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Thread: CCD Resolution of Astrophotography One Shot Cameras (OSC) vs Monochromatic Cameras?

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    While megapixels are useful in comparing DSLR cameras, they are very misleading when comparing CCD cameras. For example, consider the two cameras you are comparing:

    Starshoot Pro color: pixel size = 7.8 um array size = 3032 x 2016 (6MP)
    Starshoot Deep Space Mono: pixel size = 6.45um array size = 1392 x 1040

    The mono camera actually has the HIGHER resolution (6.45 um vs 7.8 um). When you add the fact that mono cameras in general have 4 times more resolution per mm than a color camera (because only some of the color pixels are used depending on the wavelength compared to all of the mono pixels being used), the Orion mono will have even better resolution than the Orion OSC.

    In addition, you can not do narrowband imaging (Ha) with a OSC.

    Further you have to consider the array size. The Orion OSC is very large chip (thus 6MB). But large chips require short focal lengths or strong focal reducers to get the field onto the chip. Coma & vignetting become very real problems with large chips. Most mono chips are smaller (thus lower MP's) to avoid coma & vignetting problems while giving you greater resolutions.

    While mono cameras require filter wheels & color filters, their potential (narrowband imaging, etc) are unlimited. OSC while very convenient with low learning curves are much more limited. Without starting an arguement, an OSC is like a Polaroid Camera -- easy, fast, simple. While a Mono CCD is like a SLR with interchangeble screens and lens -- harder to learn, as complex as you can get, but unlimited in potential.

    If you look at the great astroimages posted by the experts (and good amateurs), you will be hard pressed to find many taken with a OSC. Almost all are taken with Mono + filters.

    Hope this helps,

    Jim
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  3. #12
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    Okay, now I'm confused. Are you saying pixel density is the determining factor of resolution? And even if thenreesolution is high does that mean the image size will still be small?

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    The determining factor of resolution is much more complicated than I implied in my general statements above. Resolution is dependent on at least 3 factors. Probably the most important factor is the actual resolving power of your specific optics, since any camera can only produce a resolution that the optics permit. (The camera can't resolve a detail that the scope can't "see"). Thus the first step is to match a camera's ability to your particular scope optics.

    The next most important factor is pixel size. ASSUMING that your scope can handle high resolution, the smaller the pixel size in microns (um), the greater the resolving power of the camera. BUT, small pixel size is wasted if your optics can not resolve to that smaller pixel size. See above (primary resolving factor).

    The final resolving factor is atmospheric/seeing conditions. Just as you can get the best collimnation on clear stable nights (can more easily see the "airy disc), you will get the best resolution on clear stable nights...

    I don't mean to make this rocket science, but in a way it is... It is easy to use MP amount when comparing DSLR cameras. All DSLR cameras have the same chip size, so the higher the MP, the smaller the pixel size and thus the greater resolution. Since CCD camera chip sizes vary, you have to look at the pixel size.

    But a simple comparison of pixel sizes is not enough. It also depends on the resolving power of the scope's optics...

    You are right -- this can get to be very confusing...

    Hope I have not added to the confusion.

    Jim
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    Okay, I think this is starting to make sense. So even though the resolving ability of the mono camera is greater the image size will be much less?
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    So even though the resolving ability of the mono camera is greater the image size will be much less?
    Image size... This may get confusing to explain also. Naturally the total image size will be smaller for the mono camera since its CCD is smaller. BUT the size of the object may be the same size as the larger OSC chip, yet have better resolution.

    May be easier to explain using the DSLR example. Assume the DSLR has a 35mm equivalent sensor. Assume that the object you are imaging is a galaxy that takes up the center third of the sensor (12 mm). Take a snapshot. The resulting image will have the galaxy taking up the center third of the picture (12 mm), with stars and dark sky making up the other two-thirds (12 mm to either side of the galaxy) in the picture. Label this the OSC image.

    Now let your imagination run with me... Suppose you could magically remove the 35 mm sensor in the DSLR and replace it with a smaller sensor (say 12mm) WITHOUT moving the camera off the scope. Now your galaxy which is still centered takes up the whole sensor. Take a snapshot. The resulting image will have the galaxy taking up the entire picture (1/3 of 35mm = 12mm) with no stars and deep sky on either side of the galaxy. Label this the MONO image.

    Putting these two images side-by-side the OSC image will be 3 times larger than the Mono (35mm vs 12mm). BUT THE GALAXY ITSELF WILL BE THE SAME SIZE IN BOTH IMAGES. In addition, the Mono image will show much more detail (better resolution).

    So the question then becomes "which image is larger" depends on whether you are talking about the ENTIRE image or the OBJECT in the image...

    Hope I have not added to the confusion...

    Jim
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    That concept I understand. My question was relating to the size of the image itself, not the subject. Because there are less pixels to actually display with the monochrome camera this means the image size is small but the resolution is high. Is this correct? And how much can drizzling help increase the size of the 1MP image? Do you know of any high MP cameras around $1,000-1,500? Thank you for all of your help so far!
    Last edited by Mothywood; 05-17-2011 at 09:24 PM.
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    My question was relating to the size of the image itself, not the subject. Because there are less pixels to actually display with the monochrome camera this means the image size is small but the resolution is high. Is this correct?
    YES, but my point was that it is the size of the subject, not the size of the total image that is important in most cases. A galaxy in a large image will make up only a fraction of the image, whereas the same galaxy in a smaller image will make up the majority of the image. The galaxy in both images will be the same size, but because the mono uses smaller pixels (and more of them), the mono image will contain more details per square mm's than the OSC image of the galaxy.

    It is the galaxy that is the object of imaging, not a large section of sky with a small fuzzy galaxy somewhere on the image.

    This is were the issue of what really is important comes down to. It all depends on what you are imaging. Large sections of sky, or specific subjects in that area of the sky...

    On a related side-note, I am including a link to a piece of free software by Rod Wodaski called CCD Calculator. The program lets you enter your scope size, (including focal reducers), then choose different cameras and see the resulting FOV of the images that camera would take. These FOV's are super-imposed over popular images (M16, M20, etc.) to give you a real good feel for any particular camera/scope combination.

    The Orion Star Shooter is included in the list of cameras, but the real strength of the program is the ability to enter any cameras pixel size and array size, then save it under a new camera name (ie. the software never becomes outdated because you can enter any new camera into the listing).

    Here is the link:
    The New CCD Astronomy Home Page

    Take a look at the program. Download it (along with the larger image library). Play with it to get an idea of what any camera you are looking at can actually do in the real world. Worth the download.

    Hope this helps...

    Jim
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    Wow, that's really helpful, thanks! The problem with 1MP is that I cannot print anything larger than a postcard. It won't even fill my computer monitor. How much will drizzling help? I've never used it before.
    Mounts: CGX, CGEM
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    For the reasons akjudge=Jim mentioned above, I went mono with my new set-up. I posted this link on another thread but will also post it here... CCD University. The links on the L/H nav pane lead to other great pages. I think I was a bit confused by your terminology but the other Jim did a great job of explaining it.

    Your questions about debayering in OSC - here is a link that talks about the debayer methods, you may already be familiar with.

    As far as the Drizzle method - are you talking about as imposed by Maxim or by Deep Sky Stacker or is there another platform you are using? I've only tried in DSS and found in some cases I had what seemed to me to be more artifacts? But I'm not sure if I'm missing the point here. Many image processing programs like PhotoShop and Paint Shop Pro allow resizing of an image to just make it larger. I've in the past done this during intermediate post processing steps(often after sepirating color panes into component R/G/B from the original OSC), done my work, and the 'sized down' again - in my case to original resolution. IMO the results are mixed and depending on the target, I am not certain want results is actuually something in the image or created from it? I say that as I look at mono R/G/B images of same and they are different. I feel the mono R/G/B to be a higher resolution and more true represntation. But gain as mentioned above there is a lot to do with the original aperture of the imaging scope and the tru image mapping at that resolution onto the CCD chip.

    It brings another question to my mind, and that is as the other Jim mentioned, the idea of arcsecond per pixel (and your seeing limitations) and the sampling you are doing of that with any CCD - there are built in limitations. Keeping the CCD size the same you'll get a much higher resolution image of M1 in an 8" refractor than you will in a 4" refractor. IME no amount of interpolation or dithering or drizzle will compensate for the difference in aperture of the imaging scope - and in every case an monochrome original wether it was done R/G/B or Ha/OII/SII is sharper and IMO less artifacts.

    Here are some OSC images I did 'using all the tricks' with an Orion SSPv2. The only difference in these images was the final color balancing of the final post process image data. These took many hours of post processing, different to go for different details - these were done with my ~6"apo. You can compare to others you see out there.

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    crop_cwazee_M1_txt.jpg

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    klaatu2u adds some great clarity to the discussion. Thanks..

    As for Drizzle method -- my understanding is that it is a method to help remove artifacts from an image run. Basically the scope is moved 1 pixel from the previous position. This is like using a Gaussian Blur.

    My point is that Drizzle does not result in a bigger image, but rather in an image with less noise because of the movement between images (blurring).

    I could be on shaky ground (or dead wrong) so someone please correct me if needed.

    Jim
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