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"Lessons from the Masters"...first impressions

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#51 vpcirc

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Posted 19 September 2013 - 07:18 PM

Tony Hallis would tell you PixInsight is a waste of money because there's nothing it can do that he can't do in photoshop. Does that invalid what he's teaching because you don't agree? Or does he have a different approach to accomplishing what he wants? A great example might be the content aware healing brush in photoshop. I can't find anything as easy and quick to correct flaws in PI. When it comes to explaining things, it's sometimes easier for some to understand when it's explained one way, to others they may not get what's being said at all. Golf lessons are a great example, I can get what teacher says, but not the other even though both are trying to get me to correct my swing plane.
I agree with you Peter, one of the biggest mistakes new imagers make is to start out with an SCT at F10. They get frustrated really quick. I doubt many of them are reading Stan's write up though.

#52 Alph

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Posted 19 September 2013 - 07:34 PM

The confusion about this 'myth' comes from two places.
The first is that us old folks are locked into the paradigm of old-time cameras where a lower focal ratio meant more light into the camera. What is forgotten sometimes is that this was accompanied by an increased aperture by a changing iris.


You are making wrong assumption what others think. Honestly, you are the one who looks confused. You fell victim to his misinformation campaign. When it comes to imaging speed, focal ratio and pixel size rule.

#53 BlueGrass

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Posted 19 September 2013 - 09:50 PM

Got my copy today and given my schedule, will take some time to read. Given the discussions here already, I'm sure there will be a number of critics and proponents. The F ratio 'myth' has been pretty well run to ground in these forums and I don't expect much to change. What I do hope is the PS techniques will improve my slowly evolving processing skills. BTW, for those in the Salt Lake area, Tyler Allred is scheduled to speak at the SLAS November meeting...

#54 Ken Crawford

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Posted 19 September 2013 - 09:55 PM

Can't have too many processing tools in the tool Box! The problem is that powerful tools like PS - PI - Stack - ect can be used with the precision of a surgeons knife or a blacksmith's hammer! Depends on the touch!

Just don't have the touch of a blacksmith :)

#55 vpcirc

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Posted 19 September 2013 - 09:59 PM

Alf, the mars rover data shows no life currently on Mars. Since you're typically in outer space in your travels, is the rover misreporting information on the chemical composition and life on Mars?

#56 Peter in Reno

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Posted 19 September 2013 - 10:12 PM

As much as I have disagreed with Alph 98% of the time in the past, I am afraid he is correct. Focal ratio will always determine the image speed, film or CCD. That part will never change since the invention of camera.

Peter

#57 vpcirc

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Posted 19 September 2013 - 10:29 PM

Peter, everything has a cost in imaging, faster speed has a cost as well. The key is finding the right balance. You can take wonderful images at F4, but you can take even better ones at F10 if the aperture is the same. It's just harder and takes longer, but in the end the F10 image will show greater detail and resolution. RCOS doesn't make an OTA at F5 for reason.

#58 Peter in Reno

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Posted 19 September 2013 - 11:26 PM

You said it yourself, "it takes longer" at F/10 than F/4 regardless of aperture (that's what you appear to be implying). That's the whole point of shorter focal ratio is to take shorter exposure times so we can capture the images quicker and take more DSOs in shortest possible time. Who wants to image 100 hours of the same object before imaging another 100 hours of another object. I don't think focal ratio have any effect of details.

F/10 (or any other focal ratio) does not necessarily mean it will show greater details. I think you meant longer focal length (higher resolution) then it would show greater details but not focal ratio. I think you are missing the point about differences between focal ratio (image speed) and focal length (resolution).

In the case of your examples, you can take not only wonderful images at F/4 but also in great details if the focal length is longer which will require larger aperture. You will get similar details in 10" F/4 as in 4" at F/10 because both scopes have same focal length but exposure times with 10" F/4 will be shorter not because of larger aperture but also shorter focal ratio. Maintaining same short focal ratio while increasing focal length just happens to increase aperture. It's very simple math.

In your previous post, everything you talk about is focal ratio but not focal length. You seem to imply that fast speed (short focal ratio) automatically means short focal length. That's not necessarily true because you can have short focal ratio and long focal length by increasing aperture size. But don't let aperture size determine image speed.

RCOS or any other Cassegrain-like scopes at F/5 or shorter would not be practical because the secondary mirror will be too big and reduce contrast. That's one of the reasons why Cassegrain-like scopes have slow focal ratio is to have higher contrast.

Pixel size can also determine image speed. There is a cost. Larger pixel size can help reduce exposure times but at the expense of under sampling (stars would be more square). That's the another subject I don't wish to discuss in this thread.

Peter

#59 blueman

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Posted 20 September 2013 - 01:43 AM

Let's not forget about matching a CCD chip to the scope. That is as important as all the other factors. Good coverage and proper f/l and f/r to get the desired image pixel scale will give good results.
Blueman

#60 ollypenrice

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Posted 20 September 2013 - 02:37 AM

I'm due to receive a copy for magazine review and am greatly looking forward to it. When the Editor sent me the latest list of review titles it was the first on the list and I replied to his email without even reading the rest!
I did wonder how the Ps/PI thing would play out in the book.
Olly

#61 vpcirc

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Posted 20 September 2013 - 03:31 AM

That's where you're wrong Peter. Because your image scale changes how much of what you see changes. If I hold an eye chart up for you at 10 feet which line can you read? If I now move to 20 feet which line can you read? Scale definitely effects resolution and detail. I'm not confused about the difference between ratio and speed for a second. Ratio only effects scale and therefore detail you can see. Faster = smaller scale and less detail.

"An f/5 system can photograph a nebula or other faint extended deep space object in one-fourth the time of an f/10 system, but the image will be only one-half as large. Point sources, such as stars, are recorded based on the aperture, however, rather than the focal ratio – so that the larger the aperture, the fainter the star you can see or photograph, no matter what the focal ratio" Astronomics

Again, I'm saying the "magnification" is what allows you to see more detail and resolution of the image in a slower system. I do agree with you in that the imager traveling to setup is unlikely to be able to gather enough data without lots of effort. That's why I setup a concrete pier in my back yard and went to automated imaging. I got sick of trying to stay up all night. Now of course I took it a step further, and I realize not everyone has these options available to them.

#62 Alph

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Posted 20 September 2013 - 10:54 AM

Ratio only effects scale and therefore detail you can see. Faster = smaller scale and less detail.


Mike,
Focal length and pixel size effect image scale. Focal ratio has nothing to do with scale. It only determines imaging speed. If there were no diffraction then aperture wouldn't matter much. In the olden days, astronomers and physicists were assuming that aperture did not matter at all! Only the discovery of light diffraction changed that thinking.

#63 HunterofPhotons

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Posted 20 September 2013 - 11:04 AM

.....That's the whole point of shorter focal ratio is to take shorter exposure times so we can capture the images quicker and take more DSOs in shortest possible time.....
Peter


Peter,
The rate of object photons captured by a telescope is solely dependent upon its aperture.
It has nothing to do with its focal ratio.
If you don't understand these principles of physics, then I can understand your befuddlement.

dan k.

#64 Peter in Reno

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Posted 20 September 2013 - 11:18 AM

Hi Dan,

I am currently using Google to look for information you are providing and I can't yet find them. If you know where I can find them, can you provide the links to web sites?

Thanks,
Peter

PS: Don't include links to Stan Moore's article. :)

#65 Peter in Reno

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Posted 20 September 2013 - 11:20 AM

Ratio only effects scale and therefore detail you can see. Faster = smaller scale and less detail.


Mike,
Focal length and pixel size effect image scale. Focal ratio has nothing to do with scale. It only determines imaging speed. If there were no diffraction then aperture wouldn't matter much. In the olden days, astronomers and physicists were assuming that aperture did not matter at all! Only the discovery of light diffraction changed that thinking.


Once again, I am afraid Alph is correct.

Image scale = (206.3 * pixel size) / focal length

Where is the focal ratio in the formula?

Peter

#66 korborh

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Posted 20 September 2013 - 11:30 AM

.....That's the whole point of shorter focal ratio is to take shorter exposure times so we can capture the images quicker and take more DSOs in shortest possible time.....
Peter


Peter,
The rate of object photons captured by a telescope is solely dependent upon its aperture.
It has nothing to do with its focal ratio.
If you don't understand these principles of physics, then I can understand your befuddlement.

dan k.


This whole "object photons" and "myth "wording used by Stan has caused much confusion on this very basic of optical concepts i.e. f/ratio.

As Alph says, f/ratio has nothing to do with resolution or image scale. You can get any resolution or image scale with any f/ratio.

F/ratio only says about the steepness of the cone of light which translates into SNR per linear dimension on the detector.

Having Stan's myth write-up in this book really undermines its credibility.

#67 mikeschuster

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Posted 20 September 2013 - 11:39 AM

Peter,
The rate of object photons captured by a telescope is solely dependent upon its aperture.
It has nothing to do with its focal ratio.
If you don't understand these principles of physics, then I can understand your befuddlement.

dan k.


Dan, imagine adding a very long focal extender with very small pixels. Object photons will be spread out across many pixels. Signal to noise in each pixel will be small. At full resolution the image will look very noisy as each pixel may have captured only a few photons.

If you downsample the noisy image significantly in software you may recover an acceptable image, assuming that read noise was not a significant factor in the original. This downsampling is equivalent to decreasing f/ratio.

So to get a reasonably noise free image you either need a fast system, big pixels or a significant software downsampling.

Note however that faster f/ratio and big pixels is not always a good thing as both will increase the amount of sky background photons captured by each pixel. This increase may swamp the few photons from a dim star whose airy disc fits entirely within one pixel. So fast systems and big pixels are not necessarily a good way to capture dim stars.

Regards,
Mike

#68 Peter in Reno

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Posted 20 September 2013 - 11:58 AM

So far I found this link written by Steve Cannistra who is one of the authors in this book:

http://www.starrywon...com/fratio.html

I let you be the judge and decide what you think.

Peter

#69 hytham

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Posted 20 September 2013 - 12:06 PM

I just don't understand the confusion people have between f-ratio and aperture. Increasing the aperture increases the amount of light gathering capabilities. Lowering the f-ratio allows for a higher concentration of light on a smaller surface because we have affected the angle of incidence (or steepness of the light cone) to the CCD (this is where you see vignetting).

Both work in conjunction to lead to a brighter image in a shorter amount of time. A 106mm scope running at F3 is not going to collect the same amount of light as a 200mm scope running at F3. If you say it does, prove it.

I hope nobody is arguing that f-ratio alone is enough. Then there are a lot of people that have been ripped off over the years by purchasing large aperture systems.

I really don't understand the confusion here.

#70 korborh

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Posted 20 September 2013 - 12:16 PM

A 106mm scope running at F3 is not going to collect the same amount of light as a 200mm scope running at F3.


Both scopes at f/3 will capture the same amount of light per pixel. That is the utility of f/ratio - in just one simple number to give SNR per linear dimension. Use this number for this purpose and nothing more.

#71 vpcirc

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Posted 20 September 2013 - 12:20 PM

Higher focal ratios=higher magnification and smaller field of view. Lower focal ratios= Wider field of view and less magnification. The reason a lower focal ratio images faster is because more light is being concentrated. That comes at the cost of resolution and magnification. Yes you can effect some of this based on the size of the chip and the pixel size, but if all things are equal, aperture and camera, the results are as stated above. If you don't want to believe that than by all means ignore it. There are tons of google searches that will tell you the same thing.

#72 Peter in Reno

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Posted 20 September 2013 - 12:22 PM

I too have been confused about larger aperture leads to larger light gathering power but does it really mean shorter exposure times regardless of focal ratio?

This just crossed my mind and please correct me if I'm wrong. Take a flashlight and a piece of paper. Make a pinhole at the center of paper and place the paper on flashlight. The light through the pinhole does not appear to be bright. Now as you make the hole bigger, light will appear to be brighter but is it really brighter on a camera? The brightness of flashlight is always the same. The hole in the paper is the aperture. Is this analogy accurate?

Interesting link about light gathering power:

http://starizona.com...ing_theory.aspx

Peter

#73 HunterofPhotons

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Posted 20 September 2013 - 01:26 PM

So far I found this link written by Steve Cannistra who is one of the authors in this book:

http://www.starrywon...com/fratio.html

I let you be the judge and decide what you think.

Peter


He's talking about what happens for an individual pixel.
This has nothing to do with 'The F-Ratio Myth'.
Once again, let me repeat, the number of photons collected from an object is only dependent upon aperture.
Physicists are pretty sure this is true. <g>
Take two scopes of equal aperture, one f/10, the other f/5.
Whatever encompassing object they look at, they are capturing the exact same number of object photons. The only difference is that the f/5 scope is putting them in a smaller area.
Of course when you have a smaller area for the same amount of photons they will collect at a faster rate at the pixel level, but they are collecting the total number of object photons at the exact same rate.
Once again let me reiterate, Stan is talking about object photons, the total number of photons from an object.
All he has done is restate a law of physics and given it a provocative title.

dan k.

#74 Alph

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Posted 20 September 2013 - 01:44 PM

He's talking about what happens for an individual pixel.


That's all what it matters.

#75 vpcirc

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Posted 20 September 2013 - 03:04 PM

Peter you are fond of focal reducers. Take an image with it on and without it, compare the two, and see what happens to the image.






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