Help: Can this noisy image be improved?

[admin]

Hi Ivo,
Thanks a lot for looking at my data. It helped a lot!

Glad it helped!

It's possible the noise is still due to contribution of light pollution. Light pollution is becoming increasingly difficult to filter out with the switch to LED street lighting.

You should never color-calibrate a processed (stretched) image! It doesn't make sense mathematically; your data should still be linear (unless you're processing and color calibraring in StarTools).
As you can see, using something like eXcalibrator on data acquired with a light pollution filter yields far worse results than attempting to color-calibrate by known processes;

Spiral galaxies like these should exhibit a yellowish core, bluer outer rims, red/brown dust lanes and pink/purple H II areas dotted around their arms. A light pollution filter tends to kill yellow, but blue and red/orange should remain largely intact.
Stars should not be uniformly white, but should exhibit coloring according to the black-body temperature curve. With a light pollution filter, yellow will usually be absent. So at the very least, you should have red, orange and blue stars in your image.

It's worth investigating why your stars are purple. Something in your imaging train might be this causing color fringing? The FSQ-106ED is an excellent instrument and should not exhibit this sort of color fringing. Is there any other glass in between your OSC and your scope, except for your light pollution filter?

Another question that I have is when do I use manual Develop rather than AutoDevelop? I've always used AutoDevelop and like you saw, it didn't work very well. So, is it better to try Develop first before using AutoDevelop and just try both and see?

AutoDev usually does a good job with any sort of reasonable signal. It will definitely struggle however if any "real" detail stands out of the noise only marginally, like in this case. If you cannot get AutoDev to tone down the noisy background by specifying an Region of Interest and using Ignore Fine Detail parameter), a manual Develop may indeed be much easier to deal with.

[cytan299]

Hi Ivo,
I forgot, I do have a Haida UV filter right at the front of the FSQ106 to protect the lens.I'd be really surprised if it introduces purple. However, I'll try without the filter when the sky clears (unfortunately, it's inclement weather here as far as the eye can see for the next 10 days). OTOH, the LPS D1 filter that I'm using is a lot better than the LPS P1 filter I had used before. The P1 filter makes stars really look cyan. And LED street lights are now the norm here.

Pardon my ignorance on using eXcalibrator. Doesn't Startools allow me to calibrate the colour by selecting "white" stars? Since colour is always done after stretching, so why is doing it in eXcalibrator non kosher? I saw your link about colours of M101. Very interesting ...

cytan

[admin]

Hi Ivo,
I forgot, I do have a Haida UV filter right at the front of the FSQ106 to protect the lens.

Hmmm... this might just be the culprit. Definitely give it a go without (even just for a simple test).

Pardon my ignorance on using eXcalibrator. Doesn't Startools allow me to calibrate the colour by selecting "white" stars?

StarTools certainly does, and makes it seem effortless when the data is already heavily processed, but it is not the norm, nor is it possible using any other software that I know of.

Since colour is always done after stretching

Only in StarTools! (because of its Tracking/time travel capability, see also the video on that page). In any other software it is extremely important you do operations like color calibration while the dataset is still linear (e.g. before stretching!) and in the right sequence. The same goes for a number of other operations (e.g. light pollution removal, deconvolution). StarTools is the only software (that I am aware of) that allows you to perform these operations - in a mathematically correct way - at a more convenient time. It processes your image in 3D (X, Y, time) rather than 2D (X, Y). The result is unique functionality (for example Color Constancy in the Color module) and a much cleaner signal. However it is most definitely not something you can (or should!) do with simpler software like PixInsight, Photoshop or dedicated tools like eXcalibrator.

If you're a beginner, StarTools gives you a lot more freedom - mathematically - without causing you to adversely affect the final image. It's protecting you from nonsensical operations by trying to make sense of your commands mathematically on a fluid timeline. If it didn't, the engine would break as the mathematics would break down.
However, once you get into more advanced territory, it's probably a good idea to read up on Tracking, how StarTools' processing engine works and how it fundamentally differs from more basic software like PixInsight and Photoshop.

[cytan299]

Hi Ivo
Thanks for the explanations and links. It's most helpful. I agree that calibration should be done before stretching. And it is interesting that StarTools is able to do it in the end. However, I think colourization is an aesthetic and therefore subjective. I think you recognize that because you provide the sliders and alternative options to get the different colourization schemes. So although, in the end, the colour might not be scientifically correct but it may just be more pleasing artistically than scientifically.

There's still a lot for me to learn!

cytan

[admin]

Hi Ivo
Thanks for the explanations and links. It's most helpful. I agree that calibration should be done before stretching. And it is interesting that StarTools is able to do it in the end. However, I think colourization is an aesthetic and therefore subjective.

You're totally right of course. Much is entirely subjective about trying to represent color in outer space. There is no one right way or method or color balance.
Narrowband especially (for example the Hubble palette) is a great example of this; there is no one "correct" or "scientific" way to show colors and correlated detail. Anything goes! Whatever fits your goals.

I think you recognize that because you provide the sliders and alternative options to get the different colourization schemes. So although, in the end, the colour might not be scientifically correct but it may just be more pleasing artistically than scientifically.

Personally, I like to think that every image we produce is just one of infinite possible interpretations of the - otherwise - invisible data. All you and I do is guiding the viewer to what we feel is "right", whether it is with a scientific goal in mind ("see this stuff? that's the same stuff you can see in a completely different galaxy here!"), or just instilling a sense of wonder or amazement ("this thing is *bright*, so bright your eyes wouldn’t see color anymore").

There's still a lot for me to learn!

The learning never stops, even for me. There's always something new, always something to do different or improve.

Clear skies!