me if I could, because the image is heavily cropped. Perhaps my
eyes are even worse than I thought. Incidentally, my 80mm achro
suffers badly with vignetting.
raymo

[doppler]

I find that iso 3200 is the best compromise for my 1100d canon, but it has worked really well, with low noise at 6400 on a cold night. Vignetting is a camera field of veiw/ sensor size thing. My 8" f6 newt had bad vignetting with a 35mm film camera at prime focus but is good with the canon 1100d crop sensor

[andyc]

Interesting points everyone! I certainly don't wish to disparage your eyes raymo :eyepop:, so perhaps best I give examples. The first image is a blow-up crop of part of your image, showing the darkened 'haloes' round the moderately bright stars (not on the brightest diffraction-spiked stars). You may need to magnify the image to see the haloes clearly. I get the same thing out my camera, I'm >90% sure that's due to unsharp masking put on images by a setting in the camera, and leads to hard edges round some stars, and dark haloes round others. The second image is a crop from one of my image subs, un-processed, but with the default 'unsharp mask' set by Digital Photo Professional to 3. This is how the jpegs look. The 3rd image is identical, but for the unsharp mask set to zero. The haloes disappear without the strong unsharp mask.

Just this evening, I discovered how to have your camera set the unsharp masking to zero by default (so you don't have to process it out). In the menu on the back of the camera, set "picture style" from 'standard' to 'neutral'. If that works for you, it would save you doing any fancy processing!

I really need to look out the detailed discussions of ISO and dynamic range - I don't really want to be making unsupported comments!! I'll look for the articles I was reading, beacause they were excellently supported by data. The technical part if I recall was that with the ISO at 400-800, each photon filled about 1 unit of the well in the detector. Until I can find the right article, this link has some exposure-ISO data (http://photo.stackexchange.com/questions/40188/longer-exposure-lower-iso-or-shorter-exposure-higher-iso-what-gives-better)- see discussions and graphs in the first answer. I *think* that it is read noise that limits lower ISO (and the need for longer exposures of course), and then dynamic range limits the higher ISOs, and the noise becomes a larger fraction of the dynamic range. Stacking images increases the dynamic range of your whole image, but only if the stars/pixels aren't already saturated.

I'm reasonably sure that more exposure in each sub = more signal, better signal-to-noise and generally better images, but again I need to find the appropriate data to prove this - I can recall an article with a series of California Nebula images demonstrating this, but can't find it just now. Obviously that' limited by how well your set-up tracks the stars!

Sorry for the long comment and hi-jacking of your lovely image, maybe we need to start a separate thread if discussion continues?

[doppler]

Boy this beginers astrophotography section is not really for beginers, I am getting lost already?

[raymo]

Thanks again Andy, I've set the picture style to neutral, and will see what happens when the weather finally lets me do some imaging. I'm
inclined to think that maybe I'm slightly overdoing the unsharp masking
myself. I saw the haloes, but for some reason they are much fainter
on my laptop than in your images.
Sorry Rick, some of it is over my head too.
raymo

[SimmoW]

hey chaps, here's a very informative site re. exposure vs no. of subs. It shows that 20 is a good number, massive law of diminishing returns after 20. Clearly, exposure length is king.

http://www.dens-astropics.org.uk/page%2035.htm

And here's a site that demonstrates the sweetspots for dynamic range and noise. Yep, around 800 is the sweetspot for most Canons.

http://www.sensorgen.info/

[cometcatcher]

I still think there is a place for many subs. As in the hundreds if not thousands. What I've found is that for every double number of subs, there is half the noise. For instance 2 subs stacked is half the noise of a single one, 4 is half the noise of 2, 8 is half the noise of 4 etc right up to 1000 is half the noise of 500! So apart from killing the poor camera, why stop? lol.

Less noise means the image can be stretched more in the shadow areas so brings out fainter detail. (Taking the longest subs one can reasonably do without trailing of course)

Sorry Ray, I'm afraid we've totally hijacked your thread now. :whistle:

[raymo]

No worries Kevin, it's still interesting. There is a member [who's name I
don't remember] who occasionally posts stunning pics here, that are the result of 250 or more subs.
raymo

[andyc]

I went and dug out the links I was looking at (on another machine), and found that one was Simmo's first one!

Another one, by Craig Stark, has really good data about ISO and dynamic range in an article entitled "Profiling the long exposure performance of a Canon DSLR" about halfway down the article (http://www.cloudynights.com/item.php?item_id=2786), (it's pretty technical in other parts too). Look for the "Dynamic range vs ISO" section. The take-home message was a loss of 50% dynamic range per ISO stop, and no increased actual sensitivity somewhere beyond approximately ISO 400 (the exact value will vary with model).

Keith Wiley has a nice graphic about dynamic range (http://keithwiley.com/astroPhotography/imageStacking.shtml)and some interesting discussion.

If anyone tests this out, it would be interesting to hear, but that's for another time and another thread!

And sorry once again for hijacking your thread ... everyone, go admire Raymo's lovely picture :thumbsup:

[raymo]

Thanks again for the information, which I will read; and for the compliment. I don't know if I have the meaning of sensitivity wrong,
but if I make exposures of the same length using 400-800-1600-3200,
and 6400 ISO, the images get progressively brighter, and show more and more stars and detail. I assume that this would continue if my camera's ISO went higher. I realise that as the ISO rises, so does the noise, but it still seems to me that sensitivity increases with the ISO setting.
raymo