Very long focal length refractors

[OneCosmos]

I came across an interesting article yesterday that said that although modern triplets are perfectly optimised for imaging, visually they are flawed compared to the very high focal ratios of old when it comes to viewing the bright planets in particular. I’m talking about those old 80 -100mm fluorite f/12 or even f/15+ refractors.

Has anyone compared them to modern fast APOs? Does anyone close to Brisbane have one? I’d love to look through one.

[glend]

Long refractors do not usually have ultra modern glass formulations, one factor driving that is that once you get to f15 there is virtually no difference in terms of colour rendition (assuming both lenses sets are figured well, etc).
Most long refractors continue to be Doublet objectives, this provides a weight savings in comparison with Triplets and in long refractors balance can be an issue.
Don't get hung up on trying to find a long focal length fluorite objective scope, as generally speaking expensive fluorite is just not used anymore, and any legacy scopes like that might be very tightly held by owners. There are modern alternatives.
I have a very nice iStar 127mm, f12, which is Doublet using what they call R30 glass, branded as an Anastigmatic, is promises better colour rendition than traditional Doublets with usual Crown and Flint objectives. Effectively, my R30 f12 delivers colour correction comparable to f15 refractors.
See this chart for information on how longer focal lengths deliver colour correction:

https://www.cityastronomy.com/CA-ratio-chart-achro.jpg

This article explains things pretty well:

https://www.cityastronomy.com/color-correction-refractor.htm

[LewisM]

Yes, there are alternatives, but fluorite is most assuredly making a resurgence. Takahashi is going back to predominately fluorite in their refractor lineup, as are TEC and Agema and some others as glasses like FPL53 become either hard to obtain or expensive, and fluorite manufacturing simplified and not as expensive as it was (plus fluorite's AR coating issues were resolved many years ago).


I only have FPL53 scopes presently, but the image presented by fluorite is more visually appealing to most observers (warmer tones than the clinical colours in FPL). Additionally, FPL glasses DO contain fluorite - hence the F in FPL. These are fluorite enriched glasses.

[OneCosmos]

Thanks Glen,

Whilst having my coffee, dog patiently at my side, I’m browsing about this topic and whilst long ‘fracs have their advocates, there are more than a few folk who say there is virtually nothing to compare between the views through a classic f/15 and a Skywatcher ED equivalent size. The latter is easier to mount and probably way cheaper.

That isn’t the whole story of course but it would be foolish to not recognise just what good value SW refractors are these days. Pride of ownership is something else again. Being proud of a ubiquitous SW is less likely than a rare Unitron et al.

PS I suppose what I’m saying is classic long focal length refractors are great value compared to exorbitant APO triplets from the likes of APM, Astrophysics, Tak etc but not when compared to an SW ED. If long fl is better however, then that’s another story. For example achieving higher magnification with a larger EP is one reason to prefer long FL.

[glend]

Chris, you make some good points. In the SW ED range, the largest available is the 150mm f8 ED, currently selling for $2999 @ Andrews. Still is it really a long refractors in the classic sense, being an f8? It has come into some criticism because of SW's refusal to discuss the glass used in the Doublet, it's reputation for blue tinge. Early SW ED models, up to the 120mm, did use an FPL element I believe, but SW does not advertise them like that anymore. It could be argued, and has on the CN forum, that SW wanted to move to a lower cost objective, lessening the imaging community interest (after all they can push the Esprit as the scope for imagers.)
Looking at the economics, you can go to larger diametre Doublets, but many of the big objectives are expensive. Building your own long refractors is possible, as I did with my iStar. And iStar still builds good Stehl Doublets, but there is a cost.
I noticed that iStar are now offering Flourite APO objectives, but the cost for an 8" f7 is $11k USD. You can get an 8.7" f15 R35 for $4800 USD. The bargain of the bunch is imho, the 150mm f15 Achro Doublet at $700 USD, with a Strehl ratio of 0.95, and focal length of 2250mm - now that is a long refractors.

IStar website here: http://www.istar-optical.com/achromatic.html

Use the top tabs to select which type of objective you want to see, full spec are there for them all.

D&G still sell long objectives as well.

http://www.dgoptical.com/refractor.htm

Their 8" f15 complete scope, seems pretty reasonable at $3700 USD, but the shipping would be a killer, then ad GST, etc, which is why it is so cost effective to build your own, imho.

[OneCosmos]

Hi Glen,

I see iStar no longer make complete telescopes which is a shame because if I bought a lens on its own it would likely stay that way:lol:. I’d need to find someone to build it for me.

Are the D&G good? An 8” f/15 would be a sight to behold at 10ft long! Affordable too.

[glend]

Well D&G have been around a long time and seem to be well regarded on the CN forum. Perhaps asking the question on CN refractor forum might get you a good deal of owner knowledge. Can't hurt to email D&G either. They ship with UPS, who has a good track record delivering scopes to Australia, but I admit I have not shipped in one that long.
You would need to consider a mount with suitable capacity, and the need to get it up high enough for all that focal length. I admit that sometimes using my iStar that i have to sit on the floor of my observatory, so think about the room you need for a beast.

[OneCosmos]

Haha for sure. I owned the APM 152mm LZOZ f/8 Triplet until recently but I sold that and my 16” SDM to fund my 28” Normand Fullum mirror SDM which will be finished within weeks. I started this thread really thinking about small aperture long focal length scopes -like 80mm just for planetary views but tbh the setup time for that would be no less than the SDM. I’ll wait first to see what kind of planetary views I get with the dob but I doubt they’ll match the former APM 152mm for crispness although on nights of good seeing the detail should be present.

[sharpiel]

I think you'll find a divide between the visual users and the AP crowd in this debate. AP is less forgiving in terms of multi focal wavelengths compared to visual. What may be acceptable to a visual observer won't necessarily satisfy an AP user. When it comes to AP money usually buys APO, often high end APO.

[OneCosmos]

Yes, I spent the better part of 20 years as a serious imager, with a high end APO, but in contrast with the majority I moved from imaging to visual, which I absolutely love. The advantage of visual may we’ll be that we can use larger, slower and thus cheaper lenses and still operate at optimal performance.

I’ll either end up with nothing and savour my upcoming 28” dob, or invest in a ‘rare’ classic or buy a large D&G achromat

Chris

[Camelopardalis]

Regarding planetary through a small but long refractor...I think you’d find the view pleasing but lacking detail.

Aperture is king when it comes to resolving finer details on the planetary surface. And experiencing the jet stream above SE QLD :shrug:

[anj026]

I enjoy using two long focal length refractors. They are both home made using lenses from D&G. One is 6" f15 and the other is 5" f15. They are very sharp and a lot of fun to use though they both show some chromatic aberration. They are very high contrast and perform very well for their aperture. There is a nostalgic charm to these old style refractors that I enjoy. They are simple and need almost no maintenance just reasonable care.

I would expect equivalent aperture f8 fluorite doublets to perform better with reduced chromatic aberration and much easier handling with the reduced length. They would also be much more expensive.

Sadly D&G have not been making lenses or telescopes for amateurs for a couple of years now despite their website still showing open for business. As far as I know they have been busy with other contract work. I think they will still fit in repair and restoration of historical lenses where possible.

Here is my review of the 6" from many years ago.
http://www.iceinspace.com.au/41-340-0-0-1-0.html

[Slawomir]

That's interesting, because recently I have made a decision to do the same. I started this hobby nearly 10 years ago and jumped straight into astrophotography, skipping visual altogether.

Now, rediscovering the hobby as a purely visual amateur astronomer, and loving it.

I'm currently quite happily using an opposite to a very long focal length refractor, a leftover from my passion for imaging, but I would surely love to try one day something more tuned towards visual astronomy. Not sure though if I would ever dare to acquire a very long focal length refractor, purely because of logistics and ease of use. After all the complexity with setting up for astroimaging, I am all for maximum ease of use and simplicity.

[glend]

I too spent years imaging, before switching back to pure visual astronomy. I was happy to step back from the large amount of time required for serious imaging, equipment, processing, etc and just enjoy the vista of the night sky again; a heart attack was a significant push in the direction of a less stressful life.

[Merlin66]

Remember almost all the Astronomical discoveries made prior to 1900 were made using long focal length achromats (yes I know about the various reflectors which were also around....)
I don’t think they worried too much about chromatic aberrations, they just got on with the job.

[OneCosmos]

Shortly after the renaissance of my visual astronomy the lunar occultation of Saturn took place (2019). At that time I posted a somewhat lyrical account of observing that event through the refractor. For some reason though I posted on Facebook rather than in the visual observation forum on this site. It would have been more apposite here though Nd many who would appreciate it probably never saw it.

So, perhaps slightly off my own topic I’ll reproduce it below as it gives a good flavour of why I prefer visual astronomy.


As most of my astronomical friends know, I have this year decided to focus entirely on visual astronomy and forgo the camera that has been permanently attached to my refractor for the past 13 years. Although you have nothing to show afterwards to fb friends and others, visual astronomy is intrinsically more visceral and of the moment and no event more so than last night’s majestic lunar occultation of the beautiful planet Saturn.

In the absence of a camera capturing the event the only personal record of what unfolded on a cold winter’s night in Brisbane available to me is the written word.

In an age where a picture paints a thousand words and a meme can replace an essay, how is it possible to capture the sheer majesty of the event in words? Well I believe you can and even if you can’t the thrilling awe of watching it happen before you leaves a memory etched into your brain long after the photos have had their moment of glory.

As work finished for the day I hurriedly negotiated the streets of Brisbane, bristling with other souls going about their business, each lost in personal events I can know nothing about; my aim was simple, get home as quickly as possible to set up the telescope because one of the greatest shows of the solar system was to commence at around 18:30 -not much time to be ready. The choice of ‘scope was easy, as wonderful as large dobsonians can be the 6” refractor was clearly the right choice because nothing, and I mean nothing is so capable of yielding the crisp, sharp perfection that high quality refractors are rightfully renowned for. The Newtonian design does a great job of corralling vast numbers of photons but too many of them are recalcitrant and escape the path intended for them and the result can be a softness of focus that leaves you frustrated and disappointed. Not so with the refractor. When made well none of the photons escape the telescope’s focus resulting in sharp edges and perfectly focused detail.

We take the mechanics of the movements of the moon and planets very much for granted today and yet men and women grappled for hundreds and even thousands of years to explain, mathematically, why the ‘wanderers’ apparent motion seems so arbitrary and independent. Our forefathers were actually far more attuned and accustomed to the importance of their movements than we are today.

Sat there, in my garden, I carefully selected my choice of eyepieces to place in the binoviewers and could immediately see the beautiful, lonely image of Saturn, surely one of the most awe-inspiring sights available to us from here on Earth. A whole world encapsulated in that small, but perfectly sharply focused gathering of photons, it’s importance only exaggerated by its proximity to the terminator of the moon -that part that separates night from day on our natural satellite. The terminator throws all the craters and valleys in to sharp relief as the sun’s rays dance between the crenellations.

Before my eyes the moon in its relentless orbit gradually swallowed the great planet. Unlike Earth, there is no atmosphere so you don’t see a gradual dulling of the light of the planet, it is more instant and shocking. Only during an occultation or an eclipse are you really confronted with the reality of the mechanics of the solar system. Normally you know it is moving but it is like watching the hour hand of a clock. During an occultation, for a moment, it’s movement becomes real, we can at last truly experience the orbit rather than simply understand it.

To witness such an event sends a tingle down your spine, you know you are watching the laws of physics and motion made real.

I urge everyone to step outside and take in the night sky and ponder the majesty that awaits you.

[Tropo-Bob]

Chris,

Grab yourself one of these: 4" ED F11 refractor. I enjoy using mine and it was cheaper and probably better than the heritage F15 scopes. I ordered mime direct from Germany. It was here in 7 working days and cost around $1200 including freight.

[glend]

Yes TS has some great bargains in long Achro refractors, even some of their ED scopes are great value. The AUD seems to have improved against the Euro as well.

[Fox]

OneCosmos (Chris), thank you for such a wonderful story, it’s reminiscent of Terence Dickinson’s “Adventures in Refractorland”.

...my fluorite Genesis is now on the comeback...? does this mean flair jeans when using it as well? Fox!

[jahnpahwa]

This is so inspiring (and nicely written!) :) Can't wait for more clear nights at the eyepiece.
Thanks Chris.

[OneCosmos]

This is from Telescope Service, Germany?

[glend]

Teleskop-Exoress technically, but the brand is TS:



https://www.teleskop-express.de/shop/product_info.php/info/p11262_TS-Optics-102-mm-f-11-ED-Refractor-with-2-5--RAP-Focuser.html

[Camelopardalis]

Wouldn’t mind one of those myself, but how many 4” scopes can a man need? :lol:

[sharpiel]

The number you have plus 1...

[Rainmaker]

I've been using the Skylight AR101 F15 for some planet gazing recently.
It has a Televue 101mm F15 doublet objective and is giving excellent planetary views, not in the LZOS class but very nice nonetheless.

[Slawomir]

A great story and a nice write up Chris, thank you for sharing :thumbsup:

[Tropo-Bob]

Mine was from APM, however, I did not know of Telescope Service at the time, so have never compared prices.

https://www.apm-telescopes.de/en/telescopes/apochromatic-refraktors/ts-optics.html

[OneCosmos]

How about I take that LZOZ off your hands?🤣

[OneCosmos]

I just bought a ladder for the SDM that cost almost as much as this 4” refractor 🤣. How severe is the CA? Do you always know you aren’t looking at the optics money can buy?

[glend]

That is a dilemma Dunk. I considered that TS ED scope as well, and it seems a good price. However, my 127mm f12 iStar R30 sort of trumps it in my opinion, at 1524mm fl. Frankly i would jump on a D&G long Achro refractor if they were still making them but not sure I have the swing room required in the observatory.

[Tropo-Bob]

I have still not worked out how good this scope is. I was comparing it to my Tak 100 fluorite on Jupiter and the Tak was winning easily until I used a Brandon EP and suddenly this scope was showing almost as much. Paired with Brandon EPs, the scope does very well on globulars and colourful doubles; maybe even better than the Tak. I have not seen any CA with this F11 scope.

My new, light proof fence is in the background. (Overlapping boards will keep it lightproof even when the boards shrink and it also reduces outside noise.)