Try increasing gamma if dark sections aren't distinguished

Try increasing gamma if dark sections aren't distinguished

Thursday, November 27, 2014

North America and Pelican Nebulae

North America and Pelican Nebulae

The North America Nebula is a huge emission nebula in Cygnus close to the bright star Deneb.
It took me years to see it through a telescope, even from dark skies.  At 4 times the size of the full moon, i was staring right at the middle of it with my long focal length telescope.  Finally i used a filter in the small low power finder scope on top and caught it.
This image is a two frame mosaic with my shortest focal length telescope, showing the North American nebula to the left, and the pelican nebula to the right of a central patch of dust.
This image uses the "hubble pallet" with high energy oxygen as blue, low energy sulfur as red, and hydrogen as green.

not entirely happy with the framing as i've clipped the west coast in order to include the pelican :(
might try again next year

Here it is with an alternative pallet using only high energy oxygen as blue and low energy sulfur as red (hydrogen was used in part as luminance):

Here's an older close up of the "cygnus wall" aka "gulf of mexico"

and the Pelican

Takahashi FS60c @255mm CS 8.6nm Ha, AD 3nm OIII, AD 3nm SII, SX H9
2 frame mosaic total time Ha 49x5 min, OIII 74x10 min, SII 92x10 min
Newport Beach, CA 10/2014

Sunday, November 16, 2014

Photographic Light Pollution Filter Comparison: Hutech IDAS LPS-P2 vs Chroma Loglow

Having recently moved I've noticed some improvement in my skies from bortle white to bortle "off white".  At zenith on a good night, my sky quality meter reads 18.12 mag/arcsec^2.  There is a red glow to the north up to an altitude of 40 degrees and a grey glow above that to 70 degrees.  The sky is darker above and to the south.   The milky way is impossible naked eye.
I do all my imaging from these skies with an LPS filter, narrow band filter, a combination of both.  Emission nebulae are great, galaxies are difficult, but can be very rewarding (especially if there are HII regions),  reflection nebulae are very difficult and dust is impossible.
As a testament to the fact that I purchase too many filters, I was given a Chroma Loglow filter to test.  So I've compared it the the Hutech IDAS LPS-P2 filter which many (including me) consider the gold standard light pollution filter.  A quick visual inspection showed more light passing through the Chroma than the IDAS.  I initially thought the Chroma had more of a reddish cast, but realized this was on a fluorescent bulb.  Searching around for an incandescent bulb yielded similar, more neutral color balance for each with perhaps a slight blue cast to the IDAS.

Iris Nebula, a reflection nebula surrounded by dust shot through heavy light pollution:
Reflection nebulae are usually blue because the scattering is more efficient for blue light than red (this is the same scattering process that gives us blue skies and red sunsets) (Wikipedia).
The black void above, below, right, and left of the nebula is caused by intervening dust.  darker skies allow the glowing dust to be imaged.

Dumbbell Nebula, an emission (planetary) nebula, at maximum elevation with minimal gradients due to light pollution:

Here are the details of the filter test:
It should be noted that the filters were used on different nights, though the conditions were similar.  My first target was M24, low in the south, but in the direction with least light pollution, using an FS 60C at F 4.2 and a Starlight Xpress H9C camera.  The star and background signal was much higher with the Loglow, though I did not note any difference in depth or gradients in this dense star field (comparison images not shown).  The seeing was slightly better when the IDAS was used.  The combined image can be seen here in a prior post.

Dumbbell Nebula
Next up was M27, an excellent target, as it has strong OIII and Ha emissions transiting at zenith in an area where gradients and light pollution are minimized.  Using an FS 102 at F 6, I compared a Baader IR/UV filter to the IDAS and the Loglow.  There were no appreciable light pollution gradients.  The nebular emission signal was enhanced relative to the stars with both the IDAS and the Loglow.  I could not appreciate any significant difference between the IDAS and the Loglow.  I've attempted to minimize processing to color balance and matching digital development:




stack of 9x120 sec exposures, color balanced by eye followed by digital development in maxim.

Iris Nebula

Last was the Iris nebula, a reflection nebula surrounded by dust in the dreaded northern muck, again with the FS 102 at F6.  As there appeared to be a significant color difference in the reflection component, the images were color balanced with x-calibrator.  The reflection component appeared significantly more blue with the IDAS and Loglow compared to the IR/UV filter.  I am not certain whether this is the result of grey light pollution contaminating the IR/UV images or enhanced color by the IDAS and Loglow due to rejection of signal in the middle of the color spectrum.  In any event, I found the effect aesthetically pleasing.




stack of 19x5 minutes color balanced with x-calibrator, ddp in maxim.

An aggressive histogram stretch in maxim showed the worst gradient with the IR/UV and best with Loglow, though the IDAS was close.




I did shoot some luminance with the two filters, but the background signal was so high with 2 minute subs, that it reached the non-linear region of my camera, making flats ineffective.  Between hot spots and dust specks it was difficult to make any comparisons.

I compared the stellar, nebular, and background signal for a representative section of the M27 stacks and found the following:
For all of the quantities, the Lowglow had higher signal than the IDAS; the IR/UV the highest signal of all. 
However, for the ratio of stellar signal to background and nebular signal to background, the IDAS had the highest ratio, followed by the Loglow, and then the IR/UV filter.

M27 background1,2802,2302,890
nebula (color balanced)177927333388
nebula (unbalanced)167227453400

iris background3,5005,3767,257
iris star19,000 21,000 28,000
excalibrator green0.930.991.03
excalibrator blue0.760.811.16

Takahashi Fs 102 @619 mm Hutech IDAS LPS-P2/Choroma LoGlow, Starlight Xpress H9C/H9
M 27 34x2 minutes RGB, IRIS ~100 x 2 min luminance, 84x5 min RGB
9/15-9/24/14 Neport Beach, CA

Bottom Line

The two filters were comparable in terms of reducing light pollution gradients and enhancing nebular emissions.

The LoGlow seemed to be slightly better than the IDAS-P2 in terms of light pollution gradient reduction.

The LoGlow also passed more signal while the IDAS-P2 had a higher signal to background ratio. 

clear skies

Wednesday, November 12, 2014

10/23/14 partial eclipse animation

finally finished the animation of the partial eclipse in hydrogen alpha.
imaged over about two hours.
a "usb fault" caused me to miss the very beginning while trees interfered with the end.
here's the full disk animation (large file) over 2 hours:

though the sun spots were dramatic in white light, they were less prominent in hydrogen alpha at this scale.
other than the moon passing by, there was not a whole lot happening during the time period of the animation at large scale.

Here's the sunspot, AR 2192 in hydrogen alpha:

Here are 150% enlargements of two small sections that showed some activity:

Sunday, November 9, 2014

solar active region AR 12192: the monster revisited

here's a wider view of the monster sunspot grouping i caught just before the recent partial eclipse.
according to one source, this grouping, known as AR 12192 is one of the largest sunspot groupings in recorded history

fortunately, it arrived at a time when i was working on my sunspot technique and had made great strides, imaging structures i'd never been able to catch before.  specifically, the honeycomb-like granular structure on the bright surface and the and the "penumbral filaments" at the edge of the dark spot (best seen around the large spot to the right). 

"Granules on the photosphere of the Sun are caused by convection currents (thermal columns, BĂ©nard cells) of plasma within the Sun's convective zone. The rising part of the granules is located in the center where the plasma is hotter. The outer edge of the granules is darker due to the cooler descending plasma." (Wikipedia)
These convection currents contribute to the sun's magnetic field.

the penumbral filaments at the edge of the dark spot aren't that well understood (at least by me).  they seem to involve an interaction between magnetic fields of different orientations and convective flow.
magneto-convective cell reference

the sun rotates once every 27 days or so
will be interesting to see if this grouping comes back around.  

the key to the improved image was taking a huge number of frames over a short period and discarding all but the very best.  to do this i had to use a different camera with a smaller field of view and faster frame rate.  unfortunately, the field of view was smaller than this large grouping.

compare the detail (especially the fine filaments) on this small high frame-rate field to the right side above:

i tried a mosaic in an attempt to get a wider field and high frame rate, but it came out to too patchy

for reference here's a prior image at the same scale:

IMAGE improvement DETAILS
the following measures gave improved images:
1. switched cameras from DMK 51 to zwo ASI120MC which allows a much higher frame rate.
-DMK is great for wide full disk animations, but limited to 12 fps-i'd misread the framerate as 60 fps-that's the output avi :(
using a partial frame with the zwo got me to an average of 54 fps which overcame the handicap of the color camera
2. much smaller stack: took only the best 20 or so images at the upward curve to the left of registax stackgraph quality indicator.
3. kept the imaging time very short--20 seconds
4. put the solar filter (baader film) on the scope inside and immediately pointed it at the sun when i brought it outside.
-the charcoal black carbon fiber tube heats up very quickly with direct sunlight. rather than insulate, i figured the film would reflect the energy immediately, keeping the tube out of direct light. 

i think the biggest factor was limiting the stack size
which means i can reprocess some of my older images :)

did not recollimate
tried an off-axis mask prior to these changes with no improvement
no change in focus technique