F ratio - again!

[ollypenrice]

OK, I'm a former teacher so 'how to teach things' is a bit of an obsession. How about this:

Noah has an 8 inch F10 telescope and wants to speed up his image capture time. He buys a 0.5x focal reducer to bring his telescope to 8 inch F5.

His sister Jemima has the same scope but she's opinionated and ignores the 0.5 focal reducer option in favour of an invention of her own, an Aperture Increaser , which also takes her scope to F5.

Who wins?

👹lly

[Stu]

2 minutes ago, ollypenrice said:

OK, I'm a former teacher so 'how to teach things' is a bit of an obsession. How about this:

Noah has an 8 inch F10 telescope and wants to speed up his image capture time. He buys a 0.5x focal reducer to bring his telescope to 8 inch F5.

His sister Jemima has the same scope but she's opinionated and ignores the 0.5 focal reducer option in favour of an invention of her own, an Aperture Increaser , which also takes her scope to F5.

Who wins?

👹lly

So, as an observer not an imager....

.... does it depend on what size target you are imaging?

[Clarkey]

Deafening silence....

I'm not answering, sounds too much like a QI question. I can hear the klaxon now....

[Magnum]

Its all relative 😜

Well, lets see.

The first law of thermodynamics, also known as Law of Conservation of Energy, states that energy can neither be created nor destroyed; energy can only be transferred or changed from one form to another. 🥸 so using a focal reducer or barlow cant increase the number of photons entering the scope, but can either concentrate them into a smaller area or spread them out when they reach the sensor 🤪.

I dont know if thats right but it sounded good in my head, so id say the aperture increaser will win.

 

Edited May 5, 2021 by Magnum

[mikeDnight]

1 hour ago, ollypenrice said:

OK, I'm a former teacher so 'how to teach things' is a bit of an obsession. How about this:

Noah has an 8 inch F10 telescope and wants to speed up his image capture time. He buys a 0.5x focal reducer to bring his telescope to 8 inch F5.

His sister Jemima has the same scope but she's opinionated and ignores the 0.5 focal reducer option in favour of an invention of her own, an Aperture Increaser , which also takes her scope to F5.

Who wins?

👹lly

I'm going to stick my neck out and say the 8" F5 wins. I've no real idea of course as I'm thick, but isn't it more to do with focal length rather than focal ratio? So wouldn't the 8" with its 40 inch effective focal length win out over say a 10" with a 50 inch focal length? I know what my report will say, as they all said the same thing, - "...nice boy, could try harder."

[Stu]

Seems that the teacher has left the classroom! 😉

[jetstream]

9 minutes ago, Stu said:

Seems that the teacher has left the classroom! 😉

Hes trying to wrangle with the 400" diameter scope for the second condition lol!

[Mr Spock]

I believe we can use fuzzy logic to solve this:

Long (x) = { 0, if fl(x) < 40”. (fl(x)-8”)/16”., if  fl. <= ap(x) <= 80”. 1, if ap(x) > 16” }

And for compactness, let

    a = X is Long and X is Short
    b = X is Long or X is Short
    c = not (X is Long)

Then we can compute the following values.

Aperture  fl    X is Long       X is Short        a       b       c
------------------------------------------------------------------------
8”     40”      0.00            1.00                    0.00    1.00    1.00
16”   80”      0.21            0.29                    0.21    0.29    0.79

Because let's be honest, this imaging stuff is fuzzy at best...
 

[ollypenrice]

Whoa! Jemima has an Aperture Increaser, not an Aperture And Focal Length Increaser!  So there is no change in Jemima's FOV and she puts four times as many object photons on the chip. Noah has no new object photons but puts the ones he has already onto fewer pixels.

I have an aperture increaser in every camera lens in my bag.

Olly

Edited May 6, 2021 by ollypenrice

[Stu]

12 minutes ago, ollypenrice said:

Whoa! Jemima has an Aperture Increaser, not an Aperture And Focal Length Increaser!  So there is no change in Jemima's FOV and she puts four times as many object photons on the chip. Noah has no new object photons but puts the ones he has already onto fewer pixels.

I have an aperture increaser in every camera lens in my bag.

Olly

But Noah sees more sky, so has a benefit if imaging something large, no?

[Alien 13]

Assuming you are using the same sensor with these scopes then aperture will win for light collection but will not necessarily give the best results if one of the combinations is a mismatch regarding the pixel size etc.

In the Camera world f/ratio is useful for obtaining correct exposures but there is no way a Camera phone lens at f/4 is the same as a full frame f/4. Perhaps astro kit should use the "crop factor" calculations in there specs 😋

Alan

[andrew s]

You have to define the criteria for winning. Total photons collected or per pixel or mm^2 etc.

Regards Andrew 

[ollypenrice]

7 minutes ago, Stu said:

But Noah sees more sky, so has a benefit if imaging something large, no?

He sees more sky, certainly, but has a quarter of the light.

 

1 minute ago, Alien 13 said:

Assuming you are using the same sensor with these scopes then aperture will win for light collection but will not necessarily give the best results if one of the combinations is a mismatch regarding the pixel size etc.

In the Camera world f/ratio is useful for obtaining correct exposures but there is no way a Camera phone lens at f/4 is the same as a full frame f/4. Perhaps astro kit should use the "crop factor" calculations in there specs 😋

Alan

Noooo!!! Shock horror. 😁 Crop factor is the most misleading term ever introduced into photography. It's not entirely meaningless because it allows subject framing (AKA field of view) to be compared across two focal lengths and chip sizes. However, it implies a relationship between resolution and chip size when no such relationship exists. It also, I've discovered since exploring daytime photography, adds confusion regarding depth of field.  I've read numerous claims to the effect that full frame sensors give a greater depth of field. They may do, but not because they are larger sensors but because they have larger pixels.

Jemima's Aperture Increaser has no effect whatever on her image scale or her system resolution in arcseconds per pixel. These are unchanged, though she does gain in optical resolution assuming her Increaser is diffraction limited.

Noah loses system resolution in arcseconds per pixel while his optical resolution, assuming his reducer is diffraction limited, is unchanged.

1 minute ago, andrew s said:

You have to define the criteria for winning. Total photons collected or per pixel or mm^2 etc.

Regards Andrew 

True. Noah only wins if he needs the extra FOV. On all other counts Jemima wins, I would say.

Olly

[Alien 13]

6 minutes ago, ollypenrice said:

Noooo!!! Shock horror. 😁 Crop factor is the most misleading term ever introduced into photography. It's not entirely meaningless because it allows subject framing (AKA field of view) to be compared across two focal lengths and chip sizes. However, it implies a relationship between resolution and chip size when no such relationship exists. It also, I've discovered since exploring daytime photography, adds confusion regarding depth of field.  I've read numerous claims to the effect that full frame sensors give a greater depth of field. They may do, but not because they are larger sensors but because they have larger pixels.

Olly

I quite like "crop factor" provided the calculations are done correctly, works for depth of field too.

Lets say you have a full frame Canon camera with a f/4 50mm lens, you can get the equivalent image from a Canon crop body including identical depth of field and noise by applying the calculations as follows.

50mm/1.6 = 31.25mm but you also have to apply to the f/ratio so (f/4)/1.6 = f/2.5

So a 32 mm f/2.5 lens would give near identical results on a crop body to the 50mm f/4 on a full frame.

Alan 

[ollypenrice]

9 minutes ago, Alien 13 said:

I quite like "crop factor" provided the calculations are done correctly, works for depth of field too.

Lets say you have a full frame Canon camera with a f/4 50mm lens, you can get the equivalent image from a Canon crop body including identical depth of field and noise by applying the calculations as follows.

50mm/1.6 = 31.25mm but you also have to apply to the f/ratio so (f/4)/1.6 = f/2.5

So a 32 mm f/2.5 lens would give near identical results on a crop body to the 50mm f/4 on a full frame.

Alan 

In order to comment on depth of field don't you need to know pixel size? A larger pixel can move forwards and backwards in the light cone by more than a small one without going out of focus.

Olly

[david_taurus83]

Let's say both cameras are the same and are gathering 1" per pixel. Won't the larger aperture funnel more photons towards each pixel?

If it were 2 buckets collecting rain and deposting into 2 identical baffled containers, wouldn't the larger bucket fill its baffles more quickly?

[gorann]

Is not the simple answer that more aperture aways wins, you just need to find the money, and get fit enough to handle the bigger scope and the bigger mount to swing it, or build an obsy😁

Edited May 6, 2021 by gorann

[alacant]

11 hours ago, ollypenrice said:

Who wins?

Noah.

Jemima has to wait for two friends to arrive before she can lift her telescope onto the mount. Celebrating the end of lock-down, they arrive with frost bitten fingers from the pub car park. It is late. In the darkness, they forget to tighten the dovetail clamp and the telescope falls to the hard concrete floor beneath causing €4000 damage, FLO send a courier to collect, the but the damaged instrument is confiscated on arrival at Dover for not having the correct vaccine documentation. It spends the next 14 days in quarantine at a luxury 5 star hotel.

By this time, Noah has managed 36 hours on m101 and it's now behind the trees when viewed from Jemima's fixed pier.

Am I close? Do I win a gso 16" f5? I promise I'll be careful with it.

[Skyline]

I refer to Stan Moore's Focal Ratio Myth.

f-ratio myth (stanmooreastro.com)

[ollypenrice]

6 minutes ago, Skyline said:

I refer to Stan Moore's Focal Ratio Myth.

f-ratio myth (stanmooreastro.com)

My starting point as well.

Olly

[Chris]

If the aperture increaser consists of a separate mirror and tube twice the size, then I think Jemima has it 🙃 

 

 

 

Edited May 6, 2021 by Chris

[happy-kat]

What ever happens to bend the light so the telescope now has the FOV of an F5 the light is still traveling down the long tube so the original exposure length is not effected to make it in reality shorter if anything the extra glass has the potential to make it longer.

[CraigT82]

2 hours ago, andrew s said:

You have to define the criteria for winning

I'm with Andrew on this one, depends on what 'winning' is?

 

[Captain Scarlet]

I just bumped into Alice and Bob. They’re both confused, angry and sulking

[Mr Spock]

I was wrong. Fuzzy logic can’t explain this, only voodoo can...