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Which APOs are actually APO?

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#1 bokemon

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Posted 04 December 2020 - 07:38 PM

Hello folks,

I'm looking at refractors in the 100mm+ aperture range that are also fast (F<=6) but perhaps more importantly, don't have chromatic aberration problems so that imaging with a L filter the stars are the same size as under R G and B.  I'm seeing many scopes that claim "APO" but then in the user reviews, the blue channel gets bloated unless they use some kind of blue -cutting-out filter.  I will be using a monochrome camera + filter wheel.

I know that some of these newer Chinese telescopes like the Askars and Sharpstars seem to fail this test, and their reducers make the problem even worse.

How does the Esprit 100 do in terms of chromatic aberration?

Any other similar scopes?

 

The whole point is to to be able to use the L filter to do LRGB imaging.  Currently I do R G B imaging with my camera lens umm I mean achromatic refractor, but the blue channel still needs fixing.



#2 RogeZ

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Posted 04 December 2020 - 07:42 PM

I have not found a refractor that is perfectly apochromatic; if you want that type of correction you need to look into mirror scopes.

 

If you think the effect is visible in LRGB, you should see in SHO.....


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#3 Spikey131

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Posted 04 December 2020 - 09:37 PM

TV NP101 is


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#4 Berny

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Posted 05 December 2020 - 12:00 AM

I have never seen any false color in my TV101.



#5 csauer52

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Posted 05 December 2020 - 06:29 AM

I have not found a refractor that is perfectly apochromatic; if you want that type of correction you need to look into mirror scopes.

 

If you think the effect is visible in LRGB, you should see in SHO.....

I guess you've never looked through or imaged with a TOA130?


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#6 starman876

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Posted 05 December 2020 - 10:28 AM

The Tak and AP and many more high end  refractors are true APO's and a lot of Chinese triplets are becoming very good.


Edited by starman876, 05 December 2020 - 10:29 AM.

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#7 RogeZ

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Posted 05 December 2020 - 03:13 PM

I guess you've never looked through or imaged with a TOA130?

I have not used a TOA, but I have used a FSQ106 and it is not perfectly apochromatic. The Epsilon 180ED I had was.


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#8 RogeZ

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Posted 05 December 2020 - 03:14 PM

I have never seen any false color in my TV101.

Visually and photographically are vastly different, you might not be able to see it but the camera will detect it.


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#9 csauer52

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Posted 05 December 2020 - 03:55 PM

 The Epsilon 180ED I had was.

Well I hope it was, it's a reflector lol.gif

 

Maybe you had a bad FSQ? Mine is exceptional and my TOA130 is perfect for all intents and purposes. 


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#10 SilverLitz

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Posted 05 December 2020 - 05:22 PM

My limited experience, Esprit 100 is very good, but ES ED102CF (FCD1) is not.



#11 RogeZ

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Posted 05 December 2020 - 07:10 PM

Well I hope it was, it's a reflector lol.gif

 

Maybe you had a bad FSQ? Mine is exceptional and my TOA130 is perfect for all intents and purposes. 

The FSQ was great. 
 

Mind you my personal test for Apochromatism is very hard for any optic. I basically take a parfocal set of NB filters, test their parfocality on a reflector and then compare the result with the refractor. Its not scientific but its quite hard. 



#12 Richard Whalen

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Posted 05 December 2020 - 07:39 PM

I guess it depends on your definition of apochromatic. Some scopes claim apo performance, and may have it, but only in the center of the fov not over the entire fov. And the faster/larger aperture the scope, the more difficult it becomes and more correction (lens) may be required.. Along with glass to correct for other abberations like field flatners etc.

 

Trying to push 100mm aperture to imaging perfection is kind of silly to me. Just have fun with what you have. If your super serious about imaging, use a much bigger scope for real resolution. While I really have fun with my 110mm, it never is going to image the detail I can get with my 10" RC. And my 10" not close to my friends 16" or larger scopes.


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#13 f74265a

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Posted 05 December 2020 - 08:21 PM

From a physics perspective, i think full APO is impossible bc refraction bends every frequency of light slightly differently. The goal of an apo is to reassemble the spectrum back to the original image. A well executed triplet can reduce this to a level you won’t notice by aligning the focus points of three colours. Others can tell you which do a good job. A well executed doublet can come close, but it can align only 2 colours
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#14 KBHornblower

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Posted 05 December 2020 - 10:38 PM

Which ones are actually apochromatic depends on our definition of apochromatic.  A lens that brings three colors to the same focus, and has full correction of spherical aberration at two colors, could have unsatisfactory residual color if poorly executed but still pass muster in truth in advertising on a legal technicality.



#15 edif300

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Posted 06 December 2020 - 05:51 AM

The parfocal is a myth bad deserved. There is nothing parfocal other that not registrated focus variation by your system.

 

If you own the Epsilon you know what I am talking about (need refocusing after filter changes). An also there are two lenses in the light path -corrector-, one of them is an ED for some GOOD reason.



#16 RichA

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Posted 06 December 2020 - 07:53 AM

Hello folks,

I'm looking at refractors in the 100mm+ aperture range that are also fast (F<=6) but perhaps more importantly, don't have chromatic aberration problems so that imaging with a L filter the stars are the same size as under R G and B.  I'm seeing many scopes that claim "APO" but then in the user reviews, the blue channel gets bloated unless they use some kind of blue -cutting-out filter.  I will be using a monochrome camera + filter wheel.

I know that some of these newer Chinese telescopes like the Askars and Sharpstars seem to fail this test, and their reducers make the problem even worse.

How does the Esprit 100 do in terms of chromatic aberration?

Any other similar scopes?

 

The whole point is to to be able to use the L filter to do LRGB imaging.  Currently I do R G B imaging with my camera lens umm I mean achromatic refractor, but the blue channel still needs fixing.

None are.  You know the ronchi test?  Do it with any refractor, and you'll see colors on the lines.  Do it with a Newtonian reflector, no color.

 

I have not found a refractor that is perfectly apochromatic; if you want that type of correction you need to look into mirror scopes.

 

If you think the effect is visible in LRGB, you should see in SHO.....

Probably true.  Ronchi tests always seem to show colours on the lines with refractors, but Newtonians, RC's and pure reflective systems don't.


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#17 RogeZ

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Posted 06 December 2020 - 10:56 AM

The parfocal is a myth bad deserved. There is nothing parfocal other that not registrated focus variation by your system.

 

If you own the Epsilon you know what I am talking about (need refocusing after filter changes). An also there are two lenses in the light path -corrector-, one of them is an ED for some GOOD reason.

But I didn't need to refocus, that's my point. And as you pointed out, thats including the correct which induces some minor CA.



#18 tom_fowler

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Posted 07 December 2020 - 01:00 PM

As several people have observed, it is not possible by the laws of physics to create a perfect APO, one that bends all colors to the same focus point.  But it is possible to get extremely close, and true APOs such as those by TEC, AP, LZOS, and others give truly spectacular views of many types of objects.  Such premium scopes have advantages over other designs because they do not have the central obstruction, which removes energy from the central Airy peak and puts it into the rings.  The ED scopes aren't quite as good, but visually they perform well.  Photographically they do a credible job, but not as good as reflectors.  See my article here on CN, https://www.cloudyni...r-imaging-r3253, where I compare the APM/Lunt 152mm ED with two other scopes, one of which is, supposedly, an APO.


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#19 RichA

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Posted 14 December 2020 - 10:23 PM

As several people have observed, it is not possible by the laws of physics to create a perfect APO, one that bends all colors to the same focus point.  But it is possible to get extremely close, and true APOs such as those by TEC, AP, LZOS, and others give truly spectacular views of many types of objects.  Such premium scopes have advantages over other designs because they do not have the central obstruction, which removes energy from the central Airy peak and puts it into the rings.  The ED scopes aren't quite as good, but visually they perform well.  Photographically they do a credible job, but not as good as reflectors.  See my article here on CN, https://www.cloudyni...r-imaging-r3253, where I compare the APM/Lunt 152mm ED with two other scopes, one of which is, supposedly, an APO.

Almost all apos used ED glass.  I think by "ED" you mean doublets as opposed to triplets.  The only alternative is pure fluorite, which is still best of course.



#20 KBHornblower

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Posted 14 December 2020 - 11:35 PM

As several people have observed, it is not possible by the laws of physics to create a perfect APO, one that bends all colors to the same focus point.  But it is possible to get extremely close, and true APOs such as those by TEC, AP, LZOS, and others give truly spectacular views of many types of objects.  Such premium scopes have advantages over other designs because they do not have the central obstruction, which removes energy from the central Airy peak and puts it into the rings.  The ED scopes aren't quite as good, but visually they perform well.  Photographically they do a credible job, but not as good as reflectors.  See my article here on CN, https://www.cloudyni...r-imaging-r3253, where I compare the APM/Lunt 152mm ED with two other scopes, one of which is, supposedly, an APO.

What laws of physics?  I know that no available real world combinations of glass will do a perfect job, but does that necessarily preclude, in principle, the possibility of developing such glass?


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#21 Richard Whalen

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Posted 16 December 2020 - 11:44 AM

Dont mean to split hairs, but no reflector is perfectly apochromatic either as soon as you stick an eyepiece in the focuser. But other than that unless you use a really poor eyepiece you will not notice any CA, same with some APO refractor designs. I heard about a few generous focal length 4 element super APO designs (maybe GPU?) that were built? that gave Newtonian levels of perfection in focus. Once any color error is well inside the airy disk you will not see any CA visually, and at that point the spider and secondary of a Newtonian will have more effect on contrast and the view than any residual CA on a high quality APO. 

 

Observers get hung up on minut amounts of color to much, which in many cases cant be seen visual in focus. More important is the figure, collimation, and mechanics of the scope such as cell design etc. Even your own eyesight unless perfect will have a more negative effect.

 

I once had extraordinary vision, but once I hit 45 I have seen it decline and now in my 60s it is just normal 20/20, which to me feels like horrible vision in comparison.


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#22 Jon Isaacs

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Posted 16 December 2020 - 12:26 PM

Dont mean to split hairs, but no reflector is perfectly apochromatic either as soon as you stick an eyepiece in the focuser. But other than that unless you use a really poor eyepiece you will not notice any CA, same with some APO refractor designs. 

 

:waytogo:

 

On axis, eyepieces can be free of chromatic aberration..  any way, regarding splitting hairs, 

 

A human hair is about 0.002" in diameter.  The longitudinal focus error in a FPL53 triplet can be 1 part in 50,000 which is about 0.0008" for a focal length of 1000mm.

 

That's splitting a hair.  :)

 

One way to look at the level of color correction is to look at the depth of focus. At F/6, the depth of focus is 0.003", that's in green. In any event, the longitudinal focus error is 0.0008", the depth of focus 0.003, is about 4 times the error.

 

This says all the colors can be in focus simultaneously. It's another way of saying the colors are inside the Airy disk..

 

Jon


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#23 payner

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Posted 16 December 2020 - 04:16 PM

There are triplets that one will not detect CA, either photographically or visually. More important is does one's refractor exhibit SA (i.e., spherochromatism).


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#24 RichA

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Posted 16 December 2020 - 05:22 PM

Dont mean to split hairs, but no reflector is perfectly apochromatic either as soon as you stick an eyepiece in the focuser. But other than that unless you use a really poor eyepiece you will not notice any CA, same with some APO refractor designs. I heard about a few generous focal length 4 element super APO designs (maybe GPU?) that were built? that gave Newtonian levels of perfection in focus. Once any color error is well inside the airy disk you will not see any CA visually, and at that point the spider and secondary of a Newtonian will have more effect on contrast and the view than any residual CA on a high quality APO. 

 

Observers get hung up on minut amounts of color to much, which in many cases cant be seen visual in focus. More important is the figure, collimation, and mechanics of the scope such as cell design etc. Even your own eyesight unless perfect will have a more negative effect.

 

I once had extraordinary vision, but once I hit 45 I have seen it decline and now in my 60s it is just normal 20/20, which to me feels like horrible vision in comparison.

Lateral colour exists in more than a few expensive wide angle eyepieces.



#25 Bowlerhat

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Posted 16 December 2020 - 06:55 PM

Ah yes the quest for "true apos". Do you want a side of 0.99 strehl with that?




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