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Soft diffraction spikes on Jupiter at high power?

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#1 phreon

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Posted 18 November 2012 - 01:50 PM

Among other things, I viewed Jupiter last night with my AD10 Dob (10 inch, 1250mm focal length). It was a simple back-yard viewing session, cool night, reasonable seeing and Jupiter was about 70 deg Alt.

Using the included 9mm GSO Plossl, Jupiter's disk and moons were sharp against blackness, but detail wasn't that great. I tossed in my 6mm Sterling Plossl and refocused; detail was better but I was up against seeing. I'd get glimpses of the GRS, defined banding, etc. However, while using the 6mm, I noticed general glare in a 4 pointed shape much like very soft diffraction spikes. My understanding is that the prevalence of spikes should decrease as power goes up / the view dims. Though the Sterling is my newest EP, given the reviews I've read and performance I've seen against dimmer objects, I wouldn't think it's an issue with the EP.

A star test earlier did reveal that my scope is just a bit out of collimation; the diffraction rings were just a tad bit skewed. I understand planetary observation demands good collimation on a fast scope; could this have been a contributing factor? FWIW, the scope had been outside for several hours, but the fan batteries were dying.

Thanks,

Doug

#2 mark cowan

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Posted 18 November 2012 - 05:05 PM

If you have a spider with straight vanes Jupiter will indeed cast faint wide diffraction smears.

Best,
Mark

#3 JohnMurphyRN

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Posted 18 November 2012 - 05:36 PM

If you have a spider with straight vanes Jupiter will indeed cast faint wide diffraction smears.

Best,
Mark


I experience this also, more pronounced with BV. I'm glad to know what it is, thanks :)

#4 Jon Isaacs

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Posted 19 November 2012 - 05:56 AM

If you have a spider with straight vanes Jupiter will indeed cast faint wide diffraction smears.

Best,
Mark


:waytogo:

A single star produces 4 sharp diffraction spikes. A bright, extended object like Jupiter is conceptually like the sum of a great many bright stars, each of which produces 4 sharp diffraction spikes... the sum of those spikes makes 4 diffraction bands the width of Jupiter.

A curved spider produces similar bands but they are more evenly spread out over the entire image so they are not as visible.

Jon

#5 DavidC

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Posted 19 November 2012 - 04:53 PM

Yes, Jupiter looks great about midnight when it's high in the sky. Any bright object seen thru a reflector like Jupiter will produce defraction spikes B cause of the spider vanes. That's just the nature of the beast.
David

#6 DavidC

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Posted 19 November 2012 - 04:55 PM

Unless of course the spider is curved or round. That's even better, B cause you won't have any spikes then.

#7 GeneT

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Posted 19 November 2012 - 06:42 PM

I recommend comparing these viewing scenarios over several evenings. It is not unusual to have some evenings with Jupiter having razor sharp views, and others where they are blurred out. You need a night with excellent seeing to get a good read of any problems. You might collimate more than once in each viewing session to remove that variable. Is your collimation holding through the viewing arc? If your telescope was out for several hours, the ambient temperature should have been reached. Please update us following a few more observing sessions.

#8 Jon Isaacs

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Posted 19 November 2012 - 11:55 PM

Unless of course the spider is curved or round. That's even better, B cause you won't have any spikes then.


The diffraction effects are there, they are just spread out rather than concentrated.. The loss of contrast on Jupiter is probably greater from the curved spider because a curved spider needs to be thicker than an optimally designed straight vane spider.

Jon






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