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Can't seem to find a straight answer on this

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#1 Haas

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Posted 02 February 2013 - 04:36 PM

Is a refractor telescope the only one that will show a right side up image view? Or, does it show upside down? I know reflectors are inverted, but refractors are all right side up, no? What about the cat design, aren't they still reflectors, thus upside down view?
If a person wanted a scope for both celestrial viewing and horizon viewing, he would be stuck with a refractor, right?

#2 neotesla

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Posted 02 February 2013 - 04:42 PM

Refractors are inverted as well. A diagonal will flip the image in one axis so that it is vertical but still reversed left to right. For terrestial viewing you need an Amici or Porro prism that will give you a correct image.

#3 jiminaz

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Posted 02 February 2013 - 04:42 PM

I have a Celestron SE 8 that is a Schmidt-Cassegrain and the images are right side up. I know that this probably doesn't help but it is a start.

#4 Eric63

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Posted 02 February 2013 - 04:48 PM

A refractor and a Cat will give the same view and for real horizontal alignment a prism is required. I guess the addition of a diagonal in the cat makes it different from the Newt.

#5 Haas

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Posted 02 February 2013 - 04:50 PM

Refractors are inverted as well. A diagonal will flip the image in one axis so that it is vertical but still reversed left to right. For terrestial viewing you need an Amici or Porro prism that will give you a correct image.


Ok, when I was a kid, I had a 60mm refractor that I bought at a rummage sale. I know nothing was upside down, so that one you're saying must have had a prism in it to do that? Do all refractors have this prism, or do you have to make sure it has it before you buy it?

#6 neotesla

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Posted 02 February 2013 - 04:56 PM

Your refractor probably had an internal prism arrangement, like in binoculars that made the image correct aspect. If you buy an astronomical refractor, viewing straight through, will give you an inverted image in both axis. Most refractors come with a standard prism or mirror diagonal which will give you a vertically corrected image, but horizontally inverted. Most people are happy with this setup, as the more glass or mirrors in the optical path generally dims the image, or can introduce optical abberations.

#7 Jon Isaacs

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Posted 02 February 2013 - 04:58 PM

Is a refractor telescope the only one that will show a right side up image view? Or, does it show upside down? I know reflectors are inverted, but refractors are all right side up, no? What about the cat design, aren't they still reflectors, thus upside down view?
If a person wanted a scope for both celestrial viewing and horizon viewing, he would be stuck with a refractor, right?


All scopes invert and reverse the image, refractors and Cats (SCTs, Maks etc) can be used with diagonals which invert one direction. If a simple mirror or prism diagonal is aligned vertically, the image will be erect but reversed left to right.

More complicated diagonals that use roof prisms can provide images that are correct top to bottom and left to right. The complicated light path means they suffer at the highest magnifications, generally correct image diagonals are not used for astronomy.

There are image correctors available for Newtonians and other straight through scopes but they are quite rare and for astronomy, not very useful. I have one that was part of deal for a used scope. I break it out every couple of years just to see how it works.

Jon

#8 Haas

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Posted 02 February 2013 - 05:00 PM

Most refractors come with a standard prism or mirror diagonal which will give you a vertically corrected image, but horizontally inverted.



That's what I was wondering. Guess I didn't ask the question very well. So, would any Meade ETX series have at least the diagonal mirror, or do you specifically need to ask about it before you would buy anything?

#9 Tony Flanders

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Posted 02 February 2013 - 05:03 PM

Is a refractor telescope the only one that will show a right side up image view? Or, does it show upside down?


Here's the full story.

Reflectors show images upside down -- or as I would prefer to state that, rotated 180 degrees. As though you took a book and rotated it so the print is facing down, not up.

Refractors as usually used, with a 90-degree star diagonal, show images right-side up but mirror imaged, as though you're holding a book in front of you and trying to read it in a mirror. The same is true of SCTs.

This is actually the result of the star diagonal, not the telescope. All on its own, viewing straight through like an old-fashioned spyglass in a pirate movie, a refractor shows images rotated 180 degrees, just like a reflector.

You can also buy diagonals, either 45-degree or 90-degree, that will fully invert the image, showing it right-side up and not mirror reversed.

#10 neotesla

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Posted 02 February 2013 - 05:04 PM

The diagonal/mirror is part of the design of the ETX scopes I believe, as you can't view straight through, the EP port is perpendicular to the optical path.

#11 Haas

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Posted 02 February 2013 - 05:04 PM

All scopes invert and reverse the image, refractors and Cats (SCTs, Maks etc) can be used with diagonals which invert one direction.



You're saying "can be". Does that mean I should not assume any scope has a diagonal in it? I guess what I'm trying to get at is, if I were to see an ad for a refractor scope that I was interested in, I still should not assume it will have the prism or diagonal mirror in it, correct?

#12 Haas

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Posted 02 February 2013 - 05:08 PM

The diagonal/mirror is part of the design of the ETX scopes I believe, as you can't view straight through, the EP port is perpendicular to the optical path.


So, an ETX scope would give a right side up image, but incorrect left to right, do I have that right? (no pun intended :))

#13 Jon Isaacs

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Posted 02 February 2013 - 05:11 PM

You're saying "can be". Does that mean I should not assume any scope has a diagonal in it? I guess what I'm trying to get at is, if I were to see an ad for a refractor scope that I was interested in, I still should not assume it will have the prism or diagonal mirror in it, correct?



Almost all refractors designed to be used for astronomy are designed to be used with a diagonal but the diagonal is separate from the scope and fits the end of the focuser.

Some refractors come with the diagonal, some do not, you have to verify this for each scope. In general, TeleVue refractors all ship with a diagonal, Explore Scientific refractors seem to ship with a diagonal.

Jon

#14 derangedhermit

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Posted 02 February 2013 - 10:54 PM

Reflectors show images upside down -- or as I would prefer to state that, rotated 180 degrees. As though you took a book and rotated it so the print is facing down, not up.

This seems to assume a certain orientation of the observer's head to the eyepiece. I know I've "run the experiment" years ago with a 6" Newtonian OTA held over my head and in other odd positions, but I have forgotten the results :foreheadslap:

#15 Achernar

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Posted 02 February 2013 - 11:33 PM

With a diagonal, the view will be right side up, but reversed left to right. You can use an image erector with some telescopes to get a correct orientation, or a 45 degree diagonal that accomplishes the same thing for terrestrial use of a refracting or catadioptric telescope. Reflectors in general would give an reversed and inverted image.

Taras

#16 WarmWeatherGuy

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Posted 03 February 2013 - 04:21 AM

If the number of reflecting surfaces is odd then you will have a mirror image.

Without a diagonal:

A refractor has zero reflecting surface and a Newtonian or SCT has two reflecting surfaces (two and zero are both even). The image will be upside down.

With a diagonal:

A refractor has one reflecting surface and a Newtonian or SCT has three reflecting surfaces (one and three are both odd). The image will be right-side-up but a mirror image.

With a 45 Degree Angle Erect Image Diagonal:

Somehow this manages to add 2 reflecting surfaces while rotating 180° which makes everything right-side-up AND not a mirror image.

Here is a 45° diagonal.

Note that people generally use a diagonal on refractors and SCTs but not on Newtonians.

#17 Haas

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Posted 03 February 2013 - 01:34 PM

If the number of reflecting surfaces is odd then you will have a mirror image.

Without a diagonal:

A refractor has zero reflecting surface and a Newtonian or SCT has two reflecting surfaces (two and zero are both even). The image will be upside down.

With a diagonal:

A refractor has one reflecting surface and a Newtonian or SCT has three reflecting surfaces (one and three are both odd). The image will be right-side-up but a mirror image.

With a 45 Degree Angle Erect Image Diagonal:

Somehow this manages to add 2 reflecting surfaces while rotating 180° which makes everything right-side-up AND not a mirror image.

Here is a 45° diagonal.

Note that people generally use a diagonal on refractors and SCTs but not on Newtonians.


Thanks warm weather guy. That's probably the most strait forward answer I've gotten so far. Not to take away from everyone else who's answered, I appreciate all.
When we talk about diagonal, are we just talking the angle of the eyepiece?

#18 WarmWeatherGuy

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Posted 03 February 2013 - 02:15 PM

When we talk about diagonal, are we just talking the angle of the eyepiece?


The star diagonal is used to make it so you don't have to twist your head upside down to look at things directly overhead. There are two types. One has the light exit 90° from the entrance and the other only 45°. You can use either one or none at all. Both diagonals invert the image top to bottom but the 90° one does it like a mirror and the 45° one simply rotates the image 180° and does not make a mirror image.

A Newtonian has the light exit hole coming out of the side of the telescope so you don't have to turn your head upside down to view things directly overhead. This is why people don't use diagonals on those types of telescopes.

#19 Haas

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Posted 03 February 2013 - 02:21 PM

So, just out of curiosity, can a star diagonal be used on a reflector, just to get things right side up if you wanted?

#20 WarmWeatherGuy

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Posted 03 February 2013 - 02:30 PM

So, just out of curiosity, can a star diagonal be used on a reflector, just to get things right side up if you wanted?


Yes.

If you use a 90° star diagonal on a reflector the image will no longer be upside down but you would then have a mirror image.

If you use a 45° star diagonal on a reflector the image will no longer be upside down and you would NOT have a mirror image.

If you use a star diagonal on a reflector you would have to position your head higher up and you would then be looking down. You might then be tempted to rotate the star diagonal 90 degrees. In that case your image would be half way between being upside down and right side up, i.e. your image would be rotated 90 degrees.

#21 WarmWeatherGuy

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Posted 03 February 2013 - 02:34 PM

I forgot to mention that reflectors tend to have less back focus so you might not be able to focus when using a diagonal. If you design your reflector to have more back focus then the secondary mirror would have to be closer to the primary mirror and at that point the secondary mirror would have to be larger. This would block more light so this is why they don't do that.

#22 Haas

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Posted 03 February 2013 - 02:55 PM

So, just out of curiosity, can a star diagonal be used on a reflector, just to get things right side up if you wanted?


Yes.

If you use a 90° star diagonal on a reflector the image will no longer be upside down but you would then have a mirror image.

If you use a 45° star diagonal on a reflector the image will no longer be upside down and you would NOT have a mirror image.

If you use a star diagonal on a reflector you would have to position your head higher up and you would then be looking down. You might then be tempted to rotate the star diagonal 90 degrees. In that case your image would be half way between being upside down and right side up, i.e. your image would be rotated 90 degrees.


Well, by George, I think I've got it. Thanks!! :D

#23 Haas

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Posted 03 February 2013 - 03:00 PM

I forgot to mention that reflectors tend to have less back focus so you might not be able to focus when using a diagonal. If you design your reflector to have more back focus then the secondary mirror would have to be closer to the primary mirror and at that point the secondary mirror would have to be larger. This would block more light so this is why they don't do that.


Yeah, I'm not going to do it either, I was just trying to get he concept down in my head. My head can be a stubborn thing sometimes. :tonofbricks:

#24 Tony Flanders

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Posted 03 February 2013 - 04:31 PM

So, just out of curiosity, can a star diagonal be used on a reflector, just to get things right side up if you wanted?


You could in theory, but nobody does.

It's also possible to make an image erector with lenses. Those are sometimes sold with low-end reflectors.

To amplify about more about the image possibilities: In general right-side up images are more gratifying for viewing terrestrial objects. It's definitely possible to get used to birds hanging from tree branches and flying upside down; in fact your brain corrects for this better than you might imagine. But all in all, right-side up is better. Mirror imaging usually doesn't matter much except that it makes moving the telescope counterintuitive, and it's hard to read lettering on distant signs.

For astronomy, however, I (and many others) much prefer the upside-down images provided by reflectors. Astronomical objects don't really have an up and down; they rotate as the night progresses. And you can always rotate your charts to match what you see in the eyepiece.

The mirror-imaged view provided by refractors with a star diagonal, by contrast, never ceased to trip me up. Short of bringing a mirror into the field with you, you can't make your charts match what you see in the eyepiece. So I find navigation through a refractor much, much harder than through a reflector.






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