Anyone here good at Maths or Raytracing??

The Problem:

I've a 75mm diameter refractor with a focal length of 500mm. I place a 2x barlow of focal length of 75mm into the optical path at a distance of 360mm from the objective.

At what point will I have a cone of light 20mm in diamter??

You answer would be greatly appreciated.

Have fun,

The Problem:

I've a 75mm diameter refractor with a focal length of 500mm. I place a 2x barlow of focal length of 75mm into the optical path at a distance of 360mm from the objective.

At what point will I have a cone of light 20mm in diamter??

Are you looking for something less flippant than just dont use the barlow at all because the cone should be about 20mm at 360mm anyway?

Ah, yes you're right! :oops:
Went through so many calcs earlier, I just got totally confused!

Just wondering, are there ever two answers to the issue of 20mm i.e
500 / 75 = 6.666 6.666 x 20 = 133mm
i.e. apart from 500-133 = 367mm, could it also equal 500 + 133mm = 633mm??

Ok, let's try and tackle this problem from the other side:

I have a scope: 75mm / 500mm. The OTA is approx 380mm long.
How can I get a 20mm light cone to appear beyoned the end of the tube? Is it possible?

Thanks,

Ah, yes you're right! :oops:
Went through so many calcs earlier, I just got totally confused!

Just wondering, are there ever two answers to the issue of 20mm i.e
500 / 75 = 6.666 6.666 x 20 = 133mm
i.e. apart from 500-133 = 367mm, could it also equal 500 + 133mm = 633mm??

Yea the diverging cone will be at 633.

Ok, let's try and tackle this problem from the other side:

I have a scope: 75mm / 500mm. The OTA is approx 380mm long.
How can I get a 20mm light cone to appear beyoned the end of the tube? Is it possible?

Yea, at 633mm

I'm going to keep trying to avoid having to learn how to calculate barlows here

You could use a barlow to do it but it would need to be placed closer to the objective so that it focuses the light further away and so pushes the 20mm point further out. Though we are talking aout a 2" barlow here probably.

But, eh... why do you want a 20mm light cone rather than a focus? Got a sudden interest in very out of focus stars all of a sudden?

Are you worried about vignetting??
If not, then I dont understand where you are coming from?

Thanks Lads.

...I've got a cunning plan...

All will be revelaed in due course if successful.

Clear Skies,