Do It Yourself  SQM-LE Weather Proof Enclosure 

Recently I purchased a Sky Quality Meter (SQM) from the folks at Unihedron. The SQM is used to measure the sky darkness. The model I purchased is a lensed meter (SQM-LE) that measures a 20 light cone of the sky and outputs it in Magnitudes per Square Arc Second (MPSAS) and  Nake Eye Limiting Magnitudes (NELM).  The reason the LE model of the meter was chosen is it is accessible via an Ethernet cable. This allows remote measurements.

But in order for the SQM-LE to work it needs to be outside. The only problem is the manufacturer didn't  enclose it in a weather proof case. They also didn't provide any way to mount it.  Since my observatory is remotely controlled, the SQM-LE  needs to be permanently mounted, so the above two issues were a real problem. 

That meant I needed to figure out a way to mount, and weather proof the unit. Then I remembered my lightning detector...PVC pipe is a wonderful thing

This is the top of the custom SQM-LE enclosure. It's made from a 3-1/2" to 1-1/2"  PVC pipe coupling. 

The silver section is a clear plastic lens cover from a cheap dollar store flashlight. It provides a clear view of the sky and protects the SQM-LE from the elements. The inside was painted silver in order to reflect the sunlight. This was done to reduce the effects of solar heating during the daylight hours. 

The circuit board was removed from the vendor's enclosure (4 screws held the case together, and 3 screws attached the circuit board to the case inside)

After removal a small bevel cut was made on both corners of the circuit board on either side of the lens. This was needed so the lens would fit flush against the inside of the clear cover shown below.

Two small slots were cut into the inside of the stepped down end of the PVC. (10 & 4 o'clock in the image)

Notice the clear circular area of the cover is not in the center of the silver circle. This is because the lens on the SQM-LE circuit board will be off set when it is inserted.

The SQM-LE board was then inserted into the 2 slots. The insertion is a press fit, but to make sure the board stays put a dab of hot glue was applied to each side of the board. This still makes it possible to remove the board later if needed. You can see the power plug, Ethernet port and calibration lock switch. The calibration switch is left in the unlocked position so the meter can be calibrated in-situ if needed.

The SQM-LE as seen from the outside. The lens is perfectly aligned to the unpainted area of homemade cover. A bead of silicone sealant was placed between the edge of the silver cover and the PVC to keep the water out. The cover is a press fit into the PVC.

Update - 3/21/09 Anthony from Unihedron pointed out an error in my setup. I neglected to include the black factory provided lens plate. The cover can be seen in the first photo at the top of this page and has Unihedron stenciled on it. I had assumed that plate was a simple clear cover. Well that was incorrect, it turns out it functions as a reducer and is screened with metallic ink to prevent light from entering the outer rim of the lens. 

Without the plate the 20 light cone spec they have for the meter is not met. At Anthony's suggestion I cut the factory plate so it would fit inside my cover. The installed lens plate shows as a black ring between the silver and clear area in the above photo. Without the factory plate the delta MPSAS with the original factory case and my case was +.34. The delta with the factory cover installed in my case is -.04.

The 2 other main pieces that make up the rest of the cover:
-The center section is just a 3-1/2" length of 3" diameter PVC tube.
-The bottom is another 3-1/2" step down. 

The Ethernet and power cables will run through the opening in the bottom. A simple peice of screen (not shown) is used to keep the bugs out.

The assembled cover is shown below.  Since the SQM-LE is always on and enclosed it runs warm. The internal temperature sensor which is mounted just below the light sensor indicates the internal temperature is about 20 degrees warmer than the ambient outside temperature. This is OK in the winter as it will help melt any snow that falls on the cover.  

When the weather warms up the temperature inside the cover will rise.  To help with this vents holes have been cut into the PVC housing  (not shown in these images). The vents are cut at a 45 degree upward angle to prevent rain from getting in. Small pieces of screen have been glued inside and cover the holes to keep the creepy crawlers out.  Since the three housing pieces are not glued together this allows the upper section and middle section of the housing to be slightly rotated in order to open and close the vent holes. With the holes open the temperature inside the enclosure runs about 5 degrees higher than ambient when there is a slight wind, and about 9 degrees higher in still air.

The enclosure for the  SQM-LE is shown here assembled (minus the vent holes). It is important the to point out that the SQM-LE needs to be recalibrated when a additional covering is placed over the factory cover, or if the factory cover is replaced.

The calibration is done by changing the factory Light Calibration Offset value. The factory calibration info is provided on a data sheet that is included with each meter (don't lose it). The amount that the number needs to change is dependent on the properties of the new cover. 

To get that number  requires measuring the meter with the original factory cover in place before any modifications are made, and then again with the new cover installed. The key is the same light source has to be used so the variation in the readings is related to only the covers and not a change in lighting.  A Extech 401025 light meter was used to insure the light used before and after the modification was the exactly the same. 

The plastic cover I used only decreased the light going to the SQM-LE by .04 MPSAS. The folks at Unihedron provide a Java Script that allows the user to change the calibration value. As you can see from the screen copy below the jSQM program allows the user to view and change a number of parameters.

The finished unit was mounted on the bottom right side of the 120 watt solar panel. The solar panel supplies no power to the SQM-LE. The SQM-LE gets it 5 vdc power from the included AC adapter that is plugged into a weather proof outlet on the outside of the observatory.

The original placement of the SQM-LE shown above partially obstructed the solar panel as seen by the shadow. After this picture was taken the SQM-LE's mounting position was changed in order to reduce the shadowing of the solar panel. 

The bracket that holds the SQM-LE is attached to the back of solar panel support bracket by a single stainless steel screw.  The Ethernet and power cables are also tied off here.

The SQM-LE is mounted to its support bracket by means of hose clamp. 

The support bracket and hose clamp were off the shelf items purchase from the local Sears hardware store.

The SQM-LE is tilted at a slight angle to help the water roll off when it rains. 

The top and bottom sections can be easily be separated if the unit requires service.

This view shows the current mounting position of the meter as well as the vent holes.  An additional hose clamp was added to the existing one so the SQM to be attached to the bracket closer to the top. This lowered the SQM so the shadow it cast on the solar panel were greatly reduced.

The vent holes are drilled upward at a 45 angle in order to prevent water from entering the enclosure. In the image above the vents are closed. They can be opened by twisting the lower section counter-clockwise about 1/2 inch until the inner and outer holes are aligned. A screen behind the inner holes prevent bugs or debris from getting inside. (The IP address of the unit is printed on the top.)



Click on the above drawing for a larger version


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