57 replies to this topic

[Jeff B]

Again, specifically, Serial Number 009 owned by my friend Paul from Indy.  Paul brought this scope along with his AP 110GTX to my place for both DPAC and star testing.  Paul is not the original owner.  Only 20 of these scopes were produced and we believe this is the only one in the US.

 

I was struck by the scope's size and robust build.  Fit, finish and feel, were all superb. I've attached a picture of it next to a "puny" SW120ED.

 

Upon arrival, after taking it out of its case, we set in on my saw horses, took off the dew shield retaining ring, and slid the dew shield all the way to the back of the OTA, past the objective cell.  We did that to facilitate temperature acclimation of the objective, whose cell seemed just slightly warmer than the temperature in my bunker (most people call it a basement).  

 

I was really impressed with the design and construction of the objective cell....and just look at those sets of lens element radial adjustment screws just begging to be "adjusted".  We resisted the begging.  

 

The objective coatings are a beautiful(!) deep purple.  

 

I did not make my usual checks of focuser and lens block alignments as prior samples of CFF scopes that have passed my way showed everything was in order, and besides, I was too lazy to make a paper mask for the objective.   

 

After a little while we mounted it up on the test bench (no trivial task there) and aligned everything.

 

Jeff

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[Wibo]

a deep purple might not be good, hopefully the stars aren't yellowish

[Jeff B]

A 133 LPI Ronchi screen was used for all images. 

 

Here are the inside and outside of focus white light LED images as well as a green LED image at focus.  The at focus image was difficult to take and is waaay hyped in contrast to bring out as much structure in the wave front as possible (code for making it look as ugly as I can).  

 

The white light images suggest overall mild over correction with some minor/mild smoothly transitioning edge deviations from the 80% span to the edge.  The figure is what I call a compound figure but transitions to the middle zone are smooth.   There is also evidence of some longitudinal (color focus) "CA" as seen from the mild blue/red fringing on the R/L lines as well as some spherochromatism.  Overall though, this is very good chromatic performance, especially considering the "sporty" combination of  large aperture (185mm), fast focal ratio (F6.8) and an oil coupled design.  Well done.

 

The at focus image shows a generally smooth figure with evenly distributed small, fine scale textures and a mild center zone spanning about 40% of the aperture, the effect of which is most easily  seen in the outside of focus white light image.  All transitions are smooth.

 

Jeff

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Edited by Jeff B, 09 December 2023 - 01:15 AM.

[Jeff B]

A bit of an aside, I hype the contrast on the at focus images to bring out and high light structure in the wave front.  But taking these images consumes the most time as the focus and screen adjustments needed to get even illumination can be quite tedious, especially with optics which are smooth with no/little zoning.  But I persist.  Though the goal is not to have them, it's not uncommon for me to have shadows on these images.  Unfortunately, hyping the contrast, while enhancing structure on the lit areas, can hide structure in the shadows.  So I also find it useful to compare the un-processed image to the hyped image.

 

And so here is that comparison for this CFF.  Not a big difference for me but useful.

 

Jeff

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[Ben the Ignorant]

Cool but why not use a 10 lines/mm, 254 lines/inch Ronchi grating? I have one, simple to use, affordable, sensitive. It's branded Teleskop Service but it's probably a clone of others with exactly the same frame. I don't have your expertise but why?

[Darren Drake]

Is that astigmatism??

[peleuba]

Is that astigmatism??

 

Hi Darren - Are you referring to the bands not being perfectly perpendicular to the camera (straight up and down)?   

 

Probably not astigmatism.   Astig in double pass is most noticeable as you move through best focus.  The bands get chocked/distorted but then quickly straiten.

 

I am guessing the Ronchi screen may have been slightly tilted in the photo or some other alignment issue and not astigmatism.

[Scott in NC]

Nice work, Jeff.

[Ben the Ignorant]

After leaving the forum for a while doing a cleaning chore (manual tasks sometimes stimulate the thoughts), I remembered the test is a double-pass that doubles the sensitivity, so that's why a 133 lines per inch grating is enough?

[Scott in NC]

After leaving the forum for a while doing a cleaning chore (manual tasks sometimes stimulate the thoughts), I remembered the test is a double-pass that doubles the sensitivity, so that's why a 133 lines per inch grating is enough?

Yes.

[RichA]

A bit of an aside, I hype the contrast on the at focus images to bring out and high light structure in the wave front.  But taking these images consumes the most time as the focus and screen adjustments needed to get even illumination can be quite tedious, especially with optics which are smooth with no/little zoning.  But I persist.  Though the goal is not to have them, it's not uncommon for me to have shadows on these images.  Unfortunately, hyping the contrast, while enhancing structure on the lit areas, can hide structure in the shadows.  So I also find it useful to compare the un-processed image to the hyped image.

 

And so here is that comparison for this CFF.  Not a big difference for me but useful.

 

Jeff

Through all the DP stuff you've thankfully done, one thing is clear;  everything runs second to AP scopes.  

[Scott in NC]

Through all the DP stuff you've thankfully done, one thing is clear;  everything runs second to AP scopes.  

I’ve had Taks (a triplet and a quadruplet) and a TMB whose DPAC tests I’d put up there with anything produced by AP.

[Jeff B]

Thanks for the comments everyone.

 

Here are the inside/outside of focus color montages, derived from the white light images.

 

Best correction seems to fall between yellow and red so I also generated some orange images by cutting the blue channel out and reducing the yellow.  Yup.

 

I find this to be a well balanced objective, especially considering the large aperture and speed.

 

Jeff

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[Jeff B]

I thought it might be interesting to compare this sample CFF185 F6.8 made in 2017 with an earlier 160 F6.5 sample I owned for several years.

 

The 185 has a smoother figure and seems to have a little better chromatic correction overall, but the 160 is very well balanced chromatically with best correction in green with no real zoning.  The 160 was a very sharp optic capable of over 300x, seeing permitting. 

 

Jeff

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[fate187]

Nice report, Jeff! If you allow I can post my 185 #15 DPAC images as well. It is essentially of same quality with respect to optical figure and correction as the above mentioned #9 sample.

 

Regarding astigmatism in DPAC: when I see this in DPAC its usually some missalignment of the lens assembly/tube to the mirror. The star test will reveal any astigmatism. My sample #15 does not have any issues there after proper(!) cooling/acclimation.

 

BTW regarding DPAC testing and ronchi gratings: in the past I used a 200lpi ronchi grating. But newer tests I did (during bad weather, which persists ) I use a 133lpi grating for inside/outside focus tests. This yields good comparison shots to other forum member's optics. A 66lpi ronchi is used to give me the sharpest image result of the wavefront at focus.

Edited by fate187, 10 December 2023 - 08:49 AM.

[Jeff B]

Nice report, Jeff! If you allow I can post my 185 #15 DPAC images as well. It is essentially of same quality with respect to optical figure and correction as the above mentioned #9 sample.

 

Regarding astigmatism in DPAC: when I see this in DPAC its usually some missalignment of the lens assembly/tube to the mirror. The star test will reveal any astigmatism. My sample #15 does not have any issues there after proper(!) cooling/acclimation.

 

BTW regarding DPAC testing and ronchi gratings: in the past I used a 200lpi ronchi grating. But newer tests I did (during bad weather, which persists ) I use a 133lpi grating for inside/outside focus tests. This yields good comparison shots to other forum member's optics. A 66lpi ronchi is used to give me the sharpest image result of the wavefront at focus.

Yes, please do Michael.  Feel free to copy my images for any side-by-side comparisons.

 

Funny you should mention a lower Ronchi screen resolution for at focus images as I'm "looking into" that too.   Perhaps a little more like  a KE with less interference from adjacent lines.

 

Outdoor artificial test results will be next.

 

Jeff 

[peleuba]

I find this to be a well balanced objective, especially considering the large aperture and speed.

 

Me too.   I would take this lens any day, especially, like you say, because its fast.  It rather smooth, too, for an aspherized triplet.

 

As you know, Jeff, I am looking for a replacement for a 6" class scope that I had ordered that the manufacturer was ultimately, not able to deliver.   This would be a nice option.  

[Jeff B]

Me too.   I would take this lens any day, especially, like you say, because its fast.  It rather smooth, too, for an aspherized triplet.

 

As you know, Jeff, I am looking for a replacement for a 6" class scope that I had ordered that the manufacturer was ultimately, not able to deliver.   This would be a nice option.  

Trouble is, this is the only sample on US dirt.  Also, it is every bit of say, 45 pounds, and very nose heavy.  How's your lower back?

 

And no, I'm not selling my TEC 160 ED.  

[drprovi57]

These DPAC tests have been very valuable.  I have owned both the CFF200 f/6.5 (I still own and use primarily for imaging) and a CFF200 f/8 (sold, provided excellent visual views of planets, double stars, and DSO's).  I sold the CFF200 f/8 to purchase a CFF230 f/9 (9" beast for visual and planetary imaging) - expecting delivery in early 2024,  I believe the last large 9" class refractor from CFF - it has been in the design/build stage for about 18 months.   If your DPAC setup can handle a large 9" refractor, perhaps we can arrange a DPAC test - be interesting to compare with the supplied interferometric data and star/planet tests.  

 

Thanks for all your DPAC efforts - 

[Jeff B]

Outdoor Star Testing

 

The CFF was mounted, dew shield pushed back off of the objective cell, on my trusty Losmandy G11 (the photos of Paul and the OTA were taken after the testing).  We used a 50 micron artificial star placed about 252 feet away, which makes the star size about 10% of the 185's diffraction limit.  Small enough but I  still used a 2X barlow over the star's 1.25" nosepiece.  

 

It was the same day we tested Paul's AP 110GTX.  We star tested the 110GTX after we did the CFF.   It was a mostly cloudy day, fairly stable temperature (52 -55 degrees F) but with the higher temperatures during occasional cloud breaks.  Ground level Pickering seeing varied from 6 to 8 with 8 during extended cloudy periods.   Plenty good.

 

Scope, had cooled with the dew shield retracted, from an ~66 degree F basement for a little over an hour outside while we were DPAC testing the little AP and then lining the CFF and the star up (which involved at lot of trudging back and forth from the star and scope).  Testing itself took about an hour.  

 

I used all the wrong stuff:  2" AP star diagonal, cheaper 6mm ortho but also nice 9 & 7 mm  UO orthos, and good quality 2x and 3x barlows. A good combination for high power, 420x, was the 3x barlow and the 9mm UO ortho.   So, I violated some star testing "rules" but our objective was to sniff out coma and astigmatism, not spherical & color corrections, which is what the DPAC testing showed me, but.....

 

Star testing dogma states you need to let the OTA thermally stabilize prior to getting serious.  Well, not for me.  I start looking right away because a: I'm compulsive and b: I find it fascinating to watch a scope's star image morph and evolve if it is still cooling a little.  That educates me.  This scope was educational for me. Even though we were primarily looking for coma & astigmatism, I still had a gander at the spherical content, which seemed quite consistent with DPAC once the objective settled out. Early on the scope seemed slightly under corrected, then neutral, then finally, maybe, slightly, overcorrected, both with and without the green filter.  Also, in the beginning, I saw signs of mild tube currents, occasionally flattening a portion of the out of focus diffraction rings with a narrow local "bleed" at focus.  This went away.    Color also went through an evolution.  Early, I found, at high power,  moderate amounts of red/blue splashing around the airy disk, which was slightly yellow-ish.  This too subsided considerably as the objective cooled but there was a small bit of splashing left over, visible above maybe 250X, but the nice round airy disk was quite white.

 

I saw no coma at all up to 420x, though one could be fooled by a mild tube current.  I was not.  The only astigmatism I saw was a mild trefoil pattern early at high power that quickly went away.

 

I found the star test excellent, and again, especially for such a large fast, oil coupled triplet.

 

Jeff 

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Edited by Jeff B, 10 December 2023 - 03:05 PM.

[Paul Morow]

I think I would rather see one of those Celestron Models from the good old days standing next to that beautiful OTA than that fella. 

Edited by Paul Morow, 10 December 2023 - 04:16 PM.

[scoale]

I think I would rather see one of those Celestron Models from the good old days standing next to that beautiful OTA than that fella. 

 

Congratulation on a great scope, Paul.  And thanks for contributing to the ongoing education of the forum.

[Kitfox]

Paul, you are one blessed individual!  As scoale said, congratulations. And great analysis, Jeff.  I was expecting a little more of “interest” out of a beast with this kind of speed. That is one seriously obtuse cone of light rays to be corrected and generate that level of perfection.   Well done, CFF!

 

Btw, Jeff,  I was looking at the in-focus images, and trying to spot the aspherization, a la knife-edge testing.  Could you, Scott or Paul (peleuba) discuss the in-focus images a little more knowing that there is at least some deviation from a spherical figure in these optics?

Edited by Kitfox, 10 December 2023 - 10:15 PM.

[Scott in NC]

Btw, Jeff,  I was looking at the in-focus images, and trying to spot the aspherization, a la knife-edge testing.  Could you, Scott or Paul (peleuba) discuss the in-focus images a little more knowing that there is at least some deviation from a spherical figure in these optics?

Steve, I’m out of my league here, so will let the true experts answer this one. 

 

Jeff, thanks again for contributing to the education of us all.
 

[Jeff B]

These DPAC tests have been very valuable.  I have owned both the CFF200 f/6.5 (I still own and use primarily for imaging) and a CFF200 f/8 (sold, provided excellent visual views of planets, double stars, and DSO's).  I sold the CFF200 f/8 to purchase a CFF230 f/9 (9" beast for visual and planetary imaging) - expecting delivery in early 2024,  I believe the last large 9" class refractor from CFF - it has been in the design/build stage for about 18 months.   If your DPAC setup can handle a large 9" refractor, perhaps we can arrange a DPAC test - be interesting to compare with the supplied interferometric data and star/planet tests.  

 

Thanks for all your DPAC efforts - 

Thanks Jason and yes, I can test you 9" F9 CFF and would be happy to do so.  Shoot, I did my LZOS 10" F9.

 

Jeff