My take....expensive and doesn't look like a long FL refractor. Better off with a large Cass.
All I can bring to this discussion is I'm waiting on an email from Ales over at Istar on getting an 8" f12 R30 Lens. Nearly finished a 2" Pillow Block that will take an 8" with ease.
As a kid at school I got to use the 9" f15 Oddie Refractor on Mt Stromlo every clear Wednesday night for a year....sealed my fate. That and the Astro Optical Supplies catalogue....photos of Unitron's, like Steve's 6"
I have had good experience with Istar. I made a telescope with a 5" f12 R30 which is a great lens. An 8" f12 sounds wonderful Matt. I hope it comes together for you.
Back in the 90's I was fortunate enough to visit Mt Stromlo. I can only imagine what it would be like to observe through a large classic refractor like that.
I am sorely tempted to exchange my APM 152 for the new f7 180mm from APM. Markus has agreed but we're talking another $17k-more than $500 per mm😫
Do you realise that for $2300 (current price) you could have a wonderful Skywatcher MN190 Mak-Newt f5.3, 1000mm fl, . Fully corrected -no detectable coma, heavily baffled, amazing contrast, very small secondary mounted on the corrector - so no diffraction spikes. Reviewers suggest large APO performance at 1/3 the price. It is both a astrograph and a powerful visual scope. Yes i have one of these.
I have nothing against large refractors, i have a 152mm achro, and my iStar, but from a value for money perspective, true colour rendition, and apeture, it is hard to make a case for the investment required (for me) just to have an exclusive brand name on a scope. These are simply devices to enable our grasp of night sky, imho. However, i understand that for many people they become "objects d'art" and therefore the experience of having one, for them, is a reward in itself.
Do you realise that for $2300 (current price) you could have a wonderful Skywatcher MN190 Mak-Newt f5.3, 1000mm fl, . Fully corrected -no detectable coma, heavily baffled, amazing contrast, very small secondary mounted on the corrector - so no diffraction spikes. Reviewers suggest large APO performance at 1/3 the price. It is both a astrograph and a powerful visual scope. Yes i have one of these.
While this is probably a thread for refractor fanboys , I have to back up Glens comments in that I used one of these at SPSP and it is indeed a very nice solution and fantastic bang for buck in having many of the similar qualities to an APO refractor of aperture which would be nominally unaffordable , but with far less elements and you still get to have a big chunk of glass at the top end to catch dew and dust .
I saw a very refractor like high contrast image of Saturn that held up under high power. The only down side was the weight and it was up high on an HEQ6 and was swaying a bit in the wind.
As an aside to my 6" Polarex refractor, I have a project underway for my A E Jaegers 6" F15 achromat lens bought before the 1980's fire which destroyed their factory. I had a spare lens/dew shield assembly from a 6" Unitron so I machined up a lens cell for the Jaegers lens and it screws into the cradle just like the Polarex 6" lens. About all I need now is a tube around 6 1/2' diameter and about 7 feet long! It will be interesting to see how it compares to the 6" Polarex lens.
When I made my D&G scopes I found it difficult to source light weight metal tubing. So I made the tubes from thin ply. That has worked out very well however I would consider rebuilding them if decent metal tube was available at reasonable expense.
When I made my D&G scopes I found it difficult to source light weight metal tubing. So I made the tubes from thin ply. That has worked out very well however I would consider rebuilding them if decent metal tube was available at reasonable expense.
Your hexagonal ply tube, even at a seven foot length, would probably be stiffer than a thin walled steel or aluminium tube and they would require wide spaced tube rings and bars to support them. I always wondered how people baffle and paint the inside of those long metal tubes. Laying up a ply tube, at least the way you did it, seems very sensible. The test would be to check the collimation sag on the mount.
Your hexagonal ply tube, even at a seven foot length, would probably be stiffer than a thin walled steel or aluminium tube and they would require wide spaced tube rings and bars to support them. I always wondered how people baffle and paint the inside of those long metal tubes. Laying up a ply tube, at least the way you did it, seems very sensible. The test would be to check the collimation sag on the mount.
The thin ply tube is very stiff and light with no sag at all. The 6" OTA is 14kg and the 5" is less than 9kg including tube rings, finder, diagonal and eyepiece.
They just don't look like classic telescopes, not that that really matters. They perform like the real thing and at night I still enjoy using them very much.
There are a few techniques for fitting baffles to metal tubes. One way is to make a skeleton frame with the baffles at their correct spacing and then slide the frame into the tube. Painting the inside can be done with a brush on a stick or even a spray can on a stick with an extended trigger for the nozzle.
So this 8" f12 R35 might happen....specially if anyone else wants to do the big Refractor thing. Istar would build a few lenses if they had a couple of interested buyers. I know its nuts...I don't care. At current exchange rates it'll be $3-3500K for the objective in a cell....anyone interested send Ales an email at Istar optical.
If it doesn't happen thats OK I still have my 6" f12 Istar Achro.
An Istar 8" F8 ( ?) Achro lens went cheap here on IIS Trade not too long ago for well under $1000 - the glass was substandard and showed veins of inhomogeneity that could be seen in the extra focal star test, so you would have to be vigilant. The cost of optical glass skyrockets as you go larger as the cost of finding glass and value of pieces free of inclusions and veins goes up exponentially .
I'm interested in doing some more lens work sometime, and would like to do a large achromat ( F12) - I was thinking about something in 10" league . But something like that would have to go into a very large dome or use at least one folding flat to cut the tube down to something manageable and then you would have to look down from the top.
I think such a configuration would not tick the 'romance' box which I think is the great appeal of achro refractors which are basically visual instruments.
I have considered an 8" f12 for the backyard. I think it might be do-able if the weight can be kept down.
My experience with the 5" f12 R30 Istar lens proved it to be very heavy for it's aperture. An 8" R35 would need to be permanently mounted for me as I think it would just be too heavy. I would prefer a standard achromat 8" f12 for that reason alone.
Mark if you ever offer an 8" f12 coated achromat lens in cell I would be interested. D&G offer them for the price of $2395 USD plus $275 USD for simple coatings on all four surfaces. That would be over $4000 AUD and who knows how long you would have to wait for it, maybe years.
I don't know if it would be commercially viable for you to make a run of them in Australia. What do you think?
Good point on the weight of an 8" f12 amnstigmat. Ales did say in an email that he could do an 8" f12 achro FMC in a cell for USD 2,250 plus post and a wait of two months.Would likely be cheaper for two? Lots of time to work it out. Zane sold a 180 f8 recently. The only Istar lens that I have heard of that wasn't perfect.
Mark if you ever offer an 8" f12 coated achromat lens in cell I would be interested. D&G offer them for the price of $2395 USD plus $275 USD for simple coatings on all four surfaces. That would be over $4000 AUD and who knows how long you would have to wait for it, maybe years.
I don't know if it would be commercially viable for you to make a run of them in Australia. What do you think?
Andy : Probably wouldn't be viable - I would expect a hefty expense on the AR coatings , USA sourced glass and adjustable mirror cells. Probably a fast track to working for $10 an hour committing to a production run on something like that !.
I suspect what D nd G would be batch polish production runs of elements getting radii of surfaces more or less right but not worrying to do much surface figuring . They would then mix and match lenses to find pairs producing the best wavefront , then do some quick localised polishing on the rear element surface to achieve an optical null and decent star test . Just my guess for that price , but I may be wrong .
As I said I am interested in making a large achromat for fun at some stage with a view to making a large oiled triplet down the track . Oiled triplets have always appealed to me as the two oil mated surfaces have next to no surface quality tolerance and don't have to have AR coatings. So after mating there are no more complications beyond that of mounting and aligning a doublet. I have no idea why they aren't more common, but it is probably to do with less design freedom in that the mated curves of the oiled elements have to be more or less matched in radii.
No worries Mark. I understand there are risks involved in AR coating and that would naturally complicate any commercial endeavour.
The oiled triplet does make a lot of sense for one off production. Even more so if it allows you to shorten the f ratio and make the tube length more user friendly.
D&G must have ways of reducing their costs somehow. I have heard that Barry Greiner the owner of D&G makes his main income through other optical contracts and that is also why lens production can be sporadic and lead to long wait times.
R Royce advertises an oiled doublet that sounds interesting but it would be f15 and very long.
For me if I am honest with myself and considering the cost and the convenience, I am probably better off with a Mewlon Cassegrain.
Just throwing this question out there. Does anyone know what the largest non-professional refractor is in Australia -by which I mean owned by an individual? Perhaps the question should be split into the largest achromatic and the largest APO.
That would be Barry Adcock's 10" APO triplet (FPL53 based). Barry has made more than a dozen refractors, APOs and achros, doublets and triplets and this recently completed APO has now replaced his 14" Schiefspiegler as his planetary instrument of choice.
His 8" f/30 Coelostat refractor (fixed eyepiece, inside the house) is probably among the largest achromats too, if not the longest, but certainly most comfortable.
, I have to back up Glens comments in that I used one of these at SPSP and it is indeed a very nice solution and fantastic bang for buck in having many of the similar qualities to an APO refractor of aperture which would be nominally unaffordable , but with far less elements and you still get to have a big chunk of glass at the top end to catch dew and dust 
.
