Hi, i have SharpCap Pro for a few years now and still learning my way though it.
I have a bit of a very basic question that I just can't quiet get my head around.
It is under and over exposing.
I use a different program for my imaging, it kind of controls the lot, dithering, exposure, all that sort of thing automatically.
I have previously just been selecting say 180 second exposers for my Nebula targets and 300 second exposure for galaxies and just staying with that.
I want to find out how to tell if i am over exposing or under exposing in my images.
I was going to try and use the histogram function to see if tis will tell me what i am doing
I read up on the smart brain function, were you slew to your target and it measures the sky darkness there and gives you a recommendation.
I am using a asi2600mc camera, i am not sure if i am suppose to select darkest or a different color on this option.
With the asi2600mc, on the histogram, how do i tell if i am over exposing or under exposing?
This is my main question, and as you can see it really is back to a basic level, but i just can't get my head around what i should be looking at.
Any images of the histogram showing an over exposed or under expose image would be greatly appreciated, as this will also hopefully help me understand what is going on.
I have two imaging scopes now, the main one is a refractor at f7, the other is a Hyperstar at f1.9 so a lot of difference there.
I should also add a note that I am just working on gain 100 for now to make things less complicated.
Any information would be greatly appreciated,
Thanks.
Help understand over and under exposure on histogram
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bluesilver
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Re: Help understand over and under exposure on histogram
Bluesilver
See https://docs.sharpcap.co.uk/4.1/#The%20Histogram
In essence:
The grizzly details (for rocket scientists):
viewtopic.php?t=456
Bringing it all together (for humans):
viewtopic.php?t=2552
Dave
See https://docs.sharpcap.co.uk/4.1/#The%20Histogram
In essence:
- underexposed_overexposed.JPG (71.91 KiB) Viewed 2849 times
The grizzly details (for rocket scientists):
viewtopic.php?t=456
Bringing it all together (for humans):
viewtopic.php?t=2552
Dave
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bluesilver
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- Joined: Mon Oct 05, 2020 12:51 am
Re: Help understand over and under exposure on histogram
Appreciate the reply and those couple of links.
So basically the brain tells me i need a black level offset of 12, this seams to be a common every time i do this test.
The exposure differ depending on where I have the scope pointed ( the different objects that i am imaging )
If i am reading this correctly, is it just as simple as, if the histogram is hard up to the left or even half passed this stage as in half the histogram is missing, then i am underexposed and need to add more seconds to the sub?
And if the histogram is a good distance away from the left hand side, i am over exposed and need to reduce the seconds of the subs?
Is it just that simple?
So basically the brain tells me i need a black level offset of 12, this seams to be a common every time i do this test.
The exposure differ depending on where I have the scope pointed ( the different objects that i am imaging )
If i am reading this correctly, is it just as simple as, if the histogram is hard up to the left or even half passed this stage as in half the histogram is missing, then i am underexposed and need to add more seconds to the sub?
And if the histogram is a good distance away from the left hand side, i am over exposed and need to reduce the seconds of the subs?
Is it just that simple?
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admin
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Re: Help understand over and under exposure on histogram
Hi,
not quite like that regarding the histogram...
If the peak on the left is touching the left hand side (or part of it is missing off the left hand side) then you need to increase the black level (offset/brightness). The brain will suggest a value as part of its calculations, but you can also do this by looking at the histogram of dark frames. Just increase the black level until the whole peak is visible. If you think in terms of dark frames you realise that increasing the exposure isn't the right solution for this!
Once you have the black level sorted, use it for lights, darks, flats, etc without changing it - it avoids possible mistakes later.
The brain will also help suggest your exposure based on the sky background measurement. The choice of colour channel is a matter for taste and target. If you choose 'blue' or 'darkest' you will often get much longer suggested exposures because there is not much light (pollution or otherwise) in the blue channel. However, if you are imaging an H-alpha nebula then it's mostly red, so you don't care much about the blue light in the darker parts of the image (nothing there), so blue wouldn't be a good choice then. If you have vignetting at the corners of the frame, tick the box to avoid edges/corners to stop the brain from measuring the values in those darkened regions.
The actual value you get will depend on lots of factors (how much/little light pollution, how high the target is, moon glow, filters, etc), so initially you may want to run the brain for each target until you get a feel of the suggestions it makes. Don't forget that there is no harm in going for longer exposures than suggested (other than the normal troubles of guiding, tracking, satellite trains, etc) - if it suggests 5s for your hyperstar then don't feel you need to use 5s - 15s or 30s will be fine!
cheers,
Robin
not quite like that regarding the histogram...
If the peak on the left is touching the left hand side (or part of it is missing off the left hand side) then you need to increase the black level (offset/brightness). The brain will suggest a value as part of its calculations, but you can also do this by looking at the histogram of dark frames. Just increase the black level until the whole peak is visible. If you think in terms of dark frames you realise that increasing the exposure isn't the right solution for this!
Once you have the black level sorted, use it for lights, darks, flats, etc without changing it - it avoids possible mistakes later.
The brain will also help suggest your exposure based on the sky background measurement. The choice of colour channel is a matter for taste and target. If you choose 'blue' or 'darkest' you will often get much longer suggested exposures because there is not much light (pollution or otherwise) in the blue channel. However, if you are imaging an H-alpha nebula then it's mostly red, so you don't care much about the blue light in the darker parts of the image (nothing there), so blue wouldn't be a good choice then. If you have vignetting at the corners of the frame, tick the box to avoid edges/corners to stop the brain from measuring the values in those darkened regions.
The actual value you get will depend on lots of factors (how much/little light pollution, how high the target is, moon glow, filters, etc), so initially you may want to run the brain for each target until you get a feel of the suggestions it makes. Don't forget that there is no harm in going for longer exposures than suggested (other than the normal troubles of guiding, tracking, satellite trains, etc) - if it suggests 5s for your hyperstar then don't feel you need to use 5s - 15s or 30s will be fine!
cheers,
Robin
-
bluesilver
- Posts: 92
- Joined: Mon Oct 05, 2020 12:51 am
Re: Help understand over and under exposure on histogram
Appreciate the reply and explanation, Adjusting the black level using dark frames makes sense now you mention it that way.
I ill try this out with a dark frame and adjust the black level to suit.
Very much appreciated.
The main concern was trying to find out if i am over or under exposing and image.
Sort of like say imaging the Trifid nebula or the Swan nebula just for example.
I don't want to be over exposing the stars, but then again still want to capture the nebula.
I was trying to find out say if imaging at f7 and 5 minute subs, am i over exposing the stars or not?
Asi 2600mc at gain 100
Would I be correct in saying that if the histogram is just off the left hand side, it should be fine?
It is just that a few would say even at f7, if you take thousands of 30 second subs you will end up with the same result as you would be taking hundreds of 300 second subs.
But are the 30 second subs all under exposed or are all the 300 second subs over exposed and you should of been somewhere in the middle?
Or am i just making this way more complicated than it should be?
I ill try this out with a dark frame and adjust the black level to suit.
Very much appreciated.
The main concern was trying to find out if i am over or under exposing and image.
Sort of like say imaging the Trifid nebula or the Swan nebula just for example.
I don't want to be over exposing the stars, but then again still want to capture the nebula.
I was trying to find out say if imaging at f7 and 5 minute subs, am i over exposing the stars or not?
Asi 2600mc at gain 100
Would I be correct in saying that if the histogram is just off the left hand side, it should be fine?
It is just that a few would say even at f7, if you take thousands of 30 second subs you will end up with the same result as you would be taking hundreds of 300 second subs.
But are the 30 second subs all under exposed or are all the 300 second subs over exposed and you should of been somewhere in the middle?
Or am i just making this way more complicated than it should be?
Re: Help understand over and under exposure on histogram
bluesilver
All the above information is pulled together in my viewtopic.php?t=2552 document. See page 10 for a graphical representation of how to systematically derive a suitable value for offset. The bias histogram (with suitable offset) looks something like:
Remember the offset established is for a particular gain - change the gain and the offset needs to be reviewed/adjusted.
My learning was based on https://www.qhyccd.com/uploadfile/2018/ ... 334403.pdf
For a real world example, see viewtopic.php?t=7696 post #9. Because of a black level (offset) which was too low the result was a less than optimal histogram (gap=0 at the left hand side). Once the black level was increased, a better shaped histogram was obtained. The result was to capture more of the faint nebulosity compared with images in the early part of the thread.
Note: be aware of the following naming inconsistencies across camera manufacturers (these are the same thing):
Altair -> black level
QHY -> offset
ZWO -> brightness
The above information is applicable in general to CMOS cameras.
Get the offset right and apply it consistently across bias, flats, darks, lights and your data will be well calibrated. That only leaves adjusting the exposure while keeping an eye on the histogram.
For completeness, I have an Atik 314E CCD mono camera. The only control is exposure - there is no gain or offset control. Interestingly, for no effort, the bias frame looks like this:
In this case the camera electronics controls the offset.
This https://tools.sharpcap.co.uk/ is useful - I think of it as a minumum exposure calculator.
Dave
The only way to be sure is to capture both the 300s and 30s sets with your equipment, at your location and compare the results. Trawl the internet enough and you will find every possible scenario is 'the one'.Or am i just making this way more complicated than it should be?
All the above information is pulled together in my viewtopic.php?t=2552 document. See page 10 for a graphical representation of how to systematically derive a suitable value for offset. The bias histogram (with suitable offset) looks something like:
- Altair183C-bias-mean-ADU-FITS-Liberator.JPG (70.76 KiB) Viewed 2823 times
Remember the offset established is for a particular gain - change the gain and the offset needs to be reviewed/adjusted.
My learning was based on https://www.qhyccd.com/uploadfile/2018/ ... 334403.pdf
For a real world example, see viewtopic.php?t=7696 post #9. Because of a black level (offset) which was too low the result was a less than optimal histogram (gap=0 at the left hand side). Once the black level was increased, a better shaped histogram was obtained. The result was to capture more of the faint nebulosity compared with images in the early part of the thread.
Note: be aware of the following naming inconsistencies across camera manufacturers (these are the same thing):
Altair -> black level
QHY -> offset
ZWO -> brightness
The above information is applicable in general to CMOS cameras.
Get the offset right and apply it consistently across bias, flats, darks, lights and your data will be well calibrated. That only leaves adjusting the exposure while keeping an eye on the histogram.
For completeness, I have an Atik 314E CCD mono camera. The only control is exposure - there is no gain or offset control. Interestingly, for no effort, the bias frame looks like this:
- Atik314E-bias-mean-ADU-FITS-Liberator.jpg (108.4 KiB) Viewed 2823 times
In this case the camera electronics controls the offset.
This https://tools.sharpcap.co.uk/ is useful - I think of it as a minumum exposure calculator.
Dave
-
bluesilver
- Posts: 92
- Joined: Mon Oct 05, 2020 12:51 am
Re: Help understand over and under exposure on histogram
Hi, Sorry to come back to this topic.
I have just been out imaging and took a screen shot of the histogram that I am seeing
Scope is Skywatcher Esprit 150 at f7
Camera is asi2600mc
Offset or black level offset is set to 50, so that part is fine
This is taken last night in Bortle 3 area, no moon
I can't work out what I am suppose to be seeing with this histogram to know if I am under or over exposing
This is 180 second exposure of the Keyhole Nebula
Unless i am suppose to click something on the histogram to make it change shape or something?
I use this histogram to take flats and works great, with the flats I just adjust exposure untill the white line on the histogram is sitting at around 50%, that puts the red line at around 20% and the green line at around 70%
But trying to work out this one is proving a bit challenging at least for me,
Any help on this would e greatly appreciated,
Thanks.
I have just been out imaging and took a screen shot of the histogram that I am seeing
Scope is Skywatcher Esprit 150 at f7
Camera is asi2600mc
Offset or black level offset is set to 50, so that part is fine
This is taken last night in Bortle 3 area, no moon
I can't work out what I am suppose to be seeing with this histogram to know if I am under or over exposing
This is 180 second exposure of the Keyhole Nebula
Unless i am suppose to click something on the histogram to make it change shape or something?
I use this histogram to take flats and works great, with the flats I just adjust exposure untill the white line on the histogram is sitting at around 50%, that puts the red line at around 20% and the green line at around 70%
But trying to work out this one is proving a bit challenging at least for me,
Any help on this would e greatly appreciated,
Thanks.
- sharpcap.jpg (276.68 KiB) Viewed 2709 times
Re: Help understand over and under exposure on histogram
bluesilver
[EDIT: see viewtopic.php?t=7696 post #9. The Keyhole Nebula can clearly be seen at the centre of the image to the right of Eta Carinae. That image was created from 60x120s exposures. See the second histogram below the image.]
Suggestion - download FITS Liberator then you can present histograms which include minimum and maximum statistics. For my equipment, skies and the object concerned I would consider this a good histogram.
Maybe take some samples of say 5 exposures at different values and process these. That will help you understand the effects of the different exposures. Different objects will require different exposures because of varying brightness.
If you are using the Hyperstar then expect exposures to be substantially reduced.
Dave
- Gap at left hand side of histogram = GOOD. This indicates that for your chosen gain and brightness you are not black clipped.
- Histogram at right hand side is hitting the vertical axis hard = BAD. Over-exposed at 180s, reduce the exposure until the histogram falls off and just reaches the right hand side.
[EDIT: see viewtopic.php?t=7696 post #9. The Keyhole Nebula can clearly be seen at the centre of the image to the right of Eta Carinae. That image was created from 60x120s exposures. See the second histogram below the image.]
Suggestion - download FITS Liberator then you can present histograms which include minimum and maximum statistics. For my equipment, skies and the object concerned I would consider this a good histogram.
- histogram.JPG (101.08 KiB) Viewed 2706 times
Maybe take some samples of say 5 exposures at different values and process these. That will help you understand the effects of the different exposures. Different objects will require different exposures because of varying brightness.
If you are using the Hyperstar then expect exposures to be substantially reduced.
Dave
-
bluesilver
- Posts: 92
- Joined: Mon Oct 05, 2020 12:51 am
Re: Help understand over and under exposure on histogram
Appreciate the reply and advice,
I can see what you are saying now, Appears the exposure is as you said, way too long.
First time I have heard of FITS Liberator
I will have to look that up and find some videos on what it is and how to use it.
For processing though, I use Pixinsight.
I can see what you are saying now, Appears the exposure is as you said, way too long.
First time I have heard of FITS Liberator
I will have to look that up and find some videos on what it is and how to use it.
For processing though, I use Pixinsight.
Re: Help understand over and under exposure on histogram
bluesilver,
You mentioned the Keyhole Nebula. I had to look that one up. It is in the Eta Carinae Nebula. My guess as to the source of that spike on the right-hand side of the histogram ... stars! There are plenty of star clusters nearby and if your frame includes them, well that explains the spike. Generally you can't do anything about this, unless you suspect that your Gain is turned up too high. You mentioned that your exposure is 180s at Bortle 3. That sounds right. You are good to go. Start stacking. Capture 30-60 min. We're excited to see your results!
Brian
You mentioned the Keyhole Nebula. I had to look that one up. It is in the Eta Carinae Nebula. My guess as to the source of that spike on the right-hand side of the histogram ... stars! There are plenty of star clusters nearby and if your frame includes them, well that explains the spike. Generally you can't do anything about this, unless you suspect that your Gain is turned up too high. You mentioned that your exposure is 180s at Bortle 3. That sounds right. You are good to go. Start stacking. Capture 30-60 min. We're excited to see your results!
Brian