Barlow Question

[cv01jw]

I have a Williams Optics Barlow from a set of binoviewers which increases the focal length of the scope by 1.6x - it is a small barlow which screws directly to an eyepiece.

I have just got a Baader 2.25x Q Barlow which can be broken down and the small lens unit used as a screw on barlow at 1.3x.

My question:

If i fit the 1.6x barlow to the Baader unit, in place of the 1.3x unit, will this result in an increase of 2.75x?

I worked this out thus:

Baader unit is 2.25x or 1.3x, so 2.25/1.3 = 1.73

Replacing the 1.3x lens with a 1.6x would give an overall increase of 1.6 x 1.73 = 2.75

Is my maths correct?

I hope so, because it gives me a very wide range of magnifications with a single 18mm orthoscopic eyepiece

[John]

In theory, yes but the precise amplification that you will get using the 1.6x William Optics optical unit in the Baader tube will depend on the focal length of the William Optics barlow element, which I'm afriad I don't know. Barlow amplification is a product of the focal length of the barlow combined with the spacing between the eyepiece field lens and the barlow lens as I understand it. As you have all the componants, give it a try and see how it turns out 

[cv01jw]

Thanks John - approximate numbers are OK, I will give it a go later on (weather permitting).

[jock1958]

I love this forum....hadn’t thought about doing that with my Baader Q-turret lens. Will give that a go when I’m off work next

Thanks 

[Ricochet]

If the 1.6x is specified for when the barlow element is used as the binoviewer nose piece then the magnification when screwed to the Baader barlow body will be less than 1.6x. The magnification depends on the distance between the barlow element and the eyepiece. With the binoviewer between them this distance is about 100mm and I imagine the Baader barlow tube is significantly shorter than that. 

You can use drift times to determine the magnification given by any particular barlow set up. Pick an eyepiece and star and take a few drift timings to get an average. Do the same with the barlow in place and divide one time into the other to get the ratio. 

[cv01jw]

Thanks Ricochet - I will try this.  I assume using a crater on the Moon would work just as well to measure drift times?

[Ricochet]

If the crater is small enough for you to get accurate timings I guess that would work. You can try it and see if you have difficulty determining when a crater crosses the edge of the FoV. Even with point source stars this can be more difficult than you would think due to edge distortions.

[cv01jw]

Thanks, I will have a go tonight if the weather is good.

I keep saying 'if the weather is good' because I have been saying it all week and so far have managed to get out and observe through clear skies for 5 nights on the trot!  I have done more observing this week than I managed in April and May combined.

[Cornelius Varley]

Maybe of interest to you Barlow Lenses

[Louis D]

If you have extension tubes and/or a really long backyard (UK garden), you can tape up a ruler or yard stick (meter stick?) and note how much of it is visible through the eyepiece by itself and with various barlow combinations.  After you have those numbers, it's just a matter of ratios to figure out the magnification factors.

[jock1958]

Hi Guys

If I screw the lens from my Baader Q-Turret Barlow into my 5mm Vixen SLV will this give me a magnification of x243 in my Tak 100DL? 

Thats assuming the lens on its own gives x1.3? 

[Louis D]

4 hours ago, jock1958 said:

Hi Guys

If I screw the lens from my Baader Q-Turret Barlow into my 5mm Vixen SLV will this give me a magnification of x243 in my Tak 100DL? 

Thats assuming the lens on its own gives x1.3? 

Assuming the barlow element doesn't impact the lower Smyth lens group.  Carefully check for clearance before inserting the unit into any negative/positive eyepiece.  Many eyepieces of this type put the Smyth group right at the end of the barrel to minimize in-focus requirements.

[jock1958]

Thanks for that, measured the Barlow lens length and inside my Vixen eyepiece and both about 13mm. 

So looks like the Barlow lens will bottom out on the Vixen,  my BCO eyepieces have loads of clearance so no issues there.