DIY Sky Brightness Meter; Coleman Outdoor Compact Table; Lapdome; First light with Losmandy G11G

I've been tinkering with a microprocessor called an Elegoo Uno processor, and as a part of that I came across something called Phidgets. This is a collection of clever sensors and devices that can be controlled by laptops and mobile devices.

One of the sensors is a light detector. It's capable of measuring light down to 188 microlux, which just happens to match the value you would expect to measure for a nearly perfect dark sky site. That suggests it might be the basis for a night sky brightness measuring device. Fortunately it's built to report visual brightness and not full spectrum which can include a strong IR component.  The Phidget sensor is made up of two sensors; the second is IR and is used to correct the first.

The actual semiconductor sensors sits in a well with beveled sides within a plastic case. Rather than expose this to stray light I placed it in a PVC tube as shown in the picture and diagram below. This shield tube is probably too long, which means I'm blocking out some of the light cone entering the sensor compared to what it was normalized to receive.

Version 1: Long light shield

Version 1: Long Light shield

The length of the version 1 light shield was based on an incorrect calculation. Correcting the error gave a light shield length of 60mm, or about half of the Version 1 length. A few minutes of sawing gets us to Version 2, which is decidedly smaller in appearance:

Version 2 closed

Version 2 open

At this reduced length the shield is not much more than a way to keep the two caps together. In fact, it doesn't act as a shield at all; it's completely outside of the sensor's field of vision.

There are two black objects on top of Version 2. The smaller one is a combination temperature and humidity sensor; the larger is the hub used to communicate with my laptop via USB. The USB connection provides the power needed by the devices.

Thus far only Version 1 has been tested.The conditions were clear sky with a first quarter moon. (The sensor was pointed straight up and did not include the Moon in its field of view.) The reported light value was 2 millilux or about 2000 microlux. Is this a reasonable value?

It now gets a little complicated. First we need to correct for the oversized light shield. This introduces a factor of about 1.67 (a very uncertain value determined from indoor testing) so the measurement is now about 3330 microlux.

The site I was testing at was not Bortle 1, so the natural sky contributed some signal.  Assuming the site was at midpoint of Bortle 4, it was at about 520 microlux.

Next we have to add on the contribution of the first quarter moon, which takes a site from Bortle 1 to Bortle 5. A Bortle 5 sky at m = 19.7 has a value of about 1560 microlux, so the moon adds about 1370 microlux

In other words the sky starts at 520, to which we add 1370 from the moon for a total of about 1900 microlux.

Is 1900 close enough to 3340 to call it worth working on further? Yes, definitely, given the uncertainties in the numbers. If it had blown up or started a fire I'd say that it should be set aside, but this did neither.

I will continue to tinker with it, including rewriting the sample polling program to take time averages and log them. In addition to more local testing I'll also take this to this year's Nebraska Star Party, where the sky is Bortle 1 😃

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New Stuff:

1) My old field table finally fell apart after 15 years of service and has been replaced with a Coleman Outdoor Compact Table.

It seems sturdy enough and I like the solidity of the aluminum tabletop, but time will tell how well it holds up. So far the only negative I can say about it is the carrying bag; it's several times larger than it needs to be and allows the contents to flop around inside.

2) You know how when you try to make something you get the feeling that it's just not coming together and that you might be wasting your time? That sums up my attempt to make a portable light box for my laptop. I have thrown in the towel and puchased a LapDome. I haven't used this in the field yet, but it looks like it should be a great way to keep laptop light from interfering with visual observers without make the laptop difficult to use.

It seems really well made and should help with dew control, too. I kind of wish that it had side openings for cables in addition to the rear opening; my laptop has only side ports and none at the back.

One improvement I may make is to cut a board to fit the inside bottom to keep the laptop's air intakes happy.

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Lastly, the same night I was tinkering with the light sensor and watching people try to get first light with the Cherry Grove Observatory Planewave 12.5" CDK I also got some time with my new mount. It worked great! The following image is little more than a simple trial: 14x60s luminance frames and 15 dark frames. First, the full frame, reduced in size. Click to see it at 31% of the original scale:

Obviously an AT-65EDQ doesn't give the best pixel scale for an object like M51.

Here is a crop to the target so you can see it a little better. Click the image to see it full scale.

Not bad for total exposure time of 14 minutes! (Camera = SBIG ST-8300M)

Luminance is nice, but I can't wait to try out the other three Astrodon filters.😀