My 6" f7 home/club made mounting.

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My 6" f7 home/club made mounting.

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Post by OzEclipse »


Some of you may have heard me talk about the home made telescope I made when I was a teenager. I often use the OTA on my EM200 GOTO mount but today, I brushed off the cobwebs of it's original push to visual Porter-Springfield inspired mounting. The original Porter-Springfield mount designed in the early 20th century used an offset tube and an extended counterweight to divert the focus down the optical axis to a diagonal on the polar axis. This resulted in a fixed eyepiece position facilitating sketching at the telescope. The following illustrations show a newtonian and cassegrain implementation.
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The Late Great Cliff Duncan, president of the ATM club of Queensland and the designer of the whole series of mountings, used the broad bearing surface principle from the Porter-Springfield for stability but did not want to implement the polar axis eyepiece position. Cliffs design mantra was that the diameter of the bearing disc surface should be the same as the mirror. So the bearing discs on my mount are 6 inches to match the six inch mirror.

Most of the parts were cast in aluminium. Each member would buy their own Aluminium ingots at the local metal supplier and then everybody would pitch in and help with the various tasks, conditioning the sand, making the sand moulds, firing the furnace. Cliff was ingenious. His furnace used a blower fan to aspirate a mix of sump oil obtained free from his local garage and kerosene. The fuel tank was hoisted into a tree by a pulley then the fuel gravity fed down and into the industrial blower and into the furnace. The club would have "foundry" weekends when the club furnace would be fired up, sand moulds formed and laid out then poured and cooled. My nickname in the club was "Hacksaw Joe." This sounds derogatory but was given to me to acknowledge that I could cut the feeders off the castings with more precision than most of the members - a process known as fettling. The more feeder metal that stays on, the more time it takes to machine it off. But it the hacksaw ever ran into and damaged the casting, it would be discarded and thrown into the remelt pile.

Cliff owned all the equipment and generously made it accessible to club members. The foundry was in Cliffs back yard. Under his house were two lathes, drill press, linisher and other hand and machine tools. There was no milling machine but Cliff used a variety of techniques on the lathes to do what milling had to be done. There was also a small optical polishing room and an optical testing tunnel for Foucault and Ronchi testing. Final figuring was done by testing with a Coudé mask over a Foucault test.

The parts that were cast and machined were mirror cell, finder brackets, focuser base, cradle and yokes, rotation rings, bearings, angle bracket between the bearings, aluminium legs, zinc counterweight, and the ATMQ badge. The spider was fabricated (machined on lathe) from brass bar and spring steel arms were brazed into the body. Brass tubing was also use to make the upper part of the focuser.
IMG_1576-bearing.jpg
The mount has just five pieces(below).
IMG_1571.JPG
The cast aluminium pier legs have milled slots that hold the leg bolts in place so that you only need one spanner to attach the legs to the pier. I know I know, there are two spanners on the grass. :lol:
IMG_1572.JPG
IMG_1573.JPG
Despite the four 6" diam 1" thick bearing discs. The equatorial head is quite light. Below I am lifting it with only a little difficulty using the fingers on my right weak hand. I have an arthritic wrist and fingers. I have not weighed it but I'd guess 7kg.
IMG_1574.JPG
The head attaches to the pier with a hand tightened 5/8 bolt and ball washer then the final latitude angle is adjusted with 4 3/8 allen head bolts that surround the main bolt to adjust. There is no azimuth adjustment but the mount is easy enough to nudge around to the pole and it is only a push to so precise polar alignment isn't important.
IMG_1575.JPG

The counterweight is made of cast zinc and attaches by hand without tools.
IMG_1576.JPG
IMG_1577.JPG
The counterweight shaft is machined from 1.5" steam pipe. If you look carefully you can see a slot milled up the side. Inside the pipe is a 5/8" W threaded rod which is turned by the steampunk cast aluminium wheel at the end of the counterweight shaft. As the thread turns, it drives a travelling nut with a protruding fork that pushes and pulls the counterweight up and down the shaft. A really beautiful if arrangement allowing very precise adjustment of RA balance to make pushing very smooth. The large disc bearings can be pushed with one hand on the counterweight shaft to smoothly track the scope even at 300x magnification or more. The counterweight shaft is not mechanically part of the dec so using the counterweight shaft won't ever mover the scope in dec.
IMG_1578.JPG
As soon as I set this up it looked like it was about to rain so I covered the mount even though the mount is basically waterproof.

I will show you the OTA and rotation ring system tomorrow.

Joe
Last edited by OzEclipse on Wed Oct 20, 2021 9:00 pm, edited 2 times in total.
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Amateur astronomer since 1978...................Web site : http://joe-cali.com/
Scopes: ATM 18" Dob, Vixen VC200L, ATM 6"f7, Stellarvue 102ED, Saxon ED80, WO M70 ED, Orion 102 Maksutov, ST80.
Mounts: Takahashi EM-200, iOptron iEQ45, Push dobsonian with Nexus DSC, three homemade EQ's.
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Re: My 6" f7 home/club made mounting.

#2

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Very cool DIY work Joe, great story about the aluminum molding. I had never heard of the Porter-Springfield mount but after reading your description I understand why, lol. The mount is a nice piece of work though. So since the rings are fixed and part of the mount, it can only support one size OTA? Or can you just bolt on another size ring. And there is no Alt adjustment for the RA axis, so this is just for one latitude? Probably not for AP but it should work fine for visual if you can get it polar aligned.
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Re: My 6" f7 home/club made mounting.

#3

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SkyHiker wrote: Wed Oct 20, 2021 11:53 am Very cool DIY work Joe, great story about the aluminum molding. I had never heard of the Porter-Springfield mount but after reading your description I understand why, lol. The mount is a nice piece of work though. So since the rings are fixed and part of the mount, it can only support one size OTA? Or can you just bolt on another size ring. And there is no Alt adjustment for the RA axis, so this is just for one latitude? Probably not for AP but it should work fine for visual if you can get it polar aligned.
Hi Henk,
Thanks for your post.

This particular design is not designed to be motorised. It could be motorised with a lot of effort. Cliff was a master optician, when you joined the club, we made these scopes to be used visually. It a push only. The saddle plate bolts to the dec bearing, the tube yokes bolt to the saddle plate, the rotation rings sit in the yokes. In theory, you could put new yokes on the saddle plate and change the OTA. I made an adapter plate with a dovetail for the saddle so I can put it into my Tak EM200. But retrofitting this mount with a dovetail is difficult due to the big friction adjuster knob poking through the saddle plate.

The mount has ALT adjustment for the RA but doesn't have AZ adjustment. The cast legs are smooth underneath and can slide over grass so you can nudge it around and because it can't be easily motorised & used for photography, accurate PA is not a big deal. The push motion is very smooth.

Cheers
Joe
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Amateur astronomer since 1978...................Web site : http://joe-cali.com/
Scopes: ATM 18" Dob, Vixen VC200L, ATM 6"f7, Stellarvue 102ED, Saxon ED80, WO M70 ED, Orion 102 Maksutov, ST80.
Mounts: Takahashi EM-200, iOptron iEQ45, Push dobsonian with Nexus DSC, three homemade EQ's.
Eyepieces: TV Naglers 31, 17, 12, 7; Denkmeier D21 & D14; Pentax XW10, XW5, Unitron 40mm Kellner, Meade Or 25,12
Cameras : Pentax K1, K5, K01, K10D / VIDEO CAMS : TacosBD, Lihmsec.
Cam/guider/controllers: Lacerta MGEN 3, SW Synguider, Simulation Curriculum SkyFi 3+Sky safari
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Re: My 6" f7 home/club made mounting.

#4

Post by John Donne »


Very nice design Joe !
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Re: My 6" f7 home/club made mounting.

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Post by turboscrew »


Smart looking thing! We're going to hear how well it works?
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Re: My 6" f7 home/club made mounting.

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Post by OzEclipse »


turboscrew wrote: Wed Oct 20, 2021 7:48 pm Smart looking thing! We're going to hear how well it works?
Hi Juha,
Cliff was a bit of a slave driver and made me work and work on the mirror until it was perfect. I was an impatient teenager who just wanted to look through the scope. It was a valuable life lesson on application and perseverance and one that lasted long past the telescope making process. I set up the OTA with a 3.5% central obstruction secondary mirror. As a result, the focuser only takes 1.25" eyepieces. 2" eyepieces would show vignetting.

A few years ago and after many years with a sub-standard coating, I had the mirror re-aluminised and over-coated at a cost of $420. I could have bought an 8" newt for that.

After that I wrote two descriptions of the performance. First was a descriptive essay describing a whole night of observing on my own.
https://joe-cali.com/astronomy/articles ... Essay.html

Then the scope was set up next to my friend Phil's observatory, we did a side by side comparison between my 6"f7 reflector and his Takahashi TOA150 refractor. On those occasions the scope was mounted on my Takahashi EM-200 mount.
https://joe-cali.com/astronomy/articles ... ctors.html

This is the first time in many years that the scope has been back on its original Porter-Springfield mounting.

But I will also set up the OTA and do some more photos today then some observing reports with the original setup but with my now completed eyepiece collection. Some months ago, I completed my eyepiece acquisition phase purchasing Pentax XW5 and XW10 1.25" eyepieces.

My 1.25" eyepiece set now contains-
Screen Shot 2021-10-21 at 8.40.00 am.png
More coming......

Joe
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Amateur astronomer since 1978...................Web site : http://joe-cali.com/
Scopes: ATM 18" Dob, Vixen VC200L, ATM 6"f7, Stellarvue 102ED, Saxon ED80, WO M70 ED, Orion 102 Maksutov, ST80.
Mounts: Takahashi EM-200, iOptron iEQ45, Push dobsonian with Nexus DSC, three homemade EQ's.
Eyepieces: TV Naglers 31, 17, 12, 7; Denkmeier D21 & D14; Pentax XW10, XW5, Unitron 40mm Kellner, Meade Or 25,12
Cameras : Pentax K1, K5, K01, K10D / VIDEO CAMS : TacosBD, Lihmsec.
Cam/guider/controllers: Lacerta MGEN 3, SW Synguider, Simulation Curriculum SkyFi 3+Sky safari
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Re: My 6" f7 home/club made mounting.

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Post by OzEclipse »


Here is the OTA of my 6" ƒ7. It's got a few chips of paint missing. Not bad considering I originally gave it one paint job in black. When it was out in the sun and got so hot I couldn't touch the tube, I then realised how stupid that idea was and repainted it white. This paint job is almost 40 years old. After taking the pictures, I realised my seedling trays are on the grass in the background of almost every picture - spinach beet and borlotti beans!
IMG_1579 copy.jpg
IMG_1580 copy.jpg


Mirror Cell

The mirror cell is one of the cast and machined components. It is composed of two rings, one ring is screwed to the rear of the tube, the other circular component has a T profile. The mirror can be seen through the back of the cell. It is a full thickness 1:6 mirror. Even so, a 1" thick 6" diameter mirror cools quite quickly. It sits on three screws and is held from falling out by three brass tabs. If I tilt the tube past horizontal, the mirror can be heard rattling around. There is no stress on the mirror. Collimation is achieved by three pairs of push pull screws. These keep their setting quite well even when the scope is transported.
IMG_1581 copy.jpg
IMG_1582 copy.jpg

Rotation Rings

This is a close up of one cradle yoke and one rotation ring. A piece of high strength parachute web holds the OTA in place though not too tightly. The profile of the rotation rings stops the tube from sliding out allowing the eyepiece position to be easily rotated to a comfortable viewing poition regardless of the orientation of the OTA.
IMG_1583 copy.jpg

Finderscope

The original finderscope was made from a pair of 7x50mm binocular objective and binocular eyepiece mounted in 50mm PVC plumbing fittings. The original eyepiece had soft focused edges so I machined a focuser capable of taking standard 1.25" eyepieces and now use a 26mm Plossl that came with one of my small cheap scopes.

Just last week, I removed the 50mm objective and temporarily taped in place a 63mm binocular objective. A friend of mine was trying without success to use it to focus an ultrabright multicoloured 25mm LED into a smaller fibre optic bundle he was using in a nightclub lighting feature he'd been commissioned to make. He asked my advice and I told him the focal length was much too long. I found some 30mm low power microscope eyepieces from my junk box that were perfect for the job. And I had 4 of them, one for each bundle. He was so grateful, he asked if I could use the 63mm binocular objectives. Does a bear ?????? in the woods. So last week I taped it in place and it turns the finder into an 8.8x63 finder, 50% more light grasp than the original 50mm objective. Seems to have reasonably sharp stars over most the the FOV. I'll use it awhile like this and if I like it, I'll tack the objective in place with a few drops of cyanoacrylate glue then permanently glue the objective in place with epoxy resin and paint it white. The finder brackets were also cast in the club foundry.


IMG_1584 copy.jpg

IMG_1586 copy.jpg

Focuser

The focuser base is also aluminium cast in the foundry. In that are three brass tubes that slide smoothly with friction. When looking through the eyepiece, you twist gently as you push or pull.
IMG_1588 copy.jpg
IMG_1589 copy.jpg

Excluding eyepieces, the mirror kit in 1978 cost me AUD25, I recall the ingots cost $2 ea and I probably bought 10 of them. The brass for the focuser and spider cost about $10. Dad was a builder and the steel tube was an offcut from his building site.

All up, the scope in 1978-80 cost me less than $60(AUD) which I paid for in holiday jobs. Corrected for inflation, that's about about $220 (AUD) in today's Australian dollars or about USD160 today. Excluding eyepieces, that's about USD 4 per year. Good value for money in my books.

Cheers
Joe Cali
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Amateur astronomer since 1978...................Web site : http://joe-cali.com/
Scopes: ATM 18" Dob, Vixen VC200L, ATM 6"f7, Stellarvue 102ED, Saxon ED80, WO M70 ED, Orion 102 Maksutov, ST80.
Mounts: Takahashi EM-200, iOptron iEQ45, Push dobsonian with Nexus DSC, three homemade EQ's.
Eyepieces: TV Naglers 31, 17, 12, 7; Denkmeier D21 & D14; Pentax XW10, XW5, Unitron 40mm Kellner, Meade Or 25,12
Cameras : Pentax K1, K5, K01, K10D / VIDEO CAMS : TacosBD, Lihmsec.
Cam/guider/controllers: Lacerta MGEN 3, SW Synguider, Simulation Curriculum SkyFi 3+Sky safari
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Re: My 6" f7 home/club made mounting.

#8

Post by notFritzArgelander »


Thanks! My 3rd scope was a long Newtonian on a pipe fitting mount and I’m a fan. Unfortunately the scope was lost to fire. I’m glad you have yours.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
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Re: My 6" f7 home/club made mounting.

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Post by OzEclipse »


A small addition to this thread.
The secretary of the Astronomical Association of Qld was sorting out old archive material, scanning and digitising some, tossing others. He found an old high school astronomy club newsletter from the 1970's that had been lodged with the Association and noticed my name. So he sent the copy to me.

The "News" section pretty closely dates my commencement of the build as being August/Sept 1978. First light was August 1980.
Image 3.JPG
I separated the declination bearing today and took some pictures. The 1" steel shaft is located in a thick aluminium boss. The two greased outer surfaces, approximately 1/2" provide the thrust surfaces.

The nearest modern equivalent is probably the Disk Mount
http://www.discmounts.com/
untitled shoot-3215.jpg
I have also made a simple adaptor out of a piece of 6mm aluminium plate and a short Vixen rail. This allows me to attach the OTA with saddle plate, yokes and rotation rings to my Takahashi EM 200 GOTO mounting.
untitled shoot-3217.jpg
untitled shoot-3216.jpg
Joe
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Amateur astronomer since 1978...................Web site : http://joe-cali.com/
Scopes: ATM 18" Dob, Vixen VC200L, ATM 6"f7, Stellarvue 102ED, Saxon ED80, WO M70 ED, Orion 102 Maksutov, ST80.
Mounts: Takahashi EM-200, iOptron iEQ45, Push dobsonian with Nexus DSC, three homemade EQ's.
Eyepieces: TV Naglers 31, 17, 12, 7; Denkmeier D21 & D14; Pentax XW10, XW5, Unitron 40mm Kellner, Meade Or 25,12
Cameras : Pentax K1, K5, K01, K10D / VIDEO CAMS : TacosBD, Lihmsec.
Cam/guider/controllers: Lacerta MGEN 3, SW Synguider, Simulation Curriculum SkyFi 3+Sky safari
Memberships Astronomical Association of Queensland; RASNZ Occultations Section; Single Exposure Milky Way Facebook Group (Moderator) (12k members)
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Re: My 6" f7 home/club made mounting.

#10

Post by BABOafrica »


Very nice project. Thanks for all the pictures showing us how you did it.

BABO
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