Noob telescope build!

Maplesyrup (Josh) · #1 20-12-2010, 12:05 AM

I'm not sure whether to post this here or in the diy...I chose here so I wouldn't annoy the pros with my lack of knowledge (hopefully here it may be excusable), at least for the meantime.
Ok, well the hsc is done so now I have the time to move onto the project. I am hoping to build a telescope, an 8 inch dob. This will be my first scope, so I'm definitely taking a risk here ahaha!
Hopefully I can gain a greater understanding of telescopes than just buying a scope and using it.
I have already received some awesome help from mental4astro, who basically has convinced me to build my own as a budget entrance to astronomy.

Alrighty, well I have the primary mirror, a 8" f6, a 1.3" secondary mirror and a 2" focuser.
I can see how most will go together but my main concern is the distance between the primary and secondary mirror...I have no idea how far they need to be apart. The mirror states on the back F6 as the focal ratio and 47 3/4 " ...is the secondary mirror meant to be 47 3/4" away?
This scope will be built in an almost travel scope style but a bit simpler. Please any tips or help or advise on anything would be brilliant!

Thanks all
Regards
Josh

TerminallyTired (Jason) · #2 20-12-2010, 01:21 AM

Basically the 47 3/4" is the focal length of the mirror. This is how far you want your eyepiece from the mirror.

You will want to figure out what diameter tubing you will be using. If you use a tube 9" in diameter then the light has to travel 4.5 inches from the secondary mirror to the edge of the tube. So there mirrors should be about 43 1/4" apart. However you will need to take your focuser into account as well. I would guess there is another 4 inches or so there. So you would want to to reduce the distance to something like 39 1/4". Measure up your focuser assembly and decide in what tube size you are using and do the maths.

Just remember that you will want the focus point pretty close to the focuser with the draw tube fully retracted so you can use a range of eyepieces, say 4mm through 40mm.

L8r

TT

ZeroID (Brent) · #3 20-12-2010, 06:34 AM

Get NEWTWIN on your PC, put in the parameters and play with it all. Great way to learn how it all fits together. That's what I did with the 10" build. I've now got an excellent scope for my purposes and learned heaps in the process. And thanks to Mental4astro (Alex) for his encouragement and knowledge in getting me there. Play with GUIPLOP as well to understand mirror support. They are not critical to your scope build but they explain and illustrate the principles and give you confidence to go ahead. Good luck, ask anybody here for help, it will be forthcoming.

Blue Skies (Jacquie) · #4 20-12-2010, 09:03 PM

This is what I normally tell people: Imagine the focal length as a piece of string - you can't change the length of the string but you can bend it any where you like, even multiple times! However, you're building a dob, so you only want one bend in it, and you want to know where.

You need your focuser and your tube radius. Rack your focuser about half way out and measure the height from the top where the eyepieces will rest on it to where it will meet the tube. Then add your tube radius to this. This it the distance from the middle of the secondary to your eyepiece. Once you know this you know where to 'bend' your string! What ever is left will determine the length of your tube. To determine where the primary should go measure from the middle of the focuser down to the top of the primary - so where the top edge of the primary should be when its all put together.

So I always start at the top and put the focuser and secondary in first (or determine their positions, I should say) then work out where the primary should go.

For a solid tube a good rule of thumb is to have it at least the same length tube as your total focal length, and by the time you take out the bit that diverts out to the focuser you should still have enough room to put in the primary cell and have enough to shield the focuser from stray light.

I thought I had a diagram illustrating this but can't find it off-hand. I'll see if I can dig it out for you.

I should add that playing with NEWTWIN is also a good idea, it makes a lot of work look easy!

Maplesyrup (Josh) · #5 21-12-2010, 10:22 AM

Ok brilliant! Just wasn't sure about that. That string analogy is quite a good explanation!
Unfortunately I cannot run Newtwin. I don't run windows, I use linux. I'm going to try and finish the mirror cell today.
Thanks for explaining that so well and the encouragement TerminallyTired, ZeroID and Blue Skies

ZeroID (Brent) · #6 22-12-2010, 07:02 AM

Cool !!

Keep us posted on progress and ask anything anytime. The only 'factory' components on my scope are the mirrors, the focusser and the red dot finder.

I gave up trying to build a 2" Crayford but it's always on the cards and I've still got ideas about building a 'Telrad' type finder. I WILL definitely build an finder scope with illuminated reticle and right angle EP.

It's all good fun I tell ya

Maplesyrup (Josh) · #7 23-12-2010, 09:41 PM

I have finished the mirror cell and the housing box, although it will get a coat of paint tomorrow (I will post some photos to see what you all think).
ZeroId you bring up something I have neglected! Should I buy a finder scope or should I attempt to build one?
Regards
Josh

ZeroID (Brent) · #8 24-12-2010, 06:50 AM

I bought a red dot finder although I had a cheapo 5 x 24 finder scope off another old scope. RDF works well for me mostly but my skies are pretty Light polluted and I want a finderscope with right angle EP to make it easier to find those hidden stars. I use binocs to prefind my target at present but it gets hard to transfer the angle to the scope when I change back. Hence I need a finder scope with about 50 mm aperture and a 5>8 x mag to cut through the haze.
I'm trying to make it because I like building things but having trouble getting a suitable lens. Pair of old binocs would be the best start point.
A decent finder scope can be quite expensive. A Red dot finder or Telrad or similar is cheaper and quite effective. I personally would start there.

Maplesyrup (Josh) · #9 25-12-2010, 04:16 PM

Yeah I prefer the idea of a red dot or telrad finder! I will do some research into what building one and see if it is possible...pretty sure ebay has rifle red dot sights, I wonder if I could use one of them?

ZeroID (Brent) · #10 27-12-2010, 03:54 PM

Yep, they'll do perefectly. That's where it all began. Think mine cost about $29. Not worth trying to build although I might do do one just for the heck of it. Keep the workshop busy..