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Equipment Discussions >> Refractors

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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
Pooh-Bah
*****

Reged: 08/18/07

Loc: GGS
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
Postmaster
*****

Reged: 02/01/06

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
Post Laureate


Reged: 08/11/06

Loc: Utah
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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