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Cat for planets

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#201 azure1961p

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Posted 17 December 2011 - 11:20 PM

Its an interesting riddle in a way. Imagers are supposed to be sooo sensitive that every bit of detail including visual ought to be recorded. And in truth it probably is. The cam tho is only one half of the equation. The thing that blows the visual comparison out of the water is the after imaging processing. The manipulation of contrasts that can be had is utterly rutheless. A ten inch F/4 even with a huge secondary can produce images youd expect from a ten inch apo SIMPLY because those effects of central obstruction, light scatter, even minor mis collimation can be by passed with wavelet processing after so many stacked frames. Ive NEVER seen a ten inch f/4 produce images on par with a like sized refractor visually. Not even close. For after imaging processing though its all quite pedestrian to repeatedly turn out contrasty images that are virtually immaculate of reflector type optical artifacts. It blows right past them.

Lol, its not even fair!!

Pete

#202 Asbytec

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Posted 17 December 2011 - 11:37 PM

I used to image back in the early days when equipment was expensive and clunky, so I tended to think of sketching as a poor man's imaging. Well, it's not. You're absolutely correct, one can put down on paper what we see...and that's a beautiful art form. Mine suck in terms of art, and some guys are truly artists. Ah, who cares...throw your observation in the mix, regardless, and share the excitement.

I have astig, too, and wear glasses (as of a year ago.) But, maybe you find you don't really need them at the eyepiece. I was afraid to use my 12mm Ortho, but hey...no prob. No glasses needed.

Oh, and I look forward to your first light...and sketch. I am waiting with baited breath (need to brush.) Hell, maybe you'll spend more time in the observing forums, too, and not arguing much over MTF. :)

Personally, MTF is great and interesting, but I've learned not to worry about it. CATs do fine on planets, IME. In fact, it seems collimation, focus, and cool down (factors we can control) coupled with good seeing (which we cannot control, mostly) allow all scopes (CATs, Newts, and refractors) to perform closer to their theoretical MTF. And the results can be stunning, even with a little residual SA.

Anyway, back on topic. Go CATS!

#203 Twilight

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Posted 18 December 2011 - 12:36 AM

Eric I agree with you completely. If an observer is sketching you get the general idea what can truly be seen in the scope or eyepieces used. I wish I could sketch but I can't even draw a circle and would have to be warm nights at that. Cold weather turns me to stone. LOL People that can sketch really impress me!

If I could sketch that would be a time I really apprecieated the SCT and Go To.

#204 Sarkikos

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Posted 18 December 2011 - 09:48 AM

John,

Sorry about the floaters. Also, the virtues of higher magnification are lost if the scope isn't tracking (I find it hard to keep my attention focused on a particular detail when the object moves quickly across the EP FOV). Really smooth optics with first-rate coatings in the 10" to 12" class help too. My eyes aren't getting any better with advancing age, so I don't mind dropping coin on the best optics I can get. When my eyes get worse to the point that the optics don't help at the EP, or with imaging that can be visualized well enough on a computer screen, I'll sell my premium OTAs.


The floaters are merely an inconvenience, but the retinas still function very well. I can see fine detail at more moderate powers, which works out well, because I don't like uberhigh magnification.

I don't need to push the magnification to see detail in the GRS, strings of small barges, over a dozen bands, networks of festoons, white ovals, etc. I can get good results with a binoviewer, Baader M&SG filter, an apodizing mask, a C10NGT on a Dob mount and decent though not exhorbitant eyepieces. Go figure. YMMV. My sketches of the planet's surface always closely match the photos.

Mike

#205 Alph

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Posted 18 December 2011 - 01:58 PM

There are no details to observe on double stars (other than their separation). There are on planets.



This strikes at the heart of the matter.

Angular Resolution is not meaninful in terms of describing visual performance in extended objects.

Theory (and I know that is a dirty word for many people but it is hard to have a meaningful dialog without it) says that an extended image is composed of an infinite number of overlapping DIFFRACTION PATTERNS. Note, theory does not say it is composed of overlapping AIRY DISK.


You must be referring to Huygens’ principle of constructing a wavefront from point sources. The Huygens’ principle shows/explains why angular resolution is a defining factor and contradicts your statement “Angular Resolution is not meaningful in terms of describing visual performance in extended objects” Sorry, Ed. You couldn't be wronger on this one.

#206 Sarkikos

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Posted 18 December 2011 - 04:14 PM

Pete,

Ive NEVER seen a ten inch f/4 produce images on par with a like sized refractor visually. Not even close.


You've viewed planets through a 10" refractor? Which observatory was that in? :grin: That is getting up to a monstrous size for a telescope. I've looked through an 8" f/15 refractor, and that thing is a behemoth mounted permanently under a dome.

But like I said, it didn't show me anything on the image of Jupiter that I couldn't see in my 8" f/6 reflector. My 10" f/4.8 Newt shows me more. So far I'm not impressed by what a refractor can do for planets versus a stabilized, well-collimated Newt with decent optics.

Mike

#207 JJK

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Posted 20 December 2011 - 08:22 PM

Earlier in this thread, the question of usable magnification was raised. A cardiologist (who built a 32" Relay Telescope and was written up in a recent Sky & Tekescope magazine) presumably makes good use of magnification well over 300x.

http://www.youtube.c...be_gdata_player

#208 Sarkikos

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Posted 20 December 2011 - 09:07 PM

JJK,

I get a "404 not found" error at that link. "We're sorry, the page you requested cannot be found. Try searching for something else."

I suppose with an aperture of 32", the observer would be able to use magnifications well over 300x with good effect, as long as the seeing cooperated. That would be about a 2.7mm exit pupil, which would be very comfortable. And Texereau's ideal planet observing exit pupil of 0.8mm would put you at around 1000x on a 32" aperture, which would be very nice ... during a night of good seeing ... with a thermally stabilized instrument ... that tracks. It is not the high power that I object to so much as the narrow exit pupil and the difficulty of manually tracking at high powers.

If the aperture is large enough to support high power without torturing me with an exit pupil substantially below 0.8mm, and there is tracking, and the seeing is good, and the telescope is thermally stabilzed, then I have no problem with high power. Otherwise, I will make do with more moderate powers and try to use my experience and technique to make up the difference.

:grin:
Mike

#209 Vondragonnoggin

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Posted 20 December 2011 - 10:06 PM

A 32" F/4 telescope with a 4mm eyepiece is giving about 813x with a 1mm exit pupil. Still very comfortable, but that image scale has to be about nickel sized in the eyepiece. That is monstrously big. If you had steady skies and tracking, that must give the most spectacular view of planets. How comfortable would that be to look at?

:crazyeyes: :drool: :bigshock: :jawdrop:

#210 Sarkikos

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Posted 21 December 2011 - 08:16 AM

Very comfortable. There may be some discomfort, however, in transporting the scope, setting it up, and climbing up the 10' ladder to look through the eyepiece. Other than that, very comfortable indeed.

:grin:
Mike

#211 JJK

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Posted 21 December 2011 - 06:51 PM

Hi Mike,

Sorry. My iPad automatically transfers the URL to an app.

Search for "mega home made telescope" and select PART SIX.

Clear Skies,
John

#212 Sarkikos

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Posted 21 December 2011 - 08:03 PM

John,

Thanks. Here is a link to the video on YouTube:

Mega home_made telescope PART SIX

Uploaded via my trusty, crusty eight-year-old PC clone running XP.

After watching the video, I realize the ATMer will not have to transport the scope, set it up, or even climb a 10' ladder to look in the eyepiece. Yes, very comfortable indeed. Well done!

Mike

#213 JJK

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Posted 21 December 2011 - 09:13 PM

Thanks.

The Sky & Tel article on the builder/owner of instrument and the scope was pretty interesting, but it was cool to see the owner describe the system in a talk spread across seven videos on YouTube.


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