Full well capacity and why does it matter?

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?

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

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.


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).


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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Pluto finally gets a face!

I admit it, I am one of those people who has a bumper sticker that reads “Pluto is SO a planet!” I grew up with nine planets, Mercury through Pluto, and no group of astronomers (not planetary scientists!) was going to kill that childhood memory. Yes, it is irrational, sue me. Regardless of it’s current planetary status the object known as Pluto is the furthest object we have visited and taken close range photographs of, and it is not at all what we thought. Lets start with the probe we sent, New Horizons.


Launched on January 19th, 2006 from Cape Canaveral Florida on the back of an Atlas V rocket it took nine long years to reach it’s ultimate target. At around one thousand pounds Sir Patrick Moore made comment that it resembled a piano glued to a cocktail bar-sized satellite dish.


The closest images were taken at approximately 7,800 miles above the surface of Pluto on July 14th, 2015. Data took about thirteen hours to travel from the spacecraft back to Earth making the first images from this distance arriving early in the morning on July 15th, my birthday!


What is fascinating is that just when you think you know what is out there, science throws you a curve ball. We expected tons of craters since it is near the Kuiper belt, we see few. We expected a fairly featureless ball of ice, we see mountains, valleys, and a huge smooth area being called the heart. This heart shows in false color as two distinct areas that could be caused by geologic activity, or surface erosion. Amazing. There is still more data to come and it will be slowly streaming in and being processed for quite some time, around sixteen months to be a bit more accurate. Stay tuned to NASA for more!

More information on New Horizons and the latest pictures of Pluto are on NASA’s website.

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You don’t have room to carry a telescope?

One of the concerns I hear from potential astronomers is that they have to travel to a dark site to do any serious observations or astrophotography and they just don’t think they can haul that much equipment. Sometimes it is a matter of driving a small vehicle, sometimes a matter of weight they have to carry. Either way I can usually talk to them a little and help dispel their concerns.

Lets start by saying you can have a lot of fun with nothing more than binoculars which can be worn on a strap around your neck. Whether walking, riding a bicycle, a motorcycle or a car, there is no excuse for not participating with binoculars. Often I have two pair of binoculars with me when I go out for a night. They are too easy to just throw into the back of the car or in the backpack. Many astronomy clubs even have Messier events where observers attempt to view as many Messier objects as possible in one evening with only binoculars. Even for seasoned astronomers this can be a really fun event and is often combined with cookouts or outreach programs.


In many countries motorcycles are an extremely popular method of transportation for the majority of people, and indeed in the US they are fairly popular with college aged people for their great gas mileage and affordability. Unfortunately there is no way to do any astronomy beyond binoculars when you only have a motorcycle, right? Wrong. Assuming you are not pulling a trailer with your motorcycle (which would allow you to carry a full astrophotography kit without a problem) you still have room for much more than just a pair of binoculars. In fact, here are two options I can carry on my motorcycle should I want to take it for a night observing.


First is an iOptron SmartStar E R80 setup which was designed to be portable and is even featured in my Getting Started: Budget Astrophotography book. Included is the tripod, go-to mount (and they have a GPS version as well), all electronics running off battery power, camera, several eyepieces and remote shutter release. This entire kit fits without issue and will easily go on just about any motorcycle or even a bicycle as it mostly fits into a medium sized backpack.


Next is something a little bigger, an Orion 90mm refractor, and an older version is featured in my book Getting Started: Using an Equatorial Telescope Mount. While this simple EQ mounted telescope and has no computer or electronics at all, it is still a very nice little visual setup to grab and go on an unexpectedly clear night when you just want to run out for a couple hours.

So what about small cars? I drive a MINI Cooper S Countryman which although not the smallest out there I believe qualifies as a “small car”. I regularly carry my full astrophotography rig, table, two laptops, fan (for summer), blankets (for winter), chair, cooler, and on occasion a second telescope or AP setup.

room6In the above image, everything but my laptop and cooler which are in the front seat are in the back. There is still plenty of room for a complete second observing telescope/mount combination.

Family cars can be used as well without sacrificing the back seat. My 8″ dobsonian with all my eyepieces, accessories, chair, table, computer, cooler and much more all fit neatly in the trunk of my Wife’s Buick. We could still carry five full sized adults.

There certainly are limitations but you should never think that you can not do any real astronomy or astrophotography because of what you drive, ride or even if you walk.

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Comet Lovejoy C/2014 Q2

Comet Lovejoy was a fantastic comet to both image, and view in medium telescopes.

Comet Lovejoy C/2014 Q2

The image above is my best image from Comet Lovejoy 2015, a single frame taken on February 18th at 8:27pm CST. Comet Lovejoy was discovered by Terry Lovejoy with an 8″ SCT telescope on August 17th 2014. Anyone who thinks that they can’t contribute to astronomy with their little telescope should take note that Terry has discovered five comets so far with his Celestron 8″ SCT mounted on a Vixen Sphinx mount. His equipment is readily available to any amateur astronomer. Admittedly that in addition to a CCD camera is a little more money than many people want to spend (around $4,000 US to start).  It certainly is far less than what most people think of when they think of the telescopes used to discover comets and get them named after you.

Imaging & Viewing Comet Lovejoy

I had a lot of fun imaging Comet Lovejoy for two hours on one telescope while I observed it in two other telescopes (my 127mm refactor and the college’s 16″ SCT). With a reasonable quality eyepiece it was pretty easy to see and with a high quality eyepiece that helped to increase the contrast the tail just jumped out at you. When I was viewing it the comet was roughly magnitude 4 making the nucleus (the central region) a naked eye object from a dark site and easily observable with binoculars. When looking at the area it looks like just another star until you really stare and see that it is a little fuzzier than the other stars around it. Once you lift your binoculars up however, it becomes quite obvious. These are the objects that are great fun to take the kids out to see or to go knock on your neighbor’s house and let them have a look. Nothing builds interest in astronomy like this! I can only hope that the next comet to pass earth is this spectacular.

More information and images of Comet Lovejoy C/2014 Q2 can be found on Wikipedia.

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Orion 2″ Multiple Filter Wheel review

I have been using this Orion 2″ multiple filter wheel available from Amazon for quite some time and it never really dawned on me to write a review of it. I suppose I never thought of it because it is just an accessory that happens to sit between the camera and the telescope and since it never gave me any serious problems making it just vanish from my thoughts. Well, not so much.

Orion 2" multiple filter wheel

The Orion 2″ multiple filter wheel does a lot; it houses four filters (Light Pollution, Ha, O3 and S2), it has my HOTECH SCA 2 Inch Field Flattener for Refractor Telescopes attached, and it supports my camera on the end of everything. Every image I shoot (except for spectra) is shot through this attachment regardless of which camera I use. I would say that makes it pretty darn important!

The filters I use in here include the Baader Planetarium 2″ Moon and Skyglow Filter, Baader Planetarium H-Alpha (7nm) CCD Filter, Baader Planetarium 8.5nm OIII CCD Filter, 2″ and Baader Planetarium 8nm SII CCD Filter, 2″.

I picked this particular wheel because I was ordering a lot of stuff from Orion anyway so it made sense to get it from them.I chose the four position so that I could have all four filters I knew I eventually wanted mounted inside.

The 2″ was perfect because when using a DSLR or full frame CCD you can get some pretty significant vignetting of the image with smaller filters, and besides, my focuser was 2″ already so it just worked.

I decided on manual for two reasons; it is far cheaper and I really did not see the advantage of the automated one when I will be right there and can change filter any time I need to in a second or two. Yeah, I am starting to wonder how nice it would be to just program in the sequence I want and let everything happen but I am already lazy enough, I don’t really need any help.

There are a few minor complaints however. My biggest being that there is nothing to tell you which way to turn the wheel, and it will turn either direction. Did I just go from filter 1 (the LP filter) to filter 2 (HA), or to filter 4 (S2)? I solved that problem by cutting out an arrow from a fairly stiff glow in the dark tape I purchased to wrap around my tripod legs.

The side of the filter wheelAs you can see in the above image the arrow is really easy to see, even when it is no longer glowing. If the wheel is in a weird position and you can’t really see the arrow, the tape is thick enough you can easily feel where it is pointing. I would have really liked it if they made it where the wheel only turned one direction.

The filter number window My second complaint with this filter wheel can be seen in the image above. I am not sure who thought it would be a good idea to have a small silver number in a little window but they were wrong. When the telescope is in a position where the wheel is near the ground facing down, and it is freezing cold so you are wearing a ton of clothing, you can not see anything but a blur. I have had to take a picture with the flash on my phone and then zoom in on that image to figure out what the number said. Sure, if you know you started on filter 1, and you know you moved two detents over to filter 3 you should be fine. This assumes no one is talking to you, pats you on the back because they are leaving, or a rustling in the bushes behind you startles you. Trust me, it has happened more than once.

Inside the Orion 2" multiple filter wheel

Other than those little complaints the filter wheel is very well built. This is not some little flimsy piece of plastic but is solid and fairly substantial metal. The interior is well built and holds everything as it should. It has always worked as advertised and without hesitation. As I get older I find myself wondering what an electronically controllable motorized filter wheel would be like but other than that I have no problem recommending this wheel to anyone.

Yes, it is a little expensive for a manual filter wheel (around $200 as I recall), but that money is spent in this case on a well built piece of metal that if cared for even a little bit should last you pretty much forever. Remember, you can help support this blog by purchasing items through links on it such as this Orion 2″ multiple filter wheel available from Amazon.

Thanks for reading my review of the Orion filter wheel!

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