BIG dob ready for first light

[dark knight]

Looking very nice so far Mark, hmm that last photo makes me think its time to wash the mirrors in my 200p as its not been used for ages!

[mdstuart]

You know that moment when you try new jeans and they are very tight but you can just get in them! Well that is how the mirror feels.

Top tip for future reference..make the box 1 inch bigger than you think you need..

Now the truss poles.

[mdstuart]

I think I will lose a mm of so of aperture around the mirror unless I make the hole bigger. The mirror is 20.25 inches! Oh well I will still have 20 inches of aperture 🤔

Let's see how it performs first then I can decide on whether to get the sandpaper out..😀 No not on the mirror on the wood!

Edited July 3, 2020 by mdstuart

[dark knight]

Oh no Mark what a shame, think id get the sandpaper out now while everything is in the component stage, be a shame to put it all together only to find it affects the overall performance in a negative way, all looks great though

[mdstuart]

4 hours ago, dark knight said:

Oh no Mark what a shame, think id get the sandpaper out now while everything is in the component stage, be a shame to put it all together only to find it affects the overall performance in a negative way, all looks great though

Thanks but the hole is only VERY slightly too small so I will go with it for now.

All of this is just a learning experience for the 25 inch 😉

Mark

[Piero]

22 hours ago, mdstuart said:

You know that moment when you try new jeans and they are very tight but you can just get in them! Well that is how the mirror feels.

Top tip for future reference..make the box 1 inch bigger than you think you need..

Now the truss poles.

 

22 hours ago, mdstuart said:

I think I will lose a mm of so of aperture around the mirror unless I make the hole bigger. The mirror is 20.25 inches! Oh well I will still have 20 inches of aperture 🤔

Let's see how it performs first then I can decide on whether to get the sandpaper out..😀 No not on the mirror on the wood!

 

 

Hi Mark,

apart from losing ~1 mm in aperture, it seems that the mirror almost touches the internal walls of your mirror box. Even if it does not, I feel there is not enough distance to guarantee adequate air circulation around the mirror. The internal dimensions of the mirror box should be 1 inch larger on each side at least. Also, the baffle ring structure seems to be almost the size of the mirror. This can potentially cause warm air coming from the mirror to move right on the light path.

A larger mirror box can allow you to build a more adequate lateral support than three strong side pins - which can easily cause a large amount of astigmatism particularly on a large mirror like yours. I've been there on my 12" and it was a nightmare..

I don't want to sound over-critical or dismissive, but I would re-do the mirror box if I were you. A bit time consuming, yes, but less time (and energy) consuming than having to redo the mirror box, rocker box, revise the tubes, revise the telescope balancing etc later. 

Edited July 4, 2020 by Piero

[mdstuart]

Piero

Thanks for the feedback. I will pause and have a rethink. Lots of wise comments.

Mark

[mapstar]

Pretty good efforts there Mark.

Spider Maybe distorting the upper tube as you can get a lot of pressure on them.

Aperture of the box can be made larger if it needs to be 

[mdstuart]

Thank you mapstar.

I think it is fair to say there will be some aspects of my scope that are not optimised. My top uta is 1cm too small, the secondary at 90mm is a bit large etc. 

It is however my first effort at scope making!

Mark

[mdstuart]

Apparently the 75% rays are going to be annoyed with me😉 according to NEWT.

....and the clips will introduce 7nm astigmatism I think 😱 re CRUXIS

Edited July 7, 2020 by mdstuart

[mdstuart]

On the plus side the secondary has not fallen off the holder👍....yet

[mdstuart]

We can make this thread a sticky with a title of read this before making your own telescope.

[saac]

Mark I wouldn't get too hung up on these  details at the moment - the view at the eyepiece is all that matters. Given that you are the stage you are at I would press on - as you say if do find it is compromised , which I doubt it will be, then you can always go back an modify from a position of knowledge. I think you have made fantastic progress and no doubt picked up some skills and confidence along the way .  Can't wait for your first light moment. 

Jim 

[saac]

Just a thought Mark what sort of weight do you have in the mirror box - is it going to be a comfortable lift?   My mirror stays in the box and the lift is just on the limit of being comfortable, it's surprising how quickly it gets heavy. 

Jim 

[mdstuart]

Jim

Thanks for the encouragement. 

The mirror is thick and heavy. That's why the clips at 120deg are not too much of an issue. I am planning on giving it wheelbarrow wheels.

It will certainly fit in a car now so a dark sky will outweigh any design issues.

Mark

[mapstar]

12 hours ago, mdstuart said:

Thank you mapstar.

I think it is fair to say there will be some aspects of my scope that are not optimised. My top uta is 1cm too small, the secondary at 90mm is a bit large etc. 

It is however my first effort at scope making!

Mark

Nobody can criticise anyone making a functional scope Mark and you're nearly there.

Lot's wouldn't even attempt it so well done upto now.

[Stu]

54 minutes ago, mapstar said:

Nobody can criticise anyone making a functional scope Mark and you're nearly there.

Lot's wouldn't even attempt it so well done upto now.

Absolutely! It’s a fine job Mark, very fine workmanship on the dovetails and it looks great. I’m sure it will perform very well and form a basis for future projects. 👍👍

[Surreydocker]

14 hours ago, mdstuart said:

On the plus side the secondary has not fallen off the holder👍....yet

Speaking as a drummer, I have mixed feelings about this picture but if the the project is a success, the sacrifice will have been worth it ! 

[Pixies]

17 minutes ago, Surreydocker said:

Speaking as a drummer, I have mixed feelings about this picture but if the the project is a success, the sacrifice will have been worth it ! 

I still think a 70s gold-glitter wrap would improve it!

[mdstuart]

The drum does look fantastic with the coats of varnish.

Mark

[mdstuart]

So the centre of gravity is 7.5 inches above the top of the mirror box.

But wait the bearings will bring the cog down. Oh my where is the center of gravity of a semi circle.... 4r/3pi. But wait I am going to cut out shapes to make the bearings lighter..my head hurts.🤔

Thinking of bearings 32 inches in diameter which is in the 1 to 1.8 times the mirror size..that's big..

Here is my high tech cad diagram. The bearing will be a semi circle but the cereal box was too small!

Such fun.

Mark

[mdstuart]

Bearing creation with a borrowed router😀

[mdstuart]

Rocker box taking shape. I managed to inset/sink the screw heads.

That router was something else. It was snowing wood! 

Definitely learning new skills and enjoying this stage of the build. It takes a lot of time though and the dark skies are returning which is giving me a sense of urgency.

 

[mdstuart]

Looking like a telescope now.

Just a few more bits and pieces to sort. Need to put the strut in place to support the bearings etc. 

Az motion seems ok and Alt will be better when balanced with the top cage!

I made some cut outs inside the bottom box to help the mirror to breath.

The bottom box is snug on the mirror box so I need some fine spacers to stop it scratching.

Might shorten the bearings a bit at some point as the whole length is not needed.

Still learning loads as I am going through this ready for the 25 inch 😉

Mark

[mdstuart]

Getting there.

A few more tweaks to the poles, some wheelbarrow handles and a strut for the bearing and maybe something to guide the alt az movement and then back to fuzzy hunting.