54 replies to this topic

[mikeDnight]

Here's a couple of shots. The first is Rigel and its huge purple halo visible; I did not even had to take a long pose at high ISO to make it evident.

 

And the second is a long exposure of the Orion nebula. Here the halo is visible also on the smaller stars because of the long exposure. I admit I've little experience when it comes to refractors, having owned only some achromatic small ones, but this to me does look a bit... excessive.

 

PS: I took these pictures without any filter whatsoever. I noticed that band pass filters for nebulas or light pollution reduce the halo consistently becaue they block, among other wavelegths, the higher portion of the spectrum, so violet light coming from the lens is greatly attenuated.

 

What do you say? Should I contact the vendor so he can ask for explanations directly to the manufacturer? Is this a defect unrelated to glass?

I'm sorry you're unhappy with the correction of your new refractor. As a doublet at F6 it could never have the colour correction of a longer F ratio apo doublet no matter what its lens is made of. When Takahashi produced the SKY 90 using a fluorite objective, they used a wider separation of the elements than the longer FS series, so as to attain a high level of colour correction. May be you could find someone near to you who uses a Sky 90 and compare the CA in a side by side test. As far as fast doublets go the SKY 90 is amazing, but it is not as colour free as its longer sibling doublets. 

Today Takahashis fastest 100mm doublet if F7.4. It is excellent but to get better colour correction in a shorter/faster scope you'd need to look at a modern triplet unfortunately, and that would not only increase the weight but the price too.

 

Mike

[orlyandico]

Given that, seeing that M42, that is definitely an FPL-51 or Chinese equivalent. I know it when I see it (because I have one of these as well, although an 80mm f/7). My suggestion? mask it down to 80mm.  That would turn it into an f/7.5 and I'm pretty sure reduce the violet halo.

 

Or.. put a bandpass filter on it. UHC/LPR filters have a steep cut below around 450 nm and that may be good enough. You would need an LPR filter anyway.

 

If however you can send it back, and are amenable to spending more money to get a "real" APO, you may want to look at that.

 

The very common 100mm f/9 FPL53 doublet (Orion 100ED f/9, Skywatcher 100 Pro) is pretty much a true APO (I had one of those too) and if you put the common 0.8X reducer/flattener on it, you can get it down to f/7.2 which is a bit faster.

[Marcsabb]

Given that, seeing that M42, that is definitely an FPL-51 or Chinese equivalent. I know it when I see it (because I have one of these as well, although an 80mm f/7). 

Taken from the sales pitch at Omegon websie:

 

"The two element objective lens is constructed of a clever combination of OHARA FPL-53 and S-NBM51 glass. These are special types of ED glass with high and low refractive indices and superior light transmission characteristics. A particularly special feature is the use of S-NBM51 glass, which is usually employed in the optics of apochromatic fluorescence imaging microscopes. The benefit to you is that the optics provide a particularly sharp and high-contrast image!"

 

Given that they mention 'ohara FPL-53' explicitly, if they are using FPL-51, either from Ohara or a compatable glass from another manufacturer, this is a consumer fraud plain and simple. One thing is to advertise a product with some rather meaningless and slightly un-scientific marketing banter, another this is to lie about a fundamental characteristic of the product sold.

 

So I'd rather not drop this issue if I can get an answer from the manufacturer. If anybody can help me posting pictures from true, FPL-53 based doublets of roughly comparable focal rato, I would be grateful.

Edited by Marcsabb, 01 January 2017 - 04:46 AM.

[Erik Bakker]

Happy New Year everybody!

 

Marcsabb,

 

Why battle with the manufacturer? My advice would be to take the easier road to happiness and return this scope that is apparently nowhere near your expectations. Rest assured, you're not the first nor will you be the last to experience this disappointment. 

 

Marketing materials tend to be not so good in being realistic about the qualities of the product they describe. 

 

In the end, it helps if we can look at them as stepping stones on our road to telescopic happiness. I learned that the hard way in the 1980's and 1990's.

 

Part of the problem with marketing hyperbole is that we want that marketing hyperbole to be true. So we can have that wonderful great performing scope that costs only half of what other people payed for a different brand scope of the "same" quality. So we have to save less of our hard earned money. And besides, we already "knew" that people buying that very expensive top performer from brand "x" mainly payed for that name "x" , not for the quality  

 

Long story short: it will save you time and frustration if you focus on what you want and really need and how much you can afford to get that. Then make a decision on where you want to spend your budget. Ask people in the know for advice AND listen to them. Try out your scope of decider at a local star party. Or at a shop after the sun goes down. Take your own camera and mount with you if you like. In that way, you can buy based on your own personal experience instead of other people's marketing hyperbole. The moment I started doing that, I was always pleased with any new scope I bought.

[orlyandico]

I don't there are any FPL53 doublets that fast (100mm f/6).

 

The fastest FPL53 (or similar) doublet that I know of is the Tak Sky 90 (90mm f/5.6) which has fluorite and a widely-spaced doublet to improve correction. It still has CA.

 

The FPL53 doublets that I know of are the 80mm f/7.5 (Orion 80ED and friends), 100mm f/9 (Orion 100ED, Vixen 100mm f/9 ED Sf, Skywatcher Pro 100ED), and the 120mm f/7.5 (Orion 120ED and friends).

 

None of these as you see even approach f/6.  However...

 

I know that the 100mm f/9 is practically apochromatic even with a DSLR and long exposures. If you calculate the color blur of an FPL53 doublet..

 

CB = (915 x [OTA diameter MM] ) /(12000 x [OTA F-ratio] )

 

so the 100mm f/9 is 0.847 (true APO < 1)

80mm f/7.5 is 0.813

120mm f/7.5 is 1.22 (not a true APO, a semi-apo)

 

a 100mm f/6 would be 1.27

 

therefore, the color blur of the 120mm f/7.5 should be around the same as a 100mm f/6

 

You can look for photos with the 120mm f/7.5 on astrobin and check if they have as much color blur as your sample.

 

Here's a Running Man with an ST8300C (OSC) and the Orion 120ED..

 

http://astrob.in/15439/0/

 

granted it's processed, but I don't see nearly as much CA on that image (actually I can't see any).

 

Based on that, I would think your 100mm f/6 is not FPL53.

 

Question is, can you return it for a refund?

[junomike]

 

Given that, seeing that M42, that is definitely an FPL-51 or Chinese equivalent. I know it when I see it (because I have one of these as well, although an 80mm f/7). 

Taken from the sales pitch at Omegon websie:

 

"The two element objective lens is constructed of a clever combination of OHARA FPL-53 and S-NBM51 glass. These are special types of ED glass with high and low refractive indices and superior light transmission characteristics. A particularly special feature is the use of S-NBM51 glass, which is usually employed in the optics of apochromatic fluorescence imaging microscopes. The benefit to you is that the optics provide a particularly sharp and high-contrast image!"

 

Given that they mention 'ohara FPL-53' explicitly, if they are using FPL-51, either from Ohara or a compatable glass from another manufacturer, this is a consumer fraud plain and simple. One thing is to advertise a product with some rather meaningless and slightly un-scientific marketing banter, another this is to lie about a fundamental characteristic of the product sold.

 

So I'd rather not drop this issue if I can get an answer from the manufacturer. If anybody can help me posting pictures from true, FPL-53 based doublets of roughly comparable focal rato, I would be grateful.

 

Can you take a pic of the lens cell.  I find it suspicious that there's no pic of this. 

My guess is it's because it will look exactly like this.

(see 4th pic and then the FPL-51 designation under Tech Data)

 

EDIT:  Probably no need for the lens cell pic as I believe this is your scope (uses FP-51 as predicted).

 

Mike

Edited by junomike, 01 January 2017 - 09:20 AM.

[daveCollins]

Happy New Year everybody!

 

Marcsabb,

 

Why battle with the manufacturer? My advice would be to take the easier road to happiness and return this scope that is apparently nowhere near your expectations. Rest assured, you're not the first nor will you be the last to experience this disappointment. 

 

Marketing materials tend to be not so good in being realistic about the qualities of the product they describe. 

 

In the end, it helps if we can look at them as stepping stones on our road to telescopic happiness. I learned that the hard way in the 1980's and 1990's.

 

Part of the problem with marketing hyperbole is that we want that marketing hyperbole to be true. So we can have that wonderful great performing scope that costs only half of what other people payed for a different brand scope of the "same" quality. So we have to save less of our hard earned money. And besides, we already "knew" that people buying that very expensive top performer from brand "x" mainly payed for that name "x" , not for the quality  

 

Long story short: it will save you time and frustration if you focus on what you want and really need and how much you can afford to get that. Then make a decision on where you want to spare your budget. Ask people in the know for advice AND listen to them. Try out your scope of decider at a local star party. Or at a shop after the sun goes down. Take your own camera and mount with you if you like. In that way, you can buy based on your own personal experience instead of other people's marketing hyperbole. The moment I started doing that, I was always pleased with a new scope.

Erik, great reply. We are all surrounded by unfairness in life. Life has never been fair and never will be. We are also surrounded by false advertising. Those who want to separate you from your money will say whatever they think will work. It is our responsibility to take care of ourselves as best we can and minimizes our losses due to being mislead.

 

This is a fun hobby and I personally won't turn it into something that brings me unhappiness and causes me to chase down others for unfair dealings. If I make a mistake, so be it. I'll move on and learn from the mistake. So far astronomy has been one of the more meaningful things I've ever done and it is a source of enjoyment for me, not frustration.

 

EDIT: type

Edited by daveCollins, 01 January 2017 - 11:13 AM.

[Erik Bakker]

Dave,

 

From your sig I can make an educated guess about the road you traveled over the decades 

 

It's always a privilege to meet a fellow traveler 

[Marcsabb]

Yesterday, the ordeal of my new Omegon refractor took an unexpected turn for the better.

 

I was cleaning the scope, intent on repacking it and sending back to the shop, when, upon cleaning the interior surfaces of the focuser, I noticed that the cloth used to wipe it was stained with some soothy spots.

 

Perplexed as what was the cause of that stains on the cloth, I reached the inside of the tube, wiped a surface and examined the patch afterwards: same soothy marks! It was then that I had a lucky intuition: I took a long plastic stick, wrapped a patch of clean cloth around it, wet with some mild lens cleaning fluid, and I started to clean the interior surface of the lens cell. Guess what? The patch of cloth afterwards had the same kind of grayish stains on it. I had heard of some new scopes having outgassing issues due to the flat black paint used to flock the interior of the tube, but this was the last issue I was expecting from my new scope. Is it really the paint or maybe some residue from another process? The scope is made of treated carbon fiber, so God knows all the chemical processes involved in the manufacturing of the tube.

 

Now filled with hope, I placed the scope back on the tripod, I let it cool down for more than two hours outside and waited till dusk. Yesterday the sky cooperated and gave me a nice evening with very good seeing, like I rarely happen to have around my parts.

 

With the Moon and Venus still high in the sky, it was time to give a last chance to this unlucky scope. I train the scope at Venus and slide the 7mm UWAN eyepiece into the cup: I see the same half-moon image of the planet as before, except that this time something seems different. Whereas before the was some hint of aberration plainly visible on the borders, now the bright tiny object is clearly detached from the blackness of the sky around it. I switch to the 4mm eyepiece and now I'm treated with an entirely different view than before: the ugly, unsymmetrical purple halo is completely gone. Vanished! Now the planet shines in all its splendor with some hint of reddish and blue aberration at the borders giving me the impression I'm observing a tiny diamond under a magnifying lens.

 

I stare at Venus for ten minutes: the chromatic aberration is still there, but I would now describe it as "controlled and localized". 

 

Afterwards, I aim the scope at the moon and I'm greeted again with a very satisfying view. Despite being quite low on the horizon, the image remains crisp and steady thanks to the good weather conditions. Aberration at the border is minimal, with just a slight hint of red/orange hue. I still didn't believe what I was seeing, so I quickly connected my (color) astro camera to a PC and took some shots of Moon and Venus. I post them here. In case of Venus, it's a single, non retouched frame coming from a 1 minute video capture. In case of the moon, it's a simple stack of 16 frames again from 1 minute video. No post processing. I just wanted to share with you what I saw yesterday. I'd say that the pictures reproduce quite well what I was seeing at the eyepiece yesterday evening, so I'll let you all be the judge.

 

What can I say more? Finally I've the scope I was expecting to buy in the first place. A stupid, improbable issue ruined my day but hopefully now it's solved. I'll probably contact the shop anyway telling them of my experience so that it may serve as a warning.

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Edited by Marcsabb, 02 January 2017 - 04:18 AM.

[Erik Bakker]

Nice update.

 

Likely the long (over 2hrs) cooldown period allowed and the much better seeing remedied most of your issues.

 

The cleaning of a dirty lens surface is not something a new lens should need, nor is it a known cure for chromatic aberration and the likes. Plus there is no guarantee that the residue on the inner lens will not form again. So I would still seriously consider returning it.

[Marcsabb]

Nice update.

 

Likely the long (over 2hrs) cooldown period allowed and the much better seeing remedied most of your issues.

 

The cleaning of a dirty lens surface is not something a new lens should need, nor is it a known cure for chromatic aberration and the likes. Plus there is no guarantee that the residue on the inner lens will not form again. So I would still seriously consider returning it.

One would expect a uniform patina or residue to make the image a bit blurrier or darker not to impact aberration but that wasn't what I saw. Another explanation is: the patina on the lens could have acted as a colored filter, so I mistook what I was seeing as lens aberration, while it was but a simple purple tint coupled with some diffraction induced by the residues.

 

I didn't leave the scope out cooling much longer than in the past occasions. The only notable difference was the sky seeing which was arguably one of the best I've encountered in this location during the last year.

Edited by Marcsabb, 02 January 2017 - 04:26 AM.

[Erik Bakker]

Great seeing works wonders for sure.

 

Overall, are you happy with your new scope now?

[Marcsabb]

Great seeing works wonders for sure.

 

Overall, are you happy with your new scope now?

Well, let's say I'm cautiously optimistic now 

 

Regrettably, yesterday I had to quit my observing session early, before I could take some pictures of Rigel and Orion and compare to the one I've made before to check for differences. If it's anything like I've seen with Venus, then I'll be satisfied. This scope will be used mostly for camera work (both for astronomy and nature) and when I need to travel light. The intention was to have two light but capable little scopes (the other being the Intes M500 for moon and planetary work, which quickly displaced the Celestron C6 as favorite travel scope). 

 

When I searched for a new scope within the constraints of my budget, I felt this was a good combination of size, weight, aperture, F-ratio. The only unknown factor at the time was lens quality; here I'm still cautious but yesterday developments make me believe maybe it wasn't a bad choice after all. 

Edited by Marcsabb, 02 January 2017 - 04:49 AM.

[Erik Bakker]

Hope all turns out well then 

[Marcsabb]

Yesterday evening the sky went cloudy, so I have to postpone further testing. During the day I played a bit with it but the differences after cleaning the lens are more subtle. The scope works very well also mated with the Astro Physics CCDT67 focal reducer turning it in a fast F/4 optical system. 

 

When I cleaned the inside of the tube, I noticed that the scope is also internally baffled; from a purely mechanical point of view, this scope is of excellent manufacture and with a high quality finish, something that I see a bit lacking in the scopes coming out of the Syntha group (Celestron, SkyWatcher, etc.), regardless of their optical merits.

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Edited by Marcsabb, 03 January 2017 - 02:19 AM.

[Marcsabb]

It seems the patina covering the interior of the lenses was indeed the problem. See Rigel before and after. From my limited experience with ED/APO refractors (peeping through the eyepieces of friends' instruments, mostly) I'd say it's a decent correction. Maybe Omegon did employ FPL-53 glass, after all 

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Edited by Marcsabb, 04 January 2017 - 03:03 AM.

[Edwin]

Strange story, but it seems OK now.

[Erik Bakker]

Remarkable improvement. Not sure what the cause is. The right half of the split-image looks OK to me.

[Marcsabb]

Remarkable improvement. Not sure what the cause is. The right half of the split-image looks OK to me.

I'm still in doubt if I should return it. I cannot make up my mind...

[Erik Bakker]

Take the legal time you have in your country and let your heart decide 

[junomike]

 

Remarkable improvement. Not sure what the cause is. The right half of the split-image looks OK to me.

I'm still in doubt if I should return it. I cannot make up my mind...

 

Although it does appear your newly cleaned glass has made a huge improvement, the Idea of a return should come down to If you're happy with it or not?

That being said, IMO the glass is not FPL-53 but who am I to disagree.

 

Mike

[Peter Besenbruch]

The color I see in the Venus photo looks like atmospheric dispersion, mostly. Given what the OP paid for it, I might keep it, and clean again in six months.

[chrisastro8]

​​This is a remarkable turnaround in performance, and you did excellent work making that happen. 

Now that you are vested in the scope, seems like its a keeper to me.

 

Thanks again for this thread.

 

Cheers,  Chris

[Sasa]

Taken from the sales pitch at Omegon websie:

 

"The two element objective lens is constructed of a clever combination of OHARA FPL-53 and S-NBM51 glass. These are special types of ED glass with high and low refractive indices and superior light transmission characteristics. A particularly special feature is the use of S-NBM51 glass, which is usually employed in the optics of apochromatic fluorescence imaging microscopes. The benefit to you is that the optics provide a particularly sharp and high-contrast image!"

 

Given that they mention 'ohara FPL-53' explicitly, if they are using FPL-51, either from Ohara or a compatable glass from another manufacturer, this is a consumer fraud plain and simple. One thing is to advertise a product with some rather meaningless and slightly un-scientific marketing banter, another this is to lie about a fundamental characteristic of the product sold.

 

So I'd rather not drop this issue if I can get an answer from the manufacturer. If anybody can help me posting pictures from true, FPL-53 based doublets of roughly comparable focal rato, I would be grateful.

 

The use of S-FPL53 and S-NBM51 glasses explains a lot. This pair provides about 3x times better color correction than traditional achromat (distance between blue and read focii and green e-line is 1/6000 of focal length, compared to 1/1800 of traditional achromat). If you compute color factor SN2 Zeiss used to for quantification of color correction you will get a value of SN~2.5, not even semi-apo in Zeiss standard.

 

Combination of FPL53 and NBM51 glass is one of my favorite (I'm still thinking of designing and executing one more long focal lens; my first lens 82/1670mm doublet is based on S-NBM51 short flint glass). The strength is not its color correction (for example with FPL53 and BK7 you can get color correction at the level of 1/40000; combination of FPL51 glass with S-BSL7 gives 1/14000, still more than 2 times better color correction than the correction of the glass pair in your telescope). The strength is small Abbe number of NBM51 which at the end results in much mild radii (for example, if you would like to build traditional Fraunhofer-like doublet out of FPL53 and BK7, you will get 114mm radius of one inner surface; with NBM51 you will get 241mm). More mild radii means usually less spherochromatism, thinner lenses. For example 100mm f/12 doublet made out of these two glasses would be superb visual instrument (still not good for imaging, as g-line focus would be off by 0.9mm).

Edited by Sasa, 05 January 2017 - 07:52 AM.

[Sasa]

Taken from the sales pitch at Omegon websie:

 

"The two element objective lens is constructed of a clever combination of OHARA FPL-53 and S-NBM51 glass. These are special types of ED glass with high and low refractive indices and superior light transmission characteristics. A particularly special feature is the use of S-NBM51 glass, which is usually employed in the optics of apochromatic fluorescence imaging microscopes. The benefit to you is that the optics provide a particularly sharp and high-contrast image!"

 

 

BTW, it is "clever"  marketing. Statements are correct but notice there is no statement about "superior color correction". Yet, the use of FPL-53 invokes in many people that the lens will have excellent color correction.

 

I'm not bashing the scope. I think, this could be nice visual instrument, the color factor of SN2~2.5 is similar to my 63/840mm achromat where the CA is very, very small. But definitely people should not expect from it the apochromatic performance, especially for imaging.