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  #21  
Old 04-06-2015, 08:20 AM
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Ah get it, thanks
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  #22  
Old 04-06-2015, 05:48 PM
clive milne
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If refractors are a panacea, why aren't they commissioned in a contemporary professional context in apertures exceeding 4 inches?
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  #23  
Old 04-06-2015, 06:53 PM
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Originally Posted by clive milne View Post
If refractors are a panacea, why aren't they commissioned in a contemporary professional context in apertures exceeding 4 inches?
I think the simple reason for that is cost and size. An excellent 6 inch APO refractor costs around USD$12,000. An 8 inch one jumps to USD$30,000.

Plus an 8 inch refractor is getting to the permanent observatory type size and weight.

A good 8 inch reflector is a fraction of that.

Greg.
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  #24  
Old 05-06-2015, 10:23 AM
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So the same as for most of us average amateurs then

Thinking about this, there must be an overlapping, blurred zone where practice takes precedent over theory and real atmospherics make the differences irrelevant, or tips the balance one way or the other

For example, between 6 and 8" in average seeing west of the Dividing range?

Is this what is meant by "diffraction limited" ? And what stage is this actually noticeable to the eye, rather than with instrumentation?
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  #25  
Old 05-06-2015, 12:05 PM
glend (Glen)
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Originally Posted by alpal View Post
That quote by Gary can cut both ways in its relevance, yes a refractor objective is at the front and exposed to the night air - but so is the corrector plate of a SCT (dew magnets that they are), and are not all refractors equipped with dew shields these days, and they all (refractors) need dew heaters. Newts however, mostly all have fans, and have a long tube that acts as a dew shield, and being open tube do not have cool down issues that SCTs and refractors do (minimal tube currents) and for truss dobs no tube currents at all.

My point is many of these pronouncements can be turned right around by looking at all aspects of the design and performance. There is a thread over on Cloudy Nights called "Are APOs better than Newts" and of course it was stuck in the Reflector forum by a refractor guy looking for a bit of entertainment. That thread has over 300 posts now and shows no sign of dying off. I own both refractors and reflectors and while the refractors are nice, especially on planets, contrasty, no diffraction spikes, etc, they cannot keep up with a good coma corrected newt in the deep sky light gathering category. My closest hybrid scope is my new MN190 Mak-Newt which seems to combine the best of several worlds at a reasonable cost, and while being the equivalent of a 6" APO in performance (at way less $), it cannot give me the deep reach that the 10" imaging newt does. Aperture, as always, is king; and in the analysis of aperture per $, there is nothing that can match a newt. So it comes down to what aspects of performance are important to you, if it's only uncorrected star shape, and contrast then yes a large refractor will get you going, but if your after deep reach then only a reflector can give you that at a cost way less than any larger APO refractor.

Last edited by glend; 05-06-2015 at 12:43 PM.
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  #26  
Old 05-06-2015, 05:01 PM
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In the last two years, since I got into astrophotography, I never had dew on my refractors (imaging in Sydney and Brisbane), nor had any cool down issues
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  #27  
Old 05-06-2015, 10:02 PM
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Quote:
Originally Posted by glend View Post
That quote by Gary can cut both ways in its relevance, yes a refractor objective is at the front and exposed to the night air - but so is the corrector plate of a SCT (dew magnets that they are), and are not all refractors equipped with dew shields these days, and they all (refractors) need dew heaters. Newts however, mostly all have fans, and have a long tube that acts as a dew shield, and being open tube do not have cool down issues that SCTs and refractors do (minimal tube currents) and for truss dobs no tube currents at all.

My point is many of these pronouncements can be turned right around by looking at all aspects of the design and performance. There is a thread over on Cloudy Nights called "Are APOs better than Newts" and of course it was stuck in the Reflector forum by a refractor guy looking for a bit of entertainment. That thread has over 300 posts now and shows no sign of dying off. I own both refractors and reflectors and while the refractors are nice, especially on planets, contrasty, no diffraction spikes, etc, they cannot keep up with a good coma corrected newt in the deep sky light gathering category. My closest hybrid scope is my new MN190 Mak-Newt which seems to combine the best of several worlds at a reasonable cost, and while being the equivalent of a 6" APO in performance (at way less $), it cannot give me the deep reach that the 10" imaging newt does. Aperture, as always, is king; and in the analysis of aperture per $, there is nothing that can match a newt. So it comes down to what aspects of performance are important to you, if it's only uncorrected star shape, and contrast then yes a large refractor will get you going, but if your after deep reach then only a reflector can give you that at a cost way less than any larger APO refractor.


Hi Glend,
I Agree & I only use Newts. because I like imaging deep sky targets.
Newts. give the best bang for the buck in that area.

A Schiefspiegler Newtonian telescope would be interesting as there is no obstruction
but I've never looked through one.
There is a good article here on obstruction & design of Newts:

http://www.alpo-astronomy.org/jbeish...Sec_Mirror.pdf


cheers
Allan
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  #28  
Old 05-06-2015, 11:19 PM
glend (Glen)
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Thanks for that link Allan. I wonder if the calculations can apply to my mak-newt as well, which has a rather small central obstruction of 26% and no spider as itts mounted directly on the corrector plate. The mirror in the mak-newt is a spherical and not parabolic. I will hwve to read that article carefully.
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  #29  
Old 06-06-2015, 02:39 AM
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Eden (Brett)
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Quote:
Originally Posted by glend View Post
Thanks for that link Allan. I wonder if the calculations can apply to my mak-newt as well, which has a rather small central obstruction of 26% and no spider as itts mounted directly on the corrector plate. The mirror in the mak-newt is a spherical and not parabolic. I will hwve to read that article carefully.
Looking forward to seeing what you can bring down with that new Mak-Newt of yours, glen. It sure looks like a top instrument and I am surprised there aren't more of them out there and in larger aperture sizes (maybe there are? I couldn't find any, just the Skywatcher offerings.)

Between your Mak-Newt and Greg's Riccardi-Honders, we're all but doomed this winter
Brett
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  #30  
Old 06-06-2015, 06:37 AM
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Quote:
Originally Posted by glend View Post
Thanks for that link Allan. I wonder if the calculations can apply to my mak-newt as well, which has a rather small central obstruction of 26% and no spider as itts mounted directly on the corrector plate. The mirror in the mak-newt is a spherical and not parabolic. I will hwve to read that article carefully.
Yes the formula applies to all obstructions:

Contrast factor = CF = 5.25 - 5.1x - 34.1x+ 51.1x

where x is the obstruction ratio.

So the maximum contrast factor you can have is 5.25 for any telescope.


cheers
Allan
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  #31  
Old 06-06-2015, 07:54 AM
Alchemy (Clive)
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Fascinating topic,

given all seriously large professional observatories are mirrors then the questions already answered, once you get so big that atmosphere plays a part then adaptive optics and computer tricks are required so it's no longer just the scope.
Given ground based observatories are now producing Hubble esq images, anythings possible, just depends how deep your pockets are.

HOWEVER for me I have had both reflectors and refractors, for simplicity of use I prefer a refractor, no collimating, no reapplying coatings, minimal flexure ..... Grab and go then enjoy.
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  #32  
Old 06-06-2015, 08:53 AM
glend (Glen)
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After reading Jeff Beish's paper on Newtonian Design Calculations (supplied by Alpal), I have run the Contrast Factor calculations for each of my Newts:

With a theoretical perfect score of 5.25 (assuming zero obstruction - admittedly not going to happen with a newt anyway):

MN190, with 0.26 obstruction ratio, gives a Contrast Factor of 2.5168.

10" Imaging Newt, with 0.25 obstruction ratio, gives a Contrast Factor of 2.6412

16" Dob, with a 0.216 obstruction ratio, gives a Contrast Factor of 3.0773.

What can be shown from these numbers:

Firstly, the two imaging scopes have larger secondary obstructions due to the fact that they are imaging scopes and thus run larger secondary mirrors than what is required for a purely visual setup as in the 16" dob.

Secondly, using the Central Spot Energy table it seems that over 70% of the energy is concentrated in the centre spot for all these scopes. Theoretical maximum Central Spot Energy is 84% but again that would be with no obstruction, which no newt can deliver.

Thirdly, the MN190 and 10" Imaging Newt are very close in terms of contrast factor but very different in terms of diffraction - something not addressed in Beish's paper. The 10" newt has a traditional spider holding the secondary where as the MN190 has no spider and thus no diffraction spikes (beause the secondary is mounted on the rear of the corrector plate).

Finally, I am not sure how Spherical and Parabolic primaries affect the image and contrast, if at all. The MN190 has a spherical primary while the others have traditional parabolic primaries. Spherical abberation is corrected of course by the Corrector plate, and is the typical design used in SCTs and Maks. Parabolics are figured to correct spherical abberation.

Conclusions, well from a visual point of view big dobs are clearly the kings of contrast factor as long as you keep the secondary size under control as you go up in size. However, there would be a question as to how this translates to image quality as conventional imaging newt design recommends larger secondaries to provide the correct spot size for imaging sensors.

It is important to remember that these calculation are related to relative obstruction area in the pupil (see the reference paper below), and thus may have limited validity in imaging scope comparisons (especially in comparing imaging against visual scopes).


For those seeking an even more indepth look at central obstruction math here is a link for you that is beyond my aging brain:

http://www.telescope-optics.net/obstruction.htm

Last edited by glend; 06-06-2015 at 09:33 AM.
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  #33  
Old 06-06-2015, 09:30 AM
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Hi Glend,
nice post below.

I think if you can get the central obstruction below about 18%
you would hardly notice the difference between a refractor & a Newt.
Then it boils down to how good your eyepieces are.
Some eyepieces have poor contrast because they scatter the light
either by poor choice of glass or bad coatings.

It's hard to get the central obstruction down to 18%
just by the design of the Newt. as the larger the Newt is
the bigger the secondary mirror must be to obtain all
of the light cone from the primary & reflect it to the eyepiece.
To do so you would have to get the eyepiece into focus with a low profile focuser
as close as possible to the telescope tube.
In that case the telescope would be only usable for eyepiece viewing &
no good for a camera which requires more back focus as it
works at prime focus.

Therefore - you need one Newt. for imaging & another one for viewing.
If you're rich you could have an exotic oil spaced refractor
with top eyepieces & do better than a Newt. on planets.
Oil spacing gives less scatter although with the latest technology & coatings this is debatable.


cheers
Allan
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  #34  
Old 06-06-2015, 10:10 AM
glend (Glen)
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Thanks Allan. Those central obstruction ratios and Contrast Factor calculations, really cannot be used to compare different scope designs IMHO. If you were comparing two similiar sized visual newts to make a purchase decision, then yes they might be good to know.

When I look through the MN190 it is a wonderful view compared to looking through the 10" Imaging Newt, with pinpoint stars, great contrast, good shape, coma free to the edge, etc where as the Contrast Factor numbers say the 10" newt should be the better. That's the problem for me. It's not apples to apples. I'd be happier comparing the MN190 to a equivalent clear aperture APO (Something like a 6" APO), because the view I am seeing is closer to what that APO provides. Trying to get a central obstruction down to 18% in a newt to reach refractor central spot parity is only relevant where your are comparing equivalent apertures. IF you accept that you need to have more aperture in the newt to negate the obstruction differences then your getting to the heart of the matter. Re the Mn190 verse the 10" I can only assume the difference lies in the mak corrector front end, and the rest of the optical design that is not brought into consideration in that paper's Contrast Factor determination, which is a newt to newt comparison factor.

Last edited by glend; 06-06-2015 at 11:39 AM.
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  #35  
Old 08-06-2015, 09:51 AM
glend (Glen)
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Compare these images

I have completed initial imaging setup for the Skywatcher MN190, and wanted to demonstrate my last comments on Contrast Factor numbers.

Here are two images of the same object: Eta Carinea.

http://www.astrobin.com/full/180326/0/

http://www.astrobin.com/full/185615/0/


The first is taken with my 10" imaging newt with a Baader Coma Corrector on the camera (this is my lightweight f5 newt).
The second is taken with the new Skywatcher MN190 f5.3, which of course has no coma corrector on the camera. Both sub taken at 209" with the same Canon 450D Full Spectrum camera with minimal processing ( i think I did more processing with the first one, the second only has auto levels done). The first was taken at Bretti the second taken in my backyard observatory. The first was the NEQ6 on its tripod, the second the mount head was on my pier.

Love the edge to edge coma free field of the MN190. Sure it is wider field of view to the 10", and the 10" gets in a bit closer; however, I much prefer the contrast and stars in the MN190. Of course the 10" has a higher Contrast Factor - which is my point - that number doesn't tell you much at all.

Note: Submissions to Astrobin are jpegs for the public gallery.
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  #36  
Old 08-06-2015, 11:15 AM
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Hi Glen,

The photo taken with MN190 looks heaps better, especially that the one taken with 10" Newtonian looks out of focus.

But I think I will stick to my 4" doublet

Colour: http://www.astrobin.com/full/141089/D/

Mono: http://www.astrobin.com/full/148715/D/
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  #37  
Old 09-06-2015, 07:42 AM
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That's impressive Glen. I would not have though you would get round stars to the corners with a 19% obstructed scope.

As Roland pointed out in the original post I posted you need the large 40-50% central obstruction to cover a large sensor.

Its amazing how light will bend around the obstruction.

Greg.
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  #38  
Old 09-06-2015, 08:22 AM
glend (Glen)
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Originally Posted by gregbradley View Post
That's impressive Glen. I would not have though you would get round stars to the corners with a 19% obstructed scope.

As Roland pointed out in the original post I posted you need the large 40-50% central obstruction to cover a large sensor.

Its amazing how light will bend around the obstruction.

Greg.
Well Greg it does have a thick front corrector plate, and while I am not sure exactly how that bends the light ( ala a refractor objective shape) it obviously is doing a good job.
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  #39  
Old 09-06-2015, 09:38 AM
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Well Greg it does have a thick front corrector plate, and while I am not sure exactly how that bends the light ( ala a refractor objective shape) it obviously is doing a good job.
Looks really good Glen - terrific results

Greg, the SW MN gets by with a small obstruction, because the designer did not insist on full illumination away from the field centre. This will produce some vignetting, but it looks to be quite manageable. This design will still produce round stars up to the edge of the field, because coma is controlled - which means that the atmosphere determines the star shape, not the scope.

The light does not bend around the secondary at all (apart from a little diffraction at the edges). The secondary obstruction produces a light cone that has a hollow core, but it still focuses to a geometrical point. The actual point spread function of an obstructed scope will have significant energy outside of the central peak, mainly due to the diffraction of the secondary - but for DSO imaging with the scopes we are considering, that does not matter, because the spot size due to the atmosphere is maybe 10x bigger than that due to the scope (in area). Provided aberrations are controlled, what the scope does is almost immaterial. That is why a scope with large central obstruction (eg RHA) can still produce high contrast images even though the design has very low theoretical contrast - it is simply that the contrast (and resolution for that matter) is all down to the atmosphere and not the scope. In any event, contrast can be adjusted in processing - it really is not a major issue for imaging (although it is for visual).

When we compare larger scopes by looking at their images, we are actually comparing atmospheres - above about 6 inches aperture, scope performance (assuming reasonable quality) has only a modest effect on the outcome, except in exceptional seeing. In typical Australian conditions, Glen's scope should do almost as well as anything, including the AAT (although not as quickly).

Last edited by Shiraz; 09-06-2015 at 10:19 AM.
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  #40  
Old 09-06-2015, 10:26 AM
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Yes that what I meant Ray the diffraction of the secondary mirror.

When out of focus you can see donuts but when it comes to focus its a solid dot of light. Amazing. One of the confusing aspects of light and the is it a wave or is it a particle question?

Greg.
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