Why use a Barlow with a zoom lens

[Sunnydays]

 I have a 6se and used my new Celestron zoom last night; it was great? Now i know why people love them! My question is ,if the zoom lens does in theory what the Barlow would do when using a single EP, what is the benefit of using a Barlow with a zoom? I read people doing this? What magnification EP would you not use a Barlow with, and what would you use a Barlow to see?

[John]

I quite often use the zoom + barlow combination for flexible high power observing of the moon, planets and double stars. Sometimes small planetary nebulae and globular clusters as well.

My scopes, mostly, have shorter focal length than the C6 SCT though.

With the barlow lens, my zoom gives me a focal length range of 9.5mm to 3.2mm. In most of my scopes, that is a very useful range.

Having the ability to instantly adjust the focal length is very useful for finding out the optimum magnification for the seeing conditions and target.

 

[Sunnydays]

Do you start with a lets say 32mm then switch out to the Barlow and zoom?

[Sunnydays]

Can I reasonably use a zoom and a Barlow with my scope? (6se) Or am i pushing the limits already just using standard magnification with the zoom?

[Waddensky]

Simple. My 8-24mm zoom used with my 2x focal extender (comparable to a barlow) gives 4-12mm focal length. The two together give me a stepless 24mm - 4 mm focal length range to choose from, resulting in magnifications from 50 to 300x. Without the extender, this would only be 150x.

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Can I reasonably use a zoom and a Barlow with my scope? (6se) Or am i pushing the limits already just using standard magnification with the zoom?

The resolution of a 6" scope allows for about 150x magnification. Magnifying more is possible (up to about 300x), but will not resolve more details. Under normal circumstances (seeing), the maximum useful magnification is around 150-200x.

[John]

On 05/07/2020 at 20:09, Sunnydays said:

Do you start with a lets say 32mm then switch out to the Barlow and zoom?

Sometimes yes and sometimes I go straight in with the zoom / barlow combination. Depends what I'm observing.

 

 

[Dannomiss]

I have thought about the barlow question though. if my FL of scope is 750mm and I use a 5mm I piece I can get 150x Magnification.... however I could also use my 10mm EP and 2x Barlow and get 150x magnification. So my question which is similar, why would you do either? WOuld you the 10mm + barlow give you a more comfortable viewing experience or would going straight to a 5mm be better?

Its a good question 🙂

Also: I have read something on here about barlows and that with the barlows where you can take the lens of the end - if you then attach that to an Eyepiece, its not 2x now but approx 1.5x. However it doesn't work with my new BST starguider lens as it need a little extension to separate the EP lens and the barlow lens. Just a thought.

Edited July 7, 2020 by Dannomiss

[Louis D]

8 hours ago, Dannomiss said:

I have thought about the barlow question though. if my FL of scope is 750mm and I use a 5mm I piece I can get 150x Magnification.... however I could also use my 10mm EP and 2x Barlow and get 150x magnification. So my question which is similar, why would you do either? WOuld you the 10mm + barlow give you a more comfortable viewing experience or would going straight to a 5mm be better?

Its a good question 🙂

Also: I have read something on here about barlows and that with the barlows where you can take the lens of the end - if you then attach that to an Eyepiece, its not 2x now but approx 1.5x. However it doesn't work with my new BST starguider lens as it need a little extension to separate the EP lens and the barlow lens. Just a thought.

It really depends on the eyepieces.  If I've got a 5mm Huygens (not so far fetched, I've got an H6mm) and a 10mm Delos (I do) and a quality 2x Barlow (such as Tele Vue, Meade 140, GSO ED, etc.), then I'd definitely use the 10mm with the Barlow any day over the 5mm Huygens.  It will be more comfortable, sharper, etc.

However, let's say I've got a 5.2 Pentax XL (I do), I'll use it because the integrated factory solution of positive and negative design elements in it works better than the Delos/Barlow combination, it's more compact, and it will come to focus in all of my scopes.  Longish Barlows that I prefer work best in Newtonians and other scopes that don't utilize diagonals.  Shorter Barlows that do work well in diagonals have more aberrations than longer ones in my experience.

The trick of attaching the Barlow lens element to the bottom of an eyepiece works best when the field lens is well up inside the eyepiece barrel.  When they're near the bottom as in your Starguider, the field lens is intercepting the diverging Barlow rays way too soon which leads to a host of aberrations.  That's because the Barlow lens has a designed working distance related to its focal length, and you don't want to get too far inside of that distance for best performance.  It also doesn't help that the lower elements of the Starguider are acting like a Barlow in their own right, so now you're adding even more divergence to the system with that Barlow lens element.  This trick works best with positive only designs in my experience; and even then, it is hit or miss.  Often, the field of view is constricted and the outer field is distorted.

[michael.h.f.wilkinson]

Barlows are an economical way of increasing the number of magnifications at your disposal, and quality Barlows or tele-centric lenses like PowerMates or TeleExtenders do not compromise much in terms of quality, compared to the plastic horrors it has been my profound lack of pleasure looking through in the distant past. I still have some tele-centric lenses, but use these mainly for planetary, lunar and solar imaging. For observing I much prefer having a range of individual EPs, not so much because of optical quality, but due to ergonomics. Especially for planets, I really like to switch directly between EPs (preferably parfocal ones too) so I do not lose critical focus, or only have to adjust minimally. Switching Barlows as well often moves the plane of focus significantly, which might mean missing brief moments of good seeing. Combining a zoom lens with a Barlow might actually work a bit better in that respect, allowing you to choose a suitable range (e.g. 4-12 or 8-24 mm) depending on the target, by choosing whether or not to use a Barlow.

[Louis D]

1 hour ago, michael.h.f.wilkinson said:

For observing I much prefer having a range of individual EPs, not so much because of optical quality, but due to ergonomics. Especially for planets, I really like to switch directly between EPs (preferably parfocal ones too) so I do not lose critical focus, or only have to adjust minimally.

I'll sometimes leave my GSO 2" ED Barlow with the TV PBI in the focuser to double my scope's focal length all night and use longer focal length eyepieces at higher powers just to change things up a bit on some nights.

[DeathWarpedUp]

In your 6se, as your finding out I doubt there's not a lot of need for the barlow with the zoom, hence this thread.

Like a lot of questions on this forum it comes down to the limits and spec of your scope which is why it's always good to mention your scope specs when  asking about ep's ect.

A Barlow and zoom works great but generally with faster scopes where it becomes somewhat obselete, as others point out.  Slower scopes like maks and scts don't like the more extreme mags.

 

 

 

Edited July 8, 2020 by DeathWarpedUp