Staring at the Sun, 18

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A Computer Lab in a Box

8/22/2010: I've been getting some decent results by capturing full-res data with a Canon 50D, then discarding all the pixels except the red ones in Maxim DL5, then using Registax 5 to stack 20-100 of those still-massive 16-bit .TIFF files, then using Photoshop and FocusMagic to fine tune the finished image.

Looking back over the previous entries, it's easy to see that incremental improvements in hardware and technique have paid off nicely. But three fundamental problems remain with my DSLR-based workflow. First, it takes approximately all afternoon to go from snap snap snap to an image you might see here. Second, the 50D needs exposures of about 0.4s to get decent signal when barlowed to a generous image scale. That's due to three factors: it has an an infrared cutoff filter that trims too much of the deep red H-a signal along with nearby IR noise; it has a Bayer filter which means that three pixels out of four are blinded behind green or blue filters; and the telescope is operating at something like F30 rather than its native F10 because I have to use barlow lenses to get a large enough image on the on the Canon's relatively huge sensor. Third, with the Canon making such long exposures, and operating in LiveView mode, too, it cannot make several exposures per second to beat atmospheric seeing using techniques pioneered by planetary astrophotographers. I'm using a remote release and a timer to trip one frame every two seconds. The effective frame rate of 1/2 fps is at least an order of magnitude slower than the minimum rates used by successful solar / lunar / planetary imagers. You're not going to beat the seeing with such leisurely exposures spaced so far apart, not soundly anyway.

There are better, cheaper cameras around that address all these concerns. A pure monochrome camera would not have a Bayer matrix; those touted for astronomical use do not have an IR cut-off filter either. As a result, they are sensitive enough to allow shorter exposures and they have plenty of pixels because all are in play.

The smaller sensors on these cameras would allow me to record the same detail in a much smaller solar image. That means I could use the Lunt at it's native focal length (F10, not F30), and that alone provides a factor of nine in reduced exposure time. Put it all together and we have at least a four stop advantage in light intensity. And that's before considering improved quantum efficiency and possibly better A/D circuitry.

The smaller chips can be read out faster, so rates of 15-60 frames per second are not unusual (with enough light and enough computer power to receive them).

Exposures can be much shorter; there can be many more of them; and the individual exposures will be smaller and more manageable.

Here are the constraints. It can't cost a fortune. It needs to have a large enough sensor to record a field of view sufficient to get about a third of the Sun in the frame behind a 600mm EFL focal length. As a bonus, larger chips are ideal for lunar photos behind the A-P. It needs to have small pixels because all my instruments are short-focus, and I want to oversample lunar, solar, and planetary images for aggressive post-processing. Pixels can be 5 or 6 microns, tops. As for bit depth, 12-bits would be better than 8-bits based on my experience with Canon's RAW frames, but I could live with 8-bit pixels since we're going to be averaging many, many stacked frames anyway. A USB connection is required (rather than Firewire) because that's what my notebook computer has, and I'm not planning on buying another of those anytime soon.

The DMK-series of cameras made by my neighbors in Charlotte, The Imaging Source, are popular and have been used for years by solar system photographers around the world. If there is a standard, they're it. But it's an aging standard. They were contenders, but their cameras with larger chips are not particularly affordable nor particularly sensitive, and they're limited to 8-bit output. A couple of current masters (Christopher Go and Anthony Wesley) use cameras from Point Grey Research. Lumenera cameras are too pricey to consider seriously. I thought about some high-end webcam-like variations from Meade, Orion and Celestron, but I really want a larger array and more precise control than those offer.

8/24/2010: I picked the Point Grey Research Chameleon.

PGR's Chameleon is available as a monochrome camera with a 1/3-inch CCD comprised of an array of 3.75 micron pixels. It will output up to 18 frames per second at 1296x980 pixels and offers ROI (Region Of Interest) capability for subframe capture (at higher rates?) when appropriate (planets anyone?). The Chameleon uses a USB 2.0 connection. And it's priced near the bottom of the range; less than half the price of similar-sized Imaging Source cameras and only about twice that of many glorified webcams. Planetary imaging gurus Anthony Wesley and Christopher Go both use PGR cameras. They firewire models (Go uses the Flea3) behind larger telescopes. The combination lets them grab far more frames per second than the Chameleon can deliver through the USB port of my netbook behind a 5-inch refractor. Even so, the Chameleon has big advantages over what I've used previously (the Canon 50D at one extreme, a Philips Toucam at the other).

So what's the catch? Not much software in the PC universe supports Point Grey Research's cameras. There's the "demo program" included with the camera and precious little else. Or so I gather from the web. The demo program may suffice. It appears to offer all the options I know that I want now. The camera ships with a SDK which is said to allow the control software to be modified using C++. If the demo code doesn't get the job done, then I want to be able to use the SDK and provided libraries to be sure it does. That means it's (finally) time to learn some C++, and that is why this post is titled, "A computer lab in a box."


8/27/2010: Here's the box:




It's small! And it's light! No,I mean it's really small and light. No shutter, no mechanical controls, no Peltier cooling, no mirror, and no viewfinder. It's a Sony CCD, an A/D converter, supporting electronics, and a USB connection. Period. I actually think a significant amount of its weight is in the included tripod adapter which I attached to the camera just to keep from losing it. How small? This small:




An adapter from Agena Astro Products mates the camera's C/CS lens mount to a T-ring. 1.25- and 2-inch adapters already on hand take it from there. There are enough rings and adapters lying around to put it behind the solar telescope, the A-P, and the 200mm Nikkor (although that may require a little measuring and a little machining). It might also make sense just to watch eBay for a used Nikon F / C-mount adapter.

The software installed easily under WinXP SP2, although there are a few rough edges because PGR doesn't cater to end users so much as to machine vision labs, industry, and other places where IT support is readily available. Their get-started documentation calls for users to run "Setup.exe," but it doesn't exist. I selected a likely candidate and ran that instead. A few questions about an IEEE port (Firewire) required me to ignore those options since I don't use one (most PGR cameras use Firewire rather than USB). That said, the camera was autodetected nicely, the proper driver selected, and FlyCap displayed a realtime stream with no fuss or drama. The options to capture an AVI were neither particularly idiosyncratic nor especially handy (but the more I use the interface, the better I like it). I had to define the default capture directory and argue with the computer a little about its availability (FlyCap prompts for a directory but actually wants you to specify a directory and a filename for capture -- in fairness, I discovered after the fact that the help system explains that). There are plenty of options to try and many capture modes with which to experiment. My initial impression was that the software was offering me little feedback about what it was doing. In fact, there are cues and telltales on-screen, but they are easily overlooked. The situation is exacerbated by my netbook's widescreen mode; some of the controls and status indicators can are below the bottom of the page. Scroll down. I just need to play around with it.

8/28/2010. First light.

After practicing on the desktop, I installed the s/w on the Aspire One netbook, verified that it worked there, then carried the kit out to the solar telescope. I set the netbook's screen size to 1280x768 which is a little cumbersome (it insures that I can see everything, but it also means there are two levels of scrolling to deal with; one for the video adapter, one for the app). There may be a better way to live with the little screen. The camera was very easy to focus on the Sun (focus for the eye with an eyepiece in the extender, then remove the eyepiece and extension and replace them with the camera; proper focus is not far away). I focused on the limb of the Sun, then on surface detail using the FeatherTouch focuser. A box to provide a shaded work area will be very useful. I tweaked the telescope's aim, then captured two, 30 second AVI's at 15 fps at 8-bit pixel depth. The camera was set for 15 fps and that's what the little Aspire One computer captured via buffering in memory. Good deal. Thirty second runs may be excessive, as the results are half a gigabyte each and take a while to transfer over the home wifi network to the desktop for processing. It will be worth trying some ten second runs to see just how much difference it makes. There may be a dependence between the stillness of the air and the length of run required for top-shelf results.

Here's Registax 5.1 at work on my first Point Grey Research AVI capture:


screen capture


And boy, does it work! I've aligned my very first AVI capture in Registax 5.1 on several points (but not, as you can see, on the area of dusky filaments in the upper left. This image was made from the best 10% of 450 frames, aligned, stacked, and wavelet processed in Registax 5.1. To the finished .TIFF, I applied FocusMagic in PhotoShop CS4, and I rotated the image for a more "comfortable" view:



Registax's default behavior (select and stack the best 10% from an AVI) was well chosen, at least in this instance. An image made from the best 5% was better than an image made from the best 50%, but this image made from the best 10% was better than either. The 50% test ran for a comparatively long time. When imaging an extended subject like the Sun's intricate surface or the Moon, it would probably be a good idea to choose more than five alignment points. Here's a colorized version of today's best result:



I stretched the same frame without mercy to reveal limb detail. It would probably be much better if I began with 12-bit data or at least with 8-bit data collected using a more appropriate exposure. Aligning on the prominences in Registax would make a world of difference. Still, just to see what's not lost down in the noise, it's not a bad start:


I don't know what the exact exposure time per frame was. And I don't know how to find out in retrospect. This effort was just plug it in, let the camera select gain and exposure, auto-everything, capture the data for 30 seconds and then bring it inside to work it over. It's real first light, proof of concept, unrefined stuff. The air was not particularly steady. Watching the AVI and watching Registax do its thing, the Sun's image was moving all over the place. It's hard to believe this much detail could be extracted. This is very promising!

There's webwork to finish by the end of the month, then clear weather predicted for the middle of this week.

Among other solar experiments to be tried: see what happens with a single etalon. Try manually controlling gain and exposure to see how they're related to noise. Try 12-bit captures. Is it possible to use a telecompressor to get a full-disk image with the Chameleon? What happens if I increase EFL?

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                   © 2010, David Cortner