Author: Allan Hall

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Cooling a camera sensor, why and how. Part 1

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If you are into astrophotography for any length of time you will run across people cooling a camera sensor or read camera specifications for CCD cameras that include cooling information. Why would anyone want to cool a camera? What does it accomplish? Is it important?

Let’s start with what cooling a camera (actually the sensor) does.

Camera sensors are a grid of light sensors called photosites. These sensors react to light and create voltages or data which increases based on the amount of light that hits them. The resulting signal from the photosite to the computer in the camera can be the same for a bright light over a short exposure as it would for a dim light over a long exposure. There are however other differences.

One of the biggest enemies of astrophotography is image noise. Noise in a photograph is basically variations in the light and dark readings of the photosites. This causes artifacts to appear in the image that are not actually there. In astrophotography we tend to take lots of images, even different kinds of images, and then use sophisticated software to merge all of these together removing the noise and increasing the brightness and detail of the target we are shooting.

Many types of noise can be removed because certain types of noise changes from image to image while the signal (the part of the image we want to keep) remains the same. The software then removes the parts of the image (by complex mathematical algorithms that help average that part of the image out) that vary and are considered noise.

Noise can be caused by a variety of factors. Shot noise is the noise created every time the shutter is tripped. The act of powering up the sensors, opening the shutter and reading the sensors actually injects noise into the data stream. It stands to reason that the fewer shots the better as we would have less shot noise.

Thermal noise is caused by the sensor heating up and releasing electrons (as any heat source does). Unfortunately the photosites can not distinguish between electrons released as light, and electrons released as heat, so it tends to measure both. The longer the exposure, the hotter the sensor gets (to a point of course) and the higher the noise to data ratio making it harder to extract the data from the noise.

When the data is read from a photosite it is then multiplied by a factor dependent on the ISO/ASA set on that camera. This multiplication includes all data the camera reads from the photosites including any shot noise and thermal noise.

Like all things, this is a balancing act. Longer exposures reduce shot and read noise, but substantially increase thermal noise. Shorter exposures with more cool down time between them can substantially decrease thermal noise, but greatly increases shot and read noise. So what to do?

What if we could eliminate or at least substantially reduce thermal noise from our equation? We can, by cooling a camera.

Virtually all modern CCD cameras include a chip cooler to cool the chip.  This allows you to cool the sensor of the camera by 10, 15 or even substantially more degrees over ambient. To a degree, the cooler the sensor the better (like everything of course, to a point). By cooling the chip in this manner you can use longer shots without the worry of the chip heating up, thereby reducing thermal, shot and read noise all at the same time. With a DSLR we don’t have access to the chip so we will try the next best thing, cooling a camera.

Reducing these noise sources lowers what they call the noise floor. This noise floor is a level at which data is above and the black of space would be below. You need the noise floor to be lower than the level of the target at which you are shooting. The dimmer the object, the lower you need the noise floor.

There are ways of processing lots of images such that it lowers your noise floor, but if you start with a noise floor below the level of the target to begin with your data will look even better after processing.

So what if you shoot with a camera that isn’t cooled such as a DSLR? Your first option is to shoot in the winter when the ambient temperature is low enough to help cool the sensor. If you shoot outside in a non-climate controlled area, this can substantially lower the temperature of the camera sensor if you live in an area where it gets cold at night. There can be a big difference between shooting in 80 degrees and shooting in 20 degrees. Do watch out for batteries though, they do not last nearly as long at 20 degrees as at 80 degrees so you are likely to run out of battery power several times on an all night shoot.

Another option is to have the camera modified to put a cooler inside. Yes, you can get this done to many off the shelf DSLRs but normally would not as it would make the camera difficult to use for normal daytime photography. It is also not terribly inexpensive. I would be hesitant to recommend this option and would instead steer you towards spending the money on an astronomy dedicated CCD with a cooler already built in.

Lastly you could build a contraption that covers the camera with what amounts to an ice cooler with a built in refrigerator. This method of cooling a camera has the advantage of not altering your camera so you can continue to use it for other things. It is also fairly inexpensive so you can try it to see what you think without having devote a lot of money into something that you wind up not liking.

One problem with the last idea of cooling a camera is the added weight and power requirements. Adding a bunch of weight to your astrophotography rig, especially right on the end with the camera, can cause huge tracking problems. Even if you have a massive mount and balance the setup with the camera and cooler installed, the resulting wind surface can destroy images with even a minor amount of wind hitting it. It basically adds a huge sail to the tail end of your setup.

Another problem with cooling a camera such as a DSLR is how well it can be cooled. Since the cooling will be either external (through shooting at night, or through placing the camera in a cooler) or by means of an aftermarket solution that is at best, a kludge never intended by the manufacturer, how good will the results be? Are those results worth the time, effort and/or money involved in the process?

As we discussed earlier, cold batteries do not last long. When cooling a camera that uses a battery inside the camera (as most DSLRs do) remember that your battery will not last nearly as long. You might opt for a external power adapter. Even if your camera does not typically have the ability to use an external power adapter there are things made to replace the battery on some cameras with a device that plugs into an outlet.

I have for years wanted to know if cooling a camera made a real world difference, lets actually run some tests and see what happens…

Stay tuned for furture parts of this article on cooling a camera.


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Pixel size, sensor size and more

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Pixel size, sensor size and many other factors seem to complicate our choices for cameras these days. Just when you thought cameras could not get any more complex with ISO range, well depth, and active/passive cooling I’m here to throw another wrench or several into the mix.

Lets start with pixel size or pixel measurement which should really be called photosite size. The actual sensor on a digital camera is made up of light detectors called photosites. These photosites are what create the pixels in the image. Each photosite measures the light hitting that sensor and generates a signal in proportion to the amount of light hitting it that is sent to the processor inside your camera.

photosite size comparison

fig 1: Illustration of how photosite size affects light collecting ability

As a general rule, the larger the photosite size the more light it can gather simply because the larger area will be struck by more photons. More light striking the sensor means a higher signal output by each photosite. This in turn means it will require less amplification (a lower ISO) to achieve the same results as a camera with smaller photosites. You could also say that at the same ISO the camera with the larger photosites could use a faster exposure.

Since you can collect more light with a larger photosite size that also means that you have a higher signal to noise ratio (SNR). This is particularly important in astrophotography because we are always shooting an extremely dark object (nebula etc) against a totally dark background (black of space). Since there is so little contrast or difference between the object and the background, it is important to have the highest SNR possible. The reasoning is that when there is a lot of noise, it is much more difficult to extract the signal.

Think of it as audio. When you are at a live concert the band is the noise, then you try to talk to the person next to you which is the signal. This is very difficult to do at a heavy metal concert (high noise) but far easier at a concert featuring an unplugged classical guitarist (low noise). Since in astrophotgraphy you are always shooting long exposures (compared to normal daylight photography) and using high ISO values when you can, there is a lot of noise injected into the images.

A larger photosite size will have lower noise primarily because the accuracy of the measurement from a light sensor is proportional to the amount of light it collects. In other words, if a sensor collects one photon over a one second exposure it will be dramatically less accurate than if it collects one hundred photons. This occurs because every photosite has an amount of noise that happens when the sensor is read (read noise) and a certain about of noise per exposure (shot noise). This amount of noise does not substantially change from a one second exposure to a two second exposure whereas the number of photos captured doubles. More photons collected means a lower amount of noise in relation to the number of photons.

Now to be technically correct, the amount of noise does change as the exposure time changes, but it does so far less than the increase in the number of photons collected. In fact, the signal is the squared amount of noise, or the noise is the square root of the signal, whichever is easier for you to remember. If the signal is 900 photons, then the noise is 30 which gives you a SNR of 30. Double the incoming light to get 1800 photons and you get a noise of roughly 42.5 and a SNR of about 42.5. As you doubled the light collected in this example, you increased the SNR which made it easier for you to pull really dark objects out of the muck.

The next effect of a larger photosite is in dynamic range. A dynamic range is basically the amount of difference between the darkest a sensor can record and the lightest. In a previous article I discussed full well depth as being the maximum amount of light a photosite can store and that is an important player in dynamic range.

dr6

fig 2: A six part gray scale representing a low dynamic range

In figure 2 above the numbers across the top represent percentages of saturation of a photosite. Since this has such a small dynamic range everything from about 19% through 33% all reads as the same color. This is not what you want in astrophotography where the nebula is almost as dark as empty space.

dr12

fig 3: A twelve part gray scale representing a low dynamic range

In figure 3 we see that there is far more definition so there are two different shades for objects in the same range of 19% through 33%. The higher the number of shades on this chart, the higher the dynamic range. More dynamic range makes it easier to separate nebulas and empty space.

Two primary things affect dynamic range, the ISO and the well depth. Since a larger photosite size or larger sensor size typically has higher full well capacities and also require lower ISO values for a given exposure, they tend to have far superior dynamic ranges.

snr-d7000

fig 4: Dynamic range and SNR by ISO for a Nikon D7000 DSLR

In figure 4 we see how the dynamic range (DR) and SNR both drop as the ISO increases. The D7000 camera used above is what is typically called a crop sensor (APS C sensor size) camera which typically has substantially smaller photosites compared to a full frame sensor camera. To get a general idea you could say that the dynamic range of a crop sensor camera at ISO 800 is 10.75 and for a full frame camera is 11.75 at the same ISO. While this is a massive generalization (and really wrong 99.9% of the time) it does give you the right idea.

This sounds great! Are there any down sides? Maybe.

One argument for smaller photosites is that they capture more detail. This stands to reason since the same amount of light would be spread across more photosites on a smaller sensor and therefor more pixels. It would simply be a matter of more pixels in the same image meaning more detail. True enough.

The opposing view is that in most cases people do very little enlargement by cropping an existing photo. This means that a photo taken with a 20MP (mega pixel or million pixel) full frame camera would have the same detail as a photo taken with a 20MP crop sensor camera. Also true.

Another concern is that crop sensor cameras have a crop factor built in. This means if you shoot a full frame image with a 50mm lens and then put that same lens on a crop sensor camera what you wind up with is the photograph looking like it was shot with up to a 75mm lens. This is because the lens puts the same size projection of light at the sensor plane (where the sensor is) and if you have a smaller sensor, less of the image appears on that sensor. This makes the image appear to be zoomed in.

While the image being zoomed in has no real bearing on the image quality, it can really throw a wrench into things when you bolt that camera onto a telescope. An object that fit perfectly on your full frame camera’s sensor now spills over the edges on the new crop sensor camera you just bought.

The last and probably most important factor is that a larger photosite size or a larger sensor size typically cost more money. This money could be spent getting something with a better cooling system or some other feature. Only you can decide where to spend your finite resources and which features are more important than others.

 So what does sensor size have to do with anything other than maybe having a crop factor? A larger sensor simply has more room for larger photosites, or a higher number of smaller ones as compared to smaller sensors.

I hope you enjoyed my article on pixel size and sensor size!


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Five planets in the morning sky

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This morning I viewed five planets aligned and the moon in the morning sky. It was a simply amazing sight. I had to get up really early in the morning to get out to the dark site so that I could spend a little time imaging, and a lot of time just admiring the view, and still go to work the next morning.

Five planets in the sky

Screenshot from Stellarium showing the position of the five planets at approximately the time I was viewing them

The five planets that were visible were Mercury, Venus, the Moon (yes, not a planet, but still a wonderful addition to this lineup), Saturn, Mars and Jupiter in that order from west to east along the ecliptic. What you don’t hear about is that Pluto is actually there as well just to the left of Venus on the screenshot above. I was not into astronomy the last time there was a five planet alignment back in late 2004 and early 2005 and it was a little different with the order of Mercury, Venus, Mars, Jupiter and then Saturn. Depending on when you observed back then you could get the moon in there as well. I was not about to miss the 2016 planet alignment!

Three of the objects in the sky

The moon, Venus and Mercury on the morning of the 5th.

It was a cold morning, just below freezing when I arrived at the dark site. The air was calm and clear. Once I set up my equipment and let my eyes adjust to the darkness the planets just jumped out of the sky. The moon, Arcturus and Vega also begged for attention. Even with five planets in the sky the real action for me was in the rising Sagittarius which contained Mercury, Venus and the moon.

I had of course seen all of these five planets before, but only once for Mercury, and I had never imaged it. It is far too small and bright for my equipment to do anything but render Mercury as a bright point of light just like Vega, but in a wider field with its neighbor, it was spectacular.

Venus is the most difficult I have imaged before, and for my equipment I think I have a pretty amazing image. After many sessions, tons of attempts, and more hours than I care to admit I finally got an image of Venus which showed something besides a bright dot. In the image below you can clearly see the shading on the clouds that cover the planet, amazing.

Venus

My attempt at Venus, click to enlarge and see the cloud shading

This image required the use of a video camera instead of my typical DSLR or CCD cameras. Stacking hundred is images is the only way I could get something this clear of something this small. Even with this setup, Mercury is far too small to pull this off.

Mercury is the most difficult of these five planets to image and my next chance to image it with any real meaning will be the transit on May 9th, 2016. May is a terrible weather month but I will be keeping my fingers crossed. I got lucky enough with the Venus transit so I guess it could happen again.

I hope you enjoyed my article on the five planets!


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Full well capacity and why does it matter?

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What is full well capacity? The full well capacity of a camera (sometimes called pixel well depth or just well depth) is a measurement of the amount of light a photosite (the part of a sensor that collects the light for a single pixel on monochrome cameras or that is used as the luminance value for a single pixel on a color camera) can record before becoming saturated, that is no longer being able to collect any more.

Lets explain this a little more in depth before we move on. This subject can get a little overwhelming for those who are not familiar with the parts and terminology so I want to take things slow. We will start with a discussion of what a camera sensor is and how it works before we get into well depth or full well capacity.

The sensor in a digital camera is a collection of photosites arranged in a grid. These are sensors that measure the amount of light that strikes them. In simple terms, the more light that strikes the photosite, the higher the voltage, or longer the pulse width the photosite will output. The basic thing to remember is that more light means more response from the photosite.

At some point however the photosite becomes saturated, which means it has had all it can take. At this point you can continue applying light but the photosite will not register it any more. Think of this as charging a battery, once it is fully charged, continuing to charge it will not result in any additional capacity in the battery. The photosite has reached it’s full well capacity or it’s maximum well depth.

For monochrome cameras one photosite equals one pixel and you are done. For color cameras it is a little more complicated. Color cameras (called one shot color in astrophotography because you can shoot color with monochrome cameras by taking three images; one with a red filter, one with a green filter and one with a blue filter and then combining them) on the other hand use each photosite as a pixel’s luminance value (luminance is how bright a pixel is) and then take a group of four pixels covered by a Bayer matrix to show what a pixel’s color is.

Bayer matrix - full well capacity article

A representation of a Bayer matrix or Bayer filter

The Bayer matrix is a filter or set of filters that covers the photosites with colors, two green, one red and one blue for each block. It uses two greens because our site is predominately green so images should be as well. Each of the photosites in the block has its own luminance, but the colors from all four are mathematically calculated to give a color to each one.

This is why monochrome cameras that are 10MP are more accurate than color, because the 10MP cameras actually capture 10MP of data for both luminance and color (when shooting with red, green and blue filters) whereas a one shot color camera captures 10MP of luminance data and only 2.5MP of color data. The monochrome does require three times the number of images to make a color image however.

The capacity of the photosite to record light is called the full well capacity. The higher the full well capacity, the brighter the light you can record. You are probably wondering why we need to worry about bright lights and full well capacity if we are recording dim objects like nebulae? Glad you asked!

If all you cared about was the really dim nebula showing up in your image, full well capacity is not that important at all. Unfortunately the majority of time we have very bright objects (comparatively) in the image too, they are called stars. The brightness difference between the stars and the nebula is huge, so we need the capability to record both objects without the star appearing too bright or the nebula being too dim.

At this point you may say you don’t care about how bright the star appears, once it turns white on the image it is white and can’t get any whiter. This of course assumes you do not care about having star colors in your color image or are shooting monochrome. In those cases full well capacity would seem to matter less.

There is another problem that happens once the full well capacity is exceeded called bleed or bloat. Think about filling a glass with water. What happens when you overfill the glass? That’s right, it spills over on to the counter. The same thing happens with photosites in that the excess light can in effect (not literally) spill over or bleed over to the surrounding photosites. This causes the stars to bloat and seem to get larger.

We have all seen this in images. All stars are point objects, they appear as a single point of light. Sirius appears exactly the same size as Polaris, just much brighter. In many images however some stars appear much larger than others, this is where the full well capacity has been exceeded.

What is happening when you pass the full well capacity is the photosite is generating so much voltage that it is causing the photosites surrounding it to increase their voltage as well. This spreads the response out to multiple photosites which makes the star look larger than it should.

In astrophotography you want the black of space to be black, the star to be bright, colorful and appear as a point of light, and the nebula to appear bright and defined. The way this works is that we expose long enough to get the nebula to be nice and bright, while hopefully having enough well capacity to store all the light from the stars without exceeding the full well capacity and bleeding over.

The black of space is handled by the noise floor of the camera. This is outside the scope of this particular article but basically the lower the noise, the easier it is to capture smooth black space and separate that from the slightly less black nebula. The longer the exposure, the more light you get off the nebula without capturing anything from empty space. The noise floor is a mixture of several factors including things like camera read noise and shot noise.

So more exposure means more data from the nebula, but that can cause the stars to saturate the photosites exceeding their full well capacity. Where is the balance?

Setting your camera exposure is basically a matter of shooting as long as you can without exceeding the full well capacity of the sensor. If that does not give you enough exposure to capture any data from the nebula, then you will be forced to increase the exposure even while causing the stars to bloat.

This is why a higher full well capacity is something astrophotographers strive to have. Unfortunately cameras with a low noise floor, high resolution, and high full well capacity are very expensive. This is where the balance of money versus capabilities comes in. Get the best you can afford and do the best you can.

Remember when we discussed color cameras and the Bayer matrix? Remember it seemed a little out of place and not really relevant to the discussion? It was, and here is why it is important to full well capacity.

When shooting monochrome, since each photosite creates one pixel, bloating only really affects one thing, luminance. Color stands on its own because you are just measuring the luminance of that one photosite through a colored filter.

On the other hand, if a color camera has a photosite driven over its full well capacity, it too bleeds luminance data to the surrounding photosites, but the color data changes too. For example, if you measure the color using the scale of 0 being no light and 255 being full, and then the three colors being broken out as red, blue and green you come out with something that could look like 0-255-0 for an object that is pure blue.

If the star is a point light source and it is over a blue photosite on a one shot color camera, once the photosite reaches it’s full well capacity and a reading of 0-255-0 and then is still exposed to light it could spread to look more like 10-255-20 (remember there are two green to every one blue and one red). This can continue on to 100-255-200 and beyond. As you probably guessed this dramatically changes the color that is presented in the final image.

All of this so far has been theoretical as real life presents unique challenges. For example a star is never a point light source on Earth. Even on the highest mountains in the clearest weather there is always pollution, water vapor and more in the atmosphere distorting your vision. This makes it appear as if Sirius is indeed larger than Polaris. If you were in orbit, you would have a much harder time seeing a size difference between the two.

So is this all academic? Not really, we still strive for smaller colored stars, brighter nebula and blacker smoother space. The full well capacity of your camera is how you get there.

You can read a lot more information on well depth including full well capacity and other aspects of CCD cameras on Wikipedia.

I hope you enjoyed my article on full well capacity and well depth!


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Why have a separate astrophotography laptop?

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One of the best decisions I have made in my astrophotography endeavors is to have bought a completely separate astrophotography laptop for use out in the field. This is also one of the first things I tell newcomers to the hobby. They often look at me strangely which then leads to a lengthy explanation. Hopefully this article will help everyone understand the benefits.

Astrophotography laptop

Let talk about what an astrophotography laptop does. My astrophotography laptop pretty much does everything. The only computer related task that it does not do in the field is play movies to keep me entertained while imaging. My laptop does the following:

  • Drives the telescope mount
  • Makes corrections to the tracking based on input from the guidescope
  • Manages the camera including temperature control and exposures
  • Stores the images
  • Allows quick and dirty processing to make sure the target is what and where expected
  • Provides a bridge to my gamepad for manual mount control
  • Provides target management including notes on exposures, temperatures, seeing conditions and more
  • Runs many miscellaneous astronomy software programs

A good portion of this requires drivers, software components and direct hardware access. For example driving the telescope mount requires the astrophotography laptop communicate with a third party PC card installed in the PC card slot. This PC card has a serial port in it, another driver is required for that. Then the serial data has to talk directly to the hardware in the mount. On my system this is all linked into the software running the guider, the software running the mount, the planetarium software and finally, EQMOD which ties all this together.

Once you have all this working working (which in itself can be no easy feat) you certainly do not want anything to mess it up. Unfortunately there are a lot of things that can break some of your functionality:

  • Update of your drivers
  • Update of Windows (or Mac OS)
  • Software installation/updates that replace Windows DLL/OCX files (or Mac equivalent)
  • Update of other components such as Java/Flash/DirectX/.NET etc

If any of this happens, there is a good chance that one or more of your astronomy computer programs may quit working. If it does, it can be difficult to figure out what exactly did what and even more difficult to put things back like they were.

The real bummer here is that if your luck is like mine, you won’t even know something is broken until you get out into the field, get your equipment set up and are settling in for a long night when you realize you can’t track an object. Now you get to spend the next four hours tracking down the problem. At this point you will be more than ready for a dedicated astrophotography laptop!

I know I need an astrophotography laptop, now what do I do?

There is good news in that an astrophotography laptop can be an inexpensive addition to your astrophotography equipment. There is for example no reason to purchase a new unit unless you just want to. A nice refurbished (by the manufacturer) or off lease unit can often cost $300 or less. You can get a really nice unit for under $500 if you want to splurge. When you compare this to what you paid for the rest of your equipment this is not too expensive.

Some things to think about when purchasing an astrophotography laptop are:

  • If you image at a site with AC power, you may not need a battery
  • You probably do not need a large screen size, something in the 14″ range would probably be fine
  • If you are downloading images to your laptop you should consider at least a 250GB hard drive or SSD
  • If you control everything like I do make sure you get at least 4GB of RAM, 2GB is fine for camera control only
  • If you require a serial port to run your mount, make sure the unit has one or has a way to add one (PC card slots are awesome), USB to serial adapters are finicky so treat them as a last resort
  • Once you load your stuff on it you probably will not need a CD/DVD drive so an external may be fine
  • Count how many USB ports you will need, add a few extra, maybe with a USB hub
  • Relegate ports on a USB hub to mice, game controllers, etc. Try to never use a USB hub port for mount/camera control
  • If thinking about a netbook, remember some programs will not like the low screen resolution and may be difficult to use
  • Buy a Windows computer. Macs are great computers but will MASSIVELY increase the difficulty of getting things to work, there are far more astronomy programs for Windows

My favorite places to buy an astrophotography laptop include eBay (directly from the official Dell depot only) and Newegg.com. I can not count how many I have bought from these two places and have had very few issues (the least from Dell). When problems have happened they were resolved quickly.

Good luck with your astrophotography laptop!


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Keeping warm while observing or imaging

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It is time to start thinking about keeping warm while observing and imaging as winter is right around the corner. Observing is a very low energy activity. During the winter when views are the best you will be shocked at how cold you can get when you are not moving. Even in warmer climates like down here in Texas where a really cold winter night might typically hit 20F, it is amazing how cold you feel when you haven’t really moved in hours.

The key is to always take far more warmth than you expect to need and to layer. My rule is that if I am comfortable walking out my front door and sitting on a deck chair for fifteen minutes, I need at least four times that much clothing/blankets.

Yes, this sounds like overkill but if you wear too many layers you can just peel one or two off. If you do not bring enough, you have to pack up and go home. Which would you rather be prepared to do?

Clothes for keeping warm while observing

When I will be out all night and the temps will be below 30F I wear two layers of long thermals, pants, ski pants, socks, wool socks, insulated boots, shirt, sweatshirt, fleece jacket, heavy jacket, scarf, gloves and hat. As if that is not enough I carry two wool blankets and an electric blanket as well.

Silly? As long as I am keeping warm while observing you can call me anything you like. Every hour you spend out there bleeds off heat. You are far colder at the end of the first hour than if you sat on your deck for fifteen minutes. You are far colder after eight hours at a dark site than any deer hunter after sitting in a stand for two or three hours.

Thermoses

Two things that help are a thermos full of hot chocolate/coffee and another one with some hot soup. Stay away from alcohol as it may make you feel warmer right as you drink it but it actually causes your body temperature to drop (that was a really fun experiment that the television show Mythbusters did).

alcohol

What you want to avoid is anything that generates enough heat that it might affect your viewing/imaging. Things such as space heaters, or firepits are really bad ideas. Fire is particularly bad as not only does the rising heat waves and light cause problems but the ash floating in the air will cause visual/imaging issues and will tend to coat everything within a much larger area than you think. I will be the first to admit doing some binocular viewing on a crisp night with a roaring firepit is a lot of fun, but I certainly will not be setting up my serious equipment anywhere near a fire. Keeping warm while observing is not worth having to spend an hour cleaning all my equipment and having poor quality images.

If the cold begins to get to you while imaging you can always retreat to your car. I would not start it, as the lights and heat waves rising from the vehicle could cause problems with your imaging. You certainly will be warmer inside an enclosed vehicle even if it is not running. If you are at a site that has AC power you could even run an extension cord through a cracked window and use it to power your electric blanket.

I hope some of this will give you ideas for keeping warm while observing.


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Historical Astronomy Equipment

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Recently I became interested in astronomy equipment that was, shall we say, less than modern. I was enthralled with the way they used to do things we we take for granted today, such as tell time.

I live about an hour drive from Plantersville, Tx, home of the Texas Renaissance Festival. The Ren Fest as it is popularly known is like a theme park based very roughly on a renaissance time period European town with inhabitants dressing as people from all over the world in that general era. There are a ton of shops (over 300 I read somewhere), shows and rides. One can spend the day watching jousting, see a falconer, ride a camel, shoot a bow and arrow, visit a period tavern (or twenty) and listen to live music. It runs every weekend in October through November and really is a lot of fun.

This year my wife and I went and I was fascinated with a few pieces of astronomy equipment for sale I thought I would share with you.

The first item is an Astrolabe. What is that you may ask? It is a device used to calculate and predict the positions of celestial objects including the sun, moon, planets, and stars. You can think of it as one of the first handheld astronomy calculators or computers. It was also used to determine the local time, for surveying and triangulation. It was first used around 150BC and continued use into the 15th century.

An astrolabe, an early piece of astronomy equipment

The front and rear of my 4″ bronze and pewter astrolabe.

There is a shop that has a lot of this kind of stuff, all hand made in the workshop of Norman Greene from Berkeley California. The astrolabes came in a variety of sizes with the 4″ I bought being the second largest I remember. They also come in a few finishes including a monotone pewter, monotone bronze, this dual tone bronze/pewter and a gold plated monotone. I opted for the dual tone because I thought it was much better looking and easier to read. I also took the display model instead of a new one as the use gave it a lot of character. I also bought the optional stand you see in the images above.

Included with this device is a book describing a variety of basic astronomy uses such as finding the time, showing the position of stars and more. While giving the instructions is well beyond the scope of this blog post I will give you some of the instructions so that you get an idea of how it can work.

To find the elevation of the sun during the day, hold the device about waist level by the included chain or ring through the top. Rotate the pointer on the rear of the device so that the shadow of the leading part is centered on the rear part of the pointer. This gives you the sun’s elevation in degrees as read on the outer ring where the pointer is pointing (approximately 51 degrees in the image below).

astrolabe4

This calculation can be used to further determine the time of day or night (using a star instead of the sun). I don’t pretend to be an expert with this thing but I am certainly having fun learning how it works.

As an aside, mine has a single plate and there are versions with many plates that supposedly can do advanced stellar calculations. If I ever get good with this one I might move to the more advanced model, we will see.

Up next is an Aquitaine Sundial, or Shepherds Watch. This is a much simpler device to tell time which looks like a large ring. I did not purchase this from the Ren Fest as I had already spent too much money on the astrolabe and did not want to push my luck with this too as the wife was already looking at me funny. Instead I bought my Aquitaine Sundial from Amazon.

Aquitaine Sundial

My Aquitaine Sundial.

To use one this amazing piece of astronomy equipment, simply adjust the center brass ring to align the hole with the current month marked on the outer pewter colored ring. Holding the device by the included strap/necklace point the hole towards the sun and look where the bright light shines through the hole and onto the scale marked on the inside of the ring. This should show the current time.

The one downside of this simple astronomy equipment is that it has to be made for a specific latitude. The one I have is designed for about 40 degrees north latitude (about the center of the US). Unfortunately I live just over 30 degrees, which means I have to make adjustments to get it to show approximately the correct time. It is still a fun thing to whip out at astronomy events and have people ask you how it works.

Star gazing equipment like this sure does make you respect the ingenuity of renaissance scientists.

You can read more about the Renaissance period at Wikipedia.

 


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