Thousand Oaks vs. Baader Planetarium Solar Filter Films

With all the buzz about the eclipse going on I thought about revisiting the topic of solar filters. I’d previously imaged the sun very precariously using regular ND filters (10-stop, or stacked 10-stop + 6-stop) but this is inadvisable as they’re not rated for this use. I did this by shielding the entire aperture with a piece of cardboard between photos and manually jerking it out of the way just in time for each photo (based on my intervalometer’s pre-photo beeps). This was years ago and near as I can tell there was no damage to my equipment but it was still a bad idea in hindsight. I was perhaps lucky; not all conditions and filters are the same, and damage to your focusing screen, sensor, and even eyes could result (it’s largely about the parts of the EM spectrum you can’t see).

Considering instead filters meant specifically for solar photography using a camera with a super-telephoto lens, you essentially have two options — screw-on glass solar filters, and solar filter film. I haven’t experimented with the former, this post is specifically about the latter.

Two common solar filter films I saw suggested on astronomy discussion forums are sold by Thousand Oaks (TO) and Baader Planetarium (BP) and I ended up doing a side-by-side comparison.

My first purchase was a 6x6” square of Thousand Oaks film on Amazon. The Amazon listing is a little ambiguous about which of their films you’re buying, I believe I received what they call their “Silver-Black Polymer Sheets.”

For comparison, I ordered (also on Amazon) the Baader Planetarium “AstroSolar Safety Film 5.0.”

As they’re intended for use on any aperture size, both of these products require you to construct your own filter holders. There are myriad instructions online for how to do this so I’ll omit providing any of my own here. I made holders for both films using Planters 10oz peanut cans with the bottoms removed, a hot glue gun, and a lot of tape. It turned out that with a little effort the inside of the Planters cans fit nicely around the extended hood of the Canon 400mm f/5.6L USM I picked up used (see previous post). I will say that the TO film is much more sturdy, I felt less nervous building the TO film filter holder vs. the BP, which felt like handling gold leaf by comparison.

Here’s a side by side comparison of solar images through the TO and BP filter materials (TO on the left, BP on the right):

fog_before fog_after

The above photos are 1024x1024 full-resolution crops from a Canon 6D using the Canon 400mm f/5.6L USM with a Canon 1.4x teleconverter (original version, quite old). In each the sun was about half-way between center and the top of the frame (the sun moves quite fast when you’re futzing around with settings at 560mm equivalent focal length). I also rotated the BP image to align with the TO image, as the sun’s movement between filter swaps resulted in a small line-of-sight rotation relative to my pan/tilt mount (this reduces the BP image sharpness slightly).

The teleconverter undoubtedly reduces the sharpness some also, and I’m not sure if there’s a clear difference in terms of optical clarity between the two filter materials. However, the BP filter lets through way more light, so the ISO ends up being lower, hence the much higher relative amount of noise in the TO image, and better definition of the sunspot detail using the BP film. My initial interest in the BP film was after seeing a blog post somewhere else that compared the two films and showed a much sharper image with the BP film. In hindsight, I suspect that poster may have had the same challenge I did — difficulty focusing through high-ISO noise on Live View. I may have better luck running the camera tethered and using the EOS app to adjust focus, I’m not currently set up for that though, so TBD I guess. In-camera settings for each, to give you an idea about the difference in light transmission:

  • TO: f/8.0, 1/800s, ISO1600

  • BP: f/8.0, 1/1600s, ISO100

Doing the math (and excluding my LR adjustments), there’s about a 5-stop difference which is more than I would have expected. Also, the TO filter (as advertised) produced a really strong orange tint, which is maybe what you expect thinking about the sun as a cartoonish yellow orb in the sky, whereas the BP provided a nearly perfectly white image of the sun. I used the white balance eyedropper in Lightroom to bring the TO closer to white, for the side-by-side comparison, but I made no additional effort to match the white balance.

I’ve included a gallery at the bottom of this post showing the “As-Shot” and edited Lightroom settings I used for each. I didn’t think too hard about this for the editing, just played around to get some extra contrast in the sunspots. Note that some of the LR screenshots shows f/5.6; this is because I have several of the pins taped off in the teleconverter so I can still use autofocus with this slow lens. I’m shooting wide-open through the teleconverter which gives me an f/8 equivalent aperture for both photos. Also keep in mind that the “As Shot” white balance values are just what my Canon 6D decided to go with based on a small disk of light against a large black background, I wouldn’t consider either of these to be objective truths on color rendition.

In addition to loss of sharpness using the TC, I found it really difficult to focus the lens on the sun, especially with the TO film. The decreased amount of light means Live View is using a much higher ISO, so the image is just boiling with noise in 10x viewfinder crop mode, and coupling that with the 2x2 or 3x3 (?) pixel binning that occurs in Live View on this camera, it’s more of a guessing game than anything else. The backlash in the focusing mechanism makes me skeptical that using the scale reading will be a reliable means of getting focus for solar imaging.

In conclusion, I’ll probably use the Baader film for imaging because of the more neutral color rendition and better exposure flexibility, but the Thousand Oaks may go on a spotting scope or something for visual observation, e.g., during an eclipse. I think the orange disk of the sun through the TO looks quite nice, from a purely aesthetic standpoint.

The Supermoon over Austin and Why You Probably Wouldn't Notice If No One Told You

Unfortunately this month's appearance of the so-called "supermoon" was obscured by overcast, but here's a photo I snapped around this time last year of another supermoon.

A "supermoon" is what the media likes to call a full moon when it's near the perigee of its orbit, i.e. it's at its closest approach to the Earth. The Moon gets this close to the Earth once during each orbit of the Moon around the Earth, so about once every 28 days; thus the Moon appearing this large in and of itself is nothing special, but I suppose it makes for feel-good fodder on slow news days. In actuality the Moon only appears slightly larger; about 14% larger than it appears at apogee (this is when it appears its smallest), or around 6-9% bigger than the average apparent size of the Moon. Because of this small difference, combined with the fact that we only see the moon intermittently over long periods of time, and in different phases, it is highly unlikely that the average person would naturally notice the Moon was larger or smaller from week to week.

There are a few reasons why people "notice" a supermoon being bigger than a regular full moon, and it all comes down to our imperfect human perception.

First, the news reminds us to look at the Moon because it will be bigger than usual. We go out and look, and because a bigger moon is what we're expecting, we agree "well, I guess it does sorta look bigger." This is a form of confirmation bias. In truth, most of us have no intuitive sense for how big the Moon should actually appear, certainly not to an accuracy that would make us notice that the moon was 5%, 10%, or even 14% larger than at some other time we saw it.

Secondly, a full moon rises during sunset because a full moon is always on the opposite side of the Earth from the sun. Most people are out and able to see the Moon around sunset during their evening commute, so when there's a full moon (or a supermoon), most people will see it as it's rising, close to the horizon. When the Moon is close to the horizon it always appears bigger regardless of where it is in its orbit -- this is an optical illusion of sorts because suddenly we see the Moon next to objects on land and its comparative size appears bigger. This problem, sometimes called the "moon illusion" is a long-known issue with human perception and has been discussed by scholars for thousands of years. Click here for more information on the "moon illusion." Typically, doctored photographs (of which there are many on social media after the media hypes a supermoon) fraudulently increase the size of the Moon almost to the point of ridiculousness, most likely because the photographer thought that the Moon "looked really big!" and is confused as to why their photographs don't reflect what the optical illusion had them perceive, regardless of the focal length the photo was captured at. The photograph they took is indeed accurate, but being a flat image, it just doesn't play into the same part of our brain that produces that optical illusion of the Moon looking huge next to the horizon. In actuality, any time you see the sun or the moon rise behind features on land, it will appear very big.

As a consequence of being marginally closer to the Earth, a supermoon also appears slightly brighter, owing to the fact that the flux (or density) of reflected photons from the surface of the Moon is higher when we get closer to it. An analogy for this would be that your shower feels more intense when you move your face closer to the shower head. However, I would argue that most people wouldn't notice this change in apparent brightness any more than they would notice an increase in apparent size. Our eyes involuntarily adjust to low light levels all the time, and it would be impossible to make an assertion that the Moon was brighter one night vs. another without using some additional equipment (a camera with manual exposure controls would suffice). Also, the clarity of the atmosphere (depending on temperature, humidity, particulates, etc) varies frequently, adding yet another variable into the situation that we humans are not well suited for evaluating without special equipment.

To me, supermoons are fun just because they get people interested in and talking about the Moon. The Moon, while not a favorite subject of mine in and of itself, is definitely one of my favorite compositional elements in a photograph, and all the overzealous reporting in the world won't change that.

"Super" Moonset

I managed to stop by campus tonight just in time to catch the Moon setting behind the UT tower.

This is the tail end of what the media has been calling the "black supermoon," but really "supermoon" just means the moon is at the perigee (minimum altitude) of its slightly eccentric orbit when a full or a new moon occurs.

Lost in Space: Double Sighting of a Derelict Spacecraft

On the night of May 23rd, I was on a job down in a friendly south Texas town called Cuero. It was a hazy night, with high-altitude fast-moving cirrus clouds coming and going every 20-30 minutes. Not particularly ideal conditions for astrophotography, but excellent for showing off the interesting light patterns that a small town casts off. Unlike a big city, the light pollution of a small town comes from very concentrated sources, and each of them is often monochromatic. For instance, an oil services depot in the countryside may use sodium vapor lamps, while the city itself may use mercury vapor lamps. Various farming and oil operations across the landscape all use different lighting sources, which makes for a beautiful array of colors in a long exposure.

I spent 30-40 minutes filming a few short scenes from an overlook south of town (a popular spot among the locals). Being pretty busy with other projects and processing cityscape scenes of Cuero, I didn't actually have time to review this footage until last night. What I discovered was at first a curiosity to me as a photographer, but turned into something I would find fascinating as an aerospace engineer.

Early in one of my first image sequences there was a 5 frame series of streaks in the sky, all seemingly connected. Sure enough, the streaks lined up perfectly, and after stacking the frames in Photoshop and doing a little minor touching-up, I was rewarded with the following image:

If one looks closely at the top-center of the image, the two streaks can be seen. A detail shot is shown below:

Once I realized this wasn't an aircraft, I knew it was most likely an orbiting object, and even more interesting was that this orbiting object had apparently flared twice in a single pass.

(Aside: A satellite flare is a highly specular reflection of the sun off of solar panels or other glossy surfaces on an orbiting spacecraft. The spacecraft must have a very special alignment to direct the rays of the sun to an observer on the dark side of the Earth, so seeing one flare, let alone two distinct flares in a row is quite rare.)

The aerospace engineer in me took over and I set about doing some back-of-the-envelope orbit determination to be sure I was looking at a satellite. I looked up the nearby stars on my trusty star chart and estimated the observed angular distance traveled during the 50 second flare interval. I then applied some trigonometry and the definition of mean motion, and I was able to ascertain that the body was indeed moving at the correct angular rate to be in low-Earth orbit. Just by observing the direction the flares pointed in, I knew it was a highly inclined orbit also. My coarse calculations alone didn't help me identify the specific object, but they at least confirmed that what I had captured could most likely be attributed to an orbiting body.

Exactly identifying the body by continuing in this manner seemed daunting . At best, my calculations would perhaps narrow the list of potential objects down to a few hundred. I knew estimating the other orbital elements based on my crude observations would be difficult to impossible. To make matters worse, the internal clock on my camera was significantly off (and had since been corrected), so the image timestamps were useless for finding any absolute timing information. I quickly realized this could turn into a rather laborious exercise of parsing and sorting Space Track data.

Fortunately, I remembered that my favorite planetarium software, Stellarium, was bundled with a plugin that seamlessly pulls two-line elements from Celestrak to display satellites passing overhead. I knew approximately when I was out shooting that night (based on when I sent some text messages as I was packing up), and sure enough, I was able to quickly find a spacecraft that exactly fit the observed trajectory:

What I had captured was the Advanced Land Observation Satellite (ALOS), otherwise known as Daichi. ALOS is a Japanese Earth-observing satellite that was launched in 2006. At 4,000kg, ALOS is a fairly large vehicle (hence the name -- Daichi can be roughly translated from Japanese as "big"). Here's what ALOS looks like:

Advanced Land Observation Satellite. (C) Japan Aerospace Exploration Agency (JAXA)

Advanced Land Observation Satellite. (C) Japan Aerospace Exploration Agency (JAXA)

In 2011, after only five years, the ALOS mission was cut short by an unknown technical fault. It has been speculated that the vehicle sustained damage from a meteoroid impact causing the spacecraft to enter a power saving mode and become unresponsive. Despite this, ALOS is considered to be a success in that it exceeded its three year design life and collected a vast amount of high resolution imagery of the earth. This imagery is being used to construct a high-resolution global digital map which will have broad applications in mapping, natural disaster damage analysis, and water resource research [link].

ALOS has been derelict in a 700km altitude sun-synchronous orbit for the past 5 years. With no active attitude control systems, the vehicle is undoubtedly in a tumbling state, which increases the possibility for multiple flares as I was able to observe two weeks ago. At such a high altitude, ALOS will likely be in orbit for decades (or more) to come.

In yet another amazing coincidence, the successor to ALOS, ALOS-2, aka Daichi-2 was launched by JAXA at almost the SAME TIME I photographed ALOS (May 24th, 2014 at 03:05 UTC, or May 23rd, 2014 at 10:05PM CST)!

ALOS-2 also has an Earth-observing mission, but, unlike ALOS, has no optical cameras, and relies solely on radar for its imaging duties.

This wound up being a long post, but it's not often that I get a direct connection between my photography work and my aerospace engineering background. This string of coincidences was quite a treat!