20 replies to this topic

[Tulloch]

Hi there, in light of the K-man's recent posting of Uranus and Neptune, I thought I'd ask this question that has been bugging me a bit since I started in this caper. It concerns the colour correction I've been applying on Jupiter and Saturn to get them to look "good", or at least to give me the colours I'm expecting to see (and that everyone else gets). This then leads onto the "correct" colour cast to apply to Uranus and Neptune, assuming there is one.

 

Firstly, I'm using the white balance values that ASI recommends for the ASI224MC, which are WRed=52 and WBlue=95 (well, I think I am, I didn't change them from the default values and the log file says they are "off").

 

When I take take images of Jupiter, the stacked and sharpened images are quite green looking, and I have been using Registax 6's "RGB autobalance" to give me the colours it thinks are correct, this results in a nice looking image with values Red 1.09, Green 0.94, Blue 1.09 (see image below).

 

For Saturn, the suggested colour shifts are Red 1.03, Green 0.96 and Blue 1.18 (second image below). 

 

Then the question comes of what colour correction should be used for Uranus and Neptune. Obviously they are blue in colour, so using the RGB autocorrect in R6 doesn't work, you get a "white" looking planet as it bumps the red up to huge values. The histograms I get for these planets are shown below (Uranus 3rd, Neptune 4th), and since the histogram is nice and blue (or blue/green), and I'm expecting to see blue or blue/green, I don't play with the colour settings at all.

 

However, given that Darryl (and others) are seeing a rosy glow on Uranus using a mono camera and filters, should I be colour correcting my images to make them more "accurate"? I do it for the larger planets, why not Uranus and Neptune? Should I use figures similar to those I use for Jupiter and Saturn? Something else? Is there a theoretical shift that should always be applied due the scattering of light from Earth's atmosphere that is proportional to the altitude of the target? Or should I simply not bother about it too much as this is a direct result of using a colour camera in the first place? Or just be like the mice from Hitchhiker's Guide to the Galaxy, and just make something that looks good?

 

Interested in your thoughts.

 

Thanks, Andrew

 

 

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[Kokatha man]

Hi Andrew - the ASI224 camera image of Uranus I posted later in the thread you mention here was colour adjusted using rgb values from a file with said & an image of Alex Obhukov's - I think - but it was part of a thread discussion on so-called "true colours" for Uranus & Neptune which I kept some notes etc from but cannot for the life of me find atm...the thread was 4 or 5 years ago now.

 

In my current thread I note that the colour camera doesn't achieve this particular pinkish hue for the NP regions of Uranus - I'm sure you could manipulate an image to get something like it but I wouldn't bother tbh...the reason it doesn't create it might well be to do with the bleed between Bayer matrix filters in an osc, but that's pure speculation on my part: here is that fairly recent posting of Paolo's where he evinces that hue in the NP regions btw: https://www.cloudyni...s#entry9733779 

 

Paolo used a C9.25 incidentally, but it was a mono camera & filters which I think has something to do with the situation...each r-g-b filter does have fairly specific bandwidths. (various makes do vary but this is a fair bit tighter than the osc situation)

 

Incidentally the red & blue slider values are under the "More" tab in FireCapture - here's a screenshot of an older FC version with dummy cam...best I can do on the pc but they are all similar.

 

Fwiw the values for the NP brightening regions I sampled were r=170, g=157 & b=149 & for the rest r=138, g=161 & b=170 in my current r-g-b image.

 

I wouldn't get into any lather about this using the colour cam tbh...our takeaway is that the seeing itself was pretty decent (despite the forecasts!) to get the r-filter to evince (remember that word? ) contrast definition between the NP regions & the rest (I can even kid myself that it can be seen very vaguely in the g-filter image  ) so I am happy with that outcome: transparency was on the poor side & you'd know all about light pollution at low elevations with Uranus also - why I'd like to get a few degrees north in black skies before it is too late because I still think southerners can pull decent images of Uranus despite the elevation nowadays...& remember that with an iR610nm filter the ASI224MC is capable of capturing NP contrast variation if that sort of thing is any challenge to yourself..!

 

 

 

[CPellier]

Hi Andrew,

It's nice to start with more or less "fine" color settings when using a color camera, but too often people think that objective and precise values can be always applied to get an accurate color balance, and this is not the case. Variations of atmospheric transparency, variation of planetary elevations in the sky, are going to shift "good" parameters from one night to the over. So yes, you should not be afraid to adjust the coefficients during the post-processing. However, this should be done by building a personal knowledge of what actually are the reliable colors on each planet. There are several ways to learn this and it is a matter of time and experience. 

For Uranus and Neptune, one could rely on their albedo spectra, that tell us that Uranus is slightly less reflective than Neptune in blue, but slightly more reflective in red. So an adequate color balance is to get Neptune blue, and Uranus green-blue. Uranus is a tricky planet for colors because it is also a matter of color saturation, not only tint or hue, and the planet is obviously poorly colored.

Some remarks:

1) You do not precise if you are using an IR-cut filter with the ASI224MC. The absence of such a filter is likely to wreck any attemps of getting correct color coefficients.

2) Pay attention precisely of how are processed color images of Uranus and Neptune that you see on the web. Currently many observers regrettably false-color their IR image, or apply an IR luminance on a previsouly taken RGB frame. None of those processing is going to produce anything good about color balance.

Here is are some RGB images of Uranus I took over the past years with ASI224MC and IR-cut filters; they do reveal a different tint for the polar region ; being brighter in red, it is just colorless or slightly pinkish, but certainly not green or blue.

 

 

[Tulloch]

Thanks Darryl and Christophe, I really welcome your expert opinions on this. Christophe, I should have mentioned from the outset that all my images with the ASI224MC are taken with the ZWO UV/IR cut filter in place, I would only mention it if had removed the filter and included the IR part of the spectrum. They are some really impressive images of Uranus using the ASI224MC showing the presence of the cloud mass on the north pole.

 

I guess the point I'm trying to make is how do I accurately set the white balance of the camera? In terrestrial photography it's easy to place a white card in the scene and set the white balance that way, obviously it's more difficult in this case. Atmospheric scattering will tend to shift the colour of the target towards the red as it gets closer to the horizon, but by how much? Is there a standard white object that can be used for white balancing in the field?

 

With the mix of colours present in Jupiter and Saturn, it's easier to modify the colour balance to get something that look "right", but when one colour dominates (like Uranus, Neptune and I guess, Mars) can we use "known" colour shifts to get a better (or more accurate) result?

 

Using PIPP, I split a video I took into its rgb components, stacked each separately and then recombined them in Photoshop. Obviously, the intensity of the red channel is much reduced over the others, and maybe if you squint you can see a slight increase in intensity on the right hand side, but since it's so dim there is no chance of reproducing the white clouds seen by Hubble or yourselves. Maybe that's also a consequence of seeing conditions at the time I took the videos, or light pollution from the suburbs. However, if I knew that I needed to bump up the red channel by 10/20/XX% then it might be better? I would certainly colour correct the Jupiter and Saturn images above prior to posting, why not these ones?

 

Maybe I'm asking the impossible, but I'm more interested to see what others do in these circumstances.

 

Thanks again, Andrew

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[Kokatha man]

I'll (reiterate) my take on this first because I'm reading it right now Andrew.

 

That said I'd defer to Christophe' much more studied experience in these colour matters & apologise   if I have misinterpreted his own approach!

 

But from what I have experienced using the ASI224MC, imo it is necessary to make all colour adjustments in "post processing" - which is what I believe Christophe is clearly stating in his reply to you above..& I think that the colouration of the Uranian satellites in his images is another confirmatory clue of this ....

 

He is also absolutely right in that there are quite a few factors that influence the output "from the can" before any post-processing...transparency & elevation being the most influential in our own opinion also...that said I don't feel that altering the Wred & Wblue slider values in the camera for capture makes a lot of difference tbh - but I have to confess my own experiments on this particular point are not very exhaustive at all...so I say that with an element of caution. (I have used somewhat higher Wred values a few years ago - possibly for that 2015 image I reproduced in your thread above - but I am unsure tbh...)

 

I shouldn't have been quite so dismissive when I said <"I'm sure you could manipulate an image to get something like it but I wouldn't bother tbh"> because I appreciate that you wish to achieve as accurate a colour outcome as possible...& I especially know Christophe is a very strong proponent of this goal, which I ultimately believe is the right & proper approach! 

 

I have certainly followed his admonitions personally for a long time now as far as "false" or ir-rgb images of the Ice Giants, even though there are some early efforts still on our website. Having converted to his views I was surprised to see quite a lot of ir-r-g-b or ir-g-b images during this Jovian apparition...although obviously the people doing so were experiencing a lot of poorer seeing...& in an effort to chronicle the GRS & surrounding activity particularly, resorted to these false colour approaches.

 

However, it should of course be stated that by clearly declaring the composition of such combinations there is nothing intrinsically wrong with such images...

 

Finally I do think that the mono cameras exhibit a clear advantage in this particular situation as our & Paolo's (plus others) clearly demonstrate imho in that there is no need to manipulate the outcomes...& my thoughts re the "tighter" bandwidths of the individual mono filters is a prime factor in arriving at a correct colour balance...ie, without the "bleed"  of colour cameras that I mentioned in my last post here - the "true colour" is "truer" because despite "splitting" channels from a colour camera that "bleed" still influences the outcome, requiring aforesaid post-processing manoeuvres to arrive at the "correct" outcome.

 

 

 

 

[Tulloch]

Thanks Darryl, I appreciate the input.

 

It doesn't really matter to me whether the colour correction is done in pre- or post-processing, if post is the way to go, then that's fine by me. Of course, you do need to set some values for Wred and Wblue in the imaging software, but from my own limited experience (so take this comment with a good helping of salt) it appears that the values suggested by ZWO may not be optimised (for planetary at least). Do you know how they came up with these numbers?

 

No arguments from me about the advantages of mono imaging vs colour imaging, provided the seeing conditions are identical for each capture . I don't quite understand how colour can bleed across pixels (and I don't doubt your statement), but I'll do some more reading on that front.

[Kokatha man]

Perhaps "bleed" gives an incorrect notion of what I'm trying to describe Andrew - here is the response graph of the ASI224MC where the channel values are relative - "1.0" is roughly equivalent to a QE of 75-80%.

 

Also the Astronomiks 2c filter graph for their red, green & blue filters - I use these (but not for iR, instead use Baaders for those & an Astrodon uVenus for that specific filter)...the concept I'm trying to convey appears to be a bit shaky with these graphs I have to admit, but whether I'm right or wrong it has been my understanding up to now - so if I make an **** of myself, so be it...it wouldn't be the first time I have..!

 

Also an Astronomics ir/uv cut filter for good measure.

 

 

 

 

Because the graphs aren't similarly-scaled it makes it more difficult to envisage what I'm attempting to explain & of course the first graph is a relative QE one with the others transmission %'s, making it even more obtuse...but if we take 20% transmission as a (possible) colour-outcome-affecting value we can see (hopefully!   ) the following:

 

Looking at the green filter of the Bayer matrix of the ASI224MC, that (arbitrary) 20% values stops below around 465nm, with the blue of the Bayer matrix admitting light to 530nm at 20% - so we have a "mixing" (my "bleed") of the blue & green transmission (of noticeable affect if 20% is a valid figure) between 465nm & 530nm at 20%... a 65nm "cross-over" or "bleed" of blue & green. 

 

You can see my Astronomiks filters (using that purely arbitrary 20% figure of mine again) will only "mix" or "bleed" between these 2 filters from about 485nm to 510nm...a 25nm "bleed" of blue & green.

 

Taking the red & green graphs my mono filters will admit mixing between about 570nm to 595nm (25nm bleed) whilst the ASI224MC's Bayer matrix filters will pass between 565 & 645nm. (80nm bleed)

 

That's the basis of my reasoning: as said already I freely admit it is a bit shaky, especially the somewhat "apples & oranges" aspect of QE vs transmission, but I think it has some validity - feel free to tear it to pieces if it is something you don't agree with!!!

 

The last ir/uv (luminance) filter graph just demonstrates that the total light throughput is controlled between the indicated bandwidth when used with the colour camera...

Edited by Kokatha man, 29 November 2019 - 11:05 PM.

[Tom Glenn]

Also, Andrew, before you lose too much sleep over attempting to recreate "true color", keep in mind that this concept is a lot more complicated than one might initially imagine.  If we define "true color" as what would be perceived by the human eye if one were observing from a spacecraft, then you have to realize that human color perception is not fixed.  Since we are talking about bandpasses of filters, take a look at the response curve for human cone cells of the retina, which are responsible for color vision (pulled from Wikipedia).

 

 

As you can see, there is extreme overlap of the response to green and red, which accounts for much of the genetic variation in human color perception, including the majority of types of "color blindness", which is actually a misnomer, as most cases manifest themselves as deficiencies in discriminating some (but not all) shades of red and green.  Even among individuals with "normal" color vision, we can't really be sure that everyone is perceiving things identically.  The retinal cone cells have specific response curves, and it's up to the brain to then make sense of that signal and assign a color.  Add to this the discrepancies among computer displays and cameras, and you have a very hard time standardizing a "correct color", at least as far as amateur imaging is concerned. Nevertheless, the advice from Darryl and Christophe is very useful and good to follow.  

Edited by Tom Glenn, 30 November 2019 - 02:14 AM.

[Kokatha man]

...I refrained from venturing into the realm that Tom has raised Andrew because I wanted to convey my use of the term "bleed" in Post #7, which wasn't probably the best descriptor...as Nick (happylimpet) put it fairly succinctly a few years ago <"There are two factors - colour balance (ie adjusting the overall cast so that white is white and greys are grey) and the fact that the broad FWHM filters used in Bayer arrays allow adjacent red into green, blue into green, green into blue..etc etc etc. So colour saturation is diminished.">

 

Or as Christophe said in the same thread <"Yes. It's nice to adjust color balance from the start of the process, but let's not forget that it can (and must) be adjusted during image processing. So it's not a problem if the camera's color balance isn't absolutely perfect. It must just be "correct" (without any dominant tint)">

 

I won't begin to dissect Nick's comments further because I really just wanted to add a very little more to Tom's comments about colour perception: females generally have a different perception to males for one thing...& there was an extensive article by Dr. Dietmar Hagar (a surgeon & AA'er) about this with detailed explanations of cone & rod function etc which I can't find on his website any more..but, a complex issue to be sure.

 

I probably made a point about that rosy tint on Uranus that simply appeared in the image without any colour etc adjustments...& because of people like Christophe & Paolo's images etc (forgetting that Christophe was using the ASI224MC) as well as certain other images I had seen from the HST etc: in fact r-g-b imaging with a mono camera is very unusual for us...I thought it might very well be the only time we have done so, but on looking at our website I did find a single one from August 2016 - a rather bland one of uniform colouring similar to that of the more southern aspects of the planet, entirely devoid of any NP brightening.....which leads me to believe that the conditions were probably not good enough compared to the other night...   

[Tulloch]

Thanks Darryl, I now understand what you mean about colour bleed, I had forgotten that the different pixels overlap in their colour sensitivity.

 

If we are going to swap graphs, I'll put this one up, showing the relative sensitivity of the ASI290MM vs the human eye (from Tom's wikipedia reference) which I put on the same set of axes and hopefully have the same or similar units. (The ASI224MC will have a similar curve, but it is more difficult to add the rgb curves together.) So while it might be great to have perfect R-G-B filters with minimal crossover/bleed, what we really need is a spectral filter that takes into account the sensitivity of the camera system (OSC or mono+filters) to produce the sensitivity of the "standard" human eye (whatever that is), then apply an adjustment that takes into account the seeing conditions on the night for the target on the screen (sounds easy, right?) 

 

Tom, be rest assured, I'm not losing any sleep over this (other than the sleep I normally lose sitting in the dark staring a small fuzzy blob on a computer screen trying to get it to focus for the n'th time ), or aiming for perfect Colour Truth, more that I just think there might be a way to calibrate the white balance in the field, or at least work out a way to calculate some better white balance values so that minimal post-adjustment is required.

 

With regards to Christophe's statement, <"Yes. It's nice to adjust color balance from the start of the process, but let's not forget that it can (and must) be adjusted during image processing. So it's not a problem if the camera's color balance isn't absolutely perfect. It must just be "correct" (without any dominant tint)">.

 

I perfectly understand what he means, but the question I have is; How do I know how to correct the colour balance in post-processing if I don't have any reference point so I know the result is better? With my Jupiter and Saturn in the original post, it's pretty easy to tell these images are too green. However, since Uranus and Neptune were captured with the same system with similar settings, are they too green also? Maybe they are too blue? Since there are no other dominant colours in the image, does it even matter? 

 

Andrew

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Edited by Tulloch, 30 November 2019 - 05:22 AM.

[gfstallin]

As you can see, there is extreme overlap of the response to green and red, which accounts for much of the genetic variation in human color perception, including the majority of types of "color blindness", which is actually a misnomer, as most cases manifest themselves as deficiencies in discriminating some (but not all) shades of red and green.  Even among individuals with "normal" color vision, we can't really be sure that everyone is perceiving things identically.  The retinal cone cells have specific response curves, and it's up to the brain to then make sense of that signal and assign a color.  Add to this the discrepancies among computer displays and cameras, and you have a very hard time standardizing a "correct color", at least as far as amateur imaging is concerned. Nevertheless, the advice from Darryl and Christophe is very useful and good to follow.  

Thank you, Tom. As a "color blind" person, I often have to explain that I can, in fact, see colors. I also love watching people with "normal" color vision fight over which color something is. "How can't you folks agree on this? Are both of you color blind as well?" That is usually when I jump in with a ridiculous color assessment like "Mountain Stream Purple." It eases the tension. Still, I'll never have any idea what people find so inspiring about fall/autumn. "The trees turn brown and everything dies. Is that what we're getting excited about?" Rainbows are neat yellowish arches of light in the sky. I call it the "post-storm McDonald's," but nothing to risk going outside to image while lightning is still striking. "As neat as the one in the sky is, there is a double arch with a drive-thru that is open 24-hours a day if you like that sort of thing." 

 

In an old career as a cartographer of conflict and disaster zones, my colleagues would send me their maps to make sure that color blind folks could understand the color schemes. I warned that I could speak for all color blind people, though I really didn't have much information to back up that assessment. Still, their effort to make sure I understood what was going on with their work was a step in the right direction.

 

George

Edited by gfstallin, 30 November 2019 - 05:31 AM.

[Kokatha man]

I understand your concerns Andrew, I sometimes used to chide Christophe (good naturedly!) about where do we posit the nature of "correct" exactly..?

 

You can come up with a prescriptive formula for a pc screen in terms of r-g-b values I suppose, but then we'd still have arguments over who sees what...is your screen correctly calibrated, etc...

 

That said there are some general guidelines...some like to match eyepiece appearances but personally speaking I have not viewed Uranus (or Neptune) at sufficient magnitude to appreciates that aspect - even if our scope & eyes were sufficient to detect such subliminal colour variations: I'd envisage you & me both looking at Uranus through a 200" scope in near-perfect seeing & then arguing over how it appeared to each of us!

 

For my part I keep my screens regularly calibrated (most of the time) & acknowledge anyone who has made a detailed study & examined the rationale behind any conclusions (Christophe is one such person I respect in this endeavour btw) but I know now that I will never look at another r-g-b capture of Uranus from an AA scope without thinking "hey - the contrast variation should be greater in the red on a good night, so the polar regions should take on that rosy tint!"

 

We've taken quite a few r-g-b captures of Neptune btw & I can easily see that deeper blue at the ep of our S/vue 10x50 - which comes out in the image captures, but I have to say that personally Uranus is much harder for me to prescribe an ep colour appearance to! 

 

But "relax" might be the best mantra...my other main takeaway from our own session on November 27th is what a wonder new prescription glasses do to being able to view a screen from a good distance (600-700mm) which I ensured by getting the optometrist to prescribe for that distance using our own website images of Jove at the time: prior to this they were making "readers" which you use much closer, even though I had asked for one pair to be computer glasses - which they weren't!

 

It sure pays to be very explicit at these places...when I picked them up this week (a few hours before this session) I got them to find one of the vacant optometrists' rooms with a hi-def screen so that I could go online on to check everything was ok...just like I did when the optometrist was deciding what lenses I needed - at that earlier visit I told her that even the finest print on their standard (printed) appraisal chart was just not good enough for what I wanted to see, so we did the final lens order confirmation on our website!

[Tulloch]

That said I'd defer to Christophe' much more studied experience in these colour matters & apologise   if I have misinterpreted his own approach!

...

He is also absolutely right in that there are quite a few factors that influence the output "from the can" before any post-processing...transparency & elevation being the most influential in our own opinion also...that said I don't feel that altering the Wred & Wblue slider values in the camera for capture makes a lot of difference tbh - but I have to confess my own experiments on this particular point are not very exhaustive at all...so I say that with an element of caution. (I have used somewhat higher Wred values a few years ago - possibly for that 2015 image I reproduced in your thread above - but I am unsure tbh...)

Maybe I am obsessing a little over this, but I was interested in your statement that altering the Wred and Wblue values didn't seem to make a lot of difference, so I thought I'd do a little experiment of my own (being a scientist and all). So, I made up an artificial planet made from a small circle from a hole-punch and gluing it onto a piece of black card, then pointed the ASI224MC with the fish-eye lens attached at it and taking some 2000 frame videos while changing the Wred and Wblue values. Note that the test was done indoors, so there is a yellow cast from the indoor lights.

 

I started by increasing the Wred value from 52 up to 70, and then to 80. After stacking the videos, I then tried re-creating the colours of the Wred=70 and and Wred=80 captures by changing the red, green and blue values of the Wred=52 video in Registax. It turned out that increasing the red value in R6 made the image lighter as well as redder, so I instead reduced the green and blue values until the colours and brightness were reasonably well matched.

 

The results are shown below, with the various values for FC and Registax shown. Perhaps unsurprisingly, there is a direct correlation between the ratio of Wred values in FC compared to the ratio of green&blue values in Registax. For example, moving from Wred=52 to Wred=70 is an multiple of 1.35x, where the inverse is 0.74. This matches well for the g&b values reduction in Registax of 1.00 to 0.75. The same occurs when going from Wred=70 to 80, with the corresponding drop of g&b from 0.75 to 0.65. Now this is only a sample size of two so hardly a representative sample, but the trend is there.

 

Then it occurred to me that maybe someone had done something like this before, or that there were other recommended Wred and Wblue settings for these cameras out there on the net. Turns out that I didn't have to look too far after all, after a quick search on Google, I found a page from a presentation on Christophe Pellier's website entitled "Planetary imaging with a colour camera" (see 2nd image below), where he recommends Wblue as high as possible (99) and Wred between 60 - 70 for the ASI224MC. These values are close to those suggested by ZWO (R52/B95), but since increases in blue levels are still required in the post-processing steps, maybe the Wblue values in FC don't go high enough?

 

I think I'll stop there  , and think about this some more tomorrow ...

 

Andrew

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[Tulloch]

... so what to take from all this?

 

If Wblue is set to 99 and still needs to be increased in post-processing, maybe we need to ask Torsten to increase the maximum Wblue (& Wred) sliders to 200, rather than 100? 

[Kokatha man]

Obviously this concerns you more than me Andrew but I can appreciate the question even if I think you will find those other factors (specifically seeing, transparency & elevation in our experience) much more influential.

 

Your further posting had me look up some ZWO info including this https://astronomy-im...-Windows-EN.pdf but am none the wiser, especially after seeing the FireCapture Wred & Wblue setting examples therein.

 

I think (without plugging the camera in which isn't convenient atm) I have set what I thought was the "standard" as Wr=55, Wb=92 for the ASI224MC.....but I do have this posting from Sam on my home pc:

 

Sam Wen quote: <"WRed WBlue are not correct, the default value should be WRed 55 WBlue 99"> but to further complicate matters I feel sure I emailed Sam after this a got a retraction to the effect it was 55/92..!

 

It might have been in an email that I got his last comments...something about some values merely being software applications (ie, not relevant) but that & anything else is rather vague atm...

 

I'll see what else I can find when I get the time...I have literally hundreds of emails between Sam & me so it won't be easy to find which one it was, if any! 

[Tulloch]

 

I think (without plugging the camera in which isn't convenient atm) I have set what I thought was the "standard" as Wr=55, Wb=92 for the ASI224MC.....but I do have this posting from Sam on my home pc:

 

Sam Wen quote: <"WRed WBlue are not correct, the default value should be WRed 55 WBlue 99"> but to further complicate matters I feel sure I emailed Sam after this a got a retraction to the effect it was 55/92..!

 

Thanks Darryl, I didn't touch the numbers so Wred 52 and Wblue 95 are the "default" values as far as I know.

 

In May last year you thought they were the recommended values also, 

https://www.cloudyni...24mc/?p=8606642

 

I'm not sure how FC handles colour balance with the Wred and Wblue values, however it appears that the green is a bit heavy straight out of the camera and maybe we need a little bit extra blue to help balance the result better?

 

I wonder how easy it is to increase the blue slider range in Firecapture?

 

Andrew

[Tulloch]

I'm reviving this older thread with some actual data, albeit over only 2 nights but in significantly different seeing conditions.

 

DSO imagers sometimes use G2V stars for colour calibration purposes, while the advice I've seen for planetary is typically along the lines of "find a white-ish section of the planet and white balance against that". For Jupiter this is typically a white band on the planet, Saturn is the rings and Mars is the polar ice cap (reference here). Personally, I find this pretty unsatisfactory; is the band on Jupiter or are Saturn's rings really white? Are the polar icecaps on Mars pristine, with no dust settled on them or blowing around in the wind? What about Uranus and Neptune who don't have a "white" area?

 

So I thought I'd try imaging a G2V star near to a planet I was imaging so see if I could produce a reference white balance point to balance the planet image against. With all the larger planets on the other side of the world right now, I have had to use Uranus for the time being, but hope to continue with this technique next year.

 

Early results are promising, on a good night of seeing (16 Dec 2019) I was able to capture the G2V star HIP 9911, which I stacked and got Registax 6 to auto-balance the colours for me (just like I have done already for Jupiter and Saturn). The colour correction settings for the star came out pretty close to what I've used in the past for the larger planets, which was a promising start. On 22 Dec I tried a different G2V star, HIP 19767, however this session was compromised by smoke in the atmosphere from far-off bushfires drifting across the sky. This produced a significantly higher red colour correction from the other nights, which may not have been totally unexpected given that smoke particles tend to scatter red light more. The colour correction settings from Registax for the planets are shown in the table below:

 

                      red      green      blue    

Jupiter           1.09      0.94       1.09

Saturn           1.03      0.96       1.18

HIP 9911       1.08      0.95       1.08

HIP 19767     1.20      0.93       0.96

 

The effects of the colour shift from the G2V stars for the Uranus images are shown below, together with the images of the G2V stars, shown at 3x capture scale (using 3x drizzle in AS!3). The colour shifts for Jupiter and Saturn are shown in the first post at the top of this thread. To mis-quote Aristotle, one swallow does not a summer make, however I believe this technique has some promise and I hope to continue this next year once the larger planets come around again. Of course, if anyone else would like to have a crack at a G2V star next time they are out imaging the planets, feel free , I'd be interested in the results.

 

Thanks, Andrew

 

 

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[Kokatha man]

Nothing much to add to this thread atm (although Dec 16th looks the clear best...did I say that in one of your other threads lately Andrew..? ) but I thought this snippet from Bomber Bob in another forum I engage in on CN worth posting: Bob is a seasoned observer btw )

 

<"I've noticed that game nights create a sky-glow that helps me see tiny details on Mars - diminishes some of that bright orange-red, which overwhelms my retina.">

 

Interesting, without any specific connections to your smokey skies...& again with nothing really pertinent to your subject matter Tom made the comment recently that Mars is much more impervious to poor seeing conditions than probably all the other planets when it comes to imaging - which I thoroughly concur with! 

Edited by Kokatha man, 23 December 2019 - 09:29 PM.

[Tulloch]

Nothing much to add to this thread atm (although Dec 16th looks the clear best...did I say that in one of your other threads lately Andrew..? ) but I thought this snippet from Bomber Bob in another forum I engage in on CN worth posting: Bob is a seasoned observer btw )

 

<"I've noticed that game nights create a sky-glow that helps me see tiny details on Mars - diminishes some of that bright orange-red, which overwhelms my retina.">

 

Interesting, without any specific connections to your smokey skies...& again with nothing really pertinent to your subject matter Tom made the comment recently that Mars is much more impervious to poor seeing conditions than probably all the other planets when it comes to imaging - which I thoroughly concur with! 

Thanks Darryl, as you say, the 16th was certainly the best seeing of the three nights. 

 

I don't know what Bob meant by "game nights" (and I note he posted that comment in 2016 - well remembered  ), but maybe there was smoke in the atmosphere from the "game"?

 

I hope my physics is correct, from my reading it seems that smoke and other small particles in the atmosphere scatter via a Mie scattering process, rather than the normal Rayleigh scattering from gases in the atmosphere. I think that means that the red light from the planet is removed (scattered) more than the rest of the spectrum, leading to a correction required by Registax to "replace" that lost red light? Anyway, that theory fits my data really well, I sure hope it's correct  .

 

I haven't had a chance at Mars yet, so I have no comment on how easy it is to image. I've seen a number of posts saying that the blue part of the spectrum is more sensitive to poor seeing, so it's probably not surprising that Mars with its dominant red hue should be less affected. Just for interest, how do you white balance your Mars images?

 

I feel as though I'm trying to "swim against the current" with my colour calibration idea; either people don't think it's possible, or straight out don't care, but I think I'll continue until it proves hopeless. The weakest point of my whole process is that I also don't know how Registax does the RGB "autobalance" correction, but it seems to do a good job (usually). Anyway, gotta have a hobby (or hobby-horse), even if it useless .

 

Andrew 

[Kokatha man]

"game" as in a nearby sporting arena with the glare of bright lights I presumed Andrew!

 

I generally approach Mars the same way as Jupiter & Saturn, letting the "RGB Balance" of R6 do its' thing with an increase in saturation & vibrancy, which I believe assists all these modern cmos sensors where the raw outcomes do appear rather "washed out" or "wan" imho...

 

I remember getting into a prolonged debate on another forum where I was harangued slightly for my off-white Mars polar region some years back now...ultimately someone noted that this same colouration was on nearly all other imagers' captures at the time & was put down to dusty/dirty conditions, something I think you alluded to with your "pristine" comment above.

 

Saturn seems to go through significant colour-appearance changes over the seasons & I note that our images usually compare with that of other folks whose outcomes I have confidence in: this means that there might be some basis for said...or else they are post-processing in exactly the same way as me..!

 

I don't think there is anything wrong with you persisting with this investigation btw - I would suggest that a more correct way to label your outcomes might be "a colour outcome based upon the colour-balance derived from a solar-class star in the near vicinity" or somesuch rather than simply "true colour" & all that could infer, despite all the intervening factors - but I won't lose any sleep if you stick to the latter definition..!

 

This quite early Mars (for us) presented the best captures as far as resolution is concerned regarding the blue channel, I cannot recall any others we captured revealing quite so much surface detail...2014 using the ASI120MM-S.

 

 

 

[Tulloch]

Thanks Darryl, I don't think I've ever claimed "true colour", just an image calibrated against a G2V star in a similar location and atmospheric conditions, but I get your point.

 

I'm looking forward to imaging Mars next next, but kicking myself slightly for missing the opportunity in 2018, oh well. Mars won't be getting higher than about 45 degrees in 2020 near me, but that should still be good enough. Nice image of Mars btw, but I'm sure you don't need me to tell you that  .