40 replies to this topic

[Astrojensen]

Hmmm I am curious here on this offering.  Not in the sense I want to buy this new large apo, but more along the lines that the TS152 achro is so good.  I owned the latter and it was an amazing deep sky scope with very little in focus color, except on the moon.

 

See the color crossings and compare yourselves (note the x axis scale)....the apo is (marginally) better by the looks of things but I think would probably snap up the faster achro for visual over this if I was primarily deep sky observing.. 

 

TS 150mm refractor comparison.png

The apo isn't just "marginally" better, if you compare the two color crossing images, it's DRASTICALLY better. Look at the scale of the two images. The apo has around three times better color correction than the achromat. 

 

 

Clear skies!
Thomas, Denmark

[Tyson M]

The apo isn't just "marginally" better, if you compare the two color crossing images, it's DRASTICALLY better. Look at the scale of the two images. The apo has around three times better color correction than the achromat. 

 

 

Clear skies!
Thomas, Denmark

I did notice the scale, but can you describe which color crossings and why its around 3x better?

 

Regards

[j.gardavsky]

Re: Curves in #24

 

The Kasai F/5.9, and the Photoline F/8 have quite different speeds.

What I particularly don't like about the Kasai F/5.9, is the undercorrecion in blue, otherwise I would have it for the richiest field observing at the low-to-medium magnifications.

The Photoline F/8 has of course better curves, but taking it cool, I don't need F/8 for the faint fuzzies I am hunting.

 

And the Catch 22 reads: For well corrected F/5 or F/6 you optimally need 4 lenses in the objective - 2 of them ED, and a field flattener.

These are in my Leica APO Televid, but don't ask me the price,

 

JG

[Astrojensen]

I did notice the scale, but can you describe which color crossings and why its around 3x better?

 

Regards

It's not about how many color crossings there are (there are none in the Takahashi TOA design!), it's about how far from each other they come to focus on the optical axis. In the achromat, the violet rays focus 0.6mm out from the mean, while in the apo, they focus less than 0.2mm from the mean. It's the same for the yellow, in the achro it focuses 0.35mm out from the mean, while in the apo it's less than 0.75. 

 

ALL colors in the apo focuses within less than 0.35mm, while in the achromat, the spread is more than 0.9mm. 

 

 

Clear skies!
Thomas, Denmark 

[Tyson M]

It's not about how many color crossings there are (there are none in the Takahashi TOA design!), it's about how far from each other they come to focus on the optical axis. In the achromat, the violet rays focus 0.6mm out from the mean, while in the apo, they focus less than 0.2mm from the mean. It's the same for the yellow, in the achro it focuses 0.35mm out from the mean, while in the apo it's less than 0.75. 

 

ALL colors in the apo focuses within less than 0.35mm, while in the achromat, the spread is more than 0.9mm. 

 

 

Clear skies!
Thomas, Denmark 

 

Violet and yellow.....do they matter a ton for visual?  Yellow perhaps for sure for double stars, but not so much violet I'm thinking..

[j.gardavsky]

Violet and yellow.....do they matter a ton for visual?  Yellow perhaps for sure for double stars, but not so much violet I'm thinking..

The 0.486nm is not a violet wavelength, it is thevisibly bright blue H-Beta, and it matters pretty much.

JG

[Blueox4]

At that price there will be compromises obviously. I think we all have learned that you get what you pay for and there are no shortcuts. I’m sure it’s a very nice telescope but make sure you know the return policy is my advice. Good luck! 

[HydrogenAlpha]

"Should" being the operative word. IME the ES 127 (FCD-1 Triplet) does not out perform the 120ED (FPL-53 Doublet).

YMMW

Indeed, but this also means it is irrelevant whether the glass is FPL-51 or FPL-53 (or Fluorite for that matter). You can have a well-figured FPL-51 or a poorly built fluorite. 

Edited by HydrogenAlpha, 03 February 2020 - 03:57 AM.

[Carl N]

Noticed APM has some triplet 152 with 3.7" focuser on sale for under $2800.

[HydrogenAlpha]

Noticed APM has some triplet 152 with 3.7" focuser on sale for under $2800.

They are reselling the TS triplet I linked to earlier.

[Moravianus]

They are reselling the TS triplet I linked to earlier.

With cheaper shipping..

[Gniewko]

It's not about how many color crossings there are (there are none in the Takahashi TOA design!), it's about how far from each other they come to focus on the optical axis. In the achromat, the violet rays focus 0.6mm out from the mean, while in the apo, they focus less than 0.2mm from the mean. It's the same for the yellow, in the achro it focuses 0.35mm out from the mean, while in the apo it's less than 0.75. 

 

ALL colors in the apo focuses within less than 0.35mm, while in the achromat, the spread is more than 0.9mm. 

 

 

Clear skies!
Thomas, Denmark 

Hello everyone,... this is my first post here

I think, we need to take into account the greater depth of focus in slower refractor. Defocusing colour rays work exactly like defocusing whole image. I think...

https://www.innovati...or-is-too-much/

Edited by Gniewko, 07 February 2020 - 05:52 AM.

[j.gardavsky]

The focus error is a very good characteristics of the telescope performance, and actually more intuitive than Strehl.

It can also be extended for the C-e-F polychromatic characteristics.

 

Thank you for sharing the link,

JG

[Gniewko]

These plots show it very well.

In achro f/5 color lines spread is 0,5mm. In achro f/15 spread is 1mm!

Chromatic aberration in f/15 is much smaller of course

Image from mr. Sacek site https://www.telescop...ochromatism.htm

 

 

[j.gardavsky]

These plots show it very well.

In achro f/5 color lines spread is 0,5mm. In achro f/15 spread is 1mm!

Chromatic aberration in f/15 is much smaller of course

Image from mr. Sacek site https://www.telescop...ochromatism.htm

 

abera2.jpg

This is the perfect explanation.

 

What matters is, if the depth-of-focus for the C-e-F wavelengths overlap, or not.

In the above example, at F/5 they don't overlap, at F/15 they do.

And this has been actually known since the old days of the classic photography.

 

 

The CA versus depth-of-focus is best characterized by the Zeiss RC value, used when designing the astronomy telescopes by Zeiss, Lichtenknecker, and others, in the telescope making.

In the RC-optimized design of an achromatic refractor, the CA deviations are within the depths of focus for the C, e, F wavelengths.

 

Best,

JG

[junomike]

New Link as the first one (first post) no longer works for some reason.