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Newtonian Astrograph for Visual

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#1 HotRod217

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Posted 18 April 2021 - 12:06 PM

What are the disadvantages of using a Newtonian Astrograph Telescope for visual? Are there any benefits, maybe higher contrast because of the baffles? I heard that there are problems with a too large secondary mirror. In my case it would be the Orion 10" f/3.9 Newtonian Astrograph Reflector Telescope. It has a 32% secondary mirror obstruction.

 

 



#2 LuscombeFlyer

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Posted 18 April 2021 - 12:16 PM

The Orion astrograph is fine for visual work. Yes, the secondary is a little larger than strictly necessary. But pop in a wide FOV eyepiece and enjoy the wide-field views.
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#3 Tangerman

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Posted 18 April 2021 - 12:28 PM

You might also need an extension tube to reach focus for your eyepieces, and at that focal ratio, I'm told most want a coma corrector. But if you've got those, I don't see why it wouldn't work. 


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#4 Augustus

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Posted 18 April 2021 - 12:48 PM

It'll work fine, but you have to ask yourself a few questions:

 

  1. Is it really worth it? A 10" f/4 has a marginally larger maximum field of view than an f/4.7
  2. What's the mount? A Dob is going to be short and would have to be made custom or by yourself. Thus you're probably going to be using it on an uncomfortable and cumbersome GEM or an odd and expensive alt-azimuth mount.
  3. What eyepieces are you using with it? An f/4 scope really needs good eyepieces, preferably 82- or 100-degree to take advantage of the fast focal ratio without running into exit pupil issues. 


#5 jgraham

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Posted 18 April 2021 - 12:51 PM

Fast, f/4 Newtonians make fine, wide field visual scopes. However, one characteristic of these scopes is a curved focal plane and significant off-axis coma. A paracorr or other coma corrector is a nice addition.


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#6 doc_cj

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Posted 18 April 2021 - 02:17 PM

Not sure if this will add much, but it will help me in my own thought process. My hope is that it will open up the possibilities and issues for everyone.

 

I'm considering the Orion 10" Astrograph as well. Not sure if I want to go with the Newtonian or a 9.25" or 10" SCT such as the Celestron or Meade. Since I want to do both observation and astrophotography then I have to balance a lot of things. The 10" Orion, as I understand it, is excellent for the photo part, and it's cheaper. I can buy the Orion 10" Astrograph with the Atlas mount for around $2200, and with a budget of $3000 that would mean I could then add a COMA corrector ($250 - $300), a couple decent lenses for observation ($200 - $300), and still have a little money left over. 

 

I appreciate any thoughts folks might have on these items:

 

COMA corrector -- (1) Baader Rowe Coma Corrector for $209, or (2) Explore Scientific's HR Variable Coma Corrector for $329. 

 

Lens for observation -- (1) Orion Q70 2" eyepieces - 26mm, 32mm, and 38mm with 70* FOV at $289, or (2) an ultra wide FOV such as the Celestron 15mm Luminos Eyepiece - 1.25-Inch - with 82* FOV for $120. 

 

Thanks for reading and for any insight you can provide. 


Edited by doc_cj, 18 April 2021 - 02:19 PM.


#7 maroubra_boy

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Posted 18 April 2021 - 06:33 PM

I have two 8" f/4 scopes.  One is a dedicated visual instrument on a dob mount, the other is a solid tube OTA that I can stick onto a dob mount or a GEM for photo.  I also have other Newts from f/4.5 to f/5.  These f/4 units are the only ones I use a coma corrector with.  The others can show some coma but not enough to bother me so much as to want to use a coma corrector.  But this depends entirely on the eyepiece that you use with Newts.  If you see astigmatism (seagulls) in the stars along with coma (little comet tails), then the problem is an eyepiece mismatch with the scope and a coma corrector won't help you here.

 

Mind you, with these f/4 scopes, the coma corrector is really only used when I am doing low magnification stuff.  High magnification does not require a corrector.

 

Just any old coma corrector won't do however.  Coma correctors are designed to be optimised around a specific f/ratio.  So one made centred around f/5 will struggle to clean up all the coma in an f/4 scope, and one centred around f/4 can overcorrect in an f/5 scope.  Some people have criticised unfairly some correctors because they felt that the corrector did not do the job they wanted.  Dig a little further down into their reviews and you will find that they used a corrector designed for f/4 in an f/5 scope...  One f/stop does not sound like much, but with optics this is a BIG difference.

 

One can get all technical and full of jargon with all scopes.  The bottom line is these scopes can be used for visual, and as with ALL scopes they are a compromise somewhere.  I love my f/4 Newts, they have their strengths and weaknesses, and I use them accordingly and enjoy the time I spend with them.

 

Below is the sketch I did of the Veil Nebula using one of my f/4 Newts.  A 30mm 80deg eyepiece is able to fit the WHOLE of the supernova remnant in the FOV! :)  Sure the resulting exit pupil is a little large, but it's not exactly like trying to look through a brick, now is it?

 

Alex.

Attached Thumbnails

  • F4.JPG
  • K1 completed (74).JPG
  • 8in Veil LR.JPG

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#8 maroubra_boy

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Posted 18 April 2021 - 07:23 PM

 

I appreciate any thoughts folks might have on these items:

 

COMA corrector -- (1) Baader Rowe Coma Corrector for $209, or (2) Explore Scientific's HR Variable Coma Corrector for $329. 

 

Lens for observation -- (1) Orion Q70 2" eyepieces - 26mm, 32mm, and 38mm with 70* FOV at $289, or (2) an ultra wide FOV such as the Celestron 15mm Luminos Eyepiece - 1.25-Inch - with 82* FOV for $120. 

 

Thanks for reading and for any insight you can provide. 

Corrector wise, the Baader is specifically centred for f/4.  The ES one is very vague on this detail.  For an f/4 scope I would certainly prefer a corrector that is centred on f/4.  That is my preference.

 

Eyepiece wise - the Q70 line is not a good match for Newtonians, especially fast Newts.  These will show astigmatism, not just coma, and the faster the f/ratio the stronger the effects of astigmatism.  Sure they will work, but the image will not be a pleasing one off axis, and a coma corrector WILL  NOT correct astigmatism.  In an SCT however these eyepieces are a great match as they are designed for the focal plane shape that SCT's and refractors produce, a convex one.  Newt's produce a concave focal plane which is much harder to design for and more expensive to manufacture.   Eyepiece choice for SCT's, Maks and refractors is much wider than for Newt's.

 

Alex.


Edited by maroubra_boy, 18 April 2021 - 07:24 PM.

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