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CN Report: The Tele Vue NP101

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#126 Tom T

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Posted 10 February 2008 - 07:21 PM

Thanks for the comments folks, they really are appreciated.

T

#127 gripweed44

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Posted 10 February 2008 - 11:44 PM

If a focuser focus's
Is it not in focus?

Focus in the EYE of the observer is what matters
RP or CRAY- what does it matter if it works--

for you

#128 jonnyastro

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Posted 11 February 2008 - 01:29 PM

I would add that Dickinson and Dyer in , "The Back Yard Astronomer", also alluded to the NP101s superiority to the TV102 visually, stating, "optics don"t get any better than this".

#129 Dirtyharry

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Posted 11 February 2008 - 06:33 PM

OK guys, I get the message! Much as the NP101 is a nice piece of kit,I won't be parting with my TV102 anytime soon!

#130 Tom T

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Posted 11 February 2008 - 06:36 PM

Heaven forbid! Harry, the 102 is a world class apo in it's own right. Mine was an excellent piece of kit that gave me many many happy hours under the stars.

T

#131 karim

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Posted 13 February 2008 - 12:27 PM

I agree with Tom, the FSQ is mostly used for astrophotography, whereas the NP101 (or TV102 to that matter) is visual.

IMHO, the NP101 is to TV102 what C9.25 is to C10.

#132 Daniel Mounsey

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Posted 16 February 2008 - 02:39 AM

But this is really off-topic: if you want a discussion about this, the ATM and Optics forum is the place for it, or you can try Astromart if you feel like getting spanked by Roland himself.


Hi Alexis,

Tom T. mentioned some interesting points about the in-focus star test and light outside the airy disc which I found to be quite valid. Just thought I'd share some experiences I had during some observations I made. I have always respected your views and continue to, so I'm attempting to share some observations I made while testing a good number of 4" apo refractors and in this case, two, three and even four of the same exact ones on the same night at the same time. I also had some personal discussions with Markus Ludus at NEAF about the star test, in fact I just recently star tested a new FSQ106 Q for Markus before I sent it to him but let me address something first. I actually don't care what the papers say. If I can't observe the differences myself, then I don't bother.

There is actually quite a bit of difference of opinion in this industry regarding star tests these days. Since we know there are so many orders of spherical aberration, there needs to be some clarification. For example, there have been discussions about differences in the star test being visible outside of focus but not in-focus. But I ask by what standards are these images the same from a visual standpoint? What do they define as absolutely excellent? In other words it's a bit vague.

Here are some observations I made. I took two of the same scopes and focused them on the Trapezium in Orion under excellent seeing conditions. Each one was star tested on a brighter star first and then placed on the Trapezium. I want to make it clear to others that the point of using the Trapezium as a target was NOT about splitting the components as it is not a valid test. The key was to observe and study the contrast between the components and observe the strength of the light in and around the airy discs, particularly the E and F component stars. In some of the refractors I tested, a good number of them did not direct a strong amount of light in the airy discs of the fainter E and F components compared to others and the differences were quite startling to my eyes to say the least!

I'm a custom wood case builder. I can purchase three sheets of A-1 birch ply and build three of the same cases but they still will have subtle differences which can be observed. It's just simply impossible to get every one literally, exactly identical and high quality apos are no different and easily proved to be so during my critical tests with the human eye. In every single case with my tests, the star test proved absolutely dead accurate to what was observed with the in-focus image on the Trapezium and particulary the planets. In other words any optic which exibited a slightly higher degree of under-correction, proved to scatter more light outside the airy disc.

It's just simply impossible for opticians to make every optic exactly the same and thus, observers end up discussing various orders of spherical aberration, but at critical levels of observations, they are still aberrations, they still exist and some can be detected visually. My point is that three scopes with three slightly different star tests will all have a different level or degree of contrast. They are not going to be the same while trying to descern very fine planetary details and that defference will most easily be noticed side by side on the same night. My visual tests clearly validated this. BTW, the NP101's I tested were incredibly consistant in optical quality in that regard. I can understand why Tom likes the NP101, it proved to be a good scope during my observations as well.

Clear Skies! :)

#133 sixela

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Posted 16 February 2008 - 03:26 AM

Post deleted by sixela

#134 sixela

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Posted 16 February 2008 - 03:34 AM

The key was to observe and study the contrast between the components and observe the strength of the light in and around the airy discs, particularly the E and F component stars. In some of the refractors I tested, a good number of them did not direct a strong amount of light in the airy discs of the fainter E and F components compared to others


I have no issue with that - that's eating the pudding, which is its proof. Performance *in* focus is what counts. What isn't simple is correctly interpreting the *out* of focus star images.

Doing an in-focus star test isn't as simple as it sounds either, by the way, at least not if you want to detect 1/8th wave of spherical aberration. Refractors need to cool as well and will typically show spherical aberration before they're perfectly cooled (the aberrations are nowhere near as ugly a on an uncooled Newtonian, but they do exist).

My point is that three scopes with three slightly different star tests will all have a different level or degree of contrast.

If you're talking about the in-focus star test, yes. If you're talking about out of focus star test images, though, it's possible for the three scopes to have very different *out* of focus star test images and still perform identically in the *in*-focus star test. Yes, in theory what you're seeing are aberrations, but I doubt you can see them if the effect in focus is to reduce the Strehl ratio by 0.003. *Some* aberrations are much more visible out of focus than in focus, and that's the danger.


Mind you, even out of focus it's *possible* to discriminate between different orders of e.g. spherical aberration by the way ring patterns *evolve* around focus. But it certainly isn't simple.

#135 Daniel Mounsey

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Posted 26 February 2008 - 12:46 AM

Hi Alexis,

Forgive me taking so long to get back to this forum. My time has been very limited lately. I don't make any claims regarding Strehl ratios or wavefronts based on visual observations. They mean very little to me.

The issue I'm having with the star test is that since sophisticated triplets have hit the market, opticians have placed contradicting emphasis on the in-focus image and my tests clearly proved that the star test works only one way and that is, that both sides look exactly as close to the same as possible. Obviously there are limitations to this. It's just simply impossible for opticians to make every single lens the same in that manner, not to mention the various temperatures they are exposed to causing differences as well.

As you and I both know, refractors by their very nature will exhibit under-correction throughout the cooling process and it's a safer state for the optic to be in. My FS152 produces a slightly different star tests under various temperatures but the problem I'm having regarding this matter is that some opticians are trying to convince observers that two of the same refractors, one producing under correction and one that isn't are going to produce the same image and that just doesn't fly with me and my tests clearly proved it.

Most observers don't notice it and would be quite satisfied and understandably so. Focusing both instruments on a planet will reveal subtle differences that most observers seeing conditions would rarely allow them to see, not to mention that they'd need two of the same scope to see it more easily. Those last remaining hard outlines on lunar and planetary images can not be the same.

When I listen to some of the things I hear about this new star test, it begins to remind me of the magic bullet theory during the JFK assassination. Since opticians can not make all lenses the same, they have to come up with a reason as to why and those reasons have become a bit misleading. I'd be more than happy to discuss this issue with any optician. There's really nothing to debate though, all one has to do is simply look and see for themselves. :) Some optics are and "A" and there are some that are just a freak of nature that are an "A+" and they are the rare ones that purist would appreciate, provided their conditions allow.






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