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  #21  
Old 02-06-2019, 04:24 PM
Stefan Buda
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Quote:
Originally Posted by Camelopardalis View Post
Define practical size domain?
I have.
At least twice.
8" to 16" to make it easy.
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  #22  
Old 02-06-2019, 08:34 PM
FrancoRodriguez (Franco)
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Originally Posted by Slawomir View Post
My understating is that Nick is correct - for extended objects f-ratio determines the speed, not aperture.

If we take 10" f/5 and 4" f/5, the 4" will collect only 16% of the photons that 10" does. However, while keeping aperture the same, 500mm focal length will put 6.25 as many photons as 1250mm focal length (keeping aperture the same).

So if we combine both factors, aperture and the focal length: 0.16 x 6.25 = 1 On no! How can this be? 10" f/5 will be as fast for extended objects as 4" f/5? Actually, 4" f/5 will be somehow faster due to the lack of central obstruction and loss of light at the mirrors with the 10"

But for point sources, 10" will easily detect many many more stars than 4" and will most likely allow for a sharper image with tighter stars
I think you've given me exactly what I needed--thanks! "...sharper image with tighter stars." That's got to be it.
It's such a seemingly simple question, trivial in fact, but actually a little harder to answer when it comes down to it.
The awful truth is that I'm trying to convince myself THAT I COULD DO WITH A BIGGER TELESCOPE! Terrible I know, and rather embarrassing...
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  #23  
Old 03-06-2019, 06:00 AM
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Quote:
Originally Posted by FrancoRodriguez View Post
I think you've given me exactly what I needed--thanks! "...sharper image with tighter stars." That's got to be it.
It's such a seemingly simple question, trivial in fact, but actually a little harder to answer when it comes down to it.
The awful truth is that I'm trying to convince myself THAT I COULD DO WITH A BIGGER TELESCOPE! Terrible I know, and rather embarrassing...
Glad my waffle was useful

To cheer you up - I have a very nice 4” refractor and I’m also trying to rationalise the need (read: want) for a larger aperture
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  #24  
Old 03-06-2019, 08:47 AM
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Camelopardalis (Dunk)
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Originally Posted by FrancoRodriguez View Post
I think you've given me exactly what I needed--thanks! "...sharper image with tighter stars." That's got to be it.
It's such a seemingly simple question, trivial in fact, but actually a little harder to answer when it comes down to it.
The awful truth is that I'm trying to convince myself THAT I COULD DO WITH A BIGGER TELESCOPE! Terrible I know, and rather embarrassing...
Indeed, as you can see, surprisingly hard

I recommend you read up on Airy disk (even Wikipedia has a good page on this). The mathematics of optics means that the Airy disk size is related to the f-ratio of the scope, not the aperture.

Of course, the mathematics assumes perfect optics and steady (no?) atmosphere. In practice, we don’t get either, so practice is always worse than theory.

The bottom line is, a fast f-ratio scope will give you tighter stars than a slower scope. Getting a larger, faster scope is (sort of) win-win. To a point. With the increasing focal length you are more likely to notice the turbulence of the atmosphere that degrades the view/image - this is “seeing limited”.

Btw, you’re not alone in wanting more bigger...it’s called aperture fever
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Old 03-06-2019, 09:15 AM
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Quote:
Originally Posted by Camelopardalis View Post
Indeed, as you can see, surprisingly hard

I recommend you read up on Airy disk (even Wikipedia has a good page on this). The mathematics of optics means that the Airy disk size is related to the f-ratio of the scope, not the aperture.

Of course, the mathematics assumes perfect optics and steady (no?) atmosphere. In practice, we don’t get either, so practice is always worse than theory.

The bottom line is, a fast f-ratio scope will give you tighter stars than a slower scope. Getting a larger, faster scope is (sort of) win-win. To a point. With the increasing focal length you are more likely to notice the turbulence of the atmosphere that degrades the view/image - this is “seeing limited”.

Btw, you’re not alone in wanting more bigger...it’s called aperture fever
̶O̶h̶ ̶d̶e̶a̶r̶.

Let’s quote a few paragraphs from above mentioned Wikipedia article:

Mathematically, the diffraction pattern is characterized by the wavelength of light illuminating the circular aperture, and the aperture's size.

An optical system with resolution performance at the instrument's theoretical limit is said to be diffraction-limited.

The diffraction-limited angular resolution of a telescopic instrument is proportional to the wavelength of the light being observed, and inversely proportional to the diameter of its objective's entrance aperture. For telescopes with circular apertures, the size of the smallest feature in an image that is diffraction limited is the size of the Airy disk.

In short - small aperture = DSO data is always diffraction limited (unless seeing is horrible) = less detail, while large aperture = data is seeing limited = more detail.

Last edited by Slawomir; 03-06-2019 at 05:43 PM. Reason: To decrease condenscendning tone
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  #26  
Old 03-06-2019, 09:36 AM
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Of COURSE the f-ratio is related to the aperture, but did you read down as far as "Mathematical formulation", where you conveniently missed out:

The radius q1 of the first dark ring on a screen is related to theta and to the f-number by q1 = 1.22 lambda N (where N is the f-number)

(End quote). This is a commonly quoted formula involved in calculating the spot size. For the mathematically challenged, at the same wavelength of light, the Airy disk size increases linearly with f-ratio.

In practice, I agree, a small scope will be diffraction limited, but a larger one is less likely to be, especially along the east coast of Australia. I think we're essentially saying the same thing but just not seeing it that way

And FWIW, your condescending tone is not appreciated. This used to be a friendly forum and I might suggest you pull your head in.
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  #27  
Old 03-06-2019, 05:38 PM
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I hope our discussion has answered Francisco's original question. I sincerely apologise for coming across as rude geek
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  #28  
Old 03-06-2019, 07:35 PM
N1 (Mirko)
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Originally Posted by Wavytone View Post
What is totally beyond small lenses - and small scopes - is serious lunar & planetary stuff.

This is where you need scopes with resolution better than 0.5 arc second (aperture > 25 cm) and focal length > 4 metres. And the mount to track precisely during an exposure; this is far from trivial.
Crikey I didn't know the line between futile amateurism and serious (planetary) astronomy was that well defined, so thanks for clearing that up. Seriously
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  #29  
Old 04-06-2019, 12:15 PM
FrancoRodriguez (Franco)
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Thanks, you've all been most helpful. So, in a nutshell, would I get any tangable benefit of a 12.5 inch f7.5 planewave corrected dall kirkham paired with a 16803 chip? The field of view with this setup is not very different to my current setup using a 100mg APO triplet.
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  #30  
Old 04-06-2019, 08:41 PM
Stefan Buda
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Now that you provided some numbers, it is much easier to answer your question.

In a nutshell, yes the larger setup would produce better results in some respects but not all.

As you said, the field of view would be the same. The required exposure for extended objects would also be the same. The main advantage would be in resolving fine detail in nebula and the stars will be smaller relative to the larger pixels. That is a big advantage if you are going after a globular cluster, but not big if you are chasing the Running Man nebula.

EDIT: I got it wrong. Exposure time would be the same if you used the same sensor, but with the scaled up sensor the bigger scope would be better at catching up with the Running Man nebula too.

Last edited by Stefan Buda; 05-06-2019 at 06:54 AM.
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  #31  
Old 04-06-2019, 09:23 PM
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How about we reduce the argument to its absurd conclusion and apply some physics on top of the math?

Lots of luck collecting *any* photons down a 1mm aperture from a dim flux source...the shot noise will be woeful.

An aperture of 10 metres , or 10000mm *will* collect more photons. Yes there are moot points over pixel size...but without flux down the pipe, there is nothing to measure.

P.S. Infinite numbers of photons do not rain down from dim sources in the sky....bigger telescopes catch more over a finite period, which is hardly a surprise to those proposing the OWL.

Last edited by Peter Ward; 04-06-2019 at 09:50 PM.
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  #32  
Old 04-06-2019, 10:20 PM
Stefan Buda
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Peter you are absolutely correct about the physics.
That is why I was talking about "practical size domains" in earlies comments.
That is why I tried to restrict the discussion to comparing an 8" to a 16".
Francisco is talking about a scaling factor of more than 3x, involving an increase in light gathering of 9x, and that is pushing it a bit. But even so the difference between the two setups will not be very obvious when it comes to S/N ratio in extended objects.

EDIT: I got it wrong.
Exposure time would be the same if we used the same sensor on both scopes. Because we are scaling up the sensor as well, as Francisco suggested, the exposure needed to achieve the same "depth", becomes significantly shorter.

The bottom line: The larger setup, despite of having the same field of view, would have somewhat better resolution and would record faint nebulosity significantly faster.

Last edited by Stefan Buda; 05-06-2019 at 07:27 AM.
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  #33  
Old 05-06-2019, 07:24 AM
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So simply, without confusing the mathematics and physics which makes my head spin, with the mechanics: the suggestion is that if I buy a much bigger aperture , I had better be prepared for a much larger sensor with larger pixels?

It would seem that small refractors up to 800 are well paired with apsc size 4.3micron pixel sensors at 1.1 arcsecond but a much larger aperture can benefit from a full frame with 6 micron pixels ?

Where is the useful limit of tolerance , 0.8 to 1.2 “ ?


Quote:
Originally Posted by Stefan Buda View Post
Now that you provided some numbers, it is much easier to answer your question.

In a nutshell, yes the larger setup would produce better results in some respects but not all.

As you said, the field of view would be the same. The required exposure for extended objects would also be the same. The main advantage would be in resolving fine detail in nebula and the stars will be smaller relative to the larger pixels. That is a big advantage if you are going after a globular cluster, but not big if you are chasing the Running Man nebula.

EDIT: I got it wrong. Exposure time would be the same if you used the same sensor, but with the scaled up sensor the bigger scope would be better at catching up with the Running Man nebula too.
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  #34  
Old 05-06-2019, 07:56 AM
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Originally Posted by Sunfish View Post
So simply, without confusing the mathematics and physics which makes my head spin, with the mechanics: the suggestion is that if I buy a much bigger aperture , I had better be prepared for a much larger sensor with larger pixels?

It would seem that small refractors up to 800 are well paired with apsc size 4.3micron pixel sensors at 1.1 arcsecond but a much larger aperture can benefit from a full frame with 6 micron pixels ?

Where is the useful limit of tolerance , 0.8 to 1.2 “ ?
You are on the right track. If you don't match your pixel size with the scope's focal length you may waste your money. For example if Francisco decided to get the 12.5" Planewave and use his present sensor, which I presume is one with pixels of around 4 micron, he would end up with a setup that not only has a much smaller field of view, but it would require the same exposure times and the only gain, not very dramatic, would be in resolution.
The limit for sampling rate is set by atmospheric seeing and I don't think going to less than 0.8 is beneficial, it would just increase exposure time without gaining resolution.
So basically the size of your pixels set the upper limit of your focal length and from there it is just a matter of choosing the largest available aperture for that focal length.
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  #35  
Old 05-06-2019, 07:57 AM
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So, I understand the physics behind it, even while my attempts to articulate it may have been lacking previously

But I have a question - and just to be clear - because I’m uncertain and would like to understand further, not because I’m trying to be funny

What’s the deal with “extended” objects, and why are they treated differently? Surely they are photon sources subject to the same mathematics?

FWIW, I have example images of a popular galaxy taken with my 4” and with my 8” (with different cameras, so resolution +/- 10%), where the 8” shows better detail, comparable noise levels, but in 1/4 the exposure time. It’s not night and day, but I thought it was a worthwhile experiment. If anyone wishes to seem them, I will post...
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  #36  
Old 05-06-2019, 08:26 AM
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Originally Posted by Camelopardalis View Post
So, I understand the physics behind it, even while my attempts to articulate it may have been lacking previously

But I have a question - and just to be clear - because I’m uncertain and would like to understand further, not because I’m trying to be funny

What’s the deal with “extended” objects, and why are they treated differently? Surely they are photon sources subject to the same mathematics?

FWIW, I have example images of a popular galaxy taken with my 4” and with my 8” (with different cameras, so resolution +/- 10%), where the 8” shows better detail, comparable noise levels, but in 1/4 the exposure time. It’s not night and day, but I thought it was a worthwhile experiment. If anyone wishes to seem them, I will post...
The quick and simple answer is that star sizes on your sensor are determined by the f ratio alone and the size of an extended object, by the focal length (let's ignore the effect of central obstruction). That has implications regarding resolution and S/N ratio.
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  #37  
Old 05-06-2019, 10:21 AM
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Hi Franco,
In addition to the sage comments posted already, Doug from Diffraction Limited summarises the argument pretty well here:
http://diffractionlimited.com/matching-camera-optics/
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  #38  
Old 05-06-2019, 01:31 PM
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Quote:
Originally Posted by Slawomir View Post
Glad my waffle was useful

To cheer you up - I have a very nice 4” refractor and I’m also trying to rationalise the need (read: want) for a larger aperture
Bottomline you need both a widefield APO 4 inch or so or less even and a larger aperture scope for the fainter things and for greater depth in less time.

Nothing beats aperture in this game. My Honders 305mm F3.8 gets the image way faster than any other scope I have used. My CDK17 gets the galaxies and other smaller objects looking good that is not really the ideal target for the Honders even though it can do it.

One scope does not do it all. A few have come close.

Greg.
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  #39  
Old 05-06-2019, 01:52 PM
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I have to amend my answer to Francisco because I forgot to consider narrow band imaging, which is a rather dumb thing to do considering how popular it has become. It must be because I never did and don't intend to do NB in the foreseeable future.

When we put a NB filter in the light path, we severely cut down the photon flux and we really get into shot noise dominated territory. And that is where aperture really matters (as Peter so nicely pointed out). Of course RGB is subject to the same laws of physics but NB is really pushing the limits.

So, Francisco if your game is NB imaging then you will have a much more capable setup if you fork out the money for the scaled up one.
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  #40  
Old 05-06-2019, 02:21 PM
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Hmm.

Is that second chart correct or are the column annotations incorrect.?
Are we looking at image scale or pixel size in microns? Not good to show tables without units.

The top table is useful but so is this with better annotation

https://astronomy.tools/calculators/ccd




Quote:
Originally Posted by AstroApprentice View Post
Hi Franco,
In addition to the sage comments posted already, Doug from Diffraction Limited summarises the argument pretty well here:
http://diffractionlimited.com/matching-camera-optics/
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