New Scope CFF 105 F6 with .993 strehl!

[gregbradley]

I just received my CFF 105mm F6 APO oiled triplet with FPL55 glass.
It has a 3.2 inch Feathertouch rotating Focuser with microfocuser.

The most amazing part is the test report on the optics. .993 strehl! I have never heard of a strehl that high before in a scope. I am wondering if its in part a result of using the FPL55 glass which is the same performance as FPL53 but better polishing qualities.

The scope is super well made with flocked interior of the dewshield and a nice sliding and locking dewshield, pearly white paint and the scope comes with a handle, rings and dovetail plate plus a mounting point for the finder scope.

I got a field flattener designed to cover a 16803 sensor. I only now need an adapter from the flattener to the FLI cameras. Later I'll get a Starlight Feather touch focuser electronic focus unit.

It comes with a soft case. I used to have a TEC110 F5.6 fluorite and it had a TEC made lightweight focuser which was too light weight for FLI cams. I think this scope will surpass the TEC, time will tell. Its an alternative to FSQ scopes (I have had an FSQ106N and an FSQ106EDX111 both good but I wanted something different).

Photos later.

First light is likely to be using a full frame mirrorless if I can find some attaching adapters.

Very excited.

Greg.

[bigjoe]

Pics soon? Phenomenal strehl
Bigjoe

[Slawomir]

Fantastic news Greg - congratulations!

My unit of 105 has only 0.990 Strehl in green - I must have gotten a substandard unit :lol:

Looking forward to reading about your impressions with the scope in the field. Are you going to look at the stars through the glass before attaching a camera?

Echoing Joe, pics would be nice :thumbsup:

[Decimus]

Hi Greg,
Congrats on your new scope - CFF was the other brand I was considering when I bought the APM 152/1200 F7.9 LZOS which also has a certificated massive Strehl (I am sure it's the same, from memory). CFF scopes look great.

Just one question: With a CCD like the FLI 16803, how can your scope produce an image circle sufficient to cover the big square sensor? (Or do I have my facts wrong here? ) I bought a massive, dedicated WO Reducer /Flattener for my WO Gt 102, (though why did I buy a reducer when it's f6.9 anyway?)and have been informed that I can't use full frame sensor cameras with it as I will get vignetting (caused by the reducer rather than the flattener optics). Is this likely? No wonder I am still undecided about a OSC camera (goodbye QHY367C?)Maybe an APSC sensor only.

Wishing you many happy hours and clear skies with your new high quality scope!

Cheers,
Richard

[Ukastronomer]

Unfortunately we don't all live in a laboratory and in the real world no one ever would see any difference between 53 and 55 glass, Im happy with my humble Skywatcher Strehl reading 94%

I didn't go for oil spaced due to the problems with solar observing and imaging


NICE scope though :) enjoy it


.

[Slawomir]

I have the same scope (but not such large CCD), and the corrector’s lens has 80mm diameter, so vignetting should be correctable with flats I think. WO 68III flattener has 50mm lens. Greg will let us know anyway when he gets a chance to put his new baby through it’s paces.

[gregbradley]

The only reducer I have seen work on a 16803 without aberrations in the corners is the AstroPhysics TCC which is an expensive accessory.

This is a flattener. Time will tell if it covers the 16803 but I believe it was specified to just make the circle of the 16803.

The large Riccardi reducer I believe has a large enough corrected circle for a full frame sensor.

Flatteners are easier to make it seems than reducers.

Greg.

[Peter Ward]

Congrats. Looks to be a superb instrument. :thumbsup:

[Lognic04]

I'm sure it's a lovely scope, but I have to say that test report is pretty strange. "Mirror roc" is 630mm - firstly it's not a mirror :lol: And secondly, the ROC is twice the focal length in any case. This must be misprinted? The P-V surface error is supposedly 1/8 wave but the strehl is 0.993? VERY optimistic prediction. And the actual surface doesn't lie either, with there being a pretty big turned down edge. I'd be interested to see an actual star test though! Congrats on your purchase!

[Slawomir]

Well spotted Logan with the mirror. However, I would argue that for a lens made of glass with refractive index of 1.5, the RoC = exactly focal length - it is not a mirror after all. Also, if we use the stated rms to calculate Strehl with well known formula, we will arrive at about 0.993. PV error of 1/8.8 is pretty good for a lens I would say. Not sure where do you see big turned down edge either :question:

[Wavytone]

Good point Logan, and it has me scratching my head as to what was actually measured/calculated.

P-V 1/8.8 does not correlate with Strehl 0.993, no way, something wrong there IMHO. Also “surface error” is meaningless (a refractor objective has 4, 6 or more surfaces) - wavefront is what it’s really measuring - which suggests the software was written for testing mirrors - in which case the wavefront error is exactly double the “surface error” plotted on the graph at the bottom. If that’s the case the P-V wavefront error is more like 1/4 and RNS 1/10, which are still both entirely respectable.

Conversely when testing lenses the refractive index matters when calculating surface errors ... but then the P-V value is way off for the contours shown.

The contours do suggest the outer 3mm has a turned edge, but whether its worth masking off is another matter (its minor and I would not - aperture counts).

In any case it appears quite good if the measurements are to be believed, and for imaging, anything above 0.93 I'd defy anyone to tell the difference because so much depends on the seeing on the night and other factors such as flatteners, reducers and filters all of which may not be so good - and all of which contribute to the end result at the focal plane. It all comes down to the end result.

Visually you might - and only maybe - spot a difference visually side-by-side with say an AP130GTX on a night with 10/10 seeing.

[Slawomir]

Nick - there are only two surfaces to consider in an oil-spaces triplet ;)

Did you even read Greg’s opening post?

[LewisM]

There has been an ongoing war between German optician Rohr and Catalin of CFF for many years. Owners have sent Rohr their CFF optics and Rohr test results are not agreeable to those CFF releases. Rohr gives results in full-spectrum poly-Strehl, whereas CFF only gives out green Strehl - and they are QUITE different. The war still continues...

[Slawomir]

Both Roland (https://groups.io/g/tec-scopes/message/17435) and Yuri (https://groups.io/g/tec-scopes/message/17415) wrote that Rohr’s methodology is flawed and his knowledge lacking.

[Atmos]

And the war continues :lol:

Regardless of the report I’m sure you’ll have a very nice little refractor there Greg :)
I’m interested in seeing how it’ll tackle a 16803.

[multiweb]

Enjoy the new toy. :thumbsup:

[Peter Ward]

I have in my possession a lovely hand-written letter from Roland Christen regarding optical testing and quality.

BTW this is not intended to rain on Greg's parade, as I am sure the CFF scope will serve him well.... this is just to give some background information on optical testing.

AP typically deliver PV 1/10th to 1/16th wave optics. Most of this comes from a slight lack of homogeneity in the glass itself. Their Strehl cut-off of 98.4% or better....however...Strehl can be a little useless without enough data points.

AP use a super-smooth polishing technique to get the RMS figure down to 1/50th of a wave (or better!) , plus sample hundreds of data points in 5-8 orientations to produce a Strehl number they can rely upon.

As to what CFF do, I cannot say. 1/8th wave does seem a little high for such a superb Strehl number, but I am confident Greg's considerable experience with quite a few top-shelf telescopes, will quickly reveal the CFF's quality is real (or not) within a few imaging sessions.

[gregbradley]

Test reports can easily lead to disagreement with methods etc. I was not buying it on the basis of a Strehl ratio. I was buying it because Suavi's images are super sharp and show an exceptional level of detail that even a larger high end telescope does not show much more at all. I judge the results not the test reports. They should match up. Suavi's images match up with the results I would expect from a top end AstroPhysics scope. Also these CFF scopes have a 1 year waiting list which tells you something about their quality as well.
I don't know that AP are making anymore APOs apart from the 92mm F6. I wouldn't bank on it. Roland stopped making the RHA astrograph I believe.

TEC also judge strehl from the green. That's for visual people. Imagers are probably more interested in the blue and the red where false colour lives. I was surprised to see the TEC180FL was weak in the red for example although I can't say I noticed that when imaging with it. But I did notice Tak TOA150 images always seemed to get a tad more detail from the same scenes. Its a shame Tak does not make a TOA 110. They probably don't want to overlap against the FSQ which is there best seller.

The alternative scope was a Tak FSQ106EDX iv. They are in short supply and are a touch more expensive. I have had an FSQ106N and an 106 EDX iii before with the reducer and its an amazing scope but just wanted something different. Mainly to replace the lovely TEC110 F5.6 fluorite oil spaced triplet.

The main issue with that scope was the lightweight focuser, it really needed a heavy duty Feathertouch. What I liked about the TEC over the FSQ was the better colour transmission of the fluorite lens. The Tak is not as "colourful". I mentioned this to someone once and he tested his as I thought perhaps the shift to a different type of lens coating created a slight colour bias in the FSQ106ED series. His testing led to the conclusion the black internal tube paint on the FSQ does create a slight green bias which is what I was seeing in a lot of FSQ106ED images. I haven't noticed it in any of Mike's images so I wonder if Tak adjusted their coatings or internal paint or not. The FSQ has the large 88mm corrected imaging circle, has a wonderful flat field. Its weak points mechanically are the poor focuser, the poor focuser lock, the weak microfocuser, the focus shift with temperature shifts due to the large air space. Nothing is perfect and the imaging capabilities are the main thing, the focuser issues perhaps are now fixed - not sure, hence the EDX iv the earlier 3 had issues with heavy imaging trains although the EDX iii I had was perfect.

There are many nice FPL53 triplet scopes offered these days - a lot are reasonably priced compared to years ago when I was first getting high end scopes.
But as usual you pay a lot more for that last 10% of performance. Its the same with cars and everything else.

So I will be comparing it to the 2 FSQ's I have had, the TEC110 F5.6 fluorite triplet, the TEC 180FL and an AP140 F7.5.

Also I bet if I asked Roland Christen, an AP quad TCC could probably work on this scope or perhaps his flattener for the Traveller if they still make them. The alternative is the Riccardi reducer.

Greg

[Lognic04]

sigh...Obvious marketing jargon from AP - people love the term "super smooth polishing", look at zambuto for example. Once the pitch lap is on and conformed, there is no such thing as "super smooth polishing" :rolleyes: It comes down to technique.



I highly doubt AP stops figuring because of errors in the glass - It is more likely that they recognize that no one will notice or care that the optic is better than 1/10 wave. But to let an optic slip that badly as the wavefront shows... I'm confused. A massive TDE like that is a rookie mistake - if that wasn't there, sure the lens would be great, but that is sure to produce a visible issue at higher powers. As the scope is used for imaging though, it will do the job nicely. :shrug:


That's just my (most likely controversial) opinion, from someone tho has made a few optics and used an interferometer.

[gregbradley]

Not sure what people are referring to when they say the lens has a turned down edge. Is it the graph that shows the line falling heavily just before the edges of the graph?

Visually there is turned down edge. Of course the lens sits in a cell several mm so any far edge is not even exposed to light. So no issue.
It looks no different to any other high scope I have had like Taks, AP and TEC's. In fact it looks superb and the coatings are very even and a lovely bluish colour. Like an AP.

Oil spaced triplets only have 2 air to glass surfaces which is why they do them this way. They also cool down faster and are less prone to focus shift from temp drops as a large air spaced scope does. Its interesting AP have only done an air spaced triplet once that I know of - the AP160 and I have heard it wasn't as good a scope as the AP155 due to these issues of cooldown and temp focus shift etc. The 130GTX has 1 air space so it must give an extra element of improvement to bother.

TEC only make oil spaced triplets.

A turned down edge refers more to mirrors where depending on how the mirror is mounted the mirrored light surface goes all the way to the edge so a turned down edge would require masking as there is no lens cell to cover that edge and it can lead to a weakness in the image.

Anyway all this is hypothetical and the proof of the pudding will be in the imaging. The camera is a known quantity for me.

Greg.

[Peter Ward]

That's just a throwaway line IMHO.

Not everyone uses pitch.

There are "super-smooth" polishing techniques.

Teflon laps are used regularly get surfaces down to 1/100th lambda.

P.S. I was flown up to Narrabri some years ago to cover the opening of The Sydney University Stellar Interferometer (SUSI) for Sky & Space magazine.

Talking to some of the research staff and academics at the time I learned scattering was a problem for the instrument, which was solved in the end by using teflon polishing laps to get the mirrors down to 1/75th lambda or so.

P.P.S Sorry Greg....clearly not relevant to your post. Enjoy the new 'scope!

[Slawomir]

I have been asking the same Greg, as I am curious where do people see it :question:

Honestly, I am quite surprised by the number of people on IIS who are soooo eager to search for flaws instead of celebrating. If I just bought a house, car, boat, scope and a friend would come around just to point at any imperfections (real or imaginary) or give me a lecture about the brand/maker etc instead of sharing the joy as a normal sensible person would, that would probably end the friendship rather quickly :shrug:

[multiweb]

Yeah I don't think you'll have any major suprise and you'll enjoy the scope. CFF seems to have a good track record. :thumbsup:

Interesting discussion on tests and certificates. Posts linked to 'Rolando" by Suavi and Peter comments about rms vs. PV make a lot of sense. You can see how you can easily spin the marketing and show the numbers you want.

@Logan, you should email Roland at AP and tell him he's full of it. Sounds like you're ready to take over the business. :lol:

[Merlin66]

Logan,
The Chronograph solar telescope is one of the most optical demanding systems you can think of!

To reduce the light scatter additional attention was give the the edges of the lenses - to reduce diffraction scattering as well as the quality of the lens polish.
To quote from the forthcoming book " Solar Astronomy":

The objectives of professional coronographs, when it comes to refractors, are therefore made from extremely homogeneous glasses (generally fused silica), without inclusions, streaks or bubbles, and with an extremely fine polish, a high quality finish that was called previously "coronograph quality" and now "super polishing". Note that Bernard Lyot had developed with the optician Maurice Françon (discussed in Chapter 5 on instruments) the method of analyzing defects in the polishing and homogeneity of lenses, now known as "phase contrast" testing.

[LewisM]

Indeed. I agree with Yuri sand Roland. His results are most times missing fundamentally huge gaps.

You’ve been doing beautiful work with the CFF, so here’s to Greg’s success. Now if only he’d post larger images (hint hint) :)

[LewisM]

I want to see a live video stream of Roland $h!tting himself with laughter.

Yuri would just mumble “Nu da. Yasno....”

[LewisM]

Agree- we need to take each sample as it comes. Make our own assessment of the individual sample. Having enough trust in a brand to confidently buy is what drives me personally, on the basis that poor samples likely would not pass QC. It is to that end I vehemently trust Tak, AP, TEC and CFF.

Sure, CFF did have some initial issues (especially with the mirrored optics) but they worked at it and I have read only perhaps 3 negative user reviews (2 with unremediable blue halo /reflections and another with potentially out of collimation objective). Lessons learned, onwards and upwards.

[gregbradley]

I personally don't mind the different viewpoints, no offence taken and its fine that others have a different view.

I'll find out soon enough and the results will be great or not.

Just looking at the scope and a very quick look through it at the bush it looks like the business.

I now just need an adapter from my imaging train to the flattener which may take a couple of weeks!

It might work piggybacking it on the Honders so it takes the colour and the Honders takes the luminance to make image acquisition faster.

Greg.

[LewisM]

And Tak won’t even publish their scope results. There is apparently an in-house minimum standard else it is rejected or refigured. Someone once said that figure was 0.985 Polystrehl.

Didn’t Sauvi have issues with the Riccardi? Or am I mixing that up?

Oh the TEC110 - wish I had bought it from you Greg.

[Lognic04]

Was NOT my intention to bag the scope or manufacturers - i'm sure Greg fill find the scope to be excellent! It's that the numbers don't seem to check out, and the graph surprised me. I thought that a company like AP or CFF would deliver better optics for the price.
Peter, I had not heard of the teflon lap as it is practically never discussed among ATMs, and unfortunately all the results are behind a paywall. I will have to ask on Cloudy Nights - thanks! Thanks Ken as well.

TDE image is linked, see where the edge sharply turns down, in fact it might be more of a rolled down edge as it takes place over a wide area.

[Slawomir]

From the graph - about 18 nm/550nm = about 1/30 of a wavelength - seems small, to me at least ;) Why not notice a more indicative rms wavefront error of 1/73.4 wavelength - seems very smooth, even for AP standards :thumbsup:

[gregbradley]

They are a rare scope and it was a beauty. Even the focuser I could live with the flattener handled the 16803 as well. I've only seen 1 come up for sale on Astromart. It was also a very light scope. I used it to take some bird photos sometimes sitting just on a carbon fibre tripod. This new scope is a bit heavier so I wouldn't try that.

Greg.

[gregbradley]

Thanks for clarifying, I thought that was what you were referring to.
Since that part of the lens is inside the aluminium lens cell and is not part of the light stream then I don't see that as a problem. Perhaps all APO lenses are done that way???

Greg.

[Peter Ward]

I'm a Ferrari fanboy. Then there is Porche, Lambo and Mclaren...plus those annoying electric cars :D....all expensive...but all impressive performers.

My point is....we are not looking at Barina's or an old Morris here...judging from Sauvi's images the CFF will prove to be a cracker of a 'scope.

I'm looking forward to seeing your first light images Greg :thumbsup:

[Ukastronomer]

The point is that there are people out there who love their scope, I do, I bought it for me, not what lab reports said.

It is YOU who have to use it, nothing at all matters only you and your enjoyment :)



.

[DJT]

Enjoy the scope Greg. Looking forward to seeing what you do with it.

D

[RobF]

Yep, sounds like a great scope.

Hope you get the light train sorted soon and suitable skies to enjoy the new beast Greag.

[gregbradley]

Thanks David.



I am hoping the adapters are not a big deal but my 2 cameras the FLI Proline sensor is set back 21mm but the Microline is set back 15mm so it means a screw on small 2nd adapter to pack out the Proline adapter to be at the same length. He's made one like that for me before.

Greg.

[bigjoe]

Seems too much obsession with Strehl and P-V anyhow ..sooo many things to take into account measuring that.

You would expect a PREMIUM company like CFF would not let any Lens assemblies pass their scrutiny without being anything less than near Perfect ..their reputation would have been shot by now, and it ISN'T!
MANY happy uses it seems.
I too would choose CFF or Tak if imaging.

Bigjoe

[gregbradley]

Once I get the adapters sorted it should be fairly soon after that. its in the hands of Ashely at Precise Parts.

Greg.

[Slawomir]

Great to hear that Greg - looking forward to seeing the results of the first light :thumbsup:

I hope you don't mind - in another thread Nick has shared an interesting graph of 128 scopes from a Russian lab showing PV and RMS vs Strehl with some approaching Strehl of 1.

I approximately positioned the values from your certificate on the graph - blue line for PV and red for RMS - both are better than 128 scopes in this sample :thumbsup: Clearly, none of these scopes, with some approaching Strehl of 1, are better than PV 1/8 wavelength - maybe Russians got it wrong too :question: ;)

[Wavytone]

Try the updated graph - a least squares quadratic fit is more appropriate as it’s a a square law relationship in theory. Hence the problem with the intercept with a linear fit.

[gregbradley]

Thanks for the graphs. Impressive indeed.

Any statistical measure will be useless if it does not take enough samples. That is basic statistics. So it would not surprise me, as some have mentioned, that the formula used by most companies would be the one that gives the best marketing spin not the most accurate.

Greg.

[gregbradley]

I mounted the CFF 105 on top of the AP Honders last night. Luckily the counterweights just make it in balancing it with a bit left over. I was able to image with round stars with the Honders and Proline last night.

The CFF has a lovely slightly sparkly pearl finish. Its not a solid white. The rotating Feathertouch focuser is simply superb. Fabulous fine control.

I used the scope visually last night. My eyepiece collection is not that extensive but it gave pleasant views. A bit dim as 4 inch scope don't collect a lot of light. But the planets looked good. I have a collection of Meade Plossls so I'll look again tonight if its clear and use the shortest focal length I can. I have a TV barlow as well so I'll get that out.

I plan to mount the Microline 16 on it which should give a wider field than the Honders and perhaps it can be collecting colour when the Honders is collecting luminance if I can get them to see much the same spot. Its close now and I haven't really tried.

Visually though an SCT or a larger refractor are hard to beat.

I plan to spend a night or two with the Honders visually. It just means removing the imaging train and refitting the focuser which isn't a big deal but enough of a job to put it off! I think I'll do the same with the CDK again. It really needs good seeing otherwise its horrible visually.

Greg.248813

[Slawomir]

Great news with the visual test Greg.

The setup looks very impressive - so I was correct stating that I take images with a guidescope! :lol:

[gregbradley]

Hehehe :rofl: