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Observing Double Stars With An APO

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#1 Peter Natscher

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Posted 23 November 2012 - 07:40 PM

I'm new to observing with a quality APO. I've spent my last 20 years observing with many sizes of Newtonians and Cats. With my new AP175 APO and good seeing last night, double stars appeared like beautifully sharp points of color vs. what I was more used to seeing with my larger reflectors: that was pairs of stars with less color, with softer, irregular and round shapes. Perhaps with the APO, the totally black sky in the field of view is allowing the stars to show even more dramatically. Gamma Andromedae (Almach) blew my socks off with a bright gibbous moon not too far away! It's quite something to enjoy seeing this level of star light coming from light years away. It's a new ballgame for me!

#2 Asbytec

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Posted 23 November 2012 - 08:57 PM

Well, as you probably know, the peak intensity tends to be higher without an obstruction even though the Airy disc is still the same size as any 7" scope. I'd think that's one reason they appear brighter, because they are. Not sure why the sky would appear darker, though. With that premium beast, splendid views are not surprising. Congratulations.

#3 Jon Isaacs

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Posted 29 November 2012 - 07:33 AM

Well, as you probably know, the peak intensity tends to be higher without an obstruction even though the Airy disc is still the same size as any 7" scope. I'd think that's one reason they appear brighter, because they are. Not sure why the sky would appear darker, though. With that premium beast, splendid views are not surprising. Congratulations.


They would be brighter than a 7 inch Newtonian, not only because of the CO but also because of the reflectivity losses.

But compared to a significantly larger Newtonian that was cooled and collimated... I am sure a 175 mm apo is quite impressive when it comes to double stars, working to the limit when the seeing allows.

Sky darkness/brightness is a function of exit pupil. Larger exit pupils are brighter. That could be it or it could be better baffling in a light polluted environment.

Of course Almach is something doable in a poor quality 60mm scope. At the sorts of powers one uses for Almach, what one is likely seeing is not the resolution of the telescope but rather the resolution of your eye at large exit pupils.


Jon

#4 fred1871

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Posted 29 November 2012 - 07:02 PM

Apos are nice, and a 7-inch very nice!! :cool:

I had access to a 7-inch apo for some years in the past, and for double stars it certainly outperformed the very good example of a C8 I had back then - with a similar magnitude limit (better % throughput - this was in the days before XLT coatings etc).

It did better on close uneven doubles than the C8 (itself no slouch).

Gamma Andromedae is a bright'n'easy pair, but I'd expect the 7" apo to give very good colour views of it. Colour in my experience is better in an apo than in an achromat, based on several apos I used often, and several achros ditto.






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