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f ratio

alacant

By alacant
in The Astro Lounge

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barkis

barkis

Posted
Posted

Please put this thread back on a sensible footing.
If you are astronomers, then act like them, and stop fooling around, we already have a Joke Box for that, It's called 'The lounge'
where this one will end up If there is anymore stupidity displayed.
The OP should clarify what is post is all about, it looks a bit Cryptic to me. Or Crippled, whichever description fits best.

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ollypenrice

ollypenrice

Posted
Posted
2 hours ago, alacant said:

No one has said that f10 is just as bright as f4 for over 6 days now. What's going on?

The debate has been settled rather conclusively by our Swedish SGL friends who've been processing short exposure data from the slow F ratio Liverpool telescope...

Olly

Edit: here's a useful link. 

We have 67.5 minutes of exposure at F10. But we also have an excellent image of a mag 13.49 galaxy.  

F10.

Two metre mirror.

Which do you think matters most?

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barkis

barkis

Posted
Posted

Alacant, the only problem with your post, was it seemed as if there was previous reference to what you were saying.
It just looked strange, as If you just dropped it in with no explanation needed. It was easily unravelled, but the subsequent replies
were not wholly acceptable. Hence the main culprit being hidden by another Moderator. A totally unacceptable comment.

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ollypenrice

ollypenrice

Posted
Posted
5 hours ago, barkis said:

Alacant, the only problem with your post, was it seemed as if there was previous reference to what you were saying.
It just looked strange, as If you just dropped it in with no explanation needed. It was easily unravelled, but the subsequent replies
were not wholly acceptable. Hence the main culprit being hidden by another Moderator. A totally unacceptable comment.

I might be quite wrong, but as someone who tries to debunk the simplistic idea that F ratio defines exposure time in AP, I felt I was being tacitly invited to respond. I've no intention of rehearsing for the umpteenth time why aperture is the active ingredient in F ratio. Over and out on this one. Those who disagree really should explain to the professionals that they don't need big telescopes.

Olly

 

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andrew s

andrew s

Posted
Posted
14 hours ago, ollypenrice said:

aperture is the active ingredient in F ratio. Over and out on this one. Those who disagree really should explain to the professionals that they don't need big telescopes.

I totally agree but it does not explain the preponderance of small APO refractors  being used by amateur  astro-photographers. Why so few big Newtonian telescopes? 

I assume it is ease of mounting, guiding etc at a small image scale.

Regards Andrew

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ollypenrice

ollypenrice

Posted
Posted
5 minutes ago, andrew s said:

I totally agree but it does not explain the preponderance of small APO refractors  being used by amateur  astro-photographers. Why so few big Newtonian telescopes? 

I assume it is ease of mounting, guiding etc at a small image scale.

Regards Andrew

I guess so. It's more fiddlesome to get a Newt working well. In what is a fiddlesome activity at best, a refractor brings a whiff of simplicity. However, there's no denying the excellence and speed of a well fettled imaging Newt. Another factor is that smaller aperture can be mitigated by longer exposure. As a provider, I'm in the position of needing kit that will just work. While that may never be possible, using refractors is a step in the right direction!

Olly

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dph1nm

dph1nm

Posted
Posted
1 hour ago, ollypenrice said:

totally agree but it does not explain the preponderance of small APO refractors  being used by amateur  astro-photographers.

I think part of it is also field of view. Folks like those wide-field images of nebulae, and while it isn't impossible to have a large aperture and a wide field it gets expensive to manufacture optically and expensive in the amount of silicon you need for the detector!

NigelM

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barkis

barkis

Posted
Posted
On 30/06/2017 at 17:22, Davey-T said:

Good job "Doug" :grin:

Dave

Doug? Who is Doug. Dave, I think Cloudsweeper is a Doug ?.

I could change my name.from Ron. to Doug. but that would cause more confusion . ?

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Davey-T

Davey-T

Posted
Posted
23 minutes ago, barkis said:

Doug? Who is Doug. Dave, I think Cloudbuster is a Doug ?.

I could change my name.from Ron. to Doug. but that would cause more confusion . ?

Just kidding Ron, I know what your name is :grin:

Dave

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barkis

barkis

Posted
Posted
41 minutes ago, Davey-T said:

Just kidding Ron, I know what your name is :grin:

Dave

I can stand a bit of ribbing Dave, fun is good . I hope at my age, I don't yell at the wife one day,.                                "Why do you keep calling  me Ron. silly woman, when my name's Dave" ?.

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Stub Mandrel

Stub Mandrel

Posted
Posted
On 30/06/2017 at 12:40, andrew s said:

I totally agree but it does not explain the preponderance of small APO refractors  being used by amateur  astro-photographers. Why so few big Newtonian telescopes? 

I assume it is ease of mounting, guiding etc at a small image scale.

Regards Andrew

I don't question the quality of imaging achieved by big APO scopes, but having seen what the 130mm newts deliver first hand I do wonder if the superiority of small 80mm fracs over mid-size newts is largely down to the perception that price=performance. Not a frivolous comment, someone who can affords an 80mm Apo scope can probably afford a better mount, camera and software as well, so direct comparisons of cheap Newt and expensive Apo with the same gear and processing are rare.

It's also true that those imaging with these Newts tend to eb beginners (like me) and our image capture and processing skills are not as highly developed as those of moire experienced folk who have been doing things longer and perhaps developed a hankering for some 'nice glass'. I must admit, I fancy  a small frac just because they look nice...

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RichLD

RichLD

Posted
Posted
On 01/07/2017 at 20:40, Stub Mandrel said:

I don't question the quality of imaging achieved by big APO scopes, but having seen what the 130mm newts deliver first hand I do wonder if the superiority of small 80mm fracs over mid-size newts is largely down to the perception that price=performance. Not a frivolous comment, someone who can affords an 80mm Apo scope can probably afford a better mount, camera and software as well, so direct comparisons of cheap Newt and expensive Apo with the same gear and processing are rare.

It's also true that those imaging with these Newts tend to eb beginners (like me) and our image capture and processing skills are not as highly developed as those of moire experienced folk who have been doing things longer and perhaps developed a hankering for some 'nice glass'. I must admit, I fancy  a small frac just because they look nice...

I like lenses/fracs because I don't like diffraction spikes. There's no doubt in my mind that Newts offer terrific value for money though, especially from an aperture perspective. It's all down to personal taste in the end.

I only bother with widefield now as it scratches my AP itch without too much stress. I can't remember the last time I used my SW ED80, it's the Nikkor ED180s on the dual rig at f/3.6 that I've got comfy with now. I like the hassle free, small form factor, easy guiding and photon hoovering aspects of this setup for dark site trips - the 16MP QHY163m gives a very usable resolution with the lenses as well.

At least I know my f/3.6 is not mythical :evil4::icon_biggrin:

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alacant

alacant

Posted
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Posted
22 minutes ago, Kropster said:

What is the f-ratio myth

I don't think it's ever been stated. How about:

'The amount of light falling on any given sensor in any given length of time is dependent upon the focal ratio of the instrument to which it is attached.'

HTH.

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ollypenrice

ollypenrice

Posted
Posted
41 minutes ago, Kropster said:

Just to clarify this thread for the less well informed (me).....What is the f-ratio myth and is it true?

 

The F ratio myth states that exposure time always goes as the square of the F ratio, so F5 will always be 4x faster in AP than F10. Strictly speaking this isn't a myth, it's a fallacy. It is most certainly incorrect and arises from the behaviour of camera lenses where aperture varies and focal length is fixed. In this circumstance there is no fallacy. The fallacy arises when a focal reducer is used to change the F ratio. http://www.stanmooreastro.com/f_ratio_myth.htm

Olly

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