The Whitney Challenge

I don’t prefer physical challenges. Yet the Mt. Whitney stood there a very special place. It’s the highest point in contiguous United States. Beyond that, it’s also one of few accessible through a trail. This makes it a huge difference comparing to, say the highest point in China – Everest, where professional mountaineering skill is essential plus huge bucks.

I’ve been tempting to summit this mountain for a while. So back in 2014 when planning a road trip in California, I decided to stop near the lone pine for a distant look on the mountain and get more detailed information. So here it is, a map from National Geography and a picture.

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HAO_4649

The center is actually the line pine peak, Whitney is right under the flag pole

It wasn’t until I got home that I found out the image was the wrong target. And the Whitney of course is so far way. This gives you a glimpse on the length of trail, a whopping 22 miles round trip! I gradually realized this wasn’t an easy task. Each year, some people got injured along the way or worse, lost their lives here. The complication is partly due to the altitude sickness after exertion. Other times, lack of buddy to overlook each other when accident happens.

In 2016, I met Weichen, a post-doc and an avid amateur mountaineer here in Michigan. This was the perfect chance to accept this challenge. Another big push to me was so much was going on with my life that I just want to forget. In May this year we didn’t get the lottery for an overnight permit. A slot in late July was the only option for two people.

So began our training, I picked a staircase with elevator next to it. And every other day, we set off a 99 floors of simulated hike with weight. Then the next day ran for cardio. The gradual push made me comfortable with even more weights. Three days before we left for Whitney, we peaked at 11 kilometer.

We packed in all gears needed for the hike. For your reference, head light and sufficient battery is critical as you don’t want to get lost in the middle of night or fall off cliff! Trekking poles can be helpful crossing the creek and on the way down. Lastly, don’t forget to bring food, candy and water. On the website it suggests 3 liters per person per day and that really is the minimal. There is water supply along the way if you bring your filtration device. But if not, bring plenty water!

The Great Sand Dunes

Monument Valley

Navajo Mountain and Colorado-San Juan River Junction

Lake Powell, Page, Glen Canyon Dam, Horseshoe Bend and Antelope Canyon

Grand Canyon

At that time, flight to Las Vegas was the most affordable. We arrived the previous day at noon. After getting the car and filling ourselves full with buffet, we headed toward California through the Death Valley National Park.

Badwater Basin

 

Stovepipe Wells and Sunset

By the time we got to lone pine, it was already dark. We retrieved the permit from a small locker next to the visitor center. Then we immediately checked into motel just to get enough sleep for the hike next day.

Day use permit at Whitney

Sky was clear in high Sierra early next day. We left the motel at 3:30AM before dawn. There was 20 minutes’ drive from lone pine to the Whiney Portal. From there we started the actual hike at 4:15. Just after the third switchback, we encountered the first trouble at the north fork of lone pine creek. Water level was unusually high that made us to trek it through with bare foot. The standing rocks were mostly submerged.

Moon and Venus, looking back down the valley

The trail stayed on one side of mountain for the next half hour of hike until you hit the Lone Pine Lake. The first crossing has tree log bridges. After a while, we hit another submerged section without any bridge. For once more, we took off our boots to cross it.

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Me crossing the log bridge near Lone Pine lake

In the next hour, we saw the first sign of snow. The unusual heavy precipitation last winter at California has left so much snow to melt. And of course, the flooding alone the Whitney trail.

Daybreak above the Lone Pine Lake

Alone the route the trees recedes rapidly. After passing the outpost campground below the mirror lake, landscape transforms into barren rock faces. Occasionally, there’s patches of flower scatter between the melting snow.

The traverse is at the top of this huge snow patch

Then we encountered the first challenge. A part of trail completely covered in snow and we don’t have any ice climbing equipment as we didn’t expect this so late in the season. For each step forward, we made sure the trekking pole is deep solid in the snow and the other foot wasn’t slipping at all. I was scared since this was my first time attempting something so steep on ice.

The red arrow indicates the traverse covered in ice

At 10AM we finally arrived at the trail camp upstream of Consultation Lake. As we watched the huge massif and icy slope, chances of making to top at noon faded away in a flash. Altitude had already taken effect on my body when I saw my fingernails starts turning purple. We spent 15 minutes recovering ourselves with water and candy. Then we put on the sunscreen before march ahead to the endless 99-switchbacks that will quickly rise from 12,000 feet to trail crest at 13,600.

Looking back down at the Consultation Lake (right) and the Trail Camp Pond (left)

The last switchback, traversing a 40 degree ice slope

Had I succumbed to the previous scary icy traverse, then I probably wouldn’t have to suffer this near heart stopping endeavor at 13500 feet. After the last switchback, we had to traverse two more continuous patches of melting snow just before the trail crest. OMG! A 500 meter 40 degree slope is waiting for your slip at any single step. I tried not looking down. But at every step, I have to just to make sure I secured my foot at the right place.

Exhausted at the trail crest

This little guy wants my food!

At 4100 meters, I started to feel the pain in my stomach. I know I was exhausted, so was my buddy. But there was no appetite to eat. The weird feeling of thin air here is taking a toll! I essentially forced the lunch into my mouth. With the projected timing, we will at most arrive after 2PM. We dropped some of our backup supplies at the John Muir Trail Junction and we pushed on.

The Hitchcock Lakes in the Sequoia National Park

The final 2 miles to the summit was a completely different experience. Here traversing across the west flank of peaks leading to Whitney, it was dry and hot unlike the icy north slope of the valley we trekked up. For this portion, we were left with a single bottle of water for both of us. So we had to preserve as much as we can.

A final few steps toward the victory!

Oh I forgot, we brought two cans of beer! Cheers!

Finally, we made it at 2:20PM. And cheers! We brought two cans of beer, but only for photography. I sip once cause I knew alcohol would make you dehydrate even quickly. So we just left the other can inside the hut. We log our name into history and then for some serious view!

The highest 360 panorama in contiguous United State

(click for 360 link)

I took a 360 panorama on the top of Whitney, with love. There was absolutely no time to shoot a time lapse. We headed down immediately.

The needles next to the last 2 miles up

Started our way down at 3PM

At the trail crest, I felt we probably would not make it to the top. Now I had a sensation of dying due to dehydration before reaching the trail crest. Half way down, we consumed the last drop of water. It was a race against time until we dehydrate or make it back to the trail junction. There we have 1 more liter for both of us.

Some of the trail section is dangerously close the cliff edge

But to get more water, we would have to make it back down to 12000 feet at the trail camp, where most campers were. So I still have to think about that icy traverse again. We saw some daring climbers slid down the icy slope. That was really a dangerous move given melting snow with low friction and exposed rocks.

As the trail camp got closer, the hope of surviving finally overtook the fear of death. We met someone alone the route who kindly gave us filtered water. The fresh melting ice water was cold, yet so refreshing! The sun already falls behind the massif. We would have to made it faster to get back to the Whitney Portal before it’s dark again.

We skid down the frozen ice of lone pine creek

To conserve time, we follow the fresh trace of others alone the ice on the lone pine creek. We stayed away from the rocks where cracks most often formed. But in the end, there’s a moment we had to get back on the trail when getting close to the snow line. Sunset happened by the time we got close to the Mirror Lake. We got back onto the trail, turned on our head lamps and continued down the mountain.

We followed the river on the way down. GPS record

The flooding was even worse after a day of melting snow washing down in torrent. But this time, we trekked right across the creek. The cold ice actually made a beneficial effect like pain killer to our heating and swelling ankle.

The milky way came out, for the first time I’ve ever seen in high Sierra. It was so clear. With the calm air at high altitude, those stars barely twinkled! I decided to sit down for a five minute rest, just enjoy the sound of nature from waterfalls, insects, birds and the starry nights through the gaps between those tall pine trees.

We finally got back to the portal. Both cellphones battery went dry. Buddy’s GPS watch also stopped recording between the Mirror Lake and the Lone Pine Lake. It was fulfilling feeling after such an achievement. 22miles/36km round trip in snow icy condition without proper gear. It was close to midnight when we checked into hotel. I took a shower and fell asleep immediately when I touched bed. My leg still hurt the second day. That morning, I finished three plates of breakfast while watching sun light shine on the Mt. Whitney.

Sun set behind the Sierra mountain range at Owen Lake

We left the lone pine before sunset. I really need to treat myself with star gazing. Next stop was the Dante’s View over watching the Badwater Basin of Death Valley.

Along the high way, shimmering lights filled the distant void of desert. I wondered, would I one day stand on the top of that peak again or I might never wish to challenge myself with some like this. But one thing was certain, I had just left my lingering love on the top. Good night, Mt. Whitney!

The Milky Way Arch over Badwater Basin

(Click for 360)

Written on Oct 15, 2017

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CMOS Camera – P6: First Light

In July I finally got the UV/IR cut filter for this camera. I designed a simple filter rack and 3D printed it. The whole thing now fits together nicely in front of the sensor. IR cut is necessary due to a huge proportion of light pollution in the near-infrared spectrum.

Filter rack

UV/IR cut taped to the plastic rack.

With all the hardware in place, I added a single trigger exposure mode in the camera firmware. And accordingly a protocol command to issue a release on the PC software.

70SA

The camera is then attached to a SkyRover 70SA astrograph. In the camera angle adjuster, there’s a 12nm bandwidth Ha filter. This would allow me to easily reject light pollution while imaging in front of my house. Focusing through the Ha filter is extremely difficult. I chose a bright star and pulled the exposure time to maximum during liveview for focusing. Finally, before the battery pack went dry (supplying both AZ-EQ6 mount and my camera), I managed to obtain 15 frames with 5 minutes each.

NGC7000

No dark frame was used for the first light image and guiding performance was exceptional. This foiled the kappa sigma algorithm for hot pixel removal and makes the background very noisy. Anyway, NGC7000 already shows rich details!

Remarks

1. This sensor has higher dark current than Sony CMOS. Somewhat >4 folds more at the same temperature. However, doubling temperature is small. In another word, its dark current reduces quickly with cooling. Last time I observed no dark noise at –15C. Thus imaging the horsehead during winter would be brilliant here in Michigan!

2. Power issue. The sensor consumes ~110mA @5V during long integration comparing to ~400mA for continuous readout, which is minimal. However, the Zynq SoC + Ethernet PHY consumes much more than a full running CMOS sensor. Thus some power saving technique can be employed. CPU throttling during long integration/standby, powering down the fabric during standby mode, move the bulk of RTOS to OCM instead of using DDR, etc. But many of these require substantial work.

 

Anyway, I’m going to use this during the solar eclipse here in USA!

No AA filter? More of a marketing hype

Back in 2012 when D800 was released, Nikon did a bit tweaking on its antialiasing filter which led to the higher resolution D800E. A pair of birefringent crystal is organized in the parallel 180 degree to cancel out the effect. But were they worth it? As we had disassembled more camera, I decided to write a post on how these filter stack is organized.

ChipMod sent me a pair of filters on the Nikon D600. The IMX128 was scraped during monochromatic mod.

Filter set

Filters from D600: UV-IR, CMOS Cover Glass, Color Correction Stack

Back on D7000, I had shown the filter set consists of an antialiasing layer with UV-IR coating and an ICF stack sandwiched from a wave plate, a color correction glass and an other AA layer. Upon receiving the filter, I initially suspect the same. After closer examination, I found the color correction glass was actually just a single thin layer. No wave plate was glued to it. On a micrometer, it registered 0.484mm thick.

Without a wave plate, it’s impossible to spread a point into four, since the two light rays are in orthogonal polarized directions. I thought a workaround was to cut the AA filter at 45 degree instead of 0 or 90. (Here I refer to the orientation to the direction where two light rays separate. The AA filter is always cut perpendicular to the optical axis, or Z-axis, of the birefringent crystal) As such, the blue color could be mixed with red. However, upon inspection under a microscope, this was again rebutted. It turned out, the first UV-IR layer is only blurring on the vertical direction, leaving moiré as is in the horizontal direction.

AA under Microscope

Calibration slide between objective and AA1mm in 100 division

Stage setup with micrometer ruler in the vertical direction

The spread from this filter is around 5 micron and wider than that in D7k. This corresponds to a thicker crystal at 0.8mm. Now we know for sure D600 only blurs vertically. This gives the advantage to gain a bit higher resolution in the horizontal direction. The DPreview had an excellent resolution test confirming the case. D600 resolve horizontally well beyond 36, albeit accompanying color moiré. But it blurs out at around 34 in vertical directions.

Any other cameras also do this? It turns out that many other cameras follow this trend. To name a few: Sony A7Rii, Nikon D5100, and possibly other low end DSLRs all had a single AA glued to a color correction filter. One possibility is to suppress the already strong false color during video live view rising from row skipping. However, I would still argue the effect of this is minimal given the spread distance close to pixel pitch.

The material for AA filter and wave plate is usually crystalline quartz glass. Many website cites lithium niobate and that is incorrect. An argument floats around that quartz has too small a birefringent value and it requires a thick slice. This is true during the early days of digital imaging where pixel pitch were huge! (>10um) Once a proper calculation is done, the above 0.8mm thick material happens to give a close to 5um displacement. Should lithium niobate be used, it would be way too thin to manufacture. Another interesting property with quartz, or fused silica, is its UV transparent property. Based on the above transmission spectrum scan, the AA substrate material permits UV to pass when measured at corner. Lithium niobate would absorb strongly in UV just like those ICFs. Notice that without any coating, the glass itself reflects 10% of light. Again, for emission nebula imaging, you could keep the UV-IR filter.

Cooled CMOS Camera – P3: Image Quality

In the previous post I successfully obtained the test pattern with custom VDMA core. The next step will be to implement an operating system and software on host machine. In order to obtain real time live view and control, both software should be developed in parallel. Thus in this post, let’s take a look at the image quality with a simple baremetal application.

The sensor is capable for 10FPS 14Bit, 30FPS 12Bit, or 70FPS at 10bit ADC resolution. For astrophotography, 14bit provides the best setting for dynamic range and achieves unity gain at default setting. The sensor IR filter holder and the camera mounting plate are still in design. I will only provide a glimpse into some bias and dark images at this moment.

To facilitate dark current estimation, the cover glass protective tape was glued to a piece of cardboard. The whole sensor was then shielded from light with metal can lid. Lastly, the camera assembly was placed inside a box and exposed to -15°C winter temperature. During the process, my camera would continuously acquire 2min dark frames for 2 hours, followed by 50 bias frames.

Bias Hist

Pixel Intensity distribution for a 2×4 repeating block (Magenta, Green, Blue for odd rows)

The above distribution reflects a RAW bias frame. It appears each readout bank has different bias voltage in its construction. The readout banks assignment is a 2 rows by 4 columns repeating pattern, each color for each channel. A spike in the histogram at certain interval implies a scaling factor is applied to odd rows post-digitalization to correct for uneven gain between top and bottom ADCs.

Read Noise Distribution

Read Noise – Mode 3.12 Median 4.13 Mean 4.81

The read noise distribution is obtained by taking standard deviation among 50 bias frames for each pixel. Then I plot the above distribution to look at the mode, median and mean. The result is much better compared to a typical CCD.

Dark_current_minus_15

Finally the dark current in a series of 2-minute exposures is measured by subtracting master bias frame. Two interesting observations: 1. The density plot gets sharper (taller, narrower) as temperature decreases corresponding to even lower dark generation rate at colder temperature. 2. The bias is drifting with respect to temperature. This could be in my voltage regulator or in the sensor, or a combination of two.

The bias drift is usually compensated internally by the clamping circuit prior to ADC. But I had to turn this calibration off due to a specific issue with this particular sensor design. I will elaborate more in a later post. Thus to measure dark generation rate, I have to use FWHM of the noise distribution and compare against that in a bias frame. At temperature stabilization, FWHM was registered at 8.774, while a corrected bias is 8.415 e-. For a Gaussian distribution, FWHM is 2.3548 of sigma. Thus the variance for the accumulated dark current is 1.113 given the independent noise source. As such, the dark generation rate at this temperature is less than 0.01 eps. Excellent!

Preliminary Summary

The sensor performs well in terms of noise. For long exposure, the dark generation rate in this CMOS is more sensitive to temperature change than CCDs. The dark current is massively reduced when cooled below freezing point. The doubling temperature is below 5°C.

LEXP_001

An uncorrected dark frame after 120s exposure showing visible column bias and hot pixels

Scraping the Bayer, Gain or Loss? – A quantitative analysis of mono-mod sensitivity

When you are deep into astrophotography, you’d probably start doing monochromatic deep sky imaging. A typical choice would be cooled CCD imager. These CCD cameras come in a variety of size format and architecture. The most affordable are interline CCD offered by Sony and Kodak (now ONSemi). Then the expensive full frame CCD requiring mechanical shutter from Kodak. Now however, as most of my previous posts and other similar studies have pointed out, CMOS holds a much better edge comparing to CCD. The only problem is, not a lot of CMOS based monochromatic devices are out there for your choice.

CMOSIS CMV12000

One option is the sCMOS from EEV and Fairchild. But I would imagine those to be expensive. Then CMOSIS who offer global shutter ones with monochrome in various format. But their dark current (~125 eps) and read noise (>10e-) figures are not a clear competitor to CCD in any way. Sony makes small format B/W CMOS but nothing bigger than 1 inch format. As such, we saw many specialized conversion service that scrape away the Bayer filter layer these years. Unfortunately, by doing so, you essentially remove the microlens array which boost the quantum efficiency. So in this post, I’m going to investigate the QE loss and gain with such modification.

Data is kindly provided by ChipMod for this study.

The modification steps involve camera disassembly, filter stack removal, followed by prying open the cover glass, protecting the bonding wire and finally scratching the pixel array. For the last step, the scratching actually happens in layers. We’ll use IMX071 cross section EM image from Chipworks again for illustration.

image

The surface texture of an image sensor, as described by ChipMod, varies in resistance to scratching. The first layer to come off, are the microlens array indicated in green arrow. This layer is usually made of polymer. Further applying force would strip away the RGB Bayer filter as well, indicated by the red arrow. The yellow region represents the pixel pitch with the blue defining the photodiode boundary. Comparing the length of blue to yellow, we could estimate the fill factor is 50%. Because of the channel stop, overflow drain on the other axis, the fill factor is typically 40%. The gapless microlens above, focus the light rays onto the photodiode to bring the fill factor close to 90%.

image

The sensor was scraped into 3 vertical regions. From top to bottom, A: the microlens array is removed; B: both layer removed and C: the original one. Comparing A/B tells you how much light the color dye absorbs at that wavelength. A/C tells you how effective are microlens. Finally, B/C gives you the gain/loss after mod.

An identical test condition was set up with a 50F6.5 ED telescope in front of a white screen. 2 wavelength, Ha and Oiii are tested with 7nm FWHM filter in the back. Field is sufficiently flat so center regions are used to calculate mean intensity.

image

Test result

The microlens array performs as expected, it typically boost QE to 2x in native channels. Even in non-native color channel, the uLens still boost signal by 50% or more. Losing the uLens array is a major downside. But considering the absorption of color dye even in its peak transmission, stripping CFA actually minimize the QE loss. For example, in the red channel of H-alpha, signal was at 64% even though losing the uLens should impact the QE by more than half. The same is more apparent in Oiii wavelength. Because green channel peaks at 550nm, at 500nm the absorption is nearly half for this particular sensor. Thus the net result is no different from the original sensor.

In conclusion, mono-mod sacrifices some QE for resolution and all spectrum sensitivity. My estimation puts final peak QE at around photodiode fill factor, or around 45%. The state of art CMOS process maximized the photodiode aperture, making such mod less prone to loss of QE after microlens removal. This is in vast contrast with Kodak interline CCD structure where a 5 fold QE penalty should microlens are stripped away. The mod should perform well for narrowband imaging, especially for emission nebulas. However, a fully microlensed monochromatic sensor is still preferred for broadband imaging.

National Park Time Lapse – Tranquility

Since my last astrophotography road trip in California two and half years ago, I really haven’t spent anytime writing on travel and photography. Amidst the camera project and my PhD, I somehow have accumulated a pile of decent photography yet to be processed or released. But anyway, all those hard work serves to produce better images wouldn’t they. So I took a break in the previous weeks to finish off some of those leftover photo work.

Please enjoy my second time lapse compilation – Tranquility

Included are some of the time lapses I took in Big Bend NP, Mojave National Preserve, Death Valley, Picture Rock Lakeshore and Shenandoah NP. Then there’s also the Jiuzhai Valley in Sichuan, China!

In terms of astrophotography, I only got a few left in hard drive for release. The road trips I cover recently were on the east coast. With light pollution and bad weather along the way, there really weren’t many stars to be seen. Let alone for deep space imaging.

Cygnus

Wide Field Milky Way Center shot in Death Valley

As for 360 panorama, it becomes a routine for me now as the pipeline for 3×6 stitching is well established. In the meantime I start to incorparate the floor image in the stitching process.

Carlsbad CavernsThe WindowWhite SandBig BendPorcupine MountainTybee Island LighthouseShenandoahDeath Valley

Mouse over for location, Click for 360 View

The link to my first time laspe compilation is here:

The Making of a Cooled CMOS Camera – P1

As my last post had suggested, I was working on a camera design. Right now the “prototype”, as I would call it, is in the test phase. The project actually dates back to 3 years ago when we envisioned a large focal area CCD imager customized for deep sky astrophotography. At that time, the price for such a commercialized camera was so prohibitive. The most suitable monochromatic chip was the interline KAL-11002 with a size of 36 x 24mm^2. Unlike full frame CCD which necessitates a mechanical shutter for exposure control, interline could handles this electronically. However, the addition of a shielded VCCD region greatly impacts the quantum efficiency and full well capacity. Beyond that, Kodak CCDs don’t seem to recover QE well enough with microlenses, with peak at 50% and only 30% for 650nm on a B/W device. Later on we started to dig deep into the datasheet and soon we abandoned the project. The accumulated dark current in VCCD was simply too much at the slow readout speed required for decent level of read noise.

KAL-11k

The KAL-11002ABA in the original plan

What happened next was dramatic. After getting my hands on D7000 and the hacking, I was shocked by how good CMOS sensor performs. I soon realized the era for CCD in astronomy might come to an end. Sooner or later, it will too embrace the noiseless CMOS in the telescopes. When Kodak span off its imaging division to Truesense, it soon re leased its first CMOS sensor with sub 4e- read noise and CCD-like dark current. We decided to give it a try.

KAC

Got the sensor, now big challenges lay ahead. To speed up, I decided to use the microZed SOM board as the embedded controller, at least for the prototype. Thus only the power supplies and connecting PCB had to be designed. The Zynq-7010 will configure the sensor with its SPI MIO from the ARM PS side. The data will be received at the FPGA programming logic (PL) and somehow relay to the PS DDR3 memory. The data can then undergo complex calibration and save to SD card or transfered over GbE/USB.

microZed

The microZed SOM with 1GB DDR3 and various I/O

The board is then designed and fabricated with the 754 CPU socket mounting the sensor. The main PCB contains the voltage regulators, oscillator and temperature sensing circuits.

Main_PCB

Stack-up

The data lines go through a relay board, which also provides power to Zynq PL I/O banks. The whole stack is then tripled checked before applying power. After weeks of hardware and software debugging, the sensor was finally configured and running at designated frame rate. Now it’s time to work on verilog in order to receive the data. I’m going to cover that in my next part.