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What is wrong with Takahashi ?


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8 minutes ago, vlaiv said:

Don't mind me, I was being somewhat provocative with that post :D (trying to be half provocative and half funny).

But there is a point to it - maybe not that exact SW model - as you yourself pointed out - people do like nice Strehl figure, but comparing two different aperture scopes with different Strehl is another matter.

On purely optical side of things - larger aperture diffraction limited and obstructed scope can outperform smaller clear aperture with perfect figure. In practice it will also depend on atmosphere of course and target, and observing conditions, and there are also personal preferences of course.

 

It's okay I could appreciate the humour. 😁

Like you say there's so many factors involved like presence and size of an obstruction, cool down time, convenience of different designs, local seeing, etc, etc that what counts as a good scope will vary enormously from person to person. Sometimes people get a bit too focused on judging things in terms of easy to compare numbers rather than more qualitative factors and want a single easy answer about what's "best" that they can apply universally.

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Lockwood makes it simple:

"For telescope mirrors made by us to our highest standards, our optics will perform at up to 50 power per inch or beyond of aperture on nights when the atmosphere is steady enough to allow this, so long as the mirrors are properly supported, equilibrated, collimated, and reasonably clean.  When you experience a night that allows this type of use of our optics, you will forget all about the numbers."

https://www.loptics.com/faq.html

Thats pretty good for a, say, 20" newt- 1000x...+...

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35 minutes ago, dweller25 said:

I MAY be tempted if an “FC130DZ” weighed around 6.5kgs with tube rings and finder.

That, to me, would be better than the FS128

The FS128 tube is already only 7.5kg excluding finder and rings..I don't use rings as I prefer the original Tak clamshell, and that stays on the mount when I take the tube off..the finder & bracket can't be much more than 500-750gms.

To get a hypothetical FC130mm F8 at 6.5kg weight including rings and finder would be a tall order indeed, and would presumably need either a thinner tube thickness, or a slimmer tube diameter than the oversize tube of the FS128 - or both! (could be done). I personally wouldn't want a thinner tube thickness supporting a fairly robust, large doublet..:glasses12:

Dave

Edited by F15Rules
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19 minutes ago, johninderby said:

Had a SW 150PL once that I upgraded with an OO 1/12PV mirror set. An amazing planetary scope that I should never have sold. Idiot. 😢

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I always point people who want great planetary views towards Newts, they are to me the best value for money scope going.

Sadly they do not suit everyone.

I had an excellent OO 8” F/6 1/10th wave Newt and an even better Skywatcher 10” F/4.7 but had to sell both as they aggravated my bad back, hey ho 😔

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Could a 130 doublet be colour free at F/8 even if Fluorite is used ?. I'm not sure it could. If some colour is going to be present, is that a risk for Takahashi's reputation ?

I recall William Optics getting quite a bit of flack for the 110mm Megrez F/5.9 doublet because it (inevitably for an FPL-51 doublet) showed a bit more CA than people expected.

 

Edited by John
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14 minutes ago, F15Rules said:

The FS128 tube is already only 7.5kg excluding finder and rings..I don't use rings as I prefer the original Tak clamshell, and that stays on the mount when I take the tube off..the finder & bracket can't be much more than 500-750gms.

To get a hypothetical FC130mm F8 at 6.5kg weight including rings and finder would be a tall order indeed, and would presumably need either a thinner tube thickness, or a slimmer tube diameter than the oversize tube of the FS128 (could be done). I personally wouldn't want a thinner tube thickness supporting a fairly robust, large doublet..:glasses12:

Dave

It’s just wishful thinking on my part Dave.

I am getting to the point of having a trolley made to support the FS128 plus mount so I can just wheel it around….

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25 minutes ago, John said:

Could a 130 doublet be colour free at F/8 even if Fluorite is used ?. I'm not sure it could. If some colour is going to be present, is that a risk for Takahashi's reputation ?

I recall William Optics getting quite a bit of flack for the 110mm Megrez F/5.9 doublet because it (inevitably for an FPL-51 doublet) showed a bit more CA than people expected.

 

APM have a new 6: doublet coming in:

APM 6" F/8 doublet  will be built with FCD 100 and LAF53 as matching glass, we should come out with the 6" F/6 similar or slightly better

The 6"F/6 in Astro-refractor design will have a similar correction as our 152 F/8 ED, better is almost impossible. 

So best colour correction will be around 152 mm ED, good but not as good as an LZOS triplet.

Ps: You mention a minimum tolerance for a lens you are buying and one hour later.....

Edited by Deadlake
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28 minutes ago, John said:

Could a 130 doublet be colour free at F/8 even if Fluorite is used ?. I'm not sure it could. If some colour is going to be present, is that a risk for Takahashi's reputation ?

I recall William Optics getting quite a bit of flack for the 110mm Megrez F/5.9 doublet because it (inevitably for an FPL-51 doublet) showed a bit more CA than people expected.

 

My FS128 is F/8.1 and IN FOCUS it is colour free, so maybe a modern lens design could be better ?

Edited by dweller25
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38 minutes ago, John said:

Could a 130 doublet be colour free at F/8 even if Fluorite is used ?. I'm not sure it could. If some colour is going to be present, is that a risk for Takahashi's reputation ?

 

That’s an interesting point. How much harder is it to control CA at 130mm than 100mm?

I notice the Agema 130 fluorite doublet is an eye watering £6.3k in Europe - advertised with “great colour correction”. Whether that means zero CA I don’t know. It’s also 9.5kg! Lovely thing, but at those prices, I’d rather have a TOA.

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41 minutes ago, dweller25 said:

It’s just wishful thinking on my part Dave.

I am getting to the point of having a trolley made to support the FS128 plus mount so I can just wheel it around….

I do sympathise with your regarding your back, David..it must be frustrating for you..I find similar frustration with my decreasing visual acuity; fortunately it has stabilised recently, but I've had to accept my eyes aren't what they were, and train my left eye to be my main Cyclops observing eye rather than my traditional right eye. I also find binoviewing helpful as two eyes definitely seem better than one on some objects.

There is no doubt that the FS128 is a big tube for a 5", and I can see that if your back is suspect it would be much harder to handle than an equivalent 4" Tak, which are positively svelte in comparison!

The trolley sounds like a good compromise solution.

Dave

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33 minutes ago, dweller25 said:

My FS128 is F/8.1 and IN FOCUS it is colour free, so maybe a modern lens design could be better ?

Using a wide air space can give better correction with a given pair of elements so that might be an option, but it would mean a larger and heavier scope that might be a bit more prone to losing collimation.

That said, the old FS-series were a lot bigger and heavier than their equivalent modern FC-series scopes so perhaps a lightweight but highly corrected 125mm doublet could be a viable option.

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2 hours ago, jetstream said:

Lockwood makes it simple:

"For telescope mirrors made by us to our highest standards, our optics will perform at up to 50 power per inch or beyond of aperture on nights when the atmosphere is steady enough to allow this, so long as the mirrors are properly supported, equilibrated, collimated, and reasonably clean.  When you experience a night that allows this type of use of our optics, you will forget all about the numbers."

https://www.loptics.com/faq.html

Thats pretty good for a, say, 20" newt- 1000x...+...

I think here lies the problem. 

In my location, the atmosphere NEVER supports it.

And in my case the mirror was rarely equilibrated. Not 100% anyway. Maybe 90% on occasion.

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3 hours ago, jetstream said:

👍

Or Lockwood.

I dont think any would be in business long if they were merely avg optics and duds IMHO.

Lockwood would make the primary, however the secondary maybe some one else.

It's a total system so I see it would be hard for a mirror fabricator to give a system figure for a scope.

Also here is a good article why looking at a single figure can be misguided:

https://zambutomirrors.com/zambutoopticalce.html

Edited by Deadlake
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9 minutes ago, Space Hopper said:

In my location, the atmosphere NEVER supports it.

And in my case the mirror was rarely equilibrated. Not 100% anyway. Maybe 90% on occasion.

That is my concern as well, I would purchase a scope who's tube currents would never be steady.
I'd need to be retired so I could stay up all night waiting...

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1 hour ago, John said:

Could a 130 doublet be colour free at F/8 even if Fluorite is used ?. I'm not sure it could. If some colour is going to be present, is that a risk for Takahashi's reputation ?

Yes, in focus for visual John.

See here:

http://scopeviews.co.uk/TakFS128.htm

As David says, in focus the FS128 is colour free. Vega is pure white. That's what matters to me. Intra- and Extra- focal images don't..🙂

If I was an imager, it might be different..

Dave

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23 minutes ago, Deadlake said:

Also here is a good article why looking at a single figure can be misguided:

I'm not sure anyone requesting a top mirror looks at a single figure- one requirement of mine is smoothness- Terry O makes very smooth mirrors as do all the other top makers. Lockwood works with JP Astrocraft and their cells and knows what works with his thin mirrors. He tests the mirrors ( for reference, not all of them) in the cell as does Terry O on request. The magnification seen with these scopes is a testament to their optical quality, similar to Taks idea- real performance and not a battle of test results.

If a Tak, TEC or LZOS refractor can take ultra high mag, then a bigger quality scope would work well then too.

To each their own for sure.

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As an observation on all this…

…. Whilst I enjoy knowing what ‘the best’ kit is, and try to observe with the best I can afford, every bit of kit is compromised in some way shape or form so endless ‘is this better than that’ threads for me get a bit tiring. Apologies if that is a bit strong, but I had a 20 hour day yesterday so am a bit tired already! 🤪🤪

Would a 130mm f8 fluorite doublet be as colour free as an f9 or a triplet? Perhaps not. But an f9 and/or triplet would be longer, heavier and less manageable and less likely to be used (by me). I know that I love fluorite views, and the low scatter, high transmission, high contrast and quick cooling that comes with a fluorite doublet is something I would enjoy. My DC gave me noticeably better planetary views than my previous Astrotech 106mm f6.5 fpl-53 triplet which was in itself a very good scope so I’m pretty sure a 130mm Tak Fluorite doublet would be a fabulous scope.

I bought my LZOS 130mm f6 knowing that, whilst it’s a top notch scope with a high strehl (0.973), it probably gives something away to a TOA130 or an LZOS f9.2 in ultimate planetary views under excellent conditions. What it gives me back though are wider field potential and better manageability; as shown recently it’s only just longer than my DC when packed down so I’m more likely to take it away with me.

If I lived in the Atacama desert with stable dry air that would take x1000 regularly my choice might be different, but as it is, opportunity, seeing conditions, planetary altitude, observer patience and skill play a far bigger role in what I see so the tiny differences between these top end scopes are largely irrelevant. They will all give fabulous views, and I work hard to make the best of what I have, having built up a reasonable level of observing skills over the years.

Gosh, I’m terribly sorry, that’s all a bit soap boxy so please forgive me!

As you were…. 🤪🤪🤣🤣

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3 hours ago, John said:

Well said @Stu :thumbright:

The scope that has given, and continues to give, me the most from the hobby is my least expensive and probably the most compromised as well !

 

 

If a scope gets you out observing and you enjoy it then it doesn't matter what it cost or how close it gets to optical perfection.

I suspect that people who spend their time worrying about what they can't see because of the limitations of their scope will never be satisfied with what they can see.

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9 hours ago, Deadlake said:

How much was the FS128 when it was out, relative to a 100mm Takahashi? I suspect at 130 mm the TOA is popular as one of the best imagining scopes at that aperture and that's where the market is.

Note:


Takahashi should supply a lens report, LZOS and AP guarantee a minimum tolerance. 
TecnoStar also provides a minimum lens quality on their 130 mm model as well.

I paid £3,850.00 for my FS128 in 2003. Although expensive,  it was worth every penny and gave me some of the most amazing observing experiences I've ever had. At F8.1 though it was quite long, and although visually Apochromatic, it probably wouldn't suit today's imagers; but for a visual observer such as myself it was an absolute joy. Below are my FS128 being enjoyed by Gain Lee at the Astronomy Centre (Todmorden), and the map of Mars compiled from over thirty sketches of the planet during the 2003 apparition. 

 

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