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Equipment Discussions >> Reflectors

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WeltevredenKaroo
member

Reged: 03/05/12

Loc: 31°51'09" S 24°25'47" E

Big Dob fans, consider how lucky you are new

#5410208 - 09/08/12 02:42 PM

I came across an article on Dr. Wolfgang Steinicke's website about a nearly forgotten observer named Albert Marth, who discovered 557 new galaxies from the island of Malta in the first half of the 1860s. He used a 48" f/9.4 reflector. Yes, that's 9.4. An efl of 451 inches or 11,460 mm. The mount more resembles shipfitter's DIY than the glass-smooth Dobs at our star parties. All this and Ramsden 40° afov eyepieces. Does anyone see a finder on this thing? We've come a long way!

Post Extras:

Mike B
Starstruck

Reged: 04/06/05

Loc: shake, rattle, & roll, CA

Re: Big Dob fans, consider how lucky you are new

[Re: WeltevredenKaroo]
#5410342 - 09/08/12 04:04 PM

The easy part? No ParaCorr needed.

Nice website- had fun clicking thru the various pages, by observer. Yeah, truly amazing "work" these folks did! Is why i *never* use the term "work" when i refer to my own telescopic endeavors. In comparison, what we do is typically FUN!

Thanks fer posting this!

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laconicsax
super member

Reged: 10/05/10

Loc: Portland, OR

Re: Big Dob fans, consider how lucky you are new

[Re: Mike B]
#5410396 - 09/08/12 04:44 PM

Remember that the primary mirrors back then had 60% reflectivity *at best* and tarnished so quickly that the primary usually had to be removed, refigured, and re-polished every day!

48" at 60% reflectivity for the primary would give the same light as a 30" primary with standard aluminum coatings. Factor in similar light loss from the secondary, and you're looking at light grasp almost matched with a 16" mass-produced dob.

We have it so easy.

Post Extras:

JimMo
I'd Rather Do It Myself

Reged: 01/08/07

Loc: Under the SE Michigan lightdom...

Re: Big Dob fans, consider how lucky you are new

[Re: laconicsax]
#5410427 - 09/08/12 05:07 PM

Quote:

We have it so easy.

Yes, but they had much darker skies.

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Smittty692k4
scholastic sledgehammer

Reged: 07/05/11

Loc: East Bernard, TX

Re: Big Dob fans, consider how lucky you are new

[Re: laconicsax]
#5410433 - 09/08/12 05:11 PM

And that equatorial mount by today's standards + inflation would cost.....?

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Achernar
Postmaster

Reged: 02/25/06

Loc: Mobile, Alabama, USA

Re: Big Dob fans, consider how lucky you are new

[Re: WeltevredenKaroo]
#5410537 - 09/08/12 06:14 PM

A ship fitter could when presented the plans build a large telescope from steel, but even he wouldn't want to climb over 30 feet in the air just to peer into the eyepiece. At least that beast was coma free. Without a good finderscope, detailed star atlases or digital setting circles, he would have had a hard time finding every one of those 557 galaxies.

Taras

Post Extras:

jgraham
Postmaster

Reged: 12/02/04

Loc: Dayton, Ohio

Re: Big Dob fans, consider how lucky you are new

[Re: Achernar]
#5410776 - 09/08/12 09:19 PM

There is some interesting notes on the telescope here...

http://www.astrosurf.com/re/william_lassell_telescopes.pdf

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amicus sidera
Carpal Tunnel

Reged: 10/14/11

Loc: East of the Sun, West of the M...

Re: Big Dob fans, consider how lucky you are new

[Re: laconicsax]
#5410825 - 09/08/12 10:16 PM

Quote:

Remember that the primary mirrors back then had 60% reflectivity *at best* and tarnished so quickly that the primary usually had to be removed, refigured, and re-polished every day!

Sidgwick's Handbook states that silvered mirrors are good for several months to several years, at 70% reflectivity or above. There was discussion of this on the ATM forum recently; one poster on that thread, who silvered his own mirror, reported that the silvering lasted 5 1/2 years before needing to be recoated.

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Jeff Morgan
Post Laureate

Reged: 09/28/03

Loc: Prescott, AZ

Re: Big Dob fans, consider how lucky you are new

[Re: Achernar]
#5410835 - 09/08/12 10:25 PM

Quote:

A ship fitter could when presented the plans build a large telescope from steel, but even he wouldn't want to climb over 30 feet in the air just to peer into the eyepiece. At least that beast was coma free. Without a good finderscope, detailed star atlases or digital setting circles, he would have had a hard time finding every one of those 557 galaxies.

Taras

I didn't see the photo ...maybe it was a meridian scope? He just cataloged them as they drifted by. Which at 11,000 mm of focal length probably didn't take very long! Blink and you miss it.

Post Extras:

Jon Isaacs
Postmaster

Reged: 06/16/04

Loc: San Diego and Boulevard, CA

Re: Big Dob fans, consider how lucky you are new

[Re: laconicsax]
#5410836 - 09/08/12 10:25 PM

Actually light gathering goes by the area of the mirror so if both mirrors are 60% reflective. It would be the same as a perfect 29 inch or a 32 inch with 90% reflective coatings.

Jon

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laconicsax
super member

Reged: 10/05/10

Loc: Portland, OR

Re: Big Dob fans, consider how lucky you are new

[Re: amicus sidera]
#5411139 - 09/09/12 03:58 AM

Quote:

Quote:

Remember that the primary mirrors back then had 60% reflectivity *at best* and tarnished so quickly that the primary usually had to be removed, refigured, and re-polished every day!

Sidgwick's Handbook states that silvered mirrors are good for several months to several years, at 70% reflectivity or above. There was discussion of this on the ATM forum recently; one poster on that thread, who silvered his own mirror, reported that the silvering lasted 5 1/2 years before needing to be recoated.

I was talking about speculum metal mirrors. Silvered mirrors started to be used around the same time as when it was built, but Lassell used speculum metal for the 48" telescope.

I was wrong about the 60% figure though--speculum metal can get up to about 65-66% reflectivity, but does tarnish fast enough that a mirror had to be refigured and re-polished after a single night's use.

Post Extras:

amicus sidera
Carpal Tunnel

Reged: 10/14/11

Loc: East of the Sun, West of the M...

Re: Big Dob fans, consider how lucky you are new

[Re: laconicsax]
#5411707 - 09/09/12 02:01 PM

Quote:

I was talking about speculum metal mirrors. Silvered mirrors started to be used around the same time as when it was built, but Lassell used speculum metal for the 48" telescope.

I was wrong about the 60% figure though--speculum metal can get up to about 65-66% reflectivity, but does tarnish fast enough that a mirror had to be refigured and re-polished after a single night's use.

Thank you, I was mistaken, and stand corrected!

I had assumed (wrongly) that Lassell's reflector was silvered; my apologies.

Post Extras:

Scott in NC
Mad Hatter

Reged: 03/05/05

Loc: NC

Re: Big Dob fans, consider how lucky you are new

[Re: amicus sidera]
#5411732 - 09/09/12 02:20 PM

Very interesting--thanks for sharing the info about this man and his scope! We really have it so easy nowadays in comparison.

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fnowat
member

Reged: 06/12/12

Loc: Cleveland, USA

Re: Big Dob fans, consider how lucky you are new

[Re: JimMo]
#5529235 - 11/20/12 12:43 AM

Quote:

Quote:

We have it so easy.

Yes, but they had much darker skies.

But there was also coal burning/wood burning. And don't forget the Gulf Stream dumping the moisture onto them...poor buggers!

Post Extras:

Project Galileo
professor emeritus

Reged: 11/14/07

Loc: Jefferson County, Colorado

Re: Big Dob fans, consider how lucky you are [Re: fnowat]
#5529251 - 11/20/12 01:02 AM

Neat stuff. Talk about dedication and effort. My hat is off to these men.

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george golitzin
Pooh-Bah

Reged: 02/24/06

Re: Big Dob fans, consider how lucky you are new

[Re: Jeff Morgan]
#5530611 - 11/20/12 04:50 PM

Quote:

Quote:

A ship fitter could when presented the plans build a large telescope from steel, but even he wouldn't want to climb over 30 feet in the air just to peer into the eyepiece. At least that beast was coma free. Without a good finderscope, detailed star atlases or digital setting circles, he would have had a hard time finding every one of those 557 galaxies.

Taras

I didn't see the photo ...maybe it was a meridian scope? He just cataloged them as they drifted by. Which at 11,000 mm of focal length probably didn't take very long! Blink and you miss it.

No, it wasn't a meridian scope: if you look at the third photo in the link, you'll see that the observer rode in an elevated cage attached to a tower, the whole of which rested on a circular wheeled platform around the base of the telescope. Quite a lot of work for the assistant, I think!

An interesting bit of history; thanks to Dana for putting it up.

-geo

Post Extras:

Spaced
Post Laureate

Reged: 03/01/05

Loc: Tacoma, Washington, USA

Re: Big Dob fans, consider how lucky you are new

[Re: george golitzin]
#5530634 - 11/20/12 05:05 PM

Interesting article -- thanks for sharing!

Quote:

And don't forget the Gulf Stream dumping the moisture onto them...poor buggers!

The Gulf Stream's on the far side of the Atlantic from Malta, which is in the mid-Mediterranean.

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hottr6
Pooh-Bah

Reged: 06/28/09

Loc: 7,500', Magdalena Mtns, NM

Re: Big Dob fans, consider how lucky you are new

[Re: JimMo]
#5530767 - 11/20/12 06:27 PM

Quote:

Quote:

We have it so easy.

Yes, but they had much darker skies.

Only in unpopulated regions. They burned coal in those days.

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careysub
Pooh-Bah

Reged: 02/18/11

Loc: Rancho Cucamonga, CA

Re: Big Dob fans, consider how lucky you are new

[Re: hottr6]
#5536114 - 11/23/12 07:17 PM

Quote:

Quote:

Quote:

We have it so easy.

Yes, but they had much darker skies.

Only in unpopulated regions. They burned coal in those days.

Light pollution is almost entirely due to the existence of exterior lighting.

Before the advent of electricity the brightest and most numerous source of exterior lighting in history were gas lamps, introduced with the Industrial Revolution.

A bit of Googling suggests that in mid-19th century London, there were something like 100,000 gas lamps of about 60 lumens each, a total of 6 million lumens and that would have been the greatest concentration of exterior light in the entire world.

Football conference lighting regulations calls for illuminating the field with 3 million lumens.

Think about the light of two illuminated football fields as the only light pollution to speak of in all of England.

For all practical purposes, light pollution did not exist until electric lighting was invented.

Post Extras:

sslcm56
member

Reged: 07/20/12

Loc: Montgomery, Al.

Re: Big Dob fans, consider how lucky you are new

[Re: careysub]
#5536337 - 11/23/12 09:51 PM

I would give just about anything to find the funding to build a replica of that scope for public use!

Post Extras:

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