Sunday, November 29, 2015

the dolphin, sharpless 188: an exercise in faintness

Sharpless-188 (SH2-188, Simeis 22 or the Dolphin Nebula) is one of the largest planetary nebulae known (9 light year diameter).  Extremely faint, it lies in a relatively rich field in Cassiopeia, explaining the many pesky bright stars in the image.  It's an excellent example of a planetary nebula interacting with the interstellar medium.

The tiny blue star to the right and just below the big orange star near the center of the arc is thought to be the central star/white dwarf.  

It is is moving through the interstellar medium at 125 km/s (apparently really fast for such things). This creates a bright bow shock (upper right) and a faint streaming tail (lower left).

Here's an inverted image better showing the faint tail:
An interaction between the slow wind of the dying star, faster wind of the subsequent white dwarf, and rapid apparent wind due to motion through the interstellar medium is thought to account for the closing of the tail (arc connecting trailing lines of wake on either side, closing the ellipse).
More on this here:
The shaping of planetary nebula Sh2-188 through interaction with the interstellar medium
Wareing, C. J.; O'Brien, T. J.; Zijlstra, Albert A.; Kwitter, K. B.; Irwin, J.; Wright, N.; Greimel, R.; Drew, J. E.
Mon. Not. R. Astron. Soc. 366, 387–396 (2006)

Here it is in Ha only:

Here's the OIII which didn't add much to the image:

The faint tail was very difficult to capture, hence the exercise in faintness.  Binning is a way to improving signal to noise ratio by combining groups of 4 pixels in the camera into one big pixel, giving 4 times the signal for the same amount of read noise.  Here are two different images each consisting of 24 x 20 minutes exposures (8 hours).  The first binned x2, the second binned x4.

Ha binned x2, then binned x2 again in software (no read noise advantage) to 4x:

Ha binned x4:
4x binning clearly does a better job distinguishing the faint tail from the background.

Really long winded rant on noise binning etc.  (not for the faint of heart, really)

A digital camera has a chip which basically detects photons.
when a photon hits a pixel, it generates an electron*.
(* most of the time, my astronomical camera has a relatively high quantum efficiency of almost 80%, meaning 80% of the time a photon generates an electron)
At the end of the exposure, the register for each pixel is read and the measured potential is proportional to the number of photons hitting the pixel.

Things get quantum:
With very faint light, you start to see quantum effects in the images--a discrete variation in what should be a smooth area of a nebula.
The issue is actually worse than minor discrete differences from detection to detection.
When things get quantum, the detection of a signal has random variation which can be approximated by the square root of the signal.
Not a big deal for the bright core of a nebula which might give 40,000 photons per minute.
With a noise level of 200 for a one minute exposure, noise will represent only .5% of the detected value.
But with only 100 photons noise will represent 10% of the detected value.
Clearly with only 1 photon per minute, it's going to be very very difficult to separate the signal from the noise.
No problem, all we need to do is get more signal by increasing the exposure time...
most astronomical camera's are designed for very long exposure, with cooling to minimize the effect of thermal noise during long exposures (I run mine at -20C).

Formula, just for fun:
If N is the number of photons detected,
the inherent noise in the measurement will be sqrt(N)
and the signal to noise ratio will be N/sqrt(N) = sqrt(N).
So improving signal to noise is a bit of an up hill battle.
The cool thing about digital cameras is you can take one long exposure, or a series of short exposures and just add them up.  By adding up exposures night after night, you can get a weeks worth of exposure time if you want.

Light pollution:
Here's why light pollution is such a huge problem.
Increasing exposure time increases signal, but also it increases the signal due to light pollution.
Suppose we have 1 photon per minute from a nebula and increase the exposure time to 20 minutes for 20 photons.
Light polluted skies can easily give you 2500 photons which gives an inherent noise level of 50,
So the noise due to light pollution exceeds the nebula signal itself.

The skinny on narrow band filters:
But wait, all is not lost.  For the case of an emission nebula, the signal typically consists of light at a single wavelength (or a few wavelengths).  By using a narrow band filter which blocks all wavelengths but those that the nebula emits, the sky signal (which spans the entire spectrum) can easily be reduced by a factor of 100 (usually much more because light pollution is not uniform, for example there is a yellow peak due to sodium lamps).

Read noise:
Every time a digital camera takes a picture, there is a small amount of noise associated with reading the registers, typically on the order of a few electrons for a high-end astronomical camera.
Given the fact that there is noise generated by each exposure, a single long exposure, theoretically has less noise than a series of short exposures. The key to optimal exposure length is to make sure that the noise due to sky signal (light pollution in my case) far exceeds the read noise.  If this is the case, the noise associated with a series of short exposures approaches that of a single long exposure.  so sky noise can be your friend ;)

To bin or not to bin:
One issue that arises with narrow band filters is that the sky noise can be so completely suppressed that read noise becomes dominant, requiring extremely long exposures in order to efficiently stack a series of exposures.  A problem I encountered with my new camera using ultra narrow band filters at long focal length was that the background signal due to light pollution was essentially reduced to 0 even with 20 minute exposures.  Enter binning.  Binning allows you to group 4 adjacent pixels together as one super pixel giving you 4 times the signal with the read noise of a single pixel.  So you increase the ratio of signal to read noise at the expense of image resolution.

Proof is in the dolphin:
Theory's all well and good, but the question is: does binning make a difference?
For this extremely faint target I discovered binning x 4 does.

Lucky imaging in the near future:
Around the corner are cameras with zero read noise and near 100% quantum efficiency.  These cameras allow stacking of extremely short exposures, potentially keeping only the lucky few captured during the best seeing.

8" LX200R, SX Trius 694 binned x2 to 0.8"/px, binned x4 to 1.6"/px
(final image at 1.6"/px)
astrodon 5nm Ha, 3nm OIII
Ha 24x20 min bx2, 30x20 min Bx4, OIII 29x20 min bx4

Tuesday, November 10, 2015

NGC 6210 in NII-OIII: Can superturtles fly?

Here's NGC 6210, the turtle nebula in hercules:
This fairly sharp RGB-OIII image from 2007 shows at least two pairs of jets or ansae (wings).  they appear to be curving slightly, perhaps due to rotation.  Wondering if the condensations in the longer pair of jets were red FLIERS, I decided to try a deeper image in nitrogen and oxygen (NII and OIII):

This NII-OIII image suggests that the upper condensation is a red flier while the lower is not, as the lower condensation is absent in the narrow band image, but present in RGB-OIII image and the luminance (below) --probably a superimposed star.

Here's a blink of luminance (broad band including all visible wavelengths), followed by a green continuum filter (no narrow band emissions), then a stretched NII image.  Which suggests that the upper condensation is an NII red flier, while the lower a broad band star.  Not sure why i'm picking up the central glow with the continuum filter.  reflection nebula? IR leak?
NGC 6210 Luminance-Continuum-NII

There is certainly a lower condensation in the OIII, almost looks like a smeared attempt at a red flier, also note the faint outer shell to the right:
NGC 6210 OIII stretched

The NII-OIII core may represent a letter in the krypton alphabet befitting our herculean superhero*:
NGC 6210 NII-OIII linear

*Terry Pratchett fans claim to see 4 elephants (link) on the turtle's back, particularly in NII

Lastly here's a collage showing various filter images:
top: NII, OIII, NII-OIII color; linear stretch
mid: NII, OIII, NII-OIII color; non-linear stretch
bottom: continuum, luminance, NII-continuum
Answering the initial question:
with 4 wings, but only one flier, the superturtle can fly, but slowly.
QED ;)

8" LX200R, SX Trius 694 0.4"/px
astrodon 3nm NII, 3nm OIII
NII 33x20 min, OIII 35x5,6x20 min, L 155x1 min, 545x50 79x2min

Sunday, October 25, 2015

planetary nebula primer III, morphology

Planetary nebula can come in a variety of shapes.  Here are more old images demonstrating morphology.  Although relatively rare, some are nearly perfect spheres, the undisturbed exhalation of a dying star:

Abell 39:
Abell 34 "oozes faintness":
note the tiny background galaxy superimposed at the bottom edge

Most have more complex shapes.  The reason for the unusual shapes is not well understood.  Theories include interactions between the ejected gas layers, magnetic fields, planetary systems, and binary stars. 

Minkowski's Butterfly is thought to have a binary star orbiting
the white dwarf causing the central constriction with jets above
and below:

Here are a few classic bright-round-things-about-the-size-of-a-planet:

Cat Eye
Ghost of Jupiter

Clown Nebula
Blinking Planetary

Often there are faint outer layers evident on longer exposure reflecting multiple episodes of the central star shedding outer layers:
Cat eye
Blinking Planetary

Many are bipolar structures with a ring-like central constriction.  M 76 and M 57  are an example of two sides of the same coin.  

M 76 the little dumbbell left               M 57  the ring nebula right
side view of a ring                                     face on view of a ring
M57 (right) shows a bright ring with a very faint outer shell.  
Rotate that 90 degrees and you've got M76 (left) with the side view of the bright central ring appearing as a rectangle and the faint outer ring revealed as expansions blowing out on either side of the ring. 

Some have pairs of jets, ejecting material, typified by the saturn nebula:
Saturn Nebula

The jets are classically called ansae (wings), either way they are flying things.

Some of the jets have low energy NII regions at the tip aptly known by the acronym FLIERS 
(Fast Low-Ionization Emission Regions) which are more evident when a red NII filter is used:
Saturn Nebula red NII FLIERS

Blinking Planetary red NII FLIERS
IC2149, the Easter Egg Nebula
FLIERS appear to be relatively young, moving outwards at supersonic speeds.  Though poorly understood, the colorful wingtips make for nice images.
IC 4593 White-Eyed-Pea
IC 4593 displays many of the properties above and then some:

-small bright planetary

-faint outer core

-bipolar jets


-BONUS: the outer core is deformed by interaction with interstellar medium generating a bow wave upper right

More planetary nebula images here

A more scientific account of the modern concepts in planetary
nebulae morphology:

Shape, structure, and morphology in planetary nebulae

Richard A. Shaw

Tuesday, October 20, 2015

astropics: planetary nebula primer II white dwarfs, helium, and diamonds in the sky

A planetary nebula results from a white dwarf illuminating the gas that was ejected by a dying star...

Basically, a star is big ball of gas, mostly hydrogen.
The star is so massive that gravity forces the material to collapse in on itself.
The compression/temperature at the core becomes so great that hydrogen ions fuse forming helium and releasing a tremendous burst of light/energy which, in turn, pushes the gas back out preventing further collapse.  When a medium sized star starts to run out of hydrogen to fuse, it begins to collapse again until helium fusion begins, causing the star to expand and, expel portions of its outer layers.  After several iterations of this, helium fusion ceases and there's nothing left but the very hot inner core surrounded by expelled gas. 
Since fusion has stopped, the central star can be though of as dead/inert, doing nothing but cooling down until the end of time.  While one might think of them as "dead", the massive compression makes white dwarfs among the hottest "stars" at 50,000-100,000 K; the cooling takes billions of years.  This white dwarf is very small and therefore faint, but gives off much higher energy photons than most stars.  For example, ionized helium (very high energy transition) is rarely seen except in planetary nebulae around white dwarfs. 

Here are older images of two of the most famous planetary nebulae, shot with narrow band filters showing high energy blue helium in the center, teal/green oxygen further out, with low energy red nitrogen in the outer ring, or mid-substance pillars (also note the small bluish central star/white dwarf):
M 57, the ring nebula:

M 27, the dumbbell nebula:

Fun Factoids:
In the late expansion phase, the star grows to 100 times it's original size and cools, becoming a red giant.  Since the red giant is emitting light from a much larger surface, it's much brighter than the original star (example Betelgeuse, Orion's shoulder).  Conversely, a tiny white dwarf is very faint, but hot, emitting much higher energy photons which can ionize helium while the brighter red giant cannot--the photoelectric effect on a cosmic scale. 

Fusion of helium etc can continue on up to oxygen and carbon.  When the planetary nebula sheds its outer layers, it sends carbon oxygen etc into outer space, allowing future solar systems to form with higher oxygen and carbon content--cosmic recycling.  

Rather than collapsing into a white dwarf, a larger star explodes forming a supernova. 

It is thought that some of the coolest white dwarfs crystallize, and since they are primarily made of carbon/oxygen...yup, you got it, planet-sized diamonds.  Which of course is great material for comic-sci-fi, pink floyd songs, etc.

Image details:
M 57 8" LX200R, SX AO, IDAS LPR, SX H9, H9c
astrodon 5nm Ha, 5nm OIII, 3nm NII, CS 10nm HeII, OO 5nm HeII filter, IDAS LPR
RGB 29x20 min
Ha 34x20 min, NII 23x20 min, OIII 22x20 min, HeII 31x20 min

M27 NII 34x20 min, OIII 12x20 min, HeII 30x20 min
8" LX200R, SX AO, SX H9
astrodon 3nm OIII, 3nm NII, CS 10nm HeII,
Los Alamitos, CA Bortle white skies

Sunday, October 11, 2015

little gem nebula NGC 6818, planetary primer I

Planetary nebulae are clouds of glowing gas surrounding a “dying” star.  Visually, many are relatively bright grey patches (by deep sky object standard, though very faint compared to an actual planet).  Spectrographically, they typically have very strong doubly ionized oxygen (OIII) emissions compared to other nebulae.  An OIII filter is a handy tool for the amateur astronomer, enhancing these nebulae and confirming their identity.  Though not detectable visually, they also tend to be relatively strong in singly ionized Nitrogen (NII) and singly ionized Helium (denoted HeII, which I may avoid going forward due to the unfortunate confusion). 

NGC 6818 aka the little gem is a small bright planetary nebula in sagittarius.  Roughly planet sized and near the ecliptic (path of the planets through the sky) one can see why it gets the moniker “planetary”, but at roughly 6,000 light years away it’s 1/2 light year across.  The blue/teal in this image is due to OIII emissions and red due to NII emissions. 

Here’s the OIII



8" LX200R, SX Trius 694 0.4"/px
astrodon 3nm NII, 3nm OIII
NII 19x20 min OIII 24x5 min


Wednesday, October 7, 2015

planetary nebula Abell 72

Abell 72 is a faint planetary nebula
It's weak signal is almost all (teal/blue) OIII (doubly ionized oxygen)
the blue central star shows nicely in this image:
the other interesting structure in the field is the small faint galaxy pair just below the nebula.
at 979 million light years distant, the pair is quite far in the background.  it took 7 hours of exposure with a narrow band oxygen III filter to bring out the faint planetary nebula. an additional 9 hours of broad band exposure were required to bring out the faint galaxies.  interestingly, the nebula was barely detectable in the broad band image, showing the power of narrow band filters in light polluted skies.  

8" LX200R, SX-AO, astrodon 5nm OIII filter, chroma LPR filter, SX H9c/H9, ASA DDM 60

luminance 56x5 min, RGB 14x20 min, OIII 22x20 min

Monday, September 28, 2015

super blood moon

while it was a super moon (moon at closest point to earth in it's orbit during full moon) 

and a blood moon (red due to atmospheric refraction of red light during an eclipse), it was not the best eclipse experience from southern california.
the eclipse occurred just after moon rise, so the dramatic darkening which occurs during an eclipse was lost as it was still twilight when it rose.  add to this light clouds and haze on the horizon and you've a less than super eclipse.
fortunately, i had a spectacular lunar eclipse experience last april, blogged here.  

what today's eclipse did offer was an opportunity for widefield/landscape shots capturing the blood red moon as it rose...
so i took my gear on the road to a south facing beach hoping to catch it rising over the rocks in corona del mar, but it was rising too far north (i'd seen the moon rise over the rocks in mid summer)...
so i drove to the mountains (ok a hill in newport coast, but it sounds better)...

here's my attempt at a landscape wide field of the moon.  A car pulled up just in time to illuminate the foreground trees and fence in this shot (must have been Wally Pacholka's):

wide field:
nikon D 60, AF-S DX NIKKOR 55-300mm f/4.5-5.6G ED 
close up:
ZWO ASI120MC, Takahashi FS-60C F/5.9


Sunday, August 30, 2015

still more summer sun: filaments, proms and spots

used a variety of cameras telescopes and filters to capture a range of solar activity last Sunday 8/23/15.

sunspot grouping AR 2403 in hydrogen alpha compared to broad spectrum (white light):

Ha single stacked, large chip camera full disk:

the prominences projecting off the edge out into space are much more faint, so i created a composite of this and a longer exposure which burns out the center (color version):

double stacked Ha for better contrast, capturing  the filaments and prominences smaller chip camera which has a faster capture rate:

closer view of filaments and prominences upper right:


prominence only with over-saturated pixels blacked out:

Hydrogen alpha activity around sunspot grouping AR 2403:

close up version in white light showing the sun spots

Imaging details/discussion:
full disk Lunt 60 PT single stacked, on grab & go alt-azm mount, DMK 51

i switched to 60/50 double stacked as i thought higher contrast would make it easier to capture the prominence/filament juxtaposition upper right.  The single stacked Ha band width is .7 anstroms, double stacked narrows it to less than .5 angstroms.  this makes the surface filaments stand out more.  since the double stack leaves a bit of a gradient with full disk anyway, i switched to my new planetary camera ASI 120 MM-S which isn't big enough for full disk, but it has a much higher frame rate--10x faster than the DMK when cropped.

the white light capture was done with an 8" SCT, baader photographic film an IR/UV cut filter and a 550x10nm filter with the faster ASI 120 MM-S camera.  i tried 4 different filters: 540x10nm, 550x10nm, 8.5 nm OIII, 10 nm Helium.   On quick switching with a filter wheel the 550x10 seemed slightly better than the 540x10.  i then took a series of 6 20 second captures separated by a minute with the 550x10 filter hoping to capture some sort of motion.  what i found was huge variation in seeing with 4 of the 6 captures showing mush and two showing some detail, making it difficult to really conclude whether one filter was better than the other.

Monday, August 10, 2015

Sunny Sun Day

Sun showed a lot of activity on Sunday so I took a quick full disk shot with the lunt 60 on my grab and go alt-azm mount.

Hydrogen alpha


the exclusion seems to bring out the very long serpentine filament mid/upper left


Lastly here's a close up of the prominence to the right:

Lunt 60 PT single stacked, DMK 51

tried the sun spot grouping in WL, but failed miserably on saturday