the mystery of the disappearing stretch

[toryglen-boy]

Guys

Looking for some more imaging guru's info.

When i take shots with the DSLR, and stack them, when i open them up in CS4, the histogram looks ok, and the image looks as bright as it did.

However, when i stack images from the QHY8 it looks great in DSS, but then when i open them up in CS4, the image looks almost black, like a 2 second sub with no information, and the histogram is like a needle, tall and thin, and doesnt cover the entire range, using curves etc to lighten it, doesnt help

what gives?

Using Maxim DL 4 and QHY8

:thanx:

[Octane]

Probably something to do with signed and unsigned 16-bit formats.

That is how the images appear in Photoshop after being saved as PSD's in IRIS.

All you have to do is coax the details out with subtle and numerous curves and levels iterations.

Regards,
Humayun

[multiweb]

That's because you need to DDP and save it as a scaled 16bits file prior to opening it in PS. Make sure you don't clip the end of the histogram though. More is better than not enough. You can always further tighten in PS levels/curves.

[toryglen-boy]

H, i was reading that reply, and all i was thinking was ...

WTF?

:lol:

seriously, if the histogram is almost a straight line, in one point, then is there anything to coax out of it, at all ?

[Octane]

How big is the file? Email it to me and I'll have a play tonight.

Regards,
Humayun

[toryglen-boy]

about 10mb, but i will crop it down.

top man, will be in touch !!

;)

[toryglen-boy]

DDP?

there is no end of the histogram mate, there is a spike in the middle, and thats it !!

:lol:

[RB]

Duncan can you post some screen shots of the native histogram?

It sounds normal and I'll explain why.

When we stack, most of the info gets compressed into a small section of the overall available histogram, especially since you are using the QHY8 since it has 16 bit depth.
Older DSLRs only have 12 bit per colour channel and the newer ones have 14 bit per colour channel.

The more bit depth an image has, the more the histogram is compressed after stacking.

The spike that you see actually should have info in there, it's just that it takes up only a small portion of the full histogram (since it's a 16 bit camera).

What we do is we use levels first to bring the end points of the histogram in so that the spike gets stretched and eventually that spike becomes a 'bell' curve.

From there, as you know, we do the other necessary curves and other adjustments but avoid any clipping.

I haven't used Maxim DL so I can only speak from my experience with CS3.

Hope that clears it up a little.

I'd be interested in seeing the histogram and also the file.
Unless there's a simple mistake somewhere along the process or a wrong setting, it sounds like what we get when we initially open a stacked image in PShop.

[Octane]

10 MB is fine.

Email it to hq AT hqphotography DOT com DOT au

Regards,
Humayun

[toryglen-boy]

thanks for that RB, it makes *some* sense to me. am at work just now, but will post something tonight

:)

[jase]

Indeed, the others have nailed it - good advice. You've got more bit depth than what photoshop is expecting hence the 16bit image will initially look black with perhaps a few bright stars presented as dim point sources. As Marc suggests, you can do a DDP stretch and log stretch to fill the bit space prior to opening it up in photoshop, but this is nothing some levels and curves can resolve if you don't want to do any prior stretching. Also, note that while with 16bit data, you've theoretically got 65k levels of brightness, photoshop summarises this from 0 black to 255 white (similar to an 8bit space).

[toryglen-boy]

ddp?

[multiweb]

There is but you can't see it. Let's say you have a 16bits camera. The pixel intensity could go from let's say 0 to 65k. If you image something dim you'll get the sky glow at let's say 900, then your signal (faint neb) let's say from 1000 to 3000 then a gap then a few stars up to around 20k then a blank. That's it. So the range of your histogram is from 900 to 20k. Couple of spikes in the middle of the whole range from 0 to 65k. So you need to stretch your data and save it scaled. To do so you make the 900 closer to 0 and the 20k closer to 65k. DDP will do a non linear stretch. Once you're done you save as a TIFF 16bits or other the scaled data and you get your full histogram range in PS. Voila!

[toryglen-boy]

Thanks fella, your a trooper for answering my inane drivel over all this, but again, whats DPP?

you shall have your reward in Heaven

:thumbsup:

[jase]

http://www.noao.edu/outreach/aop/glossary/ddp.html - basic explanation.

[toryglen-boy]

ok, so i have been reading up, and DPP comes back as Canon software? How and where can i do a DPP stretch? i am using Maxim DL, and CS4

guys, you are making headway, and i am starting to understand this.


:D

[jase]

In MaximDL, you access the DDP features via the Filter | Digital Development menu item. Auto check the background and mid-level to start then uncheck them. MaximDL will analyse the image and determine the appropriate values. Normally these are quite aggressive so you'll want to tweak accordingly. For the background level, what ever it determines, subtract around 100 to 200 ADU counts from the value to avoid any black clipping. Adjust the mid level based on the how you wish to display the image. My recommendation is to not be too aggressive with the mid-level just stretch the data to fill out the bit space, then bring it into photoshop. Give it a whirl!

[toryglen-boy]

Jase, you're an absolute toff, and i salute you.

i will check it when i get home, although i dont remember seeing the menu you mention in maxim ... hmm !!

watch this space

:)

[TheDecepticon]

Why don't you invest in some good literature?:shrug: You are obviously very inexperienced at processing, so a good information source is a great help.:thumbsup: Try "Zone Systems". This will teach you how to stretch your image after it is stacked and many other things as well.As the others have said, what is happening is normal, it is up to you to use your processing software to tease out the detail.:)

[toryglen-boy]

yeah mate, i am aware i am asking some really dumb questions, been observing since i was 11, but only imaging in the last few months.

i like to think it evens itself out, when i can help people with stuff i know about.

usually IT issues, or guitars !!

:lol:

[jase]

Just a note on DDP. I tend to use it sparingly on luminance data, but I know others that use it as the primary means of stretching data. The process can deliver pleasing results, but for ultimate control, its hard to beat photoshop's levels and curves. The ability to display both shadows and highlights in a single process if not correctly understood can lead to trouble. Take note of the mid-level as this is where you tell the histogram to start the progressive curve to highlights.

Like all tools they have their place. I do like DDP for RGB data however. Stretching it as far as the colour data will permit, followed by heavy saturation, then introduce the layer as a softlight blend, then drop the opacity to taste. If that doesn't bring on the vibrant colours, duplicating the softlight layer again (two softlight blends) will surely get you rich colours. mmmm s a t u r a t e d c o l o u r ....

[AlexN]

Jase - Thats an awesome technique right there.. I've been having some nightmare issues trying to get nice vibrant colours from HaRGB shots, the soften blend routine as you described above is going to get a test run tomorrow! :)

I too prefer levels and curves to DDP... I use DDP at times, but generally find levels and curves creates a better, more contrasty image..

[kinetic]

Duncan,

Notwithstanding any great advice given so far but maybe
a 2 pic comparison to demonstrate 16bit histograms.

Pic 1 is a raw result of Eta Car with the histogram superimposed.
A huge amount of info is in that image.....it's all just crammed
right down the left hand end where I have arrowed.

Repeating what others have said, because you are working with
the 16bit complete range still, there is a lot of info you can tweak
and stretch with careful application of curves from this point on.....
that is...while it is still a full 16bit image.

Pic 2 is that info now stretched with a careful curve applied.
See how much was crammed down there.

Hope this helps. (btw my cam is slightly less than 16bit but the concept
still applies)

Steve

[toryglen-boy]

Steve

Thats some great stuff, and thats exactly what i get. i looked like an eejit, and i should have researched this alot more, before going down that route of CCD imaging, maybe a couple of years with the DSLR instead of 4 months, would have stood me in better stead, that said, if i can work out how you got from one image to the other, then i am off and running ... and i guess thats where the DPP stretch comes in.

:)

[kinetic]

It's like riding a bike mate, once you 'get' it you won't forget it :)
Only thing is, people like Jase just ride their bikes so much better
than me and make me look hopeless ......and Alex, well he has
a new bike every week :D:thumbsup:

Steve

[toryglen-boy]

i will stack some images from the other night, and see what i can come up with

:)

[TheDecepticon]

Hi! Sorry, they are not dumb questions!:) If you have to ask them, go ahead. I was trying to help you put a short cut in place and you can always refer back to it at any time.
He, he, that reminds me, I need to get some guitar strings for my daughters guitar!! Thanks!!:D

[toryglen-boy]

ok, so now we are getting somewhere! stacked some frames, no lights etc, just to see how this would look, i think it looks quite promising!!

all i need now is :

Some advice on the best DDP settings

and

When i save this image as a TIFF, and open it up in CS4 its all dark again, do i have to flatten the image or something? how do i get it looking like this, no matter what i open it in?

thanks!!

EDIT : i know its says the file is a PNG, i saved it in a few formats to see if anything would happen, but the file in question is a TIFF

[DavidU]

Duncans doing it ! Keep it up champ.

[jase]

Did you check auto for both background and mid level? This will provide you with the baseline settings for the image in question. Make sure there is no black boarders due to slight alignment errors or similar as these are also taken into account.

Once the above had been actioned, did you subtract 200 counts from the background? The screen stretch looks clipped, but in may not be if you zoom in. Move the red triangle (black point) all the way to the left or simply enter zero in the minimum field and hit enter. Then use the + magnifying glass to check if clipping has occurred.

I forgot to mention the DDP in MaximDL by default also sharpens your data as it stretches it. If you want to disable DDP sharpening enter the DDP dialogue box (see attachments below) and create a filter as shown. This will disable the sharpening function. Of course you may choose to keep sharpening enabled, but I prefer to use other tools for sharpening such as deconvolution, USM (unsharp mask) or HPF (highpass filter). If the image is noisy, you will certainly not want to sharpen as you stretch the data as this will simply exacerbate the noise.

[toryglen-boy]

Thanks Jase, but why does it still show up really dark in CS4, how can i flatten the image, or save teh changes, so to speak?

:question:

[jase]

Depends on how hard you've DDP stretched the data. Once you click OK, DDP is then applied. No form of imaging flatten etc required. Just save as a 16bit tiff and open in PS. Looking at the histogram on the screen stretch, I'd say you haven't applied it hard enough, but without having a good look at the data its hard to tell. On the screen stretch windows, when you move the green arrow to the left i.e decrease the white point, are there any black boarders around the image? You will want to crop these out for the best result.

Unfortunately, there is no one size fits all setting with DDP. You need to adjust it on a per image basis to get the desired result. Try stretching the data really hard and see what happens in PS. Backup your data and experiment - you've got nothing to lose!