Author Topic: ST-80 secondary spectrum reduction  (Read 130 times)

pensranbafarc

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ST-80 secondary spectrum reduction
« on: December 30, 2017, 06:13:27 PM »
There have been many discussions here and over in the refractor forum about reducing secondary spectrum in normal achromats. I have a little ST-80 (80mm f/5 doublet) that I use as a finderscope on my 16" f/6 Newt, and it exhibits the usual copious helping of unwanted color fringing. I thought it would be interesting to see if I could reduce its color down to semi-apo levels. Edward McCarthy's US Patent 2,698,555 (1955) describes methods and layouts for reducing achromat secondary spectrum that I found interesting and possibly useful, and I've attached it here, along with graphics, listings and OSLO files. As you can see from the spots, I was able to reduce the large secondary spectrum of the basic 80mm f/5 achromat to very low levels at higher focal ratios. If built, the images from either the f/6 or f/7 color corrected system would approach semi-apo performance.
This technique really makes me wonder if the seemingly hard-to-find Chromacor is based in part on McCarthy's color corrector ideas. More interesting, could Televue and other eyepiece suppliers develop a new line of McCarthy-like color correctors? They make great coma correction optics for fast paraboloids, so why not color reduction optics for refractors? I'd think they'd sell well, allowing owners of almost any conventional achromat to step up their scope performance considerably.
Something to think about.
Mike

<a href="https://www.cloudynights.com/index.php?app=core&amp;module=attach&amp;section=attach&amp;attach_id=732940" title="Download attachment">Attached File[/url]
<a href="https://www.cloudynights.com/index.php?app=core&amp;module=attach&amp;section=attach&amp;attach_id=732940" title="Download attachment">ST-80 nominal f_5 design.len[/url] 615bytes
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<a href="https://www.cloudynights.com/index.php?app=core&amp;module=attach&amp;section=attach&amp;attach_id=732941" title="Download attachment">Attached File[/url]
<a href="https://www.cloudynights.com/index.php?app=core&amp;module=attach&amp;section=attach&amp;attach_id=732941" title="Download attachment">ST-80 f_5 objective, f_6 CC.len[/url] 934bytes
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<a href="https://www.cloudynights.com/index.php?app=core&amp;module=attach&amp;section=attach&amp;attach_id=732943" title="Download attachment">Attached File[/url]
<a href="https://www.cloudynights.com/index.php?app=core&amp;module=attach&amp;section=attach&amp;attach_id=732943" title="Download attachment">ST-80 f_5 objective, f_7 CC.len[/url] 908bytes
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Lcs King

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Re: ST-80 secondary spectrum reduction
« Reply #1 on: January 08, 2018, 10:09:12 PM »
Interesting stuff here Mike.

I do have to wonder (particularly for the f/6 design), if one could just ditch the original BK7/F2 achromat and make a comparable semi-apo triplet for the same amount of time, effort and money?

schemsucopost

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Re: ST-80 secondary spectrum reduction
« Reply #2 on: January 09, 2018, 09:44:40 PM »
Right Dave, for a small achro like the ST-80, if secondary spectrum is particularly bothersome, just pay a little more and get a better lens. I'm thinking more along the lines of amateurs that already own big to &gt;&gt;BIG&lt;&lt; refractors, like D&amp;G's, ISTARs, Brandts, etc. Secondary spectrum reduction in large observatory refractors has been studied by Ross and others (see attached paper, for example). But I'm addressing the hundreds of existing "mid-range" achromatic refractors in the 6" to 12" range with significant secondary spectrum that a device from Televue or others could benefit the most from. If priced reasonably, it would not seem to me to be a trivial market.
Mike

Dave Fair

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Re: ST-80 secondary spectrum reduction
« Reply #3 on: January 10, 2018, 01:18:24 AM »
*Five* full- or near full-aperture elements? Yikes! A two- or three-element apo or semi-apo would surely be more 'efficient' and perhaps cheaper?

Ronald Bryan

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Re: ST-80 secondary spectrum reduction
« Reply #4 on: January 10, 2018, 10:33:51 AM »
Agree Glenn, for larger refractors the color reducer would certainly have to be located closer to focus and have a smaller aperture to be cost effective and practical. I've only looked briefly at moving closer to the focal plane, but will continue to explore doing so. Do you have any designs or technical publications along this line of thinking you could offer?

I just tweaked in a design for my 6" f/10 achromat that looks interesting. Here's a shot of it mounted up on my Paramount.


revekosque

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Re: ST-80 secondary spectrum reduction
« Reply #5 on: January 11, 2018, 02:44:35 PM »
And here are the plots without and with the color corrector. The McCarthy corrector does a particularly good job in bringing the g-line under control with the rest. I don't mind at all that the focal ratio went from f/10 to f/14.
Mike


Jeff Smith

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Re: ST-80 secondary spectrum reduction
« Reply #6 on: January 11, 2018, 05:47:17 PM »
There have been several threads discussing "Chromacorrs", which seems to match the intent. This thread discusses some "production" models ...

http://www.cloudynig...s-a-chromacorr/

Here is a discussion of a Chromacorr II ...

http://www.cloudynig...hromacor-ii-r17

pesorramidd

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Re: ST-80 secondary spectrum reduction
« Reply #7 on: January 11, 2018, 11:53:13 PM »
Mike,
I haven't dabbled in design, and so my remarks are just the noise of the 'armchair expert."

John Jankowski

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Re: ST-80 secondary spectrum reduction
« Reply #8 on: January 18, 2018, 05:40:01 AM »
Quote

One of the issues with the Chromacorr was it's sensitivity to alignment and position. According to Roland C, the elements of an apo triplet need to be centered within a few microns, at least in some cases. How sensitive are these designs to alignment and position?

Jon

memeforvi

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Re: ST-80 secondary spectrum reduction
« Reply #9 on: January 18, 2018, 06:03:53 AM »
Quote
Agree Glenn, for larger refractors the color reducer would certainly have to be located closer to focus and have a smaller aperture to be cost effective and practical. I've only looked briefly at moving closer to the focal plane, but will continue to explore doing so. Do you have any designs or technical publications along this line of thinking you could offer?

I just tweaked in a design for my 6" f/10 achromat that looks interesting. Here's a shot of it mounted up on my Paramount.

First light with 6in f10 refractor May 21 2016 700x1116.jpg

Hola Mike,

First thing...I really like you blue comfy chair in the background!!!

As far as this corrector goes in the current design, I would wonder
about adding 3 more thick elements and the cool down that they
would add to the total system...

Thinking that if they could be closer to focus, one smaller and cheaper
with less heat sink effect???

Just my humble thoughts...so many with trying to lighten my inventory
of optical glasses...

Best Regards,

Preston

Santosh Wolf

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Re: ST-80 secondary spectrum reduction
« Reply #10 on: January 21, 2018, 12:11:54 PM »
That chair squirts me with sleeping gas every time I sit down in it. I've never found the tank, but it appears to be inexhaustable.

rioclamabik

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Re: ST-80 secondary spectrum reduction
« Reply #11 on: January 23, 2018, 02:20:50 AM »
Hmm, I like that toy on the mount!
M.

frenafverbi

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Re: ST-80 secondary spectrum reduction
« Reply #12 on: January 23, 2018, 04:12:17 PM »
"Do you have any designs or technical publications along this line of thinking you could offer?"
***************
 Replace the relatively fast stock doublet with a long singlet???????
M.