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Is bigger better?

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#101 svdwal

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Posted 30 January 2014 - 11:11 AM

Yes, but what are the margins???

I mean, we´re talking about a 4" refractor in comparison to a C14? or are we talking about a 6" vs 8" scope?
What you say, can be true between small differences in aperture, but not between important ones.


It depends a lot on the scopes. A Ritchey-Chretien with a big central obstruction will have to be bigger than a Newton for it to beat a refractor. And a mass-market SCT with 1/4 lambda spherical abberation will have to be bigger than one with 1/10 lambda.

Apparently the heuristic of subtracting the central obstruction from the objective diameter works quite well.

#102 Jim Curry

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Posted 30 January 2014 - 12:55 PM

I have the answer to that!

Jim

#103 Chuck Hards

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Posted 30 January 2014 - 01:11 PM

I have the answer to that!

Jim


Lol, you and everyone else! ;)

#104 rcwolpert

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Posted 30 January 2014 - 01:25 PM

With 50 years of observing behind me and often pondering this question of "Is Bigger Better", I've come to the following 5 conclusions that seem to work for me:

1. A small aperture telescope out under the stars seems to always out perform a much larger aperture telescope in the garage.
2. Larger aperture provides better resolution of clusters and double stars.
3. Larger aperture provides better views of galaxies and nebulae.
4. On nights of bad seeing, both will perform about the same if the same POWER is used. If the same EYEPIECE is used, the small aperture scope may provide a much better view.
5. On nights of good seeing, both large and small aperture telescopes can have you totally awed all night long when viewing the moon, Jupiter, and Saturn.

- Bob

#105 BigC

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Posted 30 January 2014 - 01:41 PM

+1

#106 TCW

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Posted 30 January 2014 - 01:43 PM

That is quite a scope! Can it reach zenith? :lol:

#107 starman876

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Posted 30 January 2014 - 04:05 PM

For the next post it will be is smaller better :lol:

#108 starman876

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Posted 30 January 2014 - 04:07 PM

Of course all the data that has been presented are theories that were developed many years ago. I have always wondered if these same theories still hold true with all the advancements in lens making and the exotic glass that is used today.

#109 actionhac

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Posted 30 January 2014 - 06:49 PM

That is quite a scope! Can it reach zenith? :lol:


Yes I make sure the tube will reach zenith. You can see its close! I can roast marshmallows up at the other end of the tube. It settles down quickly once the sun sets. I've got a few more refinements to make to it. Its a ongoing experiment and I enjoy trying new ideas with it.

Attached Files



#110 Jon Isaacs

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Posted 30 January 2014 - 10:07 PM

A 6" APO could be very well color corrected down to about f/5. A Newtonian on that same focal ratio would get absolutely hammered in terms of contrast, fine resolution, details etc...



Have you ever seen a 6 inch F/5 apo? They might exist but I don't know of any...

Jon

#111 SpooPoker

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Posted 30 January 2014 - 10:27 PM

Here is a 6incher on a short FL:
http://www.teleskop-...S-PHOTOLINE-...

#112 Jon Isaacs

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Posted 31 January 2014 - 05:24 AM

Here is a 6incher on a short FL:
http://www.teleskop-...S-PHOTOLINE-...


It's actually a 130mm f/7 with a field flattener that can be screwed into the focuser. It's a 5 inch along the lines of the NP-127 but less intergrated and possibly not capable of visual use .

I don't know of any "native" 6 inch F/5 apos. It might be possible but the ones I see are relatively slow and within range of focal lengths where a planetary 6 inch Newtonian is possible.

Jon

#113 Petrus351

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Posted 31 January 2014 - 07:02 AM

"Only for some objects, although undermagnification is often a problem, even for experienced observers. The penalty for increased magnification is reduced field of view and brightness; faint objects grow fainter as the magnification is increased This is why larger aperture telescopes are so effective on faint objects; they provide enough light to stimulate the eye at high magnifications. For example, a 4-inch telescope will only view a globular cluster effectively at 80X, and it will appear as a blob. A 6-inch will resolve the outer stars at 130X, an 8-inch will resolve further in at 200X. 10 and 12.5-inch telescopes will make them glitter to the core at 300 and 400X".

Copy and paste from the Sky Watcher home web page.

#114 svdwal

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Posted 31 January 2014 - 12:23 PM

Of course all the data that has been presented are theories that were developed many years ago. I have always wondered if these same theories still hold true with all the advancements in lens making and the exotic glass that is used today.


Those theories still hold, AFAIK. What has changed is that ray tracing is much faster on a computer.

#115 SpooPoker

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Posted 31 January 2014 - 03:50 PM

It's actually a 130mm f/7 with a field flattener that can be screwed into the focuser. It's a 5 inch along the lines of the NP-127 but less intergrated and possibly not capable of visual use .

I don't know of any "native" 6 inch F/5 apos. It might be possible but the ones I see are relatively slow and within range of focal lengths where a planetary 6 inch Newtonian is possible.

Jon


:foreheadslap:

You are right, dunno where 130mm = 6" came from. Late night I guess. Yeah, it seems anything in APO world larger than 4" generally comes in f/7 - f/9 forms. I guess the reason would be correcting a 6" f/5 for visual would be prohibitively expensive or nigh on impossible to manufacture. A 6" f/5 on a crown flint form (achromat) would have phenomenal CA and there is only so much a triplet can do. I guess it is far easier to correct via triplet form a f/7 or f/8 than a f/6 or less.

In saying that, your TV 100mm f/5 APO does a good job on color correction - correct?

#116 CHASLX200

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Posted 31 January 2014 - 08:02 PM

Bigger is always better until ya have to move it around or set it up.

Chas

#117 starman876

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Posted 31 January 2014 - 11:02 PM

I would say bigger is better when you get around 20" and if it is your scope it is the best ever.

#118 Datapanic

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Posted 01 February 2014 - 01:01 AM

This thread belongs in the Beginners Forum!

#119 Jon Isaacs

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Posted 01 February 2014 - 04:43 AM

In saying that, your TV 100mm f/5 APO does a good job on color correction - correct?



It is excellent, even out of focus. But it's not really a 4 inch F/5.4, or at least the objective is not. It's a bit of a simplification but it's basically a long focal length apo double with a fancy ED focal reducer/field flattener at the rear of the scope. NP stands for Nagler-Petzval.

You could do a similar thing with a 6 inch apo but then you could do a similar thing with with a Newtonian but neither makes much sense.

Jon

#120 SpooPoker

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Posted 01 February 2014 - 10:11 PM

It is excellent, even out of focus. But it's not really a 4 inch F/5.4, or at least the objective is not. It's a bit of a simplification but it's basically a long focal length apo double with a fancy ED focal reducer/field flattener at the rear of the scope. NP stands for Nagler-Petzval.

You could do a similar thing with a 6 inch apo but then you could do a similar thing with with a Newtonian but neither makes much sense.

Jon


Cool sounds like an awesome scope.

Is the FOV just like a f/7 or f/8 frac or does the design get around that? I am seriously considering getting one of these for a big FOV, but if it has the same FOV as a regular 4" ED doublet, then I may sit tight on my money :)

#121 Jon Isaacs

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Posted 02 February 2014 - 09:55 PM


It is excellent, even out of focus. But it's not really a 4 inch F/5.4, or at least the objective is not. It's a bit of a simplification but it's basically a long focal length apo double with a fancy ED focal reducer/field flattener at the rear of the scope. NP stands for Nagler-Petzval.

You could do a similar thing with a 6 inch apo but then you could do a similar thing with with a Newtonian but neither makes much sense.

Jon


Cool sounds like an awesome scope.

Is the FOV just like a f/7 or f/8 frac or does the design get around that? I am seriously considering getting one of these for a big FOV, but if it has the same FOV as a regular 4" ED doublet, then I may sit tight on my money :)


The TeleVue NP-101 is a 4 inch apochromatic with a focal length of 540mm and functions as 4 inch F/5.4. But the actual objective is not F/5.4.

Jon

#122 Bomber Bob

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Posted 03 February 2014 - 09:11 PM

I'll summarize my post from another forum: A city's heat dome will affect seeing, which will affect viewing options, and consequently the scope I use on a given night.

#123 Jon Isaacs

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Posted 03 February 2014 - 09:28 PM

I'll summarize my post from another forum: A city's heat dome will affect seeing, which will affect viewing options, and consequently the scope I use on a given night.


In my experience, the "heat dome" is localized so that if the object is at a reasonable altitude, it does not affect the overall seeing. But the localized heating effects are very important, viewing over trees and building, viewing on concrete or asphalt, these can make a big difference.

Jon

#124 Bomber Bob

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Posted 03 February 2014 - 09:33 PM

"In my experience, the "heat dome" is localized..."

What I've found in my current location (the city is in a Bowl) is that winds from the north make the dome much more unstable than the warm muggy air from the Gulf of Mexico. But, just 10 miles away, it's not a factor at all.






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