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vlaiv

Odd thing about ASI1600 and Halpha

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I was trying out some NB imaging with ASI1600 and I've found this very interesting artifact of this sensor. After examining flats - I wanted to do photon count on actual Ha target to see if I can figure out surface brightness of parts of target in this band, and for this I needed normalized flat frames, so I decided to do simple normalization - divide master flat with histogram peak value - to get it normalized to range 0-1.

Problem was that I spotted this:

image.png.69217448f5abbd4e936b7411e3979e69.png

So I said to myself, you probably have dust ring in central part (where there is no vignetting) and indeed I do have bunch of them scattered across master flat, but no matter how I did my selection and how careful I was to skip dust rings, I always ended up with two distinct peaks in histogram. Then I decided to have "a closer" look master flat and see what is going on. This is what I've found:

image.png.904e3ffcb1e9acb4f681a4eb6a18429c.png

(very "zoomed" in image of master flat - individual pixels as squares)

Distinct checker pattern all over the image! So in Ha it looks like there is some sort of manufacturing artifact that creates checker pattern of "sensitivity" - QE is different for adjacent pixels.

In this particular example, first histogram peak is centered around 3120 while second one is around 3175 (these are electron counts rather than ADUs), so not great difference but regularity suggests it is related to manufacturing.

So narrowband (at least Ha) with ASI1600 - flat files (and proper calibration in general) is a must!

 

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Interesting....

I use the ASI 1600 for extreme(!) H alpha narrowband - solar imaging at 0.3A bandwidth.

I’ve never noticed a similar issue/ problem.

 

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Indeed that is odd. I will try to run some flats tests on my 1600 later today and see if mine does the same thing.

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Hi Vlaiv

Just looked at my ASI1600 Ha Flat and it has a chequerboard pattern... but on inspection so my do LRGB and Oiii, its about 1.5% difference, same as yours

 

Dave

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Is the pattern consistent ( ie pixel by pixel) across various exposures??

 

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I've run off some test flats on my 1600mm.

Ha (histogram 41%-67%): 2057162405_Ha2000.png.b3befeff6c9510e5f9c607d583357c91.png

O3 (histogram 40%-71%): 1240390240_O32000.png.f8315462b9c2d7ce8e9f4dc5a3d5bedd.png

S2 (histogram 47%-73%): 1789957989_S22000.png.5ee3ee3ca7fa152834e9c186aa94ddd8.png

Ne3 (histogram 47%-64%): 573410760_Ne32000.png.f73ce848d4421f6ea929ccda3eefc781.png

Although the pixels do vary, there does not appear to be anything like the variation in adjacent pixels that you have. (These are obviously just highly enlarged small sections from each flat - I could provide the original masters [32MB a piece!] if you wished to see them.)

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Do the enlargements cover the same area and can be compared pixel for pixel?

 

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50 minutes ago, Merlin66 said:

Do the enlargements cover the same area and can be compared pixel for pixel?

 

Each of the little squares is one pixel.

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Yeah, got that.

Is the area covered by each image identical?

I’m trying to see if a bright pixel always appears at the same X Y chip coordinate.

This would confirm the sensitivity of that particular pixel was higher or lower.

Just looking at the macro picture doesn’t give that information...

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I would have to say no. I was mainly looking to find a pattern like OP's which is not there. I will do some like-for-like samples when I get back home and see if your theory pans out.

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One thought that occurs is that there have been several versions of the 1600. IIRC the original was produced without the USB hub, then the one with the hub (which is the one I have), then the PRO version. Wonder if OP & I are using the same one.

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Some interesting points raised - I just checked one of older master flats - full spectrum (or maybe LPS filter, not sure, but not narrowband or RGB).

Again there is double spiked histogram, and checker pattern.

Mine version of camera is probably V2 - it has usb2.0 hub and screws on sensor chamber, as well as desiccant port on the side. It's not Pro - no memory buffer.

This is what it looks like:

image.png.e84ae93070e650ab5bb74446af6a801f.png

image.png.d37536a5a4f58858e3c22a01209bbde9.png

Regular L flat was taken using very short exposures - somewhere around 0.01s, while Ha used 0.5s exposures. Mentioned values of peaks - ~ 3150 are from 0-4096 range, so it would be around 77% histogram.

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Posted (edited)

Yes, that's the version I am using - it was just a thought (cost nothing, worth about that much!).

6 hours ago, Merlin66 said:

I’m trying to see if a bright pixel always appears at the same X Y chip coordinate.

OK, each of the extacts below is now 72px*54px, the top left px being (1000, 1000) from the original masterflat, resized 900x. Also attaching a histogram of the Ha one (the others all look pretty much the same with a single narrow spike). [EDIT: Forget that - brain fade - the histogram is only of the extract :iamwithstupid:] I have a feeling that the bright pixels on the Ne3 shot are more to do with the light source than the camera. The others all had exposure times in ms, The Ne3 required about 80s per exposure! I suspect there is nowhere near enough light being produced in the 3869Å wavelength by the ELP and I am going to have to rethink how I take flats for that filter.

Ha: 1681419327_Haflat.png.248b18245c0496f5d6a1db20b2426f68.png

O3: 1040225943_O3flat.png.efbfc70a7bc20dcc03bea2ae5980adf0.png

S2: 930163126_S2flat.png.d2624d5a7479e3a88c7095409997e44a.png

Ne3: 1266453035_Ne3flat.png.3b38d2d6ad22814a98b063c7802ce1fd.png

Ha histogram: 1247830035_HaHistogram.jpg.01232eb034deccfc805df4dd1ecef029.jpg

Edited by Demonperformer

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Huum. You don't have the firmware load from a OSC by mistake do you, but on a mono sensor? That reminds me allot of the pattern you get on a debayered dslr when the firmware still thinks it's an osc.

My flats on my pro don't show this at all. 

Edited by Adam J

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28 minutes ago, Adam J said:

Huum. You don't have the firmware load from a OSC by mistake do you, but on a mono sensor? That reminds me allot of the pattern you get on a debayered dslr when the firmware still thinks it's an osc.

My flats on my pro don't show this at all. 

I was not aware that you can change firmware of ASI1600, will have a look at it.

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1 minute ago, vlaiv said:

I was not aware that you can change firmware of ASI1600, will have a look at it.

I am not sure you can change it I was thinking more manufacturing error.

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