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Barlow/Eyepiece Interactions

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11 replies to this topic

#1 Phaenomena

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Posted 12 April 2021 - 04:19 PM

I have 130mm f5 newt and three eyepieces, an ES 24mm 68, an Orion 7-21mm Zoom, and an Agena Star guider 5mm. I have two barlows as well, a GSO 2x shorty and an Orion Trimag.

I'm trying to understand how each eyepiece interacts with the barlows. The 5mm works great with the 2x and with the barlow cell screwed on for 1.5x. Same for the zoom. The Trimag works with the zoom well too.

The ES 24mm is odd. It doesn't seem to play well with the 2x or 1.5x. It's easier for the view to black out, and the field stop seems fuzzy. When I use the Trimag, the field stop is sharp, and I have slightly fewer issues with blackouts. Why might this be the case? Why would the 24mm behave differently with the 3x vs the 2x/1.5x?


Edited by Phaenomena, 12 April 2021 - 04:19 PM.


#2 SteveG

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Posted 12 April 2021 - 04:25 PM

I have 130mm f5 newt and three eyepieces, an ES 24mm 68, an Orion 7-21mm Zoom, and an Agena Star guider 5mm. I have two barlows as well, a GSO 2x shorty and an Orion Trimag.

I'm trying to understand how each eyepiece interacts with the barlows. The 5mm works great with the 2x and with the barlow cell screwed on for 1.5x. Same for the zoom. The Trimag works with the zoom well too.

The ES 24mm is odd. It doesn't seem to play well with the 2x or 1.5x. It's easier for the view to black out, and the field stop seems fuzzy. When I use the Trimag, the field stop is sharp, and I have slightly fewer issues with blackouts. Why might this be the case? Why would the 24mm behave differently with the 3x vs the 2x/1.5x?

It's because the 24 has a much larger afov, so its light path is vignetted  by your barlows. The 24 is also vignetted in your Trimag, but it shows a hard edge, not fuzzy. I think the fuzzy field stop is due to the design or your GSO shorty barlow. The misbehaving exit pupil is because the barlow pushes it out further.

 

All of these issues would be resolved with a telecentric barlow, such as the Powermate or the ES Telextender.


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#3 Phaenomena

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Posted 12 April 2021 - 04:37 PM

It's because the 24 has a much larger afov, so its light path is vignetted  by your barlows.

Thanks for the response! Does the focal length have any relationship with the vigenetting, or would this be the case for all eyepieces with a large enough afov? i.e. would the same thing would happen with a 76 or 82 degree afov, regardless of focal length? And at what point would the barlow not cause vignetteing? I eventually wanted to get a large afov eypiece between 9mm and 13mm, and was hoping to use the barlows to get a little more out of it.



#4 Jon Isaacs

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Posted 12 April 2021 - 04:50 PM

Thanks for the response! Does the focal length have any relationship with the vigenetting, or would this be the case for all eyepieces with a large enough afov? i.e. would the same thing would happen with a 76 or 82 degree afov, regardless of focal length? And at what point would the barlow not cause vignetteing? I eventually wanted to get a large afov eypiece between 9mm and 13mm, and was hoping to use the barlows to get a little more out of it.

 

Generally, the focal longer focal length eyepiece are the ones that suffer from Vignetting and Extended Eye Relief. 

 

I use 16 mm and shorter Naglers (82 degree) eyepieces with 2x Barlow without any issues.  I use the 13mm and shorter Ethos eyepieces occasionally with a 2X TV Barlow as well.

 

The shorty in the 1.5x mode can be problematic. Generally Barlows improve the off-axis sharpness but not always.

 

I have to optically identical Meade version of the ES 24 mm 68 degree. With the Shorty Barlow in the 1.5x mode, it's not pretty, it messes up the view.

 

Jon



#5 SeattleScott

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Posted 12 April 2021 - 05:17 PM

It’s really about field stop not AFOV. A 13mm Nagler should be fine. Or a 9mm 100 AFOV. But a low power, wide AFOV eyepiece will vignette in most traditional barlows. The ES 24mm is designed for the widest view in 1.25” format. The opening of the barlow is narrower than the eyepiece, causing vignetting. So yeah either you get a telecentric, or just accept that you won’t be barlowing the 24mm and just save the barlows for your other eyepieces.

Scott

#6 Phaenomena

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Posted 12 April 2021 - 06:35 PM

It’s really about field stop not AFOV. A 13mm Nagler should be fine. Or a 9mm 100 AFOV. But a low power, wide AFOV eyepiece will vignette in most traditional barlows. The ES 24mm is designed for the widest view in 1.25” format. The opening of the barlow is narrower than the eyepiece, causing vignetting. So yeah either you get a telecentric, or just accept that you won’t be barlowing the 24mm and just save the barlows for your other eyepieces.

Scott

Thanks! So something like a 12.5mm Morpheus or a 8.8mm ES 82 should work with better with these barlows then.



#7 SeattleScott

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Posted 12 April 2021 - 09:26 PM

Thanks! So something like a 12.5mm Morpheus or a 8.8mm ES 82 should work with better with these barlows then.

Correct
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#8 mbhnm

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Posted 13 April 2021 - 09:11 AM

I am going to tack on to this topic as I have a Barlow question as well. For those of you who have more experience, what is the difference between using a Barlow and a lens designed for the same length? For example, what would be the difference between using my 15mm UWAN with a 3x Barlow, my 10MM MWA with a 2x Barlow, and something like a 5.5 Meade UWA. Disregarding the apple and orange and quality issues, would there be a general advantage of the 5.5 Meade or some equivalent eyepiece? Is there some significant degradation that happens with Barlows? Weighing the option of expanding my lens collection (more $) or adding more Barlow options (less $). 



#9 David Castillo

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Posted 14 April 2021 - 11:40 AM

I believe the ES24, like the Baader Hyperions, have a Smyth lens in their design and do not play well with a 3X barlow employed. The 24 will suffer from "kidney bean" effect, as you have encountered. An extended eye cup will help you keep your eye centered at the right height to prevent the black-out, or you will find it a literal pain in the neck trying to bob-and-weave for a better view.  You may also notice a very slight color shift at the field stop called a "ring of fire". Another alternative is to use the combo in a refracting telescope- no central obstruction to contend with. I suggest you leave the 24 in it's "native state" and just enjoy the wider FOV.



#10 gnowellsct

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Posted 14 April 2021 - 11:52 AM

It's because the 24 has a much larger afov, so its light path is vignetted  by your barlows. The 24 is also vignetted in your Trimag, but it shows a hard edge, not fuzzy. I think the fuzzy field stop is due to the design or your GSO shorty barlow. The misbehaving exit pupil is because the barlow pushes it out further.

 

All of these issues would be resolved with a telecentric barlow, such as the Powermate or the ES Telextender.

:) Power mate may cost as much as his whole kit.  And with his ocular in it the weight and moment arm might overwhelm the balance on his (presumably inexpensive?) 5" Newt.

 

A shorter answer is not all oculars work with all barlows.

 

Another solution is buy 5 $60 plossls to fill in the magnification gaps.   They would not cause balance problems on an inexpensive rig.  And the great thing about $60 plossls is they have a ten year reproductive cycle.  Put them in a drawer and come back ten years later, you'll have twice as many!

 

But Power mate would work for me!   In fact I have two power mates--for my solar h-alpha rig.   But all these things happened after the demons of the spirit world abducted me and told me that my days of doing inexpensive astronomy were over, and sent me out into the world to help keep employed the workers at Televue, Pentax, and other shops.

 

Greg N



#11 SeattleScott

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Posted 14 April 2021 - 12:41 PM

In general the more glass you add to the light path, the more light scatter, and less contrast. Now the top quality barlows and telecentrics have excellent polish and scatter negligible light. But there is still the hassle factor of swapping a barlow in and out, dealing with possibly not reaching focus with certain eyepieces (especially with a newt), balance issues, potentially causing excessive eye relief and blackout issues with certain eyepieces, etc.

For me I like fixed eyepieces for all the common magnifications, and use a TV barlow for reaching maximum power on nights of excellent seeing. That way I’m not paying $300 for eyepieces that I can only use on rare occasions.

Scott
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#12 mbhnm

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Posted 14 April 2021 - 06:02 PM

Thanks Scott, that's what I was thinking. One of my early purchases was a Celestron zoom with a barlow. My hope was that I could use the zoom and then add the 1.5x barlow element on good nights. Was not happy with that though, the field of view was too restrictive, I spent most of my time repositioning the scope. Then Gary Russell introduced me to wide fields and I was hooked. The zoom went into the case for the grandkids to use. While I don't have any super expensive lenses, I love wide fields. And when I have used the barlow with the wide fields, it just doesn't seem to have quite the same pop. Granted, my Celestron Omni Barlow is not top of the line, but its not a slouch either based on what others have reported. And it could be that the Omni is just not a good match for the lenses I have. I also find that getting the barlow and a lens positioned for the best view is a hassle. So, like you, I think I will focus on investing in my lens collection as I am able and skip the barlows.




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