Update on shutter repairs

After a serious series of testing the refurbished shutter modules I have learned more. There's another way for the shutter to fail, and it's sneaky. The shutter blades are very thin metal and they very quickly gnaw a bit of play around the slotted axel. There's also some slack on the planetary gear, and as a result the shutter can flop about a minute amount. The movement is very minimal, but with relatively small tolerance that's enough to cast a shadow on the edge of sensor, or have a tiny gap when closed. I wouldn't have noticed this less I had held the assembled camera in hand and looked in with the camera at the right angle. I tried a range of glues, but those didn't hold on the blade and tiny axel at all. I was running out of options when I realized the play isn't an issue, all I have to do is make sure the blades don't flail around out of position. The easiest way was to make a miniscule wavy bend on the shutter blades, so the friction of the shutter sandwich is just enough to resist the gravitational pull on the blades. After some trial and error on the test-rig I finally ended up with a friction brake that didn't hinder shutter movement even at the low-energy 2.5V 60mA pulses, yet prevented the blades from changing their angle on their own. As luck should have it, it's a clear night tonight. And not a single shutter failure, yet.


Shutter repairs

Testing a shutter
The full-sized image has lots of annotations for you to check out.
The shutter on my SXVR-H18 CCD-camera broke after three years of use, so I e-mailed the manufacturer to order one or two shutter modules as spare parts. Unfortunately they didn't have any in stock, but a day or two later I received an email saying I've been sent a box with a few old shutter modules in an unknown condition. That box arrived today in a thoroughly crushed shape, containing five nearly intact shutter modules in plenty of bubblewrap. They were apparently from various stages of manufacturing as they were all fairly different. The structure was the same in all (obviously, as they are for the same camera) but the electronics (current limiters and a parallel caps), shutter blades and mechanism substrates were all different among the modules. This was going to be fun.

I made a small 2-pin test-harness for the motor power connector to check the electro-mechanics, quickly tested all modules and none worked properly (could be from the rough handling of the package). So I disassembled all the shutters and sorted the parts based on condition. After the sorting thru the pieces I came out with three busted motors (two are needed per shutter), three pairs of warped, uncoated shutter blades and three working shutter.I had enough one the good parts pile to build at least one, most likely three, possibly even four working shutters modules. I decided to build three as making the fourth would require a bit more tools than I had taken out for the job.

The test-rig for the assembled shutter-modules was rather simple and I didn't even take a picture of it. Just a wide rubber band (visible in above image, too) to keep the shutter from over-extending and the same 2 pin connector for power from the bench supply as for motor tests. Tapping the pin-header on the shutter opens or closes the blades depending on polarity, and the motion stops against the rubber band.

The image is from testing one of the refurbished shutter modules on the camera (I should probably cut a smaller bit of antistatic mat for the computer desk as well to be on the safe side). Although all three were just fine with my test-rig, at first only one of those worked on the camera it self. Once I re-measured a rough estimate on the actual camera shutter signal, I lowered my test-rig voltage to 2.5V and limited the current to 60mA. With these I was able to repeat the performance on camera, and an hour or two later I finally had three fully functional shutter modules.

Out of curiosity I check the failed motors in more detail. The motors are small, 6mm diameter DC-motors with an equally tiny planetary gearbox. A shutter blade is press-fitted on the output shaft. Two of the broken motors were burnt or stuck and didn't turn at all, and the motor from my camera had it's planetary gearbox shred to powder, but the motor itself works just fine. I guess the cold winter nights can have an effect on polymers and they simply can't take the stress at low temperatures (below -30°C on clear winter nights).

In theory I should be able to rebuild one more shutter when the current installed one fails, and if these last at least a season each I should be OK till a proper replacement for the KAF-8300 sensor arrives. My requirements for a replacement are quite simple: no mechanical shutter, same diagonal or more, roughly the same pixel size. And a backlit sensor with better QE, especially on Ha-band, would be nice.

Now all that's left to do is to get a small bottle of argon, go to a clean place, recall how to completely clean a CCD-sensor, re-dry the humidity eaters and reassemble the camera in an argon-bath, so I don't have to worry about sublimated ice on the sensor surface.


I never bin my color channels more than luminance

And here's why: When you're working on an LRGB image set, many frequently stack the images channel by channel, and then combine the RGB to chrominance and L to luminance. In doing so, you're throwing away good luminance data.

With most of the LRGB filter sets the Red, Green and Blue filters cover combined nearly the exact bandwidth of the Luminance filter. So exposing the same duration at same binning on all filters means I can sum each RGB sequence to an additional L frame, and I've "wasted" only two frames' worth of exposure time from luminance data. If I shot the RGB with a higher binning and different exposure, all that time would be wasted from luminance and my color channels would be crappier due to worse spatial resolution.

Binning your color channels is beneficial only when you have really dark skies and the sky noise doesn't drown out the read-noise with any sensible exposure durations. If your luminance exposures are easily longer than sky limited color channels at 1x1 binning, you are wasting time and resolution by binning the colors. There's no need to expose at the shortest possible sky-limited times, it should be considered the minimum exposure time, and the maximum exposure time is set by saturation of your target on sensor.

As a case study, below are two full-resolution crops of the same spot in my M31 mosaic showing the difference. The mosaic is a 2 by 2 panels, each panel is made from 10 minute exposures, 6 for L and 5 for RGB each.

Stack of six 10 minute luminance frames
Stack of same 6 luminance frames and 5RGB-sums.
As you can see, the stack of only L frames is a fair bit noisier, and it has a faint satelite streak going thru as there wasn't enough data to weed out all traces of it.

For the lower image, each RGB-set was summed as a synthetic luminance frame, and these 11 luminance frames were stacked for the final image. Some care must be take of course, don't normalize your RGB frames when calibrating, that could throw you data a bit off.

Some image processing (like PixInsight) allow you to weight the combination based on noise modelling. This does seem to return a decent approximate of a sum, if you have normalized the color frames during calibration.