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What are undercorrected optics?

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#1 mikee

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Posted 11 January 2005 - 09:22 AM

Hi all -
I'm new to Cloudy Nights and wanted to thank everyone for all the great information that is available here especially for newbies like me. The insights have proved invaluable in helping me purchase a scope. I just ordered an Orion XT8 with a Shorty Plus barlow, variable polarizing filter and carrying case and am anxiously awaiting it - and hoping I didn't get too big a scope. It's been 25 years or since my interest in astronomy, fanned by photos in an old beat up astronomy textbook with beautiful plates from the Palomar observatory, was invaded by "life" and I feel ike a 9 year old again! In some ways I'm grateful I waited because having "mellowed" a bit, I don't have the desire to take on astrophotography as a newbie which is what I had my eye on as a young kid. I remember drooling over the Celestron catalogs thinking maybe I could somehow save up for a C-14 even though I was years away from having a job that could make the kind of money I would have needed - man I had no idea what I was thinking :p My 6th grade teacher at the time was Roger Tuthill's daughter which probably had some effect (if any of you remember his advertisments in the various magazines). haha..

Anyway, while researching countless reviews of the telescope one of the things I came across several times that I had never heard in my experience with telescopes was this: (quoted from a scopereviews.com review)
"This one has slightly undercorrected optics, not a big problem"

Since all the reviewers on different sites that mentioned this all basically said "not a big problem", I'm hoping that this is actually the case now that my credit card has been charged :grin:

Anyway, I'm curious as to what they mean by "undercorrected optics" and how this effects viewing of different objects in the night sky.

Thanks and I'm sure to be more asking and hopefully someday answering questions here at CN.

Mike

#2 werewolf6977

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Posted 11 January 2005 - 09:29 AM

I'm not sure about "undercorrected optics", but WELCOME TO CLOUDY NIGHTS!!! some-one will step up and answer your question semi-comprehensibly.

#3 erik

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Posted 11 January 2005 - 10:43 AM

mike, i certainly wouldn't worry about a review on one particular sample. the undercorrected optics that ed ting spoke about wouldn't be noticed by most observers. the reason that the orion and other imported scopes are so reasonably priced is that they use machine figured mirrors. these will give excellent performance, but aren't perfect (and NO mirror really is). in order to get a 1/20th wave accuracy hand figured mirror, you'd have to spend much more, and the differences aren't very visible. some may disagree, but that's what i think. i've owned an orion 8" newt (it was originally an xt8 dob) for years. i've done countless modifications to it, and at one point i bought a "premium" mirror. the differences weren't at all visible to me, and i ended up selling the premium mirror to a friend that was building a scope. maybe i got an excellent sample from orion, but i've looked through dozens of xt dobs, and they've all had good optics. i'm sure you'll enjoy the new scope...

#4 Jarad

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Posted 11 January 2005 - 10:56 AM

Unercorrected means that the shape of the mirror is between that of a sphere and a parabola (a parabola has a slightly steeper curve). Overcorrected means the shape is steeper than a parabola. Some places intentionally slightly undercorrect their mirrors because while a mirror is cooling down, the temperature variation tends to cause it to be slightly overcorrected until it equilibrates.

As long as the undercorrection is slight (less than 1/4 wave on the wavefront, or 1/8 wave on the surface), the scope will still be good. If it is slightly worse, then it will perform better while it cools, but get a bit worse after it is fully cooled down.

As Erik mentioned, this is quite common in less-expensive optics. To get really good optics you have to spend a bit more, or be willing to evaluate and return the cheaper ones if necessary (not all bargain optics are bad, some are quite good, they just don't have the same level of QC as the higher end ones do).

Jarad

#5 sixela

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Posted 11 January 2005 - 11:00 AM

It depends on the actual optical configuration.

For a newtonian telescope, the main mirror isn't spherical, but a paraboloid, as that is the shape which will perfectly focus an in-axis infinitely distant point source into a single point (at least ignoring diffraction).

If it's not exactly paraboloid, but too close to spherical, it's "undercorrected", and when you draw concentric circles around the mirror's optical centre, each of these circles will focus the source at a slightly different distance.

It's a bit like very, very, very mild myopia (or more precisely "spherical abberation").

As long as it's minor it doesn't *have* to be a problem - in practice, at a given aperture you cannot ignore diffraction (except at infinite aperture), and telescopes focus a point source into a disc (called Airy disc) and diffraction rings, and if the blur caused by the undercorrection is negligible in size with respect to that Airy disc, then it doesn't affect the images.

Don't worry about the reviews -- mirrors are always over- or undercorrected, even if the better ones are so correct that it would be well nigh impossible to measure it from visible light images. If the reviews say it's not a problem, then it's probably not a problem, just something noticeable (e.g. indirectly by a slight asymmetry in the out of focus images of point sources).

If you ever get you scope in a condition that shows that shperical abberation -- congratulations, you'll probably have collimated that scope perfectly ;).

#6 mikee

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Posted 11 January 2005 - 02:14 PM

Thanks all for the quick and detailed responses. You've alleviated any fears regarding the mirrors on my new telescope.

Mike

#7 Chris Schroeder

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Posted 11 January 2005 - 07:51 PM

Welcome to CN :grin:
Looks like you've got a good start, don't forget to give us a first light report. :watching:

#8 Guest_**DONOTDELETE**_*

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Posted 11 January 2005 - 08:19 PM

Welcome to CN Mike. I joined just a few days ahead of you. I've been a CN brower for years and decided it was just time to join. Listen to these folks Mike. I'm learning GOBS.

Speaking of "First Light Reports;" where would one post them? I got a new scope for Christmas, but it has been snowing/raining here for a month and the scope hasn't seen a photon yet!
Oh, stupid me..... I forgot.... new scope="No Can Do!" :foreheadslap:

#9 erik

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Posted 11 January 2005 - 08:30 PM

another quick point about these types of "abberations" found in mass produced mirrors (and premium ones, to a slightly lesser extent). on most nights from most viewing locations, the seeing conditions aren't going to allow most observers to detect a problem in a slightly under or overcorrected mirror. far more important to getting a good view is allowing the mirror to cool down, and to properly collimate it. and for deep sky objects, obviously, to view from a dark site...


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