Extension for close focus - how much do I need?

[clubjoker]

Not strictly an astro question, this, but I hope you'll let it pass, as it's a question about astro scopes.

So, I'd like to try using an APM/TMB 80mm f6 refractor for looking at closer things - wildlife, birds. It's got a with a fairly short-throw 2" drawtube.

I'm thinking that by adding an extension tube before the diagonal (planning to use a 60degree 1.25" TeleVue Everbrite), it should be able to focus on closer things.

How much of an extension tube would I need to focus on something, say, 20m away? Or 10m away? Would there be vignetting of the image?

Thanks very much

John

[knobby]

You could try using something like a toilet roll / rolled up paper taped up to carefully line up and see how long you need to achieve focus...

then measure it .

Good luck

[clubjoker]

Thanks, Knobby. I could give the experimental method a go!

I wondered if there was a way of calculating it, given it's an 80mm f/6, so 480mm focal length.

 

[John]

You would need to know a lot of detailed light path info on all the componants in the optical chain to calculate this. Experimenting with a loo roll or similar is likely to prove quicker and could be more accurate as well !

 

[Astrobits]

The formula for lenses is:

1/u +1/v =1/f

u = subject distance

v= image distance

f= focal length

re-arrange to get the distances required

Nigel

[Astrobits]

Just to clarify, any combination of lenses can be replaced by an imaginary simple lens for the purposes of  calculation of object and image positions as in the formula in my previous post.

An imaginary simple lens cannot be used for the calculation of image quality parameters.

Nigel

 

[clubjoker]

Thanks, Nigel.

I just tried that - looks like it should give a decent approximation.

subject at infinity (stars!) gives image 0.480m

subject at 10m gives 0.504m, so 2.4cm more than for subject at infinity

subject at 3m gives 0.571m, so 9.1cm extension

So looks like a 2" extension could do the trick, assuming I've got a bit of in focus and out focus when looking at stars without the extension.

 

 

[clubjoker]

John, thanks too.

I'll try the experimental toilet roll method and see how it compares to the calculation. Then I'll report back

[John]

5 hours ago, clubjoker said:

John, thanks too.

I'll try the experimental toilet roll method and see how it compares to the calculation. Then I'll report back

Interestingly my guess was that a 50mm extension might do the trick - but it was just a guess !