Aggghhh - help needed

[Radec]

Sorry for the Drama..."No drama...."

Ok, here is a conundrum.

I recently acquired an ES ED127 Apo EDF100 - lovely scope - yep, back focus for visual!

So, the focus draw tube is their now standard 45mm and they supply 2 x 20mm extension tubes.

If you screw these together they actually extend the focus tube by 37.5mm because of the thread engagement.

With my ES 2" EPs, focus on a local tree is only achieved with the two extension tubes (37.5mm total) and around 11.5<12.5mm of the focus draw tube extended plus a diagonal.

So....about 49<51mm of total extension to the draw tube - yes?

Now, trying to be logical and reduce complication, I removed one extension tube, thinking that with (originally) around 12mm of the draw tube's available 45mm used, I should be able to achieve focus with a single 18.75mm extension tube plus around 30.25<31.25mm of the draw tube used.

WRONG

Focus couldn't quite be achieved even at the very end of the draw tube (ie 45mm + 18.75mm = 63.75mm)

Now my maths may be wrong or I must be missing some important optical principle.

Can anyone help and enlighten me please?

Thanks in advance

Keith

 

[Radec]

Thanks - not very far - I am hopeful of the difference a few million miles will make John, but it still doesn't answer why the maths doesn't work with the tree!

The focal point should surely be a factor of distance at the back of the tube, whether through an extension tube or not - yet it isn't!

Puzzled......

[John]

Just to explain that I deleted my post because it did not really deal with your question.

The only explanation that I can now think of is that the light path of the extension tubes is longer than you originally thought.

 

[Stu]

Might be easiest to post up some pictures of the two setups so we can see wht is happening?

[KP82]

9 hours ago, John said:

Just to explain that I deleted my post because it did not really deal with your question.

The only explanation that I can now think of is that the light path of the extension tubes is longer than you originally thought.

 

I agree. The light doesn't travel in parallel to the barrel of the extension tube as it moves towards the focal point. Therefore its path is actually longer than the length of the extension tube. In order to work out how much longer it is, that's a complicated geometry question.

[jadcx]

You included the diagonal in the second setup (one extension only), right?

[Radec]

Yep indeed Justin - the only difference between the two was the removal of one extension tube.

I accept the light path theory, but if the focal point is (say) 55mm from the end of the retracted draw tube, surely it shouln't matter whether that is made up of one extension tube plus a lot of focus tube travel or two extension tubes and a small amount of focus tube travel....?

And yet it does!

I mean, it's not like it won't come to focus; it does. It just messes with my head!

[jadcx]

35 minutes ago, Radec said:

I mean, it's not like it won't come to focus; it does. It just messes with my head!

? me too.  It’s definitely the same tree?

As @Stu suggested, maybe some photos might help.

[Radec]

21 minutes ago, jadcx said:

? me too.  It’s definitely the same tree?

As @Stu suggested, maybe some photos might help.

Defo the same tree ? - The scope is in the garage, I have to go outside to get it and it's persisting down with the stuff sent to punish us all because I bought some gear....well, quite a lot of gear. Mind you, I sold quite a lot too, so it's their fault too. Sorry.

Photos to follow when it stops

[Radec]

Ok guys

Here are a couple of photos to show what I mean.

In the first one, the draw tube is out by 12.5mm and there are two extension spacers connected each of 20mm but together only 37.5mm.The EP reaches focus at the 12.5mm point giving plenty of in and out potential.

In the second one I have removed one extension spacer and extended the focus tube by 20mm to compensate. The EP will not reach focus!

In fact, it won't even reach focus when wound right out to 45mm!

Now I'm obviously missing something; can anyone explain what it is?

Thanks in advance

Keith

 

[Stu]

To me your extensions look longer than 20mm, nearer 30 or 40mm in comparison with the focuser drawer tube markings. Are you sure they are 20mm?

[jadcx]

I’ve tried to scale from your photos (which of course isn’t going to be super accurate) and assumed that the focus scale is accurate, but I’m with @Stu - the spacers look to be more like 40mm each.  This would explain it, as you can’t get out to the 52.5mm you would need to focus with one of the spacers removed.

[Radec]

Aaaggghhh - again - guys you are absolutely right - me a muppet! Should've thought that one through better.

They are indeed 40mm each therefore sanity has returned but carelessness on my part conquers all!

Sorry for wasting your time?

[Stu]

16 minutes ago, Radec said:

Aaaggghhh - again - guys you are absolutely right - me a muppet! Should've thought that one through better.

They are indeed 40mm each therefore sanity has returned but carelessness on my part conquers all!

Sorry for wasting your time?

No worries! Always good to get to the bottom of these things! Pictures often help! ??

[jadcx]

31 minutes ago, Radec said:

Aaaggghhh - again - guys you are absolutely right - me a muppet! Should've thought that one through better.

They are indeed 40mm each therefore sanity has returned but carelessness on my part conquers all!

Sorry for wasting your time?

Don’t worry - you should have seen how long it took me to figure out I could remove the spacer on my SX camera.  I could not work out how it was ever supposed to get focus... we live and learn.  Hopefully.

[Radec]

Thanks guys - I blame the flu?