NCP, Polaris and surrounding galactic cirrus

[RBA]

I haven't posted an image here in a while, so here I go...

This is a 2x2 mosaic wide field of the North Celestial Pole, featuring one of the best friends of astrophotographers in the Northern Hemisphere: Polaris. In fact, for us nomadic imagers, Polaris is not only our friend, but at the beginning of each session, we get on our knees and what may seem as an imager doing polar alignment, we're in fact PRAYING to the Northern Star that the session goes well!  ;D

The image also features a copious amount of galactic cirrus (some of it displaying some very cool structures), one of the oldest known open clusters (NGC 188, at the bottom-middle), and Delta Um (middle right, the second star in Ursa Minor's tail)

Here's a smaller version:

And here's a link to a larger version: http://deepskycolors.com/pics/astro/2010/11/mb_2010-11_NCP_3k.jpg

Orientation: north is IN 

If you'd like to see where the North Celestial Pole actually is, you can see it here:

The data was captured over the course of two nights next to the DARC Observatory under 21.3 mag skies (that's at the Zenith), average transparency and bad seeing, and the processing was roughly 75% PixInsight and 25% CS5. DARC is around 120 miles from my home, so that makes this a 480 miles image ;-) Not a lot of data (1h lum and 18m each color filter per frame) as I started the project when the Moon was already getting bit and setting late.

The image is also a testimony of how nice the polar scope of the EM400 mount is, as that's the only method I used both nights to polar align (no drift, etc) and as everyone knows, imaging near the pole requires a good polar alignment, but of course, this image is not near the pole but on the pole itself! The forgiving resolution of the FSQ of course does help, but still, not bad at all.

As always, I identify a number of "I shouldn't have done that" or "I should have done this that other way" during the processing, but overall, and considering how seldom this area has been photographed, I think it does it justice somehow as a display of the area, and I'm happy with the results.

 

[Hans Pleijsier]

great picture, great text. Thanks.
From now on my polar alignments get a new meaning ...

 

[Ginge]

Very nice Rogelio! And rarely seen. I enjoy it very much.

From a fellow worshipper,
Ginge

 

[cavamen]

The first time I saw the picture I was shocked: Polaris is a blue star? ???

I have done some research and Polaris seems to be a F7 spectral type (yellow-white), confirming my previous knowledge.

http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Polaris

I'm curious. How are you doing your color calibration?

Sergio

 

[RBA]

I'm curious. How are you doing your color calibration?

Regarding color balance, the ColorCalibration tool in PI following usual workflow (BN, CC, non-linear adjustment, etc), produced a blueish and somewhat purplish Polaris. G2V (it was tricky because there's another non-g2v star rigth next to the one I identified) and linear histogram alignment, all produced a greenish, blueish Polaris, more so the histogram method.  I was also going to try eXcalibrator but my FITs don't store the WCS (a requirement to run eXcalibrator AFAIK) and I stopped short of adding them manually and try...

None of the tried methods produced anything that could be considered anything white or yellowish at all.

Furthermore, a friend checked several stars in the final image (spectral type and B-V value) and concluded that the colors were "very well balanced". It wouldn't bother me a great deal if they weren't, but that's what he said.

You can have at it with the calibrated R, G and B files if you like, or maybe come up with an explanation, but reckless color balance is not.

Cheers!

 

[Juan Conejero]

F7 spectral type (yellow-white)

Not actually. Polaris is a F7 star whose temperature is 7200 K.

Using Bruce Lindbloom's CIE spectral calculator we can see that both a blackbody radiator and a CIE D illuminant at 7200 K color temperature have a strong blue dominant in terms of their relative spectral power distribution:

http://brucelindbloom.com/index.html?CIESpectralCalculator.html

To use the above calculator you need a Java-enabled browser. Check the Blackbody option at the bottom and enter 7200 as the corresponding temperature, then click on the graph (click on the graph each time you change temperature to update it).

Judging from the above data, I'd say Rogelio's rendition of Polaris is spot-on

Interestingly, a blackbody radiator at 6500 K is blue-cyan. That's why color calibration with a G2V star tends to introduce a yellow-magenta color cast.

 

[RBA]

Thanks Juan. I'm far from an expert so I deal with whatever comes out. It's good to question things and sometimes it helps us learn new things.

For whatever is worth, I replicated the color balancing on the frame that contains Polaris (remember this is a 2x2 mosaic) and here's a quick color balance walk-through and the results, only on the RGB data of the frame that contains Polaris:

1- RGB Combine (after applying DBE on each R, G and B master frames).
2- Background Neutralization
3- Color Calibration (whole image as the white reference)
4- Non linear histogram adjustments
5- Saturation (a bit strong to show colors well)
6- SCNR

Again, this is a straight-forward processing on the RGB data only, before combining RGB with the luminance and any further processing. Data was linear all the way until step 4 and of course, no masks were used at any time.

Then, there's the question I've also been asked a few times about why "my" stars show a glow around them much bigger than "usual". Many people think it's something I add during post-processing but that's not it (in fact I often tame down the glowing effect with HDRWT). And that too is a question for which I have no answer. I use Astrodon 1st gen filters but so do many other people. I have also thought that dirt in my lens might be the culprit, but again, I'm sure I'm not the only one with a "dirty" refractor. Whatever it is, it doesn't bother me, in fact  I like it, so be it  8) I just want to be clear that it's not a post-processing thing.

EDIT: Forgot to add cropping "bad" edges right after RGB combine, of course.

 

[cavamen]

Well, nicely explained. Will look Polaris next time carefully to catch the blue ray. Visually I  always see a yellowish tint.

Have fun!

Sergio

 

[RBA]

Well, nicely explained. Will look Polaris next time carefully to catch the blue ray. Visually I  always see a yellowish tint.

Sergio...

Actually my suggestion would be to image it rather than observing it. I have seen M42 visually on a 24" scope (or 22", I can't remember) and I could catch some greenish hue in it. But RGB images of M42 don't show green, at least definitely not as a major component.

And remember there's two things in play here. One is the star color, which it's more or less clear depending on what you think. The other thing is that I have to conclude there's something in my optical train that produces a rather large and intense glow around bright stars. And not only in the blue because I've seen it also in orange and red stars (see Aldebaran here or in this widefield image of Cassiopeia here where you get 5 "glows" for the price of one, only 4 of them "in the blue").

You just saw the quickly processed RGB image I posted earlier today, where that glow is anything but inconspicuous. Personally I have nothing against it, probably the opposite (I like it), but if you want "answers", it's something to keep in mind.

 

[Juan Conejero]

Hi Sergio,

Visually we see Polaris more yellowish due to the characteristics of the human vision system. Basically our vision overlaps green and red sensitivity over a large interval of the spectrum, roughly from 500 nm to 600 nm:

(Image linked from http://en.wikipedia.org/wiki/Spectral_sensitivity)

and our eye is much more sensitive to green than to blue and red:

(Image linked from http://en.wikipedia.org/wiki/Color_vision)

However this is not the case, in general, with digital image sensors and the sets of filters used in astrophotography:

(Images linked from http://www.astrodon.com/)

I don't know what filters Rogelio used for this image, but if he used Astrodon filters for example, then I think a blue-cyan rendition of Polaris is well justified for 7200 K using the average of many stars as a white reference.

 

[cavamen]

Well, I think we have all pieces of the puzzle now. 

First: our vision system "lies".

Second: PixInsight can make a good color calibration, consequent with the physics and against our vision.

This give us another insight about what is the "real" color in astrophotography.

Sergio


Rogelio: about the glow around your stars, few to say, but I see it since your first photos with FSQ and usually not centered with the star. I have seen this problem in other forum, but don't remember where and if the problem was solved.

 

[RBA]

Rogelio: about the glow around your stars, few to say, but I see it since your first photos with FSQ and usually not centered with the star. I have seen this problem in other forum, but don't remember where and if the problem was solved.

I get "filter halos" (I use Astrodon 1st gen) all the time, and it's not unusual for these to be off-center or even not perfectly round - see this b/w crop on Antares of my Rho Ophiuchus Widefield image:

For me that's not a problem, it is what it is and I rarely ever try to "fix" them anyway.

But the halos I'm talking about here are not those well-defined distinct halos but the larger diffuse halos that you can often see in most of my images when there are "bright enough" stars. My only guess is that this is likely due to light scattering in the optical train, maybe due to a dirty lens, light scattering in the detector chamber, etc. Not something I'm eagerly interested in troubleshooting (though I am curious), mainly trying to provide some pointers.

 

[twade]

Rogelio,

Awesome image!

"my" stars show a glow around them much bigger than "usual"

I would venture to bet its the Reducer causing the increased "glow".  As an experiment, I would take two images: one without the Reducer and one with the Reducer.  Don't forget to compensate for the change in f-stop so you get the same S/N ratio.

Wade

 

[RBA]

I would venture to bet its the Reducer causing the increased "glow".  As an experiment, I would take two images: one without the Reducer and one with the Reducer.  Don't forget to compensate for the change in f-stop so you get the same S/N ratio.

Thanks for the feedback, Wade. Now that's one suggestion I haven't heard! Hmm... Interesting. Yes, I may try it! Like I said this is not a concern to me,  but I'm curious.
If I get to do it one of these weeks, I'll post the results.
Thanks!
Rogelio