View Full Version here: : Which One is Faster? F/3 or F/6
So here is a question to you tech theory types. An 8-inch F/3 or an 8-inch F/6. Which system is faster for astrophotography?
Looking at Nyquist sampeling information:
Let's match the pixel micron size to our FL's
8" f/3 with a CCD camera of ~2 micron pixels
8" f/6 with a CCD camera of ~4 micron pixels
Assume that both telescopes are of the same optical design and our CCD cameras have the same QE.
Aperature is the same, and we have optimized our pixel size for our focal length for coverate of at least 2 pixels by the Airy disc. Which would be a "faster" optical system for CCD Imaging?
Although the field of view is different, whould I actually gather more nebula in a shorter period of time with the F/3? Me thinks not.
23-04-2012, 09:03 AM
This is a very popular mix-up in terminology. While f number creates different perception of the picture brightness, light gathering capacity of a telescope depends chiefly on it's aperture. 8" f3 and 8"f6 have the same aperture, hence same light gathering capacity. With f6 you will just see lesser portion of a nebula and surrounding sky, and possibly more fine detail. But you will not see "more nebula" in any of those telescopes - as they grabbed and transmitted the same number of photons. The picture will look sharper, brighter and more distorted with f3 though - due to different light distribution inside the telescope.
23-04-2012, 09:03 AM
You're operating at the same magnification in both cases and you'll have the same FOV if both cameras have the same pixel geometry (same number of rows and columns).
Both configurations will have the same effective speed. You're spreading the same number of photons over the same number of pixels in both cases - the aperture and FOV are the same.
Comparing f/ratio between scopes only makes sense if you're talking about the same sensor in both cases.
23-04-2012, 11:28 AM
The f ratio myth is real. You cannot beat real aperture. An f3 200mm aperture telescope will collect far more photons than an an F3 lens of 50mm FL say.
You did mention Niquist theory so obviously you understand sampling theory.
It is a complex balance of sampling and inherent brightness.
It is absolute heresy to say that an equivalent aperture at f/8 is as good as one at f/3 for dim extended objects!
By the way for star intensities only aperture rules. For nebulosity only focal ratio is the only arbiter.
One has to balance the the pixel size of the detector to the optic. The sampling here is important for final resolution.
What was the question again?
23-04-2012, 01:07 PM
Not really sure what the underlying question in your mind is here
However in your example - both systems will end up with the identical image scale.
If both cameras have the same number of pixels then both cameras will also have the same total field of view
However the telescopes image circle may or may not be matched to the sensors (since you said they were to be the same) and the Well depth of the 2uM sensor is going to be much smaller and certainly inferior to the 4um sensor and therefore likely have considerably more noise as a percentage of signal - ie Signal to Noise Ratio will be worse.
eg Read noise will be much the same for each and Dark noise too, but total signal will be much less in the smaller pixel sized CCD - based on area but also on the fact that there is some lost sensor area overhead in each pixel too - space for the internal wiring and the boundary edges of the silicon structures and any other etched circuitry.
So the signal to noise ratio will be significantly different - thus bit depth might end up being something like 7/8 Vs 10/11 (I'd need to work it out properly if you're interested)
The point source objects (stars) are still going to saturate the pixels in each system in similarly quick times, but you wont necessarily have the well depth to capture the faint details in the extended objects.
And you wont be able to increase the exposure time on the smaller camera to compensate because it will not have the well depth and hence dynamic range to collect any more photos/electrons.
So having the larger sized CCD is going to make all the difference in your example.
That will provide you with the best result.
Of course this is all moot if the Seeing conditions, light pollution and local conditions are not able to support it and nor is your system choice going to matter much if you cannot track and guide on the object to a high degree
So removing tracking, guiding, seeing errors, low periodic error etc are likely to make a much bigger difference - but I diverge from your posted question as stated, but i do think that is a missing element in what you are hoping to understand.
23-04-2012, 05:59 PM
I think its not that cut and dry.
Depends on the target. Some targets are widefield with bright and dim areas.
If you zoom in (F6) you may be imaging part of the dim area. If you zoom out (F3) you would be taking in more of the bright areas and therefore more signal and it would expose faster (not the dim areas though).
Of course you are assuming even performance of 2 sensors one with half the size pixels of the other and we know that is rarely the case.
9 micron pixels seems to be the allround winner in cameras. Theoretically smaller pixels should work better in a faster system but Kodak chips seem to perform best around the 9 micron pixel size. Perhaps a characteristic of their architectural design?
Kodak went to 5.5 micron pixels with their true sense sensors. These sensors only now seem to be coming out in cameras despite being around for quite some time. It takes the market a long time to accept new products like that.
Of course there are no popular cameras with either 4 or 2 micron pixels out there. The smallest Kodak is 5.4microns. Not sure about Sony - they may have some in the 4+ micron band.
My intent with the question is to simply understand the f-ratio myth better and how to explain it.
There are no 2um or 4um pixel cameras that I am aware of, but I wanted to keep some constants in the model. 200mm aperture, pixel size matched to the focal length as per Nyquist theory.
24-04-2012, 04:19 PM
Best to get it out of theory and into practise.
You have 2 approx 12 inch F3.6/8 scopes being used on this site - An APRHA by Peter Ward and a 12 inch Orion Optics UK F3.8.
They both are producing images that have not been seen with similar aperture but longer focal length.
So there is something that occurs there. Perhap the little variables of pixel matching, less effect of seeing, simpler optical paths, less prone to tube currents, efficiencies of larger pixels add up to a noticeable difference.
24-04-2012, 10:00 PM
Not sure what you mean by this - image scale is image scale.
Run some examples through to verify this.
In the example provided by the OP - The image scale is the same !
You will get EXACTLY the same image from both of them - if both cameras have the same number of pixels.
Both image scales are EXACTLY 0.69 arcsecs/pix, 7.3 x 7.3 arc mins fov on a 640x640 array.
The f6 system is concentrating its light onto larger CCD, the f3 onto a smaller one !
I didn't assume the performance of the two CCDs was the same - I said they were very different and why.
9uM pixels may be ideal for some situations, but definitely not all of them.
Sometimes 24uM pixels are best and other times 2uM pixels are best.
Its a question of selecting the ideal sampling rates and image scale for the OTA and the intended purpose. Not to mention CCD specs such as spectral response, Qe, budget etc
So yes a different choice of CCDs on the same OTA is necessary for Planetary or DSO or Widefield Vs Narrowfield etc, but that is not what was being asked.
I think the OP's choice of pixel sizes used was purely hypothetical to illustrate problem, but Lumenera have a number of 4um cameras for Astro use as do others and there are probably dozens of point and shoots in the 2um size range.
25-04-2012, 07:53 AM
Have a look here John
This bloke Roger Clark knows what he is talking about and explains things very clearly. In fact his whole site is a mine of information.
One of the references he gives is also worth a look.
It seems the f-ratio myth was really due to the reciprocity failure of film. It just does not apply as much to modern detectors that have a far wider integration time than film.
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