Jump to content

sgl_imaging_challenge_2021_2.thumb.jpg.72789c04780d7659f5b63ea05534a956.jpg

Recommended Posts

I get more confused trying to figure out why mirror diagonals flip things vertically for refractors and CATs, but Newtonian diagonals don't do the same thing for Newts.  It seems like the same principle, doesn't it?

Link to post
Share on other sites
On ‎16‎/‎09‎/‎2017 at 15:41, Louis D said:

I get more confused trying to figure out why mirror diagonals flip things vertically for refractors and CATs, but Newtonian diagonals don't do the same thing for Newts.  It seems like the same principle, doesn't it?

Interesting remark, Louis. I am going to rest a newtonian tube on a table, aim it at a landscape, and try various orientations of the focuser, plus various orientations of my head to sort this out.

Link to post
Share on other sites

I think it's to do with even/odd numbers of mirrors. In the Newtonian there are two mirrors and each mirror flips the image once, so the image you see isn't flipped (though it is rotated 180°). However, a refractor with a diagonal has just one mirror (in the diagonal) and a catadioptric with a diagonal has three, so the image is flipped.

Link to post
Share on other sites
1 hour ago, harrym said:

I think it's to do with even/odd numbers of mirrors. In the Newtonian there are two mirrors and each mirror flips the image once, so the image you see isn't flipped (though it is rotated 180°). However, a refractor with a diagonal has just one mirror (in the diagonal) and a catadioptric with a diagonal has three, so the image is flipped.

Rotated 180 or flipped, it still looks wrong terrestrially.  How many more mirrors would it take to make it look right side up and not flipped left for right?  Would they have to be oriented in a specific way like porro prisms (at right angles to each other) in a monocular?

Link to post
Share on other sites

I put a newtonian tube on a table, facing an electrical post, a pine tree to the left, and a leaf tree to the right. With the focuser held vertically, I turned my back on the landscape, and saw everything "correct" in the eyepiece. Post is upright, pine tree to the left, leaf tree to the right.

Later I rotated the tube around its horizontal axis so the focuser went from vertical to horizontal, and sat at the standard place for looking through a newtonian: on the side. The post's bottom was first in the left part of the image, and when rotating the tube, the post's bottom turned from 9 o'clock to 12 o'clock.

Will have to try that with my little SCT. Funny that a newtonian does work as terrestrial scope, but only if you turn your back on your target!

  • Like 1
Link to post
Share on other sites
3 hours ago, harrym said:

I think it's to do with even/odd numbers of mirrors. In the Newtonian there are two mirrors and each mirror flips the image once, so the image you see isn't flipped (though it is rotated 180°).

But the main mirror reflects light along its optical axis, whereas the secondary does so 90° away from the main mirror's axis. This is tougher to represent in the mind's eye than I thought!

Link to post
Share on other sites
2 hours ago, Louis D said:

Rotated 180 or flipped, it still looks wrong terrestrially.  How many more mirrors would it take to make it look right side up and not flipped left for right?  Would they have to be oriented in a specific way like porro prisms (at right angles to each other) in a monocular?

Has to be an even number for the same reason I mentioned before. You could do the Porro prism effect with 4 mirrors I think? (Edit: but this might cause distortions...)

Does anyone know how those 45 degree image erecting diagonals work?

Edited by harrym
Link to post
Share on other sites
25 minutes ago, harrym said:

Has to be an even number for the same reason I mentioned before. You could do the Porro prism effect with 4 mirrors I think? (Edit: but this might cause distortions...)

Does anyone know how those 45 degree image erecting diagonals work?

They use Amici prisms that invert and revert as well as bend the light either 45 or 90 degrees.  I think inverting and reverting is the same as rotating 180, correct me if I'm wrong.  I have never grasped how Amici prisms work their magic, though.  The 2D diagrams fail miserably to explain what is happening in three dimensions.

Link to post
Share on other sites
57 minutes ago, Ben the Ignorant said:

I put a newtonian tube on a table, facing an electrical post, a pine tree to the left, and a leaf tree to the right. With the focuser held vertically, I turned my back on the landscape, and saw everything "correct" in the eyepiece. Post is upright, pine tree to the left, leaf tree to the right.

Later I rotated the tube around its horizontal axis so the focuser went from vertical to horizontal, and sat at the standard place for looking through a newtonian: on the side. The post's bottom was first in the left part of the image, and when rotating the tube, the post's bottom turned from 9 o'clock to 12 o'clock.

Will have to try that with my little SCT. Funny that a newtonian does work as terrestrial scope, but only if you turn your back on your target!

I think the old Zuka spotting scope used a single extra diagonal type mirror to send the image to your eye while holding the scope on your shoulder, bazooka style.  I assume the final image was upright and not reversed given it's use as a terrestrial spotting scope.

Link to post
Share on other sites

I had never heard of that Zuka scope, Bing finds no picture of it, and Google finds only three, from the same Astromart page! Must be pretty rare; I'm not sure what kind of diagonal was added in order to use it parallel to the natural line of sight. I think we all need to work on our visualization skills!

Link to post
Share on other sites
28 minutes ago, Louis D said:

They use Amici prisms that invert and revert as well as bend the light either 45 or 90 degrees.  I think inverting and reverting is the same as rotating 180, correct me if I'm wrong.  I have never grasped how Amici prisms work their magic, though.  The 2D diagrams fail miserably to explain what is happening in three dimensions.

Inverting and reverting always gives an image that isn't reflected, but the angle of rotation depends on the positions of the mirrors/prisms relative to the direction you're viewing from. For example, a vertical Porro prism appears to rotate the image 180° (and reverses the direction of the light), but a horizontal Porro prism doesn't appear to rotate the image at all. This is highly counterintuitive but it seems to be correct. See the image below, and always imagine you're viewing the light coming towards you.

Double_Porro_prism.svg.png

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    No registered users viewing this page.

  • Similar Content

    • By guydive
      Hey all,
      I'm considering the 250PDS. The main use is visual, and the second is EAA (I currently use a zwo183mm). 
      What I'm hesitant about is the primary focus. since It's mostly for visual, I'm wondering on how much back focus it has compared to the standard Skywatcher 10" dobsonians? with the standard dobsonias with the same optics and focuser you already need an extension tube for most eyepieces...
      Also, there's probably a "bigger" obstruction (bigger shadow on primary)? 
    • By Goldenmole
      Good day fellow gazers at the sky! I recently joined and i would just like to say how wonderful everyone is (a special shout out to George Gearless)!
       Can anyone suggest a good reflector within my budget shown in the title? While i'm here i'd just like to say about the app, Nightshift. It is proffesional, clear and tellls you when to observe, and what you can see, for the next year! It also pinpoints your location exactly, so it is really accurate.https://play.google.com/store/apps/details?id=com.waddensky.nightshift. Anyway, that's me done! Thank you all so much for your time
    • By javaruba
      Hi all,
      After detecting a small fracture on the edge of the protective window of my ASI294MC Pro some time ago, I managed to replace it with a new one. ZWO describes the glass as "ZWO D32 AR" for the ASI294. AR probably stands for anti-reflective, not sure. I was wondering if the glass should be installed pointing in a specific direction, or is the anti-reflection both ways? I'm seeing some halos while pointing at bright stars but could be something else, see attached start image as a reference (a single frame exposure).
      Thanks in advance for any help.
      Kind regards,


    • By Planetarian
      Hi, I've got a Skywatcher Heritage 130p reflector, and if i insert anything less than 10mm eyepiece, the image won't get crisp. I guess it's normal, but as I'm very new to astronomy, I'd like to know what the sharpness depends on exactly. 
      Is that the focal length (how fast the telescope is? ) or the size of the mirror and how much light it gathers? Or both affect it the same way?  
      Are things the same with refractors in this regard? Thanks. 
    • By Pincs
      Hi, I got a skywatcher skyliner 200p dobisonian a few days ago, my first observing session was out in the cold and the telescope dewed up when I bought it back in. I then noticed that the glass on the base of the telescope(not the primary mirror inside) was smudged and had a little ring shaped scratch on it. I tried cleaning with a lense cloth but it is still slightly dcratched and smudged. Does anyone know if this makes a difference to the inside primary mirror?
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.