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For those who thought rich-field refractors were new...

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#1 RichA

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Posted 09 October 2021 - 07:13 PM

J.W. Fecker (S&T 1950) 8 inch possibly f/6.6 refractor.

 

J.W. Fecker 1950 8 inch refractor  richfield.jpg


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#2 Redbetter

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Posted 09 October 2021 - 08:50 PM

Depends on what one means by "rich-field" I suppose.  Wouldn't this have used .965" eyepieces with perhaps 21mm field stop?  That means max TFOV = 21 * 57.3 / (8 * 25.4 * 6.6) = 0.90 degrees or something like that.  That is considerably narrower than my 8" f/10 SCT in 2" mode.  It is even narrower than my 20" f/5 Dob.


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#3 markb

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Posted 09 October 2021 - 08:59 PM

A CN friend put me into a Leonard Mallan Fawcett publication from 1960, making a well designed RFT with the then new A Jaegers 5" f5 and their 32.5mm big tank Erfle. He made one cribbing the next design in the book, with a novel sliding prism body for focusing. Ads are pretty funny, by the way.

 

Despite making a couple of RFTs, I don't recall reading much about the from the late 80s onwards. 

 

I reengineered a failed RFT Jaegers f5 with a clever internal 2.25 diagonal but crippled by a period 1.25 focuser and no provision for collimating and aligning. With a 2" crayford and 30mm UFF it is amazing. My other one is done except for a needed tube trim for the straight through focuser, though I may convert it to a right angle also.

 

I never realized they released the 5" f5 that early. My big jaegers have all had 1968 or 1969 dates on the edges.

 

Incidentally, obstructed Schmidt newtonians can make terrible RFTs, In daylight or on the moon my 2" V/C cometcatcher, adapted to a 30mm 2" and a Baader MPCC, produced an obstruction shadow larger than my pupil opening.

 

I know I've seen the origin of the RFT term here on CN don't recall the details. Certainly it wasn't used by 1960 when Mallan used it in his book, unattributed.

 

EDIT The RFT term looks like it goes back to about 1940, at least, as one of the papers in the Ingalls ATM books, book 2 I think. Mine's still packed so I can't track it any further.


Edited by markb, 09 October 2021 - 09:17 PM.

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#4 markb

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Posted 09 October 2021 - 09:08 PM

As to the JW Feckler usability as an RFT, rough scaling puts that focuser at 3" or so, not sure on the turret eps. 

 

It would have been a perfect match for the 2.5 to 3 in surplus tank Erfle eye pieces that were a dime a dozen (almost) at that time. The 2.25" war trophy 10x80 flak BT eyepieces, about 27mm, 70+degree AFOV, would also have been great, not that I've seen one converted. Not Erfles, a much more complex design I think.


Edited by markb, 09 October 2021 - 09:18 PM.

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#5 RichA

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Posted 09 October 2021 - 09:31 PM

Depends on what one means by "rich-field" I suppose.  Wouldn't this have used .965" eyepieces with perhaps 21mm field stop?  That means max TFOV = 21 * 57.3 / (8 * 25.4 * 6.6) = 0.90 degrees or something like that.  That is considerably narrower than my 8" f/10 SCT in 2" mode.  It is even narrower than my 20" f/5 Dob.

Focuser is pretty big, I'd wager the eyepiece shown is at least 1-1/4" diameter. So, about 1 deg. with a 30mm eyepiece.  Plus, as others mentioned there were some larger eyepieces floating around, I had 14 salvaged periscopes from tanks in the 1990's that had the equivalent of 32mm Erfles with 65 deg fields.  But even without low-power, wide-field eyepieces,  that 8 inch will have a much larger field than typical f/15-f/18 refractors had then.


Edited by RichA, 09 October 2021 - 09:32 PM.

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#6 luxo II

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Posted 09 October 2021 - 09:57 PM

RFT's definitely go back to the 50's - in the Scientific American ATM books (Ed A.G. Ingalls) there was a chapter on this, which specifically looked at maximising the number of stars visible in a given field of view - and considered the distribution of stars by magnitude - it concluded a 6" f/5 was ideal; at that time this meant a fast newtonian - irrespective of eyepiece type.


Edited by luxo II, 09 October 2021 - 09:59 PM.

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#7 pbealo

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Posted 10 October 2021 - 07:35 AM

In the late 1800s Clark and Fitz made some "comet seeker" short focal ratio telescopes. Leslie Peletier had one 

https://www.cloudyni...rk-observatory/

 

and the Dudley Obs. in Albany NY has a 4" one.

http://dudleyobserva...ction-overview/


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#8 Tom Masterson

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Posted 05 November 2021 - 05:17 PM

I like the wording in the ad calling it semi-portable. Does that mean you need a semi to transport it?


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#9 RichA

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Posted 05 November 2021 - 05:47 PM

I like the wording in the ad calling it semi-portable. Does that mean you need a semi to transport it?

Must weight (if brass/steel) about 200lbs.



#10 ccwemyss

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Posted 05 November 2021 - 07:39 PM

I like the wording in the ad calling it semi-portable. Does that mean you need a semi to transport it?

I think it's referring to the wheels in the base. You can transport it anywhere you can roll it to. Beyond that, it's not portable. 

 

Chip W. 



#11 Jon Isaacs

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Posted 05 November 2021 - 08:46 PM

The eyepieces in the photo do not appear to be wider field eyepieces. It could have been modified but for wider fields but as configured it looks like Red's analysis is close.

 

Jon



#12 EJN

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Posted 05 November 2021 - 11:12 PM

RFT's definitely go back to the 50's - in the Scientific American ATM books (Ed A.G. Ingalls) there was a chapter on this, which specifically looked at maximising the number of stars visible in a given field of view - and considered the distribution of stars by magnitude - it concluded a 6" f/5 was ideal; at that time this meant a fast newtonian - irrespective of eyepiece type.

 

The chapter is The Richest Field Telescope - A Plea for Low Magnification by S. L. Walkden, the material for the chapter was originally written in 1914, with additional material added for the book (ATM Book II).

 

There is a photo of Clyde Tombaugh holding a 5" f/4 Newtonian he built, and also a photo of Leslie Peltier with his comet-seeker, a 6" fairly short refractor, maybe f/8.

 

It also states that Clyde Tombaugh's favorite eyepiece for the 5" f/4 was a 1" (25mm) Ramsden.

 

ClydeRichfield7inmadeforUncleLeecirca193

 

gallery_235481_7340_243459.png


Edited by EJN, 06 November 2021 - 12:02 AM.

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#13 Kasmos

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Posted 05 November 2021 - 11:22 PM

I'd be surpised if the Fecker used .965" EPs. Regardless of that, it looks like f/8-10 to me.


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#14 EJN

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Posted 05 November 2021 - 11:59 PM

I'd be surpised if the Fecker used .965" EPs.

 

Me too, I thought .965 was only found on post WWII Japanese made scopes.



#15 Terra Nova

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Posted 06 November 2021 - 09:03 AM

I'd be surpised if the Fecker used .965" EPs. Regardless of that, it looks like f/8-10 to me.

True dat! Other Feckers of the period used the standard 1.25” format. Changing gears: The illustration for the ad itself looks much older than 1950. Everything’s about it looks pre-war; I’d say 1920s-30s. I wonder if this was something they had on their books for a long time. Perhaps, made to order?


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#16 Terra Nova

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Posted 06 November 2021 - 10:14 AM

Here’s a TeleVue classic RFT:

 

https://televue.com/...uilt/#more-6483


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#17 Astrojensen

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Posted 06 November 2021 - 10:47 AM

Me too, I thought .965 was only found on post WWII Japanese made scopes.

The 0.965" (24.5mm) standard originally came from Zeiss, who used it from 1897 onwards. It may be earlier than that, but I've not been able to trace it further back. It was popularized in Japan, when Nippon Kogaku (later Nikon) and others copied pre-WW2 German telescope optical designs. 

 

And even though Zeiss (and others, like Steinheil) had small push-fit eyepieces as standards, they also had large, low-power screw-in eyepieces, 1.5" - 3" or even more in diameter, with focal lengths from 30mm to 80mm. This was something the Japanes largely forgot or ignored!  

 

There are actually very few makers today, that offers as wide a selection of eyepieces, as Zeiss did in 1920!  

 

 

Clear skies!

Thomas, Denmark


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#18 highfnum

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Posted 06 November 2021 - 10:54 AM

here are my 2 RFT's

Jeager 80mm F5

 

Jeager 125mm F5 

 

I use them they are great 

 

jaegerF5s.jpg


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#19 Astrojensen

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Posted 06 November 2021 - 11:04 AM

RFTs actually go back much earlier, to the days of Fraunhofer, or even earlier. The famous "Bonner Durchmusterung" catalogue from 1863 was based on visual observations with a 78/630mm Fraunhofer achromat with a magnification of just 10x. The original huygenian eyepiece was replaced with a Kellner. The eyepiece is enormous, compared to the small telescope, so the TFOV should have been in the 5° range, though I can find no actual numbers. 

 

csm_Fernrohr_der_Bonner_Durchmusterung__

 

https://www.bornheim...-der-astronomie

 

 

Clear skies!

Thomas, Denmark


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#20 highfnum

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Posted 06 November 2021 - 11:09 AM

i suppose my comet catcher is an RFT 5.5 inch F3.6

agree?

 


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#21 CltFlyboy

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Posted 06 November 2021 - 11:29 AM

I really have to get off my rump and finish building this Jaegers 6" f/5 RFT that I started months ago!

 

 

Attached Thumbnails

  • Jaegers 6 inch RFT.jpg

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#22 RichA

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Posted 06 November 2021 - 11:30 AM

RFTs actually go back much earlier, to the days of Fraunhofer, or even earlier. The famous "Bonner Durchmusterung" catalogue from 1863 was based on visual observations with a 78/630mm Fraunhofer achromat with a magnification of just 10x. The original huygenian eyepiece was replaced with a Kellner. The eyepiece is enormous, compared to the small telescope, so the TFOV should have been in the 5° range, though I can find no actual numbers. 

 

csm_Fernrohr_der_Bonner_Durchmusterung__

 

https://www.bornheim...-der-astronomie

 

 

Clear skies!

Thomas, Denmark

I was thinking that even a short achromat must have been a major relief from using horrifically-long singlets prior to their invention.


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#23 Terra Nova

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Posted 06 November 2021 - 11:39 AM

And even though Zeiss (and others, like Steinheil) had small push-fit eyepieces as standards, they also had large, low-power screw-in eyepieces, 1.5" - 3" or even more in diameter, with focal lengths from 30mm to 80mm. This was something the Japanes largely forgot or ignored!  

 

Clear skies!

Thomas, Denmark

The big threaded Unitron 60mm eyepiece comes to mind. This was probably copied from Zeiss.


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#24 Terra Nova

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Posted 06 November 2021 - 12:09 PM

I have always had a place in my heart for RFTs. I built 3” and 6” RFTs (both Newtonians) a couple of years after I got interested in amateur astronomy. I still have the 6”. I think it was because I got my entrance into the hobby using binoculars and the skies were so much darker back then. I’ve always loved open clusters, other types of bright and expansive DSOs, and just wandering the Milky Way, getting lost in the immensity of the star fields. That’s where the real wonder of the Infinite is for me.


Edited by Terra Nova, 06 November 2021 - 12:10 PM.

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#25 CltFlyboy

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Posted 06 November 2021 - 12:16 PM

I have always had a place in my heart for RFTs. I built 3” and 6” RFTs (both Newtonians) a couple of years after I got interested in amateur astronomy. I still have the 6”. I think it was because I got my entrance into the hobby using binoculars and the skies were so much darker back then. I’ve always loved open clusters, other types of bright and expansive DSOs, and just wandering the Milky Way, getting lost in the immensity of the star fields. That’s where the real wonder of the Infinite is for me.

Me too - there is nothing that awes me more than sweeping the skies randomly and gasping when I run across something beautiful. Nothing compares.


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