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Equipment Discussions >> Refractors

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stargazer193857
scholastic sledgehammer


Reged: 12/01/13

Loc: Southern Idaho
Are apo objectives heavier than achro objectives? new
      #6324788 - 01/20/14 06:58 PM

Does the apo objective have to be made thicker than an achro? I'm just wondering because a lot of high end refractors I've seen advertised tend to be heavier than their cheaper counter parts. I've seen this trend in binoculars too.

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ldesign1
professor emeritus
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Reged: 09/17/09

Loc: Northern Illinois
Re: Are apo objectives heavier than achro objectives? new [Re: stargazer193857]
      #6324843 - 01/20/14 07:37 PM

Well most APOs are doublets or triplets while Achros are usually single lens systems. That would account for most of the weight. The lens material may also add to the weight also. Example: fluorite glass would weigh more than cheaper glass elements.

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Tak North
professor emeritus


Reged: 11/28/07

Loc: Anchorage, Alaska
Re: Are apo objectives heavier than achro objectives? new [Re: ldesign1]
      #6324862 - 01/20/14 07:52 PM

IMO, lens weight is not the main issue here (sure, triplet objectives are heavier than a doublet, but that's probably only a small factor of the total increased weight). Most APO manufacturers tend to overbuild their tube assemblies using premium focusers, heavy tubes, dew shields, and accoutrements - probably in an appeal to the high dollar end of the market. This all adds up to a lot of weight. Many of the cheap Acros OTOH are built just the opposite - with cheap, thin walled materials, undersized tubes, etc in order to appeal to the low cost market.

For example, compare an Orion ST-80 with a TV-76. Roughly equivalent apertures, but the TV is probably 3X the weight! I highly doubt that is due to just the weight of the glass.


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jrbarnett
Eyepiece Hooligan
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Reged: 02/28/06

Loc: Petaluma, CA
Re: Are apo objectives heavier than achro objectives? new [Re: stargazer193857]
      #6324864 - 01/20/14 07:55 PM

Absolutely. Even the doublets. ED glass is heavier than standard optical glass at a given volume. You'll notice that doublet and triplet APOs are often front heavy. Not so much with achromats.

ED glass is softer too.

- Jim

Edited by jrbarnett (01/20/14 07:57 PM)


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Eddgie
Postmaster
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Reged: 02/01/06

Re: Are apo objectives heavier than achro objectives? new [Re: ldesign1]
      #6324871 - 01/20/14 07:57 PM

Quote:

Achros are usually single lens systems




Achromats are doublet lenses.


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Eddgie
Postmaster
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Reged: 02/01/06

Re: Are apo objectives heavier than achro objectives? new [Re: stargazer193857]
      #6324910 - 01/20/14 08:12 PM

A true APO will typically have three lenses, while an ED scope or Achromat will have two lenses.

Some of the glasses are a bit heavier, but this difference alone does not usually account for much of the difference.

The extra lens does though.

But also, in a Triplet, the lens cell itself tends to be longer because it has to hold a thicker set of three lenses.

It also has to have provision for adjusting the centering of the lenses.

But even here, the weight difference is not huge.

As an aggregate though it does add up.

But the weight of the lenses and cell are not the only weight.

Many APOs have focusers that are designed to handle the weight of imaging equipment. This puts a lot of strain on the focuser because it has to work smoothly with a big load.

Also, the tubes for the focuser tend to be oversized to ensure that the light cone does not get vignetted when big chip CCDs are used. This means that you also need a visual back for diagonal use while most ED and achomat scopes come with a very light weight aluminum focuser tube and a simple 2" to 1.25" reducer.

Most premium scopes these days come standard with a rotating focuser too.

Many ED and achomat scopes have inexpensive, light weight Crayford focusers, which is a great design as compared to a cheap rack and pinion, but will often lack the load carrying of the focuser on a premium APO.

All of these small differences add up.

In some cases, the company just gets the weight wrong.

I bought a Stellarvue 110ED and SV had listed the weight as 14 Lbs.

Anyway, look to the extra lens, more complicated lens cell, and heavy duty focuser and the three primary reasons why most APOs are a bit heavier than a comparable size and speed ED or achromat.


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stevew
Now I've done it


Reged: 03/03/06

Loc: British Columbia Canada
Re: Are apo objectives heavier than achro objectives? new [Re: Eddgie]
      #6325095 - 01/20/14 09:33 PM

Quote:

A true APO will typically have three lenses



Or even more...

Steve


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StarStuff1
Post Laureate
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Reged: 04/01/07

Loc: South of the Mason-Dixon Line
Re: Are apo objectives heavier than achro objectives? new [Re: stevew]
      #6325137 - 01/20/14 10:04 PM

My C102f is definitely an APO and has two lenses and weighs less than 10 lbs sans diagonal and eyepiece.

You gets what you gets.

And you pays the price.


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Mark9473
Postmaster
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Reged: 07/21/05

Loc: 51N 4E
Re: Are apo objectives heavier than achro objectives? new [Re: stargazer193857]
      #6325532 - 01/21/14 04:53 AM

Quote:

Does the apo objective have to be made thicker than an achro?



One other factor next to all those that have already been mentioned, is that if the apo has a faster f/ ratio, as they often do, then for the same aperture the lens needs to be thicker in the middle.


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Hesiod
sage


Reged: 01/13/13

Re: Are apo objectives heavier than achro objectives? new [Re: StarStuff1]
      #6325533 - 01/21/14 04:55 AM

While it is true that "APOs" are usually heavier than a similar sized "achros" (due also to a better overall built, as previously said), they are also "smaller", so in the end I found that, despite their greater weight, they could ride smaller mounts: my 4" f/10 achro does not work on a Porta, while my 4" f/7.4 ED (1 kg heavier "fully configurated", but 50 cm shorter) could be used with that small mount even at fairly high magnifications

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sg6
professor emeritus


Reged: 02/14/10

Loc: Norfolk, UK.
Re: Are apo objectives heavier than achro objectives? new [Re: StarStuff1]
      #6325922 - 01/21/14 11:15 AM

Quote:

My C102f is definitely an APO and has two lenses and weighs less than 10 lbs sans diagonal and eyepiece.

You gets what you gets.

And you pays the price.




Cannot strictly be an apo, 2 bits of glass will control 2 wavelengths, to be apo it has to control 3 wavelengths, thus the need for the 3rd bit of glass.

It may be good, very good, but strictly without the 3 bits of glass the term apo is incorrect.

As said the definition is such that it cannot satisfy the requirements with 2 glass elements.

The last sentence refers to the 3 color requirement: Defn


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jag767
professor emeritus
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Reged: 06/20/13

Loc: Massapequa, NY
Re: Are apo objectives heavier than achro objectives? new [Re: sg6]
      #6326079 - 01/21/14 12:41 PM

Quote:

Quote:

My C102f is definitely an APO and has two lenses and weighs less than 10 lbs sans diagonal and eyepiece.

You gets what you gets.

And you pays the price.




Cannot strictly be an apo, 2 bits of glass will control 2 wavelengths, to be apo it has to control 3 wavelengths, thus the need for the 3rd bit of glass.

It may be good, very good, but strictly without the 3 bits of glass the term apo is incorrect.

As said the definition is such that it cannot satisfy the requirements with 2 glass elements.

The last sentence refers to the 3 color requirement: Defn




yes 2 element can be a true apo this can and has been discussed to death! And its also completely off topic!


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Mike Clemens
Frozen to Eyepiece
*****

Reged: 11/26/05

Loc: Alaska, USA
Re: Are apo objectives heavier than achro objectives? new [Re: jag767]
      #6326441 - 01/21/14 03:35 PM

As you pass 6" aperture the triplet APO get _dramatically_ heavier than equivalent size achromats.

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Rutilus
Carpal Tunnel


Reged: 12/17/10

Re: Are apo objectives heavier than achro objectives? new [Re: Mike Clemens]
      #6327546 - 01/22/14 02:00 AM

Even my 4 inch Tak TSA-102 was too nose heavy for my liking.

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Mike Clemens
Frozen to Eyepiece
*****

Reged: 11/26/05

Loc: Alaska, USA
Re: Are apo objectives heavier than achro objectives? [Re: Rutilus]
      #6328656 - 01/22/14 03:20 PM

Isn't the achromat glass and optical prescription so well understood that someone could make a lens glass weight calculator?

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Paul Hyndman
sage


Reged: 07/13/04

Loc: Connecticut Shoreline USA
Re: Are apo objectives heavier than achro objectives? new [Re: sg6]
      #6328762 - 01/22/14 04:07 PM

Quote:

Cannot strictly be an apo, 2 bits of glass will control 2 wavelengths, to be apo it has to control 3 wavelengths, thus the need for the 3rd bit of glass.

It may be good, very good, but strictly without the 3 bits of glass the term apo is incorrect.

As said the definition is such that it cannot satisfy the requirements with 2 glass elements.





Ummm... if I'm not mistaken, the "f" in C102f refers to a fluorite element (as in the Takahashi FS series, indisputedly high quality doublet APOs).

Paul


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rnc39560
Pooh-Bah
*****

Reged: 07/23/13

Loc: MS coast
Re: Are apo objectives heavier than achro objectives? new [Re: Paul Hyndman]
      #6330723 - 01/23/14 01:40 PM



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mikey cee
Postmaster
*****

Reged: 01/18/07

Loc: bellevue ne.
Re: Are apo objectives heavier than achro objectives? new [Re: rnc39560]
      #6331232 - 01/23/14 06:48 PM

Does anyone know what Takahashi's FCT 10" triplet lens weighs?? My Istar 10" R30 doublet weighed in at 30.5 pounds!! You boys talk about front end heavy. Mike

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