Fast telescope

[vlaiv]

Please don't use fast and slow in astro photographic context to specify any sort of "speed" of capturing an image - as it is very misguiding and often wrong.

Term has significance in both visual and photographic use as has been described, but its use to denote photographic speed comes from domain of daytime photography and lens operation (on a single camera where there is plenty of light and single exposure is taken).

Comparing two scope in astro photographic domain by F/ratio and concluding that one will create image faster - is simply not true. F/10 scope can be faster than F/5 scope, and in here faster means - less time spent to make an image of equal SNR.

Btw, to address original question, In my view 6/9 so less than F/6 is fast, F/6 to F/9 is medium and above F/9 is slow scope.

[joe aguiar]

9 hours ago, Alan64 said:

"Slow" and "fast" are photographic terms.  They do not apply to telescopes, per se; only when a camera is combined or contemplated with a telescope.  If you were to attach a camera to that 32" f/2.8 Newtonian, would an image be collected fast, or slowly, given the telescope's 2276mm focal-length?

It would be fast compared to a scope that's 32 inch f10 let's say.

And we always still use fast and slow term still to describe if a scope is short or long.

That scope being fast at f2.8 is also low power wide field making it fast for the pic, if it was f10 it be much higher power and closer view and more narrow making that image much longer to take.

Yes oringinal use was for camera but we still use fast and slow to tell us if it's short medium  or long. U cant just go by focal length alone cause as different scopes and sizes focal ratio will be different 

Joejaguar 

Edited December 23, 2019 by joe aguiar

[vlaiv]

57 minutes ago, joe aguiar said:

It would be fast compared to a scope that's 32 inch f10 let's say.

And we always still use fast and slow term still to describe if a scope is short or long.

That scope being fast at f2.8 is also low power wide field making it fast for the pic, if it was f10 it be much higher power and closer view and more narrow making that image much longer to take.

Yes oringinal use was for camera but we still use fast and slow to tell us if it's short medium  or long. U cant just go by focal length alone cause as different scopes and sizes focal ratio will be different 

Joejaguar 

No, please, don't perpetuate that slow/fast scope - longer/sorter time to take the image thing as it is not true.

It just creates confusion, as a person can come and think, "look I have F/5 scope - that is supposed to be fast, and if another person with their F/8 scope can take certain image in 2h, I should surely be able to take it in less than 2h" - but that might not be the case, and people will have a wrong idea why they are failing to meet "fast scope expected exposure time".

 

[joe aguiar]

5 minutes ago, vlaiv said:

No, please, don't perpetuate that slow/fast scope - longer/sorter time to take the image thing as it is not true.

It just creates confusion, as a person can come and think, "look I have F/5 scope - that is supposed to be fast, and if another person with their F/8 scope can take certain image in 2h, I should surely be able to take it in less than 2h" - but that might not be the case, and people will have a wrong idea why they are failing to meet "fast scope expected exposure time".

 

I created nothing that's how it been for the longest time, short or long term was around since the slr camera. I didn't make up the term even tho it was created for the image its still used now.

a scope f5 takes half the time the time as an f10 this been true like forever.

people should also educate themselves, AP takes a lot time money and know how to get it right. 

joejaguar

[alacant]

1 hour ago, joe aguiar said:

It would be fast compared to a scope that's 32 inch f10

No it wouldn't!

I think you maybe confusing camera lenses with telescopes.

View the same object at the same size in each telescope and it would be exactly the same brightness.

Equally: image the same object using the same camera at the same size and you would need exactly the same time on both telescopes to record it equally.

As @vlaiv recommends, don't say 'fast' [1]

Cheers, HTH and clear skies.

[1] but if you do, then please say what your definition of fast be;)

[Stu]

5 minutes ago, joe aguiar said:

I created nothing that's how it been for the longest time, short or long term was around since the slr camera. I didn't make up the term even tho it was created for the image its still used now.

a scope f5 takes half the time the time as an f10 this been true like forever.

people should also educate themselves, AP takes a lot time money and know how to get it right. 

joejaguar

Let's all play nicely, it is nearly Christmas after all 🎄🎄

When is @ollypenrice going to pop along with a 'Ho Ho Ho, now let me tell you about the focal ratio myth'? 😁😁👍👍

[joe aguiar]

9 minutes ago, alacant said:

No it wouldn't!

I think you maybe confusing camera lenses with telescopes.

View the same object at the same size in each telescope and it would be exactly the same brightness.

Equally: image the same object using the same camera at the same size and you would need exactly the same time on both telescopes to record it equally.

As @vlaiv recommends, don't say 'fast' [1]

Cheers, HTH and clear skies.

[1] but if you do, then please say what your definition of fast be;)

sorry iam not confusing them

I never said it wouldn't be same brightness did I?? both are same size telescope so brightness is same but if its f/10 the focal length and power would be different and in f10 the item would be much closer

As @vlaiv recommends, don't say 'fast' [1] I already explained whats fast and considered slow

joejaguar

Edited December 23, 2019 by joe aguiar

[johninderby]

Why does this remind me of that old argument about how many angels can dance on the head of a pin. But seriously  it is interesting to read what fast and slow means to different people and makes you think about what it really means. 🤔

Reductio ad absurdum (aut non)

Edited December 23, 2019 by johninderby

[ollypenrice]

1 minute ago, Stu said:

Let's all play nicely, it is nearly Christmas after all 🎄🎄

When is @ollypenrice going to pop along with a 'Ho Ho Ho, now let me tell you about the focal ratio myth'? 😁😁👍👍

I've been happy to let Vlaiv and Alacant fight this corner but I suppose I ought to chime in since I'm probably guilty of having made an issue of the F ratio myth on this forum!

21 minutes ago, joe aguiar said:

I created nothing that's how it been for the longest time, short or long term was around since the slr camera. I didn't make up the term even tho it was created for the image its still used now.

a scope f5 takes half the time the time as an f10 this been true like forever.

people should also educate themselves, AP takes a lot time money and know how to get it right. 

joejaguar

Joe, what has been true forever is that, when aperture is the variable and focal length is the constant, an F5 objective has four times the area of an F10 objective so it lets in 4x as much light and so reduces exposure by 4x. Nobody is arguing with that.

The first problem arises when focal length is not constant between optics being compared. The Hyperstar would be an obvious example. Point it at a small target first at F10, then at F2.  Unlike the first situation we are now getting precisely the same number of object photons from the target. So is it faster at F2 than at F10 when there is the same amount of light in each case? Not in any way that's very useful. All that's happening is that the same amount of light is placed onto fewer pixels at F2, producing a tinier but brighter image in the same time.

And so to problem number two: if you bin the pixels at F10 / use a huge pixel camera / resample the the image downwards there will, in the end, be little to choose between the two F ratios. There is a good article on this issue here: http://www.stanmooreastro.com/f_ratio_myth.htm

Professionals build big telescopes for a reason. However, they put large pixel cameras behind them for a reason as well. 

Key point: in isolation from pixel size, focal length and aperture, F ratio is not a very useful term. When dealing with camera lenses there is no problem. When dealing with AP rigs there is room for confusion.

Olly

[joe aguiar]

it doesn't have to be complex at all

even if the terms was manilly used for Ap it still has a purpose to tell someone if their scope is if you want to call it

fast is short tube

med is med long tube

or slow meaning the scope is long compared to size

it gives the new person an idea the ratio based on that size.  2 scopes same size collect same light BUT one scope will have a wider fov since its short the other can be highter power and narrow fov.

guess if u don't agree then we can disagree to agree then

cheers

joejaguar

Edited December 23, 2019 by joe aguiar

[Stu]

18 hours ago, joe aguiar said:

it doesn't have to be complex at all

even if the terms was manilly used for Ap it still has a purpose to tell someone if their scope is if you want to call it

fast is short tube

med is med long tube

or slow meaning the scope is long compared to size

it gives the new person an idea the ratio based on that size.  2 scopes same size collect same light BUT one scope will have a wider fov since its short the other can be highter power and narrow fov.

joejaguar

That only makes sense if the aperture is fixed. I have 65mm f7.7 TAL Alkor Newt which is 500mm focal length and could be described as 'medium', and a 100mm Televue Genesis which is f5 and would be described as 'fast' but has the same 500mm focal length.

Likewise a 80mm f15 refractor with 1200mm focal length is slow, whilst an identical focal length 250mm Newt is definitely fast at f4.8.

Tube length comparison means nothing unless the aperture is the same.

[DaveS]

Posted this before, however...

This was 2 mins with an f/10 'scope. The catch? 2 metre primary, 15 um pixels binned 2x2 and cooled to -100 celsius The Liverpool Telescope on la Palma.

.

[vlaiv]

16 minutes ago, DaveS said:

This was 2 mins with an f/10 'scope

And then people go around saying - don't use that scope for AP, it is F/10 and therefore slow

[dannybgoode]

6 hours ago, vlaiv said:

Please don't use fast and slow in astro photographic context to specify any sort of "speed" of capturing an image - as it is very misguiding and often wrong.

Term has significance in both visual and photographic use as has been described, but its use to denote photographic speed comes from domain of daytime photography and lens operation (on a single camera where there is plenty of light and single exposure is taken).

Comparing two scope in astro photographic domain by F/ratio and concluding that one will create image faster - is simply not true. F/10 scope can be faster than F/5 scope, and in here faster means - less time spent to make an image of equal SNR.

Btw, to address original question, In my view 6/9 so less than F/6 is fast, F/6 to F/9 is medium and above F/9 is slow scope.

This is interesting stuff and my apologies for confusing the terms. A little knowledge and am that   

@vlaiv and others - are there any good primers/books on the more advanced and technical aspects of AP; the subject of SNR, well depth, gain iro of CMOS etc?

I have pretty much got to grips with the ‘physical’ side of AP; actually getting there whole thing set up and imaging and am keen to gain a better grounding in the technical theory behind it all. 
 

[spillage]

I am sure this is not the first debate on here regarding telesope F speed and FL and it wont be the last. Anyway I'm off to go and lazer level up my eq tripod before running a PA routine.

Although this subject can become quite confusing.

[vlaiv]

30 minutes ago, dannybgoode said:

This is interesting stuff and my apologies for confusing the terms. A little knowledge and am that   

@vlaiv and others - are there any good primers/books on the more advanced and technical aspects of AP; the subject of SNR, well depth, gain iro of CMOS etc?

I have pretty much got to grips with the ‘physical’ side of AP; actually getting there whole thing set up and imaging and am keen to gain a better grounding in the technical theory behind it all. 
 

I can't really recommend a good book on all of that, but I don't think it would require a whole book to put all there is to it either.

I learned all I know by reading articles online and applying some logic to it all - what counts for signal level how is it related to noise, how telescopes work (focuses all incoming parallel light rays to a "single point"), etc ...

Here are some guidelines for deeper understanding of it all:

- understand two main types of noise: Gaussian and Poisson

- understand four main sources of noise: read noise, dark noise, lp noise and shot noise (first one is gaussian others are poisson distributions)

- figure out formulas for sampling (focal length / pixel size stuff)

- understand how and why stacking improves SNR.

And after that just think about how things vary and what you can do with them

[Louis D]

7 hours ago, vlaiv said:

Comparing two scope in astro photographic domain by F/ratio and concluding that one will create image faster - is simply not true. F/10 scope can be faster than F/5 scope, and in here faster means - less time spent to make an image of equal SNR.

Please explain one example where an f/10 scope produces an image of equal density in less time than an f/5 scope at equivalent magnification, noise level, and level of detail.  My curiosity has been piqued.

[Stu]

29 minutes ago, spillage said:

Anyway I'm off to go and lazer level up my eq tripod before running a PA routine.

Stop right there, or you will start another debate on whether you need to level your tripod or not! 😱😱🤣🤣

[alacant]

48 minutes ago, spillage said:

level up my eq tripod

You don't need to level your tripod unless you have a fast or a slow telescope.

18 minutes ago, Stu said:

Stop right there

Phew, that was close. Almost blew it!

Merry Xmas everyone:)

[spillage]

🤪🤣

[Mick H]

All things being equal, I push down on the forward thrust to the overhead negative on the phlange grooble  to maximise the grosh pins compensatory grasp lever, and level up the eq tripod. 😉

[vlaiv]

1 hour ago, Louis D said:

Please explain one example where an f/10 scope produces an image of equal density in less time than an f/5 scope at equivalent magnification, noise level, and level of detail.  My curiosity has been piqued.

Ok, here it is:

F/10 scope is for example 6" (or rather 150mm to be precise) F/10 scope and F/5 scope is 4" (again 100mm) F/5 scope.

F/10 scope has 1500mm focal length and F/5 scope has 500mm focal length.

We put sensor A on F/10 scope and put sensor B on F/5 scope.

Sensor A is 3 times larger than sensor B and has x3 pixel size of sensor B.

Due to fact that F/10 has x3 focal length of F/5 scope and sensor A is three times larger (x3 height and x3 width) than B - both F/10+A and F/5+B will provide exact same FOV.

Due to fact that A has x3 larger pixels than B and F/10 has x3 longer FL than F/5 - again F/10+A and F/5+B will have same sampling rate / resolution in arc seconds per pixel.

So both FOV and resolution of these two setups are the same.

However light gathering surface of F/10 scope is 75^2*PI centimeters squared vs 50^2*pi centimeters squared of F/5 scope or calculated this turns out to be ~17671.5 : ~7854 = x2.25

F/10 scope will gather 2.25 more photons in same amount of time as will F/5 and those photons will be spread over same FOV and sampled by same number of pixels. Measured signal will be therefore larger by factor of x2.25 and resulting SNR will be at most larger by factor of x1.5 (at most because SNR does not depend only on signal level and shot noise, but other noise sources as well).

Here we have shown that F/10 scope when paired with carefully chosen camera and pixel size will be faster than F/5 scope with certain camera and pixel size. We have shown another important thing - speed of astrophotograpy setup depends on other factors than size of objective and focal length and thus those two alone can't be used to determine speed of of setup.

[whipdry]

8 minutes ago, vlaiv said:

Ok, here it is:

F/10 scope is for example 6" (or rather 150mm to be precise) F/10 scope and F/5 scope is 4" (again 100mm) F/5 scope.

F/10 scope has 1500mm focal length and F/5 scope has 500mm focal length.

We put sensor A on F/10 scope and put sensor B on F/5 scope.

Sensor A is 3 times larger than sensor B and has x3 pixel size of sensor B.

Due to fact that F/10 has x3 focal length of F/5 scope and sensor A is three times larger (x3 height and x3 width) than B - both F/10+A and F/5+B will provide exact same FOV.

Due to fact that A has x3 larger pixels than B and F/10 has x3 longer FL than F/5 - again F/10+A and F/5+B will have same sampling rate / resolution in arc seconds per pixel.

So both FOV and resolution of these two setups are the same.

However light gathering surface of F/10 scope is 75^2*PI centimeters squared vs 50^2*pi centimeters squared of F/5 scope or calculated this turns out to be ~17671.5 : ~7854 = x2.25

F/10 scope will gather 2.25 more photons in same amount of time as will F/5 and those photons will be spread over same FOV and sampled by same number of pixels. Measured signal will be therefore larger by factor of x2.25 and resulting SNR will be at most larger by factor of x1.5 (at most because SNR does not depend only on signal level and shot noise, but other noise sources as well).

Here we have shown that F/10 scope when paired with carefully chosen camera and pixel size will be faster than F/5 scope with certain camera and pixel size. We have shown another important thing - speed of astrophotograpy setup depends on other factors than size of objective and focal length and thus those two alone can't be used to determine speed of of setup.

Now that's sorted who believes in santa-claus? 

Peter 

[ollypenrice]

2 hours ago, vlaiv said:

Ok, here it is:

F/10 scope is for example 6" (or rather 150mm to be precise) F/10 scope and F/5 scope is 4" (again 100mm) F/5 scope.

F/10 scope has 1500mm focal length and F/5 scope has 500mm focal length.

We put sensor A on F/10 scope and put sensor B on F/5 scope.

Sensor A is 3 times larger than sensor B and has x3 pixel size of sensor B.

Due to fact that F/10 has x3 focal length of F/5 scope and sensor A is three times larger (x3 height and x3 width) than B - both F/10+A and F/5+B will provide exact same FOV.

Due to fact that A has x3 larger pixels than B and F/10 has x3 longer FL than F/5 - again F/10+A and F/5+B will have same sampling rate / resolution in arc seconds per pixel.

So both FOV and resolution of these two setups are the same.

However light gathering surface of F/10 scope is 75^2*PI centimeters squared vs 50^2*pi centimeters squared of F/5 scope or calculated this turns out to be ~17671.5 : ~7854 = x2.25

F/10 scope will gather 2.25 more photons in same amount of time as will F/5 and those photons will be spread over same FOV and sampled by same number of pixels. Measured signal will be therefore larger by factor of x2.25 and resulting SNR will be at most larger by factor of x1.5 (at most because SNR does not depend only on signal level and shot noise, but other noise sources as well).

Here we have shown that F/10 scope when paired with carefully chosen camera and pixel size will be faster than F/5 scope with certain camera and pixel size. We have shown another important thing - speed of astrophotograpy setup depends on other factors than size of objective and focal length and thus those two alone can't be used to determine speed of of setup.

HOORAY!!!! I think that's nailed it...

lly

[Louis D]

2 hours ago, vlaiv said:

Ok, here it is:

F/10 scope is for example 6" (or rather 150mm to be precise) F/10 scope and F/5 scope is 4" (again 100mm) F/5 scope.

F/10 scope has 1500mm focal length and F/5 scope has 500mm focal length.

We put sensor A on F/10 scope and put sensor B on F/5 scope.

Sensor A is 3 times larger than sensor B and has x3 pixel size of sensor B.

Due to fact that F/10 has x3 focal length of F/5 scope and sensor A is three times larger (x3 height and x3 width) than B - both F/10+A and F/5+B will provide exact same FOV.

Due to fact that A has x3 larger pixels than B and F/10 has x3 longer FL than F/5 - again F/10+A and F/5+B will have same sampling rate / resolution in arc seconds per pixel.

So both FOV and resolution of these two setups are the same.

However light gathering surface of F/10 scope is 75^2*PI centimeters squared vs 50^2*pi centimeters squared of F/5 scope or calculated this turns out to be ~17671.5 : ~7854 = x2.25

F/10 scope will gather 2.25 more photons in same amount of time as will F/5 and those photons will be spread over same FOV and sampled by same number of pixels. Measured signal will be therefore larger by factor of x2.25 and resulting SNR will be at most larger by factor of x1.5 (at most because SNR does not depend only on signal level and shot noise, but other noise sources as well).

Here we have shown that F/10 scope when paired with carefully chosen camera and pixel size will be faster than F/5 scope with certain camera and pixel size. We have shown another important thing - speed of astrophotograpy setup depends on other factors than size of objective and focal length and thus those two alone can't be used to determine speed of of setup.

So, about the photographic equivalent of moving to a larger format sensor and increasing the ISO at the same time while maintaining similar SNR.  For this to actually work, though, the f/10 would need to have a significantly larger image circle than the f/5 if the f/5 were already maxing out the fully illuminated image circle.  I'm thinking of a 4" refractor versus a 6" SCT where this probably wouldn't hold true due to the the rear baffles.