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| Messier 3. Click image for full size. |
The way things are going in terms of the weather I may not be able to resume my imaging Messier marathon project until the Wisconsin Observer's Weekend in early June.
Monday, May 20, 2013
Messier 3 from April
One advantage of long periods of cloudy weather is the time it gives you to catch up on processing. I've had a shot of M3 sitting around for almost a month. I never processed it because it was an "end of the evening" afterthought. Low priority compared to getting my new planetary imager. Now that that's settled I took a look at it today. My first pass at processing it left it too yellow, and my in my second attempt I may have overcompensated.
Image details are here.
The way things are going in terms of the weather I may not be able to resume my imaging Messier marathon project until the Wisconsin Observer's Weekend in early June.
The way things are going in terms of the weather I may not be able to resume my imaging Messier marathon project until the Wisconsin Observer's Weekend in early June.
Wednesday, May 15, 2013
Third Light with PGR Chameleon: Saturn
First light (5/12): I was using FlyCapture software distributed by Point Grey. The software behaved terribly, crashing repeatedly, but PG support was great in helping me to get the camera online. No .avi files were worth keeping.
Second light (5/13): Now using FireCapture software, which took some work to get going. The creator of the software is an active member of the FireCapture Yahoo group, and he responded to a question I posted almost immediately. Amazing support from both PG and FireCapture. Unfortunately, the night was really windy and the seeing was terrible. No .avi files worth keeping.
Third light (5/14--yes, three semi-clear nights in a row!): Everything was rolling, and the night was a little less windy. Seeing was still not at all good, but I did take some videos. Here's the "first light" f/20 image with my C925:
Awfully fuzzy, isn't it. And those rings look pretty shabby. Oh well, I'll blame the seeing.
And so another learning curve begins. I have a lot to learn about using FireCapture to optimize my imaging. So far Firecapture is far better than FlyCapture, Both in terms of stability and utility.
Second light (5/13): Now using FireCapture software, which took some work to get going. The creator of the software is an active member of the FireCapture Yahoo group, and he responded to a question I posted almost immediately. Amazing support from both PG and FireCapture. Unfortunately, the night was really windy and the seeing was terrible. No .avi files worth keeping.
Third light (5/14--yes, three semi-clear nights in a row!): Everything was rolling, and the night was a little less windy. Seeing was still not at all good, but I did take some videos. Here's the "first light" f/20 image with my C925:
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| Saturn, the planet with those big bright rings |
And so another learning curve begins. I have a lot to learn about using FireCapture to optimize my imaging. So far Firecapture is far better than FlyCapture, Both in terms of stability and utility.
Thursday, May 9, 2013
Point Grey Chameleon ready for action (clear skies?)
A long time between updates, but the weather has been incredibly awful. Clouds, Clouds, snow, Clouds.
But it was good for indoor activities like learning how to sell on eBay. My comic book sold for $300, and that along with other sales got me up to what I needed to get the Chameleon, a GSO Crayford focuser for my C925, and a nosepiece so that the Chameleon could use the focuser.
Getting the new camera to work was a brief bit of effort.The camera works well with FlyCapture software, also available from Point Grey. I made a mistake of downloading the latest beta of FlyCap, and it acted like a beta version: no video. As soon as I replaced that with the latest stable version everything worked great. The Crayford will give me zero image shift as I focus. Shift was obviously going to be a problem; the few times I tried video at f/20 it was hard to keep the target in the field of view.
Everything about Point Grey was superb. The camera was shipped via Fed Ex box in box in box; it works great, and when I wrote them asking for advice about the missing video their response was fast and professional.
Last night was the first one with the Chameleon under the stars. FlyCapture software is less than adequate, as it crashed repeatedly, and the GUI is clunky (in my opinion). I'm going to start searching for alternatives.
But it was good for indoor activities like learning how to sell on eBay. My comic book sold for $300, and that along with other sales got me up to what I needed to get the Chameleon, a GSO Crayford focuser for my C925, and a nosepiece so that the Chameleon could use the focuser.
Getting the new camera to work was a brief bit of effort.
Everything about Point Grey was superb. The camera was shipped via Fed Ex box in box in box; it works great, and when I wrote them asking for advice about the missing video their response was fast and professional.
Last night was the first one with the Chameleon under the stars. FlyCapture software is less than adequate, as it crashed repeatedly, and the GUI is clunky (in my opinion). I'm going to start searching for alternatives.
Monday, March 25, 2013
New camera deferred
Last time I wrote about I comic book that I intended to sell in order to raise some cash for a new video camera:
Silly me. The grade for the comic came in today, and it was lousy. If I'm lucky I might be able to sell it for enough to pay for the grading and shipping.I submitted it as worth between $300 and $1000.I should have my estimate within the week.
Wednesday, March 20, 2013
Comic Relief
Since I purchased a Celestron 9.25" SCT I've been thinking about upgrading from my ToUCam to something a little better. Right now I have my sights on a Point Grey Chameleon. The Chameleon is based on the Sony ICX445 and offers small pixel size, USB 2 compatibility, and a low price. It's frame rate is a little low at 18fps, but that's certainly acceptable. Reviews have been solidly positive.
Yes, there are GigE, firewire, and USB 3 alternatives that could be made to work with my laptop, but those models tend to be higher in price.
But even at $375 (+$20 or so for a 1 1/4" nosepiece), where will the money come from?
One source is an old comic book I have. I'm not a collector, but by coincidence I have one comic that is possibly worth some money--as much as $600, I hope. Not knowing a thing about comics, I have decided to get it professionally graded.
I shipped it to CGC Comics at the end of February. They will grade its condition so that a potential buyer can have confidence in what they are getting, seal it into a container and ship it back. It's been sitting in their queue since March 1, and today was scheduled for grading. CGC charges for this service on the basis of the comic's estimated worth. If the comic is thought to be worth more than it was submitted as, they will increase the initial grading charge. I submitted it as worth between $300 and $1000. I would love to hear back from them that they plan to charge more!
I should have my estimate within the week.
Yes, there are GigE, firewire, and USB 3 alternatives that could be made to work with my laptop, but those models tend to be higher in price.
But even at $375 (+$20 or so for a 1 1/4" nosepiece), where will the money come from?
One source is an old comic book I have. I'm not a collector, but by coincidence I have one comic that is possibly worth some money--as much as $600, I hope. Not knowing a thing about comics, I have decided to get it professionally graded.
I shipped it to CGC Comics at the end of February. They will grade its condition so that a potential buyer can have confidence in what they are getting, seal it into a container and ship it back. It's been sitting in their queue since March 1, and today was scheduled for grading. CGC charges for this service on the basis of the comic's estimated worth. If the comic is thought to be worth more than it was submitted as, they will increase the initial grading charge. I submitted it as worth between $300 and $1000. I would love to hear back from them that they plan to charge more!
I should have my estimate within the week.
Wednesday, February 20, 2013
Pixels, Levels and Curves--Oh My!
(I've slowly come to a sort of useful understanding of how some basic tools of image processing work. This is an attempt to put into words those concepts that are elusive to me. I hope these little essays will help a few people starting out with imaging processing. It's meant to be purely introductory)
Your imaging sensor (whether DSLR or CCD) is a digital
device based on discrete bits of information that are either on or off. Most of
what it does is an exercise in counting, not measuring.
Sensor pixels act like little buckets, and photons of light as electrons (imagine them as tiny marbles, if you want). For simplicity let’s assume that a pixel can hold 65,536 electrons. (This capacity will vary from sensor to sensor.) In binary notation, the number of electrons in this pixel can be unambiguously given by a positive 16-bit integer. (In this case we say the bucket has a 16-bit depth.)
We define black as the case where there are zero electrons in the bucket; white is when the pixel is filled to capacity.
Electrons are added to the pixel through a number of effects. Let’s consider the primary cause: Photons coming from our target object that interact with pixel to liberate an electron.
Imagine two side-by-side pixels, both getting light from a telescope pointed at an object of uniform brightness. You might reasonably assume that both pixels will fill with electrons at the same rate, and after letting a certain amount of time to pass, both pixels would have the same number of electrons in them. You would be wrong! Let’s do a little thought experiment to find out why.
Take four typical coins and flip them. Count the number of them that land heads-up and write that down for the contents of Pixel 1. Gather up the coins and flip them again, writing the new number of heads as the contents of Pixel 2. Do this nine more times, adding the new numbers of heads to either Pixel 1 or 2 as appropriate. Knowing that the chance of a coin landing head-up is 50% it’s reasonable to assume that you should have 20 heads in each tally when you’re done. Chances are you don’t, though. In fact, it’s quite likely that the two sums aren’t equal. Why? Because a collection of four coins has other possible outcomes than landing with two heads and two tails showing. The effect of these other possibilities is to change the “perfect” outcome of exactly 20 heads into a distribution peaked near 20 heads, but also having other values.
Light entering a pixel resembles this example. During a given time interval we expect a certain number to enter and produce electrons. But during each interval of time a number of different counts can possibly occur, and by the end we don’t always have the exact amounts suggested by chance.
Now imagine a field of pixels, illuminated by light coming from a uniform source (perhaps an electroluminescent flat panel). We allow light to enter the pixels until they’re about half full and then close the shutter. Do all the pixels have the same exact number of electrons in them, providing an image of uniform intensity? No, they don’t. Some have more electrons, some less, in a very random way that results in intensity that is not uniform. So the source of uniform illumination is not imaged as uniform; instead it appears a little gritty. This grittiness is called shot noise.
Another process that introduces shot noise is the spontaneous addition of electrons to pixels by dark current. Dark current results from thermal activity in your sensor that occasionally kicks an electron into a pixel bucket. The warmer your sensor is, the faster this process works. Because dark current has nothing to do with the object you’re imaging, it’s something we want to minimize. The obvious way to do this is to keep the sensor as cool as possible.
There is another way. If we could take an image in which the electrons come only from the dark current, that image would represent a sort of dark image that could be subtracted from our image of the target object. This is what shooting dark frames is all about. Dark frames are images made with the shutter closed and are essentially images of dark current. Subtracting dark frames from images made with the shutter open (light frames) goes a long way toward removing the noise present. In order to match the amount of dark current present in light frames, the dark and light frames must be made with the sensor at the same temperature and have the same exposure time.
But wait, there’s more! Noise can be generated by random processes in the sensor and its electronics, and more noise can be created during the reading and reporting of pixel counts. These can be lumped together as read noise. Read noise doesn’t depend on exposure time, and is probably not very sensitive to temperature. As for dark current noise, we can image the read noise by making a very short exposure with the shutter closed. We keep it short to minimize the contribution of dark current. These images of read noise are called bias frames.
Conveniently, every dark frame image also contains the bias image, so when dark images are subtracted from light images, the bias image gets removed, too. When you can rely on the light and dark frame temperatures being the same, bias frames are not needed. Many people apply bias frames regardless.
One more bit of terminology: The application of dark and bias frames to light frames is called calibration. Calibration can also involve flat frames and flat dark frames. We can leave discussion of those to another time.
Ding-dong! The Noisy Witch is dead? Sorry, but it’s impossible to remove her from the picture completely. Using multiple dark and bias frames to better generate the dark and bias images does help to keep her down, though. And she definitely resents increases in total exposure time.
Coming up...
Part II, In which we see that the road to the land of imaging Ahs is a Grayish-Brick Histrogram
Part III, In which we learn that even without a brain we can use Levels
Part IV, In which we find that that the heart of the matter is Curves
Other news... Our astronomy club has decided to go ahead with an Imaging Messier Marathon on April 5. Should be fun!
Pixels, Levels and Curves, Oh My!
Part I
In which a house made of light, dark and bias frames falls on the Wicked Witch of Noise.
Sensor pixels act like little buckets, and photons of light as electrons (imagine them as tiny marbles, if you want). For simplicity let’s assume that a pixel can hold 65,536 electrons. (This capacity will vary from sensor to sensor.) In binary notation, the number of electrons in this pixel can be unambiguously given by a positive 16-bit integer. (In this case we say the bucket has a 16-bit depth.)
We define black as the case where there are zero electrons in the bucket; white is when the pixel is filled to capacity.
Electrons are added to the pixel through a number of effects. Let’s consider the primary cause: Photons coming from our target object that interact with pixel to liberate an electron.
Imagine two side-by-side pixels, both getting light from a telescope pointed at an object of uniform brightness. You might reasonably assume that both pixels will fill with electrons at the same rate, and after letting a certain amount of time to pass, both pixels would have the same number of electrons in them. You would be wrong! Let’s do a little thought experiment to find out why.
Take four typical coins and flip them. Count the number of them that land heads-up and write that down for the contents of Pixel 1. Gather up the coins and flip them again, writing the new number of heads as the contents of Pixel 2. Do this nine more times, adding the new numbers of heads to either Pixel 1 or 2 as appropriate. Knowing that the chance of a coin landing head-up is 50% it’s reasonable to assume that you should have 20 heads in each tally when you’re done. Chances are you don’t, though. In fact, it’s quite likely that the two sums aren’t equal. Why? Because a collection of four coins has other possible outcomes than landing with two heads and two tails showing. The effect of these other possibilities is to change the “perfect” outcome of exactly 20 heads into a distribution peaked near 20 heads, but also having other values.
Light entering a pixel resembles this example. During a given time interval we expect a certain number to enter and produce electrons. But during each interval of time a number of different counts can possibly occur, and by the end we don’t always have the exact amounts suggested by chance.
Now imagine a field of pixels, illuminated by light coming from a uniform source (perhaps an electroluminescent flat panel). We allow light to enter the pixels until they’re about half full and then close the shutter. Do all the pixels have the same exact number of electrons in them, providing an image of uniform intensity? No, they don’t. Some have more electrons, some less, in a very random way that results in intensity that is not uniform. So the source of uniform illumination is not imaged as uniform; instead it appears a little gritty. This grittiness is called shot noise.
Another process that introduces shot noise is the spontaneous addition of electrons to pixels by dark current. Dark current results from thermal activity in your sensor that occasionally kicks an electron into a pixel bucket. The warmer your sensor is, the faster this process works. Because dark current has nothing to do with the object you’re imaging, it’s something we want to minimize. The obvious way to do this is to keep the sensor as cool as possible.
There is another way. If we could take an image in which the electrons come only from the dark current, that image would represent a sort of dark image that could be subtracted from our image of the target object. This is what shooting dark frames is all about. Dark frames are images made with the shutter closed and are essentially images of dark current. Subtracting dark frames from images made with the shutter open (light frames) goes a long way toward removing the noise present. In order to match the amount of dark current present in light frames, the dark and light frames must be made with the sensor at the same temperature and have the same exposure time.
But wait, there’s more! Noise can be generated by random processes in the sensor and its electronics, and more noise can be created during the reading and reporting of pixel counts. These can be lumped together as read noise. Read noise doesn’t depend on exposure time, and is probably not very sensitive to temperature. As for dark current noise, we can image the read noise by making a very short exposure with the shutter closed. We keep it short to minimize the contribution of dark current. These images of read noise are called bias frames.
Conveniently, every dark frame image also contains the bias image, so when dark images are subtracted from light images, the bias image gets removed, too. When you can rely on the light and dark frame temperatures being the same, bias frames are not needed. Many people apply bias frames regardless.
One more bit of terminology: The application of dark and bias frames to light frames is called calibration. Calibration can also involve flat frames and flat dark frames. We can leave discussion of those to another time.
Ding-dong! The Noisy Witch is dead? Sorry, but it’s impossible to remove her from the picture completely. Using multiple dark and bias frames to better generate the dark and bias images does help to keep her down, though. And she definitely resents increases in total exposure time.
Coming up...
Part II, In which we see that the road to the land of imaging Ahs is a Grayish-Brick Histrogram
Part III, In which we learn that even without a brain we can use Levels
Part IV, In which we find that that the heart of the matter is Curves
Other news... Our astronomy club has decided to go ahead with an Imaging Messier Marathon on April 5. Should be fun!
Tuesday, February 19, 2013
Imaging Messier Marathon list PDF
Here's a PDF I created that is adapted from Don Machholz's Go-To list in his Observing Guide to the Messier Marathon. I have added the multi-object image opportunities for those using a medium-size sensor and 700mm focal length imaging system. Have a suggested change or find an error? Leave a comment and let me know. Here's what a portion of it looks like:
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| A portion of the IMM list PDF. |
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