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Is 70mm/F24 Refractor feasible?

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#26 ed_turco

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Posted 20 September 2020 - 11:31 AM

 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 

 

 

attachicon.gif70mm f24 singlet.jpg

It's funny that when you don't know any better, you get to thoroughly enjoy substandard stuff.
 


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#27 dan chaffee

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Posted 21 September 2020 - 01:51 AM

It's funny that when you don't know any better, you get to thoroughly enjoy substandard stuff.
 

True. When I made my singlet refractors I had already made instruments that provided views that were superb.   I was more curious about how much one could see with the old instruments with their small apertures and compromised optical possibilities.  If you want to get a strong understanding of the range of what is possible to see, you really have to make many observations with scopes of different types and optical qualities and apertures and in under the full range of seeing conditions. The problem with achieving good optical quality--and getting used to it-- is that if you are at all like me, you keep looking for imperfections in instruments you have already tested to death.


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#28 DAVIDG

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Posted 21 September 2020 - 09:17 AM

It's funny that when you don't know any better, you get to thoroughly enjoy substandard stuff.
 

  That is great if you do understand what you have. The issue is when you don't and believe you have 1/10 wave scope and paid big money for it and then spent decades of one life using it but  missing out what one can really see. I have seen people use a  refractor for many many years and state the image was great but when I tested it for them found the objective in backwards. Their jaw drops when they look through the scope the way it is suppose to work.  

   It is  so much easier to find information today then it was 30 years ago, so why not spend a little time educating  oneself  so you know what you have.

   So when it comes to using a singlet as I have shown there are  cases were it works well and when it doesn't. Now people can make an educated choice if they want to use one or not.

 

 

                             - Dave 


Edited by DAVIDG, 21 September 2020 - 11:29 AM.

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#29 tank20101955

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Posted 24 September 2020 - 05:28 PM

been a while since I have seen any 74mmx1500mm achromats around, I did buy a 74mm x 3000mm that I plan on using for a folded acromat. got it from sophisticated optical tools store on aliexpress. paid $29.85 with shipping for them. these are glued achromats.


Edited by tank20101955, 24 September 2020 - 05:29 PM.


#30 tank20101955

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Posted 24 September 2020 - 05:56 PM

where can I buy telescope tube to fit a 70mm celestron astromaster focuser and objective holder?, I need 3 pcs 1000mm or 1-meter each. I am thinking of doing a sectional telescope that can be assembled with the 3 tubes and can be broken down for transport.



#31 sunrag

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Posted 24 September 2020 - 06:45 PM

Could send me the link to that Ali Express store? I am already getting a 74/3000 from Ebay but wanted to see what else is there.

#32 photomagica

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Posted 30 September 2020 - 04:18 PM

If you want a 74x1500, consider stacking two 74x3000's. If the quality of the 74x3000 lenses is good, this should give a good result.

Bill



#33 ccaissie

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Posted 02 October 2020 - 05:32 PM

I've played with OSLO designing a monofractor...using the curve formula for minimum coma and SA.

Then doing ray anaysis for some very closely spaced wavelengths, like you would get with a fairly pure filter.

 

Some interesting results.  Haven't built one...

 

I like filters for planetary detail, so using a singlet with the right filter could be surprising!



#34 DAVIDG

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Posted 03 October 2020 - 01:23 PM

 I ordered one of the 70mm achromats with a 3000ml Fl  off of Ebay and it arrived today. It is fully antireflective coated on both sides and the polish and cementing looks perfect. I did a quick focal length check to be sure the  focal length is 3000mm. I used the Sun and had to stand on a chair to get enough distance since 3000mm is 118". The image of the Sun which is over 1" in diameter came to sharp focus. Not a critical test of optical quality but encouraging. I'll double pass autocollimate test it  in the next few days to see actually how good it is.

 

                 - Dave 

70mm 3000mm fl sun test .jpg

70mm 3000mm fl.jpg

 


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#35 sunrag

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Posted 03 October 2020 - 08:12 PM

Sweet! Mine will be delivered next week.

How are you planning on making the OTA? Straight or folded?

#36 DAVIDG

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Posted 04 October 2020 - 08:48 AM

Sweet! Mine will be delivered next week.

How are you planning on making the OTA? Straight or folded?

 I am planning on using it as a straight optical path and an alternative objective in the spectroscope at Stellafane that I restored.  The original singlet objective has  a 16' focal length since your observing the Sun in monochrome light a singlet works well. It gives a diffraction limited image at any given wavelength but because of the huge chromatic aberration that causes blue to focus about 7" away from red you need to refocus the image of the Sun at each wavelength you want to observe at. 

   This 74mm x 3000ml fl is an achromat so once focused it won't need to be changed.

 

                       - Dave 


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#37 sunrag

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Posted 16 October 2020 - 09:22 PM

Hi Dave, I got my objective this week. It does look well made.

 

It is about 10mm thick at the edges of the cemented doublet. Both elements are almost same thickness but one is slightly thinner than the other. I would say the difference is less than 1mm. Normally I would assume the thin element is the sky facing one (crown), but I am not sure about this one. Do you know which side is sky facing?



#38 DAVIDG

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Posted 16 October 2020 - 10:04 PM

Hi Dave, I got my objective this week. It does look well made.

 

It is about 10mm thick at the edges of the cemented doublet. Both elements are almost same thickness but one is slightly thinner than the other. I would say the difference is less than 1mm. Normally I would assume the thin element is the sky facing one (crown), but I am not sure about this one. Do you know which side is sky facing?

 The thinner element should be the Crown and that should face the sky.

 

              - Dave 



#39 sunrag

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Posted 17 October 2020 - 05:35 PM

Thanks Dave!

 

I made some progress with the telescope. Here is a photo. The front end has a black PVC tube for the objective cell + glare shield. Tt is not easily visible in the picture but it is jutting out of the white PVC coupler at the top.

 

I was in a hurry to get it mounted and take a quick look, as last night was going to be last clear one in the forecast.

 

I ended up using 4" cardboard tubes instead of the 3" tubes that started this quest because I had only 4" tube rings!

 

I did not have time to align the optics as balancing the tube took most of my time before the clouds rolled in. I was able to take a quick peek at Jupiter and Mars, and some stars. It was not a very good first look. Jupiter looked washed out and I could not see any details. Seeing/transparency was not very good last night so that could have been partly the reason. I did notice that star images looked odd, almost triangular. 

 

I am going to spend the coming week aligning the optics and give it another try.

 

IMG_8822.JPG

 

 


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#40 sunrag

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Posted 17 October 2020 - 06:05 PM

It's funny that when you don't know any better, you get to thoroughly enjoy substandard stuff.
 

I should make this my signature line!




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