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Long focus 8” Newt

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#1 torana68

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Posted 29 February 2020 - 06:11 PM

Ok this is the other one, tube is 7’ long, 8”mirror. Everything on the OTAis very nicely made.  Four vane spider with really thin vanes.  The mirror comes in a steel mirror holder with a close fitting lid. 

The mount is a combination of pipe and car parts 🤔

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#2 torana68

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Posted 29 February 2020 - 06:14 PM

Mount photos to follow.

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#3 clamchip

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Posted 29 February 2020 - 06:32 PM

That sure is a tiny diagonal mirror!

I'll bet it performs really nice!

 

Robert



#4 torana68

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Posted 29 February 2020 - 06:34 PM

The mount is wood with steel shafts and car parts, the wheel has a gear around the outside. 

No real idea of the age of this one ,maybe fiftees?

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#5 clamchip

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Posted 29 February 2020 - 06:44 PM

Hopefully there may be some information scribed on the edge or back of the mirror.

 

Robert


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#6 Kokatha man

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Posted 29 February 2020 - 08:51 PM

Love it..!!! waytogo.gif waytogo.gif waytogo.gif

 

What type of steering wheel is that - & are they steel, or fibre timing-drive gears...with the one around the steering wheel a toothed pulley-belt...or a huge starter-motor ring gear..? :lol: 



#7 torana68

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Posted 29 February 2020 - 10:40 PM

The gears are steel from a gearbox , the wheel something British post WW2 (ish) the lg gear looks like a turned down ring gear , dunno why it’s there?

Edited by torana68, 29 February 2020 - 10:41 PM.


#8 Senex Bibax

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Posted 01 March 2020 - 07:19 AM

Rural NSW? Looks like the sort of thing my relos would have built if they were into astronomy smile.gif



#9 tim53

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Posted 01 March 2020 - 10:15 AM

The mount is wood with steel shafts and car parts, the wheel has a gear around the outside. 

No real idea of the age of this one ,maybe fiftees?

That looks like the steering wheel from about a 1953 Singer SM1500 sportscar.  I used to have one.  Not this one:  https://www.classica..._sm_10_1570.jpg


Edited by tim53, 01 March 2020 - 10:15 AM.

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#10 torana68

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Posted 01 March 2020 - 06:56 PM

Rural NSW? Looks like the sort of thing my relos would have built if they were into astronomy smile.gif

Sydney, last in Sutherland shire, owned by a Guy who worked at the University of NSW, his daughter thinks he acquired them rather than built . 



#11 Kokatha man

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Posted 01 March 2020 - 08:09 PM

That looks like the steering wheel from about a 1953 Singer SM1500 sportscar.  I used to have one.  Not this one:  https://www.classica..._sm_10_1570.jpg

Yeah - I thought old Singer or one of the similar cars of that era like a Morris etc... :)

 

Great-looking "gear"..! :lol:



#12 starman876

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Posted 01 March 2020 - 10:39 PM

you got to love scope like thatwaytogo.gif



#13 Senex Bibax

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Posted 02 March 2020 - 07:36 AM

I've got a cousin down that way, Campbelltown to be exact, but I was thinking more of my relatives up Gilgandra way. I've seen some amazing home-brewed engineering...

 

Sydney, last in Sutherland shire, owned by a Guy who worked at the University of NSW, his daughter thinks he acquired them rather than built . 



#14 Marty0750

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Posted 03 March 2020 - 05:43 AM

Wondering if the long focal ratio is because the maker was simplifying by making a spherical mirror?

 

Out of interest just taking the rough specs D=8" FL=84" and plugging these into OSLO (optical ray trace) for an ideal spherical mirror the Strehl ratio is 0.89 which is essentially diffraction limited at almost exactly 1/4 wavelength. That would give very good (but not excellent) resolution. You'd want better than 1/4 secondary too to ensure the final image is not degraded to worse than 1/4 wavelength.



#15 torana68

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Posted 04 March 2020 - 03:26 AM

I’d think not, I don’t remember seeing any short telescopes in the mid 60’s , most people I knew were chasing planet’s.

#16 Kokatha man

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Posted 04 March 2020 - 03:31 AM

I’d think not, I don’t remember seeing any short telescopes in the mid 60’s , most people I knew were chasing planet’s.

I agree - the "standard" for an 8" was f8 in the 60's here in Oz...I have an Astro-Optical pyrex mirror set from that era (late 60's) still in its' box after I decommissioned the scope I made with it then: many atm'ers went for f8 to f10 back then! smile.gif

 

Edit - & most follks strived for parabolic mirrors if they were making their own! ;)


Edited by Kokatha man, 04 March 2020 - 03:32 AM.

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#17 Marty0750

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Posted 06 March 2020 - 06:02 AM

I’d think not, I don’t remember seeing any short telescopes in the mid 60’s , most people I knew were chasing planet’s.

I went for making the 6-inch f8 because the shallow sphere was easier to parabolize. If one leaves it a sphere and it would just make 1/2 wave, hardly acceptable. Though an f10 sphere would nudge it into 1/4 wave while f12 would make it comparable to a parabolic - if you could live with a 72 inch (1.8m) long tube!



#18 clamchip

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Posted 06 March 2020 - 10:50 AM

I have a 6 inch f/13 I built around a vintage mirror.

It's fantastic optically. 

It started out as kinda a joke and I called it my 'Hillbilly Super Planetary' mostly thrown

together with scrap parts.

It soon became evident this was no joke and it became the Allenscope after the scribed

name on the back of the mirror.

Over the years I've made many mods, mostly with the diagonal mirrors. Although I have

kept it vintage, no low profile focuser, clip-less primary cell, etc.  

Robert

 

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#19 clamchip

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Posted 06 March 2020 - 11:22 AM

Here I'm going, 'Up Up and Away!' an attempt to get away from the

driveway's heat tube currents.

The 'old timers' they knew what they were doing making these long newts.

The Polychromatic Peak to Valley wave front of a 6 inch f/15 achromat is

about .140 or 1/7 wave.

A 6 inch f/13 mirror if spherical and not even parabolized is .05 or 1/20 wave.

Robert

 

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Edited by clamchip, 06 March 2020 - 11:24 AM.

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#20 torana68

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Posted 07 March 2020 - 06:44 AM

Nice 😁

#21 davidc135

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Posted 07 March 2020 - 03:48 PM

I went for making the 6-inch f8 because the shallow sphere was easier to parabolize. If one leaves it a sphere and it would just make 1/2 wave, hardly acceptable. Though an f10 sphere would nudge it into 1/4 wave while f12 would make it comparable to a parabolic - if you could live with a 72 inch (1.8m) long tube!

A 6'' F/8 sphere would be out by .26 waves wf, not perfect but pretty good.

I use the handy formula: wave front error = 22.5 x diam(ins) / F ratio^3.  David


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#22 Marty0750

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Posted 09 March 2020 - 11:48 PM

That long f13 Newtonian!  And yes the performance should be exemplary given the primary sphere is smooth and well made and good coatings.  And davidc135 you are correct your figures check out. A slip of the calculator on my part. From Amateur Telescope Optics I use metric  0.888*D/F^3

 

I made up this chart as an exercise from OSLO with spot diagrams for a 6-inch (150mm) spherical for f8, f10 and Robert's f13.  The black circle is the airy disk and the red dots are the focused theoretical photons. Also shown are the spots at 0.25 and 0.35  degree from centre of field which correspond to 0.5 and 0.67 degree full fields. I also optimized the secondary obstruction at 1.25" (32mm) 1" (25mm) and 4/5" (0.8mm) respectively.

 

Robert's f13 long newt exceeds it own parameters. Right out to the edge all photons are "within" the airy disk. That means no effects of spherical aberration, astigmatism, coma or curvature of field! 

 

 

Marty


Edited by Marty0750, 10 March 2020 - 12:41 AM.

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#23 Dave1066

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Posted 24 March 2020 - 03:03 PM

That long f13 Newtonian!  And yes the performance should be exemplary given the primary sphere is smooth and well made and good coatings.  And davidc135 you are correct your figures check out. A slip of the calculator on my part. From Amateur Telescope Optics I use metric  0.888*D/F^3

 

I made up this chart as an exercise from OSLO with spot diagrams for a 6-inch (150mm) spherical for f8, f10 and Robert's f13.  The black circle is the airy disk and the red dots are the focused theoretical photons. Also shown are the spots at 0.25 and 0.35  degree from centre of field which correspond to 0.5 and 0.67 degree full fields. I also optimized the secondary obstruction at 1.25" (32mm) 1" (25mm) and 4/5" (0.8mm) respectively.

 

Robert's f13 long newt exceeds it own parameters. Right out to the edge all photons are "within" the airy disk. That means no effects of spherical aberration, astigmatism, coma or curvature of field! 

 

 

Marty

 

Hi Marty ,

 

I'm trying to figure out Newtonian design. I'm a planetary observer and thinking about design parameters. I did a search and found this thread. If I had a 6" F11 telescope, with 1/10 PTV parabolic mirror. The mirror having a Hilux coating that transmits 97% of the light it receives. With a central obstruction of 18% what would the spot diagram look like? What would a comparable spot diagram look like for a 12% and 15% central obstruction? 

 

David



#24 Marty0750

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Posted 24 March 2020 - 04:32 PM

Hi David

 

You may already know of this online calculator for Newtonian geometry. Check the diagonal is the right diameter for the f-ratios you list so that you are not missing any light nor introducing unnecessary diffraction effects.

 

I will use the optimum diagonal sizes as well as 18%  for your f-ratios when I get the spot and wavefront diagrams posted later today.

 

Marty


Edited by Marty0750, 24 March 2020 - 04:37 PM.


#25 Dave1066

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Posted 24 March 2020 - 05:13 PM

Thanks Marty, I'm not very good with the various online calculators. Hence why I search search engines and the various astronomy forums to see if I can find the answers in plain English.

However compared to the other online Newtonian calcs, Stellafane is brilliant thanks.

David

Edited by Dave1066, 24 March 2020 - 05:17 PM.



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