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Equipment Discussions >> Refractors

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Darren Drake
Carpal Tunnel
*****

Reged: 10/09/02

Loc: Chicagoland
8 inch f/15 achro vs a C8?
      #5775223 - 04/03/13 10:52 AM

I have a good friend who plans to build an 8 inch f/15 high quality achromat one day. We all know an apo would seriously outdo a very good C8 but just how might this achro compare for planetary viewing? I suspect it would beat it on contrast of small features but the lower contrast larger planetary features might be a close call....

Edited by Darren Drake (04/03/13 10:52 AM)


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jrbarnett
Eyepiece Hooligan
*****

Reged: 02/28/06

Loc: Petaluma, CA
Re: 8 inch f/15 achro vs a C8? new [Re: Darren Drake]
      #5775279 - 04/03/13 11:12 AM

I don't think so. Rather I suspect that the damage CA does to the image will be much less than the damage a 30%+ CO will do to the image. Unlike CO, CA doesn't have the effect of moving energy form the airy disc to the rings which manifests the same way as SA in the image. The 8" f/15 will also have other advantages performance-wise. It's optics will be far above ground-based thermal sources. By virtue of its focal ratio its images will remain diffraction limited for longer periods than the C8 with its f/2 primary through bouts of turbulent seeing, and its depth of focus will make sufficient focus easier to achieve than the hair-trigger in-focus point in the SCT.

It also has a huge disadvantage. It's huge. Ridiculous to mount, transport, etc.

Regards,

Jim


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Eddgie
Postmaster
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Reged: 02/01/06

Re: 8 inch f/15 achro vs a C8? new [Re: Darren Drake]
      #5775350 - 04/03/13 11:34 AM

There is no magic in a big, fast achromat (and at 8", f/15 is very fast indeed).

CA is in fact, quite damaging.

Even at 6", and f/15 achromat will have contrast that is not much better than if you put it next to a 6" scope with a 30% obstruction, and this ignores the damage of the secondary spectrum and just looks at the contrast transfer of the scope in photopic weighting (best possible way to rate an achromat).

Ask anyone to look at page 259 of Suiter's book and the graph on contrast transfer even at 6" the contrast loss is pretty meaningful.

Assuming an excellent C8 was used to make the comparison, I think the perfomance would not be that different.

But again, ask someone with Suiters book to give your friend a second opinion.

And tell him this.

"Dude, that is why the f/15 MCT killed the long focus achromat!!!! Heck, Even Roland Christen uses an MCT for high resolution observing."

Have your friend buy an 8" f/15 MCT. He will be happier with it.

Edited by Eddgie (04/03/13 11:39 AM)


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: 8 inch f/15 achro vs a C8? new [Re: Darren Drake]
      #5775402 - 04/03/13 11:57 AM

Quote:

I have a good friend who plans to build an 8 inch f/15 high quality achromat one day. We all know an apo would seriously outdo a very good C8 but just how might this achro compare for planetary viewing? I suspect it would beat it on contrast of small features but the lower contrast larger planetary features might be a close call....




The 8 inch F/15 achromat will be about 10 feet long and require a seriously massive mount. It will also be more expensive than a C-8.. why compare equal apertures?

Why not make a practical comparison, equal hassle factor or equal dollars? Equal Hassle would be a maybe 16 inch SCT, Equal dollars, maybe a C-11...

Jon


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Darren Drake
Carpal Tunnel
*****

Reged: 10/09/02

Loc: Chicagoland
Re: 8 inch f/15 achro vs a C8? new [Re: Jon Isaacs]
      #5775430 - 04/03/13 12:13 PM

Quote:

Quote:

I have a good friend who plans to build an 8 inch f/15 high quality achromat one day. We all know an apo would seriously outdo a very good C8 but just how might this achro compare for planetary viewing? I suspect it would beat it on contrast of small features but the lower contrast larger planetary features might be a close call....




The 8 inch F/15 achromat will be about 10 feet long and require a seriously massive mount. It will also be more expensive than a C-8.. why compare equal apertures?

Why not make a practical comparison, equal hassle factor or equal dollars? Equal Hassle would be a maybe 16 inch SCT, Equal dollars, maybe a C-11...

Jon




He already has a C8 and has an 8 inch Zeiss lens. So I was just wondering on opinions of how they would compare. It's his dream to have a long big refractor one day.


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Niklo
professor emeritus


Reged: 03/29/13

Loc: Bavaria
Re: 8 inch f/15 achro vs a C8? new [Re: Darren Drake]
      #5775475 - 04/03/13 12:41 PM

Hi Darren,
I had only the chance to see through a C8 at a school observatory and I was very disappointed. Maybe it wasn't cooled long enough or was maladjusted but the contrast wasn't good. There was a 5" refractor with F/14.2 and it showed Mars better (more details, more contrast) than this C8.
I would expect that the 8" F/15 is better (more contrast) then the C8 but the main problem is the mount for this monster telescope.
Even if the 8" refractor beats the C8 (I expect it will beat it) a 10" or 12" newton could probably be better.

A refractor is a nice thing, it looks nice and it's something special but 3 m focal length is too much for an amateur. It should be a scope in an observatory.
Now I remember that I looked through a refractor at Bayrische Volsksternwarte München with 180 mm aperture and 3 m focal lenght. As far as I remember it was better than many 8 or 10 " newtons but it is a real monster telescope.

Roland

Edited by Niklo (04/03/13 12:44 PM)


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Sean Cunneen
Let Me Think
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Reged: 08/01/07

Loc: Blue Island Illinois
Re: 8 inch f/15 achro vs a C8? new [Re: Niklo]
      #5775589 - 04/03/13 01:35 PM

I directly compared a Nexstar 8 to both my Istar 6"f 15 and my current Jaeger's 6" f15 achro and the Nexstar lost. Planetary detail was bright but washed out and stars were not as tight in the SCT as in either Achro.

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Mark Costello
Post Laureate
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Reged: 03/08/05

Loc: Matthews, NC, USA
Re: 8 inch f/15 achro vs a C8? new [Re: Jon Isaacs]
      #5775682 - 04/03/13 02:10 PM

Quote:

Quote:

I have a good friend who plans to build an 8 inch f/15 high quality achromat one day. We all know an apo would seriously outdo a very good C8 but just how might this achro compare for planetary viewing? I suspect it would beat it on contrast of small features but the lower contrast larger planetary features might be a close call....




The 8 inch F/15 achromat will be about 10 feet long and require a seriously massive mount. It will also be more expensive than a C-8.. why compare equal apertures?

Why not make a practical comparison, equal hassle factor or equal dollars? Equal Hassle would be a maybe 16 inch SCT, Equal dollars, maybe a C-11...

Jon





.... And that's why I, as much as I love refractors, personally would go with the SCT (or a regular reflector), hands down.


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saemark30
Pooh-Bah


Reged: 02/21/12

Re: 8 inch f/15 achro vs a C8? new [Re: Sean Cunneen]
      #5775689 - 04/03/13 02:15 PM

The 8 inch F15 Zeiss lens will destroy any commercial telescope of same size. The smoothness and accuracy of the lens will far exceed that of any commercial telescope short of custom made optics by the likes of AP, TEC.
The brain can tuned out the CA to a large degree.
PS I can see belts on Jupiter as well as a typical C8 with a 80mm achromatic refractor.


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jrbarnett
Eyepiece Hooligan
*****

Reged: 02/28/06

Loc: Petaluma, CA
Re: 8 inch f/15 achro vs a C8? new [Re: Jon Isaacs]
      #5775744 - 04/03/13 02:52 PM

Happy B-Day Jon. And many, many more.

Your friend,

Jim


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Darren Drake
Carpal Tunnel
*****

Reged: 10/09/02

Loc: Chicagoland
Re: 8 inch f/15 achro vs a C8? new [Re: jrbarnett]
      #5775774 - 04/03/13 03:03 PM

Hey my Bday is on saturday...

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jrbarnett
Eyepiece Hooligan
*****

Reged: 02/28/06

Loc: Petaluma, CA
Re: 8 inch f/15 achro vs a C8? new [Re: Darren Drake]
      #5775789 - 04/03/13 03:11 PM

Happy B-Day to you too, Darren!



- Jim


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jrbarnett
Eyepiece Hooligan
*****

Reged: 02/28/06

Loc: Petaluma, CA
Re: 8 inch f/15 achro vs a C8? new [Re: Darren Drake]
      #5775800 - 04/03/13 03:21 PM

An 8" Zeiss lens will o-b-l-i-t-e-r-a-t-e a C8, false color and all.

I think Roland would agree.

http://www.cloudynights.com/item.php?item_id=410

"Refractor, 7" to 9" Apo, F8 to F11, or 8" to 12" achromat, F12 or longer. These scopes will cost the most, although excellent achromats can be gotten at reasonable prices from builders like D&G Optical. Inch for inch, refractors will have more light grasp, and in my own experience, an edge on contrast which is important in making out planetary detail. Achromats will have chromatic aberration effects that turn some people off but others have learned to ignore."

Note he does not equate false color with reduced contrast and, instead, says refractors (achromats and apochromats) will have an edge in all-important contrast over obstructed designs. Of course, he didn't include a single contrast transfer graph...

Regards,

Jim

Edited by jrbarnett (04/03/13 03:23 PM)


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t.r.
Post Laureate
*****

Reged: 02/14/08

Loc: Upstate NY
Re: 8 inch f/15 achro vs a C8? new [Re: jrbarnett]
      #5775813 - 04/03/13 03:28 PM

Quote:

Of course, he didn't include a single contrast transfer graph...




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Lane
Post Laureate


Reged: 11/19/07

Loc: Frisco, Texas
Re: 8 inch f/15 achro vs a C8? new [Re: t.r.]
      #5775885 - 04/03/13 04:04 PM

This is a very interesting argument, but clearly there is major disagreement on just how much CA really does destroy contrast and detail.

Has anyone done any actual side by side comparisons of any of the following scopes? A short 6" APO, short 6" Achro, long 6" Achro, 6" Dob, 6" SCT and maybe an 8" DOB and 8" SCT as well.

Before reading all this I would have sworn that the view in my short 6" achro was better in every way when compared to my 6" SCT. Certainly the lower power views of star clusters, galaxies, and nebula are much better in the Achro, but unless I am fooling myself, I think I see more detail on the moon and planets as well. Obviously there is a giant bluish/purple blob around the planet, but it just doesn't seem like it is really causing problems. I have not actually taken the 6" or 8" SCTs outside at the same time as my 6" Achro under the same sky conditions and compared them at the same magnifications. I have also not been doing a whole lot of planetary viewing either. So I guess my thoughts on how these scopes compare to each other is probably inaccurate. I need to do an actual comparison and see what kind of results I get.


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KaStern
scholastic sledgehammer
*****

Reged: 04/18/06

Loc: InTheDark
Re: 8 inch f/15 achro vs a C8? new [Re: Darren Drake]
      #5776185 - 04/03/13 06:55 PM

Hi Darren,

Quote:

He already has a C8 and has an 8 inch Zeiss lens. So I was just wondering on opinions of how they would compare. It's his dream to have a long big refractor one day.




So if he ows the lens he definitely should build the scope.
Colour aberration will cause some damage especially when vievwing Jupiter.
Remember If focussed on green light the blue and red light will be defocussed about 1,5mm.
Blue or red object detail therefore are unsharp.
But in a scope like that it is worth to build-in a colour corrector.

Cheers, Karsten


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Ziggy943
Post Laureate


Reged: 08/11/06

Loc: Utah
Re: 8 inch f/15 achro vs a C8? new [Re: Niklo]
      #5776680 - 04/03/13 10:42 PM Attachment (45 downloads)

Quote:

Hi Darren,

A refractor is a nice thing, it looks nice and it's something special but 3 m focal length is too much for an amateur. It should be a scope in an observatory.

Roland




I don't know what you base that opinion on but I have a very different opinion. I don't think a 3m focal length is too much for an amateur. And I don't believe they belong in an observatory. I still believe in the Dobson philosophy of "bringing astronomy to the public."

An 8" F/12 refractor is an easy one-man set-up, just ask Steve. That's Steve Fisher, whom many of you know, in the background.

The 9" in the foreground is a bit more work but it was also a one-man job (mine) for over 20 years.

Like someone said, you learn to ignore the color and enjoy the images of an F/15 system. Jim mentioned the advantages of having the objective off the ground and the depth of focus. Stable images are another advantage. Often the image is so steady you think your looking at a still picture.

ok, so set-up is a little work, so what? It's worth it.

I have never seen a better planetary image in any SCT, up to 14", than in the 9" Clark. I have seen some very decent Celestrons. Meades have always disappointed.


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timps
sage


Reged: 02/24/13

Re: 8 inch f/15 achro vs a C8? new [Re: Ziggy943]
      #5776844 - 04/04/13 12:12 AM

So whay kind of refractor would match a 14" SCT on planetary?

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ianatcn
sage


Reged: 05/26/10

Loc: Hampshire, UK
Re: 8 inch f/15 achro vs a C8? new [Re: Niklo]
      #5777037 - 04/04/13 05:59 AM

At the bottom of the link look at the cakes on that table. I am inviting myself to your next birthday party Roland!

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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: 8 inch f/15 achro vs a C8? new [Re: Ziggy943]
      #5777078 - 04/04/13 07:06 AM

Quote:

I don't think a 3m focal length is too much for an amateur




I am fine with it but it ought to be f/5 of faster to make the hassle worthwhile.

By the way, how do you view objects close to the horizon with that rig, the ladder looks too short.

Jon


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Ziggy943
Post Laureate


Reged: 08/11/06

Loc: Utah
Re: 8 inch f/15 achro vs a C8? new [Re: Jon Isaacs]
      #5777250 - 04/04/13 09:53 AM

Quote:

Quote:

I don't think a 3m focal length is too much for an amateur




I am fine with it but it ought to be f/5 of faster to make the hassle worthwhile.

By the way, how do you view objects close to the horizon with that rig, the ladder looks too short.

Jon




That is Steve's ladder. My 6' ladder is just in the picture ar the left. I wouldn't have a 9" F/5 or faster. I don't think a 9" F/5 or faster would make anything "worthwhile."

Besides, why would I want to look at something on the horizon? I don't usually go for objects unless they are 20° or more off the horizon with that telescope.


Edited by Ziggy943 (04/04/13 12:45 PM)


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hfjacinto
I think he's got it!
*****

Reged: 01/12/09

Loc: Land of clouds and LP
Re: 8 inch f/15 achro vs a C8? new [Re: Jon Isaacs]
      #5777256 - 04/04/13 09:58 AM

Our club has a 10" f15 achro and it's view is not impressive. A 9.25 in this case shows more details on DSO than the 10".

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Ziggy943
Post Laureate


Reged: 08/11/06

Loc: Utah
Re: 8 inch f/15 achro vs a C8? new [Re: Eddgie]
      #5777262 - 04/04/13 10:02 AM

Quote:

There is no magic in a big, fast achromat (and at 8", f/15 is very fast indeed).

CA is in fact, quite damaging.






F/15 is a good compromise for an 8". I wouldn't call it "very fast." I would call it a good length. I wouldn't go past an F/12 and would prefer it around F/20 if you can manage the length.

The eye/brain adjusts to the CA and learns to ignore it. there does come a point in faster systems where you can't ignore it and the casual observer may be bothered by it quicker than a seasoned observer.


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hfjacinto
I think he's got it!
*****

Reged: 01/12/09

Loc: Land of clouds and LP
Re: 8 inch f/15 achro vs a C8? new [Re: saemark30]
      #5777266 - 04/04/13 10:05 AM

Quote:


PS I can see belts on Jupiter as well as a typical C8 with a 80mm achromatic refractor.




Can the sct bashing stop? I can see more in my 9.25 than in an 80 mm and a 120mm and the 14" sct at our dark site had a better planetary image than any other scope ( including a 178 mm AP don't even ask about dso) A large achro is nice and looks nice but for most it's not practical. The reason we see so few large refractors is because of portability, mounting and cost.


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Ziggy943
Post Laureate


Reged: 08/11/06

Loc: Utah
Re: 8 inch f/15 achro vs a C8? new [Re: hfjacinto]
      #5777596 - 04/04/13 12:39 PM

Quote:

Quote:


PS I can see belts on Jupiter as well as a typical C8 with a 80mm achromatic refractor.




Can the sct bashing stop? I can see more in my 9.25 than in an 80 mm and a 120mm and the 14" sct at our dark site had a better planetary image than any other scope ( including a 178 mm AP don't even ask about dso) A large achro is nice and looks nice but for most it's not practical. The reason we see so few large refractors is because of portability, mounting and cost.




You do realize this is the "refractor" forum? Here we can bash anything except refractors although we do some of that too.


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Ziggy943
Post Laureate


Reged: 08/11/06

Loc: Utah
Re: 8 inch f/15 achro vs a C8? new [Re: hfjacinto]
      #5777602 - 04/04/13 12:42 PM

Quote:

Our club has a 10" f15 achro and it's view is not impressive. A 9.25 in this case shows more details on DSO than the 10".




Then it's a 10" with a real problem. The 10"refractor given any decent optics should easily outperform a 9¼" SCT.


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t.r.
Post Laureate
*****

Reged: 02/14/08

Loc: Upstate NY
Re: 8 inch f/15 achro vs a C8? new [Re: Ziggy943]
      #5777633 - 04/04/13 12:55 PM

Quote:

...and the 14" sct at our dark site had a better planetary image than any other scope ( including a 178 mm AP don't even ask about dso)




Well then, the AP 178 obviously wasn't collimated properly, cooled sufficiently or, or, or... Sound familiar?


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hfjacinto
I think he's got it!
*****

Reged: 01/12/09

Loc: Land of clouds and LP
Re: 8 inch f/15 achro vs a C8? new [Re: t.r.]
      #5777692 - 04/04/13 01:15 PM

I forgot I'm on the refractor forum, in that case a 60 mm premium triplet easily shows more details than a c14. I forgot all sct's and newts are not properly cooled and out if collimation .

Please.


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hfjacinto
I think he's got it!
*****

Reged: 01/12/09

Loc: Land of clouds and LP
Re: 8 inch f/15 achro vs a C8? new [Re: Ziggy943]
      #5777702 - 04/04/13 01:17 PM

Quote:

Quote:

Our club has a 10" f15 achro and it's view is not impressive. A 9.25 in this case shows more details on DSO than the 10".




Then it's a 10" with a real problem. The 10"refractor given any decent optics should easily outperform a 9¼" SCT.




Maybe the 10" wasn't properly cooled down or it was out of collimation


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mikey cee
Postmaster
*****

Reged: 01/18/07

Loc: bellevue ne.
Re: 8 inch f/15 achro vs a C8? new [Re: hfjacinto]
      #5777748 - 04/04/13 01:34 PM

If it's a club owned and mounted 10" refractor it's optics are more likely dirtier than *BLEEP*. My 10" refractor doesn't take a back seat to a friend's C14 or another's C11 and I can definitely tell you that. I've looked thru theirs. This comes from their mouths also. Mike

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saemark30
Pooh-Bah


Reged: 02/21/12

Re: 8 inch f/15 achro vs a C8? new [Re: hfjacinto]
      #5777856 - 04/04/13 02:24 PM

I don't want to bash the SCT but your graphs don't show the effects of real seeing conditions and tube currents and changing slope in mirrors when I moved my C8,C11 around.
I honestly saw as much detail in a clean refractor image but without the smearing in the SCTs. Some people happen to live in better climates and below the jet stream so I am open to your opinion.
But a larger 10" Newtonian will stomp little refractors when seeing permit so it all depends on the conditions.


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KaStern
scholastic sledgehammer
*****

Reged: 04/18/06

Loc: InTheDark
Re: 8 inch f/15 achro vs a C8? new [Re: Ziggy943]
      #5777890 - 04/04/13 02:54 PM

Hello Ziggy,

in a typical 4" f/12,2 flint/crown achromat when focussed on green light
the blue and red blur is 3 times as big as the Airy disc diameter.

If you double the aperture from 4" to 8" diameter the airy disc diameter
now will be as half as big. Therefore the defocussed red and blue blur
will now be 6 times as big as the Airy disc diameter.

If you double the focal length of the 8" achromat the defocussed
red and blue blur will be 3 times as big as the airy disc diameter.
It now is a 8" f/24,4 achromat.

So f/15 is slow in a 3" achromat, quiete slow in a 4" achromat,
a tad too fast in a 5" achromat where it should be f/17 to fit
in the crterion Fraunhofer postulated, but f/15 is relatively fast
in an 8" achromat.

Thanks to physics blue and red light do not interfere to green light.
so the green image of the 8"f/15 achromat will be as sharp as possible.
The defocussed blue and red image is just superimposed to the sharp green image
and causes only straylight. And of course blue or red object details
will barely be visible without filter(s) and refocus.

Cheers, Karsten


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Ziggy943
Post Laureate


Reged: 08/11/06

Loc: Utah
Re: 8 inch f/15 achro vs a C8? new [Re: KaStern]
      #5778751 - 04/04/13 10:56 PM

Karsten,


I understand that ideally refractors follow a rule of 3x D for sufficient CA control. My comment was intended for practical purposes in 8” sized refractors. An F/15 if about as slow as you want to go. My guess would be that only a small percentage of medium sized refractors actually meet the 3xD criterion. So F/15 represents the slow end of medium sized refractors. I don’t consider F/15 as fast for any telescope. YMMV

There is more to the control of CA than merely the F/ratio. I have also owned an 8" F/16.8 that exhibited more CA than the 9" F/14.9 Clark. On paper, if you just go by F/ratios, it shouldn't be that way but it's a fact. The choice of glass and the design also influence the color.


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Lane
Post Laureate


Reged: 11/19/07

Loc: Frisco, Texas
Re: 8 inch f/15 achro vs a C8? new [Re: Ziggy943]
      #5778775 - 04/04/13 11:17 PM

3x D ?

D = diameter ??

So 100mm refractor only needs a focal length of 300mm to sufficiently control CA ???


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Ziggy943
Post Laureate


Reged: 08/11/06

Loc: Utah
Re: 8 inch f/15 achro vs a C8? new [Re: Lane]
      #5778840 - 04/05/13 12:23 AM

Quote:

3x D ?

D = diameter ??

So 100mm refractor only needs a focal length of 300mm to sufficiently control CA ???




No, it refers to the F ratio. A 4" should have an F/ratio of at least F/12 and so on.


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Ziggy943
Post Laureate


Reged: 08/11/06

Loc: Utah
Re: 8 inch f/15 achro vs a C8? new [Re: Ziggy943]
      #5779405 - 04/05/13 11:11 AM

I would also argue the terms “fast’ or “slow” are independent of the color correction but are strictly dependent on the F/ratio. Convention has always called the low F numbers “fast” and the high F numbers “slow.”

If an 8” F/15 reflector is a slow system then it’s also slow in a refractor. An 8” refractor at F/15 may not achieve perfect color correction but that doesn’t make it a “fast” system. It makes it too fast to achieve perfect color correction, but it’s still a slow system.


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alancygnusx2
professor emeritus


Reged: 11/25/08

Loc: CA
Re: 8 inch f/15 achro vs a C8? new [Re: Darren Drake]
      #5779719 - 04/05/13 02:17 PM

With its less than perfectly made optics and its 34% obstruction, the C8 is going to lose on planetary detail relative to a Zeiss doublet achromat at f/15,despite the CA in the achromat. The refractor will show better planetary views in both poor and excellent seeing.

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roadi
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Re: 8 inch f/15 achro vs a C8? new [Re: Ziggy943]
      #5780275 - 04/05/13 06:10 PM

Quote:

Quote:

Quote:


PS I can see belts on Jupiter as well as a typical C8 with a 80mm achromatic refractor.




Can the sct bashing stop? I can see more in my 9.25 than in an 80 mm and a 120mm and the 14" sct at our dark site had a better planetary image than any other scope ( including a 178 mm AP don't even ask about dso) A large achro is nice and looks nice but for most it's not practical. The reason we see so few large refractors is because of portability, mounting and cost.




You do realize this is the "refractor" forum? Here we can bash anything except refractors although we do some of that too.



cool..


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Eddgie
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Re: 8 inch f/15 achro vs a C8? new [Re: Lane]
      #5780390 - 04/05/13 07:35 PM

Quote:

A short 6" APO, short 6" Achro, long 6" Achro, 6" Dob, 6" SCT and maybe an 8" DOB and 8" SCT as well.





I have done side by side with an f/8 achromat, a 6" reflector (Mak Newt) and a 6" APO, and with a C8 and a C8 EdgeHD (and various others).

Of the ones mentioned, I would rank them this way on planets.

6" APO at the top.

Tie for the 6" MN, EdgeHD 8, and C8 (The EdgeHD has near perfect optics. The standard SCT was hand selected as the best I could find...)

6" achromat. Dead last by a long shot. The least detail on Jupiter I have ever seen. In fact the MN56 I owned gave a better planetary performance on Jupiter than the 6" achormt.

Not all of these were tested side by side though many were, but all were uesd more than long enough to form a valid opinion I think.

I also owned and used a Meade 152ED. This scope gave performance that was very close to th 6" APO. I would rate it as at least a tie with the 6" Mak Newt.

I owned two different 6" f/8 achromats. The first had excellent optics, the second had a small central zone and a bit of Spherical Abberation.

Neither of these were nearly as satisfying to use on planets as any of these other scopes, including the 5" Mak Newt.

At high powers, the lowest contrast (not the smallest, but the lowest contrast) detail was completly washed out in the achromats.

Faint festoons on Jupiter were almost impossible to see in the Achromats.

Only the 6" APO will usually show small ovals with any regularity on Jupiter.

Color redition on Jupiter, Mars, and Saturn was without doubt, far superior on all of the other scopes over the achromats.

Gentle shading on the moon (OMG, is the moon loades with different shades of gray, white, and even very pale blue) was superb in the reflectors and the APO.

By comparisn, the moon was washed in a film of violet in the achomat.

For me, the 6" f/8 achroamts were at the bottom of the barrel on most targets.

How would a 6" f/15 achromat do? Better than a 6" f/8, I am sure.

As good as a 6" Mak Newt? Maybe.

Better than a 6" APO" I have my doubts.

And as for the OP, why not just use an 8" or 10" f/15 MCT?

If he already has the lens, then of course, use it.

But before someone went out and paid for an 8" or 10" achomat, they should just get a big MCT.

Let's take for example, one Mr Roland Christen.

Mr. Christen could have easily built himself an 8" f/12 ahcromat.

But what does he use for planets?

Well, he uses a 10" MCT.


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Ziggy943
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Re: 8 inch f/15 achro vs a C8? new [Re: Eddgie]
      #5786090 - 04/08/13 01:50 PM

6" F/8 achromat shouldn't even be in a test of planetary telescopes. The result is a big 'so what'.

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clintwhitman
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Re: 8 inch f/15 achro vs a C8? new [Re: Darren Drake]
      #5787635 - 04/09/13 09:09 AM Attachment (24 downloads)

LOL Silly Comparison. Sorry but if the objective is a Zeiss and the OTA is set up properly it will be able to hold 700 power or more on a good night. There is no C 8 or C14 capable of holding the same powers as a well corrected Refractor of 8" range. If you set a C8 up next to the Pearl you would have a strong urge to kick it out of your way on a good nights seeing.

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GlennLeDrew
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Re: 8 inch f/15 achro vs a C8? new [Re: clintwhitman]
      #5787716 - 04/09/13 09:58 AM

Clint,
What are you viewing that requires a 0.3mm exit pupil? If it's reasonably bright, such as the Moon or most planets, that would normally be considered 'empty magnification', with the airy Disk being rendered as a mighty sizeable blob.


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KaStern
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Re: 8 inch f/15 achro vs a C8? new [Re: clintwhitman]
      #5788014 - 04/09/13 12:43 PM

Hi Clint,

Quote:

Sorry but if the objective is a Zeiss and the OTA is set up properly it will be able to hold 700 power or more on a good night.




sorry, but 700x in a 200mm scope does not indicate anything but bad eyesight.
Normal people see a star as a tiny blob at around 1mm exit pupil,
wich is 200x in the 200mm f/15 scope.
At 0,7mm exit pupil wich translates to 286x almost 90% of the observers
see the diffraction effects:
A star will be seen as a blob of light surrounded by dark and light rings.

Every scope, even the best, are limited by diffraction effects.
That`s physics. Please have a look on this page:

http://www.telescope-optics.net/diffraction_image.htm

Cheers, Karsten


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KaStern
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Re: 8 inch f/15 achro vs a C8? new [Re: Ziggy943]
      #5788051 - 04/09/13 01:00 PM

Hi Ziggy,

longitudinal colour aberration is detemined by the glass pairing.
Shure, you can take a better glass combination, but then it is no more an achromat.

I agree, the quality of manufacture is important.
Good quality will ensure that there is no spherical aberration at the main wavelenght,
wich will be around 550nm. There will be no turned down edge,
no astigmatism due to a tilted objective, no coma caused by a lens tilted to the other lens.
But it will not make ca lesser than theortically possible.

When your 9" f/14.9 Clark is significantly better in terms of ca than the 8"f/16.8
either the Clark has a better glass pairing, or the other scope has some sort of flaw.
Enjoy your Clark, I am sure that it is a fine scope!
And where you live the skiea are much better than where I live.

Cheers, Karsten


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Ziggy943
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Re: 8 inch f/15 achro vs a C8? new [Re: clintwhitman]
      #5788675 - 04/09/13 04:46 PM

Quote:

LOL Silly Comparison. Sorry but if the objective is a Zeiss and the OTA is set up properly it will be able to hold 700 power or more on a good night. There is no C 8 or C14 capable of holding the same powers as a well corrected Refractor of 8" range. If you set a C8 up next to the Pearl you would have a strong urge to kick it out of your way on a good nights seeing.




Reminds me of an incident at Craters of the Moon national Park in Idaho. Several Idaho societies set up for public viewing there twice a year. My brother and I brought the 9" Clark on a near perfect night and were looking at Jupiter. We were set up by an 18" LMI and a 10" Newt. After looking through the Clark a friend of the 10" owner said to his friend, "don't come over here to look, you're gonna throw your telescope away." The 18" had problems. What ever they were the 18" was terrible. It showed nothing but a big white blob. I've never seen worse in an 18". I ran it up to 852x on selected double stars. (132.2x25.4/4mm Clave' eyepiece)Jupiter was excellent with a 10mm Clave' giving 340x.

KaStern, you need to experience a medium sized refractor in top form. Your formula goes out the window. I am not claiming any magical powers but Cliff is right, these refractors can hold an image at high magnifications per inch.

Edited by Ziggy943 (04/09/13 04:50 PM)


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mikey cee
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Re: 8 inch f/15 achro vs a C8? new [Re: Ziggy943]
      #5788765 - 04/09/13 05:26 PM

Zig my friend you are so spot on......as usual. I don't have perfect nights hardly at all but I do have enough that do approach airy discs with broken and intermitant diffraction rings. My 10" Istar easily holds the airy disc and rings rather stable and contrasty at 685x and 815x without breaking a sweat. Sorry but it's just the nature of the beast I guess. I currently like Strueve 215 and Iota Leonis. I'm sure by all means these aren't the only ones but a lot of my skies are milky with high cirrus and I just happen to know where to easily look for these at the meridian. Mike

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KaStern
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Re: 8 inch f/15 achro vs a C8? new [Re: Ziggy943]
      #5788769 - 04/09/13 05:30 PM

Hi Ziggy,

I experienced the following apochromatic refractors:

7"f/8 TMB, 6"f/8 TMB, 130/1200mm TMB, 115/805mm TMB, 100/800mm TMB,
102mm f/9 mm Vixen Fluorite,
152mm f/8 Takahashi Fluorite, 102mm f/8 Takahashi TSA, 80/1470mm Achomat,
110mm f/6 ED, 100/900mm ED, 120/900mm ED (all Doublets) and a bunch of really shabby short achromats.
None of the good apochromats were able to "hold an image at high magnifications".
As soon as I applied higher mags than 0.7mm AP I began to see that the image got softer.
Detail was more rounded, dimmer, colours were less pronounced.

One time I had an observer from a nearby town with me when seeing was great.
He wanted me to pusch the mag of my 8"f/6 higher and higher when we observed the Moon.
He ended by using 900x and claimed "image is great".
I looked through and though by myself "why does this guy think the dim views with rounded crater wall detail is better than using 280x,
where the same detail appeared to be brighter and harder defined?"

I can see the diffraction effects in every telescope when magnification is higher than 1mm exit pupil.
Until 0.66mm exit pupil I can tolerate this, but as soon as magnification get higher
I find the views less good due to the increasing diffraction unsharpness I can see.
This applies omly for very good seeing and very good scopes.
Refractor or reflector, unobstructed like the Apochromats or the superb Schiefs
a good friend of mine built, or my own Yolo, all of them are limited by diffraction.
Period.

Quote:

Jupiter was excellent with a 10mm Clave' giving 340x.




This gives about 0.67mm exit pupil and fits nicely in my own planetary observations with good telescopes.

Quote:

I ran it up to 852x on selected double stars.




To split doubles I take up to 0.5mm exit pupil.
I can get higher, but I do not need to.

Edit:
The best quality optics I observed with were the 4"f/8 TMB, the 4"f/8 Takahashi,
the 130/1200mm TMB and the 9" modified Schiefspiegler of my friend Kurt.

Cheers, Karsten

Edited by KaStern (04/09/13 05:34 PM)


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Ziggy943
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Re: 8 inch f/15 achro vs a C8? new [Re: KaStern]
      #5789342 - 04/09/13 11:13 PM

"As soon as I applied higher mags than 0.7mm AP I began to see that the image got softer. "



You mean 7mm ?!


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KaStern
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Re: 8 inch f/15 achro vs a C8? new [Re: Ziggy943]
      #5789957 - 04/10/13 10:38 AM

Hello Ziggy,

sorry, I meant:

"As soon as I applied higher mags than 0.7mm ep (exit pupil) I began to see that the image got softer. "

I forgot to mention both older series 5" and 6" f/9 Meade ED.
Kurt did own the 5" f/9 but after a while of tinkering to solve the decentering issues he sold that lens.
The 6" F/9 I could look through did suffer from thoses problems too.

What I try to point out is the following:
The laws of physics apply to all telescopes, including refractors.
You cannot overcome the limits diffraction causes.

Cheers, Karsten


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Ziggy943
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Re: 8 inch f/15 achro vs a C8? new [Re: KaStern]
      #5789974 - 04/10/13 10:42 AM

You're right but it sounds like those refractors had problems.

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mikey cee
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Re: 8 inch f/15 achro vs a C8? new [Re: mikey cee]
      #5790049 - 04/10/13 11:13 AM Attachment (12 downloads)

Yes even tho' my airy discs are larger and a little softer the overall picture is still quite pleasing. Stars like 72 Pegasi which are near .5" need a lot of magnification too easily enjoy them. Mike

Edited by mikey cee (04/10/13 11:24 AM)


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KaStern
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Re: 8 inch f/15 achro vs a C8? new [Re: Ziggy943]
      #5790159 - 04/10/13 12:07 PM

Hello Ziggy,

over the years I have seen many scopes that had problems.
Most of these were catadiotric telescopes. Some reflectors too.
The smallest number were the refractors.
Out of these the expensive apochromats and the long focal length achromats
were o.k. ist almost all cases. Among the relatively ED and the cheap fast achromats
I have seen several samples that had problems.
In many cases the lens cell diameter and the lens diameters did not fit right
so that there was either decentering or pinching of the lens.

Greetings, Karsten


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zawijava
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Re: 8 inch f/15 achro vs a C8? new [Re: Darren Drake]
      #5790235 - 04/10/13 12:35 PM

One of the two scopes permanently installed in my Club's Observatory is a 8" f/15 Zeiss achromat. It has been serving Club Members and the public for 11 years now and it never fails to impress with stunning Lunar and Planetary performance. And it's no slouch on Deep Sky as well. I have yet to see any Club Member 8" SCT's that have an edge over this lens. That being said, the Zeiss is permanently mounted on a AP 1200, which I feel is a minimum mounting requirement for this scope capacity wise. And it takes 3 people to mount/dismount the OTA...2 could do it but 3 is safer. The AP 1200 head is is nearly 7' high.

-Tim

Quote:

Quote:

Quote:

I have a good friend who plans to build an 8 inch f/15 high quality achromat one day. We all know an apo would seriously outdo a very good C8 but just how might this achro compare for planetary viewing? I suspect it would beat it on contrast of small features but the lower contrast larger planetary features might be a close call....




The 8 inch F/15 achromat will be about 10 feet long and require a seriously massive mount. It will also be more expensive than a C-8.. why compare equal apertures?

Why not make a practical comparison, equal hassle factor or equal dollars? Equal Hassle would be a maybe 16 inch SCT, Equal dollars, maybe a C-11...

Jon




He already has a C8 and has an 8 inch Zeiss lens. So I was just wondering on opinions of how they would compare. It's his dream to have a long big refractor one day.




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