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Weird results with Baader 2.25x barlows

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#1 Astrojensen

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Posted 27 June 2020 - 04:00 PM

I recently purchased two Baader 2.25x Q-Barlows to use in my Zeiss binoviewer on my APM 152mm ED apochromat. My first tests were with a single barlow in my 63mm Zeiss Telemator and the results were disappointing. Nearly all eyepieces vignetted quite badly, some to the degree of being nearly useless. The images were very sharp, however, so I decided to give them another chance, after having conducted various tests to verify that it wasn't due to incorrect assembly. 

 

Today I had a chance to try them out in their intended role, in the binoviewer on the APM 152 ED. It was early twilight and the Moon was high in the SW. The seeing was fairly good. First eyepieces was a pair of 25mm Zeiss OPMIs. The image was flat out shockingly good! No hint of any vignetting and sharp to the very edge! And SHARP! A pair of 20mm OPMIs behaved similarly. Elated, I began to test other eyepieces. A pair of 9mm UO orthos showed no vignetting and the 12.5mm's were also good to go. The 18mm's showed a bit of vignetting and the 25mm's showed quite a lot. I estimate that the AFOV of the 18mm was down to 40° and the 25mm perhaps 35°! 

 

And that puzzles me to no end. The 18mm UO orthos have a SMALLER field stop than the 25mm and 20mm Zeiss OPMIs, so why do the UOs show vignetting, but the Zeiss'es don't???? That doesn't even begin to make sense. Oddly, a pair of 24mm ES68's also show less vignetting than the 18mm and 25mm UO's. Again, this makes no sense whatsoever. 

 

The Q-barlows can also be used with their lens cells screwed directly into the eyepiece barrels, but the threads aren't flush with the upper end of the barrel, so this only works on some eyepieces with enough clearance inside the barrel. It works fine on the Zeiss OPMIs and the UO orthos, but the OPMIs show some field curvature, then, and the 18mm and 25mm UOs still vignette!

 

After seeing the fantastic lunar views with the 25mm OPMIs, I don't regret buying the Q-barlows, but I must say I'm quite puzzled by their odd behaviour with some eyepieces. A generic 2x Sky-Watcher barlow shows no such vignetting with any eyepiece. 

 

 

Clear skies!
Thomas, Denmark


Edited by Astrojensen, 27 June 2020 - 04:01 PM.


#2 Shorty Barlow

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Posted 27 June 2020 - 04:19 PM

I was told the Q-Turret Barlow was designed specifically for the BCO range. I don't know if that's relevant.


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#3 sixela

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Posted 27 June 2020 - 04:54 PM

It's simply a very diverging barlow (non-telecentric and with a fairly short focal length) — and it'll yield far more than 2.25x in front of a binoviewer, which only exacerbates the problem; so far from the focal plane its own clear aperture will also be tight.

It can make the ray bundles for the outside of an eyepiece's field move towards the internal baffling.

Personally in the Baader range it looks like the wrong choice for a binoviewer. The GWKs (glasspath correctors) will work far better, and the factor printed on them is for a typical binoviewer glass path length.

Edited by sixela, 27 June 2020 - 05:20 PM.

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#4 Astrojensen

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Posted 27 June 2020 - 06:48 PM

I use them AFTER the binoviewer, not in front. The reason is that my ED refractor is sharpest if used with a long glasspath and is not as sharp, if I use a corrector or barlow in front of the binoviewer. Unconventional, I know, but it is what it is. 

 

I was told the Q-Turret Barlow was designed specifically for the BCO range. I don't know if that's relevant.

Possibly. Don't know. 

 

 

Clear skies!

Thomas, Denmark


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#5 John Huntley

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Posted 27 June 2020 - 07:46 PM

I use a Baader Q-Turret 2.25x with a 7.2mm - 21.5mm zoom in cyclops mode and the results are excellent. I use this combination more than I do my Pentax XW's and Nagler 2-4mm zoom especially in my refractors. The resulting 9.55mm - 3.2mm zoom range is really useful for planetary and double star observing.

 

I don't know exactly why these two items work so well together, but they do shrug.gif

 

 

 

 

 

 

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#6 sixela

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Posted 28 June 2020 - 06:34 AM

Even just in front of an eyepiece, it is indeed one of the barlows that through divergence will cause vignetting in an eyepiece the most of any modern design I use (the cheap $30 shorties also did, but I didn't compare them; IIRC the Celesteon Ultimate-like designs also do this, but less). And yes, that's probable a side effect of being designed with only the 18mm BCO in mind as the widest field eyepiece.

Edited by sixela, 28 June 2020 - 06:35 AM.


#7 sixela

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Posted 28 June 2020 - 06:37 AM

If you don't want glasspath compensation, I'd consider the Baader VIP element in front of the binoviewer; it's basically the 2.6x glasspath corrector minus the glass path correction. But yes, the effects of the glass path will change because of the higher effective f/ratio for the bundles through the binoviewer, so I'm not sure what that will yield.

Edited by sixela, 28 June 2020 - 06:39 AM.


#8 ButterFly

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Posted 28 June 2020 - 07:22 AM

It matters where the field stop of the eyepiece is in relation to the clear aperture of the barlow.  Unless the clear aperture is right at the field stop, the clear aperture appears smaller just by geometry.  Make a circle with your thumb and index finger and move it away from directly in front of your eye.  The clear aperture gets smaller the further away it is.

 

The clear aperture of the Hyperion 2.25x barlow is slightly smaller than the field stop of a 13 Ethos.  I notice slight vignetting when screwed in as a filter.  With an ADC between the two, adding about 35mm, there is more vignetting noticable.



#9 Astrojensen

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Posted 28 June 2020 - 09:38 AM

It matters where the field stop of the eyepiece is in relation to the clear aperture of the barlow.  Unless the clear aperture is right at the field stop, the clear aperture appears smaller just by geometry.  Make a circle with your thumb and index finger and move it away from directly in front of your eye.  The clear aperture gets smaller the further away it is.

But why does the 18mm UO shows vignetting, then, when its field stop is SMALLER than the clear inside aperture of the Q-barlow????

 

 

Clear skies!
Thomas, Denmark


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#10 DRodrigues

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Posted 28 June 2020 - 10:41 AM

...

The Q-barlows can also be used with their lens cells screwed directly into the eyepiece barrels, but the threads aren't flush with the upper end of the barrel, so this only works on some eyepieces with enough clearance inside the barrel. It works fine on the Zeiss OPMIs and the UO orthos, but the OPMIs show some field curvature, then, and the 18mm and 25mm UOs still vignette!

...

ATTENTION!!! I destroyed one of these barlows cell when it got entangled in a T to 1,25" adapter...bawling.gif

 

Thomas, with the eps that are vignetting, try to use a 1,25" extension before the eps and after the barlow, and see if it gets better/solved.



#11 sixela

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Posted 28 June 2020 - 11:29 AM

But why does the 18mm UO shows vignetting, then, when its field stop is SMALLER than the clear inside aperture of the Q-barlow????


Barlows are diverging: not only does an object's light bundle pass through the focal plane at a greater distance from the axis, the principal ray in the middle of the bundle is also angled more towards the outside, and that means that the light bundle _in the eyepiece_ is displaced towards the edge, where it may encounter internal baffling.

#12 j.gardavsky

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Posted 28 June 2020 - 01:59 PM

If I am correct,

then the Baader Q-Turret Barlow has a front field flattener and at a larger distance is the negative doublet. Herewith is the negative doublet closer to the eyepiece than in the Barlows as usual.

 

This design requires a precise tuning of the eyepiece-to-Barlow distance to get the optimum performance and to avoid vignetting on some EPs.

 

Best,

JG

 

PS: The 2nd (outer) lens in the doublet is thin, and made with glass hardly scattering the green laser beam, and herewith it is difficult to see it, both in the forward- and backscattering.



#13 Sasa

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Posted 28 June 2020 - 03:37 PM

I don't recall vignetting issue with Baader 2.25x and 9mm Baader Genuine Ortho. The Barlow came with borrowed Baader 95/560mm triplet. This combo was producing very pleasing and sharp views of Jupiter. The views with Powermate 2.5x + ZAO-I 10mm and ATC 1.5x + ZAO-I 6mm were still a little bit sharper but the difference was subtle. It is also hard to say if it was due to Barlow or eyepiece.

Edited by Sasa, 28 June 2020 - 03:37 PM.


#14 Sasa

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Posted 28 June 2020 - 03:41 PM

How close was Barlow to Telementors last internal baffle? It is quite small and if Barlow was close to it, it could have introduce vignetting.

#15 ButterFly

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Posted 28 June 2020 - 07:22 PM

But why does the 18mm UO shows vignetting, then, when its field stop is SMALLER than the clear inside aperture of the Q-barlow????

 

 

Clear skies!
Thomas, Denmark

Most of the barrel is empty space in orthos.  The field stop is very likely to be very near the shoulder.  Compare the focus positions of the eyepiece to estimate where the field stop is.  The 18mm probably focuses much closer to the objective than the others.  Even though the field stop is smaller, the apparent clear aperture of the barlow could be reduced to near its size because the field stop is so much further away.



#16 Hesiod

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Posted 29 June 2020 - 11:01 AM

I have just received my sample and made a few tests.

With the BCO18mm there is a modest, but noticeable, vignetting (worse if used at 2.25x but, overall, quite tolerable); none with the BCO10mm.

I also tried three other eyepieces: a TV 32mm Plossl (vignetted worse than with the BCO18, but do not think will ever use this combination anyway), a TAL25mm Plossl (this gave the best performances among long focal EPs I tried, with just hints of vignetting, maybe because have to rack out the drawtube by almost 1cm. For this reason I use less than I should, as it is a very, very nice eyepiece), an Intes 21mm Erfle/Konig* (the worse performer, vignetted and the eye relief became too much for it to be comfortable, being already a sort of "Volcano-top" eyepiece; on the bright side, it gave almost the notorious "no eyepiece" effect which is reported with the RKE28).

 

Despite being just daytime tests (will do further to evaluate scattering by night), I was surprised by how good the lens seems to be



#17 krakatoa1883

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Posted 29 June 2020 - 12:34 PM

But why does the 18mm UO shows vignetting, then, when its field stop is SMALLER than the clear inside aperture of the Q-barlow????

 

from TeleVue page on Powermates in relation to ordinary Barlows:

 

"Vignetting can occur due to the altered ray path, when the eyepiece's lenses are not large enough in diameter to allow all the rays to make it through. Shorter Barlows, or ones with too much magnification, only exacerbate the problem because the ray path entering the eyepiece is steeper."

 


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