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8" F7.88 Dobsonian

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#1 Tony818

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Posted 05 October 2022 - 04:12 PM

Hey everyone, I'm new to telescopes so I don't know too much. Might be stupid to build a telescope for the first one but I really want to and I'll learn better that way.
I posted for help about Aperture and Focal Length and settled on an 8 inch 1600mm so F7.88. Secondary mirror 30-38mm to keep obstruction under 20% but big enough to suit the whole DIY janky setup. Hoping this will be good for planets, moons and bit of other things.

I need help on the other parts . I'm having trouble using Newt for the Web as it's asking me things I don't know yet. Specifically the Focuser to Front of Tube. I don't know what the secondary mirrors distance is from the primary so I wouldn't know the focuser from the front of the tube.
That brings me to the focuser section. I don't know much about focusers, things like their diameter. I've marked the parts I don't know in red with letters. If you guys can help me out on this, that would be awesome!
If this stuff needs to be done after buying all the parts, what focuser would you guys recommend? 

 

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#2 hamishbarker

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Posted 05 October 2022 - 09:18 PM

Do you already have the mirror or are you planning to make it?

If you can get hold of make your own telescope by Richard berry, it's a great basic ATM book.

That f ratio is going to be a fairly long tube, and not very suitable for imaging, although not impossible. Some people are using eq platforms for planetary imaging with dobsonians. (in case imaging is in your planned activities.). Also quite a slow f ratio, again, only be a concern if imagingj Advantage is very little coma.

#3 Scott E

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Posted 05 October 2022 - 09:20 PM

Don't worry about the tube length at first. Newt will give you the distance between mirrors. The minimal amount of tube you need in front of the focuser depends on how you mount the secondary. Then beyond that, you just need enough to keep stray light from going directly into the focuser from across the tube. The more the better, to a point. An excessively long tube will limit off-axis illumination. The ray trace will show you. The tube required behind the mirror surface simply depends on how thick it is and thick your cell is. You just need enough room to mount it. Again, just look at the ray trace and play around with the numbers and you'll see the effects of the changes you make.

 

You usually want to keep "B" to a minimum. Look to buy or build a low-profile focuser and see how tall it is. "C" gives you room to focus, since many eyepieces have their focal plane up inside them. As for "D", just get a 2" focuser, way more flexible, even if you don't have any 2" eyepieces now.

 

When in doubt, click on the question marks.


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#4 lonn

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Posted 05 October 2022 - 10:12 PM

I used this guy's guide to make a 8in dob. http://www.stormthec...r-telescope.htm



#5 Tony818

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Posted 05 October 2022 - 10:46 PM

Do you already have the mirror or are you planning to make it?

If you can get hold of make your own telescope by Richard berry, it's a great basic ATM book.

That f ratio is going to be a fairly long tube, and not very suitable for imaging, although not impossible. Some people are using eq platforms for planetary imaging with dobsonians. (in case imaging is in your planned activities.). Also quite a slow f ratio, again, only be a concern if imagingj Advantage is very little coma.

I just ordered it, haven't researched making one yet. That's unfortunate it's not the best for imaging but that's fine. I suppose my next one will be all about imagine deep space. For now I'll use this one as a learning experience and eye viewing for the moon. 
I've been recommended many books, I'll take a look at them all. Thanks



#6 Tony818

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Posted 05 October 2022 - 11:02 PM

Don't worry about the tube length at first. Newt will give you the distance between mirrors. The minimal amount of tube you need in front of the focuser depends on how you mount the secondary. Then beyond that, you just need enough to keep stray light from going directly into the focuser from across the tube. The more the better, to a point. An excessively long tube will limit off-axis illumination. The ray trace will show you. The tube required behind the mirror surface simply depends on how thick it is and thick your cell is. You just need enough room to mount it. Again, just look at the ray trace and play around with the numbers and you'll see the effects of the changes you make.

 

You usually want to keep "B" to a minimum. Look to buy or build a low-profile focuser and see how tall it is. "C" gives you room to focus, since many eyepieces have their focal plane up inside them. As for "D", just get a 2" focuser, way more flexible, even if you don't have any 2" eyepieces now.

 

When in doubt, click on the question marks.

I saw the ray trace, I wish it had a ruler or measurements on it. I ordered a 2 inch focuser with the mirror. I didn't know anything about it, only 30 bucks so if anything I can swap it out. I just need it to function properly, nothing enthusiast level. 
If you google Skyoptikst 2 inch Focuser, it's that one. Rack pinion. There's a bunch of brands using the same one and putting their brand on it. Like Svbony. They're all the same. 



#7 Tony818

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Posted 05 October 2022 - 11:07 PM

I used this guy's guide to make a 8in dob. http://www.stormthec...r-telescope.htm

That's the video that inspired me but I got a very bad taste because it's been over 5 years and he hasn't uploaded a single picture or spoken about quality of what he saw through it. I saw another video and the same thing, they didn't even mention what they saw through it. One after another, I stopped watching those videos. How can all these people post videos about telescopes and not show it or even mention what it looks like. Image or just looking through it must be horrible. I'm making one specifically for that reason. 



#8 dan chaffee

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Posted 06 October 2022 - 03:54 AM

That f ratio is going to be a fairly long tube, and not very suitable for imaging, although not impossible. Some people are using eq platforms for planetary imaging with dobsonians. (in case imaging is in your planned activities.). Also quite a slow f ratio, again, only be a concern if imagingj Advantage is very little coma.

Depends on what one wants to image. Maurizio di Sciullo designed  f/8 newtonians

specifically for planetary imaging and his ccd images were superb. 20 or so years

ago he was turning heads with those shots. It's the dob mount that needs to be

addressed as you suggest.


Edited by dan chaffee, 06 October 2022 - 03:57 AM.


#9 ccaissie

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Posted 06 October 2022 - 11:11 PM

Basically an 8" f/8.... which is a great scope to use.  Our club has an old 8f8 Treckerscope from the 60's.  Great planetary scope, just about like what you are building.

 

My experiences say to leave the tube a few inches longer than the numbers show and move the mirror cell until it's where you want it.  Then use fasteners for a final assembly.  Extra inches up top are good to shield light and dew.


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#10 ccaissie

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Posted 06 October 2022 - 11:16 PM

I used this guy's guide to make a 8in dob. http://www.stormthec...r-telescope.htm

I wouldn't recommend that design..



#11 ccaissie

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Posted 06 October 2022 - 11:23 PM

I just ordered it, haven't researched making one yet. That's unfortunate it's not the best for imaging but that's fine. I suppose my next one will be all about imagine deep space. For now I'll use this one as a learning experience and eye viewing for the moon. 
I've been recommended many books, I'll take a look at them all. Thanks

Visual use at first until you get it all working.  I have seen stunning lunar photos made by holding a camera to the eyepiece and snapping one. You should have a lot of fun with this scope.  Back in my early years I could never even dream of having a big scope like this.


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#12 Tony818

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Posted 07 October 2022 - 11:43 AM

I wouldn't recommend that design..

The one in the video is a regular Newtonian design no? Mine is probably going to end up like that. What part would you recommend against. I need all the information I can before the parts come in and I find the time to make it. 




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