8 replies to this topic

[Jeff B]

Oh no!  The DPAC disease has spread to bino-viewers!   

 

I made a new DPAC Ronchi screen and LED holder that works in 1.25" focuser.  One thing I've always wanted to do was have a DPAC look at what happens to spherical and color corrections when a viewer is introduced into the light path.  Visually, I've long noticed that with my faster "APO"s, like the TEC 140ED and, especially, the CFF 160 F6.5, most viewers degrade the color correction to some degree when used by themselves (no OCS, GPC or barlow in front of the viewer).  This is easily seen by me at very high powers, particularly on stars, and especially, when a viewer is stacked with a prism diagonal.  However, I've seen scant, if any really, changes in SA with the introduction of a viewer.  Now one viewer in particular, seems rather free of color error at high power with my APOs, and that is the Zeiss Sharpest APO viewer sold by Denis Levatic.  I also need to mention that I don't really see any color disturbances when using any of my viewers with my newtonians (and they always have a barlow device upfront too).   The other thing to mention is that I also see tint differences between the right and left eyepieces.  But, again, less so with the Zeiss Sharpest.

 

So I chose my TEC 160ED as my "test vehicle" due primarily to its wonderfully smooth figure, superb correction in green light and the fact it was already mounted on my test bench in DPAC.  My reference 2" mirror diagonal was an AP Maxbright, the Baader Zeiss Spec prism was the prism diagonal reference.  When testing the viewers, I tested them straight through with no diagonals.  

 

So here are some of the results, first the viewers by themselves.  The rows move from left to right with Base 160ED, Zeiss Sharpest, Denk II, then Zeiss "Biggest" (30mm CA prisms also previously sold by Denis).  The top row is for the Right eyepiece collet, the bottom row for the Left collet.  You'll notice right away that the Ziess Sharpest L&R are reversed relative to the other viewers.  This is because you can rotate that viewer's halves all the way around to the "other side", which, for this sample viewer, gives excellent collimation.  

 

My take on the color/tint differences will be in a later posting, but have at it.  Click on the image to enlarge it.

 

Jeff

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Edited by Jeff B, 17 January 2021 - 01:00 PM.

[ngc7319_20]

Result probably depends on the F-ratio of the scope being used, since that defines the size of the "pencil" or light cone through the bino.  What is F-ratio of this scope?  All looks good though...

[Jeff B]

Yes, it does.  The TEC 160ED is F8.  I will eventually repeat some of the testing with my CFF 160, which is F6.5 and if I'm into self torture that day, maybe a fast achromat, specifically my 6" F6.5.

 

Here is an interesting set where I stacked the Zeiss Biggest with my Baader Zeiss Spec Prism diagonal using the right side of the viewer.  I separated the red, green and blue colors from the white light images and are in monochrome as the bottom row.  The top row is the "Base" TEC 160ED image with the AP diagonal.  Both sets are inside of focus.  If you stare at them long enough, you will see that red and blue do indeed drop off a bit in focus relative to green with the "stacked" system, which also explains the extra fringing, which at higher powers is quite noticeable.  But also notice, there is very little, if any spherical aberration or spherochromatism difference.  It's almost all color focus shift, just like an achromat.

 

Jeff

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Edited by Jeff B, 17 January 2021 - 02:12 PM.

[noisejammer]

Very interesting, Jeff. I'm quite surprised at the fringing caused by the Denk II. How does it behave with a PxS?

 

If you have access to a set, it would be useful to run MkV's with and without a GPC.

[Jeff B]

Looking at the shots in post #1 you can readily see the tint differences between the R&L sides of the viewers.  Even the Zeiss Sharpest shows some.  This is readily seen by me at the eyepieces as well, when closing each eye individually, especially on the Moon and daylight viewing of Venus.  Many others have reported this tint difference too.  However, when I use both eyes, the merged image is just fine with the overall tint and color errors only visible at high power (over 200X with this scope).  All of the viewers are sharp too. 

 

But stacking of the viewers with the prism diagonal, just does not work for me at high power as the color errors are, to me, quite pronounced.  You see that in post 3 but you really see that more readily in the attached image montage where I have separated out the individual Red, Green and Blue color channels from the single white light images taken at focus for the "base" (mirror diagonal, no viewer) configuration (top row) and the Biggest + Prism diagonal configuration (bottom row).

 

The Base image is like that of most modern APO triplets where the longitudinal color errors (focus position differences between red, green and blue) have largely been eliminated.  You see this in the top row, particularly between the red and green images, as very even illumination across the face of each color image.  Blue is just a hair different as a  shadow is starting to come in from the right side.  At the eyepiece, the blue error is just not noticeable to me below say 250x on really bright stars and Venus and even then, I have to look specifically for it.  

 

The bottom row however, the "stacked" configuration, is a different story.  The differences in illumination evenness across the face of each color relative to the green image is obvious, especially in blue.  This is color focus "error" relative to green light, much like an achromat.  At the eyepieces using the "stacked" configuration, this is visible to me starting at moderate magnifications.  High magnifications of say Vega, show a distinct yellowing of the airy disk at high power and blue/red fringing, again much like an achromat (rather like a 3" F15 really, only "better").  However, the images are still sharp.   

 

So, with DPAC I see the same color errors that I've seen at the eyepieces when it comes to stacking a prism diagonal with a typical bino-viewer, at least with this scope (but I've seen it with other refractrors too).   This just validates what I've actually seen when observing and why I use the "stacked" configuration only for low power viewing, something that it really excels at.

 

Jeff

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[Jeff B]

Very interesting, Jeff. I'm quite surprised at the fringing caused by the Denk II. How does it behave with a PxS?

 

If you have access to a set, it would be useful to run MkV's with and without a GPC.

Funny you should ask.  I installed the power switch assembly on the Denk II and the 1.25X and 1.70X GPCs in the Zeiss Biggest (which is basically identical to the Mark V except for coatings) and reshot the right sides.  I also reshot the left side of the Biggest with the GPC's as well.  I could only image the 1.3x and 2.0x switch settings on the Denk PS/OCS assembly as the 2.8X setting gave a tiny image that I just could not keep steady in the camera.

 

As you can see the two very different barlowing systems are doing their jobs as the lines are still very straight no matter the strength of the barlow system.  

 

Not what you also see is that the warm tints remain on the right side images, despite the much slower light cones going into the viewers with the various barlow elements in front of the viewers.  My going in assumption was that the warm tint would go away.  It did not.  Hmmmm.   

 

Jeff

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[noisejammer]

Jeff, this is a very significant body of work - very well done!

 

You mention that you used a Zeiss spec prism. I assume it was the T2-01B model rather than the 2" Zeiss spec.

 

This is particularly surprising - I use the T2-01B with my Mk V's and the image 'quality' is very obviously better than I achieve with the T2-01A (dielectric mirror) which shipped with my Mk V's.

 

A couple of years back, BillP and I swopped comments over the 01B and BBHS variants - and came to realise that, at the eyepiece, we can't really distinguish them. To be honest, this did surprise me but Matt (Kunama) and Ron (RAKing) have reported the same thing. For completeness, I was using my TOA150 at around 200-250x; iirc, Bill was using his TSA102 at similar magnifications. I don't know what Matt and Ron were using.

 

So this has me wondering - why is it that the prism seems to deliver first class views when Physics says it shouldn't...

[Jeff B]

Jeff, this is a very significant body of work - very well done!

 

You mention that you used a Zeiss spec prism. I assume it was the T2-01B model rather than the 2" Zeiss spec.

 

This is particularly surprising - I use the T2-01B with my Mk V's and the image 'quality' is very obviously better than I achieve with the T2-01A (dielectric mirror) which shipped with my Mk V's.

 

A couple of years back, BillP and I swopped comments over the 01B and BBHS variants - and came to realise that, at the eyepiece, we can't really distinguish them. To be honest, this did surprise me but Matt (Kunama) and Ron (RAKing) have reported the same thing. For completeness, I was using my TOA150 at around 200-250x; iirc, Bill was using his TSA102 at similar magnifications. I don't know what Matt and Ron were using.

 

So this has me wondering - why is it that the prism seems to deliver first class views when Physics says it shouldn't...

Thanks!

 

Yes, it is the T2-01B model.  

 

In mono-vision, I see no consistent visual differences between my BBHS, AP and the T2-01B diagonals really, maybe very subtle tint ones and maybe a touch less scatter with the prism...maybe, but nothing that's comes anywhere remotely close to "AH HAH".  

 

I've a body of DPAC work yet to be published where I directly compared the T2-01B and AP maxbrite diagonals in DPAC with two scopes that I specifically chose because they have native blue focus differences relative to green.  One was an old AP 6" F10 Pre-ED triplet and the other an Orion 110ED FPL51 based F7 doublet.  Visually, with the two scopes, there were no color, spherical or sharpness differences to be seen by me between the two diagonals.  I don't recall any differences concerning "scatter".  In DPAC there were no differences in color either.  

 

But that's with those two scopes.  Some Zeiss APOs are designed to work best with prism diagonals.  I don't know of any refractive scopes specifically designed to work best with bino-viewers and their internal prisms.  The closet might be some of APM's big ED binoculars, which are really bino-scopes.

 

But what I see visually with most of my refractors is that a typical viewer does indeed disturb the focus at the blue end of the spectrum a little bit and it can be seen visually, especially when stacked with a prism diagonal.  As to whether or not it helps or hurts the system, I recon is very dependent on the core design of the objective and maybe even sample to sample variation.  

 

The other thing that stands out is, of course, that R/L tint difference, or "warmth" to one side of the viewer.  I wonder if that's more of a filtration effect versus a subtle focus effect between the colors or some combination.

 

Also, all of the viewers are of high optical quality, showing no disturbances to SA that I can detect either visually or in DPAC.  I saw no real signs of introduced astigmatism either in DPAC.

 

Finally, (and the crowd goes wild, with clearly audible gasps of "Thank GOD!"), the viewer with the least color disturbances in the bunch here is the Zeiss APO Sharpest.  I've also seen this at the eyepieces and it seems to work especially well with the 1.7x GPC, particularly with my CFF160 F6.5 triplet. I may redo some of my DPAC testing specifically with that combination, but visually that is a very effective, sharp, color free system, well suited for high powers.  

 

If there is anyone out there with optical design software who can simulate/model a bino-viewer's effects on a few refractor designs, that would be very interesting to see here.  I'd be especially interested to find out if such modeling picks up the R/L tint differences.

 

Jeff

[fate187]

Such good work and effort put into the comparison Jeff! I also want to do this comparison with my scopes and peripherals. The influence of flatteners is also interesting to me. I have to get it done though .

 

As for the different tint you observe with your binos: I can confirm this effect with my Zeiss sharpest as well. It is not too subtle I admit and is easily seen during daylight or moon observations.

 

-Michael