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OPT 8" f/9 Planet Pro Dobsonian

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#101 Jarad

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Posted 30 May 2013 - 10:10 AM

There are lots of cheaper widefield designs that work well at f8 or longer but not in fast scopes. Most are variants on the Erfle design.

Jarad

#102 derangedhermit

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Posted 30 May 2013 - 11:50 AM

There are lots of cheaper widefield designs that work well at f8 or longer but not in fast scopes. Most are variants on the Erfle design.

Jarad


Wasn't it Rutten and Venrooij in Telescope Optics who pointed out so clearly how the aberrations of typical eyepieces, wide or standard AFOV, typically swamped telescope aberrations until the Nagler was put on the market?

I'm not aware of any line of eyepieces that meet the optical standard first set by TeleVue (and improved since then) but only at, say, f/7 and higher (I don't know a resonable f-ratio; modern high-end designs go at least down to f/4 and some below. High mechanical, production, and QA standards as well.

I *would* like to become aware of them:)

As far as field curvature, isn't a field flattener designed for an optical system (like SCTs or refractors), such as often used in photography, already available, and maybe the way to go? Is there enough benefit in matching amounts of +/- field curvature in each eyepiece? It seems like one more constraint on the design. At long focal ratios, are strongly curved fields still an issue for visual use (showing my ignorance)?

#103 Jeff Morgan

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Posted 09 July 2013 - 01:09 AM

Personally, I'm for freedom. If someone wants an F/9 8", all power to him. Personally, I believe the standard 8" F/6 is the best sized 8" dob, but to each his own.


Since I am finishing a 16” scope project my mind has wandered to the next ATM project. While the choice has not been made, a strong contender is the subject of this thread - a 8” f/9 Newtonian. When one contemplates such a scope, the stereotypical “narrow field” comes to mind. But is it really true? How would such a scope fare compared to a more common f/6?

I ran a search using AstroPlanner for all DSO’s magnitude 12.0 and brighter above Declination -45. Essentially, all the large bight DSO’s available to northern hemisphere observers.

For the sake of comparison, let’s assume that the maximum true field “panoramic” True Field eyepiece for both scopes (8” f/9 and 8” f/6) is the venerable 31 Nagler T5 and we accept an object as “well framed” if it occupies no more than 75% of the True Field. A little simple math indicates a maximum True Field of 60.2 arc minutes for the f/9 scope and 90.0 arc minutes at f/6. In terms of DSO eye candy, here are the 17 objects not “well framed” in the slower scope but still “well framed” in the faster:

M7 - Ptolemy’s Cluster
M24 - Star Cloud
NGC6383
Barnard268
Barnard283
Barnard297
Barnard312
Barnard364
ESO356-4
Mel179
NGC6960
M33
Barnard30
M42
ESO351-30
Cr69
NGC598

Interestingly, going to a 41 Panoptic in the f/9 scope doesn’t change things much. Sure it shows more sky - but doesn't "frame" many more DSO's.

Not a very big list is it?

#104 Jon Isaacs

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Posted 09 July 2013 - 07:53 AM

Not a very big list is it?



Richest field viewing is a different Paradigm, a different mindset. When you think in the framing object mindset of the zoom in to see it... faster focal ratios and the big, bright views don't make much sense. So what if the F/6 provides a image that is over twice as bright, more than twice the area, we are zooming in...

But when you step back and say I am going to look at the big picture, multiple objects in the same field of view, the various large scale features of our own galaxy, the lanes and dark nebulae that obscure the nebulosity of our own galaxy, then fast scopes go places slow scopes don't.

In terms of F/6 for an 8 inch, that is determined by ergonomics. An 8 inch F/9 has a focal length of 72 inches, same as a 16 inch F/4.5, it's a lot of avoidable hassle for an 8 inch scope, but worth it for a 16 inch. With my 16 inch F/4.42, I see everything one can see in a 8 inch F/9 but it's 4 times as bright so I am seeing a lot more..

The richest field thing... not everyone appreciates it. I do.

Jon

#105 Jeff Morgan

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Posted 09 July 2013 - 10:25 PM

Richest field viewing is a different Paradigm, a different mindset.


Indeed. I just realized I fell into the trap myself - true field is only a function of primary focal length and eyepiece field stop. Not focal ratio.

But when you step back and say I am going to look at the big picture, multiple objects in the same field of view, the various large scale features of our own galaxy, the lanes and dark nebulae that obscure the nebulosity of our own galaxy, then fast scopes go places slow scopes don't.


A 2" f/10 refractor fitted with a 2" focuser would have no trouble completely framing M31. My Fujinon 10x50 binoculars do even better. It's all about focal length and field stops.

In terms of F/6 for an 8 inch, that is determined by ergonomics. An 8 inch F/9 has a focal length of 72 inches, same as a 16 inch F/4.5, it's a lot of avoidable hassle for an 8 inch scope, but worth it for a 16 inch.


Hassle? So how much does that 16 weigh vs. an 8? Cool down? Since the eye is afocal, the brightness argument requires equal magnification. Somewhat impossible to achieve in the low power (sweeping) regime.

I get your point though - we all make choices based on what we want and the inconvenience we will suffer to get there. Naturally if the objective was Milky Way sweeping one could cut the focal length back to 36" and get more. Might be more cost effective to grab the binoculars though.

Getting back to the OP, he ran across an advertisement for a 8" f/9 and was curious about it. That in turn made me a bit curious because (1) I'm looking for a new project; and (2) such a scope offers the promise of ladder-free observing but can still be hefted by one hand. The gentle light cone is easy on eyepieces. And it shows exactly - exactly - the same amount of sky as any other telescope of 72" focal length.

#106 Jon Isaacs

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Posted 10 July 2013 - 12:14 AM

A 2" f/10 refractor fitted with a 2" focuser would have no trouble completely framing M31. My Fujinon 10x50 binoculars do even better. It's all about focal length and field stops.



It's not only about field of view, it's also about image brightness, magnification and resolution. The NP-101 with it's 4.5 degree TFoV at 17x with a 5.7mm exit pupil shows M31 in ways that are not possible with 10x50s...

Hassle? So how much does that 16 weigh vs. an 8? Cool down? Since the eye is afocal, the brightness argument requires equal magnification. Somewhat impossible to achieve in the low power (sweeping) regime.



Equal magnification: that simply means the same eyepiece. It's true one is not going to use the 55mm Plossl at F/4.5 but one is also unlikely to use it in the 8 inch F/9 simply because of it's narrow field of view.

In the real world, it might be the 31mm Nagler versus the 41mm Panoptic or both scopes with the 31mm Nagler. Both provide similar fields of view, so it's a trade off between magnification and image brightness.

With either of eyepieces, the 41mm Panoptic, the 31mm Nagler in the F/9 and the 31mm in the F/4.5, the large scope will provide essentially the same true field of view.

With the 41mm Pan in the F/9, the 16 inch (with the 31mm) will be 2.5x brighter while also providing about a third greater magnification. With both having the 31mm, the larger scope will be 4X brighter.

The hassle.. it's the eyepiece height... 72 inches, that's a tall sit. The real question, what's the advantage of a 8 inch F/9 over an 8 inch F/6 or F/7? Not too much.

Jon Isaacs


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