107 replies to this topic

[Michael Covington]

Well said.  All we can test is the complete camera.  And most of these effects only show up when an image is strongly stretched, when a camera is being pushed to its limits in relatively challenging astrophotographic situations.  So it is certainly not the case that DSLRs have been discredited as useless.  But (just as with my discovery of the Nikon Star Eater back around 2006) some of them have quirks that astrophotographers need to know about and that might be a reason for not choosing a particular camera.

Informally, I'm getting the impression that the Canon 60Da was the best of all time and they haven't gotten better    That's an exaggeration, of course.

[Alen K]

Informally, I'm getting the impression that the Canon 60Da was the best of all time and they haven't gotten better That's an exaggeration, of course.

Well, the read noise is higher than more recent cameras. But if you’re shooting in such a way where that doesn’t matter…

https://www.photonst...,Nikon D5300_14

[Michael Covington]

Well, the read noise is higher than more recent cameras. But if you’re shooting in such a way where that doesn’t matter…

https://www.photonst...,Nikon D5300_14

Yes, I also have an unmodified D5300, and it performs very well.

But as you suggest, I usually shoot in such a way that it doesn't matter, following Robin Glover's advice.  His big contribution is the realization that we don't have to chase after sensors with super-low noise levels because the noise of the sensor is easily swamped by the shot noise of the sky background.

Edited by Michael Covington, 01 December 2023 - 06:15 PM.

[gregbradley]

You've probably checked this but I just wanted to say that red rings around bright yellow stars are super common in astrophotos with any camera. It's whether it's coming from the camera or the optics.

 

A lot of "APO" telescopes do not come to focus in the red channel as well as they could. I know you are using a Tak Epsilon ED 180 but that also has a 2 or 3 lens corrector in its light path.

 

Greg.

[sharkmelley]

You've probably checked this but I just wanted to say that red rings around bright yellow stars are super common in astrophotos with any camera.

Yes, you're right, it's very common to have red or blue fringes around stars.  It's an optical issue and nothing to do with in-camera raw processing.  You can verify this by using the same lens on another camera.

[gregbradley]

You might be interested Mark in knowing that apparently Sony has ditched the Star Eater filter in the A7R5. I started a thread about it.

 

That might mean the A7r5 is the best astro camera out there now. I don't know if this is also true with the Sony A74.

 

Greg.

[sharkmelley]

You might be interested Mark in knowing that apparently Sony has ditched the Star Eater filter in the A7R5. I started a thread about it.

I've replied on your other thread:

https://www.cloudyni...ter/?p=13118427

[primeshooter]

The reason for the "non-appearance" of rings in stacked images are very interesting but quite technical. I'll attempt to explain.  The rings caused by channel scaling (like we see with the EOS R) have a couple of important and interesting properties:

• Firstly, the step size of the (circular) discontinuity in pixel values is exactly 1 ADU (analogue to digital unit).

• Secondly, the position of the (circular) discontinuity in a single exposure coincides with where the pixel values match the spike or notch in the histogram.  But this only happens where the image has some kind of background gradient because otherwise there are no well defined areas in the image where these pixel values occur. Optical light fall-off (vignetting) is the most common cause of such background gradients and this results in a circle where the pixel values fulfill that criterion, leading to circular discontinuity.  Different background gradients will produce non-circular discontinuities.  The best way to visualize this is to consider that the discontinuity will follow contour lines in the background of the image.

 

Once the above 2 points are understood, there are many interesting corollaries:

• If the image noise is much larger than 1 ADU then the discontinuity will be disguised by the noise.  Moreover, instead of being a sharply defined 1 ADU step-like discontinuity, it will become more spread out.  This is part of the reason it's difficult to detect at high ISOs since the noise (in ADUs) is greater.
• Averaging many exposures together (i.e. stacking) will reduce the background noise and make the 1 ADU discontinuity easier to detect.
• If the background gradient is gentle then again the step is not sharply defined and becomes more spread out. In general, optical systems with sharp light fall-off will be more prone to this type of ring.  Optics with little or no vignetting will not show these rings.  However, note that in other cameras, concentric rings can be caused by raw data-processing issues where vignetting plays no role.
• Since the position of the ring is dependent on the background pixel values, it means the ring position will change with the recorded brightness.  Averaging together many such exposures will "dither" the ring position, spreading it out and making it difficult (or impossible) to detect.

So, as a vast over-simplification, rings generally will not be visible in a stacked image when shooting at high ISO, with non-vignetting optics and sky conditions that are changing.  Rings will be visible in a stacked image when shooting at low ISO with heavily vignetted optics where sky conditions are constant.  But a sufficient number of exposures will still need to be stacked, depending on the ISO being used for shooting. Between these two extremes, the rings may or not be apparent.

 

In my own case, I'm shooting with the Tak Epsilon which suffers from heavy vignetting.  But the brightness of my sky typically varies quite a lot during an imaging session.  Moreover, my flats are typically sky flats taken at dusk when the light levels are changing quite rapidly.  So the varying sky conditions usually give me some protection from sharply defined rings (though not from the background gradients which are caused by the red channel scaling).  The thing I did differently on this occasion was to take flats using a constant artificial light source.   So the ring you see in my image was caused by the flats.

 

All this is only a brief summary.

Mark, can I use this to surmise this on my website with credit? It is interesting that it is not just nikon with this problem now. I am itching to test ISO 800-1600 now with the Z 8 come dark skies. I expect the split sensor artefact will be calmed down by doing so, and the likelihood of rings be reduced also. (I have not had visible rings yet, luckily, however my longest data set was 2 hours so far).

Edited by primeshooter, 04 June 2024 - 03:57 AM.