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astrolunartick

20inch Dobsonian Construction

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Looks great Pete.

Can I ask why you have gone for four collimination screws rather than the customary three? Will this give you more accurate collimination?

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Looks great Pete.

Can I ask why you have gone for four collimination screws rather than the customary three? Will this give you more accurate collimination?

I experimented with 3 and 4 collimation thumb screws with a 4 vane Spider on an old scope and come to the conclusion that 4 ends up better spaced between the vanes with 3 thumb screws i found it difficult to get my fingers round the thumb screws as they are to close to the vanes. there is a small compromise,  3 is the magic number. As a circle will always lay flat on 3 points of contact. so like a 4 legged chair with one leg slightly short you just have to make the correct adjustments. ( Basically you get used to it )

Pete

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Cheers Pete I was thinking of a wobbly table with a short leg as it happens, and how this has to be braced to stop the table tilting.

So are you gonna have some flexible padding between the end of the bolts and the secondary to reduce the possibilities of the wobbly mirror?

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Cheers Pete I was thinking of a wobbly table with a short leg as it happens, and how this has to be braced to stop the table tilting.

So are you gonna have some flexible padding between the end of the bolts and the secondary to reduce the possibilities of the wobbly mirror?

hahaha  what like a beer mat.

no its all in adjustments. like on a skywatcher newtonians Primary cell they have the locking nuts next to the  collimation thumb screws. although on my secondary you kind of adjust the thumb screws to take out any wobble. Obsession telescopes used this on there 4 vanes and many other large dob companies to. so it an accepted way. i  do prefer 3 points of contact myself. But as i say i find it more comfortable with this set up.

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Great work again Pete and looks a bit of quality. Interesting your thoughts on the number of collimation screws

I'm guessing here but your going to use a flat plate for the collimation bolts to work on then an angled plate/tube to mount the secondary.

Finding time is always hard but looks like your making the best of it when you do.

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hahaha  what like a beer mat.

I Like this telescope, beer mats for collimination pads... What next, beer cans for eye pieces... :)

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 over the top and then place the hot spot on a table and align the template over the hotspot and press the template so the sellotape sticks through the holes of the template to the top side of the hotspot. Then carefully peeling of the paper back of the hotspot. I had to do this several times as it was a little tricky as the hotspot would tend to come off with the paper backing. Then i aligned the template on top of the primary mirror and with the rubber end of a pencil pressed the hotspot down until the hot spot had stuck in place . :-)

Template on mirror with hotspot  

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You referred to the Hotspot but you seem to have used the triangle spot -- not the Hotspot. Was there a particular reason why you preferred the triangle over the Hotspot?

Jason

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You referred to the Hotspot but you seem to have used the triangle spot -- not the Hotspot. Was there a particular reason why you preferred the triangle over the Hotspot?

Jason

i was going to message you today jason. which is the best with the cateye outfit. i have both thats why i ask

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I prefer the Hotspot over the triangle. It works better with the Blackcat and the XLK (or XLKP) autocollimator. The only advantage of the triangle over the Hotspot is its smaller shadow when used with barlowed laser.

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Jason

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The four adjustment screws are much easier to use push/pull on opposite screw keep thing nice and secure ,these are always used with a central locknut,against a baseplate,nearly all the commercial builders use this system,it is a very intuitive way of adjusting your secondary mirror,and the thing i like about this system is it is very simply constructed.

Regards

Mike

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You referred to the Hotspot but you seem to have used the triangle spot -- not the Hotspot. Was there a particular reason why you preferred the triangle over the Hotspot?

Jason

Hello Jason. 

Opps my mistake. I ment Triangle. silly me. i should do my research better. Could explain how they differ in there performance Please? 

Pete

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I prefer the Hotspot over the triangle. It works better with the Blackcat and the XLK (or XLKP) autocollimator. The only advantage of the triangle over the Hotspot is its smaller shadow when used with barlowed laser.

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Jason

I only just saw that. Many thanks thats very handy. Ive stuck the triangle to the mirror now so im probably not going to risk taking it off. So i will have to make do with that. 

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Hi again.  Just a small update. A good friend kindly welded up my Tailgate. So im buzzing now. had a massive grin when he turned up at my house with it. I had to put the cell components in it to see how they fit.

Ive only a couple of weeks to go and my work shop will be set up and then theres nothing stopping me completing the scope. YIPPY :)

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Great to see an update Pete and really nice work again.

Will it be standard black paint? Could have it powder coated for a really durable finish

Damian

Edited by mapstar

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Great to see an update Pete and really nice work again.
Will it be standard black paint? Could have it powder coated for a really durable finish

Damian

Hi Damian 

Ive had quite a bit of experience spraying so im going for a matt black spray finish. Rust-Oleum is good stuff.

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A couple of questions, what wood did you use for the split blocks?

I really like your secondary mirror holder and adjustment. I have nearly finished a 14 inch dobsonian but the secondary holder and spider is a botched job and I will need to redo it! Will you have a way of adjusting the spider from the sides to get the secondary accuratley centered/ adjust collimation?

David

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Looks great Pete, nice work! Roll on the rest of the build :)

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A couple of questions, what wood did you use for the split blocks?

I really like your secondary mirror holder and adjustment. I have nearly finished a 14 inch dobsonian but the secondary holder and spider is a botched job and I will need to redo it! Will you have a way of adjusting the spider from the sides to get the secondary accuratley centered/ adjust collimation?

David

Hello David. 

I used Ash hardwood for the blocks but any hardwood with tight straight grain like Ash, Beech, Birch, Maple, Mahogany should do. Your question in regards to the way the secondary spider is mounted in the secondary cage, pushed me to work on just that today. so heres some images. I used 10mm m6  round threaded sleeve nuts which i cut a slot in with a hack saw and drilled a 2mm hole  for a 2mm sellock roll pin that holds the round threaded sleeve to the spider vane.  All parts i purchased from Ebay. I will be spraying most of this matt black.

hope that helps

Pete 

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Very nice Pete it's coming together steadily

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They look amazing Pete. How thick are the vanes?

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Thanks for the reply about the spit blocks and spider mounting, it will come in very useful for ideas as to how to improve my 14 inch dobsonian. Another question, where did you get the wood? I have somewhere I can buy birch ply but not ash etc.

David

Edited by dark star

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Thanks for the reply about the spit blocks and spider mounting, it will come in very useful for ideas as to how to improve my 14 inch dobsonian. Another question, where did you get the wood? I have somewhere I can buy birch ply but not ash etc.

David

Hi again David. I got the Ash from a local timber yard. Seeing that your in london I would have thought there is a timber yard not far from you. If not you may find what you need on Ebay.

Pete

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Hi Soupy

the vanes are 0.6mm stainless steel.

Cheers Pete, I am just curious as I am not too sure whether to reuse the secondary support vanes from the cannibalised 8" scopes I have to make a suitcase dob or to start afresh... I'll keep watching to see how you go then decide ;)

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