> 1. Use a soldering iron that can hit 400, and use a needle tip
I have to disagree with that. Almost every time I've been called in to rescue a frustrated novice at the soldering bench (which has happened a lot), I've found them using the thinnest tip they could find with the temperature cranked to 400°C or higher because nothing was melting. It's fine to have an iron that can exceed 400°C (and I can't think of any I've used that couldn't), but that capability should be rarely used.
A heavier tip -- generally the heaviest that will fit between obstacles on the board -- at a lower temperature is much more forgiving.
The small tip has little heat capacity, significant resistance between the thermal reservoir (the heavy body of the tip) and the contact point, and usually very little contact area with the work. That makes the temperature at the joint highly sensitive to tip position, surface contamination, and copper geometry on the PCB. A thick trace or ground plane connection sinks heat from the joint so well that it will never melt, but a small pad on a thin signal trace will quickly get hot enough to detach the pad. Gunk on the board or oxidation of the tip adds more variability. And the high idle temperature makes for fast oxidation of the solder coat on the tip and then the plating, which creates more problems and frustration when you're new and don't recognize those symptoms.
A fat tip provides a large thermal reservoir right at the contact point and usually creates a larger contact area that's less disturbed by a bit of contamination. The much lower thermal resistance brings the joint temperature closer to the iron's thermostat setting, allowing you to keep that setting lower -- I'll go as low as 300°C for SnPb on a not-too-heavily-coppered board, maybe up to 350°C for lead-free. That gives you consistent heating of a wide range of targets and is much less likely to lift pads even with the long dwell times that often happen while a novice is learning. At the low end, it slows tip oxidation enough to make it almost a non-issue. And less flux charring means less gunk on the board to interfere with the next joint.
Predictability makes the whole learning process much smoother, and running small tips at high temperatures creates all sorts of issues that are hard to see or understand until you've got some experience behind you. When in doubt, use more copper before using higher temperature.
That only holds in the near field of the reflecting surface, which is roughly (it's a fuzzy boundary so who cares about a 2 pi here or there) where the ratio of distance to width of the surface is less than the ratio of the width of the surface to the wavelength. For wavelengths around 10^-2 m and surfaces around 10^0 m, that means distances within 10^2 m. Much further than that and the reflected wave is spreading out and falling off as 1/r^2. A radar working at ranges of tens or hundreds of kilometers is firmly in the far field.
There's still a strong angular dependence in the reflection amplitude; 1/r^2 fall-off does not imply isotropic distribution.
> Why does detection scale as the 4th root of the RCS?
Because the power returned to the antenna scales as the -4th power of the distance to the target, and detection depends on that power being greater than some threshold. If the fourth power (rather than the square) is surprising, think of the reflection as re-transmitting the power that arrived at the target. The incident power on the target goes as RCS/r^2, and the power that makes it back to the original antenna gets another factor of 1/r^2, so the return scales as RCS/r^4.
To me, all-lowercase text comes across one of two ways:
(a) I felt speed was far more important than readability (reasonable for rapid-fire short messages or constrained typing ability such as a flip phone, also a common way to imply "fuck you, my time is more important than yours" in longer forms such as email), or
> then wouldn't that require a razor's edge of criticality?
Yup. From the device description in its decommissioning plan:
The CFX was a sub-critical assembly of uranium-2-35 surrounding a Cf-252 source. The function of the U-235 fuel was to multiply the neutrons coming from the Cf-252 source, which fissions spontaneously. The CFX was designed never to exceed a Keff of 0.99. The CFX assembly yielded sufficient neutron fluxes for applications such as neutron activation analysis.
Keff is the fission neutron multiplication ratio; 1 is criticality.
The hard part isn't having a telescope, but analyzing the images for objects that have moved between successive observations. Digital astrophotography and analysis software have been getting steadily cheaper and better, which leads to more amateur comet hunters each watching more sky, which has rapidly improved the odds of catching rare objects.
I'm not sure how the progress of institutional and amateur observations compare. Obviously the big guys benefit from the same technological advancement, but I don't know whether the fraction of new objects discovered by amateurs has been growing or not. I suspect the odds of the first interstellar object being found by an amateur were still pretty long.
By my back of the envelope math, burning 600000 kcal should produce couple hundred kg of CO2. You could also make that crossing in less than a third of the time under sail, with about a third of the daily calorie consumption, for maybe a tenth of the CO2 output.
> MCUs particularly from microchip had really good documentation
Oh how the mighty have fallen. I've only worked on one major project with a Microchip MCU (PIC32MK), but their documentation and support were terrible. No detailed documentation, just a driver library with vague, sketchy API docs and disgustingly bug-ridden code. Deadlocking race conditions in the CAN driver, overflow-unsafe comparisons in timers, just intern-level dumbassery that you couldn't fix without reverse engineering the undocumented hardware. Oh, and of course what documentation did exist was split into dozens of separate PDFs, individually served, many of which were 404 unless you went hunting for older versions or other chips in the product line. It certainly cured me of any desire to touch another Microchip product.
I have to disagree with that. Almost every time I've been called in to rescue a frustrated novice at the soldering bench (which has happened a lot), I've found them using the thinnest tip they could find with the temperature cranked to 400°C or higher because nothing was melting. It's fine to have an iron that can exceed 400°C (and I can't think of any I've used that couldn't), but that capability should be rarely used.
A heavier tip -- generally the heaviest that will fit between obstacles on the board -- at a lower temperature is much more forgiving.
The small tip has little heat capacity, significant resistance between the thermal reservoir (the heavy body of the tip) and the contact point, and usually very little contact area with the work. That makes the temperature at the joint highly sensitive to tip position, surface contamination, and copper geometry on the PCB. A thick trace or ground plane connection sinks heat from the joint so well that it will never melt, but a small pad on a thin signal trace will quickly get hot enough to detach the pad. Gunk on the board or oxidation of the tip adds more variability. And the high idle temperature makes for fast oxidation of the solder coat on the tip and then the plating, which creates more problems and frustration when you're new and don't recognize those symptoms.
A fat tip provides a large thermal reservoir right at the contact point and usually creates a larger contact area that's less disturbed by a bit of contamination. The much lower thermal resistance brings the joint temperature closer to the iron's thermostat setting, allowing you to keep that setting lower -- I'll go as low as 300°C for SnPb on a not-too-heavily-coppered board, maybe up to 350°C for lead-free. That gives you consistent heating of a wide range of targets and is much less likely to lift pads even with the long dwell times that often happen while a novice is learning. At the low end, it slows tip oxidation enough to make it almost a non-issue. And less flux charring means less gunk on the board to interfere with the next joint.
Predictability makes the whole learning process much smoother, and running small tips at high temperatures creates all sorts of issues that are hard to see or understand until you've got some experience behind you. When in doubt, use more copper before using higher temperature.
reply