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Old 27-04-2022, 01:00 PM
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mura_gadi (Steve)
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thoughts on my baffle idea please.

Hello,

Will a reducing diameter tube as below help reduce stray light or will I end up bouncing more light around than I cancel?

This is for a 8"F9 newt btw.

Thanks
Steve

I am trying to focus on light dampening within the newt tube on my build. As baffles are so complicated or simply not worth doing, I needed a KISS work program.

Assuming it is easy to use "Newt for the Web"(nftw) to take a 230mm ID baffle intrusion at the focuser cage base. Record the baffle intrusion just up from the mirror face based on a smaller ID tube say 215mm. A few other baffles for each ID to plot a light path angle as well.

Make a plywood tube reducing in diameter from the 2XX mm ID at the sec. cage to the 2XX mm ID just above the mirror box as calculated from nftw.

Removing four triangles, with a little Pi for width to length of the triangles, from one end of the tube, you could set it fairly close to the suggested baffled optical path.

Also remove one layer of plywood in horizontal strips from tube. That would give me a hard 90 degree edge occasionally to block rather than bounce any remaining light. I was just going to have the groove wider than the depth and spaced less than the width of the groove.

All of this could be done while the board is laid out flat prior to painting, flocking etc then rolled into a tube. It seems fairly KISS friendly...

But with a reducing tube, will I bounce more light into the mirror than I can cancel with these ideas?
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Old 27-04-2022, 04:03 PM
glend (Glen)
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As someone who has built both Newts and Refractors, I have some experience I can pass on. Normally production Newt tubes are not baffled, but it is blackened. Exceptions would be Mak-Newts, which have a large corrector at the front of the tube.
Even very large custom Newts do not bother with baffles as they are usually open tube structures, and a shroud might be the only external cover.
Refractors are usually baffled, and there is optical science behind the spacing and diametre of the baffles. Simply, the use of Ray Diagrams easily illustrate the light path to the eye piece, and you can work out baffle diametre and placement from the diagram. Three or four baffles are plenty.

Sure there are obsessive amateur builders that try to baffle open newts, who will argue it improves contrast; however the nature of Newts with front obstruction by the secondary, your already giving away significant contrast. A good front light shield is a better option.

In a dark site environment, baffles will do nothing for a Newt, in my opinion. In the city with nearby light sources that may shine into the tube, yes it might help; however a good front light/dew shield device is likely to be more effective, and it reduces the risks of fogging secondary mirrors.
I am sure you will get contrary opinions, especially in the Cloudy Nights forum section on ATM building.

Last edited by glend; 27-04-2022 at 04:26 PM.
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Old 28-04-2022, 03:31 AM
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mura_gadi (Steve)
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Thanks Glen as someone who has built nothing, everything helps.

I have plans for a dew shield at least the diameter of the mirror in length, and the scope is being built with urban use in mind. Hence trying to grab all the 1%er's I can.

Looks like I can save some time on the cut away at least...
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Old 28-04-2022, 01:17 PM
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I think baffles are a good idea but also to add a dew tube like extension that is also baffled..I have done this to a couple of scopes although not running any at the moment as they all wore out..the dogs eat my first one..only paper and flour construct with structural paint. It goes beyond the scope (2.5 feet extention) and I believe they made a marked improvement but I made no tests to establish such or to quantify the effect..but I would say this..study the Hubble Space Telescope and note just how what I suggest appears on it...baffled way past the secondary position...I suspect there may be some merit in such an approach given that it is used by Hubble.

We don't notice the star light..just little dots..mmm yes that touch everything ...the more of the star light from the side you can restrict the better in my view...draw a diagram tracing light both with and without such a baffled extention tube..Folk with Newts dont seem to worry and I suspect that comes from their general contruction and yet in refractors you find baffles as part of the expected furniture...I don't know if it was right or wrong but I painted all the inside with flat black paint then sprinkled saw dust, let that dry and did that again...the problem is I did no before and after tests and me believing it was better may be simply be confirmation bias sort of thing..but I do think the difference was there and most noticeable.
Alex
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Old 28-04-2022, 08:24 PM
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OzEclipse (Joe Cali)
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None of my scopes have ever had baffles. Depends how much stray light you are talking about and how directional it is.

At f9, you can get away with a smaller tube but don't do too small or air flow convection currents with mess with your view and will continue through the night as the tube continues to cool. I'd recommend a normal tube diameter and just give the mirror some breathing space.

Extra length out front will help as will a single baffle at the front or maybe two, one at the front and one in front of the spider vanes.

If you have unidirectional strong light eg from a neighbour's outdoor light, a smaller tube won't help but a one sided tube hood will.
Joe
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Old 29-04-2022, 05:09 AM
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Hello,

I had thought about the baffles refractor V reflector and I am guessin' that the baffles are there to collect scatter caused by the lens in a refractor. So, not stray light per say but induced scatter by the front lens.

(Pretty much is due to costs, difficulty and being lazy. Baffling the mirror and the focuser cage components seems to be the extent of major gains as well).

Good point about airflow, it had been creeping into my design thoughts a bit, but the idea of easy baffles was all consuming apparently. Certainly tube flow should come first. So unless the baffles are removed from the apex of the tube to allow for airflow I think I'll go backwards.

I'm running NftW with the dew shield on, and had the tube at 230mm which is the same a GSO D8F6. With the dew shield set to 200mm in front of focuser, I am getting baffles from 224mm at the opening to 202mm at the mirror face.

gives me something to think about in smoothing at least... Thanks for the replies.

Last edited by mura_gadi; 01-05-2022 at 09:36 AM.
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Old 29-04-2022, 01:10 PM
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OzEclipse (Joe Cali)
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Quote:
Originally Posted by mura_gadi View Post
Hello,

I had thought about the baffles refractor V reflector and I am guessin' that the baffles are there to collect scatter caused by the lens in a refractor. So, not stray light per say but induced scatter by the front lens.

Good point about airflow, it had been creeping into my design thoughts a bit, but the idea of easy baffles was all consuming apparently. Certainly tube flow should come first. So unless the baffles are removed from the apex of the tube to allow for airflow I think I'll go backwards.

I'm running NftW with the dew shield on, and had the tube at 230mm which is the same a GSO D8F6. With the dew shield set to 200mm in front of focuser, I am getting baffles from 224mm at the opening to 202mm at the mirror face.

gives me something to think about in smoothing at least... Thanks for the replies.
You only need to make them from very thin aluminium sheet. You could get the ally CNC water jet cut or laser cut, two of the cheaper material cutting methods then black anodize them.

https://www.nexgen-laser.com/laser-cutting/

As I said in the last post, I've never had baffles so I can't say if they are necessary or of any benefit.
Joe
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