Hmmmm? So you are saying because the pixels are wider they are collecting more flux or photons per unit time directly proportional to the F ratio? Seems too simplified to me ...Ray?
Mike
I'm with Dave on this. The two geometrical/optical characteristics that matter for sensitivity are the aperture (determines how many photons get into the scope) and the angular size of the pixels (determines how many of the available photons get into each pixel).
As Dave points out, a 12 inch f8 scope with 9 micron pixels will be optically indistinguishable from a 12 inch f4 scope with 4.5 micron pixels, all else being equal, since they have identical apertures and pixel angular sizes.
I'm with Dave on this. The two things that matter for sensitivity are the aperture (determines how many photons get into the scope) and the angular size of the pixels (determines how many of the available photons get into each pixel).
As Dave points out, a 12 inch f8 scope with 9 micron pixels will be optically indistinguishable from a 12 inch f4 scope with 4.5 micron pixels, all else being equal, since they have identical apertures and pixel angular sizes.
Hmm?...ok, fair enough I guess (all else being equal)
That's true, I know, but we have to consider the whole chip area, I thought... in the same chip area smaller pixels are double than biggest one so the qe lost by one could be gained by next, I guess...right?
That's true, I know, but we have to consider the whole chip area, I thought... in the same chip area smaller pixels are double than biggest one so the qe lost by one could be gained by next, I guess...right?
You are correct Elio in that the same chip area will collect the same signal irrespective of the pixel size, but in the scenario where one camera has pixels double the size of the other camera, the resultant image will then be viewed at half of the size of the other (assuming the same output display device / pixel size is used!). So, the same SNR (neglecting read noise / etc) of the object is being represented at half the output display size. So ... speed has been increased at the expense of detail which has been convolved (lost).
Quote:
Originally Posted by strongmanmike
Hmm?...ok, fair enough I guess (all else being equal)
Mike
"all else being equal" - yes of course camera read noise, QE, etc will have some impact, and the analogy in the old film days would be film speed, hypering, etc. The difference being that film was a consumable item in those days and when people used f-ratios as a benchmark to compare the relative speed it was understood that film speed also had an impact, as well as optical efficiency (eg. size of central obstruction, etc), etc. Nowadays the CCD is an expensive and integral part of most people's systems (certainly not a consumable like film!), so ... you are right in that I was musing about a simple way to represent the relative "speeds" of systems out there by taking the pixel size into account as well
I probably confused things though with my first clumsy attempt at adjusting the f-ratio (which was incorrect - too late at night for such musings ).
OK, probably enough musing for a Sunday night
Last edited by David Fitz-Henr; 15-06-2014 at 09:37 PM.
Reason: Correction
I'm sorry David but I can't find correspondance in what you said... looking at hystograms of two different shots taken with my taka fsq85 @f5.3, one using an eos 1000D (pixel size 5,7) and one using eos 60D (pixel size 4.3), i should find the first much more illuminated than the second, but it doesn't...
I'm sorry David but I can't find correspondance in what you said... looking at hystograms of two different shots taken with my taka fsq85 @f5.3, one using an eos 1000D (pixel size 5,7) and one using eos 60D (pixel size 4.3), i should find the first much more illuminated than the second, but it doesn't...
Hello Elio, the histograms show different profiles and appear to be for different areas of the sky, and are also from cameras that may have different QE's, etc. Remember that my comments are qualified by "all other things being equal", which does not appear to be the case here.
To further clarify my musings though, I was not attempting to fully quantify the difference in imaging systems, such that one can take two similar systems and give an absolute ranking of speed between them. Rather, I was pointing out that when qualitatively discussing different systems' speeds (eg. this is a "fast" system, this is a "slow" system), that one needs to also include the pixel size in the equation as an important part of the comparison (in addition to aperture / f-ratio which is often used alone). In this way, we have at least covered all the major optical/geometrical properties of the imaging system; of course, if we include quantum / electrical factors (such as QE, cooling, read-out noise, etc) we will get an even better comparison, but this tends to be impractical in forum discussions so people will generally factor in the advantages of various cameras (eg. OSC / DSLR / CCD) based on their knowledge of the camera used.
As you mentioned English is not your first language, so you may have taken me too literally on some of my musings; You are correct, and as I have said, that other factors (eg. QE, cooling, read noise) will have an impact as well; this can of course lead to one system with slightly larger pixels being slower than a similar system with smaller pixels as you suggest.
Nice one! Great to get 3 in one night! I'd be lucky to get a shot of the moon at the moment with all the cloud!
That Ghost of Jupiter in really nice, it's an awesome colour, good amount of detail in it too
Erik
Thanks Erik
Lately the weather, moon and life have meant that I haven’t managed to get out as often as I would like to.… so, I have made the most of the limited time by utilising one of the major benefits of this scope and that is its aperture and speed, both of which contribute to getting decent images in a single night
In the end astroimaging is meant to be enjoyable and sometimes I just want an image with minimal fuss, not all images have to be ball breakers
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