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Mirror seems to magnify?

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#1 spoolboyy

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Posted 25 July 2013 - 08:31 AM

Hey all,

I have recently picked up a cheapie craigslist meade reflector (114mm, f/8) as a second scope to complement my 6" dob.

When I look into the OTA of my 6 incher, I see a handsome face at the bottom. When I look into the new scope it seems to have magnified the reflection of the aforementioned face. What's the deal with this?

Is this a different sort of mirror? Is this a bad thing? How will this affect the views through this scope (I haven't had a chance to try it yet).

Thank you in advance, any info is appreciated.

- A

#2 pstarr

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Posted 25 July 2013 - 08:49 AM

All concave mirrors will magnify when you look at them. The shorter the fl, the more it will magnify from the same distance away. The father away you get, the more it will magnify until you reach the focal point and the image will turn upside down. As to the views, thankfully you look through the scope and not into it to see objects. You will have to be the one to tell us what the views are like. Looking down the tube doesn't tell us anything on the quality of views.

#3 Jarad

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Posted 25 July 2013 - 08:59 AM

When I look into the OTA of my 6 incher, I see a handsome face at the bottom.



Ah, yes, you seem to have picked up the special model with the built-in ugly filter. :poke:

When I look into the new scope it seems to have magnified the reflection of the aforementioned face. What's the deal with this?



Both should seem to magnify. Exactly how much will depend on the focal length of the mirror and how far back you look at it from. Since both are f8, the 6" has slightly longer fl than the 4.5".

Your bigger concern should be if the face in one of them is less handsome than the other... ;)

Jarad

#4 csrlice12

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Posted 25 July 2013 - 09:02 AM

Actually, I believe the primary/secondary mirror system produces a 1:1 ratio (based on its f/l and mirror size)of the sky, it's the eyepiece that does the work of magnifying that image.

#5 pstarr

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Posted 25 July 2013 - 09:08 AM

Actually, I believe the primary/secondary mirror system produces a 1:1 ratio (based on its f/l and mirror size)of the sky, it's the eyepiece that does the work of magnifying that image.


Not when you look down the tube. That is the magnifying I was referring to and what the op was asking about. The role of the eyepiece has nothing to do with his question or my answer.

#6 Jarad

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Posted 25 July 2013 - 09:33 AM

Actually, I believe the primary/secondary mirror system produces a 1:1 ratio (based on its f/l and mirror size)of the sky, it's the eyepiece that does the work of magnifying that image.


The two work together. The primary produces a virtual image, with an image scale that is dependent on it's focal length. The eyepiece magnifies that virtual image based on its focal length. The combined magnification is the ratio of the two focal lengths: Mag = Primary FL / Eyepiece FL.

Jarad

#7 spoolboyy

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Posted 25 July 2013 - 03:03 PM

All of you made points that actually seem to make sense with what I have "observed" (no pun intended).

When I look through the eyepiece holder - my eye does not appear to be enlarged by the optics nor does my hand if I put it in front of the OTA. This seems to mean the Primary/Secondary pair are producing images together that don't appear to be magnified a significant amount. When looking face-on into the OTA, The reflections appear to be magnified.

It's possible I just haven't noticed any apparent magnification much with my other scope. The dob has a focal length of 1200mm and the EQ's is 910mm - so it stands to reason that if the amount of magnification is related to focal length (which would indicate a more curved mirror and result in light that was "more bent" all other things equal), that it would be more noticeable in scope with the shorter FL.

Feel free to comment or correct my restatement and conclusions if I restated anything incorrectly.....and thanks for the info everyone it was very informative!

- A

#8 Jarad

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Posted 25 July 2013 - 08:01 PM

It also depends on distance. Put either scope up at eye level facing horizontally, look in, and slowly back away. At first you will see your whole face, then it will get bigger until eventually you just see your eye. In fact, you can use this to do a quick check on the primary collimation - if it is properly collimated, you can back up until the secondary perfectly fits into the image of your pupil. If it doesn't, then the optical axis is not aimed directly at the secondary.

Jarad

#9 csrlice12

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Posted 26 July 2013 - 09:25 AM

Interesting....I'll have to try that out. Thanks!

#10 Nils Olof Carlin

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Posted 27 July 2013 - 04:19 AM

The primary produces a virtual image...


I disagree here. The image produced is real - you can pick it up on a film, ccd detector or screen without additional optics. A virtual image example - the face you see when looking at the flat bathroom mirror.

Nils Olof






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