Histogram and Why?

Discussion of using SharpCap for Deep Sky Imaging
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ScottD
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Joined: Mon May 04, 2020 2:45 pm

Histogram and Why?

Post by ScottD »

Hello! I am pretty new to Sharpcap but must say I absolutely love it! There are some things that my newbie mind is having difficulty with however. For one, can someone tell me why we would want to mess with the image histogram if all it does is show you what it looks like on the screen? I originally thought it was actually changing the captured image, but then looked at the exposure time and gain while changing the histogram and nothing in the camera settings were changing. Then, I read that changing the histogram does not impact the actual captured image. So, why do we mess with it?

Any advice would be greatly appreciated!

Thank you,

Scott
chongo228
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Re: Histogram and Why?

Post by chongo228 »

You can watch the histo to make sure your exposures and gain are set correctly. This can help you dial in exposure and gain for planets and avoid over exposure. For DSO you typically only have to worry about over exposing bright stars.

You want to "mess" with it so you can make sure your target is framed correctly and in focus. Focus is probably the most important here. If you're using a bahtinov mask you could expose for 6 to 10 seconds without touching the histo gram, or stretch the histo and expose for .7 seconds and get the same results. This helps you focus in real time and not have to wait between adjustments to observe the change. Until you get into autofocus stretching the histo really saves time.

And most of all.....while spending hours it gives me something to look at. Looking at an upstretched image is pretty boring.
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turfpit
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Re: Histogram and Why?

Post by turfpit »

Scott

Some background reading on the histogram https://docs.sharpcap.co.uk/3.2/#The%20Histogram.

Camera controls like exposure, gain & offset will affect the histogram and the data being saved. Other camera controls (depends on the camera) will affect the display but not the saved data. A good example is the Display Histogram Stretch which, when applied, will brighten the image displayed (useful for finding fainter objects) but does not affect the data saved to disk.

The attached document shows M42 with different exposures - 10s, 30s, 60s, 120s. Note how the image and the histogram changes as the exposure changes. This would be a worthwhile experiment to carry out with your own equipment.
M42 exposures for HDR.pdf
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Dave
ScottD
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Re: Histogram and Why?

Post by ScottD »

chongo228 wrote: Wed Oct 28, 2020 11:01 am You can watch the histo to make sure your exposures and gain are set correctly. This can help you dial in exposure and gain for planets and avoid over exposure. For DSO you typically only have to worry about over exposing bright stars.

You want to "mess" with it so you can make sure your target is framed correctly and in focus. Focus is probably the most important here. If you're using a bahtinov mask you could expose for 6 to 10 seconds without touching the histo gram, or stretch the histo and expose for .7 seconds and get the same results. This helps you focus in real time and not have to wait between adjustments to observe the change. Until you get into autofocus stretching the histo really saves time.

And most of all.....while spending hours it gives me something to look at. Looking at an upstretched image is pretty boring.
Thank you for your reply! I do understand how to set exposure time and gain and watch the smaller histogram at the bottom of the controls to make sure it's close to where it should be. I was actually talking about the histogram that appears while live stacking and not the display histogram stretch. My writing can be lazy and unclear, so my apologies for that. Just to be clear, the histogram in live stacking does not impact the actual saved image, correct? When focusing, I actually zoom in on a star and use the batinov mask. When framing I use that lightening bolt (auto stretch) in the display histogram to get a clear picture. I was really just wondering what purpose the histogram in live stacking actually served. I do like your response that it gives you something to look at! Again, thank you! Scott
ScottD
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Re: Histogram and Why?

Post by ScottD »

turfpit wrote: Wed Oct 28, 2020 11:09 am Scott

Some background reading on the histogram https://docs.sharpcap.co.uk/3.2/#The%20Histogram.

Camera controls like exposure, gain & offset will affect the histogram and the data being saved. Other camera controls (depends on the camera) will affect the display but not the saved data. A good example is the Display Histogram Stretch which, when applied, will brighten the image displayed (useful for finding fainter objects) but does not affect the data saved to disk.

The attached document shows M42 with different exposures - 10s, 30s, 60s, 120s. Note how the image and the histogram changes as the exposure changes. This would be a worthwhile experiment to carry out with your own equipment.

M42 exposures for HDR.pdf

Dave
Thank you Dave! I asked another person the same question - to be clear, adjusting the histogram in the live stacking mode does not impact the actual saved image, correct?
ScottD
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Re: Histogram and Why?

Post by ScottD »

Dave,

Thanks again! I just read again, the link you posted to the Sharpcap histogram information. I was actually very confused. I've only used this program with a dedicated CMOS camera a few times. I was always under the impression that by adjusting the histogram being show while live stacking was actually impacting the saved image. I now see that by clicking on the image histogram icon in the toolbar, that is the actual histogram being effected by adjusting the camera controls such as exposure time and gain. I do photography as a hobbiest and use the histogram, but it is directly correlated to the aperture and shutter speed settings. Again, I was very confused. Thank you for clearing things up!

Scott
Last edited by ScottD on Wed Oct 28, 2020 12:24 pm, edited 1 time in total.
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turfpit
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Re: Histogram and Why?

Post by turfpit »

Scott

This https://docs.sharpcap.co.uk/3.2/#Histogram%20Tab gives the info you need regarding the Histogram tab controls and their effect in Live Stack.

Everything you know about histograms still holds. It is just a matter of learning what is a good shape for a deep sky object.

Dave
ScottD
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Re: Histogram and Why?

Post by ScottD »

turfpit wrote: Wed Oct 28, 2020 12:07 pm Scott

This https://docs.sharpcap.co.uk/3.2/#Histogram%20Tab gives the info you need regarding the Histogram tab controls and their effect in Live Stack.

Everything you know about histograms still holds. It is just a matter of learning what is a good shape for a deep sky object.

Dave
Thanks so much Dave!
donstim
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Re: Histogram and Why?

Post by donstim »

ScottD wrote: Wed Oct 28, 2020 11:26 am
Thank you Dave! I asked another person the same question - to be clear, adjusting the histogram in the live stacking mode does not impact the actual saved image, correct?
It certainly can impact the actual saved image. Directly from the SharpCap user manual:

The Save button has 4 sub options:

· Save as 16 Bit Stack will rescale the stacked data linearly between the 0 and the maximum pixel value into the range 0 to 65535 and save this as a 16-bit FITS file. The 16-bit FITS option is the default as it gives a high bit depth image with the full range of the image used (i.e. brightest pixel is 65535).

· Save as Raw (32-bit) Stack will save the full 32-bit stack data without any scaling as a 32-bit FITS file. The maximum value in it will depend on the number of frames stacked, meaning more adjustments will be needed when viewing (without further adjustment this FITS file will probably appear black when opened into a FITS viewing application).

· Save with Adjustments will save the image with the Live Stacking adjustments applied (i.e. after histogram adjustments and colour adjustments have been applied) as an 8 or 16-bit PNG file (depending on bit depth of camera being used).

· Save Exactly as Seen will save the image exactly as shown on screen as an 8-bit PNG file. This will include the effects of both the live stacking histogram and colour adjustments and the display stretch if one is applied.

So, the latter two options will save your image as a .png file with all the live stack histogram adjustments applied. The difference between these two options is that the last one will also apply any adjustments you made using the small display histogram in the right panel and any color adjustments made using the camera controls in the right panel. It is also limited to 8 bits of resolution compared to 16 bits for the first of these two options.

Don
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