Question about histogram in photoshop

[Marko of Oz]

I'm using a DSI-1, colour camera for dso's and when I get an image into Photoshop for post processing the histogram always seems to start blocked all the way to the left rather then a nice bell shape curve that I've seen in tutorials etc.

Is that what's called clipping and am I losing data because of it? If so how do I avoid it, what am I doing wrong if anything?

The first picture shows what I mean about the histogram of a picture that is suppose to be the Pinwheel galaxy M83.
The second picture is the result of some serious curves adjustments.

thanks

Mark

[Dietmar]

hi Mark,

in the first image lots of information on that whole dataset has been clipped!
the histogram should look like a "bell" as you say.
so - the clipping has occured prior to photoshop.

what software are you using for aligning and stacking?

[Marko of Oz]

Thanks for the reply.
I'm just using Envisage for aligning and stacking and it is set to save the files as type tif.

[[1ponders]]

It does look very clipped Mark. Initially the histogram should look like the first one below (this is for faint DSOs anyway. Bright nebula will look a bit different). The histogram should be way to the left as is yours, but there should be a gap between the 0 level of the histogram any your curve. But even after stretching and adjusting most galaxy histograms will still end up being fairly bunched to the left like the second image. The exceptions are for really bright large galaxies like M31 and LMC, eg third image. The third image has been dark clipped as you can see but the histogram spread much further than the first histogram.

[[1ponders]]

:lol: I just grabbed two images of histograms I had stored away somewhere and then randomly selected the M31 image to demonstrate a wide galaxy histogram. I didn't realise image two was from M31 :lol: :P Oh well. :)

[Marko of Oz]

Ok, so I'm clipping my images at capture time. I guess I need to play with the histogram sliders a bit more in the Envisage software itself when I'm capturing.
many thanks
Mark

[[1ponders]]

Not having used envisage much (like about once or twice) I'm intrigued to know how you can clip at capture. I thought the histogram sliders only made it easier to see the object on the screen, that they didn't actually affect the saved date. :shrug: Maybe I'm mistaken.

[[1ponders]]

When in doubt Paul, read the manual. :lol:

I had a look at the Histogram Display and Controls page of the Envisage manual. One part jumped out at me. "Note: The effect of the sliders is saved in all file types, except fits and fits3P". If that's the case and you are saving as Tiffs or Bmps then I'd suggest after getting you imaged composed you open your sliders right up to 0 and 65535 before clicking start. This way you will ensure you will imaging at 16bit (you may only be using the bottom 12bit or so, but you will have much more headroom to play with when processing. Plus you won't get clipped.). The image will look very dark and faint but you will be able tobring this out in PS.

See how it goes. ;)

[peter_4059]

If you use TIF as the save format in Envisage you save the adjusted image (the impact of the sliders is saved in the image) so you can clip the image if you have the sliders set too tightly. If you use FITS you save the raw data and the sliders only imapct what you see on the screen.

[Marko of Oz]

Fits it is then, and fits liberator..
thanks all