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66mm refractor equals 203mm Mak.....

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#1 Cotts

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Posted 06 May 2013 - 10:39 AM

Last night at the Texas Star Party the seeing was quite poor. One of the last objects I checked was epsilon Lyrae - the legendary double double. In the 203mm, a TEC Mak/Cass, the view of each pair was of fuzzballs just touching with little or no dark sky between them. For a comparison I cranked my little William Optics 66mm achromat ( my finder scope) up to 120x and got exactly the same 'split' except that now there were two reasonably steady Airy Discs touching with little or no black sky between them.

An interesting example of the phenomenon oft discussed here at length - that a smaller scope can offer a steadier, more pleasing (if I may use that rather over-used term) view than a larger scope.

I suppose it is up to the observer to decide which view - fuzzballs or Airy patterns - is the more useful or aesthetic.

Dave

#2 skysurfer

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Posted 06 May 2013 - 01:23 PM

Probably a collimation error of the Mak.
I have a 250mm Dobson and a Televue Genesis and in some cases the Genesis issues crisper images of double stars than the Dobson despite the Dobson is two magnitudes brighter.

#3 Cotts

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Posted 06 May 2013 - 02:31 PM

The Mak is collimated 100% and is not at issue here. This is not the point I was trying to make.

When the seeing is at a given level a larger scope will actually resolve the bad seeing as scintillating speckles which form a circular pattern. The smaller scope cannot resolve the speckles and treats the light as if it were a single point source and produces a classic diffraction pattern.

My observations last night featured a coincidence - namely, the speckle pattern in the large scope was the same angular diameter as the central disc of the diffraction pattern in the small telescope.

This happens all the time on the observing field as larger scopes like big dobs or SCT's seem not to be able to produce 'pleasing' or 'sharp' or 'refractor-like' or 'crisp' or 'tight' stellar images like their smaller brethren when the fact of the matter is that the larger aperture is giving a truer picture of the state of the atmosphere while the smaller aperture is, to all intents and purposes, undersampling the available photons (both in terms of resolution and light-gathering) and producing images that convey not one bit more information. Furthermore, because the seeing is never constant, the larger scope will always convey more information to observers patient enough to wait for "those moments".

Dave

#4 Jon Isaacs

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Posted 06 May 2013 - 02:47 PM

Probably a collimation error of the Mak.
I have a 250mm Dobson and a Televue Genesis and in some cases the Genesis issues crisper images of double stars than the Dobson despite the Dobson is two magnitudes brighter.


Ever noticed how sometimes doubles suddenly sharpen up when a cloud passes? Part of the issue here is the response of the eye and the brightness of the image. Reducing the brightness with a neutral density filter can help...

Jon

#5 azure1961p

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Posted 06 May 2013 - 07:44 PM

I'd agree with you Dave. What few public outreach things I did way back usually went to the 70mm Ranger just because its simpler image was also often steadier . The folk mostly were non astronomers and so it was easy to please the less discerning.

Pete

#6 Chassetter

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Posted 06 May 2013 - 08:25 PM

Seems the first line in the original post may have a contribution to the observed phenomenon... :question:

#7 Tom and Beth

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Posted 09 May 2013 - 09:30 PM

Seems the first line in the original post may have a contribution to the observed phenomenon... :question:


No doubt. I piggyback a 60MM f15 on my TEC MC and poor seeing will sometimes equalize the views.

#8 ziridava

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Posted 12 May 2013 - 08:53 AM

Last night I was observing through my ''Toleascope'' 8 inch F/6 Dobsonian and my 60x700mm refractor.
(This is an old habit of mine,most of the time I use at least two telescopes for my observations.)
The only objects where the 8 inch Dobsonian showed more details were Saturn and M13.
On double stars,the small refractor outperformed the big cousin,in terms of separation and images had much more aesthetics.
In the bigger instrument the stars were just some sort of light pie.
I think my Dobsonian is having a pretty good primary,once it showed very clearly the eight shapes/touching Airy discs/ of Eta CrB and of Zeta Boo.

Mircea

#9 Ed Wiley

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Posted 13 May 2013 - 08:28 PM

Hey Dave: I know that night. As you know, we were observing only a few yards away. My C11 Edge was showing all snowballs and I can confirm that both of us were 100% collimated, I saw the SCURT results.

Of course the snowballs do not bother me, as you know, since I am reducing the them via autocorrelation, which produced reasonable (within limits of course) photographic separations. But the visual images are about as ugly as one can imagine; not a satisfactory visual impression as would be gained by using a smaller aperture.

BTW, Dave: You scope is Outstanding as evidenced by the SCURT results. Right in there with the Tak refractors.

IFY: This is the first time I imaged a SCURT target, very interesting. (SCURT: Scope Check Using Resolution Target.) First image showed I was slightly out of collimation! Wish I had one in the backyard.

Ed

#10 cildastun

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Posted 26 May 2013 - 05:17 AM

I think the phenomenon discussed is quite a well-known one. My Orion ED80 will often give better views on doubles (or a feature like the Cassini division) than larger scopes set up alongside. I had assumed this was the (ancient astronomy folklaw) reason that the "diffraction cells in the atmosphere are often quite small - 4" or so" - allowing a small scope to perform to its best, while larger scopes are struggling...

Chris






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