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Jon Isaacs
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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: Vondragonnoggin]
      #6488284 - 04/25/14 04:14 PM

Quote:

Nice try to school me about filter basics, but really you're talking to the wrong person trying to school me on it.





Eric:

Please be clear, I am not trying to school you on anything. I am trying help first time observers understand achromats, apochromats, the differences, and now, deep sky filters.


Jon


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Kevdog
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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: rsimpkins]
      #6488288 - 04/25/14 04:16 PM

Think of a prism. It spreads out the white light from the sun (or other source) into a rainbow of colors. A lens is basically a prism that varies across it's diameter.
The more precisely it is built, the narrower the band of the prism effect. (+ $$$)
The better the glass is, the more narrow the band of the prism effect. (+ $$$)
Using more lenses can narrow the band of the prism effect. (+$$$)
Better coatings can narrow the band of the prism effect (+$$$)
A longer focal length narrows the band of the prism (+ length/weight)

The more precisely each band of light is bent to be the same as the others, the clearer your image is. Camera lenses are the same. There's a reason that a $200 "kit" lens doesn't have near as sharp a picture as a $2000 lens. Better glass, better engineering, better coatings.

It's not just the purple color that's getting moved about, it's the other bands as well. That's why you lose detail and sharpness. It's just the purple gets "thrown off the edge" while the others "mix in the middle", which hurts sharpness and contrast between the color boundaries.

So a cheap achro can show lots of purple (and lots of loss in sharpness and contrast) due to average mfg tolerances, cheaper glass and coatings.
An expensive achro will greatly reduce it (better engineering, glass and coatings)
A "cheap" APO may be similar to an expensive achro if the engineering and the coatings don't make up for the extra glass (I'm not sure if these exist, but I'm betting there's at least a couple out there)
An expensive APO combines all the best stuff together to make that light converge as closely as possible.


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Jon Isaacs
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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: Tony Flanders]
      #6488291 - 04/25/14 04:18 PM

Quote:

Quote:

What a lot of great information. I am quickly becoming enamored with the idea of owning an APO refractor, largely based on the information in this thread.




APOs are wonderful, no doubt about it. But just remember that Newtonians are even more free from color, at a tiny fraction of the cost. There's a reason that Newtonians have been the tool of choice for most serious planetary observers for more than a century.






A 4 inch apo can provide the best possible views that a 4 inch telescope can provide...

But a decent 10 inch Newtonian will, if cooled and collimated, and if the seeing is decent, provide more planetary detail, more fine scale contrast. It's more effort but it pays off at the eyepiece.

Jon


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GOLGO13
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Loc: St. Louis area
Re: Achromats & Apochromats - Cutting Past The Hype new [Re: Jon Isaacs]
      #6488313 - 04/25/14 04:34 PM

We should point out the other benefit of the 10 inch dob. DSOs such as Globular Clusters come out and say hello! To me that was the biggest difference between a smaller scope and the 10 inch. And even more fun in a dark sky.

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Abhat
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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: Jon Isaacs]
      #6488321 - 04/25/14 04:39 PM

If we compare apples to apples, would 4" or 5" Newtonian cool just as fast AND be just as portable as a 4" APO refractor and provide similar CA free views?

If it is pretty close then what are other technical reasons I would want to buy 4" APO and not a 4"/5" Newtonian at 1/10th the price.


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Ed Holland
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Reged: 06/16/10

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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: Abhat]
      #6488329 - 04/25/14 04:46 PM

I also have a "beginner level" 4.5" f/8 Newtonian reflector. It gives excellent views, colour free of course . That telescope afforded me the first ever look at a Jovian shadow transit. Again, very useable instrument - especially as it was the only 'scope I had at the time.

It is easy to be spoiled


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GOLGO13
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Reged: 11/05/05

Loc: St. Louis area
Re: Achromats & Apochromats - Cutting Past The Hype new [Re: Abhat]
      #6488335 - 04/25/14 04:51 PM

Personally I don't think a 4-5 inch newt would cool as fast. especially if it were really cold out. My 4 inch refractor takes about 15-20 minutes when it is 20 degrees F. That's pretty darn fast. Mine is a doublet though...I'd imagine a triplet may take another little bit.

My guess is a 4-5 inch newt would be more like 45 minutes to an hour in that situation.

I think quality needs to be brought up here as well. Most apo refractors are of better quality all around (focusers, etc). So it's more than just the expensive glass. But, certainly you can get a decent newt for cheap. The XT6 I had only cost me $235 new (they are more like $300 now). It rivaled my 4 inch apo...not as sharp and not as good mechanics, but brighter and still very capable.

I think some of the smaller newts do not appear to have the same quality as the larger ones. Kind of depends on the company. Of course you can get a 5 inch Takahashi newt for $3000...so even with a newt you can pay for quality. Of course it is somewhat of a specialized newt for imaging.


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Dave Lee
sage


Reged: 02/14/13

Loc: Pinehurst, NC USA
Re: Achromats & Apochromats - Cutting Past The Hype new [Re: Jon Isaacs]
      #6488339 - 04/25/14 04:56 PM

Quote:

Quote:

Nice try to school me about filter basics, but really you're talking to the wrong person trying to school me on it.





Eric:

Please be clear, I am not trying to school you on anything. I am trying help first time observers understand achromats, apochromats, the differences, and now, deep sky filters.


Jon




Jon, thank you for stating this clearly, concisely, and better than I was about to.

dave


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StarStuff1
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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: Dave Lee]
      #6488384 - 04/25/14 05:21 PM

Dave, golfing will cost you as much or more than astronomy. Be careful out there!

ps: better apo than achro if same aperture and focal ratio


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Dave Lee
sage


Reged: 02/14/13

Loc: Pinehurst, NC USA
Re: Achromats & Apochromats - Cutting Past The Hype new [Re: StarStuff1]
      #6488424 - 04/25/14 05:42 PM

Quote:

Dave, golfing will cost you as much or more than astronomy. Be careful out there!

ps: better apo than achro if same aperture and focal ratio




This is interesting. In golf the expense is primarily in the "process of playing the game". While you can spend as much as you wish on high end golf equipment , ultimately the costs pale in comparison to high end astro equipment costs. But the process of 'observing' from the best of locations, while maybe involving some travel expense, tends to be low cost or free compared to "the equivalent" golf experience (as in a really high end course like Pebble Beach).

That aspect of the two hobbies is most interesting, IMHO.

dave


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*skyguy*
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Reged: 12/31/08

Loc: Western New York
Re: Achromats & Apochromats - Cutting Past The Hype new [Re: StarStuff1]
      #6488471 - 04/25/14 05:58 PM

Achromatic come from the Greek word "akhromatos" ... meaning "without color".

Apochromatic also means "without color ... except this time we really mean it ... well, at least some of the time!"


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obin robinson
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Reged: 10/25/12

Loc: League City, TX
Re: Achromats & Apochromats - Cutting Past The Hype new [Re: Abhat]
      #6488490 - 04/25/14 06:06 PM

Quote:

If it is pretty close then what are other technical reasons I would want to buy 4" APO and not a 4"/5" Newtonian at 1/10th the price.




The 4" apo makes a heck of a camera lens or spotting scope. It also is going to be a bit easier to travel with especially if you fly a lot. With an apo you have a quick setup and you don't have to mess with collimation. They also will retain their value over a long time. Just look at how the Pentax, Tele Vue and Takahashi apos hold up over time compared to a newt that was 1/10 of the price.

I'd get this in a heartbeat if I wasn't aggressively paying off some loans. http://www.vixenoptics.com/refractors/VSD.html

obin


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LDW47
scholastic sledgehammer


Reged: 03/04/12

Loc: North Bay,Ontario,Canada
Re: Achromats & Apochromats - Cutting Past The Hype new [Re: Kevdog]
      #6488701 - 04/25/14 08:15 PM

Quote:

Think of a prism. It spreads out the white light from the sun (or other source) into a rainbow of colors. A lens is basically a prism that varies across it's diameter.
The more precisely it is built, the narrower the band of the prism effect. (+ $$$)
The better the glass is, the more narrow the band of the prism effect. (+ $$$)
Using more lenses can narrow the band of the prism effect. (+$$$)
Better coatings can narrow the band of the prism effect (+$$$)
A longer focal length narrows the band of the prism (+ length/weight)

The more precisely each band of light is bent to be the same as the others, the clearer your image is. Camera lenses are the same. There's a reason that a $200 "kit" lens doesn't have near as sharp a picture as a $2000 lens. Better glass, better engineering, better coatings.

It's not just the purple color that's getting moved about, it's the other bands as well. That's why you lose detail and sharpness. It's just the purple gets "thrown off the edge" while the others "mix in the middle", which hurts sharpness and contrast between the color boundaries.

So a cheap achro can show lots of purple (and lots of loss in sharpness and contrast) due to average mfg tolerances, cheaper glass and coatings.
An expensive achro will greatly reduce it (better engineering, glass and coatings)
A "cheap" APO may be similar to an expensive achro if the engineering and the coatings don't make up for the extra glass (I'm not sure if these exist, but I'm betting there's at least a couple out there)
An expensive APO combines all the best stuff together to make that light converge as closely as possible.


Then throw in just average seeing that you get at least 50% of the time or more on those clear nights and what do you get for all your $$$$ ? Is it really worth it ? Maybe we should work that into the mix when educating newcomers ? It always seems to get lost in the expertise thrown about between experts as they "educate" those less expert !

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David PavlichAdministrator
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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: jgraham]
      #6488888 - 04/25/14 10:15 PM

Quote:

"...but at f8, it's not as bad as the faster AR6 class..."

Hmmm, the AR6 is also f/8. The ES6s are f/6.5s. I took a hard look at the ES152 before settling on a used AR6. I was concerned about the color in a faster achromat. It turned out okay for me.




My error!

David


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jgraham
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Reged: 12/02/04

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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: David Pavlich]
      #6488929 - 04/25/14 10:41 PM



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keithlt
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Reged: 12/21/13

Loc: Arizona
Re: Achromats & Apochromats - Cutting Past The Hype new [Re: jgraham]
      #6488937 - 04/25/14 10:51 PM

great info in this thread, so if a person is color blind an AR would better?

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Tony Flanders
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Reged: 05/18/06

Loc: Cambridge, MA, USA
Re: Achromats & Apochromats - Cutting Past The Hype new [Re: Abhat]
      #6489228 - 04/26/14 05:55 AM

Quote:

If we compare apples to apples, would 4" or 5" Newtonian cool just as fast AND be just as portable as a 4" APO refractor and provide similar CA free views?




As far as CA is concerned, Newtonians are always better than APOs. False color is inherent in refractors -- it may be too small for you to see, but it's always there. Conversely, Newtonians are inherently 100% color-free.

However, CA is not the end of the story. Newtonians have central obstructions, which degrade the image significantly, though in a completely different way from chromatic aberration. And unless they have first-class mirrors, they also scatter more light than refractors.

Putting it all together, I would say that a first-rate 5-inch f/5 Newtonian would deliver about the same planetary detail as a first-rate 4-inch f/6 APO. And with the aid of a fan, it would cool nearly as fast -- but probably still not as fast. In terms of portability, it would be roughly a draw. The Newtonian tube would be a smidge bigger, but it would require a shorter mount because you view from the top rather than the bottom of the tube.

However, a first-rate 5-inch f/5 Newtonian with a fan is not a standard commodity! You could get one custom-made for less than the cost of most 4-inch APOs, but it still wouldn't be cheap.

A more normal comparison is between a 4-inch f/6 APO and a standard commercial 6-inch f/8 Dob. Here, the Dob will cost much less and deliver significantly better planetary images. But it will take longer to cool and be significantly less portable.


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LDW47
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Reged: 03/04/12

Loc: North Bay,Ontario,Canada
Re: Achromats & Apochromats - Cutting Past The Hype new [Re: Tony Flanders]
      #6489289 - 04/26/14 07:39 AM

Quote:

Quote:

If we compare apples to apples, would 4" or 5" Newtonian cool just as fast AND be just as portable as a 4" APO refractor and provide similar CA free views?




As far as CA is concerned, Newtonians are always better than APOs. False color is inherent in refractors -- it may be too small for you to see, but it's always there. Conversely, Newtonians are inherently 100% color-free.

However, CA is not the end of the story. Newtonians have central obstructions, which degrade the image significantly, though in a completely different way from chromatic aberration. And unless they have first-class mirrors, they also scatter more light than refractors.

Putting it all together, I would say that a first-rate 5-inch f/5 Newtonian would deliver about the same planetary detail as a first-rate 4-inch f/6 APO. And with the aid of a fan, it would cool nearly as fast -- but probably still not as fast. In terms of portability, it would be roughly a draw. The Newtonian tube would be a smidge bigger, but it would require a shorter mount because you view from the top rather than the bottom of the tube.

However, a first-rate 5-inch f/5 Newtonian with a fan is not a standard commodity! You could get one custom-made for less than the cost of most 4-inch APOs, but it still wouldn't be cheap.

A more normal comparison is between a 4-inch f/6 APO and a standard commercial 6-inch f/8 Dob. Here, the Dob will cost much less and deliver significantly better planetary images. But it will take longer to cool and be significantly less portable.


If you leave it in an unheated / vented shed / garage at the same or close to the same temp as the outside cooldown is negligible if at all. The key is venting / air circulation in your storage area especially in the more humid climates. Also a 6" Dob is still very portable for 90% of observers.

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Tony Flanders
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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: keithlt]
      #6489309 - 04/26/14 08:08 AM

Quote:

great info in this thread, so if a person is color blind an AR would better?




No, not at all! The fundamental problem with achromatic refractors isn't the appearance of color, but the fact that only (at most) two wavelengths can be in focus at one time. The malfocused red and violet light will smear the image regardless of whether you can perceive the colors themselves.


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schang
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Reged: 04/24/13

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Re: Achromats & Apochromats - Cutting Past The Hype new [Re: Tony Flanders]
      #6489317 - 04/26/14 08:13 AM

Quote:


As far as CA is concerned, Newtonians are always better than APOs. False color is inherent in refractors -- it may be too small for you to see, but it's always there. Conversely, Newtonians are inherently 100% color-free.



I think this should be put it in a bulletin or somewhere newbies can easily see. It is delusional for them to read those glorified reviews about APOs without thinking that it is still a refractor. Same for Newtonian, where it is CA free, but it has some quirks as well that can be posted in the same space for newbies to see all at once, rather than going thru the maze of big cannons and small "rifles".


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