Question about Baader Q-Barlow

[Jasonb]

Hi all,

Another question from me, and probably one with an obvious answer!

I just got my hands on a Baader Q-Barlow. It's the one that's 2.25x if you use the full thing and 1.3x if you just screw the bottom part into the eyepiece. I'm aware that people say the bottom part alone doesn't work with some eyepieces as they don't have enough room to screw it in fully.

From that description, before I got it, I assumed there was a lens in the top part of the Barlow as well as lenses in the bottom part that screws out. But there isn't, the top part is just a tube, basically an eyepiece adapter. So, does that mean that the bottom part is the actual Barlow, and the only reason the top part is there is to increase the effect it gives, as your eyepiece is held further away from the Barlow? In other words, if the top part was longer, there would be even more of an effect (say 3x for example)? And if that's the case, what's the issue with the lower part not fully fitting into some eyepieces? Why does it need to fully fit? Let's say it gives 1.3x if it fully fits, won't it just give say 1.4 or 1.5 if it's a little further out as it doesn't screw in the whole way?

I'd appreciate someone explaining this to me, as I'm not sure if I'm missing something or not!

Thanks...

[John]

I have one of those and use it a lot.

The optical element of a barlow lens has a focal length like an eyepiece does. The distance between the lowest lens element in an eyepiece and the barlow lens together with the focal length of the barlow lens determines the amplification that you get, ie: 1.3x, 1.5x, 2x etc, etc. The further that the eyepiece lower element and the barlow lens elements are apart, the larger the amplification factor.

This works Ok within a certain range and you can actually get a variable barlow lens (Meade made one in the past) where the barlow element could be slid along the tube and locked to vary the amplification given. Quite a lot of zoom eyepieces work on a similar principle with a sliding amplification element situated below a fixed upper element both built into the body of the eyepiece with a twist mechanism to vary the distance between them.

The thing you have to watch for as you change the spacing between the eyepiece optics and the barlow optics is that the focal position of the combination tends to change as the spacing increases with the impact being that bringing the combination to focus in a scope can get awkward if the scope focuser does not have enough travel. Also the eye relief of the eyepiece can get extended to the point where finding and holding the correct eye position can get tricky. 

So that's why I said it works OK within a certain range. 

The Baader VIP modular barlow lens works on this principle with quite a wide range of amplifications being possible using various spacers. It uses a higher optical quality lens than the Q-Turret 2.25x barlow, and better fitments generally, hence the higher price:

 https://www.firstlightoptics.com/barlow-eyepieces/baader-vip-modular-2x-barlow-lens-125-and-2.html

Personally I use my Q-Turret 2.25x barlow in it's 2.25x mode with a zoom eyepiece and the whole combination works really rather well as a 9.55x - 3.2x zoom

[John]

I ought to add that using the lens element set of this barlow screwed directly into an eyepiece barrel needs a good 15mm or so clearance up inside the eyepiece barrel to get it to fit. Many eyepiece designs today have lower lens sets within the 1.25 inch barrels which reduce the amount of clearance quite considerably so you just cant fit the barlow lens in. The thread that screws into the filter thread of the eyepiece barrel is on the lower end of the barlow optical set so you have to be able to insert the whole of what is quite a thick lens set before the thread engages. Also you do not want the barlow lens set to contact the lower lens of the eyepiece.

Generally orthos and plossls and work OK with this but not more complex designs. The amplification might vary from 1.3x depending on the eyepiece used. I suspect 1.3x refers to using it with the Baader Classic eyepieces.

Hope that all makes some sense !

Edited January 7, 2021 by John

[Jasonb]

Well that all makes sense and is well explained, thanks!

So basically, as long as the threads of the Barlow can get enough 'grip' (let's say a few turns) in an eyepiece, you can still use it even if it's not fully screwed in? Assuming that the Barlow barrel above the threads has enough room inside the eyepiece and the eyepiece lenses etc. don't get in the way? And all of that assuming you can still get a decent focus point and eye relief distance?

Sounds fine to me, looking forward to trying it out now!

 

[John]

13 minutes ago, Jasonb said:

Well that all makes sense and is well explained, thanks!

So basically, as long as the threads of the Barlow can get enough 'grip' (let's say a few turns) in an eyepiece, you can still use it even if it's not fully screwed in? Assuming that the Barlow barrel above the threads has enough room inside the eyepiece and the eyepiece lenses etc. don't get in the way? And all of that assuming you can still get a decent focus point and eye relief distance?

Sounds fine to me, looking forward to trying it out now!

 

Yes, as long as the barlow element is able to be securely screwed in and it does not contact the lower eyepiece optical element, you should get some form of amplification of the eyepiece. Probably somewhere between 1.2x and 1.5x ?

I tried it with a 4mm non-Baader ortho eyepiece and it worked quite well.