5 replies to this topic

[Tulloch]

Hi all, as some of you may be aware, I'm interested in developing a colour calibration method for the planets, similar to that performed for DSO imaging. While it is usually easy to select a colour shift for the gas giants, calibrating for planets with a single hue (such as Neptune and Uranus) is more problematic as it is hard to determine the colour shift required.

 

DSO imagers use a standard G2V star to calibrate their images (see here and here for examples), so I thought I might try a similar technique for planetary. 

 

Recent conversations with Christophe Pellier (and others) have caused me to investigate a G2V star near to Uranus (similar RA but 10* higher in declination) as a possible calibration star to see if the method has any merit. The star in question has designation HIP 9911 (HD 13043), a +6.88 magnitude G2V star with a similar size and colour to the sun.

 

Last night was clear, with no wind or moon. In the suburbs of Melbourne, Australia I was able to find Uranus readily, and took a 10 minute video with my ASI224MC (with ZWO IR cut filter) on a C9.25 attached to an Evolution mount, Teleview 2.5x PowerMate with gain 375 at 10 fps, and the result looks pretty good. I then turned my attention to finding HIP 9911, which was made more difficult because I cannot see this star through the spotting scope due to light pollution from the city, and the pointing accuracy of the Evolution mount is never really that good. After a number of attempts I removed the Powermate and tried with the camera at native resolution, finding a star that looked promising on the screen. SkySafari told me that there was a 10th magnitude star (TYC 4689-0890-1) underneath HIP 9911 and what appeared on my screen looks like what I expected. I took some videos of that star, noting how high the red component was from this star in the histogram, even though I was expecting it to be more pure white. Just before packing up I suddenly remembered that I forgot to remove the IR cut filter from the PowerMate and place it on the un-barlowed camera  . Fortunately I was able to fit the filter and take a 2 minute video of the star, with gain 375 at 77fps, trying hard not to saturate the frames by keeping the gain low. 

 

I stacked the video of the star in AS!3 in the normal way, stacking 50% of the frames with 3x drizzle, and sent it to Registax 6, where I told it to auto-balance the RGB values (with no sharpening), the same method I use for the planets which normally gives good results. Registax pulled out the parameters r1.12, g0.93 and b1.08, which are not significantly different from the values it normally produces for the larger planets (an encouraging sign). The difference in the star colour can be seen in the first image below. The as-captured image of the star does look a little green, while the corrected image has lost this colour cast, however it still looks a little red to me, maybe it's supposed to be a little orange/yellow? I don't know how Registax calculates the automatic correction required, however it seemed to do a good job in the past so I continued to use it. (This image also shows that maybe my collimation isn't up to scratch as well).

 

I then applied the same colour shift to my Uranus image, which is shown below as the second image. The planet comes up looking bluer (with maybe a touch of a rosy glow on the north pole eh Darryl?  ) but the green tinge has definitely been removed, which is a similar effect to what I noticed previously with Jupiter and Saturn.

 

So finally, submitted for discussion is the completed (dare I say colour corrected?) image of Uranus and its moons shown as the third image, with the system shown 50% larger than captured. WinJupos shows the moons dead on target, better than I've seen before. No sign of Miranda unfortunately, it just wasn't far enough from the planet to be evinced.

 

Q1: Is this an accurate, fully colour corrected image of the planet? A: Hardly, I would need more specialised equipment to determine that for certain, I'm not even really 100% certain that I imaged HIP 9911 as my colour standard, let alone determining the actual colour shifts required other than letting Registax do its auto-correct thing.

 

Q2: Is the colour correction good enough as a first order correction? A: Hmmm, maybe? I'll leave that for discussion here (assuming you've made it this far, I'm shocked at how long this essay currently is  ).

 

Comments, criticisms and advice welcome.

 

Thanks,  Andrew.

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Edited by Tulloch, 16 December 2019 - 10:58 PM.

[Tulloch]

Hi again, I just noticed that there may have been an issue with the star auto-correct. I originally ran the star video through PIPP to reduce the size of the frames, but it appears to have altered the colour slightly, even though I'm pretty sure I didn't de-bayer in PIPP and checked the "Protect Bayer Pattern" option. Anyway, using an image of the star that didn't go through PIPP first gave me the colour correction of r1.08, g0.95, b1.05 in Registax.

 

This changes the planet's colour to a paler shade of blue...

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[CPellier]

Hi Andrew,

There is certainly a hint of the brighter polar region there

A few remarks:

Assuming that the star is the good one it should be observed with the same optical element train if you want accurate coefficients. The barlow lens, even if good, certainly has not a flat color response.

One more remark about calibration with G2V stars. When looking through our atmosphere, they do look plenty yellow (pale yellow). So calibrating a planetary image assuming they are pure white, is supposed to deliver a color balance as if it was observed above our atmosphere. Which might be correct, but the result will be significantly bluer than the colors we are used to note under the best terrestrial conditions. 

The result, there, is certainly not bad, since we are all going to agree that Uranus is not red but the level of blueness is to be discussed.

Uranus is a tough target to test any color balance method. First because it presents very few, if any, details in color (so you will get no landmarks as you can on Jupiter). Then because anyone has observed that planet visually under various conditions certainly has noted how complicated is its visual tint. No one is going to see Neptune otherwise than pale blue, but Uranus has shown to my eyes a wide range of tints, from greenish to almost grey-white. More puzzling is the fact that the color looks to vanish when using big telescopes (I have some hypothesis for this). I have had the chance to look at it with big telescopes under extremely transparent high mountain skies (with the 620 mm Cassegrain of AstroQueyras in France, and even the 1 meter of the Pic du Midi), and the planet was almost colorless. Last october I had another chance with a 500 mm RC of AstroQueyras. Although seeing was poor, then I saw a light blue tint on it. My conclusion is that Uranus'color balance is tweaking a bit toward long wavelenghts with big telescopes because the human vision is then shifting from scotopic (low light, no color) to photopic (bright colorfull daylight vision). And that the planet's color is less saturated than Neptune, and then the tint is difficult to grasp.
Another element (the list can be long!) is that the response of a color camera is adding its own peculiarities. My ASI224MC has the tendency, to me, to produce very blue image of blue targets, more that my eyes can see. My Uranus images taken with it look too blue for my opinion. Last winter I took some images of hot stars than my eye is seeing as pale blue (type B) or white (type O) and with that camera they all got extremely blue directly without any correction. I have attached Alnitak (Zeta Orionis, late O to early B). So:

1) The method you are experimenting would maybe be more accurate with a b&w camera and planetary RGB filters. Do you have that ?

2) Whatever is the equipment, imaging a star for the purpose of color balance would be more easy to do if the star is unfocused, like on the Alnitak example. This would avoid any saturation problem.

Attached Thumbnails

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Edited by CPellier, 17 December 2019 - 01:32 AM.

[Tulloch]

Hi Andrew,

There is certainly a hint of the brighter polar region there

A few remarks:

Assuming that the star is the good one it should be observed with the same optical element train if you want accurate coefficients. The barlow lens, even if good, certainly has not a flat color response.

One more remark about calibration with G2V stars. When looking through our atmosphere, they do look plenty yellow (pale yellow). So calibrating a planetary image assuming they are pure white, is supposed to deliver a color balance as if it was observed above our atmosphere. Which might be correct, but the result will be significantly bluer than the colors we are used to note under the best terrestrial conditions. 

The result, there, is certainly not bad, since we are all going to agree that Uranus is not red but the level of blueness is to be discussed.

Uranus is a tough target to test any color balance method. First because it presents very few, if any, details in color (so you will get no landmarks as you can on Jupiter). Then because anyone has observed that planet visually under various conditions certainly has noted how complicated is its visual tint. No one is going to see Neptune otherwise than pale blue, but Uranus has shown to my eyes a wide range of tints, from greenish to almost grey-white. More puzzling is the fact that the color looks to vanish when using big telescopes (I have some hypothesis for this). I have had the chance to look at it with big telescopes under extremely transparent high mountain skies (with the 620 mm Cassegrain of AstroQueyras in France, and even the 1 meter of the Pic du Midi), and the planet was almost colorless. Last october I had another chance with a 500 mm RC of AstroQueyras. Although seeing was poor, then I saw a light blue tint on it. My conclusion is that Uranus'color balance is tweaking a bit toward long wavelenghts with big telescopes because the human vision is then shifting from scotopic (low light, no color) to photopic (bright colorfull daylight vision). And that the planet's color is less saturated than Neptune, and then the tint is difficult to grasp.
Another element (the list can be long!) is that the response of a color camera is adding its own peculiarities. My ASI224MC has the tendency, to me, to produce very blue image of blue targets, more that my eyes can see. My Uranus images taken with it look too blue for my opinion. Last winter I took some images of hot stars than my eye is seeing as pale blue (type B) or white (type O) and with that camera they all got extremely blue directly without any correction. I have attached Alnitak (Zeta Orionis, late O to early B). So:

1) The method you are experimenting would maybe be more accurate with a b&w camera and planetary RGB filters. Do you have that ?

2) Whatever is the equipment, imaging a star for the purpose of color balance would be more easy to do if the star is unfocused, like on the Alnitak example. This would avoid any saturation problem.

Hi Christophe, thanks for your comments, observations and recommendations

 

I did intend to image the star with the Barlow in the imaging train, but I just couldn't find it!

 

You are of course correct, a G2V star should look as yellow as the sun, since the light passes through the same atmosphere  .

 

Uranus does present an interesting target, Neptune is still a possibility but it is difficult to find and dropping fast. I'm really preparing for Mars, which again is a single(ish) colour target, so determining the correct color balance may again prove difficult (I think, I missed 2018 since I only started imaging in 2019).

 

I've only been using the ASI224MC for a few months now, but to my eyes I think it produces images that are too green even with Wblue at the maximum level, maybe I should experiment a bit more with a higher Wred values in FC. Unfortunately ZWO are not interested in making even a very minor change to their SDK file so that we could get a better white balance directly from the camera (see link here for a discussion I had with them online), so we are stuck with modifying the white balance post-capture.

 

I only have the ASI224MC and my Canon 700D, so I'm trying to do the best I can with these.

 

I will try de-focusing the calibration star next time, I'll also try for HD 9986 which has a nice bright star nearby which I should be able to pick up as a pointer.

 

Thanks again, Andrew

[KpS]

Andrew, you used Rexistax to reach a whitebalance. I think that a more suitable and more accurate method will be aperture photometry. You can use it to get the R, G, and B intensities of the star free of background. From these values you determine the necessary correction factors. The Iris program by Christian Buil is suitable for this purpose.

[Tulloch]

Andrew, you used Rexistax to reach a whitebalance. I think that a more suitable and more accurate method will be aperture photometry. You can use it to get the R, G, and B intensities of the star free of background. From these values you determine the necessary correction factors. The Iris program by Christian Buil is suitable for this purpose.

Hi Karel, thanks for the information, without a dedicated spectrometer I am never going to get completely accurate results, however I hope to find a first order correction with this method.

 

I visited Christian Buil's website and will try to apply his  tools and techniques to my own situation.

 

Thanks again, Andrew