Histogram and faint objects

[yknott456]

I am experiencing a problem with using the histogram feature for faint objects (e.g., Heart Nebula, Calif. Nebula, etc.).  My usual techniques if to set exposure so that the peak of my histogram is about 1/3 of the way from the left.  For the really faint objects though, especially when using my 200mm f2.8 Canon lens, I'm not getting enough data to work with.  It has been suggested that the histogram feature converts the raw data to JPEG before displaying the histogram and that this may lead to a small amount of initial pre-stretching.  I don't know if that is true or if it is simply to do with loss of bit-depth as you go from 14 to 8 bits.  I do know by checking Astrobin that other people imaging the same objects with the same gear are using significantly more exposure time that my histograms have been leading me too.  I'm wondering if the on-camera  Canon histogram might be more accurate, albeit more difficult to look at on the telescope.  I tried to check this by copying one of my problem lights back into the camera but got an error that I wasn't authorized to do that.  I was able to copy it onto the SD chip but when I reinsert it into the camera and press the play button it doesn't show.  I know I can check this on a fresh picture in my next observing session but I'd like to resolve it before then.

 

1.  Has anyone else experienced this and, if so, how did you resolve it?

2.  Is there a way to look at the image that I copied to the chip and display it on-camera?  I'm pretty sure this can be done but don't recall just how.

 

thanks,

Al

 

 

[admin]

Hi Al,

 

1) the short answer is longer exposure.  That is the only way to push the histogram farther to the right.

2) I don't understand what you mean

 

Regards,

[astroman133]

Al,

 

On my T5i, I can look at luminance and RGB histograms for images that are on the SD card via the camera's LCD display by repeatedly pressing the Info button while reviewing the image. I am not sure what camera you are using, but I am sure that this is discussed in the camera's operation guide. I do not know if you can copy an image onto the SD card and see the histogram.

 

If your images are too dark, then you need to lengthen the exposure. Trying to image the Witch Head Nebula, for example, with 30 second exposures will not give you much data to work with no matter how many subs you collect.

 

Perhaps if you gave us an example of a target that is too dark, and the shooting parameters that you are using, someone may be able to recommend a change.

 

Another thing to try.  If you are able to find images of the same target that have been taken by others then perhaps you could try to duplicate their shooting parameters (lens, f/stop, focal length, ISO and exposure) and see what your histogram looks like.

[s3igell]

I believe the OP is ultimately referencing the issue that the Histogram is generated from the Embedded JPG Preview Image - and THAT data has the In-Camera Default Stretch applied to the Data.

It is a variation of the same Issue which leads to misleading Histograms on Flats - due to the In-Camera JPG Preview Stretch.

[yknott456]

Guylain, Rick-Yeah I had already figured out that I needed to lengthen the exposures.  s3gell understood that the reason for my post is that I am finding the histogram to be misleading when imaging faint objects.   I can get around this by taking a shot and loading the results into a program like Images Plus or the DPP and doing my own stretch to see if it is OK and then adjusting and repeating until I get what I want and then using the settings to set up an imaging run.  This however would be a cumbersome process that the Preview button and histogram avoids for the brighter objects.  I was hoping that someone might know of a work-around that would make this easier.  Maybe there isn't one.

 

Al

[admin]

Guylain, Rick-Yeah I had already figured out that I needed to lengthen the exposures.  s3gell understood that the reason for my post is that I am finding the histogram to be misleading when imaging faint objects.   I can get around this by taking a shot and loading the results into a program like Images Plus or the DPP and doing my own stretch to see if it is OK and then adjusting and repeating until I get what I want and then using the settings to set up an imaging run.  This however would be a cumbersome process that the Preview button and histogram avoids for the brighter objects.  I was hoping that someone might know of a work-around that would make this easier.  Maybe there isn't one.

 

Al

 

This is interesting, my ears are always burning when I ear someone saying they need to load the image in another app back and forth to get the right exposure.  It means I'm missing a key component.

 

Would having a histogram based on raw data versus debayered data be the missing link so you can do what you need all withing the constraint of BYE?

 

Regards,

[astroman133]

Guylain,

 

In order to get an RGB histogram of an image I think that the image would have to have been debayered.

 

Please correct me if I am wrong, but my impression (from previous conversations on this forum) is that the histogram that BYE displays is based on a JPEG image that is imbedded in the raw data.  Since the JPEG data has been stretched in the camera then the histogram does not directly relate to the raw data.  This is a problem when the user is trying to use the histogram to adjust exposure times. Displaying the histogram of the debayered, but unstretched and still linear, raw data would be most useful.

 

Now, for Al...If you did need to bring your raw image into another app to get a histogram of the unstretched, debayered data, you should not have to do that for every target.  You would only need to do it to establish the exposure duration for a typical target and then use that exposure for most, if not all, your imaging. That is what I do and it works out well for me.

[yknott456]

Hi Guylain,

 

As long as the histogram is not pre-stretched that sounds like it would be perfect.  Rick's suggestion for having a set of general exposure during for similar targets sounds good but then he has a lot more experience than I do.  I've just gotten the right (expensive) lens to start going after these faint critters.  Even with that though, being able to have an accurate histogram would still be best. Seeing conditions, especially here in Florida, can vary a lot, even in one night.

 

Al

[s3igell]

s3igell understood that the reason for my post is that I am finding the histogram to be misleading when imaging faint objects.   I can get around this by taking a shot and loading the results into a program like Images Plus or the DPP and doing my own stretch to see if it is OK and then adjusting and repeating until I get what I want and then using the settings to set up an imaging run.  This however would be a cumbersome process that the Preview button and histogram avoids for the brighter objects.  I was hoping that someone might know of a work-around that would make this easier.  Maybe there isn't one.

Perhaps you can post an Example:  Provide Exposure Details for one of your Imaging Runs, and Screenshots of the Histogram - both BYE and whichever Image Processing App you choose to illustrate your "Discrepancy" Issue.

[s3igell]

Seeing conditions, especially here in Florida, can vary a lot, even in one night.

Seeing Conditions - other than Clouds - aren't really going to cause much Variation in the Histogram.  The Light Captured is going to mainly be a combination of the Aperture/Focal Ratio and the DSLR Sensitivity/Gain (ISO) and the Intrinsic Magnitude of the Target DSO.