View Full Version here: : Barlow before the diagonal

13-05-2007, 07:13 PM

I have a bintel 2x barlow, and in my reading I have found references where people say placing such a barlow before a diagonal will make it an effective 3x barlow. I just tried it out, with a 45 degree erecting diagonal that came with my skywatcher - doesn't seem to work. I had alpha crux in the middle of field with diagonal -> barlow 25mm, then switched to barlow -> diagonal ->25mm. The latter configuration showed me no light, not even a hint of a star.

And so I conclude that it doesn't work. :P

So my question is, should it have worked, or is there a specific combination of barlow/diagonal that is required?


13-05-2007, 09:03 PM
I think this technique does work, but only on telescopes such as Schmidt-Cassegrains, (SCTs) where there is an enormous range of focus travel.

SCT’s focus by causing the moveable main mirror to slide backwards and forwards on a shaft inside the telescope tube.

Newtonians have a fixed main mirror and a more limited range of focus, restricted to the length of the focus tube, from fully retracted (in) to fully extended (out).

Refractors tend to have a slightly larger range of focus travel as the focus tube is usually longer than Newtonians, although not as long a range as SCT’s.



13-05-2007, 11:36 PM
Oh ok. I also see the reason why people put their equipment in their tag - so one has an idea of what a particular question is about ! :P

I shall fix this soon.

14-05-2007, 08:38 AM
Hi Steve,

What you tried should work BUT!!!

The possible reason for the lack of success here is, by putting the Barlow before the diagonal, that the Barlow lens itself is further into the light cone than where it was designed to be and as a result the focuser travel (out) is not enough to focus - an extension tube to move the Barlow further out will help.
Please note that magnification may be increased by moving the eyepiece further and further from the Barlow this gain comes at a price ... vignetting.
Barlows work by diverging the light beam and as they refract the light there is dispersion so if there is no chromatic aberration expect a little CA if you use an "ordinary" Barlow lens.



14-05-2007, 11:57 AM
By using the basic lens formula
1/F = 1/A +1/B where A and B are positions of the images (virtual or real) and F is the focal length of the Barlow, it could be shown that closer the Barlow focal point is to the main mirror focal plane, the greater effect it has.
Also, have a look at this link:

I also included two files here, optic is optical path simulator (very general ray tracing program), very simple and easy to use and other one is html with optical formulas... All very useful stuff ;)

14-05-2007, 09:51 PM
Thanks bojan. If I dig out my old lecture notes on optics, I should be able to work it out myself anyway.

Ah of course. Thanks :)

15-05-2007, 09:50 AM
You will be surprised what can work but play around during the day.

I combine barlows and once had 2 3x and a 2x in the 150 ar ...great for observing ants in a tree...the before and after works as it has been said its the focus range that may prevent this.... but dont be in a rush for higher mag its not all it is cracked up to be really.. the smaller the mag for the job is the approach.

15-05-2007, 10:54 AM
Hi Steve,

I never thought about doing what you suggested so I thought of giving it a try. I rumaged around and found a 2" WO right angle diagonal (a spare kicking around!) and a GS 2" ED Barlow lens (bought only for test purposes) BTW on test with a 6" apochromat it performed exceptionally well.

The ED Barlow assembly unscrews into 3 pieces and the lens cell screws into the diagonal - very neat - but requires the silver tube to be shortened and re-threaded (in work) to suite the 'scope. I now have a "x(you guess) Diagonal" beats all that length before the eyepiece.


15-05-2007, 10:59 AM

A couple of pics for the previous post.


15-05-2007, 01:59 PM
Hey thats neat, I will give that a try too! :)

Wish I just had a diagonal "kicking" around - I need a job to get some fine glass.

And yeah high mag isn't everything, but I have a f/5 scope, and to get to 150x or so (my max practical is 200x) I need a 2.8-3x barlow with my existing eye piece collection. Currently I only have a few below 100, then from 100 its a jump to 200 with nothing in between.


15-05-2007, 03:57 PM
Hi Steve,

1. Barlows have many uses and magnification ranges but I only use them to get the next eyepiece focal length in the range. I really do not know why manufacturers choose x2 as this will duplicate a given eyepiece(eg. 12mm to 6mm, 50mm to 25mm etc.) the best multiplication factor to give one the smoothest eyepiece focal length range is a factor of (twelfth root of 2) 1.23 something. The closest "Barlow" to this that I know about is the Tak x1.6 five element extender.

2. Re: "Max Magnification"

How high you take a magnification as being useful is a function of "Seeing", "Sky Transparency" and optical quality. The number max mag is 50(Aperture in inches) or 45(Aperture in inches) is very rubbery and doesn't say a great deal other than to tell us a magnification needed to see a split double star at the limit of the telescope's resolution. It tells us nothing about the contrast one would see when viewing the planets or the moon. High magnification, in good conditions, allows for scaling of the image so that one sees a "nice large" image.


15-05-2007, 10:37 PM
Same here. I can easily reach my scope's maximum of 200X (102mm refractor) with a barlow and my 5mm. Its just I want something between 100 and 200, and with my current collection of eye pieces I need a 3x, so I was hoping for some "cheap thrills".

Attempting to acquire a 6.4mm meade, to give 154X with a barlow.