sharkmelley

I downloaded L_0021 and I agree it's slightly out of focus.  The stars at the edges have "squiffy" shapes but we need an in-focus example before drawing a conclusion.  How are you connecting the camera - do you have a picture of the camera sitting on the focuser?

Mark

The star with the double set of spikes is defocused - that's the cause of the double spikes.  Was it in the centre of the image?  Is it the same star on which you used the Bahtinov mask?

I assume this is the Tak Epsilon?

Edit:  I now realise it was a crop away from the image centre.  Can you upload the original raw file somewhere.  I'll do a quick analysis.

Mark

2 hours ago, tooth_dr said:

Thanks again Mark. I’m feeling a bit more positive about the whole thing now.  Just the slotted screw heads to release  

That's right.

Don't touch the screws you circled.  Instead the 3 big bolts on the rear need to be undone - this doesn't affect the collimation.  The 3 grub screws on the rim also need to be slackened slightly.   The mirror can be removed, cleaned and put back without affecting the collimation.  It's a design masterpiece!

Mark

Now it makes sense.  A bigger sensor is a much greater challenge.  The Tak Epsilon vignetting is quite severe on a large sensor so you need very accurate flats.  The collimation and sensor orthogonality is also far more critical, affecting star shapes.  For flats I strongly suggest using dusk sky flats at first until you are sure your tracing panel and diffuser are up to this very demanding task.  As for collimation and tilt, this can take a long time to get right.  Spacing should be fine because the wide T-mount adaptor screws directly into the reducer but collimation requires very accurate tools and a lot of patience.  Even when the collimation is perfect you may find (as I did) that you need to shim the wide T-mount adaptor to fix tilt in the adaptor.  I found the Tak Epsilon cheshire type collimator worked very well because it screws directly into the focuser in place of the reducer.

Mark

I'm not clear exactly what you are saying here.  It's the first time you are using a D800E so does that mean the previous camera you used on your Tak Epsilon calibrated perfectly and had excellent shaped stars?  

Mark

The mirror impinges on the image creating a bigger area of shadow and an extra vertical diffraction spike on stars.  But there is also diffuse scattered light off the back of the parked mirror that prevents the flats ever properly calibrating the lights.  You'll soon become an expert at gradient removal !

Mark

I don't know what kind of edge glow (technically electroluminescence) the Nikon D800E has.  But it ought to calibrate out.  However using a DSLR (i.e. not mirrorless) with a fast scope like the Tak Epsilon will always leave a band along the bottom after calibration because of the parked mirror in the mirror box.  I always had problems with my DSLR until I physically removed the mirror.

Mark

1 hour ago, tooth_dr said:

Mark

I thought I would post looking for advice regarding using the Nikon with the Epsilon 180ED.  I have noticed that glow along the bottom of my bias frames, but it doesnt seem to calibrate out and I'm left with a band along the bottom of stacked image.  I took a series of 300s lights at ISO400, and dithered my captures.  I then took bias frames and flats at ISO400.  I've tried using the bias as darks, and created a bad pixel map, but all combinations of calibration do not seem to be removing the glow.  Would you mind if I asked how you dealt with this properly?

You're having problems with a Z6 ?  I wonder if the band you are seeing is something to do with the PDAF rows.

Mark

No, I haven't calculated any atmospheric correction.  I simply adjust the white balance to the stars in the image.  

Mark

Yes, it seems you have bundled white balancing and CCM into a single matrix.  There's nothing wrong with that but it's customary to separate out white balancing from the CCM.   You have calculated your matrix from a screen display but screen displays are actually fairly blue because they are based on the D65 illuminant.  So you have created a matrix that will not work so well for daylight - it will make daylight images look too red.  However, if you separate the operations of white balancing and colour correction then you will have a CCM that will work well over a wide range of conditions.

Mark

I have occasionally calibrated the colour correction matrix (CCM) for a sensor but always using a ColorChecker chart illuminated by sunlight.  It's an interesting idea to use one displayed on a screen.  Although the display won't have the same broad spectrum of the colour patches, I think it should still be possible to obtain a CCM that is in the right ballpark

Your derived CCM is this:

• new red = 1.3516376483848*r+0.033549040292064*g-0.054423382816904*b
• new green = -0.619460118029851*r+1.2330224800277*g-0.211822127381072*b
• new blue = -0.178260528770039*r-0.088350168915713*g+1.19254273096779*b

However your figures leave me a bit puzzled because when a CCM is applied to data that is already white balanced, the whites and greys should be relatively unaffected.  However your CCM would result in a strong colour cast because the coefficients in each row do not sum to unity.  Maybe you have combined white balancing into your CCM?

Mark

Comet C/2020 F3 (NEOWISE), taken from home in Kent at around 3:30am.  RedCat51 on Canon 600D.  10 exposures of 2sec at ISO 200 stacked, scaled by 2/3 and then cropped.

Mark

Use the camera manufacturer's software or if not, RawTherapee.

Mark

Your best bet is to take some additional exposures with the frame shifted.  That reflection will then move position (relative to the star field) and that will allow you to replace the existing artefact with clean data.

Mark

I use CFA Drizzle (a.k.a. Bayer Drizzle) as an important part of my DSLR workflow.  It means that Bayer interpolation no longer takes place and it results in noise that is more pleasant-looking  and more finely grained.  Such noise is easier to remove from the image and gives a better looking final result. 

Mark

14 hours ago, vlaiv said:

and here is same done on 16 bit version:

See how posterized faint regions become?

I would not expect that to happen. After converting the 32-bit image to a 16-bit image check the noise level (i.e. the standard deviation) in a small area of background of the 16-bit image.  You should find it adequately dithers the quantisation.  If so, then it means that the reduction to 16-bit is not the cause of your posterization issue.  No amount of stretching will introduce posterization in an image where the quantisation is adequately dithered by noise.

Mark

That's a clever solution and a good write-up!

Mark

Your "per pixel" metric is misleading you.  A better metric is the total number of photons captured by the whole sensor.

My advice is to choose whether you want to go OSC or Mono and then choose a large sensor.  Otherwise you will miss the field-of-view of your Canon.

Mark

I just accidentally found a document that defines gamut of a sensor.  See section 2.1:

https://corp.dxomark.com/wp-content/uploads/2017/11/EI-2008-Color-Sensitivity-6817-28.pdf

So contrary to what I thought, the concept does exist!

Mark

13 hours ago, wimvb said:

But with cmos you can't scale darks, so you don't need a master bias. 

Can you clarify what you mean here.  I agree that with set-point cooling, darks don't need to be scaled (if you match exposure times).   But that doesn't mean dark scaling can't be done for CMOS. 

Mark

I know exactly what you are trying to do here.  Funnily enough, earlier this week I had a disagreement with someone on another astro-forum who called the Bayer filters "sloppy" because their transmission bands overlap!

It's obvious to most people (but not to the contributor to that forum) that the sharp cut-off RGB filters typically used for astro-imaging are inferior for colour reproduction.  The example of trying to image a rainbow is a great example of this.  The  sharp cut-off RGB filters cannot reproduce the continuous change of colour within the rainbow.  But this is not a problem of gamut.  Gamut applies to display devices.  For instance an LED display can reproduce all the colours within the colour triangle formed by its Red, Green and Blue LEDs.  This is its gamut.  

However, a camera with RGB filters can be considered to be full gamut because it is able to record all those colours i.e. there is no colour it is unable to record unless there are gaps between the filter transmission bands.  The problem it has is the inability to distinguish between a wide range of colours  i.e. many different colours give exactly the same RGB pixel output values from the sensor.  The concept you need is "metameric failure".  This is the inability of the camera to distinguish between colours that the human eye sees as being different.  Those who test consumer cameras will report a "sensitivity metamerism index" (SMI) for the camera which is a standard way to measure its colour accuracy.

Mark

It's a good way of telling if someone has added diffraction spikes to their colour image during post-processing.  Such diffraction spikes are typically uniform in colour and won't have the colour banding effect.

Mark

17 hours ago, vlaiv said:

Issue solved! It is due to "feature" of algorithm that I implemented that tests show this behavior. Because binned result will not always match original sides I implemented "offset" which I totally forgot about

I'm glad you fixed it!

1 hour ago, vlaiv said:

No, I have not - I've done something similar to what you've done, only with images - created 4 images with gaussian noise and weighted added them in the same way one would create fractional binned pixel - and yes, that way it produces x1.8 SNR increase.

Still unable to figure out why there is x2 SNR increase once I fractionally bin and split result so there is no correlation between pixel values.

If the fractionally binned image is split into 4 sub-images then you're right in saying that there is no correlation between them.  I'm seeing the same standard deviation (on average) in each of the 4 sub-images as I do in the fractionally binned image.

Mark