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Equipment Discussions >> Classic Telescopes

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starman876
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Reged: 04/28/08

Loc: VA
Re: Is bigger better? new [Re: Bill Friend]
      #6343172 - 01/29/14 01:41 PM

I agree with the above also. the seeing in this area severly limits aperture size. The Telementor routinely out performs anything larger. One night I had the Telementor out and the Zeiss AS100/1000 and the telementor was providing better views. One thing I will have to say about quality optics. The image quality does not break down as one increases the power. I have seen that over and over again. the 12.5 portaball has optics that are that good. Most dobs and newtonians I have had were merely light buckets and good for low power viewing. The Cave is also an exception to that rule. Cave made high quality mirrors. If you check through most sales for scopes the minute you see someone with a high qaulity mirror they do let you know about it. Not so with other scopes you see for sale. People in this forum when they have a scope they are proud of they let you know it and provide pictures. I would provide pictures, but I have never been much of an imager. Maybe I am too impatient by expecting istant results.

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SpooPoker
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Reged: 06/04/13

Loc: North Bay CA
Re: Is bigger better? new [Re: starman876]
      #6343348 - 01/29/14 03:08 PM

There are so many variables with quality lenses or mirrors, I would take a lot of those quality measures (PV, RMS, Strehl and whatnot) with a large pinch of salt.

I do believe, if a mirror or lens produced errors that were truly below the diffraction limit, increasing the quality (lowering the errors) would be subject to law of diminishing returns - the image cleans up slightly better for every iteration but in a manner where the "relative gain" in bumping up the objective quality decreases. One could feasibly reach a limit where the human eye would no longer be able to detect the improvement in quality. Does the mirror or triplet or doublet lenses have to be perfect? Heck no!

Issues with most mirrors could include:
- not true parabola, mirror parabolic enough to work but spherical aberration would be present, but perhaps below lamda/4 from this effect.

- poor error performance from edges (could be masked off).

- poor secondary mirror partner (may be quite common, I took an underperforming 4.5" f/8 scope and it miraculously cleaned up when I switched secondary mirror).

- Regions of roughness, pits, and such. Causes scattering of light which affects contrast. Mirror needs to be smooth enough, but does not have to be perfect.

- Interfering light sources or scattered light due to hard edges / mirror roughness can affect the image contrast. This is particularly if the Tube is unflocked and unbaffled (quite common in the Chinese made scopes). Baffling / proper flocking helps the stray reflections quite considerably.

- Focuser issues: A common focuser even for a fast scope is the ubiquitous 1.25" 3.5" rack and pinion. Not a great focuser for the faster scope - and one that is unbaffled usually and introduces vignetting.

- The eyepiece itself. Ideally one would pick the best quality eyepiece, but more to the point, the eyepiece that would perform best for a given scope. A .96" Pentax Ortho may not be wisest choice for a 10" f/4.5 Newt - particularly if not coupled with a powermate/barlow. Similarly, a run of the mill Ultra Wide could introduce many aberrations to the optical path giving the illusion the Newt is performing poorly. Also fast scopes generally require assistance with a coma corrector - numerous under-performing Dobs I have looked through at star parties featured a run of the mill eyepiece and no coma correction in sight. No wonder the 6" f/8 Newtonian or 4" Refractor were giving the big old Dob the heave ho in image aesthetics!

I do believe that those folks left underwhelmed by the performance of their Newtonian, despite collimating and cooling down the mirror sufficiently, more than likely would run into any one of the issues above.

Usually the biggest issue though is seeing, cool down, and/or imperfect collimation - particularly for a faster scope.

With proper design, care to detail, and quality components, a Newtonian should perform beautifully. One thing it cannot do is beat the seeing and if there are thermal differences between mirror and outside, that alone will relegate even the most superbly designed instrument to one with 10 - 15X per inch limits.

I think all telescope designs perform beautifully once one understands what they are dealing with. A small quality instrument is an absolutely wonderful thing to own, as is a large instrument. They all good


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starman876
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Reged: 04/28/08

Loc: VA
Re: Is bigger better? new [Re: SpooPoker]
      #6343424 - 01/29/14 03:34 PM

so it would be best to start with a quality product and that way the only thing left would be seeing, cool down, alignment and eyepiece qaulity. Alignment in most refractors should not be a problem. Cool down time is shorter and eyepiece selection should be easy to figure out.

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Petrus351
sage


Reged: 12/17/12

Loc: Spain
Re: Is bigger better? new [Re: starman876]
      #6343487 - 01/29/14 03:54 PM

The laws of physics are the laws of physics, and the larger telescope has a bigger light gathering power and resolution.
Maximun magnification is related to aperture, and a four inch refractor, even being the most expensive in the world, can be used to a maximum of 200-250x at its best, while a 16 inch SCT can be used to a magnification of 800x.
I think the 16 incher, has to be a very, very, very bad a lemon, in order to show less than a 4 or 5 ich refractor.


Edited by Petrus351 (01/29/14 03:55 PM)


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starman876
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Reged: 04/28/08

Loc: VA
Re: Is bigger better? new [Re: Petrus351]
      #6343490 - 01/29/14 03:55 PM

and when you find a night in Washington DC you can use 800X then I would say that I would be dreaming

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Bomber Bob
Carpal Tunnel
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Reged: 07/09/13

Loc: The Deep South, USA
Re: Is bigger better? new [Re: Petrus351]
      #6343498 - 01/29/14 03:57 PM

I routinely used 400x on my D&G 5" -- even pushed it to 500x when seeing was near-perfect.

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Petrus351
sage


Reged: 12/17/12

Loc: Spain
Re: Is bigger better? new [Re: starman876]
      #6343503 - 01/29/14 03:59 PM

Quote:

and when you find a night in Washington DC you can use 800X then I would say that I would be dreaming




I donīt live in Washington, but that has nothing to do with the original question. If the seeing is bad, itīs better to play cards or to watch T.V. instead, as using a telescope in this case is not worth.
I have a C8, a C9.25 and a C11 EDGE, and I have used 500x many times with very good results, and I cannot imagine that with a 4 inch refractor.


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Bomber Bob
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Reged: 07/09/13

Loc: The Deep South, USA
Re: Is bigger better? new [Re: Petrus351]
      #6343509 - 01/29/14 04:01 PM

In fact: I consider 60x per inch the dividing line between average & above-average refractor optics.

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jrcrillyAdministrator
Refractor wienie no more
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Reged: 04/30/03

Loc: NE Ohio
Re: Is bigger better? new [Re: starman876]
      #6343513 - 01/29/14 04:02 PM

Quote:

and when you find a night in Washington DC you can use 800X then I would say that I would be dreaming




There will be nights when the larger telescope's greater resolution and magnification ability won't be used - but it will ALWAYS gather 16 times as much light and will present a much larger exit pupil at 250X or any other magnification.


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Petrus351
sage


Reged: 12/17/12

Loc: Spain
Re: Is bigger better? new [Re: jrcrilly]
      #6343522 - 01/29/14 04:05 PM

A C14 with at least 1/4 wave, "eats" any 4" or 5" refractor, even the most expensive ones.
And nowadays, any SCT has 1/4 or better optics.

Go and search for the best planetary pictures in the world and see how many of them, have been made with 4" o 5" inch refractors, and at the same scale.
The best ones have been made with C14īs, or similar apertures.

Ask Damian Peach, please.



Edited by Petrus351 (01/29/14 04:10 PM)


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Bomber Bob
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Loc: The Deep South, USA
Re: Is bigger better? new [Re: Petrus351]
      #6343535 - 01/29/14 04:12 PM

I bought my D&G 5" in 1988. At that time, a C14 with mount ran more then 4x the total cost of my kit (with Jaeger's pier mount). I think I did OK. I'll sacrifice some image size for a less budget-busting & hernia-inducing solution.

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amicus sidera
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Reged: 10/14/11

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Re: Is bigger better? new [Re: Petrus351]
      #6343536 - 01/29/14 04:12 PM

Quote:

The laws of physics are the laws of physics, and the larger telescope has a bigger light gathering power and resolution.
Maximun magnification is related to aperture, and a four inch refractor, even being the most expensive in the world, can be used to a maximum of 200-250x at its best, while a 16 inch SCT can be used to a magnification of 800x.
I think the 16 incher, has to be a very, very, very bad a lemon, in order to show less than a 4 or 5 ich refractor.





Agreed, but it would appear that both the Refractors and Classic Telescopes forums have participants who have decreed that the laws of physics be held in abeyance.


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Bomber Bob
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Re: Is bigger better? new [Re: amicus sidera]
      #6343546 - 01/29/14 04:14 PM

Laws? We don't need no stinkin' laws!

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starman876
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Reged: 04/28/08

Loc: VA
Re: Is bigger better? new [Re: Bomber Bob]
      #6343595 - 01/29/14 04:35 PM

I agree with the laws of physics. My 6" AP is pushed to close to 500 power without an issue. So with that said how can I argue with the laws of physics. Many nights of veiwing I would have both out at the same time and the C14 and the 6" AP stay head to head at the same powers. So how can I argue with anything that is said here

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starman876
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Loc: VA
Re: Is bigger better? new [Re: Petrus351]
      #6343606 - 01/29/14 04:38 PM

you really do need to look at pictures that smaller aperture scopes have taken. Pictures on Amart of the planets with high end APO's sure are awesome. I would have to argue that with some of them I have never seen anything better with a C14. Having a couple of C14's I know there limitations.

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rdandrea
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Reged: 06/13/10

Loc: Colorado, USA DM59ra
Re: Is bigger better? new [Re: amicus sidera]
      #6343617 - 01/29/14 04:43 PM

Quote:

Agreed, but it would appear that both the Refractors and Classic Telescopes forums have participants who have decreed that the laws of physics be held in abeyance.





I'm quite surprised that the Questar forum hasn't emptied to come over here and refute the laws of physics.


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rdandrea
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Re: Is bigger better? new [Re: amicus sidera]
      #6343619 - 01/29/14 04:44 PM

duplicate post deleted by rd

Edited by rdandrea (01/29/14 04:44 PM)


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wfj
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Reged: 01/10/08

Loc: California, Santa Cruz County
Re: Is bigger better? new [Re: amicus sidera]
      #6343622 - 01/29/14 04:44 PM

Lately I've been culling my scope herd ...

I take out 1 "big" scope (at the moment 8" cAVE) and 1 "small" scope (62mm Nihon Sieko) along with the "unit under test".

Both "small" and "big" happen to be best in catagory, no optical issues, well tested and used, no thermal/environmental issues on site.

"Big" lets me check accurately seeing and "what's possible".

"Small" sets a lower bound on desired performance.

If I can see more than "small" that I can confirm with "big", I can accept this as an objective measure of pragmatic quality - that bigger than "small" is better.

I'll be selling some 80-130mm scopes soon. I also have a 152mm that has an issue I'm solving, and should I solve it, it will have a point and leave the "sell" category.

The practical reality is that for bigger to be better, it can't just be bigger, but at least as better as "small", for the first cut.

Survivor round 2: "how many "small","big" ... serves to suite this hobby". Tune in next week ...


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Dan /schechter
scholastic sledgehammer


Reged: 12/21/06

Loc: Long Beach, Calif.
Re: Is bigger better? new [Re: starman876]
      #6343637 - 01/29/14 04:48 PM

This is a great question that gets discussed from time to time. I enjoy reading the responses whenever it comes up again.

Is bigger better? If we can standardize the instruments, physics gives us the answer. If we compare instruments of excellent optical quality, build, eyepieces, cooling time and great seeing, then the answer is yes, bigger is better. My greatest aperture refractor is an AP 6" f/9 Starfire EDT. It almost performs as well or better than most instruments on the telescopes field. I can hardly count how many times I viewed thru larger aperture instruments, especially refractors, and did not get better views. However, on those once in a year nights with fantastic seeing, the owner of those larger instruments had to peel me away. Their superiority was easy to see.

Another way to phrase the question, on nights of poor to mediocre seeing, "Is Bigger worse?" Early in my viewing experience, I thought so. I read the statement that since the seeing cell is smaller for smaller aperture instruments, they will outperform greater aperture instruments on nights of so so seeing. It makes great sense. I have visited many observatories with BIG telescopes and had the guide explain that the 4" refractor over in the corner quite often outperforms the main instrument. Again, this makes sense.

Then Clint found his "Pearl". A 9" f/10+ AP triplet. I have had my 6" Starfire set up close to the Pearl somewhere between 5 and ten times with differences in seeing from lousy to nice. Not once did my 6" ever outperform Clints AP 9" no matter what the seeing or the power. When the power was the same, the Pearl always outperformed my 6" AP. In fact, as the power increased, the difference became even more apparent.

Last summer I set up a Clark 3" f/10 refractor next to Jack Eastman`s 6" f/15 Clark and had the same experience. When the powers were the same, his 6" outperformed my 3". Not only that, it did better as the powers were increased. It produced a mediocre image at higher powers while the 3" produced a blur. This was during a time of seeing being from a 2 to 4 out of ten.

I was surprised at the results since I was sure that my lesser aperture aperture telescope would outperform greater aperture telescopes in poor seeing.

That experience made me readjust the statement that bigger is poorer in poor seeing. I have not figured out the ratio between apertures that has to exist before that is true, but I think that the larger instrument must have several times the aperture of the smaller instrument for that statement to be true. Clints 9" Pearl is only 1 1/2" times as large as my 6". Jack`s 6" was only twice as large as my 3". Maybe, someone in this forum knows what the ratio has to be for the effect to be true.

Again, great topic,
Dan


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Bomber Bob
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Re: Is bigger better? new [Re: wfj]
      #6343642 - 01/29/14 04:49 PM

"I'll be selling some 80-130mm scopes soon."

You wouldn't happen to have one of the Meade 4" refractors from the 1970's would you? (They had a Unitronish look, IIRC.)


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