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Aperture Fever? 8" vs 10" or 12"

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#126 Dana in Philly

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Posted 02 March 2019 - 05:20 PM

My ES 12" Truss Dobsonian arrived yesterday and I spent nearly 5 hours (with breaks) carefully assembling it.  Obviously, I need to get the assembly down to an average of 15 minutes or so...as has been posted on CN before.  It is massive compared to my SW 8" Collapsible Dobsonian.  And now we have a weeks worth of storms rolling through so not much chance to see it in action!  

Five hours?!? That's gotta be a record. What, did you pull the mirror out or something?


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#127 25585

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Posted 02 March 2019 - 05:24 PM

My ES 12" Truss Dobsonian arrived yesterday and I spent nearly 5 hours (with breaks) carefully assembling it.  Obviously, I need to get the assembly down to an average of 15 minutes or so...as has been posted on CN before.  It is massive compared to my SW 8" Collapsible Dobsonian.  And now we have a weeks worth of storms rolling through so not much chance to see it in action!  

Time to practice indoors, until its a drill you can do it blindfolded. cool.gif  Then collimate.



#128 Jond105

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Posted 02 March 2019 - 05:26 PM

Five hours?!? That's gotta be a record. What, did you pull the mirror out or something?


Lol. He did say carefully and with breaks. It's like me in the summer when I used to drink working on the house. Measure, beer, think about it, measure, beer, wonder, cut, beer, think about it, find tape measure, beer, measure, cut again. 😂 jk.
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#129 Dana in Philly

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Posted 02 March 2019 - 05:37 PM

So DNA7744, now that you've got the 12", I suspect you're going to see some seagulls around the outside of that Q70. (I have the Agena version.) Time to start saving for a coma corrector and some of those sweet ES 68° or 82° series EPs.


Edited by Dana in Philly, 02 March 2019 - 05:38 PM.

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#130 Jond105

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Posted 02 March 2019 - 05:41 PM

So DNA7744, now that you've got the 12", I suspect you're going to see some seagulls around the outside of that Q70. (I have the Agena version.) Time to start saving for a coma corrector and some of those sweet ES 68° or 82° series EPs.


I think DNA7744 already started a thread regarding eyepieces. I told him the same thing. Not sure when the vendors will go up, but ES did eventually raise their prices. That's a shame.

#131 DNA7744

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Posted 02 March 2019 - 06:23 PM

Replaced the flimsy red dot finder with a Telrad...and NO it did not take 5 hours!  Now waiting on clear skies!

 

 Telrad.jpg


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#132 F.Meiresonne

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Posted 04 March 2019 - 05:18 PM

I never use a telrad. Can't use it properly. for me i need a laser to get me going...I have a telrad , came with the Obs , i used as counterweight....on the OTA.

Seems a nice scope you have there.

Do you have a shroud for the scope, helps for contrast and for safety....you won't like dropping an eyepiece on the mirror....


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#133 Dana in Philly

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Posted 04 March 2019 - 05:31 PM

Time to practice indoors, until its a drill you can do it blindfolded. cool.gif  Then collimate.

Extra points for collimating blindfolded. wink.gif


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#134 DNA7744

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Posted 04 March 2019 - 06:41 PM

I never use a telrad. Can't use it properly. for me i need a laser to get me going...I have a telrad , came with the Obs , i used as counterweight....on the OTA.

Seems a nice scope you have there.

Do you have a shroud for the scope, helps for contrast and for safety....you won't like dropping an eyepiece on the mirror....

Shroud comes next year...probably Shroud's by Heather.  Meanwhile set-up, collimating taking 15 minutes or less...take down in 3 minutes.  Clear skies tonight!  Took it out for a spin last night with high/medium clouds and no collimating...nice view of Orion nebula and M81/82...much more detail than my 8" Dobson.


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#135 F.Meiresonne

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Posted 06 March 2019 - 02:57 PM

I would not wait too long for a shroud :

 

When i had my Obs i used it once without a shroud. For a short session i had in mind. I went inside only for 5 minutes and then continued observing. When i took it inside the mirror started to dew up  a bit. Then i noticed footprints ! Footprints of a darn cat.! The five minutes i was not around the silly animal had  jumped in the mirrorbox, the box moved probably and the cat took off, leaving minor scratches , well more sleeks actually , on the mirror.

At the time i overreacted about it , and i could slap myself for that error. Had i used the shroud it would not have happened...

 

Ah well , hardly scratched , if you don't know you will never notice them, and do not influence the view in any way

 

But the scope never goes out again without putting the shroud....


Edited by F.Meiresonne, 06 March 2019 - 03:00 PM.

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#136 seasparky89

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Posted 06 March 2019 - 06:42 PM

I agonized between the 10” and 8”, and settled on the 10”.  The 10” tube is the same length as the 8”, at least for the popular Orion scopes, but the 10” will show more.  Also, I can carry the 10” tube, and it will fit across the back seat of my Honda Accord.  IMHO, the 12” would be just too big and heavy for the above.  It’s a question of what you are willing to accept in order to get that noticeable light increase of the 12”.

 

Stan


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#137 stargazer193857

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Posted 06 March 2019 - 07:01 PM

I drive out to a 12" Craigslist sale once. I saw how big one is. I could lift the pieces individually, though they were plenty hefty. When I lowered the OTA down into the modern saddle (why did they leave the old ones?) I had to lower it exactly. I got impatient and bent my back so I could see better. I then got a bad back pain that took 2 months to go away. I did not buy the scope. But I was very impressed by the ease with which it blew away the view in an 8".

The base will not fit in a standard low trunk. The OTA will take up the passenger seat and the seat behind it, leaving the other passenger seat for the base.

But a 10" does not leave more seats. The base won't fit in a standard trunk and so goes in the front passenger seat, unless you have a hatchback.

#138 Jon Isaacs

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Posted 06 March 2019 - 08:12 PM

I would not wait too long for a shroud :

 

When i had my Obs i used it once without a shroud. For a short session i had in mind. I went inside only for 5 minutes and then continued observing. When i took it inside the mirror started to dew up  a bit. Then i noticed footprints ! Footprints of a darn cat.! The five minutes i was not around the silly animal had  jumped in the mirrorbox, the box moved probably and the cat took off, leaving minor scratches , well more sleeks actually , on the mirror.

At the time i overreacted about it , and i could slap myself for that error. Had i used the shroud it would not have happened...

 

Ah well , hardly scratched , if you don't know you will never notice them, and do not influence the view in any way

 

But the scope never goes out again without putting the shroud....

 

I use my scopes all the time without a shroud. Dew is rarely a problem but very often it's windy.  A shroud is undesirable in a moderate breeze..  (10-20 mph )

 

Jon



#139 stargazer193857

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Posted 07 March 2019 - 02:33 AM

Metal tubes have much less wind resistance. Big advantage.

#140 25585

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Posted 07 March 2019 - 06:27 AM

I would not wait too long for a shroud :

 

When i had my Obs i used it once without a shroud. For a short session i had in mind. I went inside only for 5 minutes and then continued observing. When i took it inside the mirror started to dew up  a bit. Then i noticed footprints ! Footprints of a darn cat.! The five minutes i was not around the silly animal had  jumped in the mirrorbox, the box moved probably and the cat took off, leaving minor scratches , well more sleeks actually , on the mirror.

At the time i overreacted about it , and i could slap myself for that error. Had i used the shroud it would not have happened...

 

Ah well , hardly scratched , if you don't know you will never notice them, and do not influence the view in any way

 

But the scope never goes out again without putting the shroud....

Keeps bugs away too.



#141 Jon Isaacs

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Posted 07 March 2019 - 12:54 PM

Metal tubes have much less wind resistance. Big advantage.

 

What makes you think this?

 

Jon



#142 stargazer193857

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Posted 07 March 2019 - 01:09 PM

What makes you think this?

Jon


My fluid dynamics class. For a body of given diameter, a circular cross section has half the drag of a square. An octagon is likely in between. A streamlined wing section of that thickness has about 1/10 the drag but only from that one direction. A braided, twisted wire has about 1/3 the drag of a round one. And I imagine a flappy shroud is draggier than a hard surface.

#143 Jon Isaacs

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Posted 07 March 2019 - 01:20 PM

My fluid dynamics class. For a body of given diameter, a circular cross section has half the drag of a square. An octagon is likely in between. A streamlined wing section of that thickness has about 1/10 the drag but only from that one direction. A braided, twisted wire has about 1/3 the drag of a round one. And I imagine a flappy shroud is draggier than a hard surface.

 

I have not done the calculations to determine the flow régime but my intuition is is that with these shapes and velocities, the drag is basically dependent on the cross sectional areas..

 

Jon



#144 F.Meiresonne

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Posted 07 March 2019 - 01:29 PM

If there is to much wind it not good to use the big scope. It can be windy  here in Flandres Belgium, but most of the time it drops by the evening.

In the Provence France however , there is sometimes Mistral, the whole night, uncomfortable, unpleasant , can last for days....


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#145 AnalogKid

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Posted 07 March 2019 - 07:19 PM

<snip>.  IMHO, the 12” would be just too big and heavy for the above.  It’s a question of what you are willing to accept in order to get that noticeable light increase of the 12”.

 

Stan

Not sure if it was mentioned in this long thread, but a collapsible, or truss 12" is smaller than the solid tube 10".  Of course the base is bigger all around in the 12" vs the 10"



#146 stargazer193857

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Posted 08 March 2019 - 02:11 AM

I just watched a video by Dobson. He said 4" is for the moon, 10-12" is for planets, and 16" is for galaxies. The 16" mirror looked pretty, but the 16" scope was huge. Scope size matters as much as aperture. 10" is an OK compromise.

#147 stargazer193857

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Posted 13 March 2019 - 02:01 PM

Not sure if it was mentioned in this long thread, but a collapsible, or truss 12" is smaller than the solid tube 10". Of course the base is bigger all around in the 12" vs the 10"


Likely the base has come up. Both the 10" and 12" bases are too big to go in a small or mid sized trunk. An 8" base just fits. Square bases though fit in the trunk.

If you drive a Honda Civic, 10" might mean no passengers.



On another note, the 20 pound 8" OTA is nothing to laugh at. Though easy to lift, the moment you disrespect it could be the moment you feel some back pain. I lifted the OTA this morning while still waking up and discovered this fact. I see why Jon moves a 6" in two pieces. He does not mind the extra walking and does not like taking unnecessary chances.

#148 25585

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Posted 13 March 2019 - 04:57 PM

What makes you think this?

 

Jon

Truss tubes with a shroud fitted, even if stretched tight will get buffeted more than a solid round tube with continuous curve.

 

The additional weight of a solld wall tube will also help.

 

If dirt is blown around, even at ground level, mirror surfaces are much more protected. 


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#149 stargazer193857

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Posted 13 March 2019 - 09:29 PM

A solid tube is better in every way but transportation. Most nights there is not enough wind to matter, but on windy nights, the steel tube and round particle board base is King.
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#150 Jon Isaacs

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Posted 14 March 2019 - 04:27 PM

Truss tubes with a shroud fitted, even if stretched tight will get buffeted more than a solid round tube with continuous curve.

 

The additional weight of a solld wall tube will also help.

 

If dirt is blown around, even at ground level, mirror surfaces are much more protected. 

This is something I have a lot of personal experience with.  Our place in the high desert is in pass in the coastal range between the desert and the ocean.  It is very often quite windy.  I consider 10mph or under, calm, 10mph-20mph, typical, over 20 mph, windy, over 30mph, probably time for a haul out a shorter scope or hang it up.  

 

First rule:  No shroud,  A shroud is just a big sail that catches the wind.  You see my photos of my desert Dobs, no shrouds..  A tube also dramatically increases the surface area.

 

Obsession with ladder 2014.jpg
 
with the 25 inch F/5, the scope wasn't the limiting factor, the limiting factor was my body buffeting in the wind on the ladder.  The scope was stable but I was not.  Same with the 22 inch.
 

Dust:  It's dry, very dry but it's basically a sandy hard pack. Dust is generally stirred up by ones feet and not the wind, it blows too much for there it be dust, it's all blown away.  A tube protects when one is storing the scope, covers do that too, but I don't worry about it when I am using the scope.  And mirrors are easy to clean.  

 

In a windy situation, I will take an open truss over a full tube scope every time.  

 

Jon


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