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Kolenka
Pooh-Bah
Reged: 06/01/08
Posts: 1009
Loc: Seattle Area, WA, USA
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This is a continuation of a side-topic started in this thread...
Quote:
Let us take a situation.
4" f/4 scope vs 8" f/8 scope. The 8" has 4x the light grasp or gathers 4x more light than the 4", but the image (object) is 16x larger. Because the object is 16x larger in area, it takes 16x more photons to bring the exposure to the same brightness on the chip, because the light is distributed over the area that is 16x greater. So, even though the 8" has 4x the light gathering, the exposure still requires 4x longer to achieve the same exposure.
Now, if the scope is an 8" f/4 vs 8" f/8, the image is 4x larger in area, so it will require 4x longer to expose the object due to the larger area of the f/8.
Tell me what is incorrect about this statement?
Blueman
The problem here with the statement is that you are discussing brightness in the exposure. But when dealing with AP and a CCD, we actually don't care about the real brightness of the exposure. We stretch it anyways. What we care about is the SNR and the dynamic range.
While brightness scaling is linear, SNR isn't. Not once you get the signal well above the read noise. Once you hit a certain exposure time, the characteristics of the camera, the subject, and the skyglow take over and determine your SNR. Until you reach that point, you will see a fairly linear effect on SNR. But assuming you are above the read noise, SNR is not linear with exposure time, unlike brightness.
EDIT: Craig's latest article here on Cloudy Nights discussing SNR provides a pretty good example that SNR and brightness are actually different beasts.
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Orion XX12 / Orion 80ED OTA / AT66ED
Nagler 7T6, 9T6, 13T6, 17T4, 26T5
Canon XS, TIS DMK 31AF03, AstroTrac TT320X
Northwest Astro Photoblog
Edited by Kolenka (05/05/09 03:25 AM)
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tiggere
super member
Reged: 02/22/08
Posts: 145
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I guess I'm guilty of the same thinking as blueman...I was under the assumption that from F4 to F8 is two full F-stops so it would take four times as long...as F8 is only seeing 1/4 of the light that F4 is...
The other issue is exactly how much does aperture play a role...lets use a real case scenario...lets assume Mag 7 skies and shooting M51 (I'm gonna get a good shot of that darn thing yet !!)...I am using a 6" SCT @ F6.3 (with the reducer)with 28.27 sq. inches of mirror and was wanting to move to a 8" RC @ F6 (with the reducer) with 50.27 sq. inches of mirror...the F-stops are about equal or close enough for government work anyhow...focal lengths will also be within a 100mm...so will it still take the same time (minutes)for the shot or will the aperture speed things along...and how much...
Now we can compare the 8" AT8IN Newt @ F4 and 800mm FL to the 8" RC at F6 and 1016 (or there abouts) FL...aperture is the same and its just over one F-stop by a hair...not sure the focal length will make a difference...so comparing these two how much longer(in minutes)would it take to get the same shot of M51 under Mag 7 skies...
Lets also use minutes of the sub for the answer instead of 1/3 as long as we are all looking to save time and everything we do is based on that anyhow...
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Celestron C6A-XLT
Atlas EQ_G with EQMOD / HNSKY
Logitech Dual Action Controller
Philips SPC900NC Webcam
Wilmington, NC
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Miguel Lopes
professor emeritus
Reged: 01/04/07
Posts: 694
Loc: Portugal
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tiggere, you've left out the most important thing: the CCD!
That is why the term "fast" is not accurate for CCD's, especially those where you can bin the pixels!
I think the f ratio is important for image scale, optical defects/tolerances and backfocus. Everything else does not apply to the CCD world.
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Astrology is the science for ignorants. Astronomy is the science for those who feel ignorant. - Miguel Lopes
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Nocturnal
Pooh-Bah
Reged: 09/14/05
Posts: 1022
Loc: CT, USA
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Comparing scopes with different focal lengths is difficult and gets misused in some cases. Look at advertisements for the hyperstar for example. It says exposure lengths are vastly reduced at F/2 compared to native F/10. The image scale is completely different as well so it means, IMO, nothing. To push it into the ridiculous you could cram all the light of M42 into 1 pixel and saturate it in a fraction of a second with the right optics. But you can't see any detail. Pointless.
My take on the whole thing is that you decide what image scale you want and then get the largest clear aperture you can afford or carry on your mount. Image scale depends on focal length and pixel size. Clear aperture depends on actual aperture and the central obstruction.
Twice the clear aperture (surface wise) at the same focal length will deliver twice the photons at the same image scale. *That* you can compare. *That's* why you don't see 66mm refractors mounted in those big Keck domes 
DSLRs make things more complicated because of the ISO settings. Still the physics (photons per pixel) are the same. How they translate into ADUs and noise gives me a headache so I won't attempt to understand it. CCDs are easy comparatively.
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Best,
Sander
C11-HyperStar on Atlas EQ-Q driven by EQMOD
William Optics M110 With FR-III/TRF-2008
DS2090 guide scope
QHY-8, DSI-Pro and DSI cameras
watec 802h video camera with KIWI OSD
Astro stuff: http://www.tungstentech.com
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DonR
scholastic sledgehammer
Reged: 11/15/06
Posts: 988
Loc: Georgia, USA
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Consider two telescopes with the same aperture but different focal lengths - an 8" f/5 and an 8" f/10. And consider photographing the same extended object with each, for the same exposure time and with the same camera.
In each case the number of photons reaching the sensor from the extended object is statistically the same, because that depends only on the aperture and exposure time. But with the f/5 telescope those photons cover one quarter of the area (number of pixels) on the sensor, so each pixel covered by the object receives four times as many photons from the object, which means four times the illumination, and thus four times the signal.
Once you get past the read noise, the primary contributor to noise in DSLR and CCD photography is shot noise, which increases proportional to the square root of illumination (NOT exposure time). So the image of the extended object captured by the f/5 telescope has four times the signal and twice the shot noise, compared to the image captured by the f/10 telescope. Therefore the SNR due to shot noise is twice as high in the f/5 telescope compared to the f/10 telescope.
The portion of the image containing the extended object captured by the f/10 telescope has more resolution (four times as many pixels covering the object), but only one quarter of the signal per pixel and one half of the SNR contributed by the major noise source, shot noise.
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Don
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Atlas EQ-G
Orion 8" f/4.9 newtonian
Orion 127mm Mak-Cass
Orion Skyview Pro mount
Orion 80mm guide scope
Canon Digital Rebel XT
Meade DSI
Philips SPC 900 NC webcam
http://www.pbase.com/dtreed/astrophotos
Edited by DonR (05/05/09 09:36 AM)
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tiggere
super member
Reged: 02/22/08
Posts: 145
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Sorry for not mentioning using a DSLR...I followed the link and didn't realise it wasn't in the DSLR forum...
So what we are doing is trading SNR for resolution by going to the higher F ratios with the downside of needing to increasing our shot times with the F10...and by increasing our shot times we are still obtaining our critical SNR numbers...
Here's what I think I'm wanting to know...if we take the 8" F/5 and 8" F/10 examples above using a DSLR with both set at ISO 800 shooting the exact same target under the exact same skies of Mag 7 on the exact same night...how much difference is there going to be on the sub lengths that will need to be shot to get the exact same results...
If the resolution is higher in the F/10 and you have to shoot 8 minute subs to to get to the optimal SNR and you only have to shoot 3 minute subs using the F/5 but have to shoot more of them to get to the same resolution as the F/10 then its 6 of one 1/2 dozen of another...the only advantage is the faster scope won't show guide errors as quick...
Is this correct?
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Celestron C6A-XLT
Atlas EQ_G with EQMOD / HNSKY
Logitech Dual Action Controller
Philips SPC900NC Webcam
Wilmington, NC
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Kolenka
Pooh-Bah
Reged: 06/01/08
Posts: 1009
Loc: Seattle Area, WA, USA
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Quote:
tiggere, you've left out the most important thing: the CCD!
That is why the term "fast" is not accurate for CCD's, especially those where you can bin the pixels!
I think the f ratio is important for image scale, optical defects/tolerances and backfocus. Everything else does not apply to the CCD world.
But that isn't entirely accurate either, and that was what spawned this whole discussion.
Say you have a graph of SNR versus exposure time. You would actually be able to split this graph into two parts, because SNR behaves differently in two regions of the graph. As you extend your time from 0 seconds, the noise is initially dominated by read noise, the slope of the line in this region is primarily determined by the f/ratio, or speed of the scope.
Once most of your signal is well above the read noise such that skyglow is now the dominant noise, you hit an inflection point of sorts, and the characteristics of the camera take over. The curve changes dramatically.
The only difference between an f/4 and f/8 scope of the same design and aperture, is that the f/4 will hit this inflection point sooner (closer to 0 seconds on the graph). Binning on an f/8 helps you reach this inflection point faster as well, but again, the camera takes over once you hit this inflection point.
Nocturnal, DSLRs are a pain to figure out here, partly because of their ISO and their noise. Because the dark current is so variable, DSLRs actually tend to push the inflection point out further compared to a cooled CCD, and will seem to support the idea that f-ratios matter when used. This is what I even encountered myself. Faster f-ratios gave me better DSLR shots, mostly because it was hard to get the signal well above the dark noise when shooting anything but open clusters.
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Orion XX12 / Orion 80ED OTA / AT66ED
Nagler 7T6, 9T6, 13T6, 17T4, 26T5
Canon XS, TIS DMK 31AF03, AstroTrac TT320X
Northwest Astro Photoblog
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lineman_16735
Tak-o-holic
Reged: 12/04/04
Posts: 2604
Loc: Central PA
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Can we at least finally agree that F ratios with CCD imaging are not the same as F ratios with film? The film F stop world just isn't the same as CCD's. An 8" F/8 to F/4 with a ccd is not going to provide you with a 4x better SNR with equal exposure times.
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Chris
A mount from Illinois
A scope from Japan
A camera from Cal-I-Fornia
A dog from Kentucky
A wife and kids from the "Twilight Zone"
The Geek Shed
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lineman_16735
Tak-o-holic
Reged: 12/04/04
Posts: 2604
Loc: Central PA
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Quote:
Now, if the scope is an 8" f/4 vs 8" f/8, the image is 4x larger in area, so it will require 4x longer to expose the object due to the larger area of the f/8.
Tell me what is incorrect about this statement?
Blueman
I really don't understand what you are saying here Floyd?
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Chris
A mount from Illinois
A scope from Japan
A camera from Cal-I-Fornia
A dog from Kentucky
A wife and kids from the "Twilight Zone"
The Geek Shed
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WarrenS
scholastic sledgehammer
Reged: 03/04/08
Posts: 892
Loc: Orange County New York
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I'm with you here on this one Chris. Having shot 35mm film daytime, for over 30 years, an f stop was an f stop, whether you used a 35mm f/2.8 or a 400mm f2.8 lens, at the same f stop the same amount of light hit the film (same iso & shutter of course). One F stop smaller was Half the light (photons or whatever) reaching the film. When I tried film A/P years back, I read that aperture has a definite effect on point light sources, ie stars, but F/Stop is what determined the amount of light hitting from an extended object, ie nebulae and galaxies.
Maybe it's different with CCD's, but until I hear it from an optical physicist or someone like that I'm skeptical.
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Warren
Astro-Tech 127EDT
Celestron Onyx 80ED
Astro-Tech Field Flattener
C8 (circa 1983 Orange Tube)
Atlas EQ-G, Orion SSAG
Canon 135mm F2.8
Canon 40D, Astronomik CLS clip filter
Leica, Minolta binos
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Kolenka
Pooh-Bah
Reged: 06/01/08
Posts: 1009
Loc: Seattle Area, WA, USA
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Quote:
I'm with you here on this one Chris. Having shot 35mm film daytime, for over 30 years, an f stop was an f stop, whether you used a 35mm f/2.8 or a 400mm f2.8 lens, at the same f stop the same amount of light hit the film (same iso & shutter of course). One F stop smaller was Half the light (photons or whatever) reaching the film. When I tried film A/P years back, I read that aperture has a definite effect on point light sources, ie stars, but F/Stop is what determined the amount of light hitting from an extended object, ie nebulae and galaxies.
Maybe it's different with CCD's, but until I hear it from an optical physicist or someone like that I'm skeptical.
The difference is that brightness is considered fixed in film, so you have a sweet spot you need to hit. With CCDs, we can stretch the brightness to fit a range of our choosing, so that is no longer an issue. But because we can stretch it, the SNR and dynamic range become more important, as they now determine how much final noise is left after we've stretched the image, and how much detail we've actually captured.
The response on a CCD also tends to be more linear than film, so exposing longer just increases the brightness after a certain point, and has a non-linear effect on SNR.
Quote:
Can we at least finally agree that F ratios with CCD imaging are not the same as F ratios with film? The film F stop world just isn't the same as CCD's. An 8" F/8 to F/4 with a ccd is not going to provide you with a 4x better SNR with equal exposure times.
I'll agree, I just don't want us to forget the caveat region where the f/4 *will* provide 4x better SNR with equal exposure times (granted we are talking about noisy cameras and/or short exposures for this to be true).
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Orion XX12 / Orion 80ED OTA / AT66ED
Nagler 7T6, 9T6, 13T6, 17T4, 26T5
Canon XS, TIS DMK 31AF03, AstroTrac TT320X
Northwest Astro Photoblog
Edited by Kolenka (05/05/09 12:33 PM)
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DonR
scholastic sledgehammer
Reged: 11/15/06
Posts: 988
Loc: Georgia, USA
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Quote:
Can we at least finally agree that F ratios with CCD imaging are not the same as F ratios with film? The film F stop world just isn't the same as CCD's. An 8" F/8 to F/4 with a ccd is not going to provide you with a 4x better SNR with equal exposure times.
With CCD's and DSLR's, the SNR due to shot noise will be about 2X better with the faster telescope and equal exposure times - 4X more signal, 2X more shot noise.
Film is an entirely different beast once you get past a few seconds of exposure time, due to reciprocity failure. In effect the ratio of signal to exposure time with film is no longer 1:1 because the film sensitivity decreases with exposure.
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Don
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Atlas EQ-G
Orion 8" f/4.9 newtonian
Orion 127mm Mak-Cass
Orion Skyview Pro mount
Orion 80mm guide scope
Canon Digital Rebel XT
Meade DSI
Philips SPC 900 NC webcam
http://www.pbase.com/dtreed/astrophotos
Edited by DonR (05/05/09 01:28 PM)
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DonR
scholastic sledgehammer
Reged: 11/15/06
Posts: 988
Loc: Georgia, USA
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Quote:
Once most of your signal is well above the read noise such that skyglow is now the dominant noise, you hit an inflection point of sorts, and the characteristics of the camera take over. The curve changes dramatically.
Actually sky glow isn't noise, it's signal - but it contains noise (mostly shot noise in longer exposures) and since it is the least exposed part of the frame, the SNR of the sky glow due to shot noise is the poorest SNR in the image.
Once past the read noise, the shot noise becomes the dominant noise and the SNR of all parts of the image (skyglow and subject) continues to increase with exposure time, until the entire sensor is saturated with signal. When the entire frame is white due to every pixel being completely saturated, the SNR is infinitely high, since the entire image is signal without any noise.
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Don
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Atlas EQ-G
Orion 8" f/4.9 newtonian
Orion 127mm Mak-Cass
Orion Skyview Pro mount
Orion 80mm guide scope
Canon Digital Rebel XT
Meade DSI
Philips SPC 900 NC webcam
http://www.pbase.com/dtreed/astrophotos
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Nocturnal
Pooh-Bah
Reged: 09/14/05
Posts: 1022
Loc: CT, USA
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Quote:
Can we at least finally agree that F ratios with CCD imaging are not the same as F ratios with film? The film F stop world just isn't the same as CCD's.
Sorry, I can not agree with that. F ratios are a property of the OTA and have nothing to do with the recording medium. F ratio is focal length divided by aperture.
I think the confusion comes from the adjustable iris in most cameras which yields variable F ratios at a fixed focal length. Add to that adjustable focal length and it gets more complicated.
Luckily we don't deal with that with astronomical imaging. Except in rare cases aperture is set. You can fiddle with focal length with reducers and barlows but that's it. Attaching a different camera does not change the F ratio.
I don't think it makes much sense to talk F stops at all in astronomical imaging as there is no iris but maybe someone can enlighten me.
I think in this case I benefit from having very little traditional photography baggage
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Best,
Sander
C11-HyperStar on Atlas EQ-Q driven by EQMOD
William Optics M110 With FR-III/TRF-2008
DS2090 guide scope
QHY-8, DSI-Pro and DSI cameras
watec 802h video camera with KIWI OSD
Astro stuff: http://www.tungstentech.com
My Astro Photos
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gavinm
scholastic sledgehammer
Reged: 08/26/05
Posts: 804
Loc: Auckland New Zealand
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Quote:
... Maybe it's different with CCD's, but until I hear it from an optical physicist or someone like that I'm skeptical...
You may well be hearing it from physicists, but some other people still think they know better.. 
Skeptical is good BTW. Good not to believe everything you read
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Gavin
Mt Albert Grammar School Observatory
Auckland, New Zealand
http://www.mags.school.nz/astronomy/index.html
12" LX200R F6.8 AP
SBIG ST7-XME + CFW10
Moonlite SCT focuser w/ temp
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+ other stuff
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Arkalius
scholastic sledgehammer
Reged: 08/03/06
Posts: 878
Loc: Orange County, CA, USA
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I'm not sure what the points being made here are...
In order to get the same SNR as a 5 minute exposure at f/5 using an f/10 scope, you'd have to shoot for 20 minutes (4x the exposure). I know it won't be exactly the same because read noise is static and dark current noise is dependent only on exposure time and not aperture, but neither of these noise sources should be significant in a good camera at these exposure lengths.
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-Arkalius
11" Celestron SCT on Orion Atlas EQ-G
Celestron 100ED Refractor
8" Zhumell Dobsonian Reflector
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bbasiaga
professor emeritus
Reged: 05/10/06
Posts: 724
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F stops are still the same. The same histogram with my 11" SCT and hyperstar at F2 an 1 second exposure is achieved in roughly 10s on my F7 refractor. The SCT is about 3 stops faster and about 3 squared times (9) the light gathering leads to about 1/9th the exposure. It so happens the image scale on these two scopes is very similar because my 3.5" F7 refactor is only about 100mm longer (im rounding a bit here) than my SCT with hyperstar. The difference apeture makes is how we get a given focal length at a given F ratio.
The CCD, DSLR, stretching, etc are irrelevant. Its not about any of that, because moving those things between systems (eg my DSLR on the SCT vs Refractor) removes it as a variable. The rest is a system where you can choose two, but have to live with the third. You can get the same histogram (total light gathered) with any system, and stretch it from there.
Choose your image scale (focal length) and exposure time (F ratio, faster = lower) and live with the apeture you get.
Or
Choose your Aperture and image scale, and live with the F ratio (expsoure time) you get.
Etc.
If you want to image at 500mm with 16" scope, all you've got to do is find someone who can build you an F1.2 scope! It'll give you the same histogram as your 66mm F/7.5 apo (also 500mm) in just a fraction of the time.
-Brian
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Vixen 80EDSF
Stellarvue 102ED2
Orion Atlas 11 EQ-G
15" Astrosystems Telekit w/ Discovery Optics
Lust for something Larger
Lust for something Larger than that
Past Lovelies:
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All sold to a good home
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Kolenka
Pooh-Bah
Reged: 06/01/08
Posts: 1009
Loc: Seattle Area, WA, USA
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Quote:
I'm not sure what the points being made here are...
In order to get the same SNR as a 5 minute exposure at f/5 using an f/10 scope, you'd have to shoot for 20 minutes (4x the exposure). I know it won't be exactly the same because read noise is static and dark current noise is dependent only on exposure time and not aperture, but neither of these noise sources should be significant in a good camera at these exposure lengths.
Brightness per pixel != SNR
I'd actually expect the SNR in the 20 minute exposure to be better.
Say signal can be represented by an abstract number that represents the minutes we exposed. Signal we captured from the object at 20 minutes would be 20. Signal we captured from the object at 5 minutes would be 5. Aperture determines how many photons (signal) we capture. Focal length tells us how those photons get spread out.
So if we ignore read noise and dark current for a moment, we are left with shot noise, with the SNR being Signal over sqrt(Signal) to account for the shot noise.
As we expose longer, the SNR goes up, independent of focal length! So assuming we have the same aperture, the 20 minute exposure actually has an SNR of ~4.47 in our imaginary units, while the 5 minute exposure has an SNR of ~2.2 in our imaginary units. 4 times the exposure got us 2 times the SNR.
The caveat here is that read noise and dark noise are not totally insignificant. They are pixel noise that act totally independently of our system, and dark noise scales with exposure time. As skyglow overcomes the dark noise though, they both become less significant again (as the weakest part of our signal is now becoming a larger source of noise than the pixel noise).
But, as we move this back into the realm of numbers pulled off a sensor, we see two regions of performance from the sensor: One where pixel noise is dominant over shot noise and determines your SNR, and thus raw brightness of the pixel is important (which is where f-ratio has its biggest impact). The second region is where shot noise is dominant and determines your SNR.
The point where you leave the first region, and enter the second /is/ controlled by the f-ratio of the scope, since spreading the photons out pushes this point out to longer exposure times. As does large amounts of pixel noise. If a larger portion of the signal is pushed into each well, you overcome the pixel noise faster and move into the second region that much faster. Which both binning and faster f/ratios give you.
We also tend to spend a lot of time in the first region as beginners. The amount of signal we capture is pretty low, and amounts to sitting at a point not very high above the pixel noise (if at all). So we do see similar effects. But it doesn't mean we can extrapolate them out to longer exposures, as you start seeing different effects on SNR as you expose longer, since different forms of noise become dominant.
There is no hard line either, the 'inflection point' is simply the point where shot noise becomes larger than the pixel noise. You can go a bit further than that and still see effects of f-ratio easily, but they become weaker the longer you expose.
--------------------
Orion XX12 / Orion 80ED OTA / AT66ED
Nagler 7T6, 9T6, 13T6, 17T4, 26T5
Canon XS, TIS DMK 31AF03, AstroTrac TT320X
Northwest Astro Photoblog
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Miguel Lopes
professor emeritus
Reged: 01/04/07
Posts: 694
Loc: Portugal
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Quote:
Quote:
tiggere, you've left out the most important thing: the CCD!
That is why the term "fast" is not accurate for CCD's, especially those where you can bin the pixels!
I think the f ratio is important for image scale, optical defects/tolerances and backfocus. Everything else does not apply to the CCD world.
But that isn't entirely accurate either, and that was what spawned this whole discussion.
Say you have a graph of SNR versus exposure time. You would actually be able to split this graph into two parts, because SNR behaves differently in two regions of the graph. As you extend your time from 0 seconds, the noise is initially dominated by read noise, the slope of the line in this region is primarily determined by the f/ratio, or speed of the scope.
Once most of your signal is well above the read noise such that skyglow is now the dominant noise, you hit an inflection point of sorts, and the characteristics of the camera take over. The curve changes dramatically.
The only difference between an f/4 and f/8 scope of the same design and aperture, is that the f/4 will hit this inflection point sooner (closer to 0 seconds on the graph). Binning on an f/8 helps you reach this inflection point faster as well, but again, the camera takes over once you hit this inflection point.
Nocturnal, DSLRs are a pain to figure out here, partly because of their ISO and their noise. Because the dark current is so variable, DSLRs actually tend to push the inflection point out further compared to a cooled CCD, and will seem to support the idea that f-ratios matter when used. This is what I even encountered myself. Faster f-ratios gave me better DSLR shots, mostly because it was hard to get the signal well above the dark noise when shooting anything but open clusters.
Well, if we get into SNR...
Binning increases well depth, therefore increasing dynamic range (great side effect!) and in some situations reducing quantizing error.
So a f/8 binned will have much higher dynamic range than an f/4...
--------------------
Astrology is the science for ignorants. Astronomy is the science for those who feel ignorant. - Miguel Lopes
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DonR
scholastic sledgehammer
Reged: 11/15/06
Posts: 988
Loc: Georgia, USA
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The important thing to remember is that while the number of photons that enter the aperture of any 8" telescope is the same regardless of focal ratio, it is only the photons that reach the camera's sensor that count in a photographic system. With an f/5 telescope, four times as many photons reach the sensor as with an f/10 telescope - the rest hit the OTA wall and are absorbed, reflected or diffracted, or pass right on through to the ground with a truss reflector. The same is true visually as well, but it's the exit pupil rather than the camera sensor that matters.
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Don
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Atlas EQ-G
Orion 8" f/4.9 newtonian
Orion 127mm Mak-Cass
Orion Skyview Pro mount
Orion 80mm guide scope
Canon Digital Rebel XT
Meade DSI
Philips SPC 900 NC webcam
http://www.pbase.com/dtreed/astrophotos
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