What is the light beam shape in the lens?

[Vroobel]

Hi,

I hope this is proper section for may question.

Is someone able to draw a shape of light beam in the lens presented in the picture below? The vertical thick line on the right side is called "imaging plane", so I assume that earlier it was a photo film, but now it's the sensor. The thick lines in midle indicate position of iris and let's leave it fully open.

 

[vlaiv]

I don't think it is possible without knowing parameters of the lens - like focal length.

We could perhaps make some assumptions based on shape of the lens, and say that left one is achromatic pair - telescope lens and that second is focal reducer and then ray diagram might look like this:

(black lines are at 0 degrees and red lines are at slight angle and focus at some distance from center of sensor / optical axis).

[Gfamily]

Haven't been able to find any ray diagrams for it, but there's information here that might be of interest 

https://joseph-tang.blogspot.com/2017/11/tair-3-300mm-telephoto-lens.html

https://allphotolenses.com/lenses/item/c_857.html

http://photohistory.ru/1207248188998898.html (in Russian)

[Vroobel]

On 09/01/2023 at 22:46, vlaiv said:

I don't think it is possible without knowing parameters of the lens - like focal length.

We could perhaps make some assumptions based on shape of the lens, and say that left one is achromatic pair - telescope lens and that second is focal reducer and then ray diagram might look like this:

(black lines are at 0 degrees and red lines are at slight angle and focus at some distance from center of sensor / optical axis).

Oh, my God, I forgot to write it. It's 300mm FL lens.

[Vroobel]

On 09/01/2023 at 23:02, Gfamily said:

Haven't been able to find any ray diagrams for it, but there's information here that might be of interest 

https://joseph-tang.blogspot.com/2017/11/tair-3-300mm-telephoto-lens.html

https://allphotolenses.com/lenses/item/c_857.html

http://photohistory.ru/1207248188998898.html (in Russian)

Hi,

Thank you for the links, they are useful. .

 

[Gfamily]

2 hours ago, Vroobel said:

Oh, my God, I forgot to write it. It's 300mm FL lens.

My first thought is that it was a true telephoto lens, where the primary doublet would have a shorter focal length than 300, and the negative lens would extend the focal length, giving a shorter than 300mm distance between the primary and the image plane. 

However, there is very little difference between the nominal 300mm focal length you mention and the 289mm shown on the technical spec. 

This makes me think that the second element is effectively a coma corrector for the relatively wide aperture f/4.1 lens, and the effects on any ray diagram would only be minor adjustments to the off axis image. 

[Vroobel]

I think here is misunderstanding, my lens is TAIR-3-PhS 300mm F/4.5 which is written on its tubus. When I was updating acquisition data at the astrobin I found out there that it's 299.8mm FL.

I'm confused...

 

[Gfamily]

3 hours ago, Vroobel said:

I think here is misunderstanding, my lens is TAIR-3-PhS 300mm F/4.5 which is written on its tubus. When I was updating acquisition data at the astrobin I found out there that it's 299.8mm FL.

I'm confused...

I'd reckon that the difference can safely be disregarded. What's the reason for the confusion? 

[Vroobel]

That's because of the 289mm FL and the F/4.1.

[vlaiv]

43 minutes ago, Vroobel said:

That's because of the 289mm FL and the F/4.1.

I'm still confused as what the issue is?

Effective aperture stop in diagram is located behind first lens group. First lens group is 70mm in diameter - but it's being stopped by aperture stop, and in order to see how much it's being stopped down - we need to know the power or focal length of first lens group.

Since we don't know that - we can't calculate effective aperture and thus can't calculate F/ratio.

We can do things in reverse.

We can say - ok, we accept that effective F/ratio of system is F/4.5. At 289mm of focal length (effective) - that will be 64.222mm of effective aperture.

Now we need a bit of math to calculate focal length of first group:

From above diagram we can write equation:

32.11:(70+X) = 26.15:X

or

32.11 * X = 26.15 * X + 1830.5

5.96 * X = 1830.5

X = 307.13mm

Focal length of first group is ~377mm - or it is F/5.3 achromatic pair.

We can now also work out focal length of second element (that acts as reducer) from this and spacing on the diagram ....

However - this is only true if we assume that F/4.5 is correct and that effective aperture is indeed 64.222mm

Alternatively - we can go in reverse - but to do that we need focal length of first lens pair.

[Macavity]

For a general impression, the above "thin lens" ray diagram is sufficient.
Detailed optical design references might feature the full ray tracing... for
"thick lenses"... Cardinal Points ... nodal planes all that sort of stuff?!? 😉

[Ratlet]

Slightly related but,

I wonder if the 142.34 is the distance from the rear element to the focal plane of the image?  I've thought about removing the complete lens assembly from my Tair 3 and mounting it in a tube which is shorter so I could us it as a visual scope.  It's got some pretty good chromatic correction and a very wide field so could be a very fun DIY scope.