Summary of All Product Reviews

Here is the list of all the cool products I have reviewed. I won’t waste your time with products I don’t use and love

Keithley 196 bench digital multimeter

LeCroy WaveAce scope

Field of View of our Canon 450D and Two Lenses

In a previous post, I reviewed the fields of view for the Canon 60Da with the C925 telescope. In this post, we look at the fields of view for the Canon 450D camera and  the two lens attachments.

The two lenses are 18 mm to 55 mm and 70 mm to 300 mm. The summaries’ of these all all the tools we have at the Addie Rose Observatory can be found here. Generally, we use the lenses at their hi and low focal length limits.

Here is the summary of the angular fields of view and resolutions for all combinations.

canon 450D 22.2 mm 14.8 mm 5.2 u
18 mm 71 deg 47 deg 60 arc-sec/pixel
55 mm 23 deg 15.4 deg 19.5 arc-sec/pixel
70 mm 18.2 deg 12.1 deg 15.3 arc-sec/pixel
300 mm 4.24 deg 2.83 deg 3.6 arc-sec/pixel

Field of View of our Canon 60Da and C925 Scope

In a previous post I reviewed how the angular field of view of an image sensor is related to the focal length of the optical tube and the sensor size. We then applied this analysis to look at the field of views of the small image sensors like the Skyrix I use for planetary work and for guiding.

In this post, we look at the field of view of the Canon 60Da camera we use with the C925 telescope and when we will have the new 0.62 focal reducer.

Here is the listing of the fields of view for the Canon 60Da:

canon 60Da 22.3 mm 14.9 mm 4.2 u
C925, 2350 mm 0.54 deg 0.36 deg 0.37 arc-sec/pixel
C925 focal reducer,1504 mm 0.85 deg 0.56 deg 0.71 arc-sec/pixel

Canon60Da__2000_01_01_0017The field of view of the C925 is 0.54 deg x 0.36 deg. This is a narrow field of view. In particular, it is smaller than the diameter of the moon. At its largest, the moon is 35 arc-min in diameter, which is 0.58 degrees.  This is a problem when trying to image the moon. It doesn’t fit in the frame.

Here is an example of the moon at 80% full, in the C925 using the Canon 60Da without the focal reducer. While it almost fits horizontally, it’s way too big vertically.

This is the motivation for getting the focal reducer. With the reducer, the FoV will be 0.85 deg x 0.56 deg. The moon, at 0.58 deg, should fit in this field of view. This is on the list to shoot next time the moon is out.

Field of View of our Small Imaging Sensors

In an earlier column about field of view, I showed that the focal length of a telescope really translates image size at the focal plane into an annular size of the object. The angular size, is the angular field of view. The connection is:

FoV[\deg ] = \frac{{{{\rm Im}\nolimits} ageSensor[mm]}}{{FL[mm]}}\; \times \;\frac{{180}}{\pi } = \frac{{{{\rm Im}\nolimits} ageSensor[mm]}}{{FL[mm]}}\; \times \;57.3

To estimate the angular size of the field of view of a telescope-imaging system, I just need the focal length and the imager size. Here is a handy table that combines these features and a summary of the fields of view for my instruments.

Here are the specs of the three small imaging sensors I use. I include the sensor dimensions and the angular field of view with each telescope and the size of the pixel on the sensor and its angular resolution.

low light level CCTV 8.4 mm 8.4 mm 14 u
C925, 2350 mm 0.2 deg 0.2 deg 1.23 arc-sec/pixel
C925 focal reducer,1504 mm 0.32 deg 0.32 deg 1.92 arc-sec/pixel
Orion 400 mm 1.2 deg 1.2 deg 7.22 arc-sec/pixel
Orion 162 mm 3 deg 3 deg 18 arc-sec/pixel
Skyrix 618c 4.46 mm 3.8 mm 4.4 u
C925, 2350 mm 0.11 deg 0.09 deg 0.39 arc-sec/pixel
C925 focal reducer,1504 mm 0.17 deg 0.14 deg 0.6 arc-sec/pixel
Orion 400 mm 0.64 deg 0.54 deg 2.27 arc-sec/pixel
Orion 162 mm 1.58 deg 1.34 deg 5.6 arc-sec/pixel
Skyrix 274c 8.5 mm 6.8 mm 4.4 u
C925, 2350 mm 0.21 deg 0.17 deg 0.39 arc-sec/pixel
C925 focal reducer,1504 mm 0.32 deg 0.26 deg 0.6 arc-sec/pixel
Orion 400 mm 1.22 deg 0.97 deg 2.27 arc-sec/pixel
Orion 162 mm 3 deg 2.41 deg 5.6 arc-sec/pixel

Preparing a 30 mWatt laser pointer to take out a street light

As noted, I was able to use a 30 mWatt laser pointer to take out a streetlight. But, I needed to modify the laser in three important ways to turn it into a practical solution:

  1. it had to be powered with an ac adapter
  1. it had to be mounted to the side of the fence in such a way that it could be aimed and held in position for extended time.
  1. it had to have a long power cable that could reach into my backyard and observatory dome.

image_thumb18I took my 30 mWatt laser apart and hooked it up to a power supply. The top half of the laser has a spring in the center that makes contact with the two 1.5v batteries. The shell of the laser is the second contact.

The center contact is the negative end, the outer shell is the positive end of the laser. Between the external contacts and the laser itself is some circuitry.

I am not sure what the drive circuitry is. The I-V curve, as near as I could tell, was sort of like a diode, a very sharp turn on in current with little voltage increase. The active circuitry did something, but it did not seem to be a very good constant current source.

Rather than try to take it apart and drive the laser diode directly, I decided to just drive the laser through the existing circuitry.

Experimenting using an external adjustable power supply, I found this laser did not turn on until there was 180 mA of current. Then it started lasing faintly.

It took about 2.7v to get 180 mA. At 3 v, I was getting about 330 mA and the laser was very bright. The effective resistance of the laser and circuit was about 3v/0.3A = 10 Ohms. This would determine the current flow in a typical circuit.

I left the laser on at 330 mA of current and after 30 minutes, the intensity had dropped considerably. The laser body was at least 15 degrees C above ambient. The laser itself was probably much hotter. I assume the drop in intensity was due to the increase in temperature.

I decided the current should be cranked down a bit, to about 300 mA, to keep the temperature rise a little lower. It was still pretty bright and would be bright enough to keep the lamp off.

I had a 5v cheap wall wart supply that was rated at 1 A. The ratings are usually based on a 10% voltage drop at the rated current draw. This would have been a source impedance of R = 10% x V/I = 0.5/1 = 0.5 Ohms.

I decided the simplest approach was to add a series resistor to the input of the laser circuitry to limit the current. The power supply output was 5v. With 3v across the laser with 0.3 A, the resistor would have a 2v drop at 0.3 A. This is a resistance of 2/0.3 = 6.7 Ohms.

I looked through my draw and came up with two 10 ohm resistors I could put in parallel with two 1 Ohm resistors in series. This would be 0.5 Ohms from the wall wart and 7 Ohms of resistors for a total of 7.5 Ohms. Close enough.

image_thumb22With the resistors in series with the laser circuitry, when monitoring the current through the laser, I got about 0.29 A through the laser using the wall wart.

I potted up all the cables, connectors and the resistors in a bathroom caulk silicone rubber to protect them from the environment. I added a power plug socket to the end of the cable pigtail so I could plug in the long DC power cable from the wall wart.

The laser tube was glued to a small right angle flange for mechanical support. Onto the base of this, I glued two  ¼-20 nuts. This would allow me to use a small camera tripod for the precise aiming.

Five minute epoxy makes for simple and easy construction.

I built a long extension cord from speaker cables to take the power from the dome to where the laser would be mounted.

image_thumb23The plan was to clamp a small camera tripod to the fence adjacent to the lamp post, next to my neighbor’s backyard. I needed a 40 foot cable to connect from the wall wart in my backyard to the laser.

Once everything was connected, I turned on the laser and used the tripod to align the laser to the photo sensor on the lamp post.

I was not surprised that in about 5 seconds of illumination, the light turned off. Success!

Taking out a streetlight with a laser pointer

imageThere is a very obnoxious street light located behind the dome of the Addie Rose Observatory. It is almost in the sight line to Polaris.

This make most of the views around the celestial north pole inaccessible.

The solution is to take out the light when I want to observe north.

While some of my friends suggested a B-B gun, or an arm mounted sling shot and a steel ball bearing, I wanted a process less permanent. After working through a dozen alternative ideas, I settled on shooting the light out with a laser.

imageIt turns out that many street lights have a photo sensor, usually located on their north side, away from the southern, direct sun.

In the close up picture to the left, you can see the small round circle on the base of the light. This is the photosensor. It is probably just a large area photoresistor.

When the light level drops below some pre-set threshold, the resistance goes up and this triggers a circuit that turns on the streetlight.

What if I could fool the streetlight into thinking it was daytime?

I tried shining the brightest flashlight I had on the sensor, but there was no impact. I tried shining a red laser on the sensor spot, but there was no impact. The light stayed on.

I was thinking of trying to place a mirror near the light to shine its own light on the sensor, but I realized this would just cause the light to go into some sort of oscillation.

As a last resort, I tried a 5 mWatt green laser pointer. No impact.  I was pretty confident some power level would work, it was just a matter of finding the lowest power (safest) that would still reliably turn off the light. I had a 200 mWatt laser available but didn’t want to bring out the “big guns” unless I had to.

And if this didn’t work, I knew where to get a 2000 mWatt laser if I really needed it.

imageSure enough, I mounted a 30 mWatt laser on a camera tripod and aimed it at the photo sensor. In less than 5 seconds, the lamp turned off.

The laser stayed on all through the evening and the light stayed off.

I’ve used this device a number of times to turn off the street light and shoot northern shots.

Now I have a stable technique to turn off the streetlight whenever I need to.

I get a few funny looks from my neighbors, but I’m used to that.

Creating high resolution panoramic images

A limitation with panoramic stitching on the iPhone can be overcome to create stunning, high resolution panoramic images using any camera and Hugin, a free tool.

The iPhone has a built-in panoramic mode for its camera, termed pano. After pressing the “take a picture” button, you pan the camera from left to right and record the image. It is automatically stitched, as you go, in real time. It creates pretty cool panorama images, as for examples shown here.

image

While the image is relatively high resolution, in this case 5054 x 1536 pixels, the zoom feature of the camera does not apply and since it is such a small focal length, the image doesn’t show much distant detail, even when zoomed in.

An alternative is to take individual stills, zoomed on the distant horizon, each at high resolution, and stitch them together with a software tool. In evaluating a number of the free tools available, I found that some of them do not preserve the high resolution of the originals.

The free tool I really like and use is Hugin. It is as simple as it gets, is robust, and saves the final image as a TIFF image at the full resolution. This means images can be 100 Mbyte or larger, but you can zoom in on the image and see the same detail as in each original image.

Here is an example of one of my recent panoramas focusing on Longs Peak, in Longmont, stitched by Hugin, which is 30,000 x 1815 pixels, and 105 MB in size.

image

I’ve found a few tricks that can be used to enhance the final image quality.

Use a camera with good zoom, like 20x optical. The digital zoom is irrelevant and should never be used. Always take the image with the highest resolution, like 10-15 Mpixels.

imageUse a tripod. Adjust the level so that when it is swept horizontally, the distance image stays horizontal and does not tilt. Otherwise, the panoramic image will be a little cockeyed, as in this image.

 

Since the images are imported into Hugin in alpha-numeric order of the filename and initially placed from left to right, it helps the tool, and makes a more robust stitching, if you take images sweeping the camera from left to right as well. This way the images come out in order and there is a higher chance of the pattern recognition software finding all the overlaps.

You cannot take too many images. If they do not overlap, you can’t make a panorama image. Take what you think is too many and let Hugin do the blending of the individual frames.

Once the images are imported, use the default settings for everything, This is an incredibly powerful tool. To create stunning panoramas, I am using just a tiny fraction of its full capability. But that’s what a good tool is supposed to be.

Toy Startup Enables All Kids to Be Makers

published in EE Times

Looking for that perfect Christmas gift that will give your 6- to 14-year-old a taste of the most basic engineering principle — that you can change your world if you can dream it — and stir their creative juices while having a lot of fun? Look no further than ATOMS from the Boulder, Colo. startup Seamless Toy Co.

“We want to enable kids to make their toys do things and have fun at it,” Seamless founder Michael Rosenblatt told me recently. “We created ATOMS to fill the gap between the maker community and kids, to narrow the gap between what kids can dream up and what they can make.” Continue reading Toy Startup Enables All Kids to Be Makers

Colliding Galaxies at Your Fingertips

clip_image002

When we look at a few Hubble photos of distant galaxies we may get the impression the universe is a static place. Other than the occasional supernova brightening, the galaxies we see haven’t changed much over the hundred years of observational history. But, it may well be the dynamics of energetic galactic collisions that created the variety of visually distinct galaxies we see today.

Though not so common today, images of many colliding galaxies, frozen in time, have been captured. In 2008, NASA and the Hubble Space Telescope Science Institute released a collection of 59 colliding galaxy images, shown above.

Continue reading Colliding Galaxies at Your Fingertips