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22" F3.38 DIY Scope Build


SimonfromSussex

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With my F4.5 Lightbridge Dan it works a treat. Plus as I say the baader doesn't affect the focal length which was a big plus for me.

I'm like you with 2 wide field eyepieces, mine are a 21 ethos and a 31 nag and so I have just screwed one onto the base of each.

For my 10mm ethos I don't notice any coma on the lightbridge so don't worry about it but you might have sharper eyes than me?

The baader is rated down to F3.5 so i'm hoping it will just about be ok on the new one but time will tell. 

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We are using this book Damian and that was the recommended spec:

http://www.obsessiontelescopes.com/publications/index.php

I think on Steve's (swampthing) 20" scope his were actually slightly wider than that.

All looks good but must weigh a lot as some bits of my build do.

I've gone for 1" for the alt bearings and worked it out due to the pressure on the teflon pads. I guess books have different opinions as I've been using the Albert highe book.

Also 22mm truss poles which seem to do a great job strength wise. It is my first scope build and it has an ally cell but a 60mm thick mirror and for a 24" it weighs quite a bit! F3.97 so quite a bit longer and more tolerant than your mirror.

I'm nearing the end now but its been a learning curve. Fingers crossed it will perform well I just wanted something different.

Damian

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Looking very nice Simon.

Just a quick thought, sorry didn't realise you'd get so far so quickly.

If you check out my bearings you'll notice there is a centre spigot like thingy in the centre of the quadrants. This allows for easy encoder fitting if you wish to add a Goto or push to system and a later date.

I thought I'd say now as it would be easy to biscuit joint a piece on at this stage before you finish the bearings.

imagejpg1_zps6b6a9e29.jpg

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All looks good but must weigh a lot as some bits of my build do.

I've gone for 1" for the alt bearings and worked it out due to the pressure on the teflon pads. I guess books have different opinions as I've been using the Albert highe book.

Also 22mm truss poles which seem to do a great job strength wise. It is my first scope build and it has an ally cell but a 60mm thick mirror and for a 24" it weighs quite a bit! F3.97 so quite a bit longer and more tolerant than your mirror.

I'm nearing the end now but its been a learning curve. Fingers crossed it will perform well I just wanted something different.

Damian

Sounds great Damian, I think we might be over engineering in some areas but as you say the speed is critical on mine.

Looking forward to some finished pics of your scope!!

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Looking very nice Simon.

Just a quick thought, sorry didn't realise you'd get so far so quickly.

If you check out my bearings you'll notice there is a centre spigot like thingy in the centre of the quadrants. This allows for easy encoder fitting if you wish to add a Goto or push to system and a later date.

I thought I'd say now as it would be easy to biscuit joint a piece on at this stage before you finish the bearings.

Great point Steve, cheers, could well be worth the addition now.  

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So today there was a lot of head scratching and thinking things through as (once cut) there is no going back.


I guess the key to a good dobsonian is balance and to that affect we needed to roughly calculate the torque of each side of the balance point. It’s a bit of a catch 22 because to calculate the balance you need to know the size and weight of the mirror box but to know the size and weight of the mirror box you need to know the balance but we need a rough idea to proceed so after running the figures we think roughly a 432mm (17”) mirror box will be workable. This calculates the torques as follows:


The top cage plus secondary mirror plus a decent eyepiece is about 8.1kg and the middle of the secondary is about 1480mm from the primary. The middle of the primary sits about 95mm from the bottom of the mirror box so to the top of the middle box (balance point) the distance is about 1158mm from the secondary (1480 – 337 (432-95) + 15 (1/2 mirror thickness dished out))  

The poles weigh about 4kg and will be about 1018mm (1158mm -140mm) in length.

So (if my maths is correct) we calculate the torque of the top half as:

8.1kg x 1158mm = 9,380 mm/kg

4.0kg x 509mm = 2,036 mm/kg

Total = 11,416 mm/kg


The cell weighs about 10kg and sits at the bottom of the mirror box. The mirror is about 22kg and sits 95cm from the bottom of the mirror box. Finally the mirror box (at 17”) weighs 15.07kg and can be taken from half way down the box. The book said ignore the bearings so we will??

The bottom half is therefore:

Cell + Fixings 10kg x 432mm = 4,320 mm/kg

Mirror 22kg x 337mm = 7,414 mm/kg

Mirror box 15.07kg x 216mm = 3,255 mm/kg 

Total = 14,989 mm/kg


So on a 17” mirror box we are quite bottom heavy. However we can add 3kg of weight to the top half to balance this if need be which is easier I think than adding it to the bottom.


Alternatively we might be able to go with a 16” mirror box, just. However this might make the scope look at bit top heavy even though it is having the opposite effect.

The bottom half on a 16” mirror box would be:

Cell + Fixings 10kg x 406mm = 4,060 mm/kg

Mirror 22kg x 311mm = 6,842 mm/kg

Mirror box 14.18kg x 203mm = 2,878 mm/kg 

Total = 13,780 mm/kg

The top half changes slightly 8.1 x 1184 = 9590 + 2036 = 11,626 mm/kg


We made the mirror box to 18” today to be safe and then we can cut it down once we have double checked everything.

I’m sure people will soon shout if I have made a horrible mistake!!??




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The mirror box needs a rounded edge on the bottom so these were first taken off with the router

post-13692-0-94526700-1409770482_thumb.j

We then drilled the holes for the cell fixings. This will allow the cell to pivot out if need be to take out the mirror. 

post-13692-0-62672500-1409770566_thumb.j

Then the biscuit joints which were amazingly quick with Alan's expertise...

post-13692-0-10191400-1409770655_thumb.j

post-13692-0-30382000-1409770738_thumb.j

Bit of glue

post-13692-0-46519500-1409770815_thumb.j

and finally clamped together to dry overnight

post-13692-0-29405100-1409770852_thumb.j

We then made a circle of the ground board with the router

post-13692-0-89212700-1409770933_thumb.j

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Then came the rocker box, this actually took a lot longer to plan than the mirror box but after a bit of careful thought we worked out the dimensions.

Al then took out the bearing path with the router

post-13692-0-68498200-1409771192_thumb.j

post-13692-0-72332500-1409771100_thumb.j

Then they were all cut to size and some angles taken off the top

post-13692-0-29263700-1409771270_thumb.j

A quick mock up

post-13692-0-90109400-1409771310_thumb.j

Before again jointing with biscuits and clamping up

post-13692-0-72921800-1409771409_thumb.j

post-13692-0-78319800-1409771450_thumb.j

That's going to be it for a few days as we both need to get back to the day job for a bit.

However hopefully we are not too far off a mock up very soon.......

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Wow how quickly its all come on and great build pic's Simon. Make's my 9month build look a bit snail pace.

I have spent 3 months of that grinding a 22" mirror which is also a massive challenge.

Looks to be going really well guys and well done. I shall post a build thread when I'm finished but till then I'm watching with interest.

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Thanks Mapstar, it's been a few months in the planning but I was quite surprised how quickly we have worked through it once started.

Grinding your own mirror is a massive achievement, nice one. I don't think I have the patience I'm afraid to do that but total hats off for doing it!!!

Will feel amazing when you get that first light knowing you have made it all from scratch. 

Keep going Astralstroll! Will be great I'm sure when you are done and get some light through it!!!

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Thanks Mapstar, it's been a few months in the planning but I was quite surprised how quickly we have worked through it once started.

Grinding your own mirror is a massive achievement, nice one. I don't think I have the patience I'm afraid to do that but total hats off for doing it!!!

Will feel amazing when you get that first light knowing you have made it all from scratch.

It is a massive task and I've had a lot of help and encouragement from John Nichol to get it to where it is now.The next bit is polishing and figuring which is gonna be difficult at F3.7-8?

I have to make a start on my own scope as the one I'm building is for someone else but I do wish it was mine as I've all this to do again and then again!

So fingers crossed there isn't much tweaking needed.

Damian

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