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Planetary Starmaster

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#51 Daniel Mounsey

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Posted 10 January 2006 - 04:00 AM

Dan,
This may make an argument for pushing air up the tube with a fan. By doing so, you create higher pressure in the tube and the exiting air flow would discourage warm air from drifting across the field of view above the scope. Of course, it will be pulling in air from the mirror box area and this would tend to be much less contaminated by body heat.

Viewing in NH in cold weather I always had a problem with body heat screwing up my images in my 10" dob.


This is true, and is also the reason why Ed Grissom still uses sono tubes but with a truss, the air can not flow smoothly upwards the way a sono tube carries air up. This is the drawback to the truss design. I have always implied that a sono tube behaves like an air tunnel or shaft to keep everything constant with a fan at all times.

#52 Daniel Mounsey

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Posted 10 January 2006 - 04:09 AM

To confuse matters further: air drawn from the mirror end is often air close to the ground, which may be warmer than ambient. I seem to be having this problem lately on my concrete driveway; can't pin it down for certain. :confused:

Jim


This is why I sometimes water my driveway down or place a wet rug under the scope. One soon realizes the thermal challenges Newts must face and it's easy to see why people are content with their apos. They are practically imune to thermal attacks.

#53 Daniel Mounsey

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Posted 10 January 2006 - 04:11 AM

I spoke with an observer friend today who places a large piece of thick, rubberized carpet over that part of the driveway he observes from.
Evidently you can buy them in huge sizes and thicknesses.
His completely covers the concrete and not only shields the tube assembly from rising thermals but eliminates heat-induced turbulence in the near-external lightpath!

Mike Harvey


This is also a good idea.

#54 Daniel Mounsey

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Posted 10 January 2006 - 04:23 AM

Sorry I have not addressed the 3 vane spider issue. The problem with the 18" Obsession is that there are 4 poles holding the rings together, so three new poles would have to be put in. I just rebuilt the upper cage to my Starmaster all together and tossed the four vane spider in the trash. Grissom will make spiders for the right price. He charges more than anybody, BUT ITS WORTH IT. People skimp on some of the most crucial things. Grissom by far produces the finest curved spiders you can buy. They just can't be made any better because the curves are just perfect, the finish is nice and they're as thin as a 5 sheets of paper. For an 18", he would make them a little thicker. But the area vs vane thickness is preportionate as the aperture increases. I'll see if Ed can help you.

#55 bgi

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Posted 10 January 2006 - 08:16 AM

Hi, Daniel,

Thanks much for checking! I can rebuild the cage easily enough. It's an older starmaster - looks like the cage tube is a sonotube material made prior to the switch to plastic.

I didn't see any dew control in Grissom's spider. What is the dew control solution? Batteries in the secondary? Currently, the spider on my scope has flat conductors glued to a vane to supply power to the dew control from the base drive battery (Thanks, Jarad). Hmmm...

#56 NHRob

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Posted 10 January 2006 - 09:07 AM

Sol,
What type and size reflector do you use?

#57 Jarad

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Posted 10 January 2006 - 09:10 AM

I have some more of that flat wire if you need it, Brad.... Er, I think I do - let me check around in my (very messy) workshop before you hold me to that....

Jarad

#58 Ronny Floyd

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Posted 10 January 2006 - 10:29 AM

Daniel,

Does Mr. Grissom have a website ? I have future plans to buy a curved spider for my scope and would be interested in one of his spiders.Thanks Ronny







#59 bgi

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Posted 10 January 2006 - 12:07 PM

I have some more of that flat wire if you need it, Brad.... Er, I think I do - let me check around in my (very messy) workshop before you hold me to that....

Jarad


Jarad,

Way cool. That beats scrounging around for some thin sheet copper. thanks!

#60 Sol Robbins

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Posted 10 January 2006 - 03:39 PM

NHRob,

I use a 10" stopped down to 9.6". It has a "picked" mirror from Sky Instruments. They test the mirrors from Guan Sheng and Synta. The better ones are separated out. I think they use a Foucalt test method.

Best,

#61 eddie kirkland

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Posted 27 January 2006 - 04:35 PM

Hi Daniel, I was wondering about your truss connections to the UTA and the mirror box; do you have pictures of how you did this? I'm planning a 10" f/6 4-pole dob and I love the looks of what you built. Did you use 1" or 1.25" poles?

Thanks,

#62 bgi

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Posted 07 February 2006 - 12:16 AM

Hi, Daniel,

Me again. Ping!

I would like to know how to reach Mr. Grissom. I'm looking forward to improving the view of Saturn this year (hint hint hint...)

thanks again.


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