Author Topic: Dimensions for 8" f6 reflector  (Read 683 times)

bankrybettdog

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Re: Dimensions for 8" f6 reflector
« Reply #15 on: February 02, 2018, 05:54:59 PM »
The spikes get thicker as the % of obstruction of the aperture increases.
Thinner spider vanes (note the spelling) yield thinner spikes.
The spike increases in apparent length because what light is in the spikes is compressed into a smaller area.
The visibility of spikes, because they are fainter than the star, is dependent on the same factors that influence the visibility of DSOs:
exit pupil, contrast with the night sky, etc.

For a smaller scope, the 3-vane spider is a viable alternative to 4 vanes.
Note how the curved spider puts more glow directly around the planet.
http://www.fpi-proto.../img/spikes.gif

facwindpsychco

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Re: Dimensions for 8" f6 reflector
« Reply #16 on: February 08, 2018, 12:01:32 AM »
NVR saw a glow with my spider. Maybe I'll have to make a 20min exposure to see it.When theory and experience dont match...

jingdilenma

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Re: Dimensions for 8" f6 reflector
« Reply #17 on: February 08, 2018, 03:27:49 AM »
Oh good heavens, I LOVE my 3-vane spider on my 8" f/6 dob. It replaced a 4-vane about 15 years ago.

Is anyone making those anymore? I would probably avoid Protostar even though that's where I bought mine.

scenunhadef

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Re: Dimensions for 8" f6 reflector
« Reply #18 on: February 09, 2018, 12:01:41 AM »
And 1800destiny is 3 week lead time.

Adam Washington

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Re: Dimensions for 8" f6 reflector
« Reply #19 on: February 09, 2018, 09:10:36 AM »
I made just one attempt or experiment at using curved spider.
One recommendention I remember from ''Gleanigs for ATM'' articles by Cox in ''Sky&Telescope'' is to keep the thickness of the arms at 1/50 of primary mirror diameter.
In my experiment that mean a tickness of 1.44mm so I made the blades 1.5mm thick.
What I obtained is something I don't like : the light from the spikes is spread across the whole visual field like the haze produced by the spherical abberation of a not parabolized mirror.

 bbcodeImage-js (do not remove or edit this tag)
Before , the secondary was hold by a 4mm thick arm made from a piece of welding electrode.
I'm seriously considering to go back to the one arm : it produced one spike but it was dim and it disturbed me less than the ''haze'' I reported.

Ziridava


Drew Bullets

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Re: Dimensions for 8" f6 reflector
« Reply #20 on: February 09, 2018, 11:33:22 AM »
Quote
Oh good heavens, I LOVE my 3-vane spider on my 8" f/6 dob. It replaced a 4-vane about 15 years ago.

Is anyone making those anymore? I would probably avoid Protostar even though that's where I bought mine.

I've seen them in a number of inexpensive scopes. I had a 5" f/6.5 newtonian with a 3-vane spier, and I used to sell a 4.5" with a 3-vane spider.
But I think many of the ones I've seen in the field in scopes >6" have been home-made.
I am also concerned with the ability of the spider to hold collimation as the scope points low, and this seems to be the Achilles foot of the 3-vaned curved spider,
especially as the scope gets larger than 8".