Nature Time Lapse of the World

Since the 2013 road trip I managed to compile and fill in most of gaps in this project – the project I’ve been working on intermittently for 3 years. It’s a time lapse I had started in 2010 when I was caught and shocked by the realism presented by time lapse photography. Those pioneers on Youtube and Vimeo made nature time lapse videos with moving clouds and starry night.

I made a rough calculation and I immediately realized how fast the shutter life will be consumed should I use a DSLR. A 1000 exposures for one time lapse at a interval of 6 seconds. So I started with bridge camera, but this time the image quality and lagging speed greatly reduced the success rate. Finally to only have one camera to work with during travel I started serious time lapse photography. The first series was shot in UK during my internship and I was first amazed by how transient but colorful the nature could be.

Gradually problems began to appear, the flickering issue posed a challenge to correct. Then comes the annoying vibration. Sometimes caused by winds, others by necessary manual control interruption. When more videos were taken between sunrise and set, the exposure control became a serious hurdle to smooth the transit. And worst of all, my physical perseverance to withstand the winds and bitter coldness for one hour.

Rio Grande Sunset

In 2011 the music was chosen and I start to fill in each melody with the appropriate clip. The flicker was vastly reduced with VirtualDub filter. When better clips were available, the entire edition would be moved around and discarded. In 2012 I got my new D7000, the new feature avtive D-lighting was both the curse and bless of photography. On one side it could greatly expand dynamic range for single shot JPEG, on the bad side it introduces extreme flicker between images due to its algorithm treats images individually. People moving around was suppressed with statistics filter with my coding effort. Every times when new semester started it became another down time for me to doubt if I could eventually finish this project.

At the end of 2013 I traveled to the Big Bend National Park in Texas for the final shot. Against all odds, I managed to reduce the light from cars moving by and finished this project. Please enjoy the video. For best effect, use full screen, dim lights and sounds up! If you like it, help me share it so others would view as well.

Technical Detail

Time lapses were mostly taken at 6 seconds interval. To reduce data burden, most were taken at medium resolution. The preprocessing were done at 4K resolution if applicable.

Panning done with AstroTrac pointing zenith. Camera includes D200 and D7000 with Samyang 14mm lens. One shot with Canon 6D.

Panning

Astrophotography in pure darkness

In Michigan, I could only see one nebula – “Michigan Nebula”. Nah, that’s just a joke in the amateur astronomy society here to complain about the frequency of cloudy nights in the state. For me, the complaint is real. I do not have an observatory for regular imaging. Packing such a heavy weight EQ mount and going to some dark rural site only to find cloud building up is almost frustrating and unacceptable. Now it seems a road trip every half year could offer me better opportunity with the best dark sites in the States.

So here are some examples. During the Christmas of 2013, I went to the Big Bend National Park in Texas. There’s absolutely no light pollution from almost any direction except some desert town outside the park. Terrain should perfectly shade these local glares.

At the dusk we entered the park, but from where we were staying took about 1 hour drive. The surrounding lost its colorful appearance when the last patch of sky became completely black. The headlight of our vehicle and the passing by prevent us from dark adaptation. But when we step out of the car, the brilliant zodiac light immediately catches my attention. It was so bright, even under the streetlight in a parking lot, I could see it reaching 30 degrees high in the sky. The clouds kept me blinded for 1 day and half. It was until the third night that I could view it in its full majesty. Until midnight that day, the zodiac light was still bright on the horizon.

Zodiac Light

Zodiac Light

This time, all the clouds move away to the west and it offered a clear night for astrophotography. I picked a spot near the park entrance to setup my tracking rig, and another camera for time lapse. The Orion’s belt was my imaging priority. In a 2 hour and 40 minutes total exposure, I was able to reveal all the dark nebula and dust bands adjacent to the bright M42 and horsehead.

Orion

Meanwhile, the sunset at Rio Grande Village was considered by us to be the most scenic combination after 3 days of lonely drive in desert.

Rio_grande

360 panorama – Sunset of Rio Grande

Now 6 months have passed, another opportunity took me to the Mojave Desert in California. This time, I’ve substitute the glass inside the optical glass inside with one having antireflection coating. Thus all the glare surrounding bright stars and nebula center are gone. About 10 minutes’ drive away from the small desert town Baker, I set up my AstroTrac on the sandy road of Mojave National Preserve. It was dry hot at such a low altitude. Besides the intermittent wind blowing against you, is the occasional sound from some unknown animal sheltering in the wasteland. The glare from Baker and head light of passing cars on I-15 are on my north, the Rho Ophiuchi Nebula is a perfect target. Yet under this dry heat, it was exhaustive trying to sleep inside a car. I manage to get 100 minutes of exposure in total.

Rho Oph

The Rho Ophiuchi Cloud Complex

This time I’m using the hacked firmware preserving the raw output from the sensor. Now with custom made calibration pipeline developed, I could achieve perfect preprocessing before the actual alignment and stacking.

IMX071 Characteristics

The IMX071 is a CMOS sensor from Nikon D7000 and D5100. Here’s a sensor characteristic test using data obtained by service mode hack.

This hack is done by Simeon on Nikon Hacker Group. By writing specific register value in USB PTP connection, the camera enters service mode where RAW sensor image can be obtained. The infamous Nikon median filter (star eater) during long exposure would also turn off. This mode is for hot/bad pixel remapping at Nikon Service Facility.

Once turned on, the sensor will be overscanned and output optical black and dummy pixels. Area definition is as follows:

Total Area: 0, 0 – 5054, 3357

Dummy: 0, 0 – 5039, 3357

Optical Black: 0, 36 – 4959, 3357

Light Sensitive Pixel: 4, 70 – 4955, 3357

The column 5040..5054 are the horizontal blanking region and it outputs a constant value. The dummy pixels don’t seem to have a photodiode, hence not dark current but only read noise is contained. However, dummy pixels should not be used for read noise estimation. The optical black pixels are actual pixels with light shielding, and they can be used for read noise and dark current estimation.

First, a pair of images is taken with same exposure setting against a flat lighting source. A LCD panel display with 4 layer of parchment paper is used as uniform light source. Then the RAW image is split into 4 separate CFA channel and a center 256 x 256 uniform region is cropped.

Image pair

The average pixel value and standard deviation are recorded.

Subtract

Then the second image is subtracted from the first one, taking away the variance caused by PhotoResponse Non-Uniformity (PRNU). The rest of the variance will be the double of summation from photon shot noise, dark current shot noise and read noise. Since the shutter speed is relative fast – 1/6s, thus we ignore the dark current.

2x Std EV

The read noise can be calculated from the optical black region with the same subtraction. Then the variance of read noise is subtracted from the rest. The following table summarize the characteristic at ISO 100.

ADU ISO 100 e-
AVERAGE STD DEV 2 RN 2 STD DEV Gain(e/ADU) STD PRNU READ NOISE FWC PRNU (%)
1 R 5742.589 71.602 1.05 54.746 2.241 20.812 1.7 36709 0.362
2 Gr 6767.172 71.968 1.016 2.614 1.9 41392
3 Gb 6790.208 72.343 1.014 56.048 2.595 22.893 1.9 41103 0.337
4 B 4522.246 64.144 1.027 48.005 2.199 15.708 1.6 36023 0.347

A digital multiplication is applied to Red and Blue channel, thus the sensor full well is roughly 42ke- at ISO 100. Read noise is really low, 2~3e-. Note that “DEV 2” are the summation of 2 deviation resulting from image subtraction.

Dark Current

Later, the dark current is also estimated from optical black region, yielding 15 ADU in total or ~0.14eps (electron per pixel per second) at 21°C. When cooled to outside temperature at -1°C, dark current falls to roughly 0.007eps. The doubling temperature is less than 6.5°C.

The preliminary testing suggest the Exmor CMOS is much better than previous generation interline SuperHAD CCD. More accurate testing can be done with complete Photon Transfer Curve once the black level correction is hacked in the future.

Black Level Hack – Update: 6/9/2014

Now we have found the register controlling the black level and the patch is available for both D5100 and D7000. As expected, the register is part of the Expeed Image Processor, not on sensor. With more tweaking on the Expeed preprocessing switch, we discovered the optical black region is also filtered to get rid of hot pixel and pattern noise. Digital gain above ISO1000 and color channel scaling could also be turned off.

Much work has been done to understand the sensor SPI protocol as well. So far, a register controlling the average black level after digitalization is discovered. There’s another set of 4 registers controlling the analog gain for each color channel. The camera faithfully issues the same gain setting for each ISO no matter the exposure lenght, thus linearity should be ideal.

Gain

The following table illustrates the more accurate sensor characteristics under ISO100 and ISO320. Note that with the patch, sensor is using the default analog gain setting instead of calibrated ones. Thus ISO100 is more like ISO 80 compared to factory setting.

D7000-Gain

The sensor pretty much behaves “ISO-less” with identical read noise. A little elevated read noise at base ISO is likely due to quantilization error from ADC at such a low gain. PRNU may not be accurate enough since I did not use a diffuser. Flatness is achieved by manually picking a 512×512 section from image with the most uniformity.

Read Noise

To further characterize read noise at ISO320, I used an in-house tool to take 64 bias frames without shutter actuation, much like the in camera Long Exposure Noise Reduction (LENR). The images are then converted to TIFF and imported into R Studio for analysis. For one channel CCD, all pixel went through a single output amplifier, thus read noise is identical. However this is not true for CMOS where each pixel has its own amplifier, and each column has its own CDS and ADC circuit. To address this, I calculated the standard deviation for each pixel and plot the distribution. Ideally, each pixel will have a different gain in a best mathmatical model. But measuring it with the current setting is difficult. Under the assumption that gain is the same for each channel, we have this:

RN Distribution

2.5x is the median value for the read noise distribution, meaning that half of the pixels have read noise less than 2.55e- RMS in that channel. Albeit a bit higher than scientific CMOS, which has median read noise of around 1e-, this is a lot better than CCDs.

When we save the read noise as a FITs image, more interesting results are revealed!

071_RN

Notice that the pixels having high read out noise always occur in vertical pairs, why? Well I guess the answer is due to the vertical 2 pixels read out sharing structure. Namely, 2.5T pixel. It’s possible the elevated noise in the common floating diffusion or the transistors cause elevated read noise appear in both photodiode.

Dark Current

To measure the dark current, I first checked the linearity of the dark count with exposure time. A bracketing set of exposures was taken after 40 minutes of thermo stabilization. And the average count in optical black and white point in effective pixels is checked. The test was done before the hack was available, but from the subtracted black level the linearity is nicely followed.

Dark_linearity

Dark_linearity_OB 

Now the actual dark current can be computed. A series of 5 minutes exposures were taken continuously. And the dark count can be illustrated using the following diagram. The dark current gradually increase from 0.13eps to 0.34eps at the end. It never reaches equilibrium even after 100 minutes as the battery continues to heat up.

Dark_current

As dark current accumulates with increased temperature, the distribution shifts right and becomes even dispersed due to dark current shot noise and hot pixels. dark_current

For more sensor related information, view this post.

Teaser: Nikon DSLR Black Point Hack for Astrophotography

Heads up Astrophotographers, Canon is no longer the best camera in terms of image quality in astrophotography. Today, we, the Nikon Hackers, are first able to extract the real, authentic RAW image from the Nikon D7000. The last hurdle towards serious astro-imaging. Especially for people doing narrow band where background is very dark. It will also promise greater bias and dark calibration.

This is an exciting moment, for me as an amateur astronomer at least. Here’s a quick peak of the dark frame image.

Preview

Here’s the image straight out of the camera. The DSP engine is still treating 0 as black point thus it’s pink on the screen. The histogram also looks weird due to its X-axis is gamma corrected for JPEG preview.

statistics

Average will now be brought back to around 600ADU, the setting for on sensor black level.

As for image quality, Sony Exmor CMOS has far less readout noise and FPN compared to Canon. Dark current is also in the range of 0.15eps stablized under room temperature. Under a typical winter condition, dark current is so low and comparable to cooled CCDs.

Currently we are still working on this hack and more test will come out when it’s ready. Stayed tune with Nikonhacker.

Grand Road Trip of 2013 – Part 3

Pick up from the previous post, where we left the grand canyon for the wild Utah. Yeah personally I thought Utah itself is a huge national park. From Bryce Canyon to Capital Reef and Canyonlands, it’s almost contiguous wilderness all the way to Colorado.

Yovimpa Point

Panorama From Yovimpa Point

The midday view could not be compared to a sunrise, which shines the canyon from the east. The view all the way down to Yovimpa Point were similar but gradually offers a higher vantage point. It was decided not to drive to far away from where we stayed the night for time lapse photography.

Bryce Canyon Natural Bridge

The Natural Bridge in Bryce Canyon

That night the unusual high level of airglow cast a greyish haze on the starry night. Human could only preceive color in broad daylight when cone cells are functional. With long exposure, the full color is recorded on camera where airglow shines brightly in green.

Airglow and Milkyway

The moonlit canyon was awesome in time lapse I took that night with airglow on lower horizon mimicking aurora. The location however, offers a poor view for the Rho Ophiuchi regions.

The next day we drove across the Capitol Reef National Park and headed directly for the Canyonlands and Arches in Moab.

Green River Overlook

The Green River Overlook

Time was still limited for 2 days in Moab. We only had the chance to view the delicate arch. The delicate arch trail were one of the most difficult in the park. Half way through you would need some rock climbing over the slick rock before reaching the frame arch and the delicate arch.

The Frame Arch 

The Frame Arch

Delicate Arch

Delicate Arch at Sunset

I’ve seen nice astrophotography in Arches National Park but it turned out the delicate arch trail are too strenuous for carrying equipment. With the lack of knowledge on where to go on the previous night, we drove to the panorama point to set up the gear.

Road to the Stars

The road to the stars

It turned out the Milky Way itself was bright enough. Should the Moab has less light pollution, the Sh 2-27 H-alpha region would be brighter in the view.

Color Palette of the Stars

The color palette of the sky were more difficult to image, with low altitute on horizon and LP, the SNR was low enough even with LPS-P2 filter.

Grand Road Trip of 2013 – Part 2

Yeah, a busy semester left me with little time to write this trip. But I’ll try to finish it.

The four-corners is indeed the most suitable place for astrophotography. After I got back home, it becomes clear when I looked up the dark sky finder and the monument valley is one of few Bortle Scale 1 dark sites in the US. It’s a shame they had so many spot light at hotel.

image

Light grey for scale 2 and dark grey for bortle scale 1 excellent sky

That afternoon we actually drove into the park on the sandy and gravel road. An all-wheel-drive SUV is highly recommend even in dry weather like this. And something should never be missed, a colorful sunset at the valley.

Colorful Sunset

The next morning we open up the curtain and witness in awe as sun rises above the distant valley and buttes, right at the doorstep.

Dawn at MV

Leaving the valley behind, the road ahead still challenges as we go into the grand canyon. We planed to make a stopover at Page, AZ for lunch and meanwhile viewing the horseshoe bend and the man made marvel – Glen Canyon Dam.

Horseshoe Bend

The horseshoe bend

I just could not risk another step before falling off the rim. Standing there, it’s utterly silence except the sound of your heartbeat, breath and the circling vulture.

Glen Canyon Dam

The Glen Canyon Dam view from the bridge

Quite unexpectedly, the AZ-89 had a landfall at echo cliffs and we were forced to take the additional 90 minutes detour to reach AZ-64 before climbing onto the south rim of grand canyan.

Grand Canyon Watchtower

The Grand Canyon Watchtower

The previous night’s observation overdrafted my energy, where I ended up photography the milky way in the village. It was until I got the photo that I found a overhead power line ruined my shot… Yet the rising milky way behind the clouds are absolutely gorgoues. The next morning, the sunshines the towering cliff in this way.

Sunrise on Cliffs

Morning light on the cliffs

I still remembered the first time I got here and viewed a sunset at Hopi point.

HDR at Hopi Sunset

HDR Composite taken last year

Foggy Valley

And foggy river valley at twilight

Well that’s just the best memory I had. We left the grand canyon and headed back down where we came and took the AZ-89A northeast to cross the Marble Canyon.

Map

Half way down the 89A I took a 360° panorama of the echo cliffs.Panorama

Click on image above for 360 city interactive panorama

Echo Cliffs and Navajo Bridge

Navajo Bridge and the Echo Cliffs

Marble Canyon

The Marble Canyon

Then it’s climbing up to the north rim again, crossing the national forest and finally arrived at our next stop in Kanab.

Dual Use Modification for D7000

Clearly, H-alpha astronomical imaging would greatly benefit from a modified camera. A lot more nebulas are now within feasible exposure time. Yet this pose another hurdle to use the camera in daily photography as its color balance is compeletely thrown away. Preset WB is a way to go but scene are so variable from one another that it becomes a chore to keep a sheet of white paper with me. And besides, preset WB only correct for one light source. The correction ratio for different color temperature is no longer the same with modified spectrum in the sensor. Spatial varible lighting, street lights plus moonlight as an example, will be a real chanllenge to correct. This will all add an insurmountable task in post-processing.

A genius solution will be taking advantage of the original factory filter and making it switchable! So, here’s the plan, I designed a filter rack just like the one offered by Hutech LPS front filter, which now could hold the ICF stack in it. After measuring the rack and dimension of Nikon lens mount, I drafted the 3D model in CAD and exported the final version as STL file, which is an universal format in 3D printing.

ICF-FF-N4

ICF-FF-N4-2

The 3D rendering of the filter rack. I named it ICF-FF-N4

The printing process is accurate up to 0.1mm in XY, the precision in Z is not as high. But none the less, horizontal accuracy is needed for filter mounting. It turned out the filter could just be secured inside the frame without any screw. The dent in the upper beam is reserved for an extra bump in the middle of the reflex mirror while it flips up.

IMG_6649

The waveplate must be in between the 2 antialiasing layer. Since we move the ICF from behind the dust filter to in front of it, we need to make sure the AA layer is facing the lens.

IMG_6650

Now the only thing left is to spray the filter rack black. The ICF is multilayer coated. We could now take away the clear filter installed in front of the sensor as the original focus could be restored. But a offset in the focusing system is still needed, because the AF sensor has an additional piece of glass in front of it.

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