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Old 17-06-2008, 11:27 PM
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Planetary imaging question... FL vs. Aperture

Hey... I've been looking around here and there and have seen some really well priced 2nd hand scopes available..

My question is this... all things being equal, does focal length win over aperture when planetary imaging?

Im currently using an 8" F/6 newt (1200mm FL), Would images be better out of a 6" F/10 Mak-Cas (1800mm FL) or would it just be larger, but lacking detail due to less light gathering power of the 6" vs the 8"
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Old 18-06-2008, 03:58 AM
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iceman (Mike)
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Focal length won't help if you don't have the aperture to capture the light. You could get an f/15 mak cas as well, but the aperture is tiny.

My 10" was 1250mm FL and my 12" is 1500mm. When I use my 5x powermate, with the extension given by the filterwheel it ends up that i'm imaging at about 10.5m focal length. With the small chip size of the DMK (and ToUcam) (5.6microns) it gives a pretty large image that i'm usually happy with. Of course we always want a bigger image, so sometimes we resample etc.

On nights of very good seeing I can use an extension tube in addition to the extension already given by the filterwheel, and can get up to 14m focal length. For this though, the seeing needs to be very good and the object needs to be bright enough so that you can still use a short-enough exposure. The longer the exposure (to let more light in) the more you lose the benefit of "freezing the seeing" and the features will start blurring together.

If you really want more focal length, get more aperture to go with it. Get a 12" newt or a C14 or something. Best of both worlds.
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Old 18-06-2008, 06:32 AM
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Alex, the key to detail (seeing and collimation/quality aside) is aperture. Aperture is what gives you the resolution that provides greater detail... the light is needed, but it's actually the distance between outer edges of the mirror that determines the resolution. This is how radio telescopes separated on the earth's surface can combine signals for increased resolution.

Seeing on the other hand has a greater impact the larger the aperture. That means there will be fewer nights which could be regarded as good seeing for a C14 say versus a C8. Still, if you have the aperture on a night of perfect seeing you will get the rewards.

Optical quality and collimation are equally important, but are to some degree controllable.


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Old 18-06-2008, 09:01 AM
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Increases in aperature increase not only resolution (up to the limit of the night's seeing) but also increase lightgathering. Increased lightgathering means you can run your camera on shorter exposures which help "cut through" the atmospheric light scattering by better capturing those short instances of good seeing than longer exposures will. Increased lightgathering also means you may have sufficient light to be able to used coloured filters to improve the apparent seeing and still run short exposures.

Longer focal length means greater magnification and larger image scale but you can get the same effect with very little light loss using good quality barlows or powermates.

So given a choice between increased aperature and increased focal length I would go for aperature, assuming that the quality of the optics is the same of course.
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Old 18-06-2008, 07:50 PM
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Tasco can give you 660x, maybe more, with a 60mm objective.....

I jest.

Like gravity, aperture always wins.....
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Old 18-06-2008, 08:01 PM
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I'll stick with my 8" F/6 untill I can find a good quality, well priced 12" OTA.
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