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Mirror, Dielectric, & Prism Performance Comparison

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#76 Full Sun

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Posted 18 January 2014 - 01:26 PM

I take it from reading all this that I should get a prism diagonal for my TEC 8" f/15.5 to replace the Astro-physics Maxbright which I have been using. I assume the new prism diagonal will reduce scatter and not add any false colour thanks to my loooong focal ratio of f/15.5

My primary viewing is double stars, the closer, the better and planets, in season...

Is there a recommended brand of 2" prism diagonal I should get?

Dave
------------------------------------
__----_----

Yes, get the Zeiss Baader 2in Prism.
You can try mine but then again you would have to bring your baby over to my backyard.
It is superb on my C-14 but I have yet to test it on my refractors.
(I do think the Maxbright has less scatter then some other 'el-premo'mirror diagonals that I have laser tested).
Clears
Jerry

#77 mayidunk

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Posted 18 January 2014 - 02:59 PM

Great thread and kudos for all the 'experimenters' for their hard work!

I take it from reading all this that I should get a prism diagonal for my TEC 8" f/15.5 to replace the Astro-physics Maxbright which I have been using. I assume the new prism diagonal will reduce scatter and not add any false colour thanks to my loooong focal ratio of f/15.5

My primary viewing is double stars, the closer, the better and planets, in season...

Is there a recommended brand of 2" prism diagonal I should get?

Thanks

Dave

Yes, get the Zeiss Baader 2in Prism.
You can try mine but then again you would have to bring your baby over to my backyard.
It is superb on my C-14 but I have yet to test it on my refractors.
(I do think the Maxbright has less scatter then some other 'el-premo'mirror diagonals that I have laser tested).
Clears
Jerry

I've been thinking about getting that 2" Zeiss Baader prism as well, but I just can't wrap my head around the $425 it would cost!

I currently have a Lumicon LumiBrite 2" diagonal. It has a dielectric mirror, and appears to not have issues with scatter, though it may be that it does, and I just don't perceive it. I recently got a Kunming United Optics 102mm f/11 achro that Stellarvue was selling, and thought that it might benefit from having a good, prism diagonal. However, the views through the Lumicon diagonal aren't bad at all. Because of that, I keep thinking that might be wasting my money getting that Baader Zeiss prism diagonal; sort of like trying to make "more gooder" what's already "good enough!"

Would the 2" Baader be better than the 2" Lumicon?

BTW, I'd also like to thank everyone involved with doing this comparo, especially Bill for dong all the dirty work. Your efforts are greatly appreciated! Thanks.

#78 BillP

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Posted 18 January 2014 - 03:16 PM

Remember everyone that I've just scratched the surface here with the initial report. Have a lot more tests to conduct and of course will be submitting it as an article here. So much more to come. Probably not any more interim results will be posted as time for me to get serious and ferret things out and ensure repeatability of results.

#79 Starman1

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Posted 18 January 2014 - 06:10 PM

Bill,

I have heard that a 2" diagonal tends to be a better risk because in a 1.25" diagonal there is more of a chance that the image will be degraded by edge errors in the smaller mirror. Of course, this will depend on the quality of the diagonals.

Mike


Hi Mike. I have heard this too, and I personally do not understand this logic. I have heard it lots of course...but I feel it is more a marketing thing rather than based in anything discrete. Seems it should matter little on the size of the mirror and find it hard to believe that manufacturers cannot produce smaller mirrors with quality to the edge. And if there is a general problem in the industry with the edge, then seems the solution is to use an oversized mirror, regardless if 1.25" or 2" format. At any rate, at present I take that popularism with a grain of salt. :shrug:

I almost hate to throw this in, but this is from Roland Christen, concerning the edges of the AP MaxBright star diagonal:
http://geogdata.csun...dielectric.html
Also, if you exclude the outer 1mm of the edge of my secondary mirror (it came with an interferogram), the peak-to-valley measurement reduces by over 50%!. The edge is always the issue with secondary mirrors, but it isn't the only problem after mounting. Too much pressure from material behind it, and it can become convex. And, improperly-glued, it can become astigmatic. Both of those issues are worse than the edge of the mirror problem.

#80 SteveC

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Posted 18 January 2014 - 07:19 PM

Can anyone explain how reflectivity and scatter relate?

According to AP, my dielectric Maxbright diagonal has 99% reflectivity, yet it has been implied in this thread that its dielectric coating may cause scatter.

According to TEC, my TEC enhanced aluminum mirrored turret has 97% reflectivity, yet it has been implied in this thread that it may have less scatter.

#81 Daniel Mounsey

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Posted 18 January 2014 - 08:45 PM

Yea Don, he's had that article out for quite some time. BTW, last night seeing was stunning. I rated it an 8 out of 10. Don, you should bring your Teeter to my place and set up for Jupiter soon while the timing is good. Once again I conducted tests all with high quality diagonals and the prism is absolutely incredible to say the least. It's simply crisper and cleaner than anything I've ever seen. It's like any glow you see around Jupiter in a dielectric is literally cut down more than 50% by my visual estimation. Dielectric diagonals just simply can not compete with it, even Ray Charles could see it.

I set up my FS102, FS152 and Mewlon 250 and the images just wouldn't quit. The Mewlon was stunning. So much beautiful detail and color on Jupiter while using the Mark V bino with the prism. I could see three gorgeous regions of violet ammonia ice just below the troposphere under the NEB and the RS really appears rusty this year. :)

#82 Starman1

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Posted 18 January 2014 - 08:54 PM

Can anyone explain how reflectivity and scatter relate?

According to AP, my dielectric Maxbright diagonal has 99% reflectivity, yet it has been implied in this thread that its dielectric coating may cause scatter.

According to TEC, my TEC enhanced aluminum mirrored turret has 97% reflectivity, yet it has been implied in this thread that it may have less scatter.


Well, I'll see if I can explain. Dielectric coatings can behave just like interference filters.
Here's how it works:
The light wave hits the surface. Some of it reflects, and some of it does not, and penetrates to a deeper level, where it reflects. If the crest of the second reflection's wave corresponds to the crest of the first wave's reflection, the height of the wave is augmented (constructive interference), and the combined reflection is higher.
That means a measurement of reflection intensity would be higher than any layer's reflection on its own. If the materials are correctly chosen, the reflection from the base glass can be augmented, bringing reflectivity up to nearly 98% at 550nm (green).
But that will be only at one angle.
At other angles, reflected waves will destructively interfere and some cancellation of the wave's reflection will occur.
This, however, is unlikely to happen at every wavelength. If it did, dielectric coatings would not scatter light at all and would be simply the best there is.
Some wavelengths, however, might be dispersed in the same way that light passing through a prism at the right angle is dispersed. At whatever that angle is, the reflection would be prismatic. In fact, prismatic reflection does occur at odd angles from many dielectric-coated surfaces.
If the stack of materials is chosen correctly, the reflection should be nearly pure at one angle--ostensibly 45 degrees on a star diagonal. But what happens to the light that doesn't reflect?
Well, it can pass through the material (hence my earlier comment about making the back of the mirror non-reflective), or it can reflect off layers in the stack and end up parallel to the original ray but perhaps out of phase. Or it can be absorbed by the material, bumping electrons in the material into more excited states. Said light would eventually be re-radiated at who-knows-what angle (but most likely not at visible wavelengths). And, if the surfaces are not perfectly parallel, some may be reflected in other directions.

Picture the Himalayas--resembles the surface of a mirror at a microscopic level. Now, coat that with a material which sticks to the first points encountered (the peaks) and the valleys later. The coating will be ever-so-slightly thicker on the peaks and exaggerate the peak-to-valley error. It's no big deal for a single layer, or even two or three. But make the layers 45 layers deep, and the final layer could be substantially more "rugged" than the first layer. Because of interference, reflectivity will still be quite high, but a fair amount of light will go in a variety of directions (admittedly, a small percentage).

This is analogous to a mirror with a rough surface. Contrast just isn't what it should be because a lot of light is scattered. Fortunately, the dielectric surfaces aren't as reflective, per layer, as an aluminum coating, or the scattering would be ferocious. If the surface is incredibly smooth (like the $795 quartz diagonal from Vernonscope), and the layers on top are applied using ion-assisted depostion, you could still end up with a very smooth surface and less scatter.

And, in fact, some high-end dielectric star diagonals are quite good. Just not cheap.

But if you compare it to a simple coating, like aluminum with a single layer overcoat, and have equally flat and smooth base surfaces, the odds are that the simple coatings will have a better finished surface, with less scatter.

The way I read that is that a dielectric coating needs a flatter and smoother base and greater skill in the application of the coatings. Unfortunately, the quality of the mirrors goes all over the place in star diagonals because they are made inexpensive so they'll sell. If a good after market secondary mirror, with a terrific surface, sells by itself for more that most high-end star diagonals, how much do you think the star diagonal that used a mirror of that quality would cost? Uh huh. You get the picture--it wouldn't sell at all.

So, what are the odds of a really good surface with smoothness at a cheap price? Well, higher with a prism. Partially because of volume (prisms are used in a LOT of other applications) and partially because they have less-expensive coatings and partially because they are partially refractive (I won't go into that here).

I've compared $50 mirror diagonals with $50 prism diagonals, and at f/10-f/15, the prism was brighter and sharper. In that price range (what comes with a lot of scopes), the mirror is cheaper to make so it makes the manufacturer more profit.

Many factors involved other that pure optics.

#83 BillP

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Posted 18 January 2014 - 09:42 PM

I almost hate to throw this in, but this is from Roland Christen, concerning the edges of the AP MaxBright star diagonal:
http://geogdata.csun...dielectric.html


:lol: Don't worry. I will try to not be too harsh as a doubting Thomas :lol:

Of course, I know this post well. And if RC did not have a diagonal of his own in the mix then one would not have to be concerned about any conflict of interests. But alas, he is marketing a feature of his diagonal. So I can't feel like it is a totally objective point of view. :shrug:

It is specific to only dielectrics. So that maybe gives it some strength. But even so, the coating technology market moves rather fast...much faster than other areas used in consumer astronomy. So even if the issue is true, is it still true? Part of the problem with some of the posts continually referred to is that they are getting a little long in the tooth and relating a previous era's technology (which in this case an era can be just 2 years, depending on the astro technology).

At any rate. Yup seen it. But given the age of it and the conflict of interest potential, have to give it a grain of salt in the context of the present.

#84 BillP

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Posted 18 January 2014 - 09:44 PM

I set up my FS102, FS152 and Mewlon 250 and the images just wouldn't quit. The Mewlon was stunning. So much beautiful detail and color on Jupiter while using the Mark V bino with the prism. I could see three gorgeous regions of violet ammonia ice just below the troposphere under the NEB and the RS really appears rusty this year. :)


Man!!! Did you have to post this!!! It is making my mouth water for a Mewlon!!!!!! Arrrrg!

#85 SteveC

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Posted 18 January 2014 - 10:32 PM

Don,

Thanks for taking the time to explain the scatter/reflectivity issue to me. For the most part, I considered all dielectric mirrors as basically generic astro products, never really considering that there might be differences between them. Silly me.

This thread has been very informative.

#86 Jeff Morgan

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Posted 18 January 2014 - 11:52 PM

It's like any glow you see around Jupiter in a dielectric is literally cut down more than 50% by my visual estimation. Dielectric diagonals just simply can not compete with it, even Ray Charles could see it.


:roflmao:

The scene in the Blues Brothers comes to mind where the kid tries to steal the guitar from Ray Charles and he fires the .357 Magnum across the room.

Invite everyone and call it the Ray Charles Invitational Observing Session.

I guess now I'll just have to retrofit my Newtonians with prisms ….

#87 Daniel Mounsey

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Posted 19 January 2014 - 12:22 AM

Nice post Don, thanks for sharing.

#88 Daniel Mounsey

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Posted 19 January 2014 - 09:57 AM

I set up my FS102, FS152 and Mewlon 250 and the images just wouldn't quit. The Mewlon was stunning. So much beautiful detail and color on Jupiter while using the Mark V bino with the prism. I could see three gorgeous regions of violet ammonia ice just below the troposphere under the NEB and the RS really appears rusty this year. :)


Man!!! Did you have to post this!!! It is making my mouth water for a Mewlon!!!!!! Arrrrg!


Yea, that was quite a remarkable evening. I like it when it's like that don't you? :lol:

#89 John Anthony

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Posted 22 January 2014 - 12:36 PM

Can anyone explain how reflectivity and scatter relate?

According to AP, my dielectric Maxbright diagonal has 99% reflectivity, yet it has been implied in this thread that its dielectric coating may cause scatter.

According to TEC, my TEC enhanced aluminum mirrored turret has 97% reflectivity, yet it has been implied in this thread that it may have less scatter.


Well, I'll see if I can explain. Dielectric coatings can behave just like interference filters.
Here's how it works:
The light wave hits the surface. Some of it reflects, and some of it does not, and penetrates to a deeper level, where it reflects. If the crest of the second reflection's wave corresponds to the crest of the first wave's reflection, the height of the wave is augmented (constructive interference), and the combined reflection is higher.
That means a measurement of reflection intensity would be higher than any layer's reflection on its own. If the materials are correctly chosen, the reflection from the base glass can be augmented, bringing reflectivity up to nearly 98% at 550nm (green).
But that will be only at one angle.
At other angles, reflected waves will destructively interfere and some cancellation of the wave's reflection will occur.
This, however, is unlikely to happen at every wavelength. If it did, dielectric coatings would not scatter light at all and would be simply the best there is.
Some wavelengths, however, might be dispersed in the same way that light passing through a prism at the right angle is dispersed. At whatever that angle is, the reflection would be prismatic. In fact, prismatic reflection does occur at odd angles from many dielectric-coated surfaces.
If the stack of materials is chosen correctly, the reflection should be nearly pure at one angle--ostensibly 45 degrees on a star diagonal. But what happens to the light that doesn't reflect?
Well, it can pass through the material (hence my earlier comment about making the back of the mirror non-reflective), or it can reflect off layers in the stack and end up parallel to the original ray but perhaps out of phase. Or it can be absorbed by the material, bumping electrons in the material into more excited states. Said light would eventually be re-radiated at who-knows-what angle (but most likely not at visible wavelengths). And, if the surfaces are not perfectly parallel, some may be reflected in other directions.

Picture the Himalayas--resembles the surface of a mirror at a microscopic level. Now, coat that with a material which sticks to the first points encountered (the peaks) and the valleys later. The coating will be ever-so-slightly thicker on the peaks and exaggerate the peak-to-valley error. It's no big deal for a single layer, or even two or three. But make the layers 45 layers deep, and the final layer could be substantially more "rugged" than the first layer. Because of interference, reflectivity will still be quite high, but a fair amount of light will go in a variety of directions (admittedly, a small percentage).

This is analogous to a mirror with a rough surface. Contrast just isn't what it should be because a lot of light is scattered. Fortunately, the dielectric surfaces aren't as reflective, per layer, as an aluminum coating, or the scattering would be ferocious. If the surface is incredibly smooth (like the $795 quartz diagonal from Vernonscope), and the layers on top are applied using ion-assisted depostion, you could still end up with a very smooth surface and less scatter.

And, in fact, some high-end dielectric star diagonals are quite good. Just not cheap.

But if you compare it to a simple coating, like aluminum with a single layer overcoat, and have equally flat and smooth base surfaces, the odds are that the simple coatings will have a better finished surface, with less scatter.

The way I read that is that a dielectric coating needs a flatter and smoother base and greater skill in the application of the coatings. Unfortunately, the quality of the mirrors goes all over the place in star diagonals because they are made inexpensive so they'll sell. If a good after market secondary mirror, with a terrific surface, sells by itself for more that most high-end star diagonals, how much do you think the star diagonal that used a mirror of that quality would cost? Uh huh. You get the picture--it wouldn't sell at all.

So, what are the odds of a really good surface with smoothness at a cheap price? Well, higher with a prism. Partially because of volume (prisms are used in a LOT of other applications) and partially because they have less-expensive coatings and partially because they are partially refractive (I won't go into that here).

I've compared $50 mirror diagonals with $50 prism diagonals, and at f/10-f/15, the prism was brighter and sharper. In that price range (what comes with a lot of scopes), the mirror is cheaper to make so it makes the manufacturer more profit.

Many factors involved other that pure optics.


I read this once then two more times, please be patient with me as I am trying to understand it, Roland states on the AP web site Advantages of the Astro-Physics Dielectric Coatings:
"Reflectivity - Reflectivity is above 99% over the entire 4000 to 7000 Å photo-visual range. Thin film coatings have extremely low surface scatter compared to aluminum or enhanced aluminum coatings. Dielectric coatings (if done right) will result in less scattered light. Examination with a laser source shows approximately a 5 fold reduction in surface scatter, a tremendous improvement over aluminum mirrors."

I realize he is selling a product but I'm hard pressed to believe his info is misleading. Sooooo my question is when does the aluminum or enhanced aluminum coating perform better then the dielectric ?? because how could this statement be true ?? "And, in fact, some high-end dielectric star diagonals are quite good. Just not cheap.
But if you compare it to a simple coating, like aluminum with a single layer overcoat, and have equally flat and smooth base surfaces, the odds are that the simple coatings will have a better finished surface, with less scatter."

Don, if you are reading this I am in no way questioning your knowledge on the subject, I'm just a little confused because the way I read it from AP the dielectric has a advantage over the aluminum or enhanced aluminum coating, or is this not true in all cases?? and if not then why?? why is Roland wrong in some cases if he is?? I kind of got what you said and I kind of didn't, if that makes sense.

Please keep in mind that I'm full of cold medicine and my head feels like a bubble, my mind is not fully firing on all cylinders right now.

#90 John Anthony

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Posted 22 January 2014 - 03:17 PM

After doing some more reading I guess the question for me in regards to scatterd light comes down to this, if dielectric coatings are resistant and they don't degrade like aluminium then this is the main advantage of these star diagonals.

"the mirror will have the same high 99% reflectivity 10 years from now when most aluminium coated will have dropped well below 90% and should be realumined. Markus Ludes from APM Telescopes confirms that testing reflection of good quartz protected aluminium coating showed after 10 years a reflection of about 83-85%."

"decay due to exposure and damage to the coatings in its use could significantly worsen quality of a reflective coating, resulting in unacceptable amount of scattered light."

The implication "is that dust/dirt are the dominant - and significant - factor of scatter from mirror surface in a relatively short period of time (one year, or so) in between two successive removals, with the combined scatter reaching 5%, or more. But scatter due to the coating may have high relative rate of increase with time, and could become dominant with a coating that is several years old.

And the implication would be not so with the dielectric, at least thats the way I'm understanding it.

#91 BillP

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Posted 22 January 2014 - 04:12 PM

John,

Many good points you brought out. Unfortuantely, different manufacturing processes have different lifespans. I'm not sure anyone can say with assurance the rate of reflectance loss as so many variables affect it, like humidity and protective coatings used. Here's an interesting link as example of some research - A NEW METHOD TO CHARACTERIZE DEGRADATION OF FIRST SURFACE ALUMINUM REFLECTORS.

I don't know if there is any good research yet on the longevity of dielectric coatings either. So will be interesting to see what time tells with these. But like any coating process, the more coats used then the more important clenliness becomes between the coats as particulates cause scatter. There's also research out there that shows that using ion-beam-sputtering to deposit the coatings causes less coating scatter than do evaporation techniques. My fear is that there are multiple methods to deposit coatings, whether protective or dielectric. Not knowing the specifics for each item sold, means impossible to draw much of any conclusions as to scatter, lifespans, and rate of degredations. One thing we do know though I think...a simply polished glass prism with some standard AR coatings has little to go wrong with it over time and will have the same longevity as the main objective.

#92 Starman1

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Posted 22 January 2014 - 05:04 PM

I read this once then two more times, please be patient with me as I am trying to understand it, Roland states on the AP web site Advantages of the Astro-Physics Dielectric Coatings:
"Reflectivity - Reflectivity is above 99% over the entire 4000 to 7000 Å photo-visual range.

Nope. No commercial dielectric-coated mirror actually has that transmission curve. the transmission at the extremes is not low (96%+), but not 99%.

Thin film coatings have extremely low surface scatter compared to aluminum or enhanced aluminum coatings. Dielectric coatings (if done right) will result in less scattered light. Examination with a laser source shows approximately a 5 fold reduction in surface scatter, a tremendous improvement over aluminum mirrors."

Depends on at what angle. At low angles, yes. Wavefront cancellation will occur (variable depending on wavelength) but not the "nearly on axis" scatter than some planetary gurus are talking about. The issue is different at different wavelengths. Complicating that is that not all dielectric-coated mirrors utilize exactly the same materials in exactly the same layers.

I realize he is selling a product but I'm hard pressed to believe his info is misleading.

On axis, a dielectric coating should perform magnificently. But, truly, the scatter depends on the quality of the original mirror (substrate) and the quality of the coatings applied. Even RC illustrates the downside of dielectric coatings with his interferograms.

Sooooo my question is when does the aluminum or enhanced aluminum coating perform better then the dielectric ?? because how could this statement be true ??

If the mirror is better (smoother), and the coatings are smoother, and transmission is not the only measured characteristic.

"And, in fact, some high-end dielectric star diagonals are quite good. Just not cheap.
But if you compare it to a simple coating, like aluminum with a single layer overcoat, and have equally flat and smooth base surfaces, the odds are that the simple coatings will have a better finished surface, with less scatter."

Simple: apply one coating layer--it's smoothness depends on the substrate and the application (IAD preferred). Add additional layers and with every additional layer the surface gets rougher and more irregular.
Since each layer doesn't reflect the entire wavelength spread, this could result in phase errors and overall wavefront errors. Some of the high-end dielectric diagonals, though, have really high-end mirrors and the most expensive processes of deposition. But they are still not the best that labs can do. They charge $thousands for a really accurate coating if ordered by NASA or the military because they know it will be tested and tested exhaustively.

Don, if you are reading this I am in no way questioning your knowledge on the subject, I'm just a little confused because the way I read it from AP the dielectric has a advantage over the aluminum or enhanced aluminum coating, or is this not true in all cases??

It's not true in all cases. Dielectric coatings would be preferred for dimmer objects, where cleaning is important, and, possibly, longevity (jury still out). They might not be ideal for an achromatic refractor with violet haze around the stars.

and if not then why??

slight off-axis scatter, wavefront deformation, cost/benefit ratio, etc.
Nonetheless, I would gladly accept a quartz substrate with a dielectric coating. The differences we are discussing are smaller than the differences in seeing from hour to hour.

why is Roland wrong in some cases if he is?? I kind of got what you said and I kind of didn't, if that makes sense.

Well, read my clarifications, and wait for Bill's remarks. The best thing i can tell you is that commercial star diagonals vary a LOT in quality.
Myself, I have witnessed that variation, and I would say that price isn't the only indicator of quality in that realm, but it is one. I have not seen any $100 dielectric diagonal outperform an AP or a TeleVue. Caveat emptor.

Please keep in mind that I'm full of cold medicine and my head feels like a bubble, my mind is not fully firing on all cylinders right now.

The web is full of details. Read all the reviews and comments and take it all with a grain of salt. A lot of what's said is on the level of:
"It was 0.05% better, so it completely blew away the other one. My God, how could anyone put up with such a piece of junk as that only 99.95% perfect one!"

And they're ALL junk in 3" seeing. :grin:

#93 John Anthony

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Posted 22 January 2014 - 08:01 PM

Thank you both Bill and Don, the info both of you provided did clear things up a bit. I have to be honest though, I feel a little misled, it just seems the claims from some rather big names are "possibly" misleading to one degree or another. It's not that I'm really fussing over it, I'm just trying to understand and it seemed to me that what I was reading here was in contradiction with some of what those big names were claiming. After Don's and Bill's response if I am understanding correctly it's not so much a contradiction as much as it is of not telling the whole story when stating such claims about a particular product.

I have the Maxbright diagonal along with the Baader/Zeiss prism and a number of other enhanced aluminum diagonals, I like them all and never really considered any differences until this thread, mostly used the Maxbright thinking the dielectric was the best. So it's been a learning experience for me.

#94 BillP

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Posted 22 January 2014 - 09:12 PM

Attached is a very preliminary scatter analysis I've done on 3 of the diagonals. It's been difficult to capture photographically the differences seen visually. These are afocal shots through a 5mm XO (163x) on the TSA-102 of a bright white-light source (Halogen lamp) thru a home made peep hole.

Note the difference in scatter. Also note the tonal differences. Can't swear yet to the colors as not sure the camera was doing any white balance adjusting. On next round I will manually set the white balance so it is set. In this pic the exposure is locked, as well as lens focus. So all is standardized and at same distances with only difference being the unknown on the white balance.

Anyway, easy to see the leftmost (common 99% reflective 2" diagonal) that there is a lot more scatter. Perhaps some CA also. Center shows much smaller scatter profile and darker background as well. Center is AP Maxbright. Right shows a slightly smaller scatter profile still than the AP Maxbright, a slightly darker background as well (visible in my hi res pic). Also interestingly the coffee tone!! Perhaps why the prisms seem to be excelling at planetary? Btw, the right pic is the Baader T2 Zeiss Prism.

Again, this is a preliminary set and things may change as I perfect the technique with regard to color. But the scatter is obvious, and aligns with impressions in the field.

Attached Files



#95 Asbytec

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Posted 22 January 2014 - 09:34 PM

In the left image, does the lighter background suggest scatter across that FOV or did the camera auto expose the image? I ran into the same problem taking images of my exit pupil.

Maybe normalizing the background might show the true extend of the scatter compared to the others. The possible and very tiny chromatic effect is maybe what you "should" see. If a diagonal is showing you what you should see, wouldn't that make it a good diagonal?

I understand your work is preliminary, difficult to do, and interesting. It's very much appreciated.

#96 Midnight Dan

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Posted 22 January 2014 - 10:07 PM

Hi Bill:

In the right image from the Baader T2 Zeiss, am I correct in detecting a vertical oval shape instead of the more round shape of the other two? Does that indicate astigmatism?

-Dan

#97 RodgerHouTex

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Posted 22 January 2014 - 10:48 PM

This is excellent information BillP. It is greatly appreciated that you have taken the time to do it.

#98 John Anthony

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Posted 23 January 2014 - 08:15 AM

I can't help but think that the diagonal is overlooked for the most part in this hobby, we are always directing our attention to the latest eyepiece or the scope of our desire which is always one scope away regardless of how many scopes we get. You purchase a good diaganol and that's it, time to give your attention to the latest and greatest eyepiece scope or other accessory. I am kind of kicking myself in the butt because I have the Baader T2 Zeiss Prism and haven't given it the time that I have had opportunity for in the past.

"In the end, the cleaning advantage marketers say for dielectrics IMO is a solution searching for an issue"

There may be some truth to this, one of the reasons I never really reach for the other diaganols is my actions are indeed the result of the suggestions I have read about the dielectric coatings from this or that marketer, because the names of some of these marketers carry some weight I just figured what they suggested was the best way to go. That's not to say I didn't end up with a great product it just more or less unconsciously isloated me from really giving attention to some other worthy options.

I think I am your average Joe observer so my experience is probaly pretty common.

#99 BillP

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Posted 23 January 2014 - 08:22 AM

All - remember that these are just preliminary shots. so don't draw much of any conclusion rather than interesting at the moment. I am still in process of baselining the process.

Norme - That brighter background was from the diagonal. The camera was in complete manual mode, except for the white balance which I forgot about and that was still in auto mode.

Dan - I had some slight shake on that shot...but small enough that I didn't want to retake given this was a test.

Roger - you are welcome.

#100 schang

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Posted 23 January 2014 - 09:09 AM

I can't help but think that the diagonal is overlooked for the most part in this hobby, we are always directing our attention to the latest eyepiece or the scope of our desire which is always one scope away regardless of how many scopes we get. You purchase a good diaganol and that's it,...
I think I am your average Joe observer so my experience is probaly pretty common.


You are not alone in this. When I started in this hobby 9 months ago, I specifically asked questions here for info for unbiased, technical/performance based reviews of telescope related equipment, like those you can get from camera/auto industries. I can say that I did not really get what I was looking for, and had to rely on my own technical knowledge and research/evaluation to get what I need. As you can see testing/comparing equipment is not as simple as one would have believed. Lots of reviews/first light reports can only be viewed as information only. You still need to do your own homework to sort out what will be the best choice for what you need. I am glad that Bill started this type of review, however daunting the task may be.






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