F12, 4" newt Build.

[bluemaxroe]

Hi All.

I am after advice and wisdom from those who are in the know, or those who have built Newtonians before.

I have an F12 4" mirror and I am trying to figure out the positioning of the focuser.    What baffles me is, looking online for Reflector designs/schematics there seem to be many different positions for the "focal point" position compared to the eyepiece.

can anyone steer me in the right direction?   see some examples below.

B.

Next question.

will there be a huge difference in the "in focus" position of a 25mm and 6mm eyepiece?

[SimonfromSussex]

Hi Bluemaxroe,

Different eyepieces will indeed focus at different positions.

What I did when we built my dob was work out (using another telescope) the eyepiece that required the most inward focus (and the one that required the most outward focus to see the range of the focuser was ok).

The one that needs the most inward focus can then be used to "test" the position of the focuser.

We made a truss dob (poles from base to secondary cage) so we measured them at first over estimating the length of them (you can always cut down but its hard to add more length again).

To do this take the diameter of your mirror, multiply by the focal ratio to get the focal length. Then take the radius of the mirror off of this length and measure this from the center of the primary mirror to center of the secondary mirror.

As I say we over estimated to start to be safe. 

We then put it together and tested. We were not far off but we did just have to trim the poles down slightly (a few mm) to ensure the inward focus eyepiece would reach focus.

If you are using a solid tube design however I'm not sure how you would do it? There might be a trick but chances are you need to get it right first time which might be tricky!?

Hope that helps.

[Ajohn]

The only info I have ever seen on the subject is here

http://www.cloudynights.com/topic/19323-focal-plane-in-eyepieces/

There may be more info about on the same site.

John

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[Ajohn]

Take that back. I've nosed around a couple of times recently because of something I want to make and a different search term bought up these 2

http://www.atmpage.com/epfocal.html

http://www.jim-easterbrook.me.uk/astro/ep/

John

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[bluemaxroe]

I have gone for a wooden coopered tube. 1200 long x 130 internal diameter. I created 16 strips of oak 5mm think and angled the edges. I have wrapped them into a tube by taping together.

Will find out more in the morning.

In the mean time, here are some images of the curing tube.

[mapstar]

Nice to see a different approach, I'll watch with interest

Damian

[bluemaxroe]

Ok well the tape came off without it falling apart, and it actually seems fairly strong without any internal bracing.

Looks nice sanded too.

[Ajohn]

One thing about 4in F12 is that it should work well even as a sphere.

I did a similar tube on a dobson. Hexagonal tube just using butt jointed cheap 6mm exterior ply. I cut the correct angles with a jig saw and a straight edge. I took some care to get the angle of the blade correct and used a good quality exterior wood glue. After the glue had dried I concluded that it didn't need and internal bracing - just the mirror box on one end.

  One problem - I didn't ensure that the end on the mirror box was dead square and rested that on the mirror box rather than "letting it into it"  squaring up and fixing etc.

John

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[bluemaxroe]

yes john. i could see how that could happen. i am going to cut rhe ends of the tube and then set the rear end of the mirror "box" into the back of the tube. only by a few mm but enough to ensure an approximate square position. then as i have faily long collimation screws i will be ablebto adjust from there forthe fine tuning. cant wait to get there but lots of finishing tobdo first. + a sendary spider to make. 4 vane i think.

[bluemaxroe]

Well last night I marked and cut the tube. Not perfectly straight, but not really an issue. Then today made the central part of the spider....might need to change the hardwood for aluminium, as it's very small..a 25x 25mm prism will fit on this.

[dweller25]

Looking good

[mapstar]

looking really nice there, inspiring me to make a start

Damian

[Soupy]

I didn't see where this was going to start with but now I'm intrigued... Be great to see how you get on.

[Uplooker]

bluemaxroe,

I am impressed, very impressed. I like the differing slant you have put on ATM. You certainly like to set yourself a challenge. Can I ask - "What's your trade, if you have one, joinery?" I will be watching with interest. It's good to see you have piqued Damian's (mapstar) interest and it seems, inadvertently, given him some encouragement. Good all round

Well done so far.

Ian

[SimonfromSussex]

Love the finish, looks great!

Really good job so far.

[bluemaxroe]

thanks everyone. was unsure where it was going myself when i started thing trend.

my trade is product/graphic design.

[bluemaxroe]

sorry, fat fingers and spell check...*this thread*

[mapstar]

Well done up to now a lovely creation in the making.

I find it nice to move out of my normal field of electrical maintenance engineering and do something creative, especially when like you it is that bit different.

As you say the spider boss is small but perfectly well formed and I'm sure the wood will be o.k.

Being wood can I ask how are you intending to finish it? Will it be varnish or stain?

To balance the humidity in the wood the finish should be applied both inside and out if not you may get problem's with warping or splitting and that would be a tragedy on such a nice looking scope.

Keep up the Great work but there's no rush as we'll all be watching to see how things come together with the "oakenscope"

Damian ;-)

[bluemaxroe]

Thanks Damian. I was thinking Matt varnish, both in and out, but then matt black paint on the inside afterwards. Not sure if black paint on top will do anything like you mentioned? But I do not want reflections inside, so black inside is important right?

I trial fitted the rear cell tonight. See the photo...... I might remake this in solid hardwood instead of plywood as it may hard a tendency to expand. But for now and for getting the right shape it's been good practise....

The other side of the three threads is a circular sheet of 1mm brass, solidly attached. Any suggestions on how to attach the mirror?

There is about half an inch of clearance either side of the mirror.

Suggestions welcome.

[Polar Bear]

Great build, looking forward to the next installment. 

[mapstar]

Nice little cell at the end. your progressing quickly.

The wood will be fine with the blackpaint and the ply will be more stable than solid wood.

I would've thought with the mirror being a small one would be o.k. sat on 3 cork or similar pads and some sort of clips to the brass plate like many commercial scope's.

Or you could possibly use 3 blobs of silicone to hold it depending upon the thickness of the mirror? Don't want to introduce tension and astigmatism.

Someone may suggest something else that is better suited?

Looking forward to the next part. Great stuff

[kalasinman]

Why not use your finish in and out and then flock the inside?

[mapstar]

Why not use your finish in and out and then flock the inside?

I did think that too, although 4" F12 is a long tube so didn't suggest it as it would be a bit of a struggle with the tube diameter won't be much bigger?

[bluemaxroe]

thats right. the tube is not much bigger than my arm. and i can only get just over half way down. result= i wont be flocking. i dont want to go to expensive with this build. the mirror is 12mm thick, an weighs 250grms

[bluemaxroe]

Well, tonight I have moved this forward again.

I have cut the hole for the focuser in the tube, and adapted the focuser to seat better onto the faceted faces of the tube.

I am going the let pictures tell the story of how I did it and see what you think of the end result. I am sorry about the order. I don't seem to be able to order the pictures, even though I uploaded them in order.

General order should be,

Cling wrap the components

Mix and apply P48

Remove, trim, sand, paint, test fit.

Ta da!

Night folks.