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just for FUN.....anyone care to speculate?

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#26 Slow Astronomer

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Posted 09 January 2013 - 07:30 PM

All I know is a Q7 owner was constantly amazed at the "better view" of DSO's through my Orion ST120 than through his Q7! We used the same high quality eyepieces, swapping them back and forth. We finally concluded it was the difference in contrast we were seeing. We suspect the CO in the Q7 and the difference in focal length were the cause. :question:

BTW - we did get a killer view of Uranus thru the Q7.

Clear skies, Dave

#27 GlennLeDrew

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Posted 09 January 2013 - 08:25 PM

Dave,
What kinds of DSOs appeared better in the 4.7" achro vs the 7" Mak?

I could buy larger 'fuzzies' being easier to see in the smaller scope due to the ability to achieve both a larger exit pupil and field of view. But such things as globular clusters and compact planetaries will definitely be better seen in the larger scope.

Another question. As regards the difference in contrast, can you supply some details? Was this gauged at same exit pupil diameter? Or at same magnification? On the same target? Was it considered over a range of scale, from arcsecond level planetary to tens of arcminute DSO to full field scattering/veiling glare?

One of the biggest errors made when comparing instruments is not eliminating all variables which will affect results of the test. And this becomes ever more difficult with an increase in disparity between objective aperture.

To compare optical quality and contrast transfer, the most meaningful results accrue when the exit pupils are identical. For one thing, the image surface brightness will be the same (less the differences in transmission efficiency), which allows the eye to more reliably assess contrast differences. And it's the exit pupil diameter which controls the point at which aberrations become visible, due to it being the normalizing factor for all apertures.

#28 Slow Astronomer

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Posted 10 January 2013 - 01:56 PM

Glenn, I just want to make it clear that I'm not advocating the ST120 is better than the Q7, it was just startling to see the difference in contrast. The background through the ST120 just seemed to be "darker".

The tight planetaries (NGC2392 & NGC 7662) were definitely better in the Q7 as the FL allowed higher magnification. We also got a nice view of Uranus but that's off topic.

On globs, open clusters, galaxies & M42 our gut reaction to the view was definitely "wow" compared to the Q7. Again the "darker" background in the ST120 seemed to make them pop. List = NGC 869, 884 & M31, 35, 37, 38, 44.

We didn't compare exit pupils - but I did posit this difference to the Q7 owner at the next club meeting. We did compare like targets and try to match some magnifications. We were both using high quality EP's (TV Delos, Pan, Nagler & ES100) It was a casual observing session, not a critical side-by-side comparison of the two scopes.

We had expected mediocre viewing conditions due to the latent local light dome and moisture in the atmosphere but conditions really improved as the night went on. We were able to see Orion's shield and other fainter stars with the naked eye. Something that doesn't happen too often around here. Clear skies,

Dave

P.S. It was amusing that the Q7 owner, an experienced AA, spent most of his time on my ST120 with his jaw hanging open!

#29 GlennLeDrew

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Posted 10 January 2013 - 02:38 PM

Dave,
Regarding the hanging jaw syndrome, I know what you mean. An experienced observer forgot all about his fine 4" APO when I let him spend some time with my home-made 60mm right-angle bino. With the 13mm Ethos eyepieces delivering 20.8X and 4.7 degree field, and the Ultrablock filters installed, he said he'd never seen better views of the large nebulosities in Cygnus.






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