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Is 70mm/F24 Refractor feasible?

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#1 sunrag

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Posted 11 September 2020 - 10:23 PM

Just when I thought I was done with building refractors, I got these cardboard tubes free from a neighbor who was going to throw them away. They are 76 mm ID, 85 mm OD, 1640 mm long.

 

These are by far the best cardboard tubes I have seen (including ones I purchased). Very smooth exterior, perfect round, and quite robust as the walls are 4 mm thick. Also quite long at 5'4".

 

So I wanted to see if a 70 mm achromat doublet or even singlet is available with long focal lengths, preferably > 1500 mm. Does such a lens exist?

 

Thanks for reading.

 

IMG_8735.jpg

 

IMG_8736.jpg

 

 

 


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#2 RichA

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Posted 12 September 2020 - 12:03 AM

I had a 50mm f/20.  I bought the lens from an optics company.  Turned out to be very good lens.



#3 ed_turco

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Posted 12 September 2020 - 11:56 AM

A single lens of high f/ number would be very easy to build.  Glass quality wouldn't have to be as stringient.



#4 sunrag

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Posted 12 September 2020 - 12:11 PM

Yes I agree that a singlet would be sufficient. Years ago, I too built a 50mm, F/20 using a singlet of unknown quality. It gave fantastic views of planets.



#5 dan chaffee

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Posted 13 September 2020 - 02:00 AM

It will be sharp on the moon, but you will see a lot of secondary spectrum. I have built  50 mm f/50 and 75 mm f/80 singlet lenses and still see strong secondary color in both lenses at those ridiculous focal ratios. I used both scopes for planets, the moon and doubles.   They both split doubles down to the limit and the edges of planets are sharp, but the internal features are less distinct because of the big contrast loss from secondary spectrum. And at over 30x/inch mag. they have a yellow-green cast to them. If this is just for fun and curiosity, that's one thing, but for quality viewing with that aperture, disappointing. SInglet lenses do offer a super  black background though, from the lowest scatter possible in any telescope...and why they are preferred for monochromatic coronagraph optics.


Edited by dan chaffee, 13 September 2020 - 02:00 AM.

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#6 photomagica

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Posted 13 September 2020 - 02:12 AM

You can greatly improve the strength and surface finish of cardboard tube by giving it several coats, inside and out, of varnish thinned one part varnish to two parts thinner. This will soak into the cardboard, and when it dries, stiffen the tube and make it more moisture proof. To finish the inside just soak a rag in the thinned varnish and push it through with a stick. It may take several dips and trips thought to fully coat the inside of the tube. Let the tube dry for 24 hours between coats and lightly sand any roughness on the outside off between coats.

 

Thinned varnish is a great way to stabilize MDF particle board as well.

 

If you build this, let us know how it works out.

Bill


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#7 Mark Harry

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Posted 13 September 2020 - 02:58 AM

Objective end of a singlet scope. Eyepiece end was another singlet, for fairly low magnification.
The low mag revealed a decent view, and daytime showed violet color, similar to a run of the mill achro with typical compound eyepiece.
Focusing, was one tube slid into the other- happened to be a snug fit. Aperture, roughly 2"

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  • objective end 16 to 17cent.jpg

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#8 ed_turco

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Posted 13 September 2020 - 01:41 PM

By the way, the color error found in singlets is primary, a doublet has that pesky secondary color error.

 

I made a 30mm plano convex singlet telescope long ago that literally amazed me.  At f/32, I could view daytime objects or the moon and I found it was hard to see any color error.  Boosting the magnification to 180 times brought the error out, but not as horribly as optical theory would predict.  Alberio and Polaris were easy to resolve.   The resolution on the moon was impressive, given the circumstances.   Saturn even had a ring.

 

I figured out some of the 'whys' of my results.  At the unimpressive aperture of 30mm, the telescope was impervious to atmospheric effects.  And, at f/32, all the aberrations we hear about don't seem to matter that much.

 

Galileo would have killed to get my telescope, but as he proved what he could do with his, I'd be thrilled to give it to him  smile.gif


Edited by ed_turco, 13 September 2020 - 01:54 PM.

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#9 dan chaffee

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Posted 13 September 2020 - 09:02 PM

By the way, the color error found in singlets is primary, a doublet has that pesky secondary color error.

 

I made a 30mm plano convex singlet telescope long ago that literally amazed me.  At f/32, I could view daytime objects or the moon and I found it was hard to see any color error.  Boosting the magnification to 180 times brought the error out, but not as horribly as optical theory would predict.  Alberio and Polaris were easy to resolve.   The resolution on the moon was impressive, given the circumstances.   Saturn even had a ring.

 

I figured out some of the 'whys' of my results.  At the unimpressive aperture of 30mm, the telescope was impervious to atmospheric effects.  And, at f/32, all the aberrations we hear about don't seem to matter that much.

 

Galileo would have killed to get my telescope, but as he proved what he could do with his, I'd be thrilled to give it to him  smile.gif

Yes, I stand corrected on the primary spectrum.   People react differently to out of focus color. My eyes are

quite sensitive to it even in my 60's, so the  mileage does vary.   At extreme focal ratios the Sidel aberrations for a spherical surface are so small they are not worth bothering to measure, unless the figure is far from spherical or astigmatic. I've used plain water white window glass for singlets that worked as well as they theoretically could.



#10 sunrag

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Posted 14 September 2020 - 08:32 AM

Thanks guys for your advice. Any idea where I can find a 70mm Singlet or Achro with long focal lengths? I have looked at the usual places (SurpluShed, Edmunds, Newport etc) and could not find anything.



#11 DAVIDG

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Posted 14 September 2020 - 02:28 PM

 That is going to be a tough one to find . If you decided to make a singlet you want to make it  of the Cross design were the back radius is 6x larger then the front. This minimize spherical aberration but not color. So for  a singlet made of BK7  that works out to a front convex radius (R1) of 39.0974"  and a back convex  radius of  257.2083"  with the center thickness of 0.25". 

 

           - Dave 


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#12 photomagica

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Posted 14 September 2020 - 08:42 PM

If you are not committed to too great an f-ratio, you can use a 1 diopter or #1 achromatic camera close-up lens. This will have a focal length of 1 meter so if you get a lens with 50mm of clear aperture, say a 52mm close up lens, the f-ratio will be 20. If you get a quality achromat from a good maker like Minolta, it will make a splendid objective lens. Even a non-achromat #1 close-up lens makes quite an acceptable objective lens. Try E-bay for either the achromats or the plain close ups. You should be able to find larger than 50mm.

Bill


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#13 RichA

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Posted 14 September 2020 - 09:02 PM

Just when I thought I was done with building refractors, I got these cardboard tubes free from a neighbor who was going to throw them away. They are 76 mm ID, 85 mm OD, 1640 mm long.

 

These are by far the best cardboard tubes I have seen (including ones I purchased). Very smooth exterior, perfect round, and quite robust as the walls are 4 mm thick. Also quite long at 5'4".

 

So I wanted to see if a 70 mm achromat doublet or even singlet is available with long focal lengths, preferably > 1500 mm. Does such a lens exist?

 

Thanks for reading.

 

attachicon.gifIMG_8735.jpg

 

attachicon.gifIMG_8736.jpg

I've got a 70mm singlet with an 8ft focal length. 



#14 RichA

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Posted 15 September 2020 - 12:50 AM

Super long focal length achromat:

 

https://www.ebay.fr/itm/363072045320

 


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#15 sunrag

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Posted 15 September 2020 - 07:45 AM

I've got a 70mm singlet with an 8ft focal length. 

May I ask where you got it from? Did you build a telescope with it?

 

That reminds me, last year I got a 150mm aperture, 2600mm FL from SurplusShed. But the CA was quite awful even when stopped down to 4". Also the focus was very mushy, and could not get sharp views at all.



#16 sunrag

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Posted 15 September 2020 - 07:50 AM

If you are not committed to too great an f-ratio, you can use a 1 diopter or #1 achromatic camera close-up lens. This will have a focal length of 1 meter so if you get a lens with 50mm of clear aperture, say a 52mm close up lens, the f-ratio will be 20. If you get a quality achromat from a good maker like Minolta, it will make a splendid objective lens. Even a non-achromat #1 close-up lens makes quite an acceptable objective lens. Try E-bay for either the achromats or the plain close ups. You should be able to find larger than 50mm.

Bill

That is a great tip about EBay close-up lens. I found several good matches. Thank you!


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#17 sunrag

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Posted 15 September 2020 - 07:50 AM

Super long focal length achromat:

 

https://www.ebay.fr/itm/363072045320

I am tempted!



#18 RichA

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Posted 15 September 2020 - 09:56 AM

May I ask where you got it from? Did you build a telescope with it?

 

That reminds me, last year I got a 150mm aperture, 2600mm FL from SurplusShed. But the CA was quite awful even when stopped down to 4". Also the focus was very mushy, and could not get sharp views at all.

Got it out of a piece of labware.  Unknown what its function was.  Never actually built it into a fully-fledged scope, just a cardboard carpet tube. 



#19 RichA

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Posted 15 September 2020 - 09:57 AM

May I ask where you got it from? Did you build a telescope with it?

 

That reminds me, last year I got a 150mm aperture, 2600mm FL from SurplusShed. But the CA was quite awful even when stopped down to 4". Also the focus was very mushy, and could not get sharp views at all.

Yes, we forget that a 4" singlet in the 1700's might have had a 100 foot focal length to cope with colour.


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#20 ccaissie

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Posted 18 September 2020 - 12:03 PM

Just when I thought I was done with building refractors, I got these cardboard tubes free from a neighbor who was going to throw them away. They are 76 mm ID, 85 mm OD, 1640 mm long.

 

These are by far the best cardboard tubes I have seen (including ones I purchased). Very smooth exterior, perfect round, and quite robust as the walls are 4 mm thick. Also quite long at 5'4".

 

So I wanted to see if a 70 mm achromat doublet or even singlet is available with long focal lengths, preferably > 1500 mm. Does such a lens exist?

 

Thanks for reading.

 

attachicon.gifIMG_8735.jpg

 

attachicon.gifIMG_8736.jpg

A singlet works, and with a well chosen filter you could do some nice planetary viewing.   Filters are common for planetary work.



#21 DAVIDG

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Posted 18 September 2020 - 12:32 PM

 That depends on what you define as works. Here is the spot diagram for the 70mm f/24 Cross design singlet I worked up. The Cross design minimizes spherical aberration but does not fully correct for it  when you use spherical surfaces and R2 is 6x larger then R1. Even if you refocus for blue and red those wavelength have even more spherical then in the green. You still have a huge amount of chromatic aberration which the spot diagram shows. The black circle is the size of the Airy disk so any light outside is not diffraction limited.  If you use filters, even very narrow ones your still not at a  diffraction limited image and you have thrown away a lot of light. If you want to eliminate spherical over a  narrow set of wavelengths at these F-ratios  you need  to aspherize a surface or if you want to stay with spherical surfaces you need to greatly increase the focal length to many many feet.

   Were singlets work is for true monochrome viewing or imaging as in solar work. The Hale spectrohelioscope at Stellafane use a 3" diameter singlet of the Cross design with a 16 foot focal length. At that focal length and aperture the image is diffraction limited over the wavelength that is used but you still have to refocus for each wavelength and the difference between were blue comes to focus vs is red is around 7".

 

                 - Dave 

 

 

70mm f24 singlet.jpg


Edited by DAVIDG, 18 September 2020 - 08:13 PM.

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#22 DAVIDG

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Posted 18 September 2020 - 12:37 PM

 Here is 74mm achromat with 3000mm ( 118" ) focal length on Ebay for not much money https://www.ebay.com...cAAAOSw9J9d4L~M

 

                         - Dave 


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#23 sunrag

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Posted 18 September 2020 - 05:14 PM

Super Find!!

Thanks  Dave. I bought it.


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#24 photomagica

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Posted 18 September 2020 - 10:14 PM

Super Find!!

Thanks  Dave. I bought it.

Yes - great find. Let us know how it works. Any thoughts on eyepieces??

Bill



#25 sunrag

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Posted 20 September 2020 - 09:27 AM

Yes - great find. Let us know how it works. Any thoughts on eyepieces??

Bill

Since this is F40 (!), it is going to very easy on eyepieces.

I have Celestron and Meade Plossl eyepieces that I can try out. I have 40mm, 32mm, 26mm, 20mm.

I think 20mm would the shortest I can use.

 

Update: the 74mm/FL3000mm has shipped from China. I expect it to get in my hands in 4 weeks. I will let you know how it works out.


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