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Old 25-07-2019, 07:43 PM
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xelasnave
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Making a baffled dew tube.

OK this is ruff but I hope you get the idea.

The idea is to baffle a dew tube to restrict stray light as well as help with a dew problem...but for me it is all about restricting stray light..a bright star or a street lights though not in he field of view sends light all around ...this thing minimises the stray light.

Hubble use this principle.

I had the idea and made one but was happy to later find Hubble was into the same idea...If they use it there must be something to it ...right?

I hope the pictures give you some idea but please note this is not a real one but some paper cutouts to help understand how I make them..I can't show a photo of the one I use on the 80mm cause it is up North and I am in Sydney.

So a cardboard tube with disk's either end one housing a smaller cylinder that fit your scope.

I place rubber strips in this one to have it fit well.

The baffles are created by folding a strip of cardboard and cutting small triangles out so it can fold in and form a ring and the uncut side you glue to the wall of the large cylinder.


I make them from cardboard and then layer it all up using flour water and salt and additional sheets of paper to make it strong..you can add resin to make it really strong but paint and lots of paper layers usually works..put in as few baffled say one each three inches..paint with black paint ...you can add sawdust of course and more paint.

Sorry this is so scant but I hope you get the idea...you can of course cut out circles for the baffled but you use heaps of sheets of cardboard...also you can just cut semi circles and add them in but I find the triangle thing works well..you can just cut lines but getting them organised can be a pain.
Any questions?
Alex
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  #2  
Old 25-07-2019, 08:14 PM
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Notice the baffling and how far it extends.
Alex
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  #3  
Old 26-07-2019, 07:45 AM
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Here is a shot showing the dew tube on the eighty mm.
Alex
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Old 26-07-2019, 10:35 AM
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mental4astro (Alexander)
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Alex,

Good idea. It is something I've been doing with a couple of dew shields I have made for some of my own scopes.

However, I do have a concern about the quality of the edge of your baffle rings. They really should be a single smooth, round edge, not straight edges. The straight edges will produce diffraction patterns that will appear as scatter or brightening of the field, or worse still as spikes coming off the stars. This is the problem with the straight vanes of Newtonians and some Cassegrain designs, such as RC's and Dall-Kirkham's. The use of curved vanes eliminates these spikes.

If you look inside refractors and some Maksutovs, you will see the same baffle rings you suggest, but these are all very smooth, regular and perfectly round. It is not by accident but by design for a reason.

Making smooth, regular round baffle rings is a bugger of a thing to make.

Another trick that helps in the same way as baffle rings is coating the inside of the dew shield with course sand before painting it with matt black. Very effective too.

The pics I've attached are of a dew shield I made, with a pair of baffle rings and sand.

Alex.
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Old 26-07-2019, 11:23 AM
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Yes you are correct and I am glad you raised this.

I should have covered that aspect but simply forgot in my rush to put up the thread.

Mine could be a little better in the unit I am using but I did round things up.

I plan making a much better one that will incorporate a light box..it's on the to-do list..I bought the lights many months ago...I started one for the eight inch that was not perfect and I destroyed it...one of my perfection is only just good enough moments...

What I have been considering is another system which involves having the rings printed at a print shop but I have yet to price that approach.

What would be nice is aluminium disks with nice sharp edges and maybe down the track that could come to pass . . Maybe carbon fibre but I have not worked with carbon fibre as yet.. should not be too hard but again so much I have to do...I have the observatory to finish which is nearly there but two piers to construct as well..

Nevertheless I really believe the minimisation of stray light is just so important that in time I will probably get more professional with these dew tubes...

So you are a sand man and I favour saw dust (less weight) do you have a reason to use sand over saw dust is my question.

Further I am so glad to have an endorsement from someone as capable as your self.

I promote the idea so often and end up feeling no one thinks much of the idea...Having Hubble do it is good but having you do it is even better.

I would have thought that someone would be making them and selling them really.

Thanks for your input I greatly appreciate you doing so

Alex
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Old 26-07-2019, 11:39 AM
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I used storm pipe 140mm (Bunnings) for my dew shield.
It was machined to fit better on my 400mm f/2.8 Canon lens.
And painted with black spray inside.


As to baffle (aperture actually, to get f/4), I used black plastic foil for folders (OfficeWorks). I cut it using tool like this one.
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Old 26-07-2019, 12:01 PM
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I like that tool. Thanks for pointing me in a better direction.
Alex
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Old 26-07-2019, 01:22 PM
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mental4astro (Alexander)
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Quote:
Originally Posted by xelasnave View Post
do you have a reason to use sand over saw dust
Simple, really, it's what I had at hand

Weight saving with using sawdust? Minimal really, and insignificant when looking at the overall weight of the dewshield alone and where it is going on the scope.

The tool Bojan mentions is great for cutting Laminate materials to make these baffle rings!

Another benefit of these baffle rings is they add rigidity and support to the tube they sit inside of.

I have a second dewshield that I use on both my SCT and as dewshield extension on the insulative dewshield I made for my Mak. This one though had just the one baffle ring, but does have sand behind the ring. The first pic below shows this second dewshield extended, the second pic shows it retracted back over the Coreflute insulation/dewshield around the Mak.

Alex.

PS, the third pic is of how I finished that first dewshield I made

Alex.
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Old 26-07-2019, 02:27 PM
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Re weight saving...yes I made that statement without comparing the two obviously I need to get more data before I make any assumptions.

I have saved a nice pile of sawdust which I plan to add to some clay that I am preparing...as certain African tribes use such an approach to make pots that they merely "fire" in a camp fire...I want to see what results I can achieve...what will I do with it..make a set of individual chess pieces and a crucible to melt bronze ..I know what else would one do with a good batch of clay.

Weight saving was a fanatical pursuit in my racing days...I would drill out even the brake shoes (we did not have disk's brakes back then)...my entire front brake assembly ended up weighing less than the inner tube...and man could you notice the difference

I certainly like your art work thanks for sharing.
Alex
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Old 26-07-2019, 04:02 PM
JA
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Hi guys,

An even better improvement in contrast could be had by using a series of rectangular apertures for the light baffles , sized so as to be slightly larger than the image and just big enough not to crop it, but still in the required proportion as determined by the aspect ratio, typically 3:2 for most current dslrs or even 4:3 or 1:1 for some cameras.

For a 3:2 aspect ratio sensor there would be close to a 42% reduction in stray (unusable to the image) light entering the optical system. Of course the edges of these baffles close to the edge of the image should be sufficiently far away so as not to generate any diffraction effects on the edges. This could be achieved by fine tuning.

One could even use a crude barn door arrangement (I wouldn't) to limit extraneous light.

Best
JA
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Old 26-07-2019, 05:16 PM
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Greetings JA....That sounds most interesting. I am expecting someone to say that it can not work but on the face of it you have my attention.

Have you tried this?

I suppose one could cut out a rectangle to go at the end of the tube (like the one I already use) and trial it.

I am getting very excited when you say 42% ...42 is of course the magic number.
Alex
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