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Gregorian Mak


Tweedledee
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I'm hoping someone can clarify something for me...

 

If a Gregorian (spot secondary) Maksutov telescope is used with a 90 degree diagonal, does it provide a normal erect image ie, suitable for normal terrestrial viewing?

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Thanks, but I think I confused myself and everyone else by inserting *spot secondary*, so please ignore that.

 

Just to make my question clearer, the unusual optical configuration I am talking about is basically as per the picture. Imagine the following optical diagram with a meniscus lens (which makes no difference to the orientation) and a 90 degree mirror diagonal. What is the orientation at the eyepiece?

 

1920px-Gregorian_telescope.svg%20(1).png

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18 minutes ago, Smithysteve said:

This is the sort of question I would pm you about Pete! ??

Haha. It's doing my head in thinking about it. :wallbash:

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After further thoughts I will stick my neck out and say the views will be reversed in both planes.

Or would it????  :blink:

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In a Greg Mak the secondary is moved outside the primary focus point so this becomes the reverse of a Mak Cass where the secondary is inside the focus point. As the secondary is outside the focus point this results in a correct image.

 

A Greg Mak has a concave secondary mirror and this acts like an erector lens and turns the final image upright and correct, hence why they used to be used for birding.

 

 

 

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That's brilliant Mick, thanks. :thumbsup::thumbsup:

 

I originally thought, and hoped that would be the case, but searching the internet suggested various different orientations, some obviously in error. Then I kept confusing myself trying to trace through the optical path. :)

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2 hours ago, Doc said:

In a Greg Mak the secondary is moved outside the primary focus point so this becomes the reverse of a Mak Cass where the secondary is inside the focus point. As the secondary is outside the focus point this results in a correct image.

 

A Greg Mak has a concave secondary mirror and this acts like an erector lens and turns the final image upright and correct, hence why they used to be used for birding.

 

 

 

 

Brilliant explanation mick.

My only question is if it is a corrected image would the diagonal not reverse it again. 

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Maybe Graham that's the bit I'm still thinking about. I'm sure the above is correct but then Pete has an AMICI diagonal which if places inside the light path shouldn't make no difference to the original spec, I think.

 

 

 

 

 

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37 minutes ago, Doc said:

Maybe Graham that's the bit I'm still thinking about. I'm sure the above is correct but then Pete has an AMICI diagonal which if places inside the light path shouldn't make no difference to the original spec, I think.

I wanted to use a mak for high power work alongside and aligned with my F5 correct image refractor, not instead of. The 2" TS Amici prism on the refractor cost a fortune, far more than the scope, so I don't want to buy another. If I can get a mak that gives the same image orientation in an ordinary 90 degree mirror diagonal that would be nice. :)

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I'm sure Mick is correct that the image would be correct in every way, but I am still having difficulty understanding this.

 

Studying the ray diagram above, it becomes obvious that the image formed at the focal plane is definitely both upside down and left/right reversed.

 

This is then complicated by several factors that I am trying to get my head around. 

 

1. The flat diagonal mirror is placed before the light reaches the focal plane.

 

I've just looked through one and confirm that flips the image vertically and leaves left/right unchanged. So at the focal plane, the image is now right way up but still L/R reversed.

 

2. The focal plane is then examined by the eyepiece when it is properly focused.

 

Looking at news print through a 56mm Meade Super Plossl doesn't change the orientation, ie everything is readable as normal.

 

4. The light passes through the persons eye lens and is focused on the retina.

 

This shouldn't change point 3 as I've just checked.

 

5. Does the brain then turn this image upside down?

 

Have I just eliminated point 5 as per point 3 and point 4?

 

Following through the above, the final image is right way up but left/right reversed.

 

What am I missing??

 

My internet searches on this are very contradictory.

 

My brain hurts. :o

 

Can anyone untangle my thinking please?

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I blame Mick for all this. :P

 

Mick kindly agreed to sell me his Losmandy AZ8, and I am very pleased with this top-of-the-range Alt Az mount. :)

 

When I went over to collect the mount, I was more excited about seeing Micks own superbly engineered alt az mount and also the tour of his brilliant five star workshop. He does a nice mug of coffee as well. Thanks Mick. :thumbsup:

 

After setting up the AZ8, I got to thinking that it looked a bit lob sided, and could do with being loaded up a bit more with a second scope. The right scope could be a correct image mak.

 

I thought it blended in nicely as a focal point in the room, but my wife has banned it to the garage. :)

IMG_20171103_201445.jpg?raw=1

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Your new mount does look very nice Pete as does your lovely lounge. I always thought it looked a little lop sided as well, I used to put my batteries on a dovetail bar on one side so I could run my dew controller without a tangle of wires.

 

As for your brain hurting, well mine is to after reading your post :lol:

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Pete if you really want to tease your brain.

If you rotate the diag through 90 degrees so you are looking through it at the 9 or 3 o'clock position instead of the 12 o'clock position the image should flip over and the left / right orientation will be reversed.. :facepalm2:

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On 03/11/2017 at 14:03, Graham said:

 

Brilliant explanation mick.

My only question is if it is a corrected image would the diagonal not reverse it again. 

A bit more research has shown that Graham is correct. :thumbsup:

 

The image formed by a Gregorian Maksutov without a diagonal is a completely correct image suitable for terrestrial viewing. If a 90 degree mirror diagonal is added, the image will be left/right correct but upside down. If an amici prism is added, the image will be left/right reversed and upside down. Maybe it is possible, if there is sufficient back focus, to rig up some combination of lenses, prisms or mirrors to get a completely correct image to a 90 degree viewing position, but this would be prohibitively expensive and very unwieldy.

 

In a refractor, Maksutov Cassegrain or Schmidt Cassegrain without a diagonal the image is left/right reversed and upside down. Add a 90 degree mirror diagonal and the image is still left right reversed but the right way up. An amici prism will completely correct the image. So I think that terrestrial or birding scopes are only in these 3 telescope types using an image correcting prism at 45 or 90 degrees. A Gregorian Maksutov could only be used terrestrially as a straight through scope.

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While searching through numerous websites about telescope image orientation, I have found several sites reprinting identical incorrect orientation information which can be confusing and certainly had me scratching my head until I got it straight in my mind. I suspect that the websites with errors had probably all copied from the same erroneous online information without checking further. 

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Rob an AMICI diagonal will not work the image will appear reversed and upside down. The only way round it is as Pete said a combination of mirrors configured to give a correct image but that's a lot fussing around and I doubt very much it will work.

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You've got me contused now....

 

In any Catadioptric scope the primary mirror will flip the image upside down,

then the secondary will flip it the right way up again...?

 

So if you use a mirror diagonal it will flip it again.

So surely the Amici won't effect that, but only correct a left to right image ? :unsure:

 

I will try with my own SCTs and report back.......

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5 hours ago, Bino-viewer said:

You've got me contused now....

 

In any Catadioptric scope the primary mirror will flip the image upside down,

then the secondary will flip it the right way up again...?

 

So if you use a mirror diagonal it will flip it again.

So surely the Amici won't effect that, but only correct a left to right image ? :unsure:

 

I will try with my own SCTs and report back.......

Cheers Rob. You are correct in what you say, but Gregorian Maks are a different configuration of catadioptric scope, so the amici prism won't do the trick.

 

The cheap amici prisms in the link you gave above are tiny. They have a clear aperture of only 20mm. I don't think I own an eyepiece with a field lens as small as 20mm. :)

 

This is what I call an amici prism, and this would be the animal for the job, check out the price on that... :o

https://www.teleskop-express.de/shop/product_info.php/info/p7947_Baader-2--90--Amici-Prism---astronomical-quality.html

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12 hours ago, Doc said:

Well done Pete that is some detective work you have done there.

 

So what's the plan then?

Cheers Mick. Well, I used to be indecisive, but now I'm not so sure. :lol:

 

The Gregorian Mak is not suitable for my needs, but maybe an ordinary Mak or SCT with Amici prism would nicely balance up the AZ8.

 

I did come across some instructions for making a nice lightweight, long focal length, folded scope - on an origami website. :D

 

At least I have now finally worked out exactly which way is up. :rolleyes:

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