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Explore Scientific 12" f/5 Truss Tube Dobsonian Telescope



 
 
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  #31  
Old January 5th 17, 09:23 PM posted to sci.astro.amateur
Martin Brown
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Posts: 1,707
Default Explore Scientific 12" f/5 Truss Tube Dobsonian Telescope

On 05/01/2017 11:13, wrote:
On Wednesday, January 4, 2017 at 12:34:25 PM UTC-5, Martin Brown wrote:
On 01/01/2017 08:43, palsing wrote:
On Saturday, December 31, 2016 at 11:40:32 PM UTC-8,
wrote:


Whereas, "rich field telescope" is a sloppy, vague term which has
come to refer to a short-focus telescope, used at low-power. IOW,
it's meaningless.


Any scope can be made to operate in the RFT regime if you allow for
unreasonably large eyepiece sizes. Basically it is RFT when the exit
pupil is around 6mm (or 7mm if you are being optimistic).

Lowest usable magnification 2" eyepiece as a function of f ratio is

f4 25mm 100 AFOV
f5 31mm 80 AFOV
f6 38mm 70 AFOV
f8 50mm 55 AFOV

And if cost is no object a 3" draw tube and the latest Explore
Scientific 30mm 100 AFOV on the f5. Yet to see one in the UK.

So any of the following magnification x aperture are nominally RFT

8 x 50
16 x 100
24 x 150
32 x 200 (probably about the optimum)
40 x 250

My local sky brightness is not ideal for this low magnification though.
In a truly dark sky it would be most impressive.

It is a more polite term for a light bucket. A bit more effort having
gone into optimising it for wide field views usually at the expense of
on axis sharpness in all but the premium models.


For planets, I would go for the 12-inch f/5 Newt over the 5-inch f/12 refractor. YMMV.


Depends a lot on whether the 12" f5 Newt mirror has a turned down edge.
Many cheap light buckets have less than stellar performance at higher
magnifications when they were only designed for use as RFTs deep sky.
Stopping it down a bit will improve the image quality no end but won't
fix the lamentable collimation errors that they also tend to have.

A 5" f12 refractor will do quite a decent job visually on planets.
(but also costs a fair bit more than a Dobsonian Newt)

--
Regards,
Martin Brown
  #33  
Old January 6th 17, 08:58 AM posted to sci.astro.amateur
Chris.B[_3_]
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Posts: 1,001
Default Explore Scientific 12" f/5 Truss Tube Dobsonian Telescope

On Thursday, 5 January 2017 21:23:48 UTC+1, Martin Brown wrote:

Depends a lot on whether the 12" f5 Newt mirror has a turned down edge.
Many cheap light buckets have less than stellar performance at higher
magnifications when they were only designed for use as RFTs deep sky.
Stopping it down a bit will improve the image quality no end but won't
fix the lamentable collimation errors that they also tend to have.

A 5" f12 refractor will do quite a decent job visually on planets.
(but also costs a fair bit more than a Dobsonian Newt)

--
Regards,
Martin Brown


I think it only fair to point out that elderly eyes are often limited to an exit pupil of 5mm or less.

The difference between one low powered telescope's star filled field and another, is likely to be subtle.

Aperture matters for planetary resolution provided the seeing allows it to be used to full effect. Some argue that waiting for a sharp moment in a larger telescope will show far more detail than enjoying almost continuous, but less detailed, views in a smaller instrument. Greater light gathering power is often an embarrassment for planetary/Lunar work.

There is a special class of reflectors with cooled, high quality primaries and small secondaries which compete strongly on planetary/Lunar detail. Long focus Newtonians have a good reputation for seeing fine planetary detail. Light pollution may matter much less for such observations but good seeing conditions are a must. Many larger aperture instruments are hampered by poor seeing.

The devil is always in the details. Wide field views of stars are much less demanding of optical quality and thermal management than specialist "planetary" instruments. A diaphragm [baffle] just in front of the mirror may/will solve turned edge problems if the rest of the mirror is smooth and well corrected.
  #34  
Old January 6th 17, 11:00 AM posted to sci.astro.amateur
[email protected]
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Default Explore Scientific 12" f/5 Truss Tube Dobsonian Telescope

On Friday, January 6, 2017 at 2:58:39 AM UTC-5, Chris.B wrote:

Aperture matters for planetary resolution provided the seeing allows it to be
used to full effect.


Incorrect, obviously.

Some argue that waiting for a sharp moment in a larger
telescope will show far more detail than enjoying almost continuous, but less
detailed, views in a smaller instrument.


They aren't arguing, they are stating a fact.

Greater light gathering power is
often an embarrassment for planetary/Lunar work.


Poor light-gathering power embarrasses small telescopes used for highly detailed planetary and lunar observing. (That said, small telescopes have other advantages, especially for newbies.)

I have actually done side-by-side comparisons of small, longer focus refractors with larger, shorter-focal-ratio Newts during planetary/lunar observation.

In each case the larger outperformed the smaller, regardless of atmospheric seeing. YMMV.



  #35  
Old January 6th 17, 11:21 AM posted to sci.astro.amateur
Martin Brown
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Posts: 1,707
Default Explore Scientific 12" f/5 Truss Tube Dobsonian Telescope

On 05/01/2017 21:03, wrote:
On Thursday, January 5, 2017 at 3:23:48 PM UTC-5, Martin Brown wrote:
On 05/01/2017 11:13,
wrote:
On Wednesday, January 4, 2017 at 12:34:25 PM UTC-5, Martin Brown wrote:


It is a more polite term for a light bucket. A bit more effort having
gone into optimising it for wide field views usually at the expense of
on axis sharpness in all but the premium models.

For planets, I would go for the 12-inch f/5 Newt over the 5-inch f/12 refractor. YMMV.


Depends a lot on whether the 12" f5 Newt mirror has a turned down edge.
Many cheap light buckets have less than stellar performance at higher
magnifications when they were only designed for use as RFTs deep sky.
Stopping it down a bit will improve the image quality no end but won't
fix the lamentable collimation errors that they also tend to have.


You are making a strawman argument, at best.


ROFL You know that what I say is true but won't admit it.

A 5" f12 refractor will do quite a decent job visually on planets.
(but also costs a fair bit more than a Dobsonian Newt)


The 12-inch will have well over twice the resolution and well over five times the light gathering power of the 5-inch.


Light gathering doesn't make any difference on planets and certainly not
when the seeing and systematic errors in the optical surfaces prevent
the effective use of high magnification anyway.

Using a webcam and registax the seeing at least can be eliminated but
there is nothing much can be done about intrinsic systematic errors.

--
Regards,
Martin Brown
  #36  
Old January 6th 17, 12:42 PM posted to sci.astro.amateur
[email protected]
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Posts: 9,472
Default Explore Scientific 12" f/5 Truss Tube Dobsonian Telescope

On Friday, January 6, 2017 at 5:21:56 AM UTC-5, Martin Brown wrote:
On 05/01/2017 21:03, wrote:
On Thursday, January 5, 2017 at 3:23:48 PM UTC-5, Martin Brown wrote:
On 05/01/2017 11:13,
wrote:
On Wednesday, January 4, 2017 at 12:34:25 PM UTC-5, Martin Brown wrote:


It is a more polite term for a light bucket. A bit more effort having
gone into optimising it for wide field views usually at the expense of
on axis sharpness in all but the premium models.

For planets, I would go for the 12-inch f/5 Newt over the 5-inch f/12 refractor. YMMV.

Depends a lot on whether the 12" f5 Newt mirror has a turned down edge.
Many cheap light buckets have less than stellar performance at higher
magnifications when they were only designed for use as RFTs deep sky.
Stopping it down a bit will improve the image quality no end but won't
fix the lamentable collimation errors that they also tend to have.


You are making a strawman argument, at best.


ROFL You know that what I say is true but won't admit it.


No, what you say is a gross mis-characterization of short focus reflectors, ie a strawman.

A 5" f12 refractor will do quite a decent job visually on planets.
(but also costs a fair bit more than a Dobsonian Newt)


The 12-inch will have well over twice the resolution and well over five times the light gathering power of the 5-inch.


Light gathering doesn't make any difference on planets


ROTFLMAO! You haven't done much actual observing have you? Must be that atrocious British weather.

and certainly not
when the seeing and systematic errors in the optical surfaces prevent
the effective use of high magnification anyway.


Again, a strawman.

Using a webcam and registax the seeing at least can be eliminated but
there is nothing much can be done about intrinsic systematic errors.


And another strawman.

  #37  
Old January 12th 17, 12:40 PM posted to sci.astro.amateur
Martin Brown
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Posts: 1,707
Default Explore Scientific 12" f/5 Truss Tube Dobsonian Telescope

On 06/01/2017 11:42, wrote:
On Friday, January 6, 2017 at 5:21:56 AM UTC-5, Martin Brown wrote:
On 05/01/2017 21:03,
wrote:
On Thursday, January 5, 2017 at 3:23:48 PM UTC-5, Martin Brown wrote:
On 05/01/2017 11:13,
wrote:
On Wednesday, January 4, 2017 at 12:34:25 PM UTC-5, Martin Brown wrote:

It is a more polite term for a light bucket. A bit more effort having
gone into optimising it for wide field views usually at the expense of
on axis sharpness in all but the premium models.

For planets, I would go for the 12-inch f/5 Newt over the 5-inch f/12 refractor. YMMV.

Depends a lot on whether the 12" f5 Newt mirror has a turned down edge.
Many cheap light buckets have less than stellar performance at higher
magnifications when they were only designed for use as RFTs deep sky.
Stopping it down a bit will improve the image quality no end but won't
fix the lamentable collimation errors that they also tend to have.

You are making a strawman argument, at best.


ROFL You know that what I say is true but won't admit it.


No, what you say is a gross mis-characterization of short focus reflectors, ie a strawman.


Yeah! Right! Lets just do the math.

300mm @ f5 fl = 1.5m

Light cone at furthest possible secondary position 30mm + 48mm = 78mm to
fully illuminate a typical 2" wide field deep sky eyepiece.

Even without the ubiquitous turned down edge so typical of a never mind
the quality feel the width light bucket mirror intended mainly for deep
sky observation to match the wide field requirement of a 2" focuser the
secondary obstruction is over 25% on a 12". That is way more than is
acceptable for seeing low contrast fine planetary belt detail.

But don't take my word for it:

http://www.brayebrookobservatory.org...m/c-o%27s.html

and certainly not
when the seeing and systematic errors in the optical surfaces prevent
the effective use of high magnification anyway.


Again, a strawman.

Using a webcam and registax the seeing at least can be eliminated but
there is nothing much can be done about intrinsic systematic errors.


And another strawman.


Anything you disagree with you simply dismiss as a strawman.

Last one I looked through which ISTR was a 14" f5 mirror that appeared
to have been silicone or maybe PU glued onto a crude 18mm plywood base.
Collimation was a joke. The Galilean satellites looked like seagulls
flying round a blob with only the north and south equatorial belts
actually visible. It was easily outclassed by a 4" refractor or an 8"
SCT both present. The refractor sometimes showing an Airy disk on stars.

It did a lot better on faint fuzzies but the very poor collimation still
meant that stars were not exactly round even at low magnification.

--
Regards,
Martin Brown
  #38  
Old January 12th 17, 04:06 PM posted to sci.astro.amateur
[email protected]
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Posts: 9,472
Default Explore Scientific 12" f/5 Truss Tube Dobsonian Telescope

On Thursday, January 12, 2017 at 6:40:21 AM UTC-5, Martin Brown wrote:
On 06/01/2017 11:42, wrote:
On Friday, January 6, 2017 at 5:21:56 AM UTC-5, Martin Brown wrote:
On 05/01/2017 21:03,
wrote:
On Thursday, January 5, 2017 at 3:23:48 PM UTC-5, Martin Brown wrote:
On 05/01/2017 11:13,
wrote:
On Wednesday, January 4, 2017 at 12:34:25 PM UTC-5, Martin Brown wrote:

It is a more polite term for a light bucket. A bit more effort having
gone into optimising it for wide field views usually at the expense of
on axis sharpness in all but the premium models.

For planets, I would go for the 12-inch f/5 Newt over the 5-inch f/12 refractor. YMMV.

Depends a lot on whether the 12" f5 Newt mirror has a turned down edge.
Many cheap light buckets have less than stellar performance at higher
magnifications when they were only designed for use as RFTs deep sky.
Stopping it down a bit will improve the image quality no end but won't
fix the lamentable collimation errors that they also tend to have.

You are making a strawman argument, at best.

ROFL You know that what I say is true but won't admit it.


No, what you say is a gross mis-characterization of short focus reflectors, ie a strawman.


Yeah! Right! Lets just do the math.

300mm @ f5 fl = 1.5m

Light cone at furthest possible secondary position 30mm + 48mm = 78mm to
fully illuminate a typical 2" wide field deep sky eyepiece.

Even without the ubiquitous turned down edge so typical of a never mind
the quality feel the width light bucket mirror intended mainly for deep
sky observation to match the wide field requirement of a 2" focuser the
secondary obstruction is over 25% on a 12". That is way more than is
acceptable for seeing low contrast fine planetary belt detail.

But don't take my word for it:

http://www.brayebrookobservatory.org...m/c-o%27s.html

and certainly not
when the seeing and systematic errors in the optical surfaces prevent
the effective use of high magnification anyway.


Again, a strawman.

Using a webcam and registax the seeing at least can be eliminated but
there is nothing much can be done about intrinsic systematic errors.


And another strawman.


Anything you disagree with you simply dismiss as a strawman.

Last one I looked through which ISTR was a 14" f5 mirror that appeared
to have been silicone or maybe PU glued onto a crude 18mm plywood base.
Collimation was a joke. The Galilean satellites looked like seagulls
flying round a blob with only the north and south equatorial belts
actually visible. It was easily outclassed by a 4" refractor or an 8"
SCT both present. The refractor sometimes showing an Airy disk on stars.

It did a lot better on faint fuzzies but the very poor collimation still
meant that stars were not exactly round even at low magnification.



You seem to forget that -I- never mentioned "mirrors glued to plywood" and that -I- get to decide details about the telescope, the mirror and the manner in which those are assembled, managed, used and adjusted.

Quit making strawman arguments, brown.

  #39  
Old January 12th 17, 05:04 PM posted to sci.astro.amateur
[email protected]
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Posts: 9,472
Default Explore Scientific 12" f/5 Truss Tube Dobsonian Telescope

On Thursday, January 12, 2017 at 6:40:21 AM UTC-5, Martin Brown wrote:
It was easily outclassed by a 4" refractor or an 8"
SCT both present.


No doubt, one could use a magnifying glass or other simple lenses, to contrive a 4-inch refractor.

It did a lot better on faint fuzzies but the very poor collimation still
meant that stars were not exactly round even at low magnification.


Support that mirror properly, collimate the scope and then get back to us.

 




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