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The Definitive Newtonian Reflector

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#1 ed_turco

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Posted 15 May 2015 - 08:25 AM

Amateur astronomers and telescope makers have debated from time immemorial the advantages and disadvantages of different telescope designs.  In particular, mountains of hard copy and electronic articles are available on the merits of refracting and reflecting telescopes, more recently, apochromatic refractors vs. Newtonian reflectors. This debate has become rather rancorous (Newtonian telescopes as APO "killers" comes to mind.) and unscientific, to say the least. And when all is said and done, in a discourse without loaded words and acrimony, a discussion devolves to one concerning perfect optics.  And isn't this what we all want or wish we had?


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#2 catboat

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Posted 15 May 2015 - 08:48 AM

Great work, Ed.  I’m glad you could get this posted here.  Lots to think about.  Thanks.


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#3 Jeff Morgan

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Posted 15 May 2015 - 09:01 AM

A great back-to-basics article that is a refreshing change from the fad and fashion of todays Newtonians.


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#4 NHRob

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Posted 15 May 2015 - 09:01 AM

Love the article and the scope.  ATMing at its best.  

The dark bucket is a nice idea.

 

Rob


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#5 catalogman

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Posted 15 May 2015 - 09:39 AM

Does the optical window in this scope dew up easily?

 

There is another way to eliminate diffraction from the spider vanes:

 

http://www.cloudynig...und-telescopes/

 

                                                                                  

                                                                                   -- catalogman


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#6 ed_turco

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Posted 15 May 2015 - 09:51 AM

Thanks to all for reading the real article after all this fuss.  The answer about the window dewing up is a good one; that is one reason why the telescope has a dewcap.  It serves this function as well as blocking scattered light.

 

ed



#7 ed_turco

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Posted 15 May 2015 - 09:54 AM

With regard to the Pfund design, there is still spider diffraction; as described in one of the ATM books, (ATM-2?)  a curved spider isdescribed as a "diffraction spreader arounder."  The optical window has none of these problems.

 

ed



#8 GShaffer

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Posted 15 May 2015 - 10:52 AM

Worth the wait Ed.......

 

I did see a couple references I need to obtain and absorb.  I have had Suiter's book for some years now.......

 

One question really at this moment......what is inside the dark bucket?


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#9 Galicapernistein

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Posted 15 May 2015 - 11:26 AM

"Curved spiders remove these spikes, but spread their light through the entire field with a very subtle haze. I think of the above problems as a subtle "mud" that degrades contrast. Interestingly, these effects can not be readily subjected to mathematical analysis. One might not directly sense this haze, but it is there and surely lowers contrast. "

Nicely said. There is a subtle mud in most reflector images that APO users are probably more susceptible to noticing because of the clean images they're used to, and spiders are a big cause of that mud. An optical window on a newt is an idea whose time has come. Great article.
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#10 droid

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Posted 15 May 2015 - 01:29 PM

excellent article Ed, bravo


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#11 ed_turco

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Posted 15 May 2015 - 01:39 PM

"Curved spiders remove these spikes, but spread their light through the entire field with a very subtle haze. I think of the above problems as a subtle "mud" that degrades contrast. Interestingly, these effects can not be readily subjected to mathematical analysis. One might not directly sense this haze, but it is there and surely lowers contrast. "

Nicely said. There is a subtle mud in most reflector images that APO users are probably more susceptible to noticing because of the clean images they're used to, and spiders are a big cause of that mud. An optical window on a newt is an idea whose time has come. Great article.

Well thank you!!  My article ofers some proof that  all APOs are not created equal, and that they can be afflicted by all sorts of stuff -- their own kind of "mud."  Some these shoter focus APOs  can be suspect.

 

After all, the manufacturers don't exactly give us an optical aberration analysis printout for all to see; this kind of knowledge owned by them.   Even as a consumer, I still can sympathize with the makers; why should they give away the specs on their telescopes so others can make them?

 

One different point.  Please remember  I am reading questions and comments in three different CN locations and I can forget where I saw a particular question:  One question was "What is inside my Dark Bucket?"

 

The dark bucket has been internally flocked and it has a a slap and a dash of Krylon 1601, I think.  It is also baffled so what no light reaches through the bottom of the Dark Bucket.

 

For those who are baffled at this point, I can offer a more facetious answer to the question  --  I store my dark in there for later use! 

 

 

ed   :lol: :bow: :flowerred:


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#12 GShaffer

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Posted 15 May 2015 - 03:09 PM

That was me a couple messages back that asked what's inside the "dark bucket", it really was the only question the article left hanging for me lol

 

I was looking for the 1st answer but enjoyed the 2nd answer :)


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#13 Kerman

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Posted 15 May 2015 - 03:16 PM

Very helpful, thanks!


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#14 catalogman

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Posted 15 May 2015 - 05:54 PM

With regard to the Pfund design, there is still spider diffraction; as described in one of the ATM books, (ATM-2?)  a curved spider isdescribed as a "diffraction spreader arounder."  The optical window has none of these problems.

 

ed

 

In the Pfund design (also called a "reverse Newtonian"), the diagonal secondary is replaced by a full-aperture

flat and becomes the first element on the light path (see the figure in #5 above). There is no spider mount in

this design.

 

The eyepiece is behind the oversized flat -- for a 1-1/4" O.D. eyepiece in your 6", the perforation in the flat

would result in an obstruction of 4.3%. This would eliminate most of the 13.7% obstruction and most of the

diffraction in your scope.

 

                                                                                                                           -- catalogman


Edited by catalogman, 15 May 2015 - 06:00 PM.

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#15 BigC

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Posted 15 May 2015 - 06:27 PM

One of the things I like about Schmidt-Newtonians is the lack of spider vanes.

 

Ed is an inspiration . 


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#16 ed_turco

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Posted 16 May 2015 - 09:46 AM

 

With regard to the Pfund design, there is still spider diffraction; as described in one of the ATM books, (ATM-2?)  a curved spider isdescribed as a "diffraction spreader arounder."  The optical window has none of these problems.

 

ed

 

In the Pfund design (also called a "reverse Newtonian"), the diagonal secondary is replaced by a full-aperture

flat and becomes the first element on the light path (see the figure in #5 above). There is no spider mount in

this design.

 

The eyepiece is behind the oversized flat -- for a 1-1/4" O.D. eyepiece in your 6", the perforation in the flat

would result in an obstruction of 4.3%. This would eliminate most of the 13.7% obstruction and most of the

diffraction in your scope.

 

                                                                                                                           -- catalogman

 

 

 

With regard to the Pfund design, there is still spider diffraction; as described in one of the ATM books, (ATM-2?)  a curved spider isdescribed as a "diffraction spreader arounder."  The optical window has none of these problems.

 

ed

 

In the Pfund design (also called a "reverse Newtonian"), the diagonal secondary is replaced by a full-aperture

flat and becomes the first element on the light path (see the figure in #5 above). There is no spider mount in

this design.

 

The eyepiece is behind the oversized flat -- for a 1-1/4" O.D. eyepiece in your 6", the perforation in the flat

would result in an obstruction of 4.3%. This would eliminate most of the 13.7% obstruction and most of the

diffraction in your scope.

 

                                                                                                                           -- catalogman

 

After Googling the Pfund design,  I see what it is  and what it does.   I must remark that a perforation in any optic counts as an obstruction; I had a problem visualizing  this fact when I was much much younger, but it is so.   Think about a normal Newt and look through an eyepiece out of focus and you'll see the usual doughnut with the hole in the middle, representing the shadow of the diagonal.  Now take this Newt and have the mirror cored (This is a thought experiment,  please!) so that the mirror now has a huge opening much larger than the diagonal).  Try the out of  focus trick again, and you will see that that image of the doughnut has a much larger than it did before.   This is the effect I describe, a perforation acting as if it were an obstruction.  

 

Central obstruction is measured as the ratio of the diagonal diameter divided by the mirror diameter.  It is NOT the ratio of the areas.   Many ATMs, including me, have fallen into that trap.

 

On another point, the image of a star with an obstruction less than 18% is virtually no different (Suiter) than an image in an unobstructed telescope.  Given that, there is going to be no difference at the eyepiece with a 4.3% obstruction either.  By the way, there is still going to be diffraction in any telescope.  According to physics, It's the Law!!!!


Edited by ed_turco, 16 May 2015 - 09:49 AM.

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#17 svdwal

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Posted 16 May 2015 - 01:59 PM

I would like to know if you can order these improvements, like not having stray light is the most important, super smooth optics next and so on. The scope as presented here just won't fit on my balcony, and not in my car either. But I am keen to ad as many improvements to my truss dob as possble.


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#18 dag55

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Posted 16 May 2015 - 04:28 PM

Thanks Ed for the great article, and for the information. I am building a 8" f-7.5 Newt. and have been deliberating what method to best eliminate or reduce diffraction caused by the sec. spider whether curved, or thin wires or a single stalk and your solution looks promising. I have made good inroads for thermal control in my Newts. I could see the advantage of the optical window and with some engineering one could place one with vent slots around the outer mounting ring, along with the dark bucket could be vented at a 90* bend in the end of it for further thermal removal and constant fan operation which would allow the mirror to track the falling temperature as well.

 

Now here is my problem, I am not a mirror maker and at 60 I doubt I will ever be, where could I get an acceptable optical window for my project?  My tube is 10" od, might a an old meade  schmidt-Newtonian widow work?  Just thinking of an economical solution as the optical windows they use are coated already.

Thanks again Dane


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#19 dag55

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Posted 16 May 2015 - 04:50 PM

Well the meade 8" optical window doesn't look promising as the sec. obstruction at 39% is to large for my small 19% obstruction that the 1.52" sec. I have.
Dane
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#20 Joe Eiers

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Posted 16 May 2015 - 08:10 PM

Thank you for the excellent article!  Could you post more pictures please?  I'd also love to here more results with planetary images.

 As far as the various light scattering issues, I have no doubt, but I am unsure how they effect the final image.  As I read the article I kept thinking about all the time I compared a friend's AP 6" F/12 Super Planetary to my 10" F/4, a C8, and my 6" F/8 newt.  I had many many hours to compare over years.  Even though my 10 was a home made, slap together dob, it always showed more detail as might be expected with so much more aperture.  Quite often the C8 was at least as good. The old 6 was image for image.  I'm NOT trying to start an argument or for a minute devalue your excellent and detailed research; I am just thinking that for me that final image of Jupiter and Saturn was so close I wouldn't have ever justified buying one myself (I had one on order, and cancelled it after 8 months).

  Although I salivate at doing everything you talk about here, I can't help going back and forth over the thought that it will end in bringing new fine details.  Perhaps my image is a bit lighter with all those improvements but all the detail is still there??  I dunno. 

  Pushing 60, and having an 8" mirror  I want to finish from 40 years ago, you've lit a fire under me.  I will endeavor to follow your lead and see what I come up with.  It's a nice, doable goal.

  Bravo for what you've done here!  

     Joe


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#21 ed_turco

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Posted 16 May 2015 - 10:47 PM

I would like to know if you can order these improvements, like not having stray light is the most important, super smooth optics next and so on. The scope as presented here just won't fit on my balcony, and not in my car either. But I am keen to ad as many improvements to my truss dob as possble.

You are in a tough spot with your telescope already bought and designed for you.  In any telescope the quality of the optics comes first.  The second issue  in order would be spider diffraction and with a truss Dob, you can do little about adding an optical window to deal with this issue.   This leaves scattered light and you've probably done as much as you can with a dark shroud.

 

It is sad that there will be telescopes that cannot be altered to deal with the issues I have described, but your question is one of the best in try to deal with an already acquired telescope.  I wish I could offer you more.

 

 

ed



#22 ed_turco

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Posted 16 May 2015 - 10:56 PM

Thanks Ed for the great article, and for the information. I am building a 8" f-7.5 Newt. and have been deliberating what method to best eliminate or reduce diffraction caused by the sec. spider whether curved, or thin wires or a single stalk and your solution looks promising. I have made good inroads for thermal control in my Newts. I could see the advantage of the optical window and with some engineering one could place one with vent slots around the outer mounting ring, along with the dark bucket could be vented at a 90* bend in the end of it for further thermal removal and constant fan operation which would allow the mirror to track the falling temperature as well.

 

Now here is my problem, I am not a mirror maker and at 60 I doubt I will ever be, where could I get an acceptable optical window for my project?  My tube is 10" od, might a an old meade  schmidt-Newtonian widow work?  Just thinking of an economical solution as the optical windows they use are coated already.

Thanks again Dane

You are creating some good ideas here, but a Meade "window" is actually a correcting lens and not the plane-parallel glass we would want.   I'm afraid that the only way to get the window you want is to make one.  I guess you might luck out and find someone with an optical window is the size you want, but don't count on it.  There might be a way in that you get one of these Meade or Celestron lenses and grind them to a plane-parrallel condition.

 

Please remember I'm just throwing out some ideas for you at this point.

 

 

ed


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#23 svdwal

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Posted 17 May 2015 - 03:31 AM

@ed_turco

 

I built it myself and have done one rebuilt already, so changing things is not a problem. But I can only work on it during the summer months, not having a shed. 

 

Regarding the optical window, it could be done in a truss scope, the main problem being keeping it dew-free. From where I generally observe, dew is a problem one cannot ignore. But if the window is 20% of the solution, I can still come within 80% of an optimal solution, in a scope I can take with me to a proper dark place.


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#24 dag55

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Posted 17 May 2015 - 06:23 AM

Thanks Ed, maybe can get someone in the future to make one, who knows I might learn how to push glass, retirement isn't that far off.
Again thanks for your article, Dane
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#25 Jeff Morgan

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Posted 17 May 2015 - 03:09 PM

Supposing I wanted to learn the techniques for fabricating an optical window. What text(s) would you recommend I study?


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