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THE BAADER BBHS-SITALL SILVER DIAGONAL


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The Baader BBHS-Sitall Silver Diagonal

by: William A. Paolini, February 13, 2016

Fig 1: LEFT - Baader 2" BBHS Silver; CENTER - Astro-Physics 2" MaxBright Dielectric; RIGHT - Baader 2" Zeiss Prism.

 

1. Overview

 

The new Baader Broadband Hardened Silver (BBHS) diagonal, available as both non-T2 (pictured) and T2-threaded in 2" and as T2 in 1.25" sizes, promises to bring Silver back to the mainstream for diagonal mirrors.  Silver has always been an excellent coating for mirroring surfaces, better than aluminum having higher reflectance across the visual spectrum with better transmission in the red color spectrum over aluminum.  In addition, both silver and aluminum are easier to maintain a flat wave front since they have less layers compared to full dielectrics (even the tightest coating tolerances can become problematic when 100 or more layers are deposited as with dielectric mirrors).  The disadvantage of silver however is that it is subject to tarnishing if not protected very well from moisture; but the new BBHS technology purportedly overcomes the tarnishing issue through the use of a dielectric overcoat to create a durable protective layer for the silvered mirror.  With this dielectric overcoat the BBHS silver diagonal is purported to have the same life expectancy as high quality protected aluminum when cleaning is performed with the same care and attention recommended for all precision optical surfaces.

 

In addition to the use of silver for the mirroring, the Baader BBHS also uses Sitall glass for the mirror's substrate.  Sitall is a crystalline glass-ceramic possessing ultra-low thermal expansion properties and a coefficient of thermal expansion of only 0±1.5 x 10−7/°C in the temperature range -60 to 60°C.  Sitall has been used in the making of primary mirrors for the Russian Maksutov telescopes, was selected by Lytkarino Optical Glass Factory (LZOS) for the manufacture of the 91 primary mirror segments of the Southern African Large Telescope (SALT), and was used for the primary and secondary mirrors of the VLT Survey Telescope.

 

Baader claims their BBHS diagonals offer peak reflectivity over a much extended spectrum compared to conventional dielectrics.  The typical premium-level dielectrics, like the Astro-Physics MaxBright diagonal and Baader Maxbright star diagonal, claim peak reflectivity in the visual spectrum from approximately 400nm to 700nm.  The new BBHS technology claims extending that performance far into the infrared with peak reflectivity extending further into the visual spectrum past 700nm to far outside the visual up to 2000nm.

 


Diagonal

Size

Type

Reflectivity Claim

Wave Front Claim

Body Build

Baader BBHS

2"

Silver Mirror

Not specified

Not specified

Diecast and CNC Machined Magnesium

Astro-Physics MaxBright

2"

Dielectric

>99%

 

Inconsistent*

CNC Machined

Baader Zeiss Prism

2"

Zeiss Specification** Prism

Not specified, (Baader Phantom Group Coatings)

Not specified

CNC Machined

Fig 2: Comparative diagonal features.

* -    Marketing on AP website makes no wave front claim.  Reference to 1/10 to 1/20 wave over the portion of the mirror in the light path apparently based on forum conversations.  No specification in these claims if this is before or after dielectric coating.

** -   Per Baader Planetarium, many production divisions of the former government owned Carl Zeiss Jena have been privatized.   The Zeiss core enterprise today concentrates on production of finished ultra-high end optical and medical devices.  Many of the thousands of standard components are delivered to Jena "just in time" like any car factory works today.  The opticians and skilled people that once made up the company are still alive (and kicking) and as a result many highly capable optical companies have developed in this region, still producing optics in the old Zeiss tradition.  Baader has people employed who formerly worked at Zeiss Jena and they still use their contacts to existing optical shops there for the production of their prisms and all optical components for the Mark V binocular,  all produced to the original exacting Zeiss specifications and processes.

 

 

2. Build, Form Factor and Features

 

The Baader BBHS diagonal's all metal construction, which is magnesium for lighter weight and faster thermal acclimation, feels robust with excellent fit and finish.  The 2" nose has a very shallow and narrow undercut and at no time did I experience any difficulties when removing it from the focusers on the Takahashi, Vixen, or APM/Lunt telescopes I used for testing.  The Baader Clicklock mechanism provided outstanding operation, easily proving to be the most ergonomic method I have ever used to secure eyepieces into a diagonal.  With a very short twist of the Clicklock mechanism eyepieces were securely held or effortlessly released. 

 


      Fig 3: Bottom view.

 

From a weight perspective, all three diagonals had a similar heft.  I did not have a precision scale to do exact weightings, but their feel was very close with the Zeiss prism perhaps being slightly heavier than the other two.  Internally, the Baader BBHS diagonal uses flat black micro baffling for light suppression in the nose piece, with side walls in the interior housing smooth and flat blackened.

 

Fig 4: Top view of the Baader BBHS diagonal

 

The BBHS diagonal I used for this test was not their T-2 version, but what they call the BBHS-Sitall Zenith Mirror diagonal (Baader product # 2456115).  It comes standard with the 2" Clicklock mechanism at the eyepiece end and a 2" nosepiece for the focuser.  While the nosepiece unscrews and uses threading like their other T-2 diagonals, the 2" Clicklock adapter does not attach using T-2 threads but is held in place by six small hex key set screws that secure themselves to a non-threaded housing.

 

Fig 5: Disassembled for cleaning.

                                                                   

Light path measures reveal the Baader BBHS, as configured, was the longest of the three diagonals (note that its light path length is approximately the same as the Baader Maxbright Dielectric as measured in my 2014 article: "Mirror vs. Dielectric vs. Prism Diagonal Comparison").  While longer, observationally I encountered no issue with running out of infocus using any eyepiece or with binoviewers with the optical corrector accessory (OCA) attached.

 

Diagonal

Measured Light Path (mm)

Baader Zeiss 2" Prism

100

Astro-Physics 2" MaxBright Dielectric

103

Baader 2" BBHS

112

Fig 6: Diagonal light path measures.


3. Observational Field Tests

 

Fig 7: Baader BBHS diagonal on Lunt 152 ED-Apo with Pentax 30mm XW eyepiece.

 

Observational testing was conducted in a suburban location in Northern Virginia, west of Washington, D.C., where the light pollution level varies, depending on the particulates and water vapor in the atmosphere, between light to moderate. Limiting magnitudes at this location vary on Moonless nights from magnitude 4 to magnitude 5.5.  For this review the diagonal was tested in three telescopes over the course of several months:

  • Lunt152 f/7.9 ED-Apochromat refractor
  • Takahashi TSA-102 f/8 Super Apochromat refractor
  • Vixen 81S f/7.7 Apochromat refractor

 

In the above telescopes, the primary eyepieces used for observing were the Pentax XW line and the Baader Morpheus line.  The Baader Zeiss prism diagonal and the Astro-Physics MaxBright dielectric diagonal were included for comparison purposes as these two diagonals were used in the original diagonal comparison report in 2014 of twelve different diagonals, Mirror vs. Dielectric vs. Prism Diagonal Comparison

 

3a. Thermal Acclimation

 

Since my tests were conducted in the winter, my initial tests were related to thermal acclimation time.  In my 2014 comparison of various diagonals, I noted that some diagonals exhibited astigmatism while they were cooling to the ambient outdoor temperatures.  With a temperature delta of 28º F (i.e., indoor temp of 74º F and outdoor observing temp of 36º F), the time to acclimate where the on-axis image showed no levels of astigmatism were as follows:

 

Diagonal

Time to Acclimate

(minutes)

Baader 2" BBHS

7.0

Baader 2" Zeiss Prism

9.5

Astro-Physics 2" MaxBright Dielectric

11.5

Fig 8: Diagonal thermal acclimation times for a 28º F temperature delta.

When the diagonals showed astigmatism while cooling, stars could not be brought to precise focus and the slightly defocused star showed the characteristic oval that changed orientation by 90º when defocusing between slight in-focus to slight out-focus.  With its magnesium construction and virtually zero-expansion Sitall mirror, the BBHS showed the quickest cool down characteristics of the three diagonals.  Interestingly, the amount of astigmatism during cool down was also comparatively small in the BBHS and Zeiss prism compared to the Astro-Physics MaxBright dielectric.  As a result, both the BBHS and Zeiss prism showed the least amount of star point deformation during cool down.  Of course, all scopes were fully acclimated showing perfect star points and airy disk patterns with no diagonals in the train before this test was conducted.

 

3b. Stray Light Control and FOV Illumination

 

As noted in my 2014 test of various diagonals, not all diagonals maintain a fully illuminated field of view (FOV) when using a 2" eyepiece that has a field stop that renders the maximum true field of view (TFOV) capability of the 2" barrel.  For this test, I used the Pentax 40mm XW with a field stop of 46.5mm.  All diagonals presented a visual FOV that appeared well illuminated with no detectable vignette of the FOV or dimming near the field stop.  The field stop also showed sharp and distinct with all diagonals.  At no time were any stray light artifacts observed during the testing.

 

3c. Scatter

 

In my 2014 diagonal comparison, I assessed that the Baader Zeiss prism diagonals tested and the Vernonscope Silver diagonal were the ones that showed a markedly less amount of scatter compared to the dielectrics and aluminized diagonals tested.  While the premium dielectrics like the Astro-Physics MaxBright and Baader Maxbright dielectrics were better relative to scatter than other dielectrics, they still showed more scatter than the Baader Zeiss prisms and Vernonscope silver.  Using the same Astro-Physics MaxBright diagonal from that test in this current test, the Baader BBHS visually showed a level of scatter that was slightly less.  However, the Baader Zeiss 2" prism diagonal still slightly less scatter than the BBHS.  As example, scatter halos around stars and planets showed slightly less extent and slightly less brightly in the BBHS compared to the dielectric.  This difference was close, but still visually detectable.  And where the scatter difference apparently had more impact was in observing close double stars.  Using the 81mm Vixen 81S Apo with a low power eyepiece so the split of Rigel was more difficult, through the dielectric the separation was just observable with a very slim line of gray-black separating the two stars.  With the BBHS the visual of the split appeared very different, with the gray-black region between the stars being more fully black and also thicker, making the split observation all the easier. Moving to some closer doubles in Orion, the results were consistently similar.  One one double in Orion that was too close to split at the magnification I was using so that it appeared as just a non-descript elongation of a single mass through the dielectric, with the BBHS this same double showed instead as two distinct touching balls.  This improved performance was very exciting to see.  My presumption is that this performance difference was due to narrow angle scatter differences nearest the stars, but it could very well be other factors that are involved.  Regardless of what the main driver for the difference is, scatter or some other attribute, what was important was that the BBHS was more effective at splitting close doubles and provided a more enjoyable observation as a result.

 

3d. Faintest Stars and Nebula

 

For this test I approached it two ways: first I observed clusters with faint stars to see if any of the faintest threshold stars showed in one diagonal any better than another, and secondly I observed the greatest extent of nebula that were visible (i.e., did any diagonal show nebula extending to a further extent).  Many amateurs in casual discussions usually refer to differences like this as being due to transmission differences.  However, although transmission may indeed be a player, it must be realized that other factors can also contribute, such as contrast and scatter.  So rather than getting bogged down in trying to determine the optical or manufacturing drivers behind any observation outcome difference, it is best in my opinion to just focus on the observations.

 

Looking for faintest stars, I observed the Double Cluster, the Orion Nebula, and the open clusters in Auriga.   I was not expecting much of any differences relative to seeing faintest stars as all the diagonals were top shelf equipment.  To my surprise however, I discovered differences immediately.  When observing the Double Cluster, the faintest stars that were at the threshold of vision and only detectable with adverted vision in the BBHS and Zeiss prism, were completely invisible in the Astro-Physics dielectric!  This performance difference was repeated observing other open clusters and was repeatedly encountered when observing the Trapezium in M42, where the F component remained illusive in the dielectric but was plainly visible in the BBHS and to a slightly lesser extent in the Zeiss prism. 

 

Moving to M42 and M43 to observe any differences in the extent of their nebulosity, the BBHS and Zeiss prism both showed more of the wings of M42 extending into the distance, and more pronounced and brighter nebulosity around M43.  The caveat relative to nebulosity though is that when the magnification was lower and exit pupils larger, all three diagonals appeared to perform the same.  So it was only when more magnification was employed that the performance differences presented themselves.  In the case of the Orion observation, the nebula looked visually the same when operating at an exit pupil of 1.25mm using a 10mm XW eyepiece (120x in the Lunt 152 and 82x in the TSA-102).  However, when I switched to a 5mm XW with a resulting 0.63mm exit pupil in both refractors, the nebulosity of M42 extended notably further when observed through the BBHS or Zeiss prism than through the Astro-Physics dielectric.  The Zeiss prism showed the furthest extent of the nebulosity, but it was only very slightly more.

 

3e. Rendition of Colors

 

Initially I had not planned for any testing of how colors were portrayed by any of the diagonals.  My presumption was that they should all be relatively the same so I doubted there would be any notable differences to warrant a test.  This all changed when I began my planetary testing observing Jupiter.  On my first evening out observing Jupiter, I used the Astro-Physics dielectric diagonal first. The observation was excellent with Jupiter giving up a ton of details through the Lunt 152 Apo.  The GRS was also well in view in all its swirling glory.  When I switched to the BBHS diagonal, I was shocked as the view was obviously different, and the GRS appeared markedly more saturated displaying a beautifully bright and rich red-pink color!  This richer color of the GRS was not slight at all, making it appear surprisingly more contrasted and colorful against its surroundings.  In comparison through the dielectric, the GRS looked lackluster and pale in comparison and much less contrasted to its surroundings.   I switched diagonals several times, and repeated the observations on other evenings and the difference remained.  The BBHS silver mirror was definitely showing colors more vividly than the dielectric, and even a little better than the prism as well.  After seeing this performance relative to colors, on other evenings I decided to observe several clusters that contained orange-red stars, like those within M37 as example.  Now attuned to look for how vividly colors are shown, it was apparent that the silver diagonal was doing an exceptional job compared to the dielectric as all the clusters I visited showed their orange and red stars much more vividly.  Seeing all these star colors so much better then reminded me of a quote from Vincent van Gogh where he said, "The night is even more richly coloured than the day…  If only one pays attention to it, one sees that certain stars are citron yellow, while others have a pink glow or a green, blue and forget-me-not brilliance. And without my expiating on this theme, it should be clear that putting little white dots on a blue-black surface is not enough."  Sometimes we get so caught up in the details, we forget about the colors.  Using the BBHS definitely reminded me just how colorful many targets can be when all components of the optical train are up to the task.

 

3f. Image Crispness

                                

My final tests were relative to how sharp or crisp the views were between the diagonals, and if there were any differences readily visible between them.  For these tests I limited them to lunar and planetary (Jupiter) observing only to make the assessment.  Overall, all three diagonals produced what I felt were equally sharp and distinct views with no notable differences.  Aside from differences in contrast and color saturation, all diagonals showed sharp and crisp views from low to higher magnifications of approximately 250x.

 


4. Conclusion

 

Fig 9: Image inset: Jupiter; NASA, ESA, and A. Simon (Goddard Space Flight Center)

 

The purpose of this evaluation was to test the performance of this latest offering from Baader relative to the diagonals I previously tested in my 2014 report, Mirror vs. Dielectric vs. Prism Diagonal Comparison.  In that previous comparison there was a silver diagonal that proved to be exceptional, but with a prohibitive cost, the Vernonscope 1/20th wave quartz star diagonal.  The new Baader BBHS technology was therefore intriguing as it promised the performance of silver without the purported longevity worries and at a price commensurate with the non-silver competition.

 

Overall, all three diagonals provided superlative results with earned reputations.  However, the Baader BBHS dielectric protected silver diagonal distinguished itself by pulling in fainter stars, showing minimal scatter, and presenting colorful stars and planetary features more richly colored, with its silver technology besting the defacto standard for high performance dielectric diagonals.  Its Clicklock mechanism provided a level of ergonomic ease far surpassing other locking technologies I have used.  It clearly demonstrated low levels of perceived scatter, the ability to bring into view the dimmer of stars in clusters than the other diagonals, the ability to make more authoritative double star splits, and the ability to show the faintest extents of nebula.  All these attributes were highly welcomed and they clearly enhanced my observations.  Most surprising however, was how brightly and vividly the BBHS technology portrayed the colors of stars and of planetary features, showing colors more richly saturated and more beautifully bright than even the best dielectric technology diagonal could muster.  The views through the BBHS of brightly colored stars accentuated in familiar clusters, and of a richly colored GRS coursing its way across Jupiter were nothing less than truly memorable. 

~ ~ ~

About the Author

William "Bill" Paolini has been actively involved in optics and amateur astronomy for 45 years, is author of the desk reference on astronomical eyepieces: Choosing and Using Astronomical Eyepieces which is part of the Patrick Moore Practical Astronomy Series published by Springer of New York, has published numerous product reviews on major online amateur astronomy boards, and volunteers with public tours at a famous vintage Clark refractor site.

Bill's professional background is as an officer in the U.S. Air Force and as a computer scientist, holding a Bachelor’s degree in Computer Science and a Master of Science in Education. He has worked for the U.S. Department of Defense, the U.S. Department of Commerce, the Federal Trade Commission, the Federal Reserve, the World Bank, and a variety of commercial corporations in the information technology, information technology security, and telecommunications industries.

Bill has been observing as an amateur astronomer since the mid-1960's, grinding mirrors for homemade Newtonian telescopes during the 1970's and eventually owning, using, and testing several hundreds of eyepieces in a wide variety of telescopes from Achromatic and Apochromat refractors to Newtonian, Maksutov-Cassegrain, and Schmidt-Cassegrain designs. Today he enjoys observing and testing new equipment from his suburban home west of Washington, D.C., where his primary amateur astronomy pursuits are lunar, planetary, bright nebula, open cluster, and globular cluster observing.

 

***

This article is placed in the public domain with no use restrictions when presented in its entirety.  Quoted excerpts are permitted without request with the citation: "William Paolini, The Baader BBHS-Sitall Silver Diagonal".

Images without credits are by the author and © William Paolini, 2016.  All rights reserved.

Please direct all questions or requests for permission to use images to the author at wapaolini@hotmail.com.


  • doctordub, cwright, roadi and 16 others like this


224 Comments

Baader makes that contention as well and recommends dielectrics for white light solar.  I personally prefer prisms for when I do white light solar. Not sure how much of the contention though is worrying about nothing or has some actual merit.  Not sure if any medical studies have been done on this. Probably not.  But always better to be save than sorry. But for night time...dielectrics are still a diagonal of last resort for me  :lol:

So after reading Bill's report I immediately recognized this Sitall design as a game changer for refractors. But I also picked up the SCT adapter for my C14, after selling more stuff, and then I started to think that this mirror is doing nothing for SCTs.

 

First question: is the scopes secondary silver?

 

When the light hits the secondary hasn't the damage already been done? Or can this improved mirror still pull out the better using whatever light it recieves? And what about a binoviewer? With a corrector? Does this corrector even allow the silver to work? Or does binoviewing omit this mirror altogether?

 

Will definitely not use this mirror for solar! Shows how fragile it really is. I give it 2-3 years use.

So after reading Bill's report I immediately recognized this Sitall design as a game changer for refractors. But I also picked up the SCT adapter for my C14, after selling more stuff, and then I started to think that this mirror is doing nothing for SCTs.

 

First question: is the scopes secondary silver?

 

When the light hits the secondary hasn't the damage already been done? Or can this improved mirror still pull out the better using whatever light it recieves? And what about a binoviewer? With a corrector? Does this corrector even allow the silver to work? Or does binoviewing omit this mirror altogether?

 

Will definitely not use this mirror for solar! Shows how fragile it really is. I give it 2-3 years use.

 

I'm not understanding.  What shows how fragile it is?  And if the diagonal's silver is completely coated and sealed by the dielectric over-layers, how is the silver going to deteriorate?

It will still deteriorate.  It will just take much longer.  Even overcoated aluminum eventually corrodes.

 

By the way, it seems the Zeiss prism Baader 90° diagonal #2456117, labeled BBHS might have a protected silver coating on the reflective surface after all.  Yet another reason it's so expensive (~$499.)

 

Bill,

For white light solar, you could just add an IR filter if you feel uncomfortable with the IR transmission characteristics of silver.

It has been highly recommended to not use this mirror for visual light solar due to IR. I was going to sell off my diagonals but kept one Dialetric for this reason. The mirror has been absolutely incredible and I haven't even been able to use it properly. Seeing has been the worst all year and I can still make out richer color in my 152 on Jupiter, plus the view will generally include red and blue on the disk when seeing is this bad.

But there is no stray color!

Moons would shimmer violently before with red and blue around their disks. Now there is significantly less scatter and a trace of red and blue.

Can't wait for a decent night!

Add this creation to the Lunt 152, Denk Binotron/LOA being the most revolutionary products for us in astro.

It will still deteriorate.  It will just take much longer.  Even overcoated aluminum eventually corrodes.

 

By the way, it seems the Zeiss prism Baader 90° diagonal #2456117, labeled BBHS might have a protected silver coating on the reflective surface after all.  Yet another reason it's so expensive (~$499.)

 

Bill,

For white light solar, you could just add an IR filter if you feel uncomfortable with the IR transmission characteristics of silver.

Which is better for planetary?

 

I would return the mirror for the prisim.

Bill,

For white light solar, you could just add an IR filter if you feel uncomfortable with the IR transmission characteristics of silver.

 

 

I have zero concerns.  UV would be more of a problem IMO.  IR you will know when it is excessive.  But at the tiny transmission levels of the ASTF filters should be zero issues.  I have not yet tried the BBHS for white light solar...I use my Zeiss prisms for that typically, plus I only do solar a few times a year at most.

Which is better for planetary?

 

I would return the mirror for the prism.

 

 

 

Planetary is often filled with personal preferences.  That being the case, only you will be able to determine which.  I don't have enough seat time with planetary comparisons with both so cannot say which I would prefer.  Anecdotally though the BBHS seems to be staying in the focuser more...but I would never let go of the prism.  Not an either-or thing for me as both are excellent and both have their strengths.

Even overcoated aluminum eventually corrodes.

 

That is true.  While I often hear of folks re-aluminizing and coating their mirrors, I seem to rarely come across threads saying similar for the secondaries.  Seems odd as they would deteriorate similarly correct?  So things like the TV Mirror diagonals and the Tak 2" Mirror diagonal would all have limited life times.

The differences between diagonals are so small as to be un-
noticeable if BillP hadn't brought it up.
Kinda like the differences between eyepieces at the top level,
or scopes.

The best+ the best+ the best= a significant difference. Maybe.

Yes...many of the differences are not so overt.  However, a difference being small does not mean it is necessarily insignificant.  Small levels of scatter can often mean the difference between seeing a feature and not being able to see it at all.  I would never have believed or known this had I not done all these comparative tests over time.  Examples that come to mind are small barges on Jupiter, orthographic clouds over Olympus Mons on Mars, storms in the belts on Saturn, subtle colorations and hues on double stars, etc. 

 

I personally attach more importance to the optimization of the accessories, like the eyepieces and diagonals, rather than the main objectives.  Reason being is that over time I inevitably upgrade the objectives as I may be able to afford, so the scope is a little more transient.  The things like diagonals and eyepieces and such though, once you have a winner it tends to stick around.  So more likely for me at this point in my journey for the accessories to go through a bunch of scopes over time rather than for a scope to go through a bunch of accessories.

 

From a diagonal perspective now I have the BBHS, the 2" Zeiss Prism, and the 1.25" Zeiss Prism.  IMO these three represent a range of optimized diagonals.  And of course on the eyepiece side I have the XWs and XO and ZAO for planetary so nothing going to top them for my observing needs.  So for me now, the main objective is all that gets varied as the accessories are now fixed (unless something new comes along that can supplant them, but doubtful). 

Bill,

I presume you know how to do a star test of a telescope.

You would find it illuminating (pun intended) to do a star test on a scope without a star diagonal and then again with the diagonal.

Over the years, I've owned many scopes that used star diagonals--Maks, SCTs, refractors--and many star diagonals from many companies, but only 2 star diagonals that had NO visible effect on the star test image.

The #1 problem, as is the case for secondary mirrors, is a slight convexity to the star diagonal's mirror, which imparts a slight magnification to the star diagonal, usually a bit of astigmatism, and a touch of spherical aberration.

Alas, I own neither of those star diagonals today.

I should buy a dozen different ones and do a comparison again.

 

Light scatter is just one problem, and it's far from unique to dielectric star diagonals.

    • jrbarnett likes this

Why would a guaranteed flat Newtonian Secondary or a Diagonal have a slight amount of convexity?  First I've heard of this.

 

I've not done a star test on the varying diagonals.  However, when doing testing with very tight doubles, I was surprised that the doubles showed split with the BBHS and touching with no black space at all between them when using the Dielectric.  Sure didn't expect that.  Was able to repeat that performance too on various evenings.  Have no idea why that would be.  Surprising though.

 

Which is better for planetary?

 

I would return the mirror for the prism.

 

 

 

Planetary is often filled with personal preferences.  That being the case, only you will be able to determine which.  I don't have enough seat time with planetary comparisons with both so cannot say which I would prefer.  Anecdotally though the BBHS seems to be staying in the focuser more...but I would never let go of the prism.  Not an either-or thing for me as both are excellent and both have their strengths.

 

Care to share these strengths/differences Bill? Thank you.

The substrate could simply be flatter on one than the other.  A distinct possibility not related to the coating material.

 

First: the process of making a flat can have the unintended effect of rolling off the edge of the mirror.  They're held on the edge in a fixture that is not PERFECTLY rigid on the nanometer level.  So the rotating table under the mirror can tilt the mirror ever so slightly.

 

Second: the process of coating the mirror with a large number of cavities of dielectric coatings also tends to roll off the edge of the mirror.

Roland Christen had a good article on this. It's because the coating thickness is not uniform over the mirror's surface and because of heat applied during coating.

 

Third: if you look at a lot of Danny's (PINBOUT) posts here on CN, he has run into a lot of secondary, flat, mirrors with slightly convex surfaces.

It is not uncommon.  He shows the interference fringes on a lot of them, and it isn't pretty.

 

Fourth, one of the reasons a 2" diagonal is recommended on scopes small enough that the entire clear aperture of the diagonal is not illuminated is simply to avoid the edges of the mirror, where the majority of the optical problems (like TDE) lie.  So, a good reason to use a slightly oversized secondary mirror in a reflector and an oversized mirror in a star diagonal.

 

If you hung out on the ATM Forum or REFLECTORS Forum a bit, you'd see all the reports on bad mirrors, mostly secondaries, but also star diagonals.

 

And, as I mentioned, a star test done with and without them usually reveals the problems caused by the star diagonal.

 

One of the reasons I recommend Antares Optics secondary mirrors is that they are tested AFTER coating, so WYSIWYG.

As for star diagonals, if you get a chance to test 4 of the same brand/model some time you'll see there is a variation from unit to unit just as there is with primary mirrors.  All too often, people blame the primary mirror in inexpensive scopes for poor image quality when it is really the secondary at fault.

    • Daniel Mounsey likes this

 

Which is better for planetary?

 

I would return the mirror for the prism.

 

 

 

Planetary is often filled with personal preferences.  That being the case, only you will be able to determine which.  I don't have enough seat time with planetary comparisons with both so cannot say which I would prefer.  Anecdotally though the BBHS seems to be staying in the focuser more...but I would never let go of the prism.  Not an either-or thing for me as both are excellent and both have their strengths.

 

Bill, I am the lucky owner of a AP175 APO.  Could you do a list of strengths (pros) and cons for each.  I am wondering if I am missing out on something here!  Maybe it is time to replace my 2" quartz dielectric.

The substrate could simply be flatter on one than the other.  A distinct possibility not related to the coating material.

 

First: the process of making a flat can have the unintended effect of rolling off the edge of the mirror.  They're held on the edge in a fixture that is not PERFECTLY rigid on the nanometer level.  So the rotating table under the mirror can tilt the mirror ever so slightly.

 

Second: the process of coating the mirror with a large number of cavities of dielectric coatings also tends to roll off the edge of the mirror.

Roland Christen had a good article on this. It's because the coating thickness is not uniform over the mirror's surface and because of heat applied during coating.

 

Third: if you look at a lot of Danny's (PINBOUT) posts here on CN, he has run into a lot of secondary, flat, mirrors with slightly convex surfaces.

It is not uncommon.  He shows the interference fringes on a lot of them, and it isn't pretty.

 

Fourth, one of the reasons a 2" diagonal is recommended on scopes small enough that the entire clear aperture of the diagonal is not illuminated is simply to avoid the edges of the mirror, where the majority of the optical problems (like TDE) lie.  So, a good reason to use a slightly oversized secondary mirror in a reflector and an oversized mirror in a star diagonal.

 

If you hung out on the ATM Forum or REFLECTORS Forum a bit, you'd see all the reports on bad mirrors, mostly secondaries, but also star diagonals.

 

And, as I mentioned, a star test done with and without them usually reveals the problems caused by the star diagonal.

 

One of the reasons I recommend Antares Optics secondary mirrors is that they are tested AFTER coating, so WYSIWYG.

As for star diagonals, if you get a chance to test 4 of the same brand/model some time you'll see there is a variation from unit to unit just as there is with primary mirrors.  All too often, people blame the primary mirror in inexpensive scopes for poor image quality when it is really the secondary at fault.

Personally I wish they'd gone with fused quartz or Zerodur rather than Sitall.  Sitall quality has been a roller coaster over the last few decades.  Zerodur has been much more consistent.  Fused quartz polishes smoother than either and is harder.

 

But honestly a high quality aluminum coated with quartz overcoat Pyrex or Supremax mirror, in my experience, when clean and new, surpasses prisms, and dielectrics on any substrate.  Not so much after some use and years though.

 

Regards,

 

Jim

So after reading Bill's report I immediately recognized this Sitall design as a game changer for refractors. But I also picked up the SCT adapter for my C14, after selling more stuff, and then I started to think that this mirror is doing nothing for SCTs.

 

 

FWIW we recently (about a week ago) got the BBHS diagonal for our C9.25 Evo SCT- and it produced a very noticeable improvement on the GRS on Jupiter- like Bills review I was surprised just how noticeable a difference...  (for full disclosure- it was replacing a 'cheap' GSO 2" quartz dielectric diagonal)

 

Unfortunately haven't had a chance between time and weather to take the scope anywhere dark yet (Jupiter was observed from an orange-light-polluted back of the house) to comment on improvements for any DSO stuff.

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Disciplus55
Apr 21 2016 05:07 AM

Hi All,

Not quite a feedback on the BBHS diagonal only, but more on an overall set up :

Last night, me and my friends from our astro club made an observation session at a secondary school from 9pm to 11pm, we had 4 scopes and about 20 enthusiastic kids who wanted to see the Moon and Jupiter. I had my TSA 102 with a Leica zoom (with Powermate) and my BBHS diagonal. The other scopes where a SW Mak 180 mm with ES 82 eyepieces and a TS Dielectric diagonal. Then a SC of 250 mm and a 200 mm Newtonian. All the kids where dropping some "wow, awesome, nice, etc..." on all scopes.. but almost all of non shy ones, quite a lot of them actually, when looking at Luna or Jupiter on the Tak where claiming things like : "Wow, here, we see much better, it's much nicer"... it was all said...  :grin:  : sharper image, way more contrast, a ton of details, vibrant colors, no CA... All 4 magnifications were close, all scopes were on temperature and collimated, but all the rest was different : aperture, scopes formulas, eyepieces and diagonals... but all in all, the resulting image was so obvious that the kids noticed the difference for their first view of the Moon and Jupiter of their lives !  :grin:

I shall compare only the diagonals during some nights in May.

 

...almost all of non shy ones, quite a lot of them actually, when looking at Luna or Jupiter on the Tak where claiming things like : "Wow, here, we see much better, it's much nicer"... it was all said...  :grin:  : sharper image, way more contrast, a ton of details, vibrant colors, no CA... All 4 magnifications were close, all scopes were on temperature and collimated, but all the rest was different : aperture, scopes formulas, eyepieces and diagonals... but all in all, the resulting image was so obvious that the kids noticed the difference for their first view of the Moon and Jupiter of their lives !  :grin:

 

Very cool :waytogo:   From a design standpoint, the apochromatic refractor is the purest possible with the least impacts to the light.  The resulting character this imparts to the view may be hard to quantify, but subjectively it is an easy tell.  So not surprised at their reactions.  And of course, the TSA is magical anyway. :lol:

    • mikeDnight likes this
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SiriusLooker
Apr 28 2016 08:01 AM

Bill..

  Yep, sorry I also would agree strongly with Jim Barnett and his feedback.. As matter of fact, after reading your nice article of testing, and (Before) reading Jim's feedback, I was also going to remark, after having over 40yrs experience, and (Now), observing 325 to 350times each year, that there are too many differences in one unit versus another, not just primary mirrors, but both secondary and diagonals.. My friend recently change out his old Secondary with Antares high quality secondary , and his Starsplitter 18inch performance went up a 3 fold!  I will recommend an Antares secondary for someone (who has a reflector), to upgrade before changing primary, or something else. 

  Just my 2cents input.

  Dennis

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SiriusLooker
Apr 28 2016 08:07 AM

Sorry, (Correction),, I meant to say   ......strongly agree with Starman1 feedback and not Jim B.    sorry for confusion..

Sorry, (Correction),, I meant to say   ......strongly agree with Starman1 feedback and not Jim B.    sorry for confusion..

 

No need to be sorry as this is a hobby of "preferences" and not "facts".  And not I strongly agree with Don also...on some points...on other points less...some points not.  He made a lot of points! :lol:   It's all good.  Only one adage I feel stands as a best practice, "To you own eyes be true."  For me, in several areas the silver has been phenomenally better.  Others I have spoken with as well once they brought those aspects into the forefront of their observations.  It's all about what we like once we see it.

So can I gather that this new Baader BBHS diagonal which seems to be ordained as the newest product with mystical powers is not recommended for SCT's because of its optical design?

But to fully articulate my point, is this only viable for a refractor telescope?

 

Mark

I would not see why that the visual attributes I have seen with this in reactors should not also translate to SCTs.  The "mystical" powers may not, but all the others should (i.e., more vibrant colors, lower scatter compared to dielectrics).  I did not test it in an SCT since I recently let go of mine FWIW.



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