Big DSLR sensors are stitched together

[MarkS]

Did you know that lithography masks used for modern sensors are not big enough to cover a full-frame sensor, so the mask operation is done in two halves?  Nor did I but it has some very interesting implications.

I have wasted a huge amount of time trying to track down the cause of the vertical discontinuity in my master flat, seen here stretched:


Here I have colour saturated the flat:


There is horizontal colour banding on the left hand side which is absent from the right hand side.  Similar effects have been appearing on the left hand side of my images and are very difficult to fully remove.

At least I now know the cause!

Another nice example can be seen in this diagram "Figure 30. Nikon Df. Sensor stitching structure" at the bottom of this page:
http://www.rawdigger.com/howtouse/rawdigger-histograms-overexposure-shapes

Mark

[RobertM]

What a great piece of detective work, god only knows how you tracked that one down !

Does the issue 'flat' out or is the final image compromised ?

I must admit that I haven't noticed that on my flats yet but it's probably because I haven't looked hard enough.

Robert

[MarkS]

What a great piece of detective work, god only knows how you tracked that one down !

Does the issue 'flat' out or is the final image compromised ?

I must admit that I haven't noticed that on my flats yet but it's probably because I haven't looked hard enough.

Robert

I spent weeks trying to get to the bottom of this one.  Eventually I realised that every flat I took (on the Tak, C11 and lenses) showed the same features, as did all flats I have previously taken at all stages of the various mods I performed.  So it wasn't caused by my mod.

I eventually posted a question on Cloudy Nights and on DPReview and got similar replies from some very knowledgeable folk. 

The issue doesn't seem to completely flat out.  Here is an uncalibrated sigma stack that shows the horizontal banding (it's the Shark in Cepheus).  I've applied modest PixInsight DBE as a final step.


Unfortunately, not all the banding calibrates out. So I still don't know where that leaves me

Mark

[Carole]

How very annoying, and well done sussing it out.

Do you capture your DSLR data to the SD card or download it onto the laptop?

I am asking because do you recall a conversation some time ago where someone on the forums discovered that if you leave your SD card in the camera whilst downloading data to the computer then this would cause horizontal banding (I have the explanation filed away somewhere).  If you take the SD card out it greatly reduces it, I have tried this and it does appear to improve/alleviate the situation.  I am only mentioning this in case it gives you any thoughts on the horizontal banding though of course these issues might be completely different with the A7S.

Carole

[Mac]

The sensor is a sensitive dynamic device. Until each pixel is read (and digitised) its value can change. The primary way pixel values change is when photons hit them. However, other electronic disturbance can also affect them. If the circuitry is insufficiently de-coupled/isolated then the signals being generated during the process of writing the data (to memory, SD card, USB, or wherever) might affect the values of the pixels yet to be read, and give rise to odd effects...

But thats  different issue compared to noise being introduced whilst copying from sd card to pc.
I agree that until the data is pulled off of the chip and saved to the card then there is a possibility of corruption, but that corruption is not the fault of being saved to the sd card
that is the due process of reading and storing the data, which is a different cause. I found the original post on astroshed which dosent go in to any detail except one persons view that it might have been the slow cards and that was just a hunch.

#back on topic#

If these sensors are stitched together and you can reproduce these images then a flat and bias must be able to eliminate the artifacts, otherwise that would mean
that there is something fundamentally wrong with the way we have been processing images.

So at the moment we produce 4 files,
Bias    Technically should include all the readout noise present in EVERY image (including the split)
Flat     Photographs all the debris in the optical train
Dark   Photographs all the noise present
Light   All the juicy bits.

Now i know we dont produce a dark for our flat as it is technically not noisy (no long exposure noise, no heat noise ect) the only noise present is the bias,
is the bias file to blame and is it consistently noisy with the strip being present in each image? I know it should be there in every image but are the internal electronics
and file process doing anything?

Remeber Nikon cameras do not produce a RAW image it is always processed, Canons do produce a RAW image, does the sony fall under the nikon side of not producing a true raw image? and this is a software processing artifact not a true sensor artifact?

If it was a true sensor artifact then it should be processed out with our normal chain of processing, if its software (camera) related then does it appear in every image.
i.e the camera says oh look shortish image, process out the line as it will be seen, longer image na dont bother as it will be hidden by light data, so in our light frames there is no line as its already been incamera processed, but in the bias and or flats its there, which then introduce the banding to the final images.

Just a thought

Mac.

[MarkS]

Mac,

I agree, they ought to calibrate out but they don't.  So the investigation continues. 

The banding in the flat of a stitched sensor is just an interesting fact I discovered along the way. 

Mark

[Fay]

What would we do without you, Mark? You will get there in the end

Fay

[The Thing]

What would we do without you, Mark?

We'd all do our astrophotography in blissful ignorance!

[Mac]

 

[Fay]

 

[MarkS]

Here's a couple of interesting examples, both white balanced and extremely stretched to show the effect.

Firstly a flat taken with an opaque acrylic sheet over the telescope mouth pointing upwards to a cloudy sky in daylight:



Now, an hour's worth of data with the same opaque acrylic sheet over the telescope mouth pointing upwards to a cloudy sky at night:



So clearly those dark stripes will never calibrate out.  But what on earth is causing them and why do they appear in all my deep-sky images?

Here's the result of using the daylight flat to calibrate the nighttime one and stretching:



You can now clearly see the stripes I get in my images.  I'm running out of ideas.

Mark

[Carole]

That's pretty bad, I think you have no alternative but to either get in touch with the manufacturer with your results and see if you can get any help from them (swap a sensor or something), but maybe the next one could be even worse, or to attempt to process out the problem every time which will be really annoying.  However this is extremely stretched, are you going to stretch your image this much? 

I also wonder why this only seems to affect one side of the sensor. 

Also what is the cause of that circular ring in the daylight stretch flat and also the image of the uncalibrated Shark image?

The recent image you presented (Horsehead) did not show this artifact, however of course underneath it all you know it is there and it must restrict your processing to some extent.  Are you sure that it doesn't get processed out when you calibrate flats with image data?

Very annoying for an otherwise excellent camera.

Carole

[JohnP]

Mark,

Think Carole could be right - Any chance of passing this by someone at Sony?

The stripes are really hard to see in your horsehead head image - Can barely make out a hint of the top one - If I stretch it then yes its there but I guess in your search for perfection its annoying... Also looks like on your stacked image there is also bands starting to show at bottom left. Is there any chance you could get a fellow AS7 user (Robert maybe) to repeat the tests to see if he gets same results? Maybe its just your sensor..? I don't suppose there is a chance that its linked in anyway with your Ha mod of the camera? Just trying to think of a few things for how you can move forward with this. The litho mask you mentioned is also really annoying.. theres a real left/ right defining line in flats


John

[Fay]

the trouble with cameras and scopes etc, it is not a level playing field.  They are never all the same, some are better and worse than others

[MarkS]

Think Carole could be right - Any chance of passing this by someone at Sony?

The stripes are really hard to see in your horsehead head image - Can barely make out a hint of the top one - If I stretch it then yes its there but I guess in your search for perfection its annoying... Also looks like on your stacked image there is also bands starting to show at bottom left. Is there any chance you could get a fellow AS7 user (Robert maybe) to repeat the tests to see if he gets same results? Maybe its just your sensor..? I don't suppose there is a chance that its linked in anyway with your Ha mod of the camera? Just trying to think of a few things for how you can move forward with this. The litho mask you mentioned is also really annoying.. theres a real left/ right defining line in flats

The artifact from the sensor left/right stitching varies quite a lot. I know of a couple of examples of Sony A7S (Robert's included) that have no artifacts at all.  I know of another couple (mine included) with obvious artifacts.

I'm doing a few more tests to definitively check if those dark bands are being caused by some kind of light leak.

The Horsehead doesn't show the problem too much because I deliberately didn't stretch it as far as I wanted to.  I have a few other images (including the Shark) which are affected quite badly and are waiting for me to create a correction frame.  At least I now know a correction frame is possible by taking "flats" under a dark sky.

Mark