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tech / sci.astro.amateur / Re: For that 10ft scope you always wanted

SubjectAuthor
* Re: For that 10ft scope you always wantedW
`* Re: For that 10ft scope you always wantedQuadibloc
 +- Re: For that 10ft scope you always wantedW
 `- Re: For that 10ft scope you always wantedMartin Brown

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Re: For that 10ft scope you always wanted

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Subject: Re: For that 10ft scope you always wanted
From: wsnel...@hotmail.com (W)
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 by: W - Fri, 25 Mar 2022 13:43 UTC

On Friday, March 25, 2022 at 5:05:25 AM UTC-4, RichA wrote:
> https://www.aliexpress.com/item/4000640306730.html?spm=a2g0o.productlist.0.0.6f302331VGkoXt&algo_pvid=c0b2e880-ec51-4bb6-b05a-f14d156bc3c9&aem_p4p_detail=20220217225309433165067225700107795225&algo_exp_id=c0b2e880-ec51-4bb6-b05a-f14d156bc3c9-9&pdp_ext_f=%7B%22sku_id%22%3A%2210000004778721146%22%7D&pdp_pi=-1%3B42.26%3B-1%3B-1%40salePrice%3BCAD%3Bsearch-mainSearch

_________
Not all lenses are meant to be used as telescope objectives.

Re: For that 10ft scope you always wanted

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Subject: Re: For that 10ft scope you always wanted
From: jsav...@ecn.ab.ca (Quadibloc)
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 by: Quadibloc - Fri, 25 Mar 2022 16:41 UTC

On Friday, March 25, 2022 at 7:43:39 AM UTC-6, W wrote:

> Not all lenses are meant to be used as telescope objectives.

I can attest to that, as I wear eyeglasses, and I also own a camera - in addition to a
smartphone with a camera.

However, a lens that is 74 mm in diameter, but with a focal length of
3,000 mm, I would suspect, _is_ intended to be used as a telescope
objective.

But maybe not. 74 mm diameter is the kind of aperture one would associate
with a department store special telescope. If diffraction limited, a telescope
with such an objective could magnify up to 180x. Divide 3000 by 180, and
you get 16 2/3. They do make eyepieces with focal lengths that big, so one
_could_ use that lens in a telescope, impractical though it might be.

John Savard

Re: For that 10ft scope you always wanted

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Subject: Re: For that 10ft scope you always wanted
From: wsnel...@hotmail.com (W)
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 by: W - Sat, 26 Mar 2022 16:33 UTC

On Friday, March 25, 2022 at 12:42:00 PM UTC-4, Quadibloc wrote:
> On Friday, March 25, 2022 at 7:43:39 AM UTC-6, W wrote:
>
> > Not all lenses are meant to be used as telescope objectives.
> I can attest to that, as I wear eyeglasses, and I also own a camera - in addition to a
> smartphone with a camera.
>
> However, a lens that is 74 mm in diameter, but with a focal length of
> 3,000 mm, I would suspect, _is_ intended to be used as a telescope
> objective.
>
> But maybe not. 74 mm diameter is the kind of aperture one would associate
> with a department store special telescope. If diffraction limited, a telescope
> with such an objective could magnify up to 180x. Divide 3000 by 180, and
> you get 16 2/3. They do make eyepieces with focal lengths that big, so one
> _could_ use that lens in a telescope, impractical though it might be.

This lens is probably useful in solar spectroscopy, maybe as the objective of a spectrohelioscope or as a collimating lens.

It could make an interesting camera obscura, maybe?

https://bonfoton.com/pages/faq

If one wanted to make it into a "practical" telescope, fold it with a couple of quality first-surface mirrors...

Re: For that 10ft scope you always wanted

<t22bee$5u9$1@gioia.aioe.org>

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From: '''newsp...@nonad.co.uk (Martin Brown)
Newsgroups: sci.astro.amateur
Subject: Re: For that 10ft scope you always wanted
Date: Wed, 30 Mar 2022 20:35:41 +0100
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 by: Martin Brown - Wed, 30 Mar 2022 19:35 UTC

On 25/03/2022 16:41, Quadibloc wrote:
> On Friday, March 25, 2022 at 7:43:39 AM UTC-6, W wrote:
>
>> Not all lenses are meant to be used as telescope objectives.
>
> I can attest to that, as I wear eyeglasses, and I also own a camera - in addition to a
> smartphone with a camera.
>
> However, a lens that is 74 mm in diameter, but with a focal length of
> 3,000 mm, I would suspect, _is_ intended to be used as a telescope
> objective.
>
> But maybe not. 74 mm diameter is the kind of aperture one would associate
> with a department store special telescope. If diffraction limited, a telescope
> with such an objective could magnify up to 180x. Divide 3000 by 180, and
> you get 16 2/3. They do make eyepieces with focal lengths that big, so one
> _could_ use that lens in a telescope, impractical though it might be.

A very long time ago some early telescopes had objectives and eyepieces
with insanely long focal lengths for planetary work made form the best
glass that they had available. Striae were a big problem back then.

Professor Allan Chapman had a nice talk about it. A copy might be hidden
somewhere on this website but I can't find it in a quick search.

https://societyforthehistoryofastronomy.com

There is brief mention of them at p27 in this edition of their journal.

https://societyforthehistoryofastronomy.com/wp-content/uploads/2016/10/shabulletin20.pdf

But there is or possibly was a full talk with diagrams...

--
Regards,
Martin Brown

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