Focal Extenders & Barlows

[Mandy D]

I have a 2 inch Explore Scientific 2x focal extender and an RVO 2x ED Barlow that I am trying to use with my RC6. I have 100 mm of extension tube fitted, but it appears I need another 75 mm to get focus with either of them. I'm not using a diagonal as the aim is imaging, but to test all of this I am using an eyepiece. I can achieve focus with my DSLRs and no extender or Barlow with either 75 or 100 mm of extensions.

Is this expected and if so, what is the most practical solution?

Edit: FLO have 50 mm M90 extensions at £39 each, 25 mm at at £35 and an 80 mm 2" extension tube for £35.

I am thinking that the M90 extensions will result in a more rigid setup than the 2" eyepiece extension tube as they go in front of the focuser and are larger diameter and screwed into place. If I go for 2 x 50 mm, rather than a 50 and 25 this will allow me to have fewer threaded connections to achieve the required 175 mm total and increase rigidity for just £4 more and gives me up to 200 mm.

The advantage of the eyepiece extension tube is that it will also work with my Newts.

Is my proposal realistic? I can always get the eyepiece extension tube for the Newt as well at a later date.

Edited November 3, 2022 by Mandy D

[Don Pensack]

It is expected and 99% of the time, people will use a star diagonal in the scope for visual use (so it can point up and you can still use it).

The star diagonal will use 100-120mm of the rear focal length.

The answer is simple--for visual use, use a star diagonal.

[Mandy D]

1 hour ago, Don Pensack said:

It is expected and 99% of the time, people will use a star diagonal in the scope for visual use (so it can point up and you can still use it).

The star diagonal will use 100-120mm of the rear focal length.

The answer is simple--for visual use, use a star diagonal.

Thank you. As I said in my post, this is all purely for imaging, not visual. 

So, the Barlow would have to go after the diagonal to maintain it's multiplication factor, but the focal extender can go anywhere in the imaging train is my understanding. So if I put another 75 mm of extension tubes on the back of the scope and the rest of the gear behind that, I'm good to go? No diagonal needed.

[Don Pensack]

Whatever extensions will work are fine for imaging.

Just be aware of:

--reflections inside the extensions

--sag in the focuser with the long extensions

--possible vignetting of the edge of the field in a deep sky image due to the long tube.

[Mandy D]

1 hour ago, Don Pensack said:

Whatever extensions will work are fine for imaging.

Just be aware of:

--reflections inside the extensions

--sag in the focuser with the long extensions

--possible vignetting of the edge of the field in a deep sky image due to the long tube.

So,  the M90 extensions between the OTA and the focuser will be stiffer than anything after this, thus reducing sag, plus no vignetting as the light cone has a smaller cone angle before it passes through the Barlow or focal extender. Also, with a Barlow, this arrangement will leave it's magnification factor unchanged: Is that correct?

 

[Don Pensack]

On 05/11/2022 at 10:17, Mandy D said:

So,  the M90 extensions between the OTA and the focuser will be stiffer than anything after this, thus reducing sag, plus no vignetting as the light cone has a smaller cone angle before it passes through the Barlow or focal extender. Also, with a Barlow, this arrangement will leave it's magnification factor unchanged: Is that correct?

 

The magnification of a Barlow increases with distance away from the lens, so it will only be its rated power at the opening of the Barlow and any extensions above it will increase the magnification.

You have to know the focal length of the Barlow to know by how much, or simply time the passage of a star with and without a Barlow, drive off.

A 2x PowerMate will, more or less, have a fairly uniform magnification with distance from the lens, from 2.0x at 0mm from the lens to only 2.05x at 100mm away from the lens:

https://www.televue.com/engine/TV3b_page.asp?id=53&Tab=_app