Barlow before diagonal?

[robhatherton]

I'm using a Revelation 2.5x Barlow with 12, 25 and 32mm eyepieces in my Omni xlt 120 refractor. The 2.5 Barlow with the 12mm gives about 200x magnification which is as much as would be practical most nights. However, if I put the Barlow before the diagonal I get quite a lot more magnification and can still achieve focus. Can anyone tell me what the likely magnification is and is this a viable solution for nights of excellent seeing when I might want more magnification?

Or just a daft idea?

[Luke]

My guess would be you would be getting about 300x magnification, as placing a 2x barlow before the diagonal is said to give about 3x typically.

I don't see anything wrong with putting the barlow before the diagonal, so long as everything feels secure.

300x is the max mag I like to use myself on planets in top conditions, though some prefer lower and others higher. It's nice to have the option. I have used 300x in my 120mm but not sure if that is slightly over its resolving limit - no doubt someone will know?

[alan potts]

Never heard of that before, nice one and well found out, anyone know if the same applies to a Powermate? I would go and see but it is cloudy for the first time in a week, hope you have the clear bit to-night

Alan

[JamesF]

It probably won't work with a powermate because they give a parallel light "cone" to the best of my knowledge.  It works with a normal barlow because the cone is still diverging.

I use the effect (though with an extension rather than a diagonal) to get more magnification out of my 2.5x barlow when imaging.

James

[JamesF]

"magnification" is a bad choice of word there.  I shouldn't have used it.  To get "a larger image at the focal plane" would have been better.

James

[Stu]

Never heard of that before, nice one and well found out, anyone know if the same applies to a Powermate? I would go and see but it is cloudy for the first time in a week, hope you have the clear bit to-night

Alan

Yep, James is right, the PowerMate magnification factor doesn't change with distance as the light 'rays' are parallel after passing through it. For the same reason, they don't affect eye relief either.

You can go the other way with a Barlow if the lens element can be unscrewed, if it is attached to the bottom of an eyepiece ie closer to the ep than normal it gives a lower multiplying factor.

Just don't ask what happens when you stack two barlows because you will get two very different answers!! Runs for cover....

Stu

[faulksy]

Never heard of that before, nice one and well found out, anyone know if the same applies to a Powermate? I would go and see but it is cloudy for the first time in a week, hope you have the clear bit to-night

Alan

come on alan, I thought everyone new that,  

[avtaram]

A Barlow placed between the focuser and the diagonal will give a magnification that is 1.5 times that of the one stated on the Barlow.

A 2.5 X Barlow will become a 3.75X Barlow.

Avtar

[alan potts]

Faulksy,

No I didn't honest. I had a barlow when I first started astronomy when I was 18, but that is a long time ago now. Maybe why I didn't know is because it don't work with a Powermate, I only remember I have one when posts like this come up.

Alan.

[JamesF]

A Barlow placed between the focuser and the diagonal will give a magnification that is 1.5 times that of the one stated on the Barlow.

A 2.5 X Barlow will become a 3.75X Barlow.

Surely it depends on the length of the optical path through the diagonal?  I reckon that could vary by as much as 30% through different brands and designs of diagonal.

James

[Cornelius Varley]

The power of a barlow is a function of the distance to sensor / eyepiece. Increasing the distance by 1 barlow focal length increases the barlow power by a factor of 1.

[BillP]

There are 2 ways you ca know for sure what the magnification will be:

1. FORMULA -- Barlow Magnification (M) = 1 + (D / FL).  D is the distance from the top of the Barlow housing (or the shoulder of the eyepiece if you are extending the eyepiece away from the Barlow) to the center of the Barlow lens elements.  FL is the published Focal Length of the Barlow (ignoring the sign as it is usuall negative).  So as an example, I have the APM 2.7x ED Barlow.  They have given me the magnification factor, and if I measure the distance from the top of the Barlow to the center of where the double elements are I find it is 105mm, so what is its focal length?  Just use the above equation and solve for FL instead.  So the formula rearranged is

FL = D/(M-1).  So then for my APM 2.7x ED Barlow its focal length = 105/(2.7-1) = 62mm.  So if I decide to put the APM ED Barlow in front of my diagonal, and I measure the additional distance added by the diagonal as 100mm, then my new "D" is 200mm from the diagonal + 105mm that I measured for my Barlow, so 305mm.  Now I can just use the formula to calculate the Barlows new magnification:  M = 1 + (D/FL) = 1 + (305/62) = 5.9x.

Now the above method may be too cumbersome for some folks, especially if you are not formula inclined.  So the next method is easy and fun and very intuitive and deadly accurate. 

2. FIELD TEST - During the daytime aim your scope at a tape measure and count the centimeters that are visible through the eyepiece without any Barlow.  Note the tape measure must be positioned so it bisects the center of the FOV.  So let's say you observe your tape measure going across the center of the AFOV and you count the tic marks and the result is that the AFOV is 20 CM in diameter.  Now place the Barlow in front of the diagonal and reobserve the tape measure and count the number of centimeters visible across the center of the AFOV.  Let's say you count 4.5cm.  All that is left to do is to divide your first observation by your secoond to get the resulting magnification factor of the Barlow in that configuration: 20cm / 4.5cm = 4.4x.

[BillP]

FYI, t he above has an error:

This:  " and I measure the additional distance added by the diagonal as 100mm"

Should read: " and I measure the additional distance added by the diagonal as 200mm"

Some day, I will be able to edit my posts :-(

[robhatherton]

Thanks for the replies: so I should get around 300x (whatever I thought was magnification!?) without any further loss of quality subject to excellent viewing conditions?

And I thought a Powermate was some sort of workbench!