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8 inch f/15 achro vs a C8?

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#1 Darren Drake

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Posted 03 April 2013 - 09:52 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....

#2 jrbarnett

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Posted 03 April 2013 - 10:12 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

#3 Eddgie

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Posted 03 April 2013 - 10:34 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.

#4 Jon Isaacs

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Posted 03 April 2013 - 10:57 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....


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

#5 Darren Drake

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Posted 03 April 2013 - 11:13 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....


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.

#6 Niklo

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Posted 03 April 2013 - 11:41 AM

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

#7 Sean Cunneen

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Posted 03 April 2013 - 12:35 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.

#8 Mark Costello

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Posted 03 April 2013 - 01:10 PM

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.

#9 saemark30

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Posted 03 April 2013 - 01:15 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.

#10 jrbarnett

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Posted 03 April 2013 - 01:52 PM

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

Your friend,

Jim

#11 Darren Drake

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Posted 03 April 2013 - 02:03 PM

Hey my Bday is on saturday...

#12 jrbarnett

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Posted 03 April 2013 - 02:11 PM

Happy B-Day to you too, Darren!

:grin:

- Jim

#13 jrbarnett

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Posted 03 April 2013 - 02:21 PM

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

I think Roland would agree.

http://www.cloudynig...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

#14 t.r.

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Posted 03 April 2013 - 02:28 PM

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

:roflmao:

#15 Lane

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Posted 03 April 2013 - 03:04 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.

#16 KaStern

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Posted 03 April 2013 - 05:55 PM

Hi Darren,

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

#17 Ziggy943

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Posted 03 April 2013 - 09:42 PM

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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#18 timps

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Posted 03 April 2013 - 11:12 PM

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

#19 ianatcn

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Posted 04 April 2013 - 04:59 AM

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

#20 Jon Isaacs

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Posted 04 April 2013 - 06:06 AM

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

#21 Ziggy943

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Posted 04 April 2013 - 08:53 AM

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.


#22 hfjacinto

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Posted 04 April 2013 - 08:58 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".

#23 Ziggy943

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Posted 04 April 2013 - 09:02 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.


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.

#24 hfjacinto

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Posted 04 April 2013 - 09:05 AM

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.

#25 Ziggy943

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Posted 04 April 2013 - 11:39 AM


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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