DSLR, Histograms in RawTherapee and APT (plus Auto-Stretch)

[Chickpea]

Hi,

I am trying to better understand the use of histograms when deciding exposure lengths.

I am using APT to capture with a Canon 400D (OTA is a Skywatcher 200P which is f/5).

I took some 60s subs (@ ISO 400) of M27 a couple of nights ago and have been looking at them in Rawtherapee (which is completely new to me). Below is one of the images with the Linear histogram displayed, I have also included the Linear-Log and Log-Log histograms.

My limited understanding is that I want the peak to be a third of the way across the histogram, so perhaps roughly in the orange box in the image below. Is that correct and is that when viewing in linear mode?

I am hoping to shoot M27 again tonight and I shall be trying out the Histrograms feature in APT. My understanding is that I want the peaks to be in the 2nd or 3rd of the five regions in that tool

I noticed that the APT histogram tool has an auto-stretch feature. As I intend to stack in DSS and then process in StarTools, should turn auto-stretch on in APT or leave it off (or does it not matter?)

Any thoughts would be much appreciated. I have also included the raw file that I viewed in RT.

L_M27_3_1962_ISO400_60s__NA.CR2

 

[ONIKKINEN]

2 hours ago, Chickpea said:

My limited understanding is that I want the peak to be a third of the way across the histogram, so perhaps roughly in the orange box in the image below. Is that correct and is that when viewing in linear mode?

The histogram 1/3rd of the way rule is just a guideline, but it doesn't actually mean anything and cant be used to gauge whether an exposure is good enough if you want to be pedantic about it. After all you could double your ISO and get a much brighter histogram which surely is much better? Well, not necessarily and often DSLRs have a point of diminishing returns at some ISO after which read noise no longer decreases linearly (not sure about the 400D, some searching suggests ISO 400).

If you really wanted to do the math you would need to know the electrons to ADUs conversion ratio at the chosen ISO value, the amount of read noise in electrons and some other technical gibberish. These values are not given by DSLR manufacturers so there wont be an easy way to do all this, hence the common tip of histogram 1/3rd of the way through as an indication of a good enough exposure (its not, but better than nothing or taking 1s frames). For what its worth the 60s exposure you have linked here looks ok to my eyes. Maybe a little bit on the dark and noisy side, you could try exposing for 2 or even 3 minutes if your guiding can handle the extra length. If not, take a few hundred subs like this and there should be a nice M27 in the end.

 

2 hours ago, Chickpea said:

I noticed that the APT histogram tool has an auto-stretch feature. As I intend to stack in DSS and then process in StarTools, should turn auto-stretch on in APT or leave it off (or does it not matter?)

Not familiar with APT, but if its just a visualization thing then it doesn't matter. If it actually applies some kind of stretch to the raw data and saves it modified then leave it off.

[Chickpea]

Thanks very much for the advice.

I watched yesterday  this video with Robin Glover which covered the maths that you mentioned. It useful to know that I won't be able to get the read noise data from Canon. I had hoped to use the Sensor Analysis tool in SharpCap, but have not yet managed to get the DSLR driver working.

I appreciate the advice regarding 1/3rd guidance and also the feedback on the image. With regards to the Histogram tool in APT, I suspect it is visualisation only, and does not affect the RAWs but for now I will leave it off when shooting.

I took your advice on increasing the exposure and last night I captured for around 2 hours on M27, shooting 120s subs @ ISO 400 (plus 100 bias and 25 darks).

This is the histogram in APT that was getting at the beginning

By the end of the session, the peak had moved left, so that the left side of it was just overlapping the edge of the first segment. The is the one of the first subs in RT.

And this is what I managed to get after stacking in DSS and fiddling around in StarTools (which I am still very new to). It is just a screengrab because I haven't bought a licence yet.

 

 

Compared to the previous capture session, I was actually able to extract some colour, which was pleasing.

I think that the exposure was probably much better, but I am still manually focusing and the guiding was bit hit and miss last night.

Thanks again for all the advice, and if you have any further thoughts, they would be very much appreciated.