Sunday, March 27, 2016

clown nebula, scientific jelly bean pallet

Narrow band imaging is a technique astronomers use to demonstrate the spatial distribution of specific ion emission lines in an astronomical object (usually nebula).  Rather than using red, green and blue filters (which allow relatively broad wavelengths) to construct an image, astronomers use narrow wavelength filters that isolate emissions due to specific ions.  In either case, the digital camera gives a grey scale image (the camera simply counts photons), which is assigned to a specific color in post processing.  For traditional RGB images the choice of color assignments is clear.  But for narrow band images, the imager may record emissions from very different ions which are the same color (as far as the eye can tell).  For example hydrogen Ha, nitrogen NII, and sulfur SII are all red.  If one were to create an image from these filters assigning each to red, the image would convey no information about the distribution of the individual emissions—the data would be lost.  The alternative is to assign several of the emission lines to completely different colors in order to maximize color contrast, making the image more informative re: ion distributions.    The classic “Hubble pallet” assigns SII to red, Ha (also red) to green, and OIII (teal) to blue.  An alternative pallet, often used used by the Hubble team for planetary nebulae, assigns NII to red, OIII (teal) to green, and Helium (royal blue) to blue.  Interestingly the main mission of the Hubble was to image planetary nebulae, but the classic “Hubble pallet” is not used for planetary nebulae.  Here’s a prior post using the classic Hubble pallet.  This technique was initially dubbed “false color” imaging by scientists.  However there was such a visceral public backlash against “false colors” that astronomers (mindful of funding) now use the term “scientifically assigned colors”, a wonderful euphemism.

On coming up with a more meaningful 3 color pallet:
In a previous post on the clown nebula, I discussed the relative contributions of Ha and NII emissions to the image, but used a relatively traditional pallet for the image, assigning NII to red and OIII to teal (the “true colors”).  I subsequently came up with a more meaningful 3 color pallet in order to illustrate the differential Ha and NII emissions (both red): I assigned NII to red, Ha to green, and OIII to blue, then balanced the NII and OIII on the "smile", making it orange.  The net effect was a variation in the color of the NII globules depending on the ratio of NII to Ha with red having more NII, and green more Ha relative to the "smile".  Also note the blue (OIII) section just above the central star, and red NII outer rim of the "nose" upper left.

In more quantitative terms, for the linear images I measured about 200 adu NII in the smile and 700 adu with the 5 nm Ha (Ha+NII), which implies 500 adu Ha, so more Ha signal than NII.  Over all, I find the NII provides more contrast and, arguably, a more interesting image.
Imaging details in previous post.

It also reminds me of jelly beans :lol:

Thursday, March 24, 2016

Double transit and GRS on jupiter

On 3/21-22 Jupiter put on a juggling display
Io, Europa and their shadows crossed Jupiter's face along with the great red spot over the course of a few hours

unfortunately, the seeing was mediocre causing distortions
mid shadow transit, only the double shadows are evident:
ironically, the transiting moons are obscured by the bright surface of Jupiter while their shadows are obvious.

as the moons rotate off to the right edge of the planet,
they become evident casting shadows just before their transits end:

here's the animation of the second half of the show, over ~1.5 hours:

PS i've got confirmation from the amateur community that the GRS has been getting a bit more red since 2012. it's also slowly shrinking according to NASA.

celestron nexstar 8 GPS (8" SCT on a wedge)
27x 3 minute captures with firecapture @ 200 fps (exposure limited at 45% histogram with gain 64)
stacked in autostakkert, Drizzle 3x, then reduced to 1.5x
sharpened in registax 6
Southern California
mediocre seeing
Mid(UT): 3/22/16

Tuesday, March 22, 2016

jupiter's redder red spot, detail on ganymede

jupiter's up and in prime form right now
here's my first of the season:

might be my imagination, 
but the red spot (which had been fading to tan) 
looks more red to me this year

here's an interesting one i think i neglected to post from 2 years ago look closely at Ganymede transiting the face of Jupiter:

The black dot lower left is the shadow of Io which is further left and below Jupiter.
Just below the shadow, Ganymede is crossing the face of Jupiter.  A close look at Ganymede shows the bottom is white while the top is slightly brown, demonstrating detail on one of Jupiter's moons.  Although not much, I never thought i'd get that from a backyard telescope.

More to come, including an animation of last night's double transit.

nexstar 8 GPS (8" SCT on a wedge)
3 minute capture with firecapture @ 50 fps (exposure limited at 95% histogram with gain 64)
stacked in autostakkert, Drizzle 3x, then reduced to 1.5x
sharpened in registax 6
Southern California
3/17/16 fair seeing

Ganymede transit image from 3/30/2014

Sunday, March 6, 2016

clowning around with narrow band filters :)

I'd planned to image a very faint planetary binned x 4, but felt compelled to shoot something binned x 1 when I realized the seeing was good.
NGC 2392 was the only bright planetary i could think of off of the top of my head
so decided to give it a shot unbinned with the NII filter
so decided to give it a shot unbinned with the NII filter which gave great detail in the reds

the source of the NII globules is still a bit of a mystery

Kitchen sink pallet--NII red, Ha green, OIII blue, He magenta:

just for fun i gathered a few exposures with a variety of filters (thanks to bilgebay for the 3 nm Ha data):
minimal processing, just digital development, except the last.
note the lack of detail in the OIII, with even less in the HeII
while NII + OIII is remarkably similar to Ha
(bill w's conjecture  Ha~NII+OIII for PN, ~Ha+OIII for emission nebulae).

Filter digression:
In general, the narrower the bandwidth of the filter, the better the signal to noise ratio (assuming your exposure is long enough to bright out background signal).  I touched on this in prior post covering the rationale behind narrow band filters in light polluted skies.
However, narrower isn't always better.  traditional "Ha" filters typically capture Ha emissions at 656 nm and NII at 658 nm (older papers refer to them as Ha + NII).  The new astrodon 3 nm Ha filters, in theory cut out a portion of the NII signal, which is not desirable when imaging planetary nebulae.

bilgebay recently captured an image with very similar equipment, but used a 3 nm Ha filter (656 nm).
he was kind enough to send me the Ha data for comparison as i was curious to see how much loss of NII signal (658 nm) affected the image.  not quite a perfect comparison as he used >4x the exposure time for his subs, while i probably had better seeing.  you can see the comparison in the mosaic above.  the difference isn't as great as i'd expected, but there is definitely better contrast between the "eskimo's fur" and the circular nebulosity in the Ha.

8" LX200R, SX Trius 694 binned 0.4"/px
astrodon 5nm Ha, 3nm OIII, 3 nm NII, chroma 4 nm He
Ha 5x5 min, OIII 8x5 min, HeII 14x 5 min, NII 30 x 5 min, L 14 x 1 min
eastbluff, CA

no calibration except luminance ;)

Ha 3 nm 7 x 20 min
Celestron C8 Edge HD, Atik 460 EX Mono, astrodon 3 nm Ha
Bilgebay Observatory, Mugla, Marmaris, Turkey
interestingly, sedat used also used an 8" SCT with the Sony ICX694 chip.