2" ED Barlow measurements at 1.25"

[Mr Spock]

I'm aware there are calculations you can do to determine the focal length of a Barlow and to see what effect different lengths of tube etc have on the magnification.

Being someone who prefers metrics to theory, I decided to do a bit of testing. Same as always, I used a refactor (this case my 102mm) aimed at a ruler some distance away. I first measured the field size in a number of eyepieces, then repeated with various extension lengths. Here's what I found with 1.25" eyepieces.

Barlow with original 'flush fitting' adaptor (indented top) - x2.0

Barlow with 10mm thick adaptor - x2.15

Barlow with 10mm adaptor and 35mm extension tube - x2.57

Barlow lens assembly screwed to Revelation 2" diagonal, with 10mm adaptor - x2.91. Note: this requires a long extension tube on the focuser to reach focus.

It seems to me with different length of extension tube you could have a Barlow for all occasions.

Here's the test subject:

 

[John]

1 hour ago, Mr Spock said:

I'm aware there are calculations you can do to determine the focal length of a Barlow and to see what effect different lengths of tube etc have on the magnification.

Being someone who prefers metrics to theory, I decided to do a bit of testing. Same as always, I used a refactor (this case my 102mm) aimed at a ruler some distance away. I first measured the field size in a number of eyepieces, then repeated with various extension lengths. Here's what I found with 1.25" eyepieces.

Barlow with original 'flush fitting' adaptor (indented top) - x2.0

Barlow with 10mm thick adaptor - x2.15

Barlow with 10mm adaptor and 35mm extension tube - x2.57

Barlow lens assembly screwed to Revelation 2" diagonal, with 10mm adaptor - x2.91. Note: this requires a long extension tube on the focuser to reach focus.

It seems to me with different length of extension tube you could have a Barlow for all occasions.

Here's the test subject:

 

Meade used to make just that - a variable  amplification barlow lens:

The Baader VIP modular barlow is also used that way through different extension combinations.

 

[Mr Spock]

Having conducted successful tests, I have a 35mm extension tube on the way. With my 102mm and 9/7/6mm orthos, it gives x204, x262 and x306 - a perfect spread. Out of my orthos I find the 9mm and 7mm especially crisp and clear, perfect for planets.

That 2" ED Barlow is rather good, so it will be great to give it some use.

It also occurs to me I could screw the Barlow nosepiece directly into the 35mm extension. I've no idea what magnification that will give - maybe x1.5? I'll find out in a few days 

[Don Pensack]

6 hours ago, Mr Spock said:

I'm aware there are calculations you can do to determine the focal length of a Barlow and to see what effect different lengths of tube etc have on the magnification.

Being someone who prefers metrics to theory, I decided to do a bit of testing. Same as always, I used a refactor (this case my 102mm) aimed at a ruler some distance away. I first measured the field size in a number of eyepieces, then repeated with various extension lengths. Here's what I found with 1.25" eyepieces.

Barlow with original 'flush fitting' adaptor (indented top) - x2.0

Barlow with 10mm thick adaptor - x2.15

Barlow with 10mm adaptor and 35mm extension tube - x2.57

Barlow lens assembly screwed to Revelation 2" diagonal, with 10mm adaptor - x2.91. Note: this requires a long extension tube on the focuser to reach focus.

It seems to me with different length of extension tube you could have a Barlow for all occasions.

Here's the test subject:

 

You missed some:

--Barlow lens attached to bottom of 2" eyepiece.

--Barlow lens attached to bottom of 2"/1.25" adapter

--Barlow in its own tube in front of the star diagonal

 

 

[Stu]

I use extension tubes to vary the magnification I get with binoviewers. I use an AP Barcon optical element fitted with T2 quick changers and then just add one or two extension tubes to vary the power. I find it much easier than changing eyepieces in the binoviewer,  cheaper too!

[Mr Spock]

1 hour ago, Don Pensack said:

Barlow in its own tube in front of the star diagonal

x3.68

It would require a long extension on the focuser to reach focus. Maybe just a bit too much weight on such a setup?

[Mr Spock]

Just an update now the 35mm extension is here.

The 35mm extension plus 10mm 2" to 1.25" adaptor and 2" Barlow measures at x2.52 which is perfect. It means in my 102mm the 9mm ortho gives exactly x200 
So for the 9mm, 7mm and 6mm combination I get x200, x257 and x300 - nice.

The Barlow nosepiece screwed into the 35mm extension = x1.85 and is of no use at all 

[Mr Spock]

As a practical test, tonight I managed to use all options.

The Barlow + extension is a bit big and unwieldy, but it works.

Putting the 35mm extension into the focuser tube means both screwing the nosepiece to the diagonal, and placing the Barlow before the diagonal, come to focus.

[Mr Spock]

One of the options has proved itself valuable tonight.

The Barlow in front of the diagonal with my 13mm LVW gives x202 with the 102mm f7 ED. Views of the moon tonight were so sharp with that combination it easily beat single eyepieces.