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Why no ED 80mm 400mm?

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#1 Jobryant

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Posted 18 March 2010 - 01:26 PM

With how popular the ST80s appear to be how come no one has came out the an 400mm ED ST80? Seems like people would jump all over an ED Orion ST80.

#2 BillP

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Posted 18 March 2010 - 02:21 PM

Celestron made the Onyx 80mm EDF which was 500mm -- a doublet which combines a Fluorite low dispersion glass with high density crown glass (cemented I believe). Original price I believe was quite high. Then went on sale once discontinued for $399 or $499 I believe. on the used market they pop up once in a while for $300. Quite an excellent scope. Easily handles 208x in good seeing (I had mine out last evening with the 5mm XO and 2.5x Barlow) and gave wonderfully crisp image of Mars and Saturn and kept pace quite nicely with my much more expensive 80mm triplet (APM 80/480).

When its price fell to the $400 mark it did indeed sell out in no time. So keeping the price point low is important as well.

#3 RodgerHouTex

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Posted 18 March 2010 - 03:16 PM

I really like my EON 80 which is sold by Orion. It's a 500 mm. focal length ED with FPL-53 glass. Maybe you should take a look.

#4 Al Miller

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Posted 18 March 2010 - 03:34 PM

Don't know shy ther are no 80mmED with 400mm FL BUT, Astro Tech has a nice 80mm ED at f/6 (480mm FL) and a 72mm ED f/6 (432mm FL) that look very nice. I just took possession of a very nice Stellarview 70mm, f/6 (420mm FL) a few days ago. The 70mm and 72mm OTA's are not significantly smaller than an 80mm. OH... Meade has the series 5000, 80mm f/6 also.

#5 Joe Aguiar

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Posted 18 March 2010 - 04:00 PM

Guess they figure there will be too much colour at F/5 even if you used 53 glass, but i guess that would be kinda cool to be that short though. I think people would buy it.

Joe

#6 astroneil

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Posted 18 March 2010 - 04:53 PM

Hi,

Reading the latest issue of Britain's 'Astronomy Now' magazine, I see a company called Eklipse have launched an 80mm F/5.5 ED scope called - wait for it, "the Ekinox 805ED" So, close enough to an ST80 in Apo form.

Cheers,

Neil.

#7 Edwin Quiroga

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Posted 19 March 2010 - 07:14 AM

Hi,

Reading the latest issue of Britain's 'Astronomy Now' magazine, I see a company called Eklipse have launched an 80mm F/5.5 ED scope called - wait for it, "the Ekinox 805ED" So, close enough to an ST80 in Apo form.

Cheers,

Neil.


Here you are!

And it is just "poa" sterling pounds!


Seriously, it looks a nice and interesting product... :thinking:

#8 Edwin Quiroga

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Posted 19 March 2010 - 07:22 AM

Well, they could be more original regarding the name... "Ekinox".

#9 Jobryant

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Posted 19 March 2010 - 10:00 AM

Wonder how well the Ekinox would perform visually? This is the 1st time I've ever heard of it. 440mm is now the lowest fl for an ED 80mm scope I know of but if I had a choice I still think I'd opt for a LOMO 80mm at 480mm. With a difference of only 40mm the Ekinox would have to be an excellent performer or very inexpensive for me to choose it over a LOMO. Now if the fl of the Ekinox was 400mm or lower then I'd be more then interested.

I just don't know why a company hasn't made a 400mm or less ED80 yet. Even if there was a tad more color then a longer fl ED it should be better then the non-EDs st80 available now. Maybe there wouldn't be enough demand for one or as mentioned earlier just too hard to make without color at such a short fl. My dream scope would be a 350mm ED80 with nice optics but this will probably never happen that's why it's my dream scope.

#10 Edwin Quiroga

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Posted 19 March 2010 - 01:21 PM

I think that 400mm or less FL is very difficult for color correcting for an affordable ED doublet systen. Maybe for that reason you do not see any scope with that specs.

#11 jrbarnett

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Posted 19 March 2010 - 04:46 PM

The Stellarvue Nighthawk Next Generation is close at 560mm focal length.

I suspect no one has done an even faster ED 80mm doublet because the radii of the elements would have to be so steep that it would be hard to figure using machine grinding and polishing adding expense beyond the increased glass cost, would weigh quite a bit more due to glass type, and likely show lots of unmitigated spherical aberration.

Just a guess,

Jim

#12 David E

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Posted 19 March 2010 - 04:55 PM

I suspect no one has done an even faster ED 80mm doublet because the radii of the elements would have to be so steep that it would be hard to figure using machine grinding and polishing adding expense beyond the increased glass cost, would weigh quite a bit more due to glass type, and likely show lots of unmitigated spherical aberration.

Just a guess,

Jim


I think that's a good guess. :ubetcha:

#13 Keithdrengen

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Posted 24 April 2010 - 04:59 AM

Here is an artificial startest of the Ekinox f5.5
http://www.teleskop-...on-ED80-005.jpg

#14 Jon Isaacs

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Posted 24 April 2010 - 05:28 AM

Why no 400mm FL ED-80... This is my thinking...

Color correction gets more difficult as the focal ratio is decreased. The FPL-53 based doublets like the ED-80, presumably the TV-85, as well as the various Chinese scopes like the 80mm Megrez II FD are all around F/7 and IME, for visual use, offer nearly perfect color correction. F/7 seems to be about the limit a FPL-53 based 80mm doublet.

A significantly shorter focal length doublet or an objective based on a lower abbey number ED glass will show some signs of chromatic aberration. To get the really good correction at F/6, a triplet objective seems to be required.

The step from F/6 down to F/5 would push the limit once again. If one really wanted an 400mm F/5 ED scope, and I don't know quite why that would be, it could be done with a Petzval design like the TV NP-101 but then the question becomes what is the purpose of going to F/5? The OTA is going to be about as long as an F/7 scope.

Faster focal ratios cost, whether it is in cash, weight, and/or color correction, one pays a price. And personally, I think F/7 is a nice place to be for visual observation. It still provides a reasonably wide field of view, my 80mm F/7 with the 555mm focal length gives nearly a 4.5 degree field of view with a 31mm Nagler. And F/7 makes it easier on the eyepiece...

That's my two cents

Jon

#15 Keithdrengen

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Posted 24 April 2010 - 06:14 AM

The f5.5 Eklipse, to me, is a lightweight and short alternative to a F5 acromat, with atleast better correction. I wonder how long an acromat would have to be to have the colorcorrection of the 80mm f5.5 eklipse/Ekinox?
http://www.teleskop-...on-ED80-005.jpg

#16 jtpowers

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Posted 24 April 2010 - 07:31 AM

I know! I have been bugging Uncle Al to produce an NP-81 (+/- a couple mm's) for a while now, but so far he hasn't listened... :fingerscrossed:

#17 Jon Isaacs

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Posted 24 April 2010 - 08:14 AM

The f5.5 Eklipse, to me, is a lightweight and short alternative to a F5 acromat, with atleast better correction. I wonder how long an acromat would have to be to have the colorcorrection of the 80mm f5.5 eklipse/Ekinox?
http://www.teleskop-...on-ED80-005.jpg


At 400 Euros, it does look like a lightweight alternative to an F/5 achromat with better color correction. But the photos show that the color correction is less than perfect.

Myself, I am not attracted to an F/5 80mm, F/7 is still plenty manageable, with the right eyepiece offers a 4.5 degree field of view and will perform nicely at high magnifications.

Jon

#18 Keithdrengen

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Posted 09 May 2010 - 06:37 AM

The f5.5 Eklipse, to me, is a lightweight and short alternative to a F5 acromat, with atleast better correction. I wonder how long an acromat would have to be to have the colorcorrection of the 80mm f5.5 eklipse/Ekinox?
http://www.teleskop-...on-ED80-005.jpg


There is also the Vixen a80ss, wonder what owners of that Vixen think about this chinese f5.5 ED

#19 EdZ

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Posted 09 May 2010 - 06:52 AM

make some assumptions and this becomes a simple calculation

assuming same size aperture and
assuming color correction curve centered the same for each scope
if the achromat is corrected to 1/2000 F and the ED scope is corrected to 1/4000 F,
then the achromat needs to be twice as long as the ED scope to make the color blur circle the same size.

Of course, we do not know what the correction of either is, so it's just an assumption.

edz

#20 Keithdrengen

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Posted 09 May 2010 - 04:18 PM

Thankyou EDZ
Other than more color, how is image detail in Vixen A80ss compared to a semiapo? Mushier above 50x?
Edit: well sorry if this got into offtopic..






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