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Equipment Discussions >> Refractors

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la200o
Carpal Tunnel


Reged: 09/09/08

Loc: SE Michigan, USA
Re: Doublet or Triplet? new [Re: Sky Muse]
      #5561529 - 12/08/12 09:22 PM

So I guess the answer to Jon's question is that you've spent zero time at the eyepiece of an NP101 and therefore know nothing about them. The NP101 in fact is not only a fine astrograph, but a superb visual telescope; I'd suggest taking a look through one before offering opinions based on no experience whatever, otherwise it seems like you're just trolling.

Regards,
Bill

Edited by la200o (12/08/12 09:24 PM)


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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: Doublet or Triplet? *DELETED* new [Re: la200o]
      #5561536 - 12/08/12 09:27 PM

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KWB
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Reged: 09/30/06

Loc: Westminster,Co Elev.5400 feet
Re: Doublet or Triplet? new [Re: Sky Muse]
      #5561794 - 12/09/12 01:39 AM

Last warning in this thread:The idea is for it not to focus on personalities between posters,which is a sure fire thread wrecker but simply about experience with and opinions of using various examples of astro gear,with plenty of room left over for our novice members to ask questions.

If any more agenda pushing as to equipment bashing continues, this thread will be well on it's way to being locked.


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Doublet or Triplet? new [Re: Sky Muse]
      #5562114 - 12/09/12 10:06 AM

Quote:

None, for I bought an FS-102 instead, and for visual only. The NP-101 is a "photo-visual" petzval, with "photo" being its strength and intent per design. If I want low-powered views, there are perhaps oculars to accomplish same with the F/8. Besides, I have my eyes, my binoculars and a 6" f/5 Newtonian to enjoy low-powered views when desired.




As someone who has many hours at the eyepiece of an NP 101, I can say that its strength are its wide, flat field combined with essentially perfect color correction. It is an excellent performer at high magnification, the planets and double stars are clean and crisp. At F/5.4 it does require well corrected eyepieces to show its best.

There are certainly other fine 4 in retractors out there and the all around high level of performance of the NP 101 may not be desired or needed but it sure is sweet.

Jon


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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: Doublet or Triplet? new [Re: Sky Muse]
      #5562125 - 12/09/12 10:11 AM

The public are drawn to refractors for a variety of reasons, and though this list does not presume the gamut, consider nonetheless, although not necessarily in order of preference...

1. Simplicity of design, therefore, less maintenance.
2. Portability.
3. Overall quality.
4. Apertural requirements, combined in varying focal ratios and per observing interest.
5. Direct observation versus indirect.
6. Solar observation.
7. Military and civil observation.
8. Imaging.
9. Childhood memories.
10. ...

Lastly, if for nothing else, the fact that it's a refractor, and the original telescopic design.

Alan


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jrbarnett
Eyepiece Hooligan
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Reged: 02/28/06

Loc: Petaluma, CA
Re: Doublet or Triplet? new [Re: la200o]
      #5562537 - 12/09/12 02:14 PM

I guess he was "lucky" in his uniformed analysis. Having looked through several NP-101s as well as TV-101s and Gensises, I think he nailed it.

Just because someone can drive a car, doesn't mean they know squat about its capabilities. Take my mom, for example. She drives a Porsche Cayenne S. Now she couldn't tell you the first thing about its cornering limits, top speed, or how it stacks up in this regard to other vehicles. I've never driven her Porsche, but without ever having driven it, I do know more about its capabilities (as well as how to better utilize them) than she does. Were she to post on a performance automobile forum making claims about her Porsche's capabilities, she'd actually be the one trolling as she hasn't a real clue.

The moral of the story is this: use of an item, telescope, automobile or anything else, should not automatically be equated to being knowledgeable about the item. Likewise, the suggestion that someone who has not used that item cannot, nonetheless, have valid knowledge about it is preposterous. The scientists and engineers that built the space shuttle never flew it, but you can be darned sure that they know much more about the shuttle than any of the pilots.

Regards,

Jim


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bratislav
sage


Reged: 09/07/06

Re: Doublet or Triplet? new [Re: Jon Isaacs]
      #5562918 - 12/09/12 06:30 PM

Quote:


As far as doublets being the best.. they have significant field curvature and less than perfect color correction though in the smaller sizes/slower focal ratios it may not be apparent. They may be the best compromise for certain observers, but triplets and Petzvals can provide more perfect views.




Actually, doublets, triplets, quadruplets and even quintuplets will all have the same field curvature (approx 35% of focal length), as long as all of those lenses are bunched up front.

The only way to flatten the field is to have another lens group widely spaced, closer to focus.
That then becomes a near focal plane field flattener (the way AP and TEC do it), an integrated positive lens group (Petzval, the way TeleVue does it), or negative lens group (as in Takahashi FSQ variants as well as some older Pentax designs).
The key is that second lens group is much closer to focus.

Besides, color correction of doublets is not actually the problem. Triplet does not give more room to a designer to improve color that much (it does, but comparatively very little; residual color is directly proportional to difference in partial dispersion, so given ED and main mating element(s), residual longitudinal color aberration will be very similar in an objective with 2 and 3 elements).
What triplet DOES buy you is the ability to distribute power, and make radii much more shallow compared to a doublet. This results in vastly reduced spherochromatism.
(triplet can also allow the designer to push the correction farther into violet/UV - insignificant for visual, could be important for CCD work)


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la200o
Carpal Tunnel


Reged: 09/09/08

Loc: SE Michigan, USA
Re: Doublet or Triplet? new [Re: jrbarnett]
      #5563017 - 12/09/12 07:24 PM

Quote:

I guess he was "lucky" in his uniformed analysis. Having looked through several NP-101s as well as TV-101s and Gensises, I think he nailed it.

Just because someone can drive a car, doesn't mean they know squat about its capabilities. Take my mom, for example. She drives a Porsche Cayenne S. Now she couldn't tell you the first thing about its cornering limits, top speed, or how it stacks up in this regard to other vehicles. I've never driven her Porsche, but without ever having driven it, I do know more about its capabilities (as well as how to better utilize them) than she does. Were she to post on a performance automobile forum making claims about her Porsche's capabilities, she'd actually be the one trolling as she hasn't a real clue.

The moral of the story is this: use of an item, telescope, automobile or anything else, should not automatically be equated to being knowledgeable about the item. Likewise, the suggestion that someone who has not used that item cannot, nonetheless, have valid knowledge about it is preposterous. The scientists and engineers that built the space shuttle never flew it, but you can be darned sure that they know much more about the shuttle than any of the pilots.

Regards,

Jim




l entirely disagree. These labored analogies are preposterous.

Regards,
Bill

Edited by la200o (12/09/12 07:26 PM)


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KWB
Postmaster
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Reged: 09/30/06

Loc: Westminster,Co Elev.5400 feet
Re: Doublet or Triplet? new [Re: la200o]
      #5563101 - 12/09/12 08:37 PM

Time to let this thread simmer down a bit so it's being locked for the time being subject to another moderator review.

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