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Gavster

Astro Physics 130GTX - First Light

34 posts in this topic

15 hours ago, Highburymark said:

Thanks John. That's cleared that up then. 🤔

:) 

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15 hours ago, Highburymark said:

Thanks John. That's cleared that up then. 🤔

Not really, my FC-100DL is rubbish :grin:

 

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1 hour ago, mikeDnight said:

Hi John,

My late friend Phil was obsessed with comparing spot diagrams of various designs of telescope. He spent a lot of his time over many years designing many different kinds of telescope. From what I remember about the spot diagrams he showed me, if a telescope keeps its natural aberrations within the Airy disk, it is essentially perfect. Most telescopes come nowhere near this kind of perfection, though Apo refractors tend to be among the best. Often the aberrations between similar scopes such as the DL and DC, though being noticeably different on paper, still come close to, or within the confines of the Airy disk and so both scopes will offer essentially the same level of performance, or at least it would take a very expert eye to notice any real difference in image quality. For a 6" achromat to give a perfect image it would need to have a F ratio of around F25, yet at F15 it performs stunningly, so it illustrates how forgiving the visual view is. SCT's produce very poor spot diagrams and this is quite noticeable in the star image produced, where the star never quite seems to snap into focus, yet the SCT remains popular. As you pointed out, focal length does play a part. Short F ratio Newtonians suffer from numerous destructive aberrations outside the Airy disk, where as, long focal length Newts have very tightly constrained aberrations and deliver near perfect images. It seems to me that we're all so easily distracted by graphs, spot diagrams and Strehl that we forget the most important factor which Gavin touched on when he said he's "Loving" his AP130. ❤💜💛

Mike

Interesting Mike and I'm sure it holds good :thumbright:

Of my 4 refractors, I'm fondest of the Vixen and the Skywatcher simply because I've owned them a long time. They and the 12" (especially that scope) are the ones that have contributed most to my enjoyment of the hobby because they have been with me longer, of course.

While the images through the Tak DL and the TMB / LZOS 130 F/9 are technically the best I've ever seen for those apertures, they have yet to become "friends" in the way the others have. These scopes will show me things that I have already seen slightly better than I have seen them before. The 12" dob is still capable of showing me new things. Gotta respect that :thumbright::grin:

None of this is surprising though - I've been in the hobby too long to nurture unrealstic expectations of kit these days :rolleyes2:

How am I going to get some new "kicks" ? - subject for another thread I think.

Sorry to rabbit on in your AP 130 thread Gavin. You have wonderful scopes mate - treasure them :grin:

 

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On 2017-5-15 at 00:29, John said:

It's funny with scopes. Last year I bought 2 "dream" scopes, the Tak FC-100DL and the TMB / LZOS 130 F/9. They are both superb instruments and undoubtely the best scopes of their respective apertures that I've ever viewed though and arguably in the top few ever made at those apertures for amateur use.

You would have thought that the old, slightly scruffy gold tubed ED120 Skywatcher would have had it's marching orders ?. But no, I keep coming back to the Skywatcher of all things. I guess that it's the whole package that is "right" with that one which includes size, weight, ease of mounting, cool down, ruggedness as well as optical ability way above it's price band. Still trying to figure it out ..... :dontknow:

 

 

Hi John,

The few reviews I've read of the first (gold) SW Ed scopes (both 100 and 120mm) seem to really rave about them. I wonder if that is because, as a crucial new product for Synta at the time they were so important and they really really tried to make them to an outstanding quality? 

Having said that, I know many with the newer versions love theirs just as much. I do think there is something quite classy about the older gold tubes though..I personally prefer them to the black diamond of the Pro Series or gloss black of the Equinox. 

Now, if they offered them in classic white with black trim, I might even become a convert! 😈😈😀😀

Getting back to Gavsters original post, that Starfire must be a complete cracker if it is doing even slightly better than the TEC.. 

Not a bad "problem" to have, comparing two of the best fracs on the planet🔭🔭😀.

Dave

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On 2017-5-16 at 21:17, John said:

I think the DL has a slightly tighter spot diagram, whatever that means. Probably more to do with the different focal ratio rather than differnt figuring quality though.

The official blurb at the launch said this (Google translated from the original Japanese, so blame that !):

"In recent years of refraction telescope, but Photo visual system short focus apochromat has become the mainstream, it is the emergence of these and differing length personality focus two ball fluorite apochromatic telescope FC-100DL.
FC-100DL in the FC type which arranged the street flow light lens of the name behind, worthy of long focal refraction of f / 9.0, chromatic aberration, you get high spherical aberration less contrast image. Specifically, halo is approximately 40% reduction of the g-line against FC-100D of f / 7.3 (purple), and has realized the 97.5 percent of about 4% increase in the Strehl ratio that indicates the high magnification performance. (Data calculated)
This aberration data is almost the same number as the three pieces ball apochromat, coma is coupled with stable image plane of the zero, I think that you understand that it is a telescope that has a high-eye viewing performance."

 

 

John, that does sound as though the DL is claimed to be optically "better"....as a long time fan of good long focus achros, I grew up reading that the shallower curves on longer FL objectives were/are easier to polish and figure to a high standard? Also, even on an ED "apo", longer focal lengths reduce CA even further, maybe light scatter too?

I'd LOVE to see an F11 or F12 115-125mm ED Fluorite doublet tested 😉..

Two other things I still like about long achromats...you get more depth of focus, ie more range of "sweet spot", within which you can get perfect focus. This is such a boon at high powers: on my D&G 5" F15 I had a moonlight dual speed focuser, but even with the original 2.7" AP focuser I could always get a snap to focus, even up to x300 on good nights. On my FS128 the snap to focus range is much less.

The second noticeable property is that Airy disks are larger with a long focus achro, really tight, but larger, you really get the "twin bullseye's in black" effect on doubles like epsilon Lyrae.

But long, big fracs are a real pain to handle sometimes, especially when viewing near the zenith, and as you get older.. Gavin, if you find the Tec140 more of a handful than the AP130, imagine the fun and games you'd have with a 2 metre long tube!😱😱

Dave

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Posted (edited)

8 hours ago, F15Rules said:

 

But long, big fracs are a real pain to handle sometimes, especially when viewing near the zenith, and as you get older.. Gavin, if you find the Tec140 more of a handful than the AP130, imagine the fun and games you'd have with a 2 metre long tube!😱😱

Dave

Dave, yes I did wonder why short focal fracs are so much more popular than f9+. However even the FC100DL I have I find a bit of a handful particularly when looking near zenith. The flexibility of the 130gtx just zipping around all areas of the sky easily on the mount without having to change observing position is rather nice. 😊

Edited by Gavster
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I reckon its the rise in popularity of imaging that has driven the industry to produce faster apo refractors. Thats why scopes such as the Tak FC-100 DL fluorite doublet and the TMB / LZOS 130 triplet F/9.2 are different - they are squarely aimed at the visual astronomer.

The original Tak FC100N was a F/10 fluorite doublet. Nikon and Pentax used to produce similar "slow" ED doublets some way back.

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2 minutes ago, John said:

I reckon its the rise in popularity of imaging that has driven the industry to produce faster apo refractors. Thats why scopes such as the Tak FC-100 DL fluorite doublet and the TMB / LZOS 130 triplet F/9.2 are different - they are squarely aimed at the visual astronomer.

The original Tak FC100N was a F/10 fluorite doublet. Nikon and Pentax used to produce similar "slow" ED doublets some way back.

Of course you're right John - I forgot about those imaging people!!

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12 hours ago, Gavster said:

Of course you're right John - I forgot about those imaging people!!

It's best to forget about "those imaging people!" However, they may have done visual observers a great service, by causing manufacturers to design faster, high quality optics. The comfort and greater range of usefulness of a good ED or Apochromat is godsend. I'd much rather use a tube of a metre or less than an unwieldy monster that tremors with every passing moth. With modern, well corrected eyepieces, or if preferred, a simple ortho or plossl with a barlow lens, they'll deliver both wide rich field views and razor sharp, colour free, high power views. I love em! :icon_biggrin:

 

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