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Why specify glass type if you don't specify optical accuracy?

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#226 Wildetelescope



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Posted Today, 10:43 AM

It has nothing to do with being a perfectionist or being a nit-picker.  It's not being a perfectionist when the chromatic aberration smears the contrast and detail.  As was said, it's not a great planetary scope but it's not an abject failure either. 


As I said, I am of two minds:


- If you have one, use it, use it for the planets, use it for deep sky, double stars.. 


- If you are thinking of buying a scope and the planets are a priority, this is probably not the scope you want.  With a mount, you are looking at $500 plus.  $500 will buy a much more capable planetary scope than this 120mm F/5 achromat.   You know that, I know that, Daniel knows that.  


Chromatic aberration is a real thing, it represents light that is out of focus.. It doesn't matter whether you are aware of it, it is light that is not focused and it affects the contrast.


So, if someone came to you and said, "I live in the city, I am interested in viewing the planets and I have about $500 to spend, what would you recommend?"   


A 120mm F/5 Achromat would not be high on the list.  I am not playing favorites and being a nitpicker, just being realistic and honest. It's someone else's money, my experience certainly points to a 6 or 8 inch Dob or the 120mm F/8.3 version of the ST-120.  



I remember my first scope very clearly.  It was a long focal length 60mm refractor, caked with mud and dirt, it has one two element eyepiece, no finder. The mount was missing major parts.  I paid $5 at a garage sale.  I washed it off, cleaned up as best I could. I decided the mount was beyond repair. Somehow I managed to strap the OTA to a worn out department store photo tripod and I was in business. 


It certainly wasn't much of a scope but it was enough so that early one morning, out on the Arizona desert, I was able to get a glimpse of nebulosity that later turned out to be M42.  That was the spark that lit a fire that became a passion.  


The "beginners mind" does not mean remaining ignorant or limited by your tools. "It refers to having an attitude of openness, eagerness, and lack of preconceptions when studying a subject, even when studying at an advanced level, just as a beginner would."  


It means learning to see, see what is there.  In the context of chromatic aberration, it means seeing chromatic aberration without preconceptions that it is either good or bad, rather determining that by seeing, by careful observation as Daniel has mentioned.


A beginner seeking a better observing tool can certainly certainly part of a beginning mind. As experienced amateur astronomers, beginning amateurs come to us for advice.  This means dropping any agenda I might have and try to help them, educate them so they can have reasonable expectations, make reasonable choices.  Knowing something about the color correction of a telescope is part of that choice.  



The discussion here has been interesting and lively, and possibly useful to the beginner, if they sort through the noise.  The OP's question is a variation on a more fundamental question asked by backyard astronomers of all levels, which is "how do I know that what I am buying is good?"   This is not an unreasonable question to ask, especially from folks who are just starting out, and it is asked often.  The answer for most experienced folks is "let me look through it for a couple nights and I will tell you:-)"  For the rest of us, it is often less straight forward, and the barrage of information/opinions can be overwhelming.  The challenge is, the definition of "good", is based partly on precise technical definitions, and partly subjective considerations that are dependent on the users individual circumstances and expectations.  There is no really good way to separate these(hence the "experienced person's answer").  Which is why there are an infinite number of multi-page threads addressing this question.  


As I understand it, Dan M. makes the case(valid in my opinion) that by and large, MOST refractors(and other types of scopes) are for the most part quite good today.  Putting aside quality control issues, the amount of chromatic aberration one sees in an ED scope using ANY glass type is really vanishingly small so that it should not be a detrimental experience to someone just starting out in the hobby, whether they are interested in visual or imaging applications.   There are so many OTHER things to learn in either discipline that it will be some time before minor lateral color aberrations will likely start to impact a new users' experience.   To that end, discussions of things like glass type, Strehl numbers, etc...   serve more to confuse and frustrate many folks, rather than help them, since they often have not even figured out yet what they enjoy looking at with the telescope.


A second, parallel line of thought is, as always, articulated by Jon quite well.  Again, as I understand it, Jon makes the case(also valid in my opinion) is that one cannot discount the fact that optical quality is a function of materials, design, craftsmanship, cost, and importantly how you intend to use the scope.  To make an informed decision a perspective buyer should consider all of these.  For a given design, glass type DOES make a difference in the level of color correction and lateral chromatic aberration WILL have some impact on the quality of your image.  If you can see it, there is a certain level of blur associated with it, and depending on what you want to observe (planets, double stars vs wide field scanning the Milky Way) that could impact your viewing experience depending on your expectations.   


What I hope that folks following this thread understand is that these two lines of thinking are NOT really at odds with each other.   What  I have always appreciated about Jon's posts is that he is always very careful to provide as much context as he can regarding his recommendations and how he uses his own scopes that fall in the price range/design in which I am interested.   It is that level of detail that he(and others here) have provided over the years that have helped ME navigate the very real issues that Dan M. raises, as I was/am learning more about the different aspects of this hobby.  And sort through the noise.




*For this discussion I focused on CA, because that is what glass type generally addresses, is often the most visually obvious form of aberration.  As others point out there are other aspects of the optics that are as important or in some opinions more so.

#227 Bomber Bob

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Posted Today, 12:58 PM

The Original Question:   Why specify glass type if you don't specify optical accuracy?


Was answered in the first half-dozen posts or so:  Marketing Strategy


(Think:  gluten-free   I bet half the folks who buy based on this term have no real idea why they should / shouldn't consider it as a factor.)


First Time Refractor Buyers:  You get what you pay for.  Refractors are about the most expensive scope design per inch of aperture.  We have 1000s of posts & threads on CN to help you understand why that is; and, to help you make the best choice if you really want a refractor.

Edited by Bomber Bob, Today, 12:58 PM.

#228 Mitrovarr



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Posted Today, 04:18 PM

I don't know if I'd say you get what you pay for. Telescopes suffer pretty badly from diminishing returns at the top end. You can easily spend twice as much to get from 95% to 99% theoretical performance.

Edited by Mitrovarr, Today, 04:19 PM.

#229 Bowlerhat



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Posted Today, 05:01 PM

I don't know if I'd say you get what you pay for. Telescopes suffer pretty badly from diminishing returns at the top end. You can easily spend twice as much to get from 95% to 99% theoretical performance.

That can be said for eyepieces too, or diagonals but I'd say you get what you pay for both.


Optical accuracy doesn't really click well to me. Some thoughts: not many can read nor care enough, and it can be a problem for them too. I think few can guarantee it though, I remember tak has one scope which guaranteed reaching x strehl minimum.


There could be falsified report, or inaccuracies too.


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