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Western Crises

Starman

By Starman
in Imaging - Lunar

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Astroman

Astroman

Posted
Posted

Rog - that was at f/20 (f/10 + 2x Barlow) and the seeing was cr*p!

I had this fight once before on another forum. A Barlow does not increase the overall focal ratio of the system, only the focal length of the particular eyepiece. Yes, the length of the barlow has a small effect, but not double.

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Jamie

Jamie

Posted
Posted

Interesting but I'm too tired for a fight :)

So when imagers state that their system is working at f/20, f/30, etc. - they are wrong or is this just convention?

Pete

This is interesting because i have always beleived that if like me you have an F9.4 scope when

i put my 4x image mate on then my imager the image would have been taken at F 37.6 or there abouts.

Could you please talk me through the ins and outs of the F ratio for future reference.

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OXO

OXO

Posted
Posted

So the formula is still the following?

How to calculate projection focal lenght?

Take the following formula (in mm):

PFR (Projection focal ratio)=TFR (Telescope Focal Ratio) x M (Magnification)

M=(ECD (Eyepiece-CCD distance) - EFL (Eyepiece Focal Lenght))/EFL

For example:

A f/6 telescope, a 10mm eyepiece, and a distance of 50mm.

M=(50-10)/10

M=4x

PFR=TFR x M

PFR=6x4

PFR=24

A 10mm eyepiece, with a distance of 50mm between the eyepiece and CCD, you'll get a f/24 system.

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Ant

Ant

Posted
Posted

Nice one Pete!

As Russ said your images are always stunning...

With regard to the F/Ratio debate...

Shurely a Barlow is the opposite of a Focul reducer? So if a focul reducer brings the F/Ratio down from F/10 to F/6.3 (for example) then a barlow would increase it F/10 to F/20 (for a 2 times) ?

Ant

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Astroman

Astroman

Posted
Posted

So the formula is still the following?

How to calculate projection focal lenght?

Take the following formula (in mm):

PFR (Projection focal ratio)=TFR (Telescope Focal Ratio) x M (Magnification)

M=(ECD (Eyepiece-CCD distance) - EFL (Eyepiece Focal Lenght))/EFL

For example:

A f/6 telescope, a 10mm eyepiece, and a distance of 50mm.

M=(50-10)/10

M=4x

PFR=TFR x M

PFR=6x4

PFR=24

A 10mm eyepiece, with a distance of 50mm between the eyepiece and CCD, you'll get a f/24 system.

This is correct for an eyepiece projection system.  To be completely accurate, you have to call it "Overall Effective Focal Ratio", because it refers to several elements lumped into one overall system.  The difficulty and confusion lies in mixing telescope parameters, photographic terms and eyepiece projection terms which share terminology but not definitions of the same words.

When inserting a barlow in a strictly visual sense, it doubles the focal length of the eyepiece only, since the image still comes to focus at the field stop of the eyepiece.  The focal length of the telescope is altered only by the physical length of the barlow.

When eyepiece projection is used, it is literally "projecting" the focal plane of the image behind the field stop of the eyepiece to the film plane, be it film or CCD chip.  Since the film plane is incapable of changing its focal point, ie it must come to focus on the plane, the system must be recalculated to account for it.  That's what the formulae above are for.  Relatively small changes in the distances involved and the focal length of the eyepiece used produce radical changes in the overall effective focal ratio of the system.

So, you have focal length, how far from the main optical component the image comes to focus, and focal ratio, the focal length divided by the diameter of the main optical component.  Normally, the ratio is the focal length of the aperture divided by the eyepiece FL.  When you add "other stuff", barlows, ep's and distances between them all, you change the overall FL.

See?

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FLO

FLO

Posted
Posted

When inserting a barlow in a strictly visual sense, it doubles the focal length of the eyepiece only, since the image still comes to focus at the field stop of the eyepiece... 

That ties in with what I know about modern short focal length eyepieces.  That is, that they are essentially longer focal length eyepieces with built-in barlows that shorten the focal length but retain the longer eye-relief ....  :scratch:

Steve :)

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