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Equipment Discussions >> Refractors

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stanislas-jean
Carpal Tunnel


Reged: 10/22/08

Re: Achro Vs Apo new [Re: Jim Curry]
      #5751522 - 03/23/13 10:43 AM

Interested to have seen a picture of the clark refractor helping Mr O'Meara rotation of Uranus measure.
What I would say, the right refractor is the model which can help you to capture the banding system on Uranus.
We know lot of apos having not this ability with 6" size.
I had the 6" refractor, achro also, that make me with the possibility to.
They are reasons for this result. CA, except if very present, is not a concern on this subject, but the ability to keep images stable at the eyepiece with 375-400x and acurate is recgnised into a refractor design.
One way of the light in the tube is also very important with regards the air currents influence, the elevation of the objective is important for reducing the local seeing (coming from the ground). The design contributes to keep acurate and stable the images at the eyepiece.
The achros are not dead still.
Stanislas-Jean


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Eddgie
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Reged: 02/01/06

Re: Achro Vs Apo new [Re: Lane]
      #5751631 - 03/23/13 11:28 AM

Quote:

I don't know that a lot of detail is lost on a planet because of some CA being present.




Let me assure you that CA can be very damaging. For example, in large, fast acromats, CA can do more damage than a big central obstruction in an SCT.

Now the question becomes on what a "Lot" of detail would be. I one considers the loss of detail from a scope with a 33% obstrutction to be a "Lot", then one would have to say that for the fast achromat, a "lot" of detail is also being lost.

Now I personallly think the loss of detail from obstructions is quite a bit less than most people make it out to be, but it is what it is, and a 6" f/8 achromat is no better than a 6" SCT in this regard.


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Eddgie
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Reged: 02/01/06

Re: Achro Vs Apo new [Re: timps]
      #5751690 - 03/23/13 11:51 AM

I am laughing about the irony that is CN.

While this thread goes on, there is another thread about masking down a 6" f/8 achromat to 80mm. There seems to be some belief that the view might be prettier or better when using this scope as an 80mm achromat than when using it as a 6" f/8.

That makes me laugh so hard in the context of your post. Here we are on CN debating whether it is preferable to mask a 6" f/8 achromat to 3.25" for planets on one page, and on another page, someone wondering if a 6" f/10 scope would be better than their 5.25" APO! Ha. Funny to me! (maybe not to anyone else though).

That should answer your question about 6" f/10 becauase at f/10 you really haven't reduced CA enough that the green weighted poly-strehl is much more than diffraction limited (if that).

Even at f/15, the poly-strehl of an f/15 achromat is no better than you would get from a 6" reflector with a 20% obstruction, which is only going to have the contrast transfer of the aperture that you already have.

Why do people think large achrmats are so great? They talk about obstructions like they are the devil's work, but somehow think CA is a trivial abberation? Even at f/15, you are already falling behind a 6" f/8 20% obstructed reflector.

I don't get it. 6" achrmats are simply not great scopes from an absolute performance standpoint unless you make them f/18 or longer. An MN66 will be equal to it even at f/18.

At 6", and f/10, you would do almost as well with a C6.

Anyway, it is just funny to me. On one page the forum is talking about the benefits of masking down a 6" f/8 achromat to 80mm to improve planetary performacne (or at least make the view "more pleasing") and on another page, we are talking about trading the other way by going up an inch in aperture and accepting contrast lowering CA and some people thinking that it may be worth it!

Is it really just me?

Sorry. Sometimes we get so deafend by the clashes of tiny swords on CN that it is comical.

Dude, I have no interest in what you buy, and I hope you find happiness.

My own key to being a happy planetary observier only came wtih patience and lots of aperture.

But for others, it maybe it can be had using a 6" f/8 achomate masked down to 3.25 inches.

Don't look for us to guide you.

We are all going in to many different directions.


Edited by Eddgie (03/23/13 12:23 PM)


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mark8888
Carpal Tunnel


Reged: 09/24/10

Re: Achro Vs Apo new [Re: Eddgie]
      #5751748 - 03/23/13 12:12 PM

I love it, "deafened by the clashes of tiny swords". That's a great line.

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stanislas-jean
Carpal Tunnel


Reged: 10/22/08

Re: Achro Vs Apo [Re: Eddgie]
      #5751782 - 03/23/13 12:28 PM

The poly strehl of a F15 6" achromat would involve a L/3 around aberration level. But this is concerning contrast levels matter not typically an optical acuracy.
Even with this kind of level I doubt that the 6" sct will be on a similar overall status and will show more.
The use of color filter will improve this average result for a significant gain.
What is researched is the detail access with regards to an aperture and observing in colour channels is not a bad idea.
Unless the esthetic images are only researched therefore for our money bag a simple reflector would be enough with its parallel inconveniences (air currents, acuracy need 2 times better, sensitivity to ground seeing).
What is highlighted is contrast level through posts but there is also resolution ability to accomplish.
If the resolution ability is present with a contrast level still enough to our own eyes we have the scope OK.
Stanislas-Jean


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dan_h
Carpal Tunnel


Reged: 12/10/07

Re: Achro Vs Apo [Re: Eddgie]
      #5751907 - 03/23/13 01:30 PM

Quote:

I am laughing about the irony that is CN................Is it really just me?

Sorry. Sometimes we get so deafend by the clashes of tiny swords on CN that it is comical.

Dude, I have no interest in what you buy, and I hope you find happiness...............

We are all going in to many different directions.







Finally a post that makes sense!

dan


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Eddgie
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Reged: 02/01/06

Re: Achro Vs Apo [Re: stanislas-jean]
      #5751920 - 03/23/13 01:35 PM

Quote:

The poly strehl of a F15 6" achromat would involve a L/3 around aberration level. But this is concerning contrast levels matter not typically an optical acuracy.
Even with this kind of level I doubt that the 6" sct will be on a similar overall status and will show more.




You may have misread what I said or didn't say. Nowhere did I say a 6" SCT would be the equal of a 6" f/15 achromat.

Here is what I said:

Quote:

At 6", and f/10, you would do almost as well with a C6.





I said a 6" 20% obstructed reflector would be about the equal to a 6" f/15 achromat, and I said that an MN66 would be about the equal of an f/18 6" achromat.

And I am the one on the other forum thread that said that reduceing a 6" f/8 scope to 3.25" by a mask would likely not really improve the absolute deatil ability, but that may not be the goal. The goal in that case could have been to get a more pleasing view.

I think though that the goal of the OP is to see more absolute detail and he is willing to do this even if there is some CA.

You can correct me if I am wrong of course, but in the case of even a 6" f/15 achromat, the poly-strel would likely limit it to about the same as his current 5.25" super APO.

And that means that my contention that the 6" f/15 achromat would not really blow him away would be correct because it would not even result in any improvement over where is at right now.

Certainly not enough to blow him away.

Edited by Eddgie (03/23/13 01:38 PM)


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stanislas-jean
Carpal Tunnel


Reged: 10/22/08

Re: Achro Vs Apo [Re: Eddgie]
      #5752068 - 03/23/13 02:59 PM

We are on a certain relativity of facts.
Frankly the 6" sct will not blow the 6" achromat regarding the ability to capture very fine low contrasted details.
It's always matter of numbers characterised on the concerned optics.
The 6" sct will blow the refractor in blue channel
not in green and red channels especially with the use of coloured filter even wide,
the contrast level will be better in the 6" sct for coarse feature and the gain will go to be loosed for fine details.
For being more acurate.
Now apo will blow the achromat and the sct on coarse detail by the contrast level greater but for very fine details we will have to search the gain, sometimes not present.
This is well illustrated here on the sky for each design by visual mean.
The relative superiority of the long achromat stays on the fact that mostly the optic is located 2.5meters above ground.
This makes over a year observation period some gain on the seeing level at the eyepice, usually one step of danjon scale until 2 steps some nights and not so exceptionnal.
A step represents statiscally 20% gain or lost (the difference noted on FTM curves respectively). All of these for visual.
Stanislas-Jean


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beanerds
sage


Reged: 07/15/08

Loc: Darwin Australia
Re: Achro Vs Apo new [Re: orion69]
      #5752368 - 03/23/13 05:49 PM

My Zeiss gives a more natural hue to oblects compared withg my Takahashi , I seen the same thing when using my ( now sold ) 127mm f7.5 NG triplet and my now owned 127mm f8 Istar achro .
True there is CA but in my eyes the very well corrected Achromats ( Zeiss and Istar ) give a more natural view especially Luna and Jupiter .
Might be my eyes aint what they used to be ? but I am not kidding .
Brian.
Quote:

Quote:


Not an easy call , Achro's are more natural .





You are kidding, right?




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beanerds
sage


Reged: 07/15/08

Loc: Darwin Australia
Re: Achro Vs Apo new [Re: azure1961p]
      #5752437 - 03/23/13 06:24 PM

Hi Peter , its a very hard thing to get across in words but I have had both my Zeiss and Takahashi out side by side many times for comparo's using my Zeiss orthos in both to get the same ( well close ) magnifications .
But every time the Zeiss gives a better view , perhaps more natural is the wrong term but it just looks nicer in the achro and you have to realise that the CA in the Zeiss is almost invisable under normal use .
I am not the only one to see this in my 2 scopes many friends see it as well , but without the ambigious side by side comparo , it will not be seen I think .
Sure the Tak is smaller , lighter and way easier to set up but to me the views are more like a laboratory experiment , so clean and sterile it looks unreal ? very hard to describe .

See my last answer here and I mention the same thing was seen using my now sold 127mm NG Apo and my 127mm Istar achromat , the apo was better for sure but not as the price differance dictates , hence I sold the apo and am now very happy with my Istar achromat .
CA does not bother me and I beileve every one's different in their tollerance , you just got to find yours .

I also have a gineric 150mm f8 Saxon ( Synta ) achro in NZ that my Nephew is using while I am here in Australia working ,with a correctly fitted 'Aries Chromocor' and that scope is as good visually up to 300x ( after that the AP struts its stuff with ease ! ) as a friends 152mm f8 AP apo , is the AP worth 10x as much as my Synta ? we regularly did side by side comparos but thats another story .

Diffiuclt to explain , and we wont mention that not everyone has multy $1000's to spend on their hobby .
Brian..
Quote:

Quote:

Quote:


Not an easy call , Achro's are more natural .





You are kidding, right?




I don't think so. What's interesting is that he attests it not his opinion alone with the apo showing a clinical view and the achromatic a more natural view. I'm curious to know what this natural impression consisted of. It's an interesting take. CA had to be seen but despite that a natural quality over the color free image was appreciated. I'd like to hear the details - if he's only respond. The notion is interesting and at no swipe to the apo.

Pete




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Sean Puett
Carpal Tunnel
*****

Reged: 09/06/10

Loc: always cloudy, washington
Re: Achro Vs Apo new [Re: stanislas-jean]
      #5752499 - 03/23/13 07:10 PM

I have a size limit on refractors of 4". There ARE one or two scopes I would violate this limit for but, they are both apos. Maybe the vixen NA140. Anyway, sidetracked again. Long achros are not for me. The well made ones cost more than a newt two to 3 times their size, in some cases and the newt will blind you with so much brightness and detail on planets. By the time you mount that long achro, you could have a nice Zambuto powered dob giving you fine beautiful planetary detail. The dob would be easier to set up and therefore would be used more often.
I love these threads. They show how people love their telescopes. The long achro lovers defend theirs, apos lovers do the same. Newt guys add their two cents and everyone brings up the flaws of the others. People get mad thread gets locked. Then one month later, it starts again.

I read them every time. The right answer is... Try them all at star parties. Then buy the one you like. You may not see CA as easily as someone else or you may not care. It may drive you nuts like it does to me. Don't forget to try a dob. My z8 did really well on planets. My Z12 is better but, you need to use a filter or you will lose your night vision.


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SteveSMS
super member


Reged: 03/29/08

Loc: Jersey Shore USA
Re: Achro Vs Apo new [Re: timps]
      #5752523 - 03/23/13 07:22 PM

Hi Timps,

I have an 8" f7 Newt with truly exceptional optics and it is stunning on planetary. The real keys, imho, are collimation and thermal management. I use the fan set up that Alan Dyer wrote about in S&T years ago and it is killer. It constantly scrubs the boundary layer in front of the primary. I love refractors too, I own 5 from 80mm to 150mm but the 8 " Newt and this new EdgeHD 8 are far superior to 120ED when it comes to planets. I do believe that an 8" APO would be mind blowing but I cannot even dream about something that expensive. Even an 8" achro with a Chromacorr would be way too much for me as I have no way to set it up permanently. Huge mounts are just too much of a hassle for my observing style.

Clear Skies,

Steve


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jrbarnett
Eyepiece Hooligan
*****

Reged: 02/28/06

Loc: Petaluma, CA
Re: Achro Vs Apo new [Re: Joe Bergeron]
      #5752539 - 03/23/13 07:35 PM

Yeah, who needs automobiles and airplanes? The whole world was explored with sailing ships, from horseback, on foot and by canoe!



Regards,

Jim


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Achro Vs Apo new [Re: timps]
      #5752676 - 03/23/13 08:48 PM

Quote:

But would't a Newtonian configured for planetary also have a long focal length? (F9 for ex.)




Why?

Aperture and optical quality are the important factors. With a fast Newtonian using a driven mount, coma is not an issue, without a driven mount coma correctors do the job. Essentially perfect mirrors are available at focal ratios under f/4.

When comparing apos and achros, it comes down to all around performance and practical issues. It is possible to build an achromat that offers planetary views that are similar to an apo. It's is also possible to build and achromat that is as compact and offers the same wide true field of view as the same apo. But those two achromarts are very different scopes.

If one likes purple, go with a singlet...

Jon


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Achro Vs Apo new [Re: beanerds]
      #5752716 - 03/23/13 09:04 PM

Quote:

My Zeiss gives a more natural hue to oblects compared withg my Takahashi




I asked this once, I will ask it one more time. How does the achromat compare to a Newtonian in terms of "naturalness" of color?

If you are using the achromat to filter/tint the colors, I guess that is fine I don't see that as being more natural if natural means an accurate representation of the actual image.

I am like the other poster who believes refractors are best 4 inches and under. Refractors get better as they get smaller, reflectors get better as their aperture increases. Color correction in a 30 mm f/5 is pretty good... There's not much anyone can do to correct the color in 10 inch f/5 refractor..

Jon

I am like


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beanerds
sage


Reged: 07/15/08

Loc: Darwin Australia
Re: Achro Vs Apo new [Re: Jon Isaacs]
      #5752937 - 03/23/13 11:05 PM Attachment (39 downloads)

Its all in the eyes of the beholder Jon .
If you want a planet buster its gotta be a long throw Refractor or Newt or MAK or CAT or.....
The quality of the optics , columination etc mean everything when trying to glean the smallest planetry details , as I guess you already know this ?

When it comes to a planet buster my home made ( by me ) 10 inch f10 newt in this photo is the cat's whiskers .
Its the alloy tube with the oblong cut outs and in 2 pieces is very hard to beat . It has 3 large clips to join it togrther .
These optics have been thru many incarnations over the years from a 500lb concrete EQ monster on wheels to its much more user friendly Dob mount of today .
Its out in only 3 trips and acclimitizes fast with the open tube .
Its secondary is 22mm in dia so its almost an unobstructed 10 inch f10 , and it shows jupiter as good as any scope I have ever looked thru .

I think I am qualified enough to claim that my little 63mm Zeiss f11.4 gives a more natural Luna/planetary view .

Note my 150mm 'Chromocored' Saxon ( Synta ) in this photo and the Gold truss tube EQ mounted Newt ( 125mm f17 ) another great scope I made , she has a 3/4 inch secondary , tiny .

One more thing look at my signiture and see I also have a Takahashi M210 , SKY90 and Istar 127mm f8 Achro .So yes I say the Zeiss achro gives a more natural hue to my eyes .

As you can see I like telescopes , but all I am trying to get across is that a very well made Achromat will perform very , very well and my Zeiss is just that . One of the best ever made .

ps.the 10 and 5 inch newts and the 150mm refractor and ED80 in this photo are at my sister's in NZ and get use by my Nephew there , the rest are here with me in Australia.
Enjoy .
Brian.


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Achro Vs Apo new [Re: beanerds]
      #5753136 - 03/24/13 01:45 AM

Quote:

Its all in the eyes of the beholder Jon . If you want a planetbusteritsgottabea long throw Refractor or Newt or MAK or CAT or..... The quality of the optics , columinationetcmeaneverything when trying to glean the smallest planetry details , as I guess you already know this ?





What I do know is that the most important factor in getting the good planetary views is the seeing... Great optics are not much use unless the atmosphere is stable enough to let the optics do their stuff.

Regarding the "naturalness of the color". As I see it, in this context, natural means the most like nature. Jupiter is part of the natural world. So, since a Newtonian has the most perfect color transfer, then it seems with q Newtonian to compare the Zeiss and Tak to, you would be in the position to make that call.

In my experience, apos more closely render colors than long focus achromats. I do like the views through my 60mm x800 Asahi-Pentax but truthfully, at the small exit pupils I use for viewing the planets with that scope, colors are dimished in comparison to a Newtonian, the eye sees more color in brighter images.

Jon


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buddyjesus
Carpal Tunnel
*****

Reged: 07/07/10

Loc: Davison, Michigan
Re: Achro Vs Apo new [Re: Jon Isaacs]
      #5753181 - 03/24/13 02:21 AM

try each kind at a star party along with filters if anyone has them lying about. I recommend a #8 light yellow to reduce chromatic aberration. We are all crazy, just crazy in different ways.

My uncle was an aperture rules guy and recommended I get a big reflector. I used a small reflector(all budget would allow) for a few years and didn't like it for optical reasons. I traded up in aperture and quality by getting a medium length 4" achromat.

Why not the other kinds? Can't afford an apo, don't like the image shift when focusing cassagrains, and can't observe standing up if I got a large dob. I also have a soft spot for GEM mounts. At the end of the day, I knew I would be happier with an achromat and that guided my purchase. I love the looks of the long achromats.

So ask yourself, what would make you happiest? Can always change your mind later, but til then live YOUR current dream.


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Jeff B
Carpal Tunnel
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Reged: 12/30/06

Re: Achro Vs Apo new [Re: timps]
      #5753389 - 03/24/13 08:37 AM

Quote:

The Tak obviously showed a little more detail and didn't require as much affort to see it but is that worth the $5,500 extra?





Basically, very basically, if you can afford it and want it, it IS worth the extra money.

If you can't, you make a statement about how big of a dob you can get for less money.


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stanislas-jean
Carpal Tunnel


Reged: 10/22/08

Re: Achro Vs Apo new [Re: Sean Puett]
      #5753851 - 03/24/13 12:30 PM

Frankly it is not matter to be lover in apo achro reflector.
Just do tests and compare them on situations, obviously actual.
Reason why I found the long achro interresting in use with more.
I have also a 305 which is used deeply only 2 or 3 nights a year. Never more sothat a 6" stays always very profitable when the seeing (tube and local) is minimised.
Think global not on paper only, with a money bag for getting usually the more details we can.
Stanislas-Jean


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