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"Night time" color correction in refractors

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#1 ValeryD

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Posted 06 December 2012 - 01:56 AM

Last years I see in some refractor (all sourced from China) designs that color correction curve shifted to the blue end of spectrum instead of worsening correction in the orange - red part of spectrum.
The designers/salesmans telling to public that at night time the sensitivity of a human eye in red part of spectrum fall down to 2% - so this part of spectrum is no longer important.

They are correct ONLY if these telescopes (with shifted to blue correction curve) are used for totally night observing - dim stars and dim DSOs. This all. On all other objects, which create enough illumination on retina so, that an eye can see color(s) of an object, these telescopes will work poorer, than telescopes with color correction curve which corresponds to a day light vision. They will work especially poorly on Mars. An observer will never be able to achieve a really good focus. He will always try to re-focus the telescope to achieve better sharpness and contrast with no luck however. I got an old Japan made ED doublet refractor 125mm F/8 with Hoya FCD1 - LaK10 glass combo with color correction curve shifted to the blue part of spectrum. Light blue F line (486nm) has common focus with yellow "d" line (588nm) instead of C line (656nm) .
The result is that I can see the moon with slight reddish borders in craters boundaries between shadows and illuminated pars, I see Jupiter relatively free of blue halo with trace of violet however, but not very sharp and I always feel the necessity to adjust the focus to make the planet sharper with no luck.
Mars was the worst - the image never was really sharp, very unpleased view. 100mm F/7 ED refractor (Jinghua made) with good color correction in blue-red and as is in violet, showed Mars totally color free and with more details than this 125mm ED APO with shifted to violet-blue color correction curve. Saturn was neutrally good.

OTOH, I have the possibility to observe through Zeiss AS refractor 130mm F/15 with color correction e-d-C, where green and red have common focus close to yellow light focus. All planets look sharp with full wealth of their own details. The only drawback in that I see brighter violet-bluish halo around Jupiter and moon limb. Such AS objective looks less well color corrected, but it works MUCH better than the scope of the same (almost) aperture with shifted to blue-violet curve.

Both scopes work EQUALLY good on DSO and stars dimmer than 2 magnitude.

The resume: shifting the color correction curve to violet-blue in achromats, in semi-apochromats and entry level ED apochromats WORSENS their performance for VISUAL works. Period!

This color correction curve shift serves only to the goal to make these scopes looks better color corrected that they are in the reality - a pure marketing step to stimulate the selling over other refractors with correct day vision color correction curve.

#2 chboss

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Posted 06 December 2012 - 02:23 AM

Interesting thoughts Valery, has come up and was discussed in some german forums over the past year.

best regards
Chris

#3 ValeryD

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Posted 06 December 2012 - 02:33 AM

Interesting thoughts Valery, has come up and was discussed in some german forums over the past year.

best regards
Chris


And what the results of these discussions? Can you give us some direct links to there with translation in English?

Thanks.

#4 astroneil

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Posted 06 December 2012 - 04:44 AM

Last years I see in some refractor (all sourced from China) designs that color correction curve shifted to the blue end of spectrum instead of worsening correction in the orange - red part of spectrum.
The designers/salesmans telling to public that at night time the sensitivity of a human eye in red part of spectrum fall down to 2% - so this part of spectrum is no longer important.

The are correct ONLY if these telescopes (with shifted to blue correction curve) are used for totally night observing - dim stars and dim DSOs. This all. On all other objects, which create enough illumination on retina so that an eye can see color(s) of an object, these telescopes will work poorly, poorer than telescopes with color correction curve which corresponds to day light vision. They will work especially poorly on Mars. An observer will never be able to achieve really good focus. he will always try to re-focus the telescope to achieve better sharpness and contrast with no luck however. I got an old Japan made ED doublet refractor 125mm F/8 with Hoya FCD1 - LaK10 glass combo with color correction curve shifted to the blue part of spectrum. Light blue C line (486nm) has common focus with yellow "d" line (588nm) instead of C line (656nm) .
The result is that I can see the moon with slight reddish borders in craters boundaries between shadows and illuminated pars, I see Jupiter relatively free of blue halo with trace of violet however, but not very sharp and I always feel the necessity to adjust the focus to make the planet sharper with no luck.
Mars was the worst - the image never was really sharp very unpleased view. 100mm F/7 ED refractor (Jinghua made) with good color correction in blue-red and as is in violet, showed Mars totally color free and more details than this 125mm ED APO with shifted to violet-blue color correction curve. Saturn was neutrally good.

OTOH, I have the possibility to observe through Zeiss AS refractor 130mm F/15 with color correction e-d-C, where green and red have common focus close to yellow light focus. All planets look sharp with full wealth of their own details. The only drawback in that I see brighter violet-bluish halo around Jupiter and moon ring. Such AS objective looks less well color corrected, but it works MUCH better than the scope of the same (almost) aperture with shifted to blue-violet curve.

Both scopes work EQUALLY good on DSO and stars dimmer than 2 magnitude.

The resume: shifting the color correction curve to violet-blue in achromats, in semi-apochromats and entry level ED apochromats WORSENING their performance for VISUAL works. Period!

This color correction curve shifting serves only to the goal to make these scopes look better color corrected that they are in the reality - marketing step to stimulate the selling over other refractors with correct day vision color correction curve.


Dear Valery,

You have a point there. I think it's true.

Jupiter also will appear washed out a bit in these instruments.

Regards,

Neil. ;)

#5 ukcanuck

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Posted 07 December 2012 - 05:33 AM

Hi Valery, this is interesting stuff...

You said you noticed this last year...were the refractors new examples or had they been around awhile? I don't wish you to name names or anything, I'm just simply curious if you see this optical tweak as being something new. :question:

#6 suburbanskies

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Posted 07 December 2012 - 10:08 AM

Thank you Valerie for explaining this. I have always thought "why doesn't anyone talk about this"?

I'm afraid that telescope makers are tempted to sell scopes that look good in a test (hey, so little blue bloat) but really performs worse if you are interested in planetary observations!

Hopefully refractor fans are more knowledgeable now...

Mark

#7 7331Peg

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Posted 08 December 2012 - 01:25 AM

For what an APO costs, I would be a whole lot less than thrilled if I found that was the case. It's not as if Jupiter and Mars are unlikely objects.

Another reason to think achromatically.


John :refractor:

#8 ValeryD

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Posted 08 December 2012 - 02:15 AM

Hi Valery, this is interesting stuff...

You said you noticed this last year...were the refractors new examples or had they been around awhile? I don't wish you to name names or anything, I'm just simply curious if you see this optical tweak as being something new. :question:


I didn't say I noticed this effect last years. I noticed it immediately when I tested that japanese 125mm F/8 ED doublet.

I wrote that I noticed that last years some manufacturers/salesmans started to promote their newly designed "apochromats" with a trick of color correction curve shifting to the blue in trade off of worsening correction in orange - red.
30%, 50% of "improvement", "less colors seen" etc etc - all these marketing hypes to stimulate peoples to buy "new" product with WORSER properties than traditional ones.






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