I just received a reply from Sam at ZWO. He confirmed my suspicion that Gamma is applied in software, after digitization. He recommended always keeping Gamma at 50 for imaging.
He did not comment on changing Brightness (i.e. Offset) to 10 (from the default of 0), so I guess that he sees no danger in that. My theory for why the default is 0 is based on the fact that the ASI120MC was designed for lunar and planetary work -- these are all bright objects. Using the camera for DSO imaging was not in the design spec.
M27 with ASI120MC
Forum rules
Please upload large images to photo sharing sites (flickr, etc) rather than trying to upload them as forum attachments.
Please share the equipment used and if possible camera settings to help others.
Please upload large images to photo sharing sites (flickr, etc) rather than trying to upload them as forum attachments.
Please share the equipment used and if possible camera settings to help others.
Re: M27 with ASI120MC
Brian
With the ASI120MC imaging planetary objects, I set Brightness=0 to get the classic 'whale' shape for the (logarithmic) histogram. No data is lost with the histogram touching the LHS as there is no data in the black surrounding the planet. See viewtopic.php?f=16&t=256 and viewtopic.php?f=7&t=211.
With the ASI120MC imaging deep sky objects, I set Brightness in the range 3..10 - whatever is needed to pull the histogram away from the LHS to start at about 10% on the horizontal scale. There may be nebulosity data lurking in that region that would be lost if the histogram touched the LHS.
I leave Gamma=50, as seems to be generally recommended on the ZWO User Group and by the owner. I adjust Gamma in post processing (GIPM 9.5, which handles 16-bit TIFFs).
I had a look at the image you mentioned, with the low gain. He did have 3.5 hours of capture. I will have to try this on M31 as I am suffering from core burnout with gain at 50%. I suppose the only way is to systematically test against the same object by using the same number of exposures but varying the gain in say 10% increments.
So much to learn .....
Dave
With the ASI120MC imaging planetary objects, I set Brightness=0 to get the classic 'whale' shape for the (logarithmic) histogram. No data is lost with the histogram touching the LHS as there is no data in the black surrounding the planet. See viewtopic.php?f=16&t=256 and viewtopic.php?f=7&t=211.
With the ASI120MC imaging deep sky objects, I set Brightness in the range 3..10 - whatever is needed to pull the histogram away from the LHS to start at about 10% on the horizontal scale. There may be nebulosity data lurking in that region that would be lost if the histogram touched the LHS.
I leave Gamma=50, as seems to be generally recommended on the ZWO User Group and by the owner. I adjust Gamma in post processing (GIPM 9.5, which handles 16-bit TIFFs).
I had a look at the image you mentioned, with the low gain. He did have 3.5 hours of capture. I will have to try this on M31 as I am suffering from core burnout with gain at 50%. I suppose the only way is to systematically test against the same object by using the same number of exposures but varying the gain in say 10% increments.
So much to learn .....
Dave
Re: M27 with ASI120MC
Dave,
Looks like the weather is finally beginning to cooperate. They are calling for cooler, sunnier days for the next 7 days. (I'll believe it when I see it!) For the past couple months I've been given only two good nights per month on average.
A couple weeks ago I imaged a star cluster with the gain set at 25, offset 0, and RAW16. Up until then I only used gain 50 and RGB24. After stacking in DSS I launched PixInsight and saw a green glow at the extreme LHS of the histogram that pretty much covered the entire frame. (I did do dark frame subtraction at the proper temperature.) The glow is not uniform; there are large areas where there are vertical stripes. I came away with the feeling that I was looking at electronic artifacts instead of anything real. I initially attributed it to the effects of moonshine but then I got feedback from Sam at ZWO. He was quite clear that I should boost Offset when running RAW16.
Tonight I am going to redo darks throughout the night in order to capture frames at various temperatures. Exposure: 120sec, Gain: 25, Offset: 10. Then beginning tomorrow I am going to image as many galaxies as I can for the next 7 days: Pegasus, Andromeda, Pisces, and Triangulum. Hopefully I will get something good and send you some links at AstroBin.
Brian
Looks like the weather is finally beginning to cooperate. They are calling for cooler, sunnier days for the next 7 days. (I'll believe it when I see it!) For the past couple months I've been given only two good nights per month on average.
A couple weeks ago I imaged a star cluster with the gain set at 25, offset 0, and RAW16. Up until then I only used gain 50 and RGB24. After stacking in DSS I launched PixInsight and saw a green glow at the extreme LHS of the histogram that pretty much covered the entire frame. (I did do dark frame subtraction at the proper temperature.) The glow is not uniform; there are large areas where there are vertical stripes. I came away with the feeling that I was looking at electronic artifacts instead of anything real. I initially attributed it to the effects of moonshine but then I got feedback from Sam at ZWO. He was quite clear that I should boost Offset when running RAW16.
Tonight I am going to redo darks throughout the night in order to capture frames at various temperatures. Exposure: 120sec, Gain: 25, Offset: 10. Then beginning tomorrow I am going to image as many galaxies as I can for the next 7 days: Pegasus, Andromeda, Pisces, and Triangulum. Hopefully I will get something good and send you some links at AstroBin.
Brian
Re: M27 with ASI120MC
Good luck with the skies Brian. Might be worth starting a new thread or threads for your results/findings.
Re: M27 with ASI120MC
Dave,
Yes, it is a good idea to start a new topic. I think I will call it "RAW16 and ASI120MC" or something like that.
I am so glad that I resisted the temptation to do lights of the galaxy du jour. Instead I spent the night doing darks. With gain 25 I began with offset 10. Instead of blindly assuming that offset 10 was the silver bullet, I brought up the file in FITS Liberator to inspect the minimum and maximum values. From experience I already knew that hot pixels would come in at 65520. What I didn't expect was that the minimum value was in the 2000 to 3000 range. That is terrible dynamic range: ln(65520/2500) / ln(2) = 4.7 bits (yuck). I know from reading the ZWO manual that gain of 25 should give me a dynamic range of 9 bits. So clearly I would be clipping the RHS of the histogram. I think that the effect that it would have is on star color -- it would be lost.
So I stepped down the offset from 10 to 5 and recollected darks. This time FITS Liberator told me that the minimum value was in the range of 1000 to 1500. The dynamic range is getting better but not great: ln(65520/1250) / ln(2) = 5.7 bits (better).
I decided to change the offset from 5 to 3. This time FITS Liberator told me that the minimum value was in the range of 250 to 500. Now the dynamic range is much better: ln(65520/375) / ln(2) = 7.5 bits. I decided to keep offset 3 and then collected darks until sunrise over what turned out to be a 3 degree Celsius temperature range.
For comparison I looked back at my darks that I collected last week using gain 25 and offset 0. In the cooler end of the temperature range FITS Liberator was telling me that the minimum value was 16, and it rarely deviated much from that value for each frame forward. I remember thinking to myself at the time: "Wow, I'm seeing the full 12 bits of the A/D!" (that is, ln(65520/16) / ln(2) = 11.99 bits). And then I took lights of NGC 188 and saw that eerie green "electronic" glow at the low-end of the histogram. Hmmm, so the manual says that gain of 25 should give me 9 bits of dynamic range but my darks are giving me 12 bits. So what happened to those 3 bits? They must be getting clipped at the LHS of the histogram!
In summary, my methodology is this: for a given gain I will find through experimentation by collecting darks which is the optimal offset so that the dynamic range is in agreement with the ZWO manual.
I will let you know how it goes.
By the way I thought of another good reason to boost the offset to prevent clipping on the LHS of the histogram: we are lessening the effects of quantization in the faintest wisps of nebulosity. If we say that 12 bits covers the range from 16 through 65520 then that means that 1 stop of dynamic range in the low end covers only 16 digital levels (i.e. values from 16 to 32), and 1 stop higher from that covers only 32 digital levels (i.e. values 32 to 64), etc. By boosting the offset you get many more digital levels in the faintest areas. But you don't want to boost the offset too high or else you clip the RHS of the histogram! Well, as it is, this is my understanding of things at this time. I could be proven wrong.
Brian
Yes, it is a good idea to start a new topic. I think I will call it "RAW16 and ASI120MC" or something like that.
I am so glad that I resisted the temptation to do lights of the galaxy du jour. Instead I spent the night doing darks. With gain 25 I began with offset 10. Instead of blindly assuming that offset 10 was the silver bullet, I brought up the file in FITS Liberator to inspect the minimum and maximum values. From experience I already knew that hot pixels would come in at 65520. What I didn't expect was that the minimum value was in the 2000 to 3000 range. That is terrible dynamic range: ln(65520/2500) / ln(2) = 4.7 bits (yuck). I know from reading the ZWO manual that gain of 25 should give me a dynamic range of 9 bits. So clearly I would be clipping the RHS of the histogram. I think that the effect that it would have is on star color -- it would be lost.
So I stepped down the offset from 10 to 5 and recollected darks. This time FITS Liberator told me that the minimum value was in the range of 1000 to 1500. The dynamic range is getting better but not great: ln(65520/1250) / ln(2) = 5.7 bits (better).
I decided to change the offset from 5 to 3. This time FITS Liberator told me that the minimum value was in the range of 250 to 500. Now the dynamic range is much better: ln(65520/375) / ln(2) = 7.5 bits. I decided to keep offset 3 and then collected darks until sunrise over what turned out to be a 3 degree Celsius temperature range.
For comparison I looked back at my darks that I collected last week using gain 25 and offset 0. In the cooler end of the temperature range FITS Liberator was telling me that the minimum value was 16, and it rarely deviated much from that value for each frame forward. I remember thinking to myself at the time: "Wow, I'm seeing the full 12 bits of the A/D!" (that is, ln(65520/16) / ln(2) = 11.99 bits). And then I took lights of NGC 188 and saw that eerie green "electronic" glow at the low-end of the histogram. Hmmm, so the manual says that gain of 25 should give me 9 bits of dynamic range but my darks are giving me 12 bits. So what happened to those 3 bits? They must be getting clipped at the LHS of the histogram!
In summary, my methodology is this: for a given gain I will find through experimentation by collecting darks which is the optimal offset so that the dynamic range is in agreement with the ZWO manual.
I will let you know how it goes.
By the way I thought of another good reason to boost the offset to prevent clipping on the LHS of the histogram: we are lessening the effects of quantization in the faintest wisps of nebulosity. If we say that 12 bits covers the range from 16 through 65520 then that means that 1 stop of dynamic range in the low end covers only 16 digital levels (i.e. values from 16 to 32), and 1 stop higher from that covers only 32 digital levels (i.e. values 32 to 64), etc. By boosting the offset you get many more digital levels in the faintest areas. But you don't want to boost the offset too high or else you clip the RHS of the histogram! Well, as it is, this is my understanding of things at this time. I could be proven wrong.
Brian
Re: M27 with ASI120MC
Some interesting research here Brian.
This is the histogram from my M27 capture:
Capture was with a colour space of 1280 x 960 = ~1.2 million pixels.
The RHS of the graph shows that ~13 pixels are over-exposed (out of 1.2M). So personally, I would not be too worried about. However, if the histogram was hitting the LHS, then I would have been losing some data - hence using Brightness to get the gap at the left.
This is all a combination of exposure, gain and brightness to get a reasonable histogram and the same set of figures might not be optimal on consecutive nights. The only way is to systematically adjust one of the settings, process and compare the images side by side. It is useful to have some 'guideline settings' as a starting point.
I have had the good fortune to have face to face discussions with a couple of world renowned astro-imagers and for them it is all about histogram. I have large planet guidelines but am still trying to find what works for DSOs. I suppose there will be different sets of settings for the various types of DSOs.
As you pointed out, the ASI120MC is described as a planetary imaging camera. So I am fortunate with the M27 I obtained from a $150 camera (although I haven't finished with M27 yet).
I suppose it is all about putting the hours in - look at my M27 from 1 year ago in my initial post and I was really pleased at the time I captured that.
Dave
This is the histogram from my M27 capture:
- M27-histogram2.PNG (7.44 KiB) Viewed 3593 times
The RHS of the graph shows that ~13 pixels are over-exposed (out of 1.2M). So personally, I would not be too worried about. However, if the histogram was hitting the LHS, then I would have been losing some data - hence using Brightness to get the gap at the left.
This is all a combination of exposure, gain and brightness to get a reasonable histogram and the same set of figures might not be optimal on consecutive nights. The only way is to systematically adjust one of the settings, process and compare the images side by side. It is useful to have some 'guideline settings' as a starting point.
I have had the good fortune to have face to face discussions with a couple of world renowned astro-imagers and for them it is all about histogram. I have large planet guidelines but am still trying to find what works for DSOs. I suppose there will be different sets of settings for the various types of DSOs.
As you pointed out, the ASI120MC is described as a planetary imaging camera. So I am fortunate with the M27 I obtained from a $150 camera (although I haven't finished with M27 yet).
I suppose it is all about putting the hours in - look at my M27 from 1 year ago in my initial post and I was really pleased at the time I captured that.
Dave
Re: M27 with ASI120MC
That's a good point, Dave. I now see how my approach differs. As you described, the pros compose the DSO like a professional fashion photographer would compose a photo shoot of a model by adjusting ISO, exposure, f/ratio at the camera in order to achieve a particular effect. For example, to achieve the "glamour look" there must be a sweet spot of settings (known within the profession) for achieving an image that one would instantly recognize as that genre.
Me, I am looking for the center lane of the highway: a combination of settings that records the image faithfully according to the capabilities of the camera. If I want a particular effect I will do it in PixInsight. It might not be as good as it could get if I composed it at the camera but it gives me something that the pros don't have: flexibility. It's hard to take a glamour shot composed at the camera and turn it into something else -- they have to call back the high-priced model for another photo shoot!
Me, I am looking for the center lane of the highway: a combination of settings that records the image faithfully according to the capabilities of the camera. If I want a particular effect I will do it in PixInsight. It might not be as good as it could get if I composed it at the camera but it gives me something that the pros don't have: flexibility. It's hard to take a glamour shot composed at the camera and turn it into something else -- they have to call back the high-priced model for another photo shoot!
Re: M27 with ASI120MC
I am attending a lunar imaging workshop run by Damian Peach on 2nd Sept. I will have a few deeper questions for him based on our discussions, now I know some of the questions to ask. In the meantime enjoy this http://www.damianpeach.com/deepsky.htm.
Re: M27 with ASI120MC
Dave, thanks for the link! A wealth of information. Where on his website can I see the workshops he offers?
The subject of Lunar photography is a perfect example of the differences in methodology. Let's say that I want to capture Tycho on the terminator. Furthermore I want to bring out the detail in the shadows and I don't care if I blow out the sunlit portions. That is a good example of needing to compose at the camera. My method is a balanced approach that would yield some detail in the shadows and some detail in the sunlit parts and grays in between. In post-processing I would stretch the histogram to bring out the detail in the shadows (and blow out the sunlit parts) but the big difference is this: My image would have fewer digital levels in the shadow areas whereas if I composed at the camera I could get many, many more levels.
I can see in the case of lunar and planetary photography that it is a great skill to have to compose at the camera to achieve the desired result. Because the frame rate is so high you can tweak the gain and the offset and then get to see the histogram update quickly. For Deep Sky with 5 minute subs, you tweak a setting, and then wait for up to 10 minutes for feedback. I guess with experience your first guess can be quite accurate. And lest I forget, after you tweak the settings then you have to capture darks! For me at this time I prefer to settle upon middle-of-the-highway settings for a given gain, and then capture the darks on a cloudy night! I guess I am lazy.
The subject of Lunar photography is a perfect example of the differences in methodology. Let's say that I want to capture Tycho on the terminator. Furthermore I want to bring out the detail in the shadows and I don't care if I blow out the sunlit portions. That is a good example of needing to compose at the camera. My method is a balanced approach that would yield some detail in the shadows and some detail in the sunlit parts and grays in between. In post-processing I would stretch the histogram to bring out the detail in the shadows (and blow out the sunlit parts) but the big difference is this: My image would have fewer digital levels in the shadow areas whereas if I composed at the camera I could get many, many more levels.
I can see in the case of lunar and planetary photography that it is a great skill to have to compose at the camera to achieve the desired result. Because the frame rate is so high you can tweak the gain and the offset and then get to see the histogram update quickly. For Deep Sky with 5 minute subs, you tweak a setting, and then wait for up to 10 minutes for feedback. I guess with experience your first guess can be quite accurate. And lest I forget, after you tweak the settings then you have to capture darks! For me at this time I prefer to settle upon middle-of-the-highway settings for a given gain, and then capture the darks on a cloudy night! I guess I am lazy.
Re: M27 with ASI120MC
The imaging workshops are here http://astrophotographycourses.co.uk/courselist.html, held in Banbury (near Oxford), UK. The lunar imaging workshop was moved from 19th August to 2nd September. Each workshop is £30 for a morning + afternoon session, around 6 hours in total.
I have attended Deep Sky Imaging, High Resolution Planetary Imaging, Solar Astronomy, Remote Astro-Imaging - all worth while. The planetary imaging workshop was attended by Martin Lewis, http://www.skyinspector.co.uk/home and Chris Garry, https://sites.google.com/site/astropipp/gallery - I found it interesting to find out how these people approach things. The Deep Sky Imaging workshop gave me the kick to get started on writing the SC manual.
Six hours of lunar imaging should cover the points you have brought up.
I have attended Deep Sky Imaging, High Resolution Planetary Imaging, Solar Astronomy, Remote Astro-Imaging - all worth while. The planetary imaging workshop was attended by Martin Lewis, http://www.skyinspector.co.uk/home and Chris Garry, https://sites.google.com/site/astropipp/gallery - I found it interesting to find out how these people approach things. The Deep Sky Imaging workshop gave me the kick to get started on writing the SC manual.
Six hours of lunar imaging should cover the points you have brought up.