A Proper Amateur Refractor!!

[Rusted]

Congratulations on having a gorgeous instrument to play with Rod.
Though I wouldn't want to confirm image orientation in a hurry. 

A long lecture:

The sheer scale of such a professional instrument should be noted by the over-ambitious refractor "wannabe."

The great length of a "normal" straight OTA means that the moment is huge. [mass x distance from the fulcrum]
Which easily exceeds the ability of most commercial mountings to keep things under control.
Forget anything under £10K if you want to go commercial equatorial. Or you will regret it from first light until you sell your "white elephant."

D&G has a list of suitable mountings in its FAQ.  I'd say it is anything but conservative and assumes a sturdy pier of adequate height.
Plus very deep pockets. Or the skill to build an adequate mounting from scrap. Or know a "tame" builder like Peter.

The real reason for APOs existence [IMHO] is the total lack of affordable commercial Equ. mountings with the ability to carry a 6"+ classical achromat.
They offer living room finishes instead of heavy engineering. Because they know the vast majority of owners will want to carry the mounting outside.

So they can't sell REAL mountings because so few will ever want to buy them. They bring a decorative, folding pocket knife to an artillery gunfight!
iStar optics sell far more short focus lenses than classical. We still gape in awe at classical refractors because there are so few of them.

Long OTA? Add a prism or a heavier eyepiece and you must immediately compensate to achieve OTA balance.
Sliding weights help but add to the moment. You can't put then on a silly, short saddle because they have no moment sited there.

You certainly don't want to be repeatedly lifting such a weight loaded instrument on and off a high mounting.
I had to make a multi-pulley hoist just to change OTAs in my dome to avoid literally killing myself.

That also means the mounting must be fully secured during every change of instrument. Or give up regular changes as I did, in the end.
I hadn't built in positive shaft locks and relied only on the big wormwheels and multipoint friction clutches.

The first time I tried swapping OTAs, without tying off the mounting counterweights, the 7" nearly batted me right out of the slit!
Then you have to change heavy counterweights to match the new OTA. I use a sturdy prop to support the Dec. shaft slightly above horizontal.
Without it I'd be completely powerless to change OTAs. I use multiples of 5kg weights.
Because I couldn't handle anything heavier while perched on a sturdy stepladder.

A 7" f/12 is hardly a big instrument by refractor standards.  You only start getting serious at 8".  Which means f/15, or more, if you want false colour under control.
What does a 7" REALLY look like?  This is only the stubby dewshield. The REAL one is 10" in diameter and nearly 2' long. [2.5x aperture.]
BTW:  I'm about 5'10" and 73 years old.

[F15Rules]

8 hours ago, RodAstro said:

Yep F15 rules

Here is a picture of what I own and use day and night over the past 15 years. Mine is one of three that is in private hands, only two are working.

The 6" lens is in the class of apochromatics visually and is of steinheil design, flint first.

One of the great things of F15 refractors is they have wonderful flat fields and are very tolerant of eye position at the eyepiece, unlike modern short apos, even F8s.

This is the public observatory at Brisbane as it shows the size of the scope. The scope was designed to fit a six foot observatory, mine is in a seven foot observatory and hard to get a photo that shows its size.

Cheers Rod

Fascinating scope, Rod. I take it that's a folded design?

If that was in Brisbane and you're in Derbyshire, it sounds like there is a story there that deserves a thread of it's own! 

Thanks for posting 🙂

Dave

[F15Rules]

8 hours ago, RodAstro said:

 

Deleted (double post)

Edited November 8, 2020 by F15Rules
Double post

[F15Rules]

Rusted,

Ok, you've convinced me..

Giant refractors are actually malevolent mechanical monsters bent on the destruction of the puny mortals that drool over them!😱

I'm sticking with my tame little FS128 5" F8!! 😄

Dave

 

[RodAstro]

Hi all

Yep as you say Rusted f15 is a big operation I have a crane stored away for working on mine.

Yes it is folded and has the bonus of fixed eyepieces. Orientation is not a big problem, you just let the object drift then you know where you are.

As you say Rusted mounts are the biggest problem for large refractors another plus for the folded design is you end up with a correct size mount, not very portable though. The RA axis of my mount is the basis for the 36" Zeiss cassegrain so I have no problem piggy baking other scopes. 

Out of interest I would like to hear views of others with long focal length refractors on eyepieces they use. 

 

Cheers Rod 

[F15Rules]

1 hour ago, RodAstro said:

Hi all

Yep as you say Rusted f15 is a big operation I have a crane stored away for working on mine.

Yes it is folded and has the bonus of fixed eyepieces. Orientation is not a big problem, you just let the object drift then you know where you are.

As you say Rusted mounts are the biggest problem for large refractors another plus for the folded design is you end up with a correct size mount, not very portable though. The RA axis of my mount is the basis for the 36" Zeiss cassegrain so I have no problem piggy baking other scopes. 

Out of interest I would like to hear views of others with long focal length refractors on eyepieces they use. 

 

Cheers Rod 

Hi Rod,

I used a lot of (much smaller) F15-F16 refractors in the past, and found them all pretty good with most eyepieces, due to the sheer focal length of the scopes: the joke used to be that a decent long frac would deliver up great images through the bottom of a Coke bottle!

With my D&G 5" F15 I found this to be largely true, but because of the relatively narrow field such a focal length delivers, I preferred eyepieces with a wider field of view, such as Pentax XW and XL ranges (70deg and 65deg respectively), Vixen LVW (65), and Explore Scientific 24/28 and 34mms..the 34mm in particular, gave superb lower power views of clusters such as the Double Cluster in Perseus.. I say lower power, because the D&G, with a focal length of 1920mm still magnified the 34mm to about 57x.

I also loved using traditional high quality orthos on Lunar, Planets and double stars..the long frac's depth of focus meant that dual speed focusers were really not essential, and the simple design and great transmission of orthos such as the Baader Genuine Ortho (BGO) range could yield some breathtaking images under steady skies. Sadly, my aging eyes now prefer a bit more eye relief, and I find binoviewers much more comfortable for many types of observations.

In more modern designs I find the Baader Morpheus range offer a great combination of wide field (76 deg), contrast and sharpness and value for money. I think they would be outstanding in a good long focus refractor.

Nowadays, it seems that most people seem to want shorter and shorter scopes and wider and wider fields, requirements that only the newer, more exotic designs of eyepiece can satisfy, such as Ethos and others of 100 degree fields or more. My own Tak FS128 at F8 is seen nowadays as a fairly slow focal ratio scope, but when I was a young man any refractor of under F10 was seen as pretty fast! 

I will always admire long achromats with a well figured lens as delivering some of the purest, most contrasty and sharpest images you can get.

Dave

Edited November 8, 2020 by F15Rules

[RodAstro]

Hi Dave

Very interesting, no mention of naglers.

Yep I love the Zeiss Orthos I have the full set and two of each for the bino head and a 40mm and a 63mm Zeiss huygens that is fantastic. These all came with the telescope and are always my preferred lenses to use, everything is so flat and sharp, I see no reason to have a wider field of view if it is full of distortions. The Zeiss bino head has a 1.5 times correcting barlow that is removable so eye relief is not a major problem with the orthos until you are using the 6mm and 4mm, 843X for the 4mm so not used often.

The dilemma I have found is that the more modern designs of wide field eyepieces and flat field eyepieces are pretty awful at F15. They seem to be designed to correct some of the aberrations in short focal length scopes so once you get to f15 they are actually creating aberrations.

When I first got the scope I was all up for getting a 31mm type 5 Nagler as we had one in the club and I loved it in all the newtonians we had but when I used it in the F15 it was awful and probably one of the worst eyepieces I have ever used in this scope.

I put this down to the flatness of field the Zeiss F15 has, they originally came with a 4"X5"  plate camera. I have found less expensive wide field eyepieces work better at F15, by chance I picked  up a SW panaview 38mm and found this to be very good with only about 10 percent of the field having slight distortions so bought the 32mm and 26mm for wide field and have used them for the past few years as the best I could get but the other day I decided to try a cheap Ascension 30mm 80 degree secondhand off ebay (I could always sell it again) and it is excellent, you have to look hard at the edge to find any distortion.

In the old days when most of us built our telescopes and used ex-military eyepieces, there was a lot of talk about matching your eyepieces to your telescope, that idea seems to be long forgotten.  

Your lovely FS128 f8 has the dilemma of being in the middle of the two extremes but Tak has made some excellent eyepieces for it and as you say the Pentax and Vixen W lanthanums should be very good as they were designed for your focal length scope and are lovely eyepieces.

Rod  

[Zermelo]

Some amazing photos.
Almost like the good old days:

[RodAstro]

Yep just like the good old days but a lot easier to do a meridian flip noerdays 🤣

[Peter Drew]

@RodAstroI have a 8.5" F12 achro refractor and a 5" F15 triplet APO, both self built with existing optics on large self built mounts.  I also have an Ealing-Beck asymmetric equatorial mount similar in size to your mount which easily carries a 16" SCT and a piggybacked SW 150ED refractor.  In the main I use zoom eyepieces on these telescopes, Baader and TV Nagler types plus a selection of TV plossls.  Lowest power eyepiece is a Vixen 42LV.     🙂  

[markse68]

3 hours ago, RodAstro said:

Yep I love the Zeiss Orthos I have the full set and two of each for the bino head and a 40mm and a 63mm Zeiss huygens that is fantastic.

Hi Rod, amazing setup you have- you’re very lucky! Are these the CZJ or the ZAOs or do you have the incredibly rare observatory grade orthos i’ve read about?

Mark

[RodAstro]

Hi Peter that Eling Beck is a lovely mount.

A 16" Meade replaced my Zeiss at St Andrews Uni they said the optics are good but the mount has never worked properly so never gets used, it has had numerous boards and has even been back to Meade, Just not in the same league as these professional scopes.

We also use some Clave true Plossls that my friend has we can never make a decision between them and the Zeiss orthos both are excellent.

I will have to come and visit you sometime I have been meaning to for years. 

[RodAstro]

Hi Mark Full set of the original brass Abbe Carl Zeiss Jena Orthos, they are so sharp and flat to the edge you can use the field stop as an occulting bar.

Picture of my set here with the bino head and rotating turret and all the fittings for afocal projection and the schott rotating filters.

Interestingly for scopes that were to be used in very northerly latitudes Zeiss also supplied a set of paired Huygens eyepieces to be used in the bino head for planetary work as you can use their field curvature as an  atmospheric dispersion corrector by placing the planet in the correct part of the field, they work very well.

Cheers Rod

 

[DaveS]

Interesting discussion.

I think the push for short focus triplet / quadruplet (Or even more complex) 'scopes comes from us imagers who want wide fields, with 0.95+ Strehl across the spectrum and right into the corners of a 16803 sensor.

For this imager, although I have a hankering after something mad like a Vixen VSD 100, realistically I'm happy with what I have.

[markse68]

37 minutes ago, RodAstro said:

Hi Mark Full set of the original brass Abbe Carl Zeiss Jena Orthos, they are so sharp and flat to the edge you can use the field stop as an occulting bar.

Picture of my set here with the bino head and rotating turret and all the fittings for afocal projection and the schott rotating filters.

Interestingly for scopes that were to be used in very northerly latitudes Zeiss also supplied a set of paired Huygens eyepieces to be used in the bino head for planetary work as you can use their field curvature as an  atmospheric dispersion corrector by placing the planet in the correct part of the field, they work very well.

Cheers Rod

 

Thanks Rod- what a beautiful set! 

Interesting about the ad “correcting“ eps- I have the opposite problem with the xws 😂

Mark

[RodAstro]

About the Atmospheric dispersion, it is an old trick many planetary observers used and worth trying with Jupiter and Saturn being so low at the moment I have found that many of the cheaper 70 degree eyepieces are ideal for this.

All you do is find the centre line of the dispersion then maneuver the planet along this line from the centre of the eyepiece field, one way the dispersion will get worse the other side of centre the dispersion will decrease until it has gone and the planet appears much sharper. You only need a AD unit for photography, although you could probably use the eyepiece method afocaly with a large enough camera chip where you can select your ROI on any part of the chip, I have never tried it though.

[DAVE AMENDALL]

Some amazing fracs on this posting. I drooled over this Byers  on youtube  some time ago. The video is still on there.  " TELESCOPE FOR SALE  Research Grade 12" F12.2  Achromatic  Refractor"

If he ever sold it ,I simply don't know..................Dave

[RodAstro]

Yep that is nice will show a lot of colour fringing though but stopped down to 10" f15 or even 8" F18 on planets it would be very nice.

The Bayers mount is very nice.

[maw lod qan]

Great post and amazing images.

I'm in awe thinking how you man handle those huge telescopes.

[John]

This has developed into a fascinating thread.

If I had been able to mount the Istar 6 inch F/12 really well (as per @Rusted's post) I think it would have performed superbly. The best that I could give it was an EQ6 mounted onto a Meade Giant Field Tripod (3 inch diameter steel legs). Even that setup weighed in at getting on for 50 kg when fully assembled. The scope made the usually rather hefty EQ6 seem rather puny

I think a Fullerscopes Mk IV would have been better but I could not find one when I needed it !

 

My inspiration for a large refractor has always come from Sir Patrick Moore's 5 inch F/12 Cooke refractor:

Back in 2016 I was fortunate to be able to purchase a TMB/LZOS f/9.2 130mm triplet which, although shorter than the traditional refractors mentioned in this thread, was still a long scope for a triplet apochromat and, as In understand it, was designed by Thomas M Back to be a specialist planetary instrument. Even at F/9.2 this scope presented some mounting challenges but with the purchase of a lovely pre-owned T-Rex mount in February this year, that problem is solved and the scope is now delivering to it's full potential.

This may well be the largest aperture refractor that I own for the foreseeable future:

 

Edited November 9, 2020 by John

[Rusted]

Patrick Moore's 5" refractor had a Charles Frank mounting.
Recognisable by the spherical housings. It also had a nice, solid pier by the look of it.

I have a lovely old, drawing office, drawing board with a 4" cylinder, hydraulic rise and fall on a flared iron base.
It weighs a ton but was far too short for a proper refractor and the rise and fall had no more rise but an excess of fall.
So I use it to support my weather station's wind vane and anemometer on a very tall, windsurfing mast.

Edited November 9, 2020 by Rusted

[Rusted]

Nice instruments John.

The T-Rex needs an offset counterbalance by the look of it.  Berry, offset fork style.
You'll need a large clamp which doesn't mar the surface of the mounting cylindrical section.
With a threaded rod, [stud/all-thread] on the opposite side to the telescope, to carry the counterweight.
Work out the weight of the telescope and its offset to calculate the moment required.
Lead is dense and compact.

What about a black, gutter downpipe clamp in PVC?
They are set up to have a sturdy screw fixing to the wall.
Some are about the right size too.

The next image is of my all DIY 5" f/15. Schott BK7/F2.
I wrote my own ray tracing software to refine it.
Built my first optical grinding and polishing machine to work it.
The glass blanks cost as much as most telescopes do today!

The scrapyard, scaffolding tube tripod lacked torsional rigidity.
It needed a raised fork because I had only one scrap length of tube to saw up.

The telescopes movements were wonderfully smooth until it reached the black hole at the zenith.
1lb pull on a fisherman's spring balance in every direction. Perfect!

I saw the major 1970s comet of the time through the 5".
The lead counterweight was melted in an aluminium saucepan on my wife's brand new "Natural Gas" cooker.
I am still reminded of this "crime against domesticity" half a century later.
1970s?

[maw lod qan]

Melting lead on SWMBO's stove a crime!??

You wouldn't believe the things I've done in the house with me hunting, just to get out of the mosquitos after sunset.

Sink, bathtub, all fell victims of my hunting success!

[DaveL59]

apparently putting greasy mechanical parts in the dishwasher is a no-no too 😉 

[Davey-T]

3 minutes ago, DaveL59 said:

apparently putting greasy mechanical parts in the dishwasher is a no-no too 😉 

Can't see the problem, I once degreased my Mini engine block in the bath ready for rebuild 😂

Dave