Barlow nose piece in a diagonal - measured results.

[Mr Spock]

I have a Meade x2 140 Barlow which supposedly gives x3 when the nose piece is taken off and screwed directly into a standard 1.25 diagonal. Having measured Barlows before, I've found they don't give the specified magnification. This is due to eyepiece placement. As this placement is different in different eyepieces, the only way to see what magnification you're actually getting is to measure it.

The test was performed using an 80ED and a ruler some distance away. Measurements were taken using an iOptron 1.25" diagonal with and without the nosepiece to give the ratio.

These are Circle-T Orthoscopics. 

25mm     x2.36
18mm     x2.40
12.5mm  x2.55
9mm      x2.67
7mm      x2.75

A couple more as a control:
LVW 22mm    x2.65
NLV 10mm     x2.68

None of them any where near the expected x3, but nonetheless very useful, particularly at the short focal lengths. I did also test a 32mm Plössl, but in this configuration it vignettes.

[vlaiv]

I would not be a bad idea to measure barlow focal length (or maybe even look it up online)?

You only need two measurements to determine it, I think - one in "standard" configuration and one screwed in diagonal. Diagonal optical path is also required (but easily measured with/without difference).

Barlow magnification formula is given as (again, if I'm not mistaken):

magnification = 1 - barlow_distance / barlow_FL

(where barlow focal length is negative number - as it is diverging lens)

For x3 magnification - you should place eyepiece at twice the barlow focal length then we have 1 - (2xbarlow_fl) / (-barlow_fl) = 1+2 = 3

[Louis D]

Using a 14mm Pentax XL which focuses at the shoulder as near as I can tell, I get 2.4x natively for the Meade 140 2x Barlow, 1.6x with the nosepiece screwed into the filter threads, and 3.0x with the nosepiece screwed into an Arcturus binoviewer's nosepiece.

I've never tried it screwed into my WO 1.25" diagonal alone, so no measurement there.  I did manage to get 1.0x using the nosepiece spaced 45mm ahead of a 0.5x focal reducer which was then screwed into the front of the binoviewer.  That leads to field curvature, but a nice wide field of view.

[Don Pensack]

What about using the #140 in front of the diagonal in its own tube?  Should be 3x there.

[Mr Spock]

1 hour ago, Don Pensack said:

What about using the #140 in front of the diagonal in its own tube?  Should be 3x there.

Thought of that. But, the thread on the Meade tube isn't the same as the diagonal.

[Mr Spock]

A few more measurements - I'll average these across the eyepieces. The first agrees with findings from a few years ago.

Meade Barlow as is - x2.3

Nose piece in diagonal, and tube used in the diagonal - x3.4

Given I'm looking at a scope with a 900mm focal length, the nose piece in the diagonal works just fine. Rounding the mags on the 12.5mm, 9mm and 7mm (the results aren't 100% accurate so rounding is fine) I get x180, x270 and x350. Perfect for the moon, planets and doubles.

 

[Don Pensack]

20 hours ago, Mr Spock said:

Thought of that. But, the thread on the Meade tube isn't the same as the diagonal.

You misunderstood.  I meant use the entire Barlow in front of the diagonal.  That won't work of course, if your diagonal threads on, but you talked about threading the lens to the diagonal, so I assumed you had a refractor-style diagonal

that has a forward tube section.  In other words: scope >> 1.25" visual back >> barlow complete >> diagonal >> eyepiece.

[Mr Spock]

I see. I'll try that to see if it works..

[Mr Spock]

I get the same as the latter - x3.4. Which is right if you think about it; it's the same light path length Barlow to eyepiece.

[Mr Spock]

Just realised - the 7mm + x3.4 in the 80ED is x300  Looked really dark though.

[Mr Spock]

More experiments, this time using a 2" x2 Barlow and 2" diagonal with the nose piece screwed into the diagonal

x2.5 - the nose piece in the diagonal

x3 - the nose piece in the diagonal and the Meade tube used as an extension. 

x3.1 - the nose piece in the diagonal with the 2" tube used as an extension.

I'm going to try this 'live' when the moon next appears.