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I thought I knew all about binoviewers...

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#1 Astrojensen

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Posted 07 April 2020 - 03:43 PM

But apparently not. But let's not get ahead of ourselves. Let's jump back in time a couple of days.

 

Saturday night was beautifully clear here and I was observing the Moon, high in the sky, with my APM 152mm f/8 ED and Zeiss binoviewer. The seeing, unfortunately, was not all that good. I began playing with my many barlows and GPCs, to find the best combinations. 

 

Now, I've always learned that the best way to use a binoviewer is to use a barlow *ahead* of the binoviewer, since this means that the binoviewer will see a less steep light cone (much longer f/ratio) and this will mean that it will introduce far less aberrations. It also means that you can use a long focal length eyepiece with good eye relief, which will be more comfortable to look through. Overall, this should give the best image.

 

Or so I thought. 

 

One of the combinations was a 2" 2x ED barlow attached to the 2" nosepiece of the binoviewer, with two 25mm Zeiss microscope eyepieces, and inserted into a 2" Baader/Zeiss prism diagonal. The image was indeed very fine. The measured magnification of the barlow in this configuration is ~2.65x (depending a little on the location of the image plane in the eyepieces) and the resulting magnification was ~127x. 

 

On a whim, I tried a combination of Baader/Zeiss T2 prism and binoviewer, with no barlow/GPC and a pair of 9mm UO VT orthos. 

 

Imagine my shock, when the image was unquestionably better than the first combination! It was crystal clear, with fantastic resolution and contrast. I hadn't really been aware of it before, but there seemed to always be a small amount of stray light somewhere in the image, sometimes across all of it, sometimes quite obviously so, when I was using a barlow or GPC. This was completely gone now. Going back to the barlowed configuration and it was suddenly all too apparent that it was a little bit washed out. The difference was not always huge, but it was definitely there. Sometimes it WAS huge, namely when the diverging lunar rays hit something bright and shiny inside the binoviewer. I've more or less always accepted this as a part of the game, accepting it as unavoidable, but when using the non-barlowed setup, there was NO such reflections and the image was stunningly good.  

 

I tried many different combinations now and always found the simple setup with prism+binoviewer+eyepieces to be markedly superior - at least in this telescope. I did a star test of the prism+bino setup and it was BETTER without a barlow or GPC, which flies in the face of everything I thought I knew. It was sharp and clear even with a 4mm ortho! 

 

I know that my APM works better with a large 2" prism diagonal than it does without one (or with a mirror diagonal), but apparently this goes so far as to also be true for a prism PLUS binoviewer! I never expected this.

 

The shortest focal length eyepiece pair I could try was 7mm and that was with a 7mm UO VT ortho in one eyepiece holder and a 7mm Kokusai Kohki 0.965" ortho in a 0.965"/1.25" adapter in the other! Amusingly, the apparent fields and focal lengths matched PERFECTLY and a perfect merging could easily be achieved! Even more amusingly, the 0.965" ortho was distinctly a little sharper and brighter. Overall the image was extremely sharp, contrasty and pleasing to behold. I tried a 6mm Baader Classic Ortho, the highest the seeing would support in the fleeting moments of best sharpness, and the image was still extremely sharp, when the seeing allowed. I've earlier had some mixed results with the 6mm BCO, but it performed extremely well in the bino and 6" APM ED.

 

This throws a monkey wrench into all my eyepiece plans. My initial tests suggested that a pair of good 15mm's and 12mm's, barlowed to the desired magnifications, would be the best approach, but after the results from Saturday night, I guess I can scrap this. Now I need pairs of short focal length orthos or other high resolution, high-contrast eyepieces. I guess going the ortho route is the easiest one. Getting another 6mm BCO seems to be the logical first step. Getting more UO VT's would be good, but they're not available new anymore and don't pop up all that often on this side of the pond, so I guess I'll go with some Fujiyama orthos instead. Comments and suggestions welcome.

 

Attached (slightly unsharp) bonus pic of my 6" APM looking at the Moon. 

 

gallery_55742_4772_45576.jpg

 

 

Clear skies!
Thomas, Denmark


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#2 John Miele

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Posted 07 April 2020 - 03:57 PM

Maybe you are finding an example of the "less glass is better" principal? I prefer to use a shorter fl eyepiece alone instead of a barlow whenever possible. I only use barlows with Binoviewers in my refractors because I can't reach focus without them. I don't trust barlows. Maybe I'm irrational but I always suspect they add their own abberations to the party and scatter a bit more of the light?


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#3 StarAlert

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Posted 07 April 2020 - 04:14 PM

Very interesting, indeed.
Do you think you’d be able to bring a 5mm or 7mm DeLite into focus without the Barlow? 



#4 StarAlert

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Posted 07 April 2020 - 04:19 PM

And how much back focus does your 152 have?



#5 Kunama

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Posted 07 April 2020 - 04:19 PM

The only time I used a 'barlow' ahead of the binoviewer/diagonal is with the Tak Extender 1.5x which did not seem to degrade the view at all.

 

I now use the Baader Maxbright T2 mirror with my Zeiss binoviewer and Tak LE eyepieces with the TMB152/1200, I have tried it with and without the GPC1.25X with very little difference at high magnification in favour of the GPC.



#6 junomike

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Posted 07 April 2020 - 05:07 PM

And how much back focus does your 152 have?

180mm as per the specs

which should be plenty when combined with a T2 Prism.



#7 slavicek

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Posted 07 April 2020 - 06:47 PM

Astrojensen, thank you for proving the "less glass is better" principal. I personally just believed to my "gut feelings" and I went to great lengths to eliminate the "correctors" = barlows used with the binoviwers. Denis007 help me machine short custom eyepiece holders, I myself machined down the focusers in my TAKs. All this so I can go: Telescope > Baader T2 prism diagonal > Binoviwer > eyepiece. Sometimes I wandered if this was worth the efford, well it looks like it was worth it.



#8 BillP

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Posted 07 April 2020 - 07:04 PM

Wow.  This does fly in the face of things.  So you are saying these are your two configurations:

 

1) 152 ED Objective --> 2" Prism Diagonal --> 2x Barlow --> Binoviewer --> Longer FL Eyepieces

 

2) 152 ED Objective --> 1.25" T2 Prism Diagonal --> Binoviewer --> Shorter FL Eyepieces

 

And with these, configuration #2 was sharper and higher contrast visually?  That is not what I experiene at all with my TSA-102.  If I use configuration #2 then I see a lot of SA in the view with a 6mm eyepiece.  Configuration #1 eliminates the SA from all that prism path.  Very odd that the 152 ED would be behaving differently!  But live and learn.  I've only used my 152 ED in config #2, but never for planetary and just with a 9mm eyepiece pair (Morpheus).  So more for DSO viewing.  Have not really tested in in high power between configurations 1 and 2.

 

PS - I am surprised that the 2" 2x ED Barlow you have only changes to 2.65x after all the glasspath of the binoviewer.  That seems way too little.  The TV 2x Barlow, as example, when used before the WO binoviewer measures 3.5x!!


Edited by BillP, 07 April 2020 - 07:06 PM.


#9 Jeff B

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Posted 07 April 2020 - 08:26 PM

Hah hah, this is just one reason this hobby is so fun for me.  The unexpected.  Especially with bino-viewers.  

 

What viewer are you using?

 

Jeff



#10 BillP

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Posted 07 April 2020 - 09:45 PM

Currently I am using and old WO Binoviewer, but am hoping I can afford a Maxbright II when they are available so I can use a 24 ES 68 without vignetting.



#11 Astrojensen

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Posted 08 April 2020 - 11:21 AM

Maybe you are finding an example of the "less glass is better" principal? I prefer to use a shorter fl eyepiece alone instead of a barlow whenever possible. I only use barlows with Binoviewers in my refractors because I can't reach focus without them. I don't trust barlows. Maybe I'm irrational but I always suspect they add their own abberations to the party and scatter a bit more of the light?

In this particular case, I guess you're right, since this scope apparently doesn't like barlows in the binoviewer setup, but I normally happily use barlows and have several of them.

 

 

Clear skies!
Thomas, Denmark



#12 Astrojensen

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Posted 08 April 2020 - 11:23 AM

Very interesting, indeed.
Do you think you’d be able to bring a 5mm or 7mm DeLite into focus without the Barlow? 

I think so, unless the Delites need an especially large amount of back focus, compared to other eyepieces. The scope has 180mm back focus. 

 

 

Clear skies!

Thomas, Denmark



#13 Astrojensen

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Posted 08 April 2020 - 11:25 AM

Astrojensen, thank you for proving the "less glass is better" principal. I personally just believed to my "gut feelings" and I went to great lengths to eliminate the "correctors" = barlows used with the binoviwers. Denis007 help me machine short custom eyepiece holders, I myself machined down the focusers in my TAKs. All this so I can go: Telescope > Baader T2 prism diagonal > Binoviwer > eyepiece. Sometimes I wandered if this was worth the efford, well it looks like it was worth it.

Hold your horses. I am only saying that I find it to be the case *with this particular telescope*! I am NOT saying that it is a general rule. I would STRONGLY advice that you investigate yourself how YOUR scope works with and without barlows/GPCs and binoviewers. 

 

 

Clear skies!

Thomas, Denmark



#14 Astrojensen

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Posted 08 April 2020 - 11:31 AM

Wow.  This does fly in the face of things.  So you are saying these are your two configurations:

 

1) 152 ED Objective --> 2" Prism Diagonal --> 2x Barlow --> Binoviewer --> Longer FL Eyepieces

 

2) 152 ED Objective --> 1.25" T2 Prism Diagonal --> Binoviewer --> Shorter FL Eyepieces

 

And with these, configuration #2 was sharper and higher contrast visually?  That is not what I experiene at all with my TSA-102.  If I use configuration #2 then I see a lot of SA in the view with a 6mm eyepiece.  Configuration #1 eliminates the SA from all that prism path.  Very odd that the 152 ED would be behaving differently!  But live and learn.  I've only used my 152 ED in config #2, but never for planetary and just with a 9mm eyepiece pair (Morpheus).  So more for DSO viewing.  Have not really tested in in high power between configurations 1 and 2.

 

PS - I am surprised that the 2" 2x ED Barlow you have only changes to 2.65x after all the glasspath of the binoviewer.  That seems way too little.  The TV 2x Barlow, as example, when used before the WO binoviewer measures 3.5x!!

Yes, I am finding that config #2 is sharper and higher contrast. I was indeed surprised by it, as it was not what I had expected, either. 

 

The barlow magnification is measured, so it is definitely real. The focal length of the barlow is around 100mm, so the 2.65x, when the barlow is used about 5cm's in front of a binoviewer with a path length of around 110mm is very close to expected. A small 1.25" 2x barlow with a focal length of about ~70mm in front of a similar length bino with a nosepiece should indeed be close to 3.5x and I've found my own 2x 1.25" barlow to work very close to this magnification (I think I measured it to 3.65x at some time). 

 

Remember that the "focal length" (it doesn't have an actual focus, of course) of a barlow is from the middle of the lens and to the plane where the magnification factor is exactly 2x. When you move the focal plane outwards another whole focal length, the magnification increases to 3x. Yet another focal length and the magnification is now 4x. 

 

 

Clear skies!
Thomas, Denmark


Edited by Astrojensen, 08 April 2020 - 11:34 AM.

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