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Equipment Discussions >> Refractors

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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: Are achromats enjoying a come back? new [Re: VNA]
      #5523536 - 11/16/12 10:09 PM

I think about that very refractor as an achromatic companion to my apochromat, but I don't want to think about the mount it would require.

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jrbarnett
Eyepiece Hooligan
*****

Reged: 02/28/06

Loc: Petaluma, CA
Re: Are achromats enjoying a come back? new [Re: VNA]
      #5523546 - 11/16/12 10:19 PM

The problem with the "law of diminishing returns" is that, as applied to telescopes, it is entirely subjective.

For example, observer A and B hate false color. Observer B also hates reflectors. Observer C dislikes small aperture and also dislikes false color. Observer D doesn't mind false color doesn't mind reflectors, but would like to save as cash as possible. The four are on budgets of $1000 They are asked to consider four scopes; a 12" f/5 Dob for $1000, a 6" f/8 achromat for $500, a 4" f/15 achromat for $800 and a 3" f/6 apochromatic triplet for $1000 (all real life scopes, available today).

Let's apply the law of diminishing returns to these three observers. Observer A hates false color. His personal cost-benefit analysis says that he could go either 3" f/6 APO or 12" f/5 Dob, avoid dreaded false color, and get what he perceives to be "value" with either scope for his money the extra he spends to avoid his dislike (false color) is a small price to pay; in fact it is a necessary price for him to pay to be satisfied. Observer B has only one choice, because for him, reflectors don't cut it, and he goes with the 3" APO. For him, it is the only choice that makes sense given his preferences. Observer C, too, has only one choice. 3" won't cut it, so he goes for the $1000 12-incher. Observer D is tolerant of false color, and above all values economy. He could go foe either the 6" f/8, get more aperture and false color, but also pocket $500 or the 4" f/15 and get some, but a bit less false color, but less aperture and have $300 less left over. He goes for aperture and $500, and buys the 6" f/8. Each of these observers has applied a version of the law of diminishing returns (given the limited scope pool in the example) and come up with a different answer.

A "law" that, when applied, results in different answers, isn't all that useful as an analytical tool.

- Jim


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Daniel Mounsey
Vendor (Woodland Hills)
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Reged: 06/12/02

Re: Are achromats enjoying a come back? new [Re: chboss]
      #5523558 - 11/16/12 10:27 PM

Just an FYI. Be careful using a dew zapper on the end of your dew shield like that. They will KILL your images since heat currents pour off them like a furnace. If you saw it in the streak test, it would shock you! Back on topic, I also love achromats.

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Abb
super member


Reged: 11/19/10

Loc: Windsor Ontario Canada
Re: Are achromats enjoying a come back? new [Re: Daniel Mounsey]
      #5523695 - 11/17/12 12:19 AM

Or, one can simply avoid ""law of diminishing returns" or any other "laws" and buy whatever he or she wants and not care what anyone else thinks of their choice

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stanislas-jean
Carpal Tunnel


Reged: 10/22/08

Re: Are achromats enjoying a come back? new [Re: chboss]
      #5523856 - 11/17/12 04:32 AM

I donot share exactly the same view.
You could go to the fabricator with the test results and it will be also a non productive discussion mostly.
My feeling is that the scope tested 150mm apo is under the fabricator standard and at the margin of the tolerance standard. Apo doesn't mean 0 spherochromatism aberration even on a 10 000 OTA. OK.
However the second OTA tested still same fabricator and diameter is on the max narrow possible tolerance. Don't imagine that we can get 80% of the fabrication serie like this, just few units.
The "bad" OTA is not a lemon because giving high contrasted views in integrated light (the B and R channels on that global view having a confidential influence visually) but the possible applications are limited.
It shall be difficult to find such OTA with more than 90% strehl in B channel, except built especially, and leaving less performance at the opposite end of the spectrum.
Frankly serious studies are undertaken with reflectors with applications possible from the UV until the IR channels.
For refractors achromats are well enough on planets, a 6" is collecting say already 80% the events occuring, they are cheap enough and enoughly performing. An excellent entry level OTA.
After a good reflector will help for second step performance at the edges of the visible spectrum.
Stanislas-Jean


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wh48gs
Carpal Tunnel
*****

Reged: 03/02/07

Re: Are achromats enjoying a come back? new [Re: chboss]
      #5523958 - 11/17/12 08:51 AM

Quote:


For your comparison I give you this link to another AP telescope a 155EDT from the early 90's with a bit more information:
http://www.astro-foren.de/showthread.php?10849-APO-von-Astro-Physics-mit-Glas...




There is no comparison between f/9 and f/6.3 apo of a similar design - the difference in the magnitude of spherochromatism is huge (aperture size is much less of a factor, with the error being nearly proportional to it).

Both units are well corrected close to the optimized wavelength. Being 0.89 Strehl @635nm indicates good - possibly true apo correction level in the red - for the f/6.3. The RMS error and the longitudinal aberration plot indicate about 1/3 wave p-v of mainly pure secondary spherical (changes with the 4th power of zonal height longitudinally, as opposed to primary spherical which changes with the square) @475nm. Considering that the error typically increases significantly faster toward the violet end, the error @435 (g-line) is probably sufficiently larger than 1/2 wave p-v to prevent the objective to be considered a "true apo" according to the technical definition (that wouldn't make it "useless").

However, considering low eye sensitivity in this spectral range, it would not necessarily - and in this case probably wouldn't - prevent it from achieving better than 0.95 polychromatic photopic Strehl, which is a more reliable criterion of the correction level (the shift in eye sensitivity toward blue/violet under the typical observing conditions would push the Strehl down, but we can only guess to what extent).

Also, the French test is somewhat odd in that it gives Zernikes in microns (usually in units of wavelength when evaluating optics). For some reason, it accentuates the primary spherical term, while discarding piston, tilt and defocus. That doesn't make much sense, since any of these terms can be, and often are, used in modeling the wavefront. Any term alone can be more or less offset by other terms, hence its value alone is a poor indicator of its actual magnitude, whenever values of other terms are not negligible relative to it. For some reason, the (eye) pupil size is added to the Zernike data which, again, indicates that the software was set for ophthalmology, and that Zernikes were calculated from the error in diopters (any longitudinal aberration can be expressed in diopters). Since the pupil is 12% larger for the 473 wavelength, it could imply that the nominal error here is about 25% larger (proportional to the square of pupil diameter) than what it really is, or that the actual error is closer to 1/4 wave p-v (but that, on the other hand, wouldn't agree with the longitudinal plot).

Rohr's display isn't impressive either: a bunch of graphs, photos, interferograms, screen snapshots, and what not - including that no-good RC_Index (which, basically, compares apos and achros based on the spectral defocus at 0.707 zone while entirely neglecting type and magnitude of spherical aberration). All that is needed for evaluating correction level in a refractor is polychromatic Strehl and longitudinal aberration plot accompanied with the RMS error at selected wavelengths (corresponding p-v wavefront maps don't hurt).

Vla


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Binojunky
Carpal Tunnel
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Reged: 12/25/10

Re: Are achromats enjoying a come back? new [Re: wh48gs]
      #5524112 - 11/17/12 10:40 AM

Well all I know is that the previous post lost me altogether, and after observing Jupiter, the Plaides and Orion last night with my lowly 4.5" Starblast and seeing a meteor or two with my refracting eyeballs I want no stinking apos in my back yard, DA.

Edited by Binojunky (11/17/12 10:45 AM)


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stanislas-jean
Carpal Tunnel


Reged: 10/22/08

Re: Are achromats enjoying a come back? new [Re: wh48gs]
      #5524133 - 11/17/12 10:48 AM

I think the zernikes as you wrote are expressed in nm and it is easy to express them in wavelength, after the bulletin could be converted, but we have those and interresting to read anyway.
It is important to highlight the fact that 3 channels are explored and corresponding to the tricolor imaging set.
This is oriented to imaging (where aberrations are involving sensitive imbalances on the final image) and anyway the measure machine get the polynome that must be read as polynome.
These are bulletins but what are your conclusions about them and the OTA behind? excellence, lemon, can be improved, etc...
The average one is the result assembly of the fabricator having the last 3 lenses only available?
Stanislas-Jean


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chboss
professor emeritus


Reged: 03/24/08

Loc: Zurich Switzerland
Re: Are achromats enjoying a come back? new [Re: stanislas-jean]
      #5524770 - 11/17/12 05:44 PM

Quote:


Frankly serious studies are undertaken with reflectors with applications possible from the UV until the IR channels.





So your recommendation is, if you are serious, to buy a reflector?
Well each telescope has its use I guess and rest my case.

best regards
Chris


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rwiederrich
Goldfinger
*****

Reged: 11/17/05

Loc: Always Dark skies of Belfair W...
Re: Are achromats enjoying a come back? new [Re: chboss]
      #5524853 - 11/17/12 06:31 PM Attachment (44 downloads)

Refractors have been coming back for me since 1970.....

Currently.......this'll do.

Rob


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chboss
professor emeritus


Reged: 03/24/08

Loc: Zurich Switzerland
Re: Are achromats enjoying a come back? new [Re: wh48gs]
      #5524864 - 11/17/12 06:38 PM

Quote:


There is no comparison between f/9 and f/6.3 apo of a similar design - the difference in the magnitude of spherochromatism is huge (aperture size is much less of a factor, with the error being nearly proportional to it).




Hi Vla

I am fully aware of this fact. It was on purpose that I linked the 155EDT because it is an ideal visual telescope at f9 and Stanislas is always talking about visual.
You should ask him why he pulled out the test result for the AP 130EDF that is optimized for CCD work...

best regards
Chris


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iceblaze
Carpal Tunnel
*****

Reged: 10/15/11

Loc: 33.9° N, 118.4° W Lawndale, ...
Re: Are achromats enjoying a come back? new [Re: rwiederrich]
      #5524924 - 11/17/12 07:15 PM

Quote:

Refractors have been coming back for me since 1970.....

Currently.......this'll do.

Rob




Fantastic! Love the chalk board

-James


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rwiederrich
Goldfinger
*****

Reged: 11/17/05

Loc: Always Dark skies of Belfair W...
Re: Are achromats enjoying a come back? new [Re: iceblaze]
      #5525050 - 11/17/12 08:39 PM

Quote:

Quote:

Refractors have been coming back for me since 1970.....

Currently.......this'll do.

Rob




Fantastic! Love the chalk board

-James




I write down the nights images and exposure times on that.....it makes for great reference.....plus during outreach with the boyscouts it makes a good class room.

Rob


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orion61

*****

Reged: 10/20/07

Loc: Birthplace James T Kirk
Re: Are achromats enjoying a come back? new [Re: Nippon]
      #5525088 - 11/17/12 09:03 PM

Quote:

There seems to be a resurgence in interest in the achromatic scopes. If so what are some of the reasons do you think. I have a Vixen A 105 M. I bought it because I wanted a 4" scope with good optical figure for doubles and planets. The color correction is good and I'm pleased with it. But part of my reason was just the classic appeal of a good 4" inch achro.



I woudn't say replace but the newer acros do perform to a higher degree in color correction and final figure.
But they will never replace the premium refractors where the additional performance comes in close to a year of secondary education cost.
Even ED's have dropped in price to where I bought one,
It has fantastic contrast and color correction.
but a 5k APO will NEVER beat a 5K DOB in performance,
A 20" tracking DOB with an 8" off axis mask will give outstanding performance on non Stellar objects..
I have a 5" Celestron Prototype from the 90's that is pretty good. I could never justify the cost! But I can
an ED, with the performance of it mmeade 102 ED/APO but we all know it is Possibly a "Semi" APO.
If I had money laying around I couldnt find anything else to use it on I might buy a Premo APO.
But remember there are those who buy the most expensive
items just because there are no others more expensive.


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mikey cee
Postmaster
*****

Reged: 01/18/07

Loc: bellevue ne.
Re: Are achromats enjoying a come back? new [Re: rwiederrich]
      #5525413 - 11/18/12 01:46 AM

Quote:

Refractors have been coming back for me since 1970.....

Currently.......this'll do.

Rob


Rob my friend that is a real "says it all" pic! Mike

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rwiederrich
Goldfinger
*****

Reged: 11/17/05

Loc: Always Dark skies of Belfair W...
Re: Are achromats enjoying a come back? new [Re: mikey cee]
      #5525427 - 11/18/12 02:01 AM

Quote:

Quote:

Refractors have been coming back for me since 1970.....

Currently.......this'll do.

Rob


Rob my friend that is a real "says it all" pic! Mike




Us old guys...need to use any *analogies* we can muster. that chair is a dental chair and it can go down nearly to the floor....perfect! Thanks bro.


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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: Are achromats enjoying a come back? new [Re: rwiederrich]
      #5525436 - 11/18/12 02:10 AM

Now that is a "Great Refractor"!

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stanislas-jean
Carpal Tunnel


Reged: 10/22/08

Re: Are achromats enjoying a come back? new [Re: chboss]
      #5525568 - 11/18/12 06:17 AM

You see the question is we find good models and also some only average. When the good one is here, keep it.
Now my recommendation, if I can raise one, is to waive to a good reflector, an 8" will suffice for making similar or better. We find excellent corrector also for the the correction of the coma.
The website that I gave here was to highlight the fact that we can met, good units and average ones sold as good. For the investment performed it is a pitty and abnormal.
How many people knows that their models are average? This is disturbing me somewhere and the results brought considered as perfection so well eared should be considered out of concern. The problem is to acquire pertinent data, not only pretty.
Stanislas-Jean


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jrbarnett
Eyepiece Hooligan
*****

Reged: 02/28/06

Loc: Petaluma, CA
Re: Are achromats enjoying a come back? new [Re: Sky Muse]
      #5525816 - 11/18/12 10:28 AM

A "great refractor" is a refractor with aperture that is on par with a medium-sized amateur Newtonian reflector.

- Jim


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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: Are achromats enjoying a come back? new [Re: jrbarnett]
      #5525885 - 11/18/12 11:12 AM

So true, with a "great reflector", that's really not so great after all, requiring a minimum of sixty inches, i.e. Mt. Wilson.

Alan


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