R35 optics?

[nobody]

Hello

While searching online for a suitable achromat, for a new ATM project, I discovered iStar offers R35 lenses.
By now I must have read everything ever written online about iStar in every conceivable language.
So far I have still found no independent confirmation that the R30/35/50 designation offers better colour correction than a standard achromat.

Individual observers have such varying false colour tolerance that reviews must always be read with considerable caution.
My own Celestron/ Synta 6" F8 seems to vary in its violet haloes depending on my mood, the object and the seeing.

I have other, smaller refractors, so know enough about false colour to know what to expect from different CA ratios.

The claimed advantages for Rx optics are obvious:
A much shorter and lighter OTA is more easily stored, handled and is very much easier to mount equatorially.
This can really make or break the point of ever making a purchase in some cases.
A longer OTA has considerably greater moment and will usually disappoint on an inadequate mounting.

The [claimed] R35 colour correction is not entirely a free lunch though.
One pays a premium for the R35 optics by having to go down one lens size for much the same price as the achromat size one above.
So you lose the next larger achromat: Which offers greater light gathering power, resolution and potential power. [Plus bragging rights!]
All at the cost of a poorer CA ratio and much greater weight and bulk of the bigger lens once mounted in its OTA.

Do not read this post as remotely a criticism of iStar, its claims or any of its products.
I just want confirmation [encouragement to purchase?] that choosing the next smaller lens in R35 [rather than the next bigger achromat] will be a wise choice.
 
Do you own a more normal focal ratio R30/35/50 lens? I'm not talking about F5 or F6 in huge apertures.
What about 6-8" R30/35 lenses in f10-f12?
Is the R35 premium worth the investment at the cost of greater aperture in a normal achromat?

Thanks for your patience


BTW: I couldn't get the private email Contact service to Ales or Mike, on the iStar website, to work.
The addresses are HTML so cannot be used for normal emails.
Hence this rather public appeal for experienced advice.
Feel free to delete this post and go to private mail.

[Ales - iStar Optical]

Im happy to see another person with an agenda and time and motivation to discuss it here on ISC!
First of all, my business address is [e-mail] and contact for Mike Harden is [e-mail] There is a security reason for not posting our direct addresses on our website. Sorry for the inconvenience everyone!

Well, there are several people who own similar size and FL achromat and tested their R35 or R50 against them. The prime example is the huge 250 F11 R30 purchased by about 10 astronomers during past 3 years, only one of these happy owners posted the actual results publically. His name is Mike Carman and look for his posts. Mike is a very experienced observer and he and few others confirmed that Rx lenses do indeed offer a substantially better false color correction compared to a standard achromatic doublet.
To further support our claims, I posted spot diagrams of standard achromats and R35 doublets side by side on several forums including this one. Spot diagrams don't lie, they show exactly what to expect from the lens quality-wise and CA control-wise.
Rx lenses use heavy flint which most people call ED glass. So one would expect a better color correction and higher resolution based on just this fact alone.

In addition to the Rx line, we are now preparing 160 F8 and 180 F9 Semi APOs. Technically, these work at around R80 level, but I decided to market these as Semi APOs simply because thats what they are.

In your other post you mentioned 180mm refractor to be the ideal size scope. I fully agree with you. This is why one of these new 180 F9 Semi APOs or 180 F11 APOs will be my new scope of choice for my own home.

I tend to disagree with your comments about 204mm refractors being too heavy and too bulky. Just take a look at our newest, completely redesigned (to the last bolt) Phoenix WXT 204-6LT. This scope weights less than some of the first iStar 150 F12 OTAs. The scope is very compact and even me with busted shoulder can lift this 13Kilo OTA onto my mount. Optically, this scope offers best CA correction in its FL and aperture class simply because we designed this doublet using a special flint. We sold around 20 lenses and OTAs during past year and so far all customers were extremely happy with quality and performance of this refractor.
I do agree that 8" refractors from other manufacturers are much heavier and anything above F8 is hard to transport and mount. This is the reason Im seriously considering development of several new models in Semi APO and R50 range in 204 and 228mm clear aperture sizes, with main objective to keep the overall size and weight at easily manageable level. Will see how things go with these in 2016 and 2017.

Thank you again for stirring up these usually calm waters of ISC, keep writing, and everyone else please keep responding and coming up with your own questions and comments. I will gladly answer every time I have a little free time,

regards,

Ales

[nobody]

Hi Ales,

Many thanks for your very full and frank response to my queries regarding Rx optics.
It is rare to have such a direct response from a company owner.

This is the very first time I have heard anything about unusual flints in the context of your Rx products.
Your skilled optician seems to be obtaining increasingly better CA correction.
I wrote my own ray-trace software and played with interesting glass combinations well over 20 years ago.
Your latest Rx prescriptions have focal lengths and apertures which should [theoretically] have awful CA-ratios by traditional optical standards.
With
reasonable commercial luck for your company, we may all look forward to
affordable, yet still easily manageable, refractor optics in quite spectacular apertures!

With so many lenses to choose from my true "agenda" was simply to improve my chances of finding the correct lens for me.
I wish I were slightly nearer to making a choice from all those available.
I demand the highest optical quality, unblemished QC, APO-like colour correction, an easily manageable OTA but I want it almost free of charge.

Having jumped on the Synta optical "revolution" myself I can confirm there were no magical glass in their popular 6" F:8s.
They produced the violet haloes one would normally expect from conventional achromats but were still usable with minus-violet filters.
Though the results were somewhat variable between examples the average quality was just high enough for their purpose.

My problem is that having a 6" refractor already seriously affects my eventual choice of lens.
My desire now is to increase the visibility of fine planetary and lunar detail without the need for a massive OTA.

The refractor is a very forgiving instrument for those of us who cannot leave the instrument set up permanently.
My natural conservatism, aided by a chronic case of cynicism, suggests I should always [always!] avoid short focal lengths.
This may be your real problem in popularising well corrected but short focus Rx optics. 

Early adopters, with the funds to risk on such purchases, are probably your best salesmen.
Word of mouth has always been the best advertising a company never has to pay for up front.
It just needs unquestionable product quality and the inevitable costs of absolute quality control from example to example.

To be truly convincing salesmen the early pioneers must avoid the fan-boy mentality.
These days they must share high quality astro images to convince even [old] cynics like me.
Fortunately you have some highly skilled imagers who can take our breath away.
And, doing so with optics which "shouldn't even work" at their huge apertures and "frightening" focal ratios.

I know I can go out and buy a traditional long lens, by a reputable maker and can [almost] guarantee the results I expect.
iStar manufactures and sells such achromats and they have a fine reputation.
The trouble is that old cynics like me can no longer manage the huge lifts involved in setting up a classical refractor each time.
The shorter Rx instruments have so many immediate and obvious advantages for us.
But, people like me are terribly afraid the optical correction claims are all just more commercial hype. [There's a lot of it about!]
We have been brought up on Sidgwick and Conrady and CA ratios >3 and the rigidity of thought that f:15+ is the only true way.
How do you convince me that I can fulfil all my optical desires with a telescope not much different in size from my Synta F:8 "Kaleidioscope? That 180/8 looks so tempting I can almost see those little craters in Plato from here.
Conrady must be spinning in his grave!
 

[boomboom]

Hi nobody,

Interesting name there...I went from a 6" f8 Synta, which according to more than a few was a very good one, to an Istar 6" f12 achro. A world of difference in the optics. The Istar made the Synta look like rubbish, which it wasn't. I kept the 6" f8 for more than a year but never looked through it again. It's a shame that you don't/can't go longer with the weight. I got around that by doing an ATM on my scope. I managed to knock 3kgs of the standard Istar f12 tube. I figure that my f12 is roughly equal to an f8 r35?

My ideal refractor is a 204 f12 r35...although a 180 semi apo sounds very, very, very tempting. Currently building the mount to take it.

Matt

[nobody]

Hi Matt

Thanks for an interesting comparison between the Synta f8 and 6" f12.
6" @ f12 is a nicely corrected lens, as is, but with the option of a minus violet filter when desired for [over?] critical users.
One would expect a considerable reduction in violet between the two lenses.
Smoothness of polish and a decent optical figure on the iStar lens should certainly aid image quality compared with Synta's earlier offerings.
Your analysis of Rx optics is pretty close. The 35% difference changes the f8 into an F11 colour correction wise.

6 x 8 = 48" x .35 = 16.8 + 48 = 64.8/6 = ~f11.
iStar may use a different method of Rx calculation but this simple way works for me.

So your 6" f12 is actually slightly better than the f8 R35 but quite close to it in a shorter package.
The CA ratio changes from a "just bearable" 1.3 @ f8 to a better 1.83 at f11.
Your true f12 has a solid 2 CA-ratio.
CA ratio = lens focal ratio divided by the aperture.
The downside for a true f12 is the need for a better mounting to handle the increased moment arm.
The R35 gives you the colour correction equivalent of a longer focal length without the extra weight, length and bulk.

Inspired by the TCR design I have been experimenting with hollow alloy beams and spars to make a much lighter OTA using Birch play rings/baffles.

I agree on the desirability of the 8" @ f12 R35
CA ratio @ f12 = 1.5. But as R35 = an f16 equivalent with a CA ratio of a solid 2.
The same correction as your 6" f12 in an 8" lens! Nice!

Istar doesn't offer an astigmat at f12. They have settled on f8.
8 x 8 = 64 x .35 = 22.4 + 64 = 86.4/8 = f:11 equivalent with a CA ratio of 1.37.
About the same colour correction as a 6" f8. I'd much prefer an f10 minimum using R35.

The 180mm [7"]0 R35 @ f12 is the equivalent of an f16 with really excellent colour correction in this aperture.
Again, Istar has dropped the longer focus lenses in favour of f8 in the R35 range.
They obviously know their market much better than I do.

[mikey cee]

Welcome aboard Nobody. Well I'm the first guy with the big Istar 10" F/11 R30 lens as I commissioned Ales and optician Zdenek to custom make the very first one! I had at that time a Brandt 8" F/13.3 lens custom made way back in 1979 for a sale price of $995. Can anyone today fathom that amount?? So I had so many years under my belt with that lens that the views were etched into my head especially Jupiter and the Moon. So I was very very aware of the CA "intensity" if you will. Then the opportunity came along to upgrade to a larger scope thanks to Istar ads where I read that I could have a custom lens made to my specs as far as the focal length was concerned. This I nneded due to the fact that I was going to incorporate it into my existing OTA. Ales recommended the ED flint instead of the traditional F2 glass. It would be much heavier so more rear tube weights were needed but luckily I had a huge custom pillow block mount with 3" shafts that could handle all that extra load. Well keeping in mind the level of CA with an 8"/ F13.3 vs. a 10" F/13.3 you'd know going out the gate there would be way more CA period.....duh a no brainer. But this new lens was to be a shorter F/11 to mirror the needed focal length for my existing OTA. Guess what on first light? I was awed by the fact that the CA was no more pronounced than with my 8" F/13.3. Hard to believe but true. Jupiter showed only a slight difference in residual color owing to the different color tweaking formulae used in the R30 process....more of a "magenta" hue verses "violet" as with the Brandt. So to say I was tickled "pink" is an understatement. Recently I acquired a Chromacor N. Thanks to the outstanding figure of this lens coupled with the Chromacor N I'd have no qualms with putting it up against a 10" apo. I'd bet it would take a trained eye to see much if any differences.....especially for the difference in costs!! So obviously my advice is to go ahead and make your purchase. If you aren't satisfied Ales will refund your money..... I think. Mike

[nobody]

Hi Mike & thanks for the warm welcome. 

So, you see pink but everybody else sees green? [with envy]

Your description of the matching/reduced colour correction is fascinating.

The eye is arguably more sensitive to magenta than it is to violet.

Your original Brandt lens = 8" @ F/13.3 = 106" FL which gives a CA-Ratio of 1.66.

That's equivalent to a normal 6" achromat @ F:10 for those who measure these things in affordable sizes. 

BTW:Your Brandt lens would cost you $3270 today given 1979-2015 US inflation rates.

www.usinflationcalculator.com/

Let's assume a straight R30 colour reduction for your big iStar:

Your iStar is a 10" @ F/11 = 110"  FL.  110 x 1.3 = 143" [new] FL equivalent for R30 colour correction.

F/14.3 / 10 = 1.43 equivalent CA-Ratio.

Which would closely match a 8" achromat @ F/11.5. Or a 6" @ F/8.5.

Can we assume that iStar greatly exceeded their promised Rx correction to well beyond R30? 

Assuming an exact subjective CA match with the Brandt lens you might well be looking at R50!  

Or... silly question alert: Did you open up the baffles, or move them backwards, to suit the larger aperture?

That might explain the dramatically improved CA. 

Only joking, of course... you did remember to move the baffles back...  didn't you?
  

[mikey cee]

Yes I added one baffle to the front and moved the other three rearward plus one inside the front end of the focuser. Ales has told me in the past that it actually tested out at R35 instead of R30 but labeled it as such due to the variances in the R30 batches while guaranteed an R30 minimum. Yeah I made a mistake earlier. Brandt's lenses were $995 and went on sale for $695. Mike

Attachments:

[mikey cee]

In this pic I've added another 8x50 finder at 180° to the initial one. I've ditched the filter wheel and now only use a pair of Baader Mark III zooms with a Baader 2" prism diagonal. Notice the large grip 4" handles for sluing and to rotate the focuser through 12 click stop settings or anywhere in between. That way all bases are covered plus I don't have to loosen any attachments in order to rotate for my viewing angle. Also took advantage of Ed Byer's closeout sale and installed a 10" drive.....beeeeeeautiful!!

Attachments:

[nobody]

Hi Mike

Thanks. An absolutely gorgeous instrument! Congratulations!
Your 10" refractor eclipses many of the instruments dotted around the world's historical observatories.
Your pillow block mounting looks sturdy enough for the task and certainly looks the part.
The whole thing has wonderfully timeless appearance, in keeping with historical examples.
Yet does not ignore the march of progress in both metal and optical technology.
Thanks for sharing your experience and for the superb images.
Nothing is remotely as impressive as a large, classical refractor.
Nor as demanding of adequate support.

You don't show a stepladder in your images of your observatory.
Even my humble 6" f/8 needs one to observe below about 35 degrees.
If my pier were any lower [about 6'] I'd have to lie on my back when viewing at the zenith.
[Instead of which I can sit comfortably.]
Your huge variations in eyepiece height must make mine seem paltry.
Were you never tempted to fold your refractor?

[mikey cee]

No Nobody the thought never appealed to me even with Brandt's literature on doing so. Yes I have an observing "throne" and a step ladder. I'm not completely out of the loop...yet anyways. Mike

[nobody]

Hi Mike

You have the perfect set-up and real observing comfort too.
The slide-in, adjustable seat height is so simple yet practical.
I have just discovered your build thread on here with lots more great pictures.
Truly impressive in both mechanical detail and utterly amazing optical performance!

However tempting it might be to follow your lead I do have to be practical.
I still have to carry anything I do build out to the mounting on its 6' high massive, welded steel pier.
So, in a badly needed bout of self-discipline I balanced a 7' long x 4" plank in the mounting rings.
Just to get a feel for the likely scale of my planned OTA.
I'm certainly glad I did because it really pushes the envelope of acceptable eyepiece height at the zenith.
This is despite the 7' height of the mounting rings with the declination axis horizontal.

Quite an eye opener, even having tried my 5" f/15 on the same mounting.
I used a galvanised steel, spiral wound, ventilation pipe for that.
Not very pretty but really quite practical on both stiffness and weight.
I lined the deliberately oversized rings with closed cell foam strips cut from a gardener's kneeling pad.
This gave me plenty of grip on the tube and easily absorbed the spiral seam.
Highly recommended for those who place cheapness and practicality above cosmetic appearance.
This tubing is both cheap and easily available for those who want to check the likely scale of any planned OTA.

What my "walking the plank" exercise taught me is that I can't manage more than an equally balanced 7' long OTA.
After that things really do get a bit silly!
[This ignores the dewshield length as it is weightless with no effect on balance.]
A valuable lesson learned before raging aperture fever took too firm a hold!

This is not the place to go into great depth but the Rx optics really do seem to offer me a way out.
I can get more aperture than my 6" f:8 but still have the colour correction of a classical refractor!
I wanted to believe that the love for Rx was just manufacturer's hype.
Having gone back through this forum and read online reviews it really does seem to work.



My 5" F/15 tube on my vintage mounting is actually a foot shorter than my planned iStar OTA.

[dkahraman]

Ales:

I just purchased my conventional achromat 220mm f/15. It is on its way to Parallax Instruments where the OTA will be built around it. I also have a 12.5" f/8 Newtonian tube assembly with a very small secondary and a low profile focuser. I plan on having lots of fun comparing the two on the planets. I am sure the 8.7" IStar will not disappoint but will it beat the 12.5"f/8 Newtonian?

It is now 2021 going on 2022 where is IStar in the development and marketing of the Rx line of doublets?

Dan Kahraman

Hi Ales:

I read that the price of Lanthanum glass had gone up 4 fold thus affecting the roll-out of these "denser" flint glasses.

My apologies for not being aware of this development since your post on the anticipated time-line in the roll-out of these special achromats.


Dan Kahraman