How much does my barlow multiply by?

[Franklin]

Just wondering if anyone out there knows of a simple formula for calculating the multiplication factor of a barlow, ie. what is the relationship between the distance of the barlow element from the eyepiece field lens?

For example; I have a x2 barlow and when used next to the eyepiece it will increase the focal length of the scope by x2, doubling the magnification of the eyepiece. But if I place the barlow in front of the diagonal, increasing the distance between the barlow and eyepiece, the multiplication factor increases. By how much, I do not know but there must be a connection between the distance and the multiplication factor.

Any ideas?

[vlaiv]

You need to know focal length of barlow element

Formula is 1 + distance / focal_length

You can calculate focal length of barlow by measuring distance at which it gives x2 magnification.

Say your barlow has 50mm of focal length then it will give x2 at 50mm distance. But if you put it at 100mm distance it will provide you with x3 magnification because mag = 1 + 100/50 = 1 + 2 = x3

 

[Mr Spock]

There is a formula, but I prefer to measure the actual magnification.

So, for my 2" Barlow and 2" diagonal I get:

Flush with top of Barlow - x2
+10mm 2" to 1.25" adaptor - x2.15
+10mm 2" to 1.25" adaptor + 35mm extension tube - x2.57
+10mm 2" to 1.25" adaptor, Barlow nosepiece screwed into diagonal - x2.9
+10mm 2" to 1.25" adaptor with Barlow in front of diagonal - x3.68

The last two require an extension tube between the focuser and the diagonal to reach focus.

[Macavity]

There are quite a few websites that mention the "Barlow formula" (qv):
https://astunit.com/astunit_tutorial.php?topic=barlow

The main issue is that, if you start changing the distances between the
various optical elements by a lot, it may be hard to find focus at all! 🙄

Perhaps best not to depart too far from the design configurations?
But then, no harm in a bit of (slowly incremental) experimentation... 😏

Edited December 23, 2021 by Macavity

[Franklin]

22 minutes ago, vlaiv said:

Formula is 1 + distance / focal_length

Thanks Vlaiv, that looks like it will work. I'm hoping to use an extension tube with my Baader x2.25 barlow to try and increase it to something like x4/x5. It's to use with a 12mm ortho microguide for attempting double star measurements. From what I've read a scope focal length of 3000-5000mm is advised but my longest scope is 1122mm, hence my need to ramp up the power. The microguide gives about 100x at native but I think I need to get it up to around 350x or higher so the linear measurment on the reticle can be gauged more accurately. I'm keeping an eye out for 5x powermate but until then I will experiment with what I have.

[Mr Spock]

I suspect when you increase the focal length the light cone becomes shallower, and, as with most optics, that puts less of a strain on it. My 2” ED certainly doesn’t suffer for it. 
 

[vlaiv]

5 minutes ago, Franklin said:

Thanks Vlaiv, that looks like it will work. I'm hoping to use an extension tube with my Baader x2.25 barlow to try and increase it to something like x4/x5. It's to use with a 12mm ortho microguide for attempting double star measurements. From what I've read a scope focal length of 3000-5000mm is advised but my longest scope is 1122mm, hence my need to ramp up the power. The microguide gives about 100x at native but I think I need to get it up to around 350x or higher so the linear measurment on the reticle can be gauged more accurately. I'm keeping an eye out for 5x powermate but until then I will experiment with what I have.

There you go - good practice for measuring things with microguide

Find distant object and feature on it and measure angle with microguide - then add barlow and extension until you get x4-x5 angle on that same feature.

That way you don't need to measure focal length of barlow - you can simply find setting where it gives you x4 or x5 magnification you are after.

[Franklin]

30 minutes ago, Macavity said:

There are quite a few websites that mention the "Barlow formula" (qv):
https://astunit.com/astunit_tutorial.php?topic=barlow

Thanks for the link, that's exactly what I'm looking for.

[Mr Spock]

Have a look on ENS. There are a number of x5 Barlows on thrre. 

[Franklin]

2 minutes ago, vlaiv said:

There you go - good practice for measuring things with microguide

Find distant object and feature on it and measure angle with microguide - then add barlow and extension until you get x4-x5 angle on that same feature.

That way you don't need to measure focal length of barlow - you can simply find setting where it gives you x4 or x5 magnification you are after.

See, that's why SGL is great.

Doh, why didn't I think of that!

So if I measure a luna crater without barlow (say crater is 3 divisions on scale). With x2 barlow crater should cover 6 divisions on scale. That will be much simpler for me. Thanks.

[vlaiv]

1 minute ago, Franklin said:

See, that's why SGL is great.

Doh, why didn't I think of that!

So if I measure a luna crater without barlow (say crater is 3 divisions on scale). With x2 barlow crater should cover 6 divisions on scale. That will be much simpler for me. Thanks.

Yep - and you don't even need to do it during night. Find something far away - like church tower or power line mast or whatever and figure out during the day configuration that gives you wanted magnification.

[andrew s]

A simple way to calculate magnification is to measure the diameter of the exit pupil as see by looking into (from a small distance) the eyepiece.  Divide this into the size of the entrance pupil i.e. normally the objective diameter and that's the magnification. Only need it roughly focused at infinity.

Can be done by the fire in daylight! 

 

Regards Andrew 

Edited December 23, 2021 by andrew s

[Don Pensack]

A very easy way:

#1.Time the passage of a star across the field in the eyepiece.  Convert to seconds.

#2.Time the passage of the same star across the field with the Barlow in place.  Convert to seconds.

#1÷#2 (i.e.1/2) = magnification factor of the Barlow for that eyepiece.

Since different eyepieces have their focal planes in different places within the eyepieces, the magnification factor will vary slightly

for every eyepiece.  Parfocal eyepieces will all have the same magnification factor.

[Mr Spock]

I like to do the measuring inside where it's light and warm  I measure by sight but you could just as easily use an iPhone or similar.

I take readings for a number of eyepieces and average them out. The 2" Barlow +10mm adaptor + 35mm extension had a range of x2.56 to x2.58 which is a pretty good +-0.4% variance. So I'm happy with calling it x2.57 - perfect for my needs