416XT Icing

Meade 416XT CCD Camera - Problems With Icing

What Icing Looks Like...

Those cute little snow-flake patterns in the top-left corner, and the iceberg towards the center are sure signs that the desiccant in the camera is saturated. Note also the myriad white spots in the areas around the major crystals, these are also tiny ice crystals producing a frosted surface. Why should this happen? - well, as the chip cools any water vapor condenses out on the coolest surface, which of course is the chip face. Tiny particles of dust act as focus points for the crystal formation. The above image was taken with the camera at -10 deg C, things only got worse as the temperature dropped to -20. My camera is only about one year old and hasn't really had heavy use so I was surprised that the desiccant had failed so soon. Whilst the camera body is reasonably well sealed against the ingress of moisture it is by no means air-tight. Temperature changes within the camera cause the air to expand and contract, alternately creating a positive pressure and partial vacuum. This in turn causes a flow of air in and out of the camera body through small gaps bringing with it moisture (hence the need for the desiccant in the first place - it might have been assembled in dry air conditions, but the expectation is that water vapor will eventually get inside).

Disassembly

The 416XT is supposed to be returned to the Meade factory (or selling agent in the UK) to have the desiccant replaced, but I figured it should not be too much of a job to replace it myself. The SBIG ST-series cameras have a desiccant 'ring' which is user replaceable, in contrast the 416XT has 5 small bags of silica placed around the Peltier chip beneath the circuit board (see below). To gain access to replace them it's necessary to take the front of the camera off and lift the circuit board out of the camera body.

Before you consider dismantling the CCD camera find somewhere clean to work. Upon re-assembly you don't want particles of dust left inside the camera as these could (will) end up on the CCD chip's imaging surface causing black spots on all your images. Comments later in this article describe the methods I used to clean the camera before re-assembly. At one stage it was necessary to take the disassembled camera into the workshop to drill and tap some holes in the camera body, but of course this is not normally required for simple replacement of the desiccant. For this particular job I placed the chip + circuit board (which cannot easily be detached completely from the camera body) into a plastic bag to mimimise the dust problem, and also vacuumed the camera out afterwards. Note that in the picture below the 6-pin block connector towards the top provides the electrical connection to the shutter. There appear to be 2 thermal sensors (probably thermistors), one attached to the CCD chip, and the other to the side of the camera body, Both have dual red wires terminating in an adhesive thermal compound to attach them to their respective targets - this needs to be replaced for the sensor to work correctly.

	416XT with the shutter mechanism removed
	Front of the camera and shutter mechanism.

I was a bit puzzled as to what was holding the circuit board in place within the camera body, there were no securing screws used yet it seemed well seated. Eventually I used two small screwdrivers in the slots at the side of the CCD chip and used them as levers to gently lift the board vertically upwards. The picture below shows that the only things holding the board in place are the 8-pin block connector and a sticky layer between the aluminum heat sink and the top surface of the Peltier chip (the latter being the primary method to secure the board). The white rectangle in the middle of the camera body is the Peltier chip.

416XT Circuit board lifted from camera body.

Well, that 'sticky' substance was going to be a problem. It looks for all the world like double-sided sticky tape, but if it were it would act as insulation between the heat-sink and Peltier. Not a great idea. Clearly then, it must be something else - like some sort of thermal transfer sticky tape - but in any case I didn't have any to replace it with. My intention therefore was to use conventional heat-transfer compound between CCD chip and Peltier, and use an alternative method to secure the board within the camera (or rather, the CCD chip - as this is the important component that needs to be kept orthogonal with the camera body and in close contact with the Peltier). Note that I've left one of the silica bags in place to show you what they look like, 5 of these are rather unceremoniously stuffed beneath the main circuit board. I thought of attempting to dry them out but seeing as I'd gone to the trouble of dismantling the camera the least I could do was use new silica. That amber-coloured sensor is a mystery to me (middle-left, connected by one red and one black wire). It appeared to be another temperature sensor, and the flat face looked as though it should be positioned against a plane surface. However, I noticed it earlier through a gap in the board (prior to it's removal) and it did not appear to resting against anything specific.

What I fancied doing was replacing the internal desiccant with a cartridge-like affair which could be screwed into the body without disassembling the camera. In this way the desiccant could be changed easily and quickly. I went over the camera body looking for a suitable position to drill and tap a hole for the cartridge, unfortunately without success. To go in through the front was not possible because of the shutter mechanism, and there was insufficient 'depth' in the inner cavity of the camera body to gain access from the side (it would have had to go through the fins anyway). The rear of the camera appears sealed, and I could not be certain of the route the leads took between the outer rear connector socket and its internal connector block, it would have been risky drilling a large hole through the back as I might have severed a wire.

The only other option was to construct an internal chamber to hold the desiccant, and this is shown below (upper pictures before anodizing for clarity):

	The internal desiccant cartridge.
	The cartridge in final position.
	Anodised and loaded with desiccant. On the right is heat-transfer compound, and the home-made vacuum brush for cleaning the inside of the camera prior to assembly.

I started with a 4" dia x 3/8" thick disk of aluminum, turned it down to fit the cavity in the camera body, machined the central cut-out to fit around the Peltier, and then hollowed it out to form the chamber for the silica gel. The top was made from a 1/16" slice of 4" dia bar, drilled to accept 11 small screws engaging tapped holes (10BA) in the chamber walls, then I finally cut out and profiled the center to match the chamber. The rim of the top is of larger diameter than the chamber because the camera body cavity is stepped (look at the third photo above). This allowed me to place 3 screws in the periphery which engage three 8BA tapped holes in the camera body, thus the silica chamber is secured in place and doesn't rattle around. Further, I made the depth of the silica chamber just 10 thou deeper than the corresponding cavity in the camera body, so when the 3 screws which hold the chamber in place are tightened the bottom face is positively pressed against the back of the camera body. I placed heat transfer paste between the chamber base and camera body so that the aluminum chamber would remain at the same temperature as the rest of the camera body (obviating the possibility of it heating up and acting like a 'storage radiator'). The top plate was drilled as shown in the picture, the chamber is filled with silica and then a layer of filter-paper placed over the top. Finally, the plate was screwed down to seal the desiccant in place. The filter paper prevents dust from the silica gel escaping into the camera body, but is permeable to water vapor. Excess paper was simply trimmed off with a sharp blade to finish the job.

With careful measurement I was able to drill and tap the top plate to accept three more screws passed through the existing holes in the outer edge of the main circuit board, small rubber O-rings were placed over the screws between board and camera body so as not to stress it when the screws were tightened. These screws now hold the circuit board in place (instead of the sticky layer or whatever it was), and also keeps the CCD chip heatsink in close contact with the Peltier underneath.

A further modification was to make a thin aluminum shield that goes around the CCD chip on the top surface of the circuit board. I've often noticed that one corner of a dark-frame is bright, one hypothesis being that heat generated by a transistor near the CCD was responsible for this artifact (this particualr artifact is a common finding with 416s). The shield is designed to prevent the chip picking up radiated heat from board components, or at least ensure it's evenly distributed, and is held in place on the board with silicone rubber. If you also decide to make this modification take care the shield does not short any connections on the board and is not too high that it interferes with the shutter.

Heat shield for the CCD chip. Note also the 2 new screws at the upper and right of the picture, these now hold the main circuit board in place.

Re-assembly

As mentioned above, the critical thing is to keep the inside of camera body CLEAN and dust-free. If working at home cover your work area with polythene and keep the camera components covered with the same until it's ready to go back together. How to remove the dust from inside the camera body? I used a domestic vacuum cleaner with a 3/8" rubber tube inserted into the suction pipe with a rag stuffed in the end to roughly seal the pipe (this gives enough suction for what we need here). On the end of the pipe I placed the brush taken from one of the those camera lens cleaning 'puffers' - the ones with a squeezy bulb with a brush having a hollow stem. This technique allows the brush to dislodge dust particles and the vacuum will then remove them. Note: it's important that the brush head is also clean and grease-free, especially going over the face of the CCD chip. I washed mine thoroughly in 100% pure ethanol and allowed it to air-dry before use (after doing this, never touch the brush fibers again by hand to avoid contaminating it with oils, and keep the brush head in a sealed container for future use).

Upon re-assembly of the circuit board watch that you don't trap a wire between the CCD chip's heat-sink and the Peltier - those 2 components need to sit firmly together for efficient transfer of heat, and contact needs to be flat otherwise the CCD chip face will not be square to the camera axis (i.e., you will not be able to focus an image on it). The heat-transfer paste is essential, but it only requires a little as the gap between the two parts should be very small (a couple of thou only). Make sure the 8-pin block connector on the underside of the board engages the pins correctly as you press the board down into place, the best measure is to look at the edge of the heatsink and the edge of the Peltier - these are parallel when orientated correctly. In my case I have 3 screws to hold the board in place, if you are doing a straight swap of new silica bags for old you will have to arrange the silica bags so none of them prevent good contact between heat-sink and Peltier (it's not a great arrangement IMHO).

Connect the 6-pin block connector from the front plate to the shutter/CFW socket on the circuit board. There is a large neoprene O-ring which sits in a groove and seals the front of the camera when the shutter plate is bolted back on. You need to take care not to trap this O-ring - make sure it's well seated all around in it's groove. I placed just a thin smear of high-vacuum silicone grease over the O-ring before re-assembly, this stuff just hates water and gives an excellent seal against moisture. Further, it is none out-gassing (that's why it's called 'high-vacuum' grease) so will not deposit residue inside the camera. Replace the small socket screws in the front and tighten each carefully, watch for un-expected resistance to tightening as this may indicate the O-ring has slipped out of it's groove and is trapped.

After re-assembly it takes 3-5 days for the desiccant to do it's job and absorb all moisture from the interior of the camera. You can reduce this time to zero by purging the camera with dry nitrogen, though to be honest I can't see how this might be done as there are no valves (inlet and outlet) on the camera body, I suppose you could provide your own by drilling and tapping into the camera body but it doesn't seem worth the effort.

OK, the final job is perhaps not as convenient nor quick to replace as the external cartridge idea, but at least it's still relatively easy to change the desiccant as needed (I don't know how much Meade charge for doing the job), and anyway, there is more desiccant in the chamber than was contained within the original 5 bags so it should last longer.

As an aside, the picture below shows my add-on heatsink which attaches to the rear of the 416XT. This gives me a delta-T of -40 deg C, and there is provision for attaching a fan which should take the working temperature even lower. The fan shown is a small CPU fan which can be powered either from the LX200's AUX socket (with a suitable resistor to reduce fan speed) or directly from a 12V battery.

	Add-on heatsink, machined from aluminium and anodised. It's attached to the rear of the 416XT with 3 screws and heat-transfer paste between.
	Auxilliary cooling fan (12V computer CPU cooling fan) provides small but sufficient airflow to get the maximum cooling from the fins.

Follow-up Report:

Some 9 months after making this modification the camera is once again showing signs of icing - not as severe as that shown above, but with experience I can now spot the tell-tale signs of frosting on the chip. This is a bit disappointing as I expected to get at least 12 months of frost-free use out of it, but I never took any precautions to store the camera in dry conditions so perhaps I've got what I deserved. Even so, I think there must be a significant leak into the camera somewhere to saturate the desiccant so quickly.

Source of the icing - the dessicant is saturated, clearly shown by it's pink colouration (it's self-indicating, blue when dry and pink when wet).

In the picture above the old silica gel is mostly pink, a certain sign it's saturated with water, but at least it shows it been working well over the 9 month period. It could be dried but as I have a large bag of the stuff I'll just replace it with new, the actual volume of material used is small. Taking the camera apart and replacing it is just a 30 minute job, but I thought I would make one further modification to enable the camera to be purged with dry gas after re-assembly. All this required was a pair of nipples set into the camera body to take the flow of gas, and screw caps to seal them afterwards.

	Setup to drill and tap the holes for the two gas nipples. The sticky tape catches the chips and metal dust from the machining operations. The CCD chip and PCB are protected inside the plastic bag.
	The camera is back in one piece and being purged with dry Argon/CO2.

The construction of the camera meant the only place to install the gas nipples was through the fins, the upper of the two pictures above shows the machining setup. Sticky tape was placed beneathe the exit hole to catch the swarf and metal dust, the CCD chip and it's PCB were sealed in a plastic bag. A 15/64" slot drill was used to cut two holes either side of the body and centered on one fin (the slot drill leaves a flat seating for the nipple to screw down onto) followed by drilling and tapping the inlet/outlet holes 4mm. The nipples were turned from 7/32" stainless steel, having a sufficient height to facillitate the attachment of the endcaps. The nipples were screwed tightly in place with a drop of Loctite #601 on the threads. The endcaps have a rubber pad on the inside to give a gas-tight seal on the nipples. A mixture of 80% Argon/20% CO2 was used to purge the camera body for a bout 15 minutes, this was the only dry inert gas I had available (it's MIG welding gas for aluminium). This procedure is shown in the lower of the two pictures above.

Well, we'll see how long it lasts this time.

Some General Notes on Cooling:

Electronically cooled cameras such as the 416XT make use of a clever device called a Peltier chip. The Peltier does not actually get intrinsically cold, but when an electric current is applied it has the property of being able to 'move' heat from one of it's surfaces to the other - one side gets hot whilst the other gets cold. In fact, because it's necessary to put energy into the device to make it work the average temperature actually increases! To get a cooling effect the 'cold' side of the Peltier is placed in close contact with the CCD chip, whereas the 'hot' side is placed in contact with a heatsink having a much larger surface area (usually the camera body itself which has deep fins). The large surface area allows the heat transferred from the cold side of the Peltier to be dissipated to the surrounding air.

The 416XT camera uses something like 2AMPs at 12Volts - or around 24 watts of power at maximum cooling (the Peltier will perhaps account for about 75% of this at a guess). This is a LOT of extra heat to get rid of and it has to be dissipated somehow - this is in addition to the latent heat being removed from the CCD chip. A design limitation comes into effect with any electrically cooled camera, whereby additional 'cooling' of one side of the Peltier requires more energy to be put in than can be dissipated by the cooling fins. In the end it's all a question of relative surface areas, the larger and more effective the cooling fins are in relation to the CCD chip then the more heat that can be dissipated. It's possible to add second stages to this process, whereby additional Peltier devices remove heat from the 'hot' side of a single Peltier. This just concentrates the cooling effect to a smaller target area (I believe the 416's 2-stage cooling works like this). Nevertheless, additional Peltiers require yet more energy to run, and thus even larger cooling fins to dissipate the additional heat. The more efficient the system is, then the better it will perform within it's design limitations. Efficiency can be measured as the ability of the system to transfer heat - aluminium itself is very efficient at conducting heat (which is why it's used for heatsinks), whilst an air gap is quite poor and so Heat-Transfer Compound is used to bridge such gaps (this is why it's used between the CCD chip and the Peltier in the 416XT described above). The small fan I show above helps in still-air conditions where normal convection currents fail to provide the maximum heat dissipation from the camera's cooling fins. The additional airflow ensures that the air in contact with the fins is at ambient temperature and thus lowers the fin temperature closer to ambient too. Efficiency can be described by the cooling system's 'delta-T' parameter, which simply states the temperature difference between the radiator (camera body) and target (CCD chip).

Below is a graph showing cooling efficiency using my own 416XT, which has additional heatsinking and a fan.

The starting point (time 0 on the x-axis) for the graph above was -10.1 deg C, stabilised for 45 mins at 65% power (yes, I know it would have been better to start at zero power!). Anyway, ambient temperature was 24.0 and the camera temperature was 25.3. Then the power was switched to 100% cooling and temperature measurements taken every 5 minutes for 1 hour. You can see that the immediate response was a drop in CCD chip temperature (lower trace), which reached a maximum some 10 minutes later (minimum temperature achieved was -17.2 deg C). However, accompanying this was a slower rise in the temperature of the camera body (upper trace) and this continued for 30 minutes (in other words, the additional heat generated by the Peltier running at 100% was slowly heating up the camera). In response to the delayed increase in camera temperature that of the CCD chip also increased, eventually the system stabilised at -15.7 deg C.

The delta-T is the difference between the camera body temperature, and the CCD chip temperature - which in this case is 43 degrees C (at 3.3 degrees above ambient). The very best that could ever be achieved with this system (without further active cooling) is to maintain the camera body at ambient - which would give the same delta-T (-43 deg) but with a lower CCD chip temperature of -20.5.

The effect of the cooling fan

The second graph (below) illustrates the effect of the small cooling fan shown in the earlier photograph. In this example, the ambient temperature was 20.8 deg C (still air conditions), and the requested camera temperature was -15 deg C. This should have just been within the capacity of the camera to maintain (delta-T is -35.8 deg C) provided the camera body temperature did not exceed ambient by more than about 4 degrees. The camera was allowed to stabilise at the requested temperature (1 hour after activation) and then the fan was switched off. You can see the dramatic rise in camera body temperature and also a corresponding delayed increase in the CCD chip temperature. In fact what happened was the temperature stabilised at -15 deg C (with the fan on) at 87% power consumption, switching the fan off caused the power fed to the Peltier to increase to 100% in an attempt to maintain this temperature. The extra power merely made things worse as the additional heat generated could not be dissipated, and the whole camera began to heat up. The CCD chip temperature finally stabilised at -12 deg C at 100% power, 3 degrees above the requested -15 deg C.