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Peter in Reno
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Re: "Lessons from the Masters"...first impressions new [Re: vpcirc]
      #6089660 - 09/19/13 10:18 AM

I just want to point out that I never bash this book (except for focal ratio myth). I said if you are a heavy PS user, this is the book to buy and read. I had to return the book for a refund because it won't help PixInisght users (except for one chapter written by Rogelio but it described very few PI tools and there are lots of fantastic PI tools).

Peter


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pfile
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Re: "Lessons from the Masters"...first impressions new [Re: Peter in Reno]
      #6089779 - 09/19/13 11:35 AM

fair enough, my short skim thru the "look inside" was misleading. there's a lot of PS in this book. but i don't think it's entirely useless, at least for me. i know zilcho about planetary imaging and there's some stuff in there on that topic. plus just seeing how others process images is useful regardless of the software involved.

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HunterofPhotons
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Re: "Lessons from the Masters"...first impressions new [Re: Peter in Reno]
      #6089783 - 09/19/13 11:38 AM

Quote:

..... I don't believe Stan's focal ratio myth was ever checked or approved by the professionals.....
Peter




Look in any physics book.
The rate of photons collected from any object is solely dependent on aperture, not focal ratio.
It's actually quite simple.

dan k.


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vpcirc
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Re: "Lessons from the Masters"...first impressions new [Re: HunterofPhotons]
      #6089845 - 09/19/13 12:20 PM

An easy to understand explanation from Celestron
The focal ratio is the ratio of the focal length of the telescope to its aperture. It’s calculated by dividing the focal length by the aperture (both must be in the same units). For example, a telescope with a 2032mm focal length and an aperture of 8" (203.2mm) has a focal ratio of 10 (2032/203.2 = 10) or f/10.

It’s variously abbreviated as f-stop, f/stop f-ratio, f/ratio, f-number, f/number, f/no., etc.

Smaller f-numbers will give brighter photographic images and the option to use shorter exposures. An f/4 system requires only ¼ the exposure time of an f/8 system. Thus, small focal ratio lenses or scopes are called “fast” and larger f/numbers are called “slow”. Fast focal ratios of telescopes are f/3.5 to f/6, medium are f/7 to f/11, and slow are f/12 and longer.

Whether a telescope is used visually or photographically, the brightness of stars (point sources) is a function only of telescope aperture - the larger the aperture, the brighter the images. Extended objects will always appear brighter at lower magnifications. The main advantage of having a fast focal ratio with a visual telescope is that it will deliver a wider field of view than slower f-numbers.


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Peter in Reno
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Re: "Lessons from the Masters"...first impressions new [Re: HunterofPhotons]
      #6089900 - 09/19/13 12:50 PM

Would you actually suggest to others to throw away their small aperture and "fast" (e.g. F/4) APO and replace it with large aperture and "slow" (e.g. F/10) scope so that large aperture/slow scope would actually reduce exposure time? Isn't that what Stan Moore pretty much is suggesting?

If I am missing the point, please explain.

If you would like me to stop discussing about focal ratio myth, I will be happy to stop. But the focal ratio myth is part of the book. I do not want to start the war about this and end up locking this thread.

Peter


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hytham
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Re: "Lessons from the Masters"...first impressions new [Re: Peter in Reno]
      #6090402 - 09/19/13 05:55 PM

Peter, as a newbie (me) one of the best things I have *discovered* is the use of multiple tools to improve upon the images. There is no one tool to rule them all at this time and having the skill set to call upon the proper tool for a specific job will show an increase in, for a lack of a better term, *better* processing.

I'm so bloody new to PS (heck imaging in general) that this book is a great centralized reference to work on that particular skill.

My advice, give it a shot. It's $35 and compared to your financial *investment* into this hobby it is peanuts. Heck, the cost of photoshop is nearly the same as one of my 50mm square filters.

Now onto f-ratio. The f-ratio does not affect the rate at which photons are collected, but affects the concentration of those photons on to the area of the CCD. The decrease in f-ratio (F10 to F5) ultimately affects the angle of incidence thereby affecting the concentration of photons onto a smaller area of the CCD allowing for an increase in SNR. As we increase our f-ratio (F5 to F10), the constant rate of photons does not change (it's that simple), but what changes is the area which the photons are now spread across is much larger leading to a decrease in signal to noise ratio. Less photons per photodiode keeping us very close to that noise threshold.

The rate of photons is based on aperture, and concentration of photons is based on the angle of incidence to the surface area of the CCD.

Peter, a more appropriate test would have been to compare two imaging systems with the same aperture, different focal lengths and matching scale. I'm willing to bet the difference would not be as different as you think.

A good test would be to compare the Veloce RH 200 against my FSQ. Both produce similar image scales (with my KAF 16803 it is 3.1"/pixel vs. 3.5"/pixel) but with wildly different apertures.

My very humble .02.


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HunterofPhotons
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Re: "Lessons from the Masters"...first impressions new [Re: Peter in Reno]
      #6090437 - 09/19/13 06:09 PM

Peter,
The take-away from any discussion of this is that for scopes of equal aperture the number of object photons captured per rate of time is equal.
It's a law of physics. <g>
For scopes of equal aperture but differing focal ratios, the scope with the lower focal ratio is capturing a larger field of view and therefore more photons of a larger field, but it is still capturing the same amount of photons of individual objects.
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.
What we are talking about in Stan's 'myth' is scopes of equal apertures.
The second area of confusion is that we are talking about object photons, not total photons. This was clearly stated by Stan, but many people seem to pass over this.
What you get with a lower focal ratio is a wider field of view and a decreased resolution of your chosen object.
What you get with a higher focal ratio is increased resolution and a narrower field of view.
Let's say you want to image The Ring Nebula. With a lower focal ratio you will capture the nebula in a smaller size with a large surrounding star field. With a larger focal ratio you will capture a larger image of the The Ring, but less of the surrounding star field.
For equal apertures you will have captured the same number of photons. They are just distributed over differing numbers of pixels. If you shrink the longer focal ratio image to the same size of the lower focal ratio image it will be the same.
The one caveat in this shrinkage is the contribution of read noise. Since the longer focal ratio image is distributed over more pixels than the shorter focal ratio image, read noise could become a contributing factor. If, however, you are taking sky-limited images, (which I hope you are), then this is not a significant factor.
What most imagers do is to frame their objects by focal ratio and expose appropriately. In other words, if you want to image The Veil Nebula, then go for the wide field of view with a lower focal ratio (or do a mosaic with the long focal ratio).
If you want to do a finely resolved image of The Ring Nebula and skip the surrounding star field, then, by all means go for the longer focal length.
Each focal length has its advantages, you just have to tailor them to the object of interest.

dan k.


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vpcirc
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Re: "Lessons from the Masters"...first impressions new [Re: HunterofPhotons]
      #6090515 - 09/19/13 07:03 PM

Very well put and explained!!! I think where people get confused is short focal length seems easier, but because all the tracking and guiding errors are hidden. The longer the focal length and image scale, the more apparent your problems are, including post processing. I think of it kind of like TV, if they do a close up of a face you can see every blemish there. If the shot is at a moderate distance, you wouldn't even notice!

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Peter in Reno
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Re: "Lessons from the Masters"...first impressions new [Re: HunterofPhotons]
      #6090521 - 09/19/13 07:07 PM

Generally speaking, all I am saying is if the goal in astronomy is to image deep sky objects that usually require long exposures due to low light from target objects is to aim for lower focal ratio whether the focal length is short or long. It pretty much guarantees to capture decent images at reasonable exposure times.

I know it's difficult to own one telescope and one camera to work with all objects since different objects have different sizes (e.g. one size fits all). That's not easily possible. Not every one has money or resources to own several telescopes of different focal lengths at reasonably low focal ratios and several cameras of different pixel sizes to better match to the optics. I understand all that.

It's possible that Stan's write up is poorly written that can cause people to mis-interpret his writings. It's very important for every one to understand otherwise people could end up buying the wrong telescope and equipment because of mis-understanding of other people's write up or articles. For example, Stan's comparison of two images may be mis-leading since it does not say whether both images are equally or unequally stretched. It also does not say if same camera was used. It does not even say what the object is. The object could have been bright enough for the signal to be well above camera's read noise for large focal ratio and stretch the image enough to match the low focal ratio image (less stretched). If Stan did that, then that may invalidate the test. I feel Stan's article has lots of holes that needs to be filled. In other words, Stan's article is lacking details.

Dan, your write up is excellent and better than what Stan explained. I pretty much already knew what you said. I was having a hard time deciphering Stan's write up.

Peter


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pfile
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Re: "Lessons from the Masters"...first impressions new [Re: Peter in Reno]
      #6090531 - 09/19/13 07:14 PM

not to cast aspersions at the masters but that piece by stan is not very well written. i think that contributes to the confusion.

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vpcirc
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Re: "Lessons from the Masters"...first impressions new [Re: pfile]
      #6090591 - 09/19/13 08:18 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.


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Alph
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Re: "Lessons from the Masters"...first impressions new [Re: HunterofPhotons]
      #6090605 - 09/19/13 08:34 PM

Quote:

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.


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BlueGrass
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Re: "Lessons from the Masters"...first impressions new [Re: Alph]
      #6090805 - 09/19/13 10:50 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...

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Ken Crawford
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Reged: 06/02/09

Re: "Lessons from the Masters"...first impressions new [Re: BlueGrass]
      #6090812 - 09/19/13 10:55 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


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vpcirc
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Re: "Lessons from the Masters"...first impressions new [Re: BlueGrass]
      #6090819 - 09/19/13 10:59 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?

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Peter in Reno
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Re: "Lessons from the Masters"...first impressions new [Re: vpcirc]
      #6090832 - 09/19/13 11:12 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


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vpcirc
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Re: "Lessons from the Masters"...first impressions new [Re: Peter in Reno]
      #6090860 - 09/19/13 11:29 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.

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Peter in Reno
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Re: "Lessons from the Masters"...first impressions [Re: vpcirc]
      #6090924 - 09/20/13 12:26 AM

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


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blueman
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Re: "Lessons from the Masters"...first impressions new [Re: Peter in Reno]
      #6091039 - 09/20/13 02:43 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


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ollypenrice
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Re: "Lessons from the Masters"...first impressions new [Re: Peter in Reno]
      #6091058 - 09/20/13 03:37 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


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