6" Achromat dilemma

[Ryachu]

I've had this 6"/F15 achromat in the back of my shed for the past 30 years. It was made for me by a colleague who had made several large mirrors and fancied a go at a refractor. I bought the BK7/F2 blanks from Germany and meanwhile we spent
weeks poring over the maths. The numbers were sent to Edinburgh Observatory and to our amazement a kind person there put it through their ray tracing software and confirmed the curves by return of post ! My friend spent the winter grinding and testing while I went skip diving. Once the lens was finished I presented him with a large piece of aluminium, he wearily fired up his Myford, and the resulting lens is in the photos.

I don't have a clue what to do with it. Is there somewhere it can be tested ? What sort of (2.2m) tube would I need ? Will it fit on an EQ6 ? Will the lack of lens coatings mean disappointment ?

Any advice would be gratefully received.
 

[Peter Drew]

It's difficult to see any option other than to use it for a telescope.  One main difficulty would be acquiring a tube for it, seamless aluminium tubing is rare in less than 3mm wall thickness in suitable diameters so pretty heavy, thinner used to be available from farming suppliers as irrigation tubing which was seam welded.  I built my 8.5" refractor by joining together two 8" reflector tubes, two 6" f8 tubes could do the same.  The lens could be tested by mounting it on one end of a wood plank and an eyepiece on the other end and focusing on an artificial star, if your colleague made good mirrors he should have had little difficulty in making a decent achromat.  Lack of coatings is no big deal, preferable but not essential.    🙂  

[plyscope]

There may be a small loss in light transmission due to lack of coatings but still the lens has potential to be a wonderful telescope. 

If you cannot find a suitable metal tube it is possible to make a lightweight tube from thin high quality plywood. Some years ago I purchased a 6" f15 lens from the US and made a tube for it. There is a description on this link.

https://www.iceinspace.com.au/41-340-0-0-1-0.html

Since then I made other telescopes using a similar method. The method is described in more detail in this thread about a 5" f12.

https://www.cloudynights.com/topic/325327-5-f12-istar-refractor-project/#entry4171500

It is also possible to fold the telescope using high quality flat mirrors to shorten the tube.

Mounting a long tube like this requires a solid mounting. An EQ6 is probably not suitable.

[plyscope]

This book has plans and instructions to make a tube and a mount for a 6" f15 refractor.

http://www.company7.com/books/products/berrybuild.html

Edited February 8 by plyscope

[Sunshine]

Nice lens! if the telescope doesn’t work out for whatever reason, I bet you could ignite a woodpile very quickly with that lens 😅

Edited February 8 by Sunshine

[Rusted]

I used commercial, straight seamed, steel, dust extractor tubing for my 6" and 7" iStar refractors.
This material is no heavier than aluminium being very thin 0.3mm but very stiff.
The seam is neat and easily placed in the gap in typical, hinged, Skywatcher style, tube rings.
Tube rotation is no longer possible but refractors rarely need this.

This ducting is used in woodworking factories and similar. Where dust build up is a fire risk.
Unlike the spiral tubing it looks like a professional telescope. Being smooth and thinly galvanised.
Little risk of rust even after years standing outside in the rain. It could be painted if you like.
I get mine in sizes up to 40cm diameter from the scrap heap of a local furniture factory.
It costs me about £2 per 2m length donated to the Friday cake fund.

It comes in two meter lengths with a very subtle [trumpet like] flare on each end. 
These flares are used to join the tubes end-to-end in extractor systems using a simple, ring clamp.
Hack-sawing off only one end flare allows you to slide a plywood or aluminium counter-cell along the tube.
Without the risk of it falling off the far end of the tube. I use rings and counter-cells of laminated marine ply.

Years ago I made a lightweight tube out of thin marine ply glued and layered around a series of ply rings.
The rings must be halved so they can be knocked out without damaging the lightweight tube.
This was for my 5" f/15 refractor where I made the lens from BK7 F2.
I converted a classical ray tracing book to BBC Basic to design the lens.
Finding several errors in the original tome and identifying errors in the first Astro-Physics triplet prescription.
This was before the availability of ray tracing software on the InterWeblet. 

Refractors are long and heavy at each end where it matters. Moment!
The heavy objective in its cell at one must be matched by the focuser to balance.

To get it off the ground I suggest a plywood, altazimuth, counterbalanced, offset fork mount.
Known as a Berry mount. Because he published on it in last century. It uses Dobsonian principles and bearings.
Don't sneer. There are those who love their Berry style mountings for their high end APO Triplets!
You can add an equatorial platform just under the Berry head if you want tracking.

My home made equatorial mounting has 2" stainless shafts in self aligning bearings.
It is entirely made from scrap aluminium. Using studding to compress and stiffen the box sections.  

 

[davidc135]

It would be quite a beast and probably needs a permanent observatory to be enjoyably easy to use.

I have the necessary kit including a large optical flat and would be happy to test the objective for only  postage cost.

I'd be cautious about sending the lenses by post however well they are packaged although the two lenses of a 4'' Tak apo crossed the Atlantic twice without mishap. If the two lenses are taken out of their cell and individually wrapped before being double boxed I expect that they'd be OK.

Orion Optics UK provide a testing service for mirrors and probably also for refractors.

David

[Ryachu]

I am incredibly grateful for the fantastic response. I've been plagued with guilt for doing nothing with the lens after all the work my colleague put into it. He retired and moved away a long time ago so we've lost touch.

Inspired by the resourcefulness and handiwork seen on here there is fresh wind in my sails. The plan is to knock together a test rig before committing to any major construction. I've made a "sledge" to clamp to the inner stile of an extension ladder. The cell holder is nearly done I just need to sort out the other end. My ED-80 can act as a temporary donor for the parts at the eyepiece end.

I've lost the notes from all the maths we did so the air gap is going to be a guess. The three bits of paper are just to prevent the elements touching.

Once again thanks for all the wisdom. I might take advantage of your kind offer David, I'm just up the road in Much Birch !
 

[davidc135]

Looking forward to following progress and you're welcome to pop over any time.

David