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Equipment Discussions >> Equipment

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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6312383 - 01/14/14 02:02 PM

Quote:


Your suggestion isolates a single parameter, which is often the best method of course, then seeks to discover a relationship. I am all on board with that. But the obvious weakness of that approach is that it does little to account for a full system test. It just gets us a single step closer in what is a loooong journey. Isolating and testing all the variables in a system test, then arriving at a working theory on how they all discretely interact would be a huge undertaking. Not something anyone can do here of course.

I prefer to cut to the chase and get an answer to a system question…is color induced and becomes a factor in the f/6-f/8 range (popular market range for APOs), does planetary performance improve, is threshold brightness impacted for DSO? My process gets me an answer. The more scientifically controlled process you describe gets us a little more knowledge on one parameter and no good way to relate that very accurately to a practical setup.




Bill:

We do what we can do they way we can do it.

That said: Isolating a relationship of interest, making a hypothesis and then investigating the relationship allows one to develop an understanding and quantify the relationship so that it can be applied to generalized, real world situations. There is good reason to carefully investigate the relationship between the chromatic aberration of a prism diagonal and the focal ratio of the telescope.

In this case, a simple analysis says that there should chromatic aberration caused by a prism diagonal, the diagonal is glass, glass exhibits dispersion so light entering the glass from air at an angle will show dispersion, the colors will separate. Add the fact that the light coming from the objective enters the diagonal over a range of angles whose maximum is approximately F/2, there is good reason to expect that a prism diagonal will exhibit chromatic aberration and that it will be a function of the focal ratio of the telescope.

Indeed, generally, there seems to be an awareness that there is a real effect here. A more involved analysis, one I am incapable of without a great deal of further study, could put real numbers on this because the parameters necessary for the analysis are well established.

Making some measurements, qualitative or quantitative, can provide the sufficient data necessary to generalize the information.

We do what we can do the way we can do it...

When you "cut to the chase" you can answer the question for a particular system or a few systems but because of the many uncontrolled variables, one cannot generalize the conclusions unless a great number of comparisons.

Starting simple, keeping things simple, isolate the variable, understand what is going on, move on...

We do what we can do the way we can do it.

Jon


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Chucke
member


Reged: 03/12/10

Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6312573 - 01/14/14 03:31 PM

Back in the 1960's there was a test published in S&T regarding the use of prism vs mirror diagonals for Newtonians. Perhaps someone could look it up. As I recall, there seemed to be a sort of cutoff point in terms of f-ratio where the prism began to introduce noticeable chromatic aberration.

Chuck


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schang
scholastic sledgehammer


Reged: 04/24/13

Loc: columbia, sc
Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Chucke]
      #6312678 - 01/14/14 04:15 PM

I do not know if this is what you mentioned, but I found it on the old CN forum:

http://www.cloudynights.com/item.php?item_id=82

http://www.cloudynights.com/item.php?item_id=1854


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schang
scholastic sledgehammer


Reged: 04/24/13

Loc: columbia, sc
Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6312707 - 01/14/14 04:30 PM

Bill:

I admire you for taking up this experiment. Although I'd like to comment that the task would be easier if you can narrow your goals to either

1) compare the different types of diagonals using a couple of scopes of different f-ratios, preferably low and fast ratios to be able to provide you a discernible visual differences (Hey, we got to trust your eyes to do this, and we do not know how good yours are ).

2) compare different brands with only one scope of your choice.

And, do you trust that each of these diagonals are the best they can be? Can there be a small lemon in there? So I'd suggest that you take two to three diagonals of each brand for a more statistically sound analysis.

Now you see why I admire you in doing this...

Best regards.


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Daniel Mounsey
Vendor (Woodland Hills)
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Reged: 06/12/02

Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Starman1]
      #6312774 - 01/14/14 05:03 PM

Quote:

Jon,
When you are talking chromatic aberration, you have to stipulate that you are looking for it "on axis", since many, if not most, eyepieces exhibit some lateral chromatic aberration when stars near the edge of the field, even in reflectors.




True Don, but it's still relative between the prism and the mirror.


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: schang]
      #6312818 - 01/14/14 05:21 PM

Quote:

Bill:

I admire you for taking up this experiment.




I second that...

Jon


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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Daniel Mounsey]
      #6312970 - 01/14/14 06:51 PM

Quote:

Quote:

Jon,
When you are talking chromatic aberration, you have to stipulate that you are looking for it "on axis", since many, if not most, eyepieces exhibit some lateral chromatic aberration when stars near the edge of the field, even in reflectors.




True Don, but it's still relative between the prism and the mirror.



Meaning what? That if it's more visible in a prism than a mirror the prism is adding some lateral chromatic aberration?


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BillP
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Reged: 11/26/06

Loc: Vienna, VA
Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: schang]
      #6313876 - 01/15/14 08:45 AM

Quote:

And, do you trust that each of these diagonals are the best they can be? Can there be a small lemon in there? So I'd suggest that you take two to three diagonals of each brand for a more statistically sound analysis.




Shien,

Hi. This point comes up every now and then regarding field test comparisons. In truth though, getting a few of each unit vs just one does little statistically. As example, presuming the manufacturer make 1000 diagonals in their production run, how many samples would I need to get results with an accuracy of say +/- 10% (presume at the 95% confidence level)? My sample size would have to be 88 diagonals of each brand!! If I have 4 samples of each diagonal, then the accuracy of the results, statistically, comes to +/- 50%...so a coin toss!

Field tests, IMO, cannot be made into quantitative research. It is at its heart a qualitative process and IMO needs to be conducted and evaluated in that context. To come to any more firm conclusion then really requires that any one field test be evaluated in the context of all the others available to try to draw any more firm conclusions. Single field tests therefore become individual data points really. Field tests IMO become most valuable when teamed with a comprehensive quantitative bench test because the field test more or less informs us of the complexities of the richer operational environment and the need to add a multitude of variables into the mix for subsequent quantitative research to help develop a more comprehensive and accurate model to what is going on. The interplay of things is where it really happens for the operational realm. So in this test, while I am looking at CA on-axis with prisms at 2 focal ratios, one has to realize that what is driving the visual CA can be much more than simply the prism, as the optical chain and the target used all have their nuances that differ (e.g., predominent spectra of the target being observed, level of correction of the main objective, amount of atmospheric induced color which can not be known easily, thermal stability/acclimation of the diagonal itself, varying iris pupil dialations during observeration readings, etc. ,etc.). There are really quite a number of variables that can influence outcomes, but just can't be adequately measured and quantified in anything we amateurs do.


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schang
scholastic sledgehammer


Reged: 04/24/13

Loc: columbia, sc
Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6313975 - 01/15/14 09:32 AM

I agree what you said about the numbers of variables that can affect outcomes, that is why I admire you for taking up this experiment. The key is how you can extract the test information in a meaningful way, which is often not so trivia if the data is not treated carefully. In other words, an early computer parlance, garbage in/garbage out when a deck of punch cards fed into a card reader. No offense here.

I mentioned about using multiple samples of each diagonal for statistic variance analysis. This is not about sample size. it is about pooled standard deviation estimate to provide us some degrees of confidence level in analyzing the data. Without it, it will be difficult to conclude anything. As you know, there are quality variation within a brand of the same product. You may get one that is below par which you then conclude that it is not good compare to other brand, but it might in fact on par or better if you test enough of them. The test number of such sample needs not be very large, normally three will be sufficient in a designed experiment. Sometimes, two will be ok if you have more brands to be compared and the variance of brands is small. If it is large, you won't be able to tell the difference at all unless the test sample size is large. The question is : would you rather do a single sample test and not be able to draw a reliable conclusion, or do limited, genuine repeat tests to get what you need to know?


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BillP
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Reged: 11/26/06

Loc: Vienna, VA
Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6314037 - 01/15/14 10:12 AM

Update: January 14 Observations --

Had a several hour break in clouds last evening which was surprising as unexpected. And wow what a break, it was so very clear out and the Moon and Jupiter closely positioned made a perfect opportunity for some testing. Last night I chose just the prisms in the f/6.25 APO for some CA evaluations using the Moon, and some Planetary detail evaluations using Jupiter.

On the Moon, I observed the limb for on-axis CA, and then the shadows of crater walls near the limb for any coloration other than black. My first observation was straight through without diagonal to confirm no CA on the limb and black shadows. I originally used the 3.5mm XW, but the lateral color on the limb near the field stop was a distracting element for me so decided would be better to choose an eyepiece that showed no lateral color. I moved to an 8mm AP-SPL with a TV 2x, but again, a very little lateral color visible from the Barlowed combination (something I have noted before regarding Barlows), so decided against that. Eventually settled on the 5 XO and 4 Supermono as these both showed no limb color regardless of where in the FOV. So gone was my idea of having comfortable eye releif for the tests

At f/6.25, I was able to coax a very small amount of color when in focus from the prisms, all of them. Most visible in the Vixen and Celestron, the Tak then showed a notch less, and finally the two Baaders showed the least. The Vixen and the Celestron were the only two where it was enough that there was a very slight blue cast to some crater wall shadows, but it was very little really. The Tak and Baaders did not really show this and was easy to get a nice black shadow in crater walls. Took a little more coaxing with the Tak, but it was right on the heels of the Baaders. Really, IMO the amount of CA on the limb was really inconsequential it was that slight. But still, there if you looked hard for it.

Moving to Jupiter was treated to some spectacular performance using the 80mm scope. The 4 main belts were strongly visible, a rich amount of detail and structure within the 2 main equatorial belts was visible, including a nice storm, and the polar regions were also quite richly portrayed with gradations of shading and also the NNTeB peaking through, although not sharply defined and a little more ethereal.

All the prisms were showing Jupiter details exceedingly well. The Celestron and Vixen were not quite as defined as the others, lacking the more finely etched views from the others, but still quite good. The Tak and two Baaders were fairly on-par in the details and crispness being shown. I also had the impression multiple times during the evening that the Baader 2" prism was giving a brighter view than the others. I never nailed this down to confirm, I just recall my reaction each time I put it in the first look gave me that impression up front. When I get arounf the threshold brightness tests will see how this impression fares.

With the prisms putting on such a good show, and having completed my primary mission for the evening, I decided to try some the mirrors just for fun. I brought out the 1.25" mirrors first and started using them. The first impression I got was, "wow...more scatter!" So it was rather obvious that the prisms had a less distracting scatter profile. I was also surprised that the details on Jupiter were definitely softer than they were in the prisms. The NNTeB was not showing through the 1.25" mirrors, and any structure within NEB and SEB was hinted at only as a shading or brightness difference only. So no more defined structure visible within the main belts. I spent a good amount of time using the TV and Vernonscope and Astro Tech mirror and dielectrics and getting their baseline and then switching in a prism and it held consistently all evening that the prisms were showing more details and showing them more etched. The Vernonscope performed the best of the 1.25" mirror and dielectrics btw, giving a slightly better level of detail and crispness, but still not up to what the prisms were showing.

I then decided to bring out two of the 2" diagonals, the Baader dielectric and the AP Max. The AP Max went into the focuser first. To my disbelief the AP was showing astigmatism on-axis! How could this be?? I then went in and got the Baader dielectric and it too showed this! That was too much of a coincidence however, so I figured that the 2" diagonals must need some thermal acclimation time. Inside the house it's 70 degrees and outside it was about 45 degrees. I had never observed thermal acclimation on 2" diagonals before so was a bit of a shock. The 1.25" diagonals have never shown this for me. At any rate, after some time the Baader dielectric and AP MX settled themselves and the astigmatism vanished. Unfortunately, they did not in the end perform much of any better than the 1.25" mirror/dielectric diagonals as far a detail visibility and crispness on Jupiter. They did come across as being a very little bit better, but it was a difficult tell. When I move to the larger TSA-102 for testing perhaps they will pull ahead. One thing that was a bit obvious, was that the AP Max definitely came across as having less of a scatter profile than the others. But even so, the prisms were the better performers for planetary. If I had to characterize it, I would say that if I went out yesterday observing with just the mirror/dielectrics I would have come in at the end of the night thinking that it was a very good evening for observing Jupiter, much better than average. With the prisms, my impression of the evening would have been that it was simply a spectacular night...what I would call a rarer evening. So all performed well, but the prisms made it seem like it was more special evening for planetary observing.

Anyway, a productive and revealing evening with diagonals as the testing begins.


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Eddgie
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Reged: 02/01/06

Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Jon Isaacs]
      #6314044 - 01/15/14 10:17 AM

Somewhat related.

I used my binoviewer in my new Stellarvue 110ED and there was a lot of chromatic aberration (using a Baader T2 Maxbright mirror and T2 Prism).

When I installed the Baader 1.25x GPC into the light path, a device that was designed by Roland Cristen specifically to offset the chromoatic an spherical chromatism induced by the binoviewers themselves, the CA improved very dramatically.

There is no question in my mind that the prisms in the binovoiewr were inducing considerable color error at f/7.

Ah, but when I went back and forth between the T2 Max Mirror and the T2 Prism, the result was much harder to see, though once again, I feel as if there was less CA in the Mirror, but the scope is so well corrected that there is not much CA left with GPC in place.

So, I do believe that the prism is indeed not the right choice for a fast refractor.

And I have used mirrors with my 6" APOs and never had any issues with CA, even in the Televue 101.

Personally, I think the test will be a fun read, but I doubt very seriously it will influence me one way or the other.

I have read dozens of tests over the years that tended to overtstate the performacne benefits of this or that product. Dielectric coatings, XLT coatings, this or that eyepeice, this or that filter, and on and on.

As for dielectrics though, when they first came out, there was all this rave about how much better they were (and I found no difference between them and enhanced aluminum), but I have dielectrics that have been cleaned many, many times with absolutly no sign if damage.

I have intentiallly abraded dielectric coatings.

My findings? Oh, you can scratch anything if you try hard enough.. I was able to scrach a dielectric mirror coating by pressing down in it with the tip of a steel screwdriver, but withough resorting to such extreme measures, I found these coatings to be extremely durable.

That is why I would recommend delectrics. No performace difference that I could see, but the knowledge that a clean diagonal is important for planetary observing and that taking the fear out of the cleaning process is priceless. You can clean these over and over and over again, and not worry about damaging them. They are that tough.


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BillP
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Reged: 11/26/06

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: schang]
      #6314060 - 01/15/14 10:24 AM

Quote:

The question is : would you rather do a single sample test and not be able to draw a reliable conclusion, or do limited, genuine repeat tests to get what you need to know?




More is always better However, I am thankful and feel oh so fortunate to even get one unit given how expensive some of this gear is. So this is not about designing a comprehensive test, but more about doing what we can given the generosity of others. I feel quite fortunate we even got one unit from the field of excellent diagonals here! So "take what we can get, and do what we can" is the mantra for field tests we do as amateurs IMO. All that can be asked really.

Now, if people want to start contributing money so we can build an amateur astronomy consumer reports of sorts, where we can do things like you suggest, then that would be way cool! Would be nice to have an independent and funded consumer group to tackle product reviews. Couldn't be done with loaners from the vendors though as that would add some level of conflict, so really all the units would need to be purchased to be fair. Would be an expensive proposition. Heck, I don't think even Consumer Reports uses multiple units for any of their tests. Expensive undertaking for consumer wares. Plus some vendors might not appreciate it, exposing their quality variability. Given how agressive some are this testing group would have to be very well insulated and independent.


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BillP
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Reged: 11/26/06

Loc: Vienna, VA
Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Eddgie]
      #6314075 - 01/15/14 10:34 AM

Quote:

As for dielectrics though, when they first came out, there was all this rave about how much better they were (and I found no difference between them and enhanced aluminum), but I have dielectrics that have been cleaned many, many times with absolutly no sign if damage.




I myself am not convinced there is a cleaning advantage. I feel it was more marketing. I never had any issues cleaning my mirror or prism diagonals in the past, not any issue cleaning the mirrors in my old SLRs back in the day. If one follows proper procedure, unlikely you will scratch anything. And today, given how tough multicoatings are and that many a prisms and mirror diagonal are available that are multicoated, unlikely much issue even when not being totally careful cleaning those. In the end, the cleaning advantage marketers say for dielectrics IMO is a solution searching for an issue...like barrel undercuts FWIW, I clean my 1980's Celestron prism diagonal all the time, and surface is still quite great


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Eddgie
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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6314182 - 01/15/14 11:28 AM

Well, then, clearly you would be wasting your money to buy a dielectric (though now the do not cost any more than enhanced aluminum).

But unless you have some way to test this, then it is a moot point.

But that is why the MilSpec almost demands dielectric coatings these days. They are harder and more scratch resistent than quartz.

When does it matter?

When you are in a desert country fighting a war and bathed in silicone dust.

And you clean your optics grabbing whatever rag you have hadny and spitting on it and whiping it off.

That is what dielectrics are able to withstand.

It is hard stuff. Very hard stuff.

But you will get some result and I am sure you will give us a table with 7 or 8 tiers of performance and maybe you will even abrasion test them....

Look forward to what I am sure will be an entertaining review.

Have fun!!!


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BillP
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Reged: 11/26/06

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Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Eddgie]
      #6314368 - 01/15/14 01:08 PM

Lots of folks talk about dielectric hardness, but I've yet to find any real data on this. If you have links to actual hardness rating or abbrasion rating of dielectrics used for mirroring, that would be useful. Similarly would be interesting for multicoatings as well. Lots of marketing, lots of anecdotal, no data

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schang
scholastic sledgehammer


Reged: 04/24/13

Loc: columbia, sc
Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6315080 - 01/15/14 06:21 PM

Testing costs money that is for sure. For an industry that is large, it is feasible to do multiple tests, by various non-profit institutions on the same product per the defined performance attributes, from which the consumers can compare and decide. This is the case for camera industry where you see various testing labs using testing equipment and field evaluation for lens and body performance. For us telescope users, it is not feasible, which I understand. I really think that telescope optical equipment can be tested the same way as camera lenses using similar test equipment; and in a lot of cases, the manufactures are willing to loan their new products to the testing institutions for PR relations/consumer info. I hope eventually, telescope industry will be going that way.

Edited by schang (01/15/14 08:25 PM)


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skullpin
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Reged: 03/13/09

Loc: Edmonton, Alberta, Canada
Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6315113 - 01/15/14 06:39 PM

Bill,

Your results mirror my observations (pun intended).

I compared a Tak 1.25" prism, Baader Zeiss 2" prism, and a AP MaxBright in a f/6 refractor on Jupiter last year. I found that the prisms did produce some noticeable chromatic aberration, though nonetheless seemed more detailed than the MaxBright. I found the Tak prism created less chromatics than the 2" Zeiss.

Keith


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Daniel Mounsey
Vendor (Woodland Hills)
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Reged: 06/12/02

Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: Starman1]
      #6315573 - 01/15/14 10:42 PM

Quite frankly Don, I haven't had any color issues with prisms while doing tests at F-5.9.

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The Mighty Mo
professor emeritus


Reged: 10/12/13

Loc: South of North, North of South...
Re: Mirror, Dielectric, & Prism Performance Comparison new [Re: BillP]
      #6315661 - 01/15/14 11:44 PM

Thanks for taking this on and the first preliminary report here Bill. I do have a question, what maximum magnification did you manage to achieve in the prisms, esp the 2" Baader? I ask because I seem to remember reading here once that the prism diagonals were only good up to around 200x, which if true would make them no good to me for planetary use. I'm hoping that was an incorrect statement about them being limited.

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Fomalhaut
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Reged: 08/16/08

Loc: Switzerland
Re: Mirror, Dielectric, & Prism Performance Comparison [Re: Daniel Mounsey]
      #6315879 - 01/16/14 05:57 AM

Quote:

Quite frankly Don, I haven't had any color issues with prisms while doing tests at F-5.9.




Daniel, I do not doubt your findings, but I think they should not be generalized: Mr. Wolfgang Rohr of astro-foren.de (temporarily out of service due to installation of a new software) tested several Zeiss APQ-100 f/6.4, some of which were originally designed for the use with and some for the use without additional glass-path. Those designed with glass-path showed color-issues (CA) without a prism and those designed for the use without glass-path provided color-issues when used with the Zeiss prism-diagonal. Only when used the appropriate way (what they originally were designed for), they performed (also interferometrically) as perfect apochromats.

In this context, outcoming results ("Prisms or mirrors best suited for a certain refractor?") do not only depend on the refractor's f-ratio, but also on the color-correction of its objective-lens - be it intentionally designed the way it is or not.

Chris


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