Understanding the histogram view

[metastable]

I'm back with another question, this time related to the histogram view. I was under the impression that the histogram would slowly move to the right as more data was stacked. From what I've seen over the past few sessions is that it seems, to me, that the histogram shows movement in the left portion of the data, left of the peak, for about the first 10 frames and then settles to almost no movement over the course of the live stack. But throughout the live stack the peak does not move. I took some screenshots at various total exposures. The reason the peak doesn't line up exactly is because I restarted the stack at some points.

Edit: After thinking about it a bit more it seems to me that after a long session I'm simply making a high quality 15s exposure rather than some additive outcome thats close to the total exposure amount. From my perspective it seems like I would need to do a much longer exposure to boost the signal greatly and then rely on the stacking to bring the noise down.

[admin]

Hi,

ideally, in uniform conditions, the histogram will not change during live stacking. While SharpCap adds the data from individual frames together in the internal representation of the stack, before calculating the histogram or displaying the image, it divides the data by the number of frames so far, resulting in the *average* values being used for the histogram and display.

This means that you don't see the image brightening as the stack progresses further, instead you see the noise level in the image reduce as more frames are averaged together. As the noise reduces, you may be able to apply a stronger stretch to the image to bring out faint detail without the noise being too obvious, but that requires a manual adjustment of the histogram stretch and doesn't happen automatically.

In older versions of SharpCap, sometimes you would see the histogram grow to the left of the peak during stacking - this was a result of drift/rotation leading to darker areas at the edges of the stacked image. The very latest versions of SharpCap 4.1 ignore the data from the edges and corners of the image when calculating the histogram, so unless you have a large amount of drift/rotation, this effect should no longer happen.

cheers,

Robin

[metastable]

Oh! I see now. That makes perfect sense and aligns with my observation. Thanks for clearing that up!

[metastable]

Hey Robin,

I had another thought about the live stacking display. When an image is saved from the live stack, say the raw 32bit, is the output the internal representation, or does SharpCap also average out before saving? Also, would it be possible to have an option to display the non-averaged data?

Thanks,
Alejandro

[admin]

Hi Alejandro,

when you are using the 'Default' stacking option then the 32 bit save is the sum of all the stacked pixel values at that position so far. This of course differs from the average only by a factor of the number of frames stacked so far, so there is no difficulty converting between the two. Saving the sum keeps all possible information in the saved file, whereas the average would throw away some information that is in the fractional parts of the pixel values.

Note that if you are using the 'Sigma Clipped' stacking then the saved values are the average, but they are either scaled up or saved as floating point values (depending on options in the saving settings), so the fractional parts of the average pixel values are not lost.

cheers,

Robin