use and insights into histograms, what do they mean?

[alcol620]

Hi folks

Can someone point me in the direction of an easy to understand explanation of astrophotography histograms and how they are interpreted and used. 

Thanks

[VikN46]

Hi, I found these tutorial a big help to get me started although based on Photoshop

 http://www.budgetastro.net/tips-and-tricks.html

[Leveye]

Histograms are easy to read. You never want to go too low with them or too high. PS lets you set points so you don'r clip things and "distort" your picture. Midrange holds the most details to our eyes. Try an s curve.

[alcol620]

Hi VikN46 & Levene

Thanks for posting. The budgetast tutorial looks useful, and it gives lots of info on using Photoshop - thanks

Levene, not sure what you mean by go too low or too high, or setting points etc. I am new to all this have have to start from basics.

Regards

Alec

[VigdisVZ]

The budget astro videos are pretty good, I learned a few things from them, even though I have worked with photoshop professionally off and on for 12 years. The techniques used in AP are different.

Basily the histogram is a summary of how many pixels of each brightness you have, and a tool to alter the brightness and contrast in an image.

If you have a lot of flat histogram to the left (dark side) you can probably trim away some by moving the baseline closer to the "heap", but dont overdo it as you will easily destroy that fine data background in the darkness and make your image "burned out".

Like every tool in the box, you need to apply them very carefully as each "edit" degrades the final result. Image editing is all about presenting the interesting stuff while lessening the excess data.

It all becomes much easier when you have more/longer subs.

[alcol620]

Thanks everyone, still a lot to learn, but certainly a bit clearer now

Thanks again

Alec

[lensman57]

the

Hi folks

Can someone point me in the direction of an easy to understand explanation of astrophotography histograms and how they are interpreted and used. 

Thanks

The left side is pure black or pixels of zero value in simple terms, the righth side is pure white or saturated pixels, the mid point is the 50% point. If you push the histogram to the left so the shaded area of the graph touches the left axis then you are clipping the data if you put it to the right then you are saturating. In AP the histogram produced by the camera provided that the sub length was correct is represented by a very narrow graph as the dynamic range has been compressed for data representation, this data needs to be " stretched " by the use of the levels and curves in PS or other methods in AP software so the histogram is as stretched as possible without clipping or saturating the pixel values, ie data. In practice you find that in most cases the stretched histogram is normaly to the left of the mid point but well stretched out and in case of an RGB  image the left hand shoulders of the R,G, B graphs meet up at the same point for a good basic colour balance.

Hope this helps a bit.

A.G

[IanL]

Try this, everything you wanted to know about the histogram, etc...:

http://www.blackwaterskies.co.uk/2013/12/how-to-interpret-image-histogram.html

[alcol620]

Thanks IanL  

Excellent reference page. Has certainly helped my understanding.

Thanks again. Everyone

What would we do without the internet?

Alec

[Pete Presland]

Try this, everything you wanted to know about the histogram, etc...:

http://www.blackwaterskies.co.uk/2013/12/how-to-interpret-image-histogram.html

very nice link, explains it very well.

[Alien 13]

I have never realy looked before but when i import a 16bit image into PS CS2 the histogram only shows 0 to 255 which is the 8 bit levels anyone know why?

Alan

[Thalestris24]

Hi

I think CS2 handles 16-bit / channel files (when selected via 'Mode') but only gives you a coarse adjustment of them - hence only 256 levels.

Louise

[Alien 13]

Thanks Louise.

Alan

[Thalestris24]

Thanks Louise.

Alan

Hi Alan

Well 'I think'  I'm correct! I made assumptions based on the interface - I've not looked deeply into it as I mostly use StarTools. FITS Liberator gives a 65536 level histogram but only displays in black and white. I use it for quick inspections of FITS files sometimes. It would be much better if it had a decent sized image display area! Heaven knows why they made it with such a small box!

Louise

[alcol620]

Thanks everyone for your help. Having spent hours trying to get to grips with this I am well pleased with an output. See attached images of M13. Both taken with ASI120MC, EQ5 pro Goto and Celestron 8SE

The poorer image, taken the next night looks to be somewhat out of focus. I may have left the 0.5 focal reducer off the camera for this, hence the more magnified and poorer image. Seeing was similar both nights, Only took 50 lights and 10 darks on each occasion.

I did take some shots of the Ring Nebula, but these are so poor that even with my new found Photoshop and Deepskystacker knowledge, I can get no meaningful output from them.

Thanks again everyone and to Doug for the tutorials at Budgetastro.net