Help with exposure

[AdamJL]

Hi all

Just want to understand proper exposure. Below are two images of the Lagoon Nebula taken at different locations (Bortle 5 and Bortle 3) with different exposures.
I took the one at a Bortle 5 site first, wasn't happy and went to a darker site to take the second as I wanted better exposure.

Bortle 5 image
ISO 800, 90 seconds

Bortle 3 image
ISO 2500, 120 seconds


I'm trying to understand why the histograms are both so close to the left. I was aiming to get the second histogram over to the right so I could stretch the image. The ISO is 3 times more sensitive, and the shutter speed is 1.3 time slower. Same scope.
How on earth do I get that histogram over to the right so I can edit it better? I can't push my stacked files at all because the histogram is so close to the left :(

Cheers
Adam

[AdamJL]

no one? :(

[Sunfish]

I think as long as the histogram is not hard up against the left hand side so data is cut then that is fine. Perhaps more details about the camera used would make things clearer. With faint astro the histogram is always going to be over to the left or some parts will be blown out or noisy.

[bojan]

Have a look at this link (http://www.iceinspace.com.au/forum/showpost.php?p=478944&postcount=37)... It might be helpful to you.

[middy]

Hi Adam,

Your histograms are perfectly fine. You are probably making the mistake of thinking the histogram represents your signal, and I made the same mistake for a long time until I came across an article on the internet that explained it all clearly. The peak of the histogram does not represent your good signal (the stars and nebula), it represents the background. If you think about it the vertical axis shows the count of pixels in the image that have a particular luminosity value. The typical astro image is predominantly dark with a sprinkling of stars and the nebula/galaxy. It makes sense that the histogram is over to the left always because the number of dark pixels far outweighs the number of light pixels. The good bits of the signal, that we are interested in, are all down the right side of the histogram and out along the tail.

Comparing your two histograms, you can see the peak shifts slightly left on the image taken in the darker skies. This is expected because the sky has less light pollution so the background is darker, hence the histogram peaks at slightly lower luminosity values. If you look at the right hand tail of the two histograms you can see the longer exposure in the dark skies now extends out past 64 now because the nebula is a lot brighter and has a lot more data to it. This is all the good, juicy data that we want.

If you plotted the histograms using a log scale for the y-axis you would see it even more clearly. This is the reason they say to leave the right slider (white point) alone when adjusting levels because you can easily drag it too far to the left resulting in much of your good data being set to luminosity 255 (i.e saturated).

Cheers,
Andrew

[jahnpahwa]

Great explanation Andrew! cleared it up for me, at least :thumbsup:

[AdamJL]

How do you stretch the image though, if you have a tiny gap on the left?
I guess this is where I have to learn processing for astro.

My camera is a 5D Mark IV.



Very helpful, thank you!



Hi Andrew. Terrific post, thank you very much for that. I have trouble stretching the histogram in PS with Levels because it's so close to the left, but I'll try Star Tools soon, see if that can help.

Thanks again!
Adam

[Startrek]

Adam
Startools is great software for the beginner and advanced, I’ve been using it with great results with both my Canon 600D for 3+ years and more recently my new ZWOASI2600MC OSC
Very important with Startools
Stack your subs ( Raw Cr files not jpegs ) in Deep Sky Stacker but DON’T touch after stacking ( don’t adjust curves, colour or anything ) just save as a fits file ready to load into Startools. In order for Startools to work its magic, it requires clean linear data from the stacker.

Deep Sky Stacker stacking software is free ( I can send you a basic procedure for stacking if you wish )

Startools is not expensive and your licence allows you to receive the upgrades etc.. I can send you a basic workflow but there’s plenty of tutorials on line also

Cheers

Martin

[Sunfish]

As Martin says, download DSS and import your images as raw files. Make sure DSS recognises your raw files as those from your camera.

Stack them in DSS with default settings and then save without modifications although you can slide the adjustments so you can see how it looks first in case there are bad files included. Inspect each sub first to make sure you do not include any with any movement..

If you are using PS you can save as a 16bit tif and then use exposure , contrast saturation and black settings in LR or PS.

If you are using astro tools such as Startools or Astropixel Processor and so on save the field as fits files without modification. It is better to to get the settings for background colour alignment and auto adaptive combination right in DSS if you can and the white balance in the camera should be fixed on daylight.

[AdamJL]

thanks chaps.

I've been using DSS for a bit, it's indeed a great bit of software. I tend to go into Photoshop from there, as it's what I am used to from my "normal" photography hobby.

Just not getting great results, so will definitely try and learn Star Tools

[AdamJL]

thanks everyone. My first attempt at editing. I had some issues again with the histogram. Opening in Photoshop showed the histogram HARD up against the left (clipped blacks) yet the image contained no clipped blacks at all. Switched to 16bit mode and it gave me room on the left to stretch.

Still a heck of a lot to learn here. This was only 11 unguided subs due to high wind (shooting into the wind). I took about double that.
No darks, no bias, only lights and flats (might try it again with all four, I just didn't get good results earlier)

120 seconds, ISO 2500, Canon 5D IV
Starwatcher 80ED, no flattener, on EQ6-R Pro
Polar aligned with PoleMaster, and 1 star aligned to Jupiter :D

stacked in DSS, edited in Photoshop

[Wilsil]

Hi Adam, What I do in PS is go to Curves and stretch it by dragging the curve up. This will move the histogram to the right.
Then go to Levels and bring the black point (left) back to the right.
You can do this a few times and s different each time.
However I am still learning a swell. :-)

[AdamJL]

Hi Will

thanks for the advice!
Interesting you do a curves stretch first. Didn't think of doing that, I've been hitting levels first. Makes sense :) I'll try it next

Cheers
Adam

[Sunfish]

You do not need a bias but you do need darks , hopefully at the same temperature exposure time and iso. Looks pretty good anyway.