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Equipment Discussions >> Refractors

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clintwhitman
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Reged: 01/01/07

Loc: CALI SoEasyACavemanCanSlewIt
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
scholastic sledgehammer
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Reged: 04/18/06

Loc: InTheDark
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
scholastic sledgehammer
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Reged: 04/18/06

Loc: InTheDark
Re: 8 inch f/15 achro vs a C8? [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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Reged: 08/11/06

Loc: Utah
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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Reged: 01/18/07

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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
scholastic sledgehammer
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Reged: 04/18/06

Loc: InTheDark
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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Reged: 08/11/06

Loc: Utah
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
scholastic sledgehammer
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Reged: 04/18/06

Loc: InTheDark
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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Reged: 08/11/06

Loc: Utah
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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Reged: 01/18/07

Loc: bellevue ne.
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
scholastic sledgehammer
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Reged: 04/18/06

Loc: InTheDark
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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Reged: 10/06/07

Loc: Wells, Maine 04090
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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