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Choosing a better barlow

Stub Mandrel

By Stub Mandrel
in Imaging - Discussion

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Stub Mandrel

Stub Mandrel

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x3 barlows seem to be a bit thin on the ground.

I have an x3 barlow element which appears to be that from a Revelation Astro x3 ED barlow. It is a two-element 'long tube' design*. Presumably it is similar in performance to the Ostara and Meade  ones for about the same price.

The next 'step up' appears to be either the Skywatcher 3 element at ~£55 of the Celestron 3 element Xcel at ~£70. The xcel looks flash and the skywatcher is very plain, but would have the advantage of a built in t-thread. How do these compare quality wise?

Next price point is the Explore Scientific 4-element at about £90

I'm guessing the TAL  'unobtanium' barlows fit around here?

The Televue Barlows are two element but like all Al Nagler designs have hyperbolic optics " reduce all aberrations to well below the airy disc in an f/4 system. " for £125.

A televue powermate is definitely outside my price bracket, as £200-300 for a 2.5x or 4x is in the 'I could buy something else that would add more to my capabilities' zone.

 

*comparing a short x2 and long x3 design side by side, the optics seem to be the same 'strength' - doesn't surprise me as I have an M42 teleconverter that gives x2 or x3 by adding in an extension piece that also shifts the position of a sprung optical element. So one other option is a quality x2 barlow, and just fit it in the x3 position...

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vlaiv

vlaiv

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As far as I understand any barlow (that being negative lens rather than telecentric assembly) varies amplification with change of distance. You can use x2 barlow element and get it to work on x3 just by moving sensor away from barlow element.

This how ever has a few consequences. It needs additional in focus travel and there is a point where barlow element starts acting as aperture stop if not wide enough (this also depends on barlow focal length and F/speed of beam coming in).

I have no clue if changing the distance affects spot diagram though (I guess that would depend on barlow lens design - glass types of elements and what figure they have).

There are two more contenders to add to your list:

APM x2.7 - well suited for newtonians as it corrects coma (focal length of this barlow is 62.9mm I believe - so if you place it at 125.8mm from sensor you should get x3 and barlow element should be placed about 42mm "inside" regular focus plane).

Baader VIP x2 - this one is modular and has t2 connection so easily adapted for AP (it was actually designed for AP - large illuminated field) - focal length of 64, so you need 128mm distance, barlow position 42.5mm "in" counting from regular focal plane.

Both of above are considered to be top line barlows.

You can find useful info on barlows here:

http://www.brayebrookobservatory.org/BrayObsWebSite/BOOKS/BarlowLens.pdf

And when reading above document - there is additional drawback when using barlow "out of spec" - with magnification higher then design, field curvature increases - not so important for planetary but for other applications it might matter.

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johninderby

johninderby

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You’d be hard pressed to tell any difference between the Explore Scientific focal extender and the  Powermate.

BTW despite Telescope House calling it a barlow the ES isn’t. It is a proper telextender. Could be Telescope House wants you to spend more on a Powermate.  ?

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Stub Mandrel

Stub Mandrel

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My setup (with a filter wheel and ADC) puts the barlow element at about 150mm from the imaging plane.

I will do some calcs and  work out exactly what magnification I'm getting, I can then reverse calculate the focal length (how can a negative lens have a focal length?) of the barlow . Is the correct formula: Barlow Focal length =  element to sensor distance / (actual magnification-1)

Naturally a good replacement will have a similar focal length.

I understand that unlike a barlow spacing has little impact on a powermate - what about a telextender? As I want to improve quality (not magnification) without spending a fortune the ES options seem good BUT if they are 'parfocal' like a powermate I need the x3 and if they are like a Barlow, I need the X2.

@FLO suggested the Baader Classic 2.25, which gives 1.3 when screwed into an eyepiece and should give around 3 with my setup. Apparently it's a three element design and quite sharp with good contrast.

As it's reasonably cheap  and will screw straight into my setup in the right place, I think it will be worth a punt.

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vlaiv

vlaiv

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1 hour ago, Stub Mandrel said:

My setup (with a filter wheel and ADC) puts the barlow element at about 150mm from the imaging plane.

I will do some calcs and  work out exactly what magnification I'm getting, I can then reverse calculate the focal length (how can a negative lens have a focal length?) of the barlow . Is the correct formula: Barlow Focal length =  element to sensor distance / (actual magnification-1) 

Naturally a good replacement will have a similar focal length.

I understand that unlike a barlow spacing has little impact on a powermate - what about a telextender? As I want to improve quality (not magnification) without spending a fortune the ES options seem good BUT if they are 'parfocal' like a powermate I need the x3 and if they are like a Barlow, I need the X2.

@FLO suggested the Baader Classic 2.25, which gives 1.3 when screwed into an eyepiece and should give around 3 with my setup. Apparently it's a three element design and quite sharp with good contrast.

As it's reasonably cheap  and will screw straight into my setup in the right place, I think it will be worth a punt.

Yes, negative lens also has a focal length (and it is negative - hence the name :D )

image.png.5aecd67d766723c1116c11bd76b63c06.png

As you see, parallel rays diverge after negative lens, and focal length is on left side - usually positive focal length is on right side (convention that principal ray goes from left to right). Negative focal length just means that if you trace rays to a single point - you need to go in "opposite" direction than light would travel.

Formula that I use (and you can rearrange it to calculate needed value) is: amplification = 1 - distance / lens_focal_length

(you need to use negative value for lens focal length, as it is negative)

So focal_length = - distance / (amplification -1) as you said.

I guess Baader Classic 2.25 is a good barlow (from what I've read, I did not use it myself), and by the looks of it, it has focal length somewhere in range of 50mm (could not find actual value, but from images, it looks like barlow body is around 60-65 mm long so 1 + 65/50 = 2.3 while 1+60/50 = 2.2 - if this is true, you will have to shorten your barlow to sensor distance to about 100mm).

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Stub Mandrel

Stub Mandrel

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45 minutes ago, vlaiv said:

Yes, negative lens also has a focal length (and it is negative - hence the name :D )

image.png.5aecd67d766723c1116c11bd76b63c06.png

As you see, parallel rays diverge after negative lens, and focal length is on left side - usually positive focal length is on right side (convention that principal ray goes from left to right). Negative focal length just means that if you trace rays to a single point - you need to go in "opposite" direction than light would travel.

Formula that I use (and you can rearrange it to calculate needed value) is: amplification = 1 - distance / lens_focal_length

(you need to use negative value for lens focal length, as it is negative)

So focal_length = - distance / (amplification -1) as you said.

I guess Baader Classic 2.25 is a good barlow (from what I've read, I did not use it myself), and by the looks of it, it has focal length somewhere in range of 50mm (could not find actual value, but from images, it looks like barlow body is around 60-65 mm long so 1 + 65/50 = 2.3 while 1+60/50 = 2.2 - if this is true, you will have to shorten your barlow to sensor distance to about 100mm). 

I think you are probably right, but I suspect I'm already imaging at rather more than x3.

Comparing images from two nights apart, same scope same camera, Jupiter is 61 pixels without the barlow, 190 pixels with it.

Magnification = 190/61 = 3.11.

Barlow focal length = - 150/2.11) = ~71mm

If your size estimates are right (and they probably are) that would give me a magnification in the region of 3.3 to 3.5.

 

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Rob

Rob

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I have 3 Barlows.. All are really good.

Orion Shorty Plus ED 2x  - Been with me for 10 years now. GSO ED 2.5 x Barlow - Fantastic performance . Antares Achro 3 x times. For a non ED it works perfectly and would not know the difference TBH

Rob

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Rob

Rob

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However if you ever see the GSO 1.25" x 2.5 ED Barlow come up on Astroboot in the future. i strongly recommend it. A 2 x Barlow is a work horse tool in the kit bag. And as you have pointed out the cost on these can be disturbing!. I have used many in my time, up till now the Orion Shorty Plus was the best in the  2 x field. But the GSO has the edge to be sure!. 

https://agenaastro.com/gso-2-5x-apo-barlow-lens.html

That 3 x being an ED & GSO I'm sure will serve you well. And at silly money a great purchase!

P.S I have no affiliation with Astroboot. I just love a good deal for Astronomers when it comes along. I have made some awful choices and paid top money over the years. Now is my time to stop others making them. :)

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Rob

Rob

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Just now, Stub Mandrel said:

I've nabbed it -)

Good Stuff.. You made a good purchase there. Funnily enough I jumped in AB around 40mins ago to see if it was still there! :)

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