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8 inch f/15 achro vs a C8?

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#26 Ziggy943

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Posted 04 April 2013 - 11:42 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".


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

#27 t.r.

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Posted 04 April 2013 - 11:55 AM

...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... :grin: Sound familiar? :p

#28 hfjacinto

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Posted 04 April 2013 - 12:15 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.

#29 hfjacinto

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Posted 04 April 2013 - 12:17 PM

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 ;)

#30 mikey cee

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Posted 04 April 2013 - 12:34 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. :smirk: Mike

#31 saemark30

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Posted 04 April 2013 - 01:24 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.

#32 KaStern

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Posted 04 April 2013 - 01:54 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

#33 Ziggy943

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Posted 04 April 2013 - 09:56 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.

#34 Lane

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Posted 04 April 2013 - 10:17 PM

3x D ?

D = diameter ??

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

#35 Ziggy943

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Posted 04 April 2013 - 11:23 PM

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.

#36 Ziggy943

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Posted 05 April 2013 - 10:11 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.

#37 Alan A.

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Posted 05 April 2013 - 01:17 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.

#38 roadi

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Posted 05 April 2013 - 05:10 PM


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.

:grin: cool..

#39 Eddgie

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Posted 05 April 2013 - 06:35 PM

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.


#40 Ziggy943

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Posted 08 April 2013 - 12:50 PM

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

#41 clintwhitman

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Posted 09 April 2013 - 08:09 AM

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. :roflmao:

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#42 GlennLeDrew

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Posted 09 April 2013 - 08:58 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.

#43 KaStern

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Posted 09 April 2013 - 11:43 AM

Hi Clint,

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...ction_image.htm

Cheers, Karsten

#44 KaStern

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Posted 09 April 2013 - 12:00 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

#45 Ziggy943

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Posted 09 April 2013 - 03:46 PM

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. :roflmao:


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.

#46 mikey cee

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Posted 09 April 2013 - 04:26 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

#47 KaStern

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Posted 09 April 2013 - 04:30 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.

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.

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

#48 Ziggy943

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Posted 09 April 2013 - 10:13 PM

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



You mean 7mm ?!

#49 KaStern

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Posted 10 April 2013 - 09:38 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

#50 Ziggy943

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Posted 10 April 2013 - 09:42 AM

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






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