21 replies to this topic

[Paul C-I]

Hi Folks,

 

Need a little reassurance. Have an 8" F12 refractor lens. When drawing a full size light cone for baffle measuring of baffle aperture, should the cone go to a point at the focal length or please remind me of an equation I'm sure I used a few years ago when constructing a smaller refractor. (image scale at focal length?)

 

All of your posts / advice greatly appreciated in advance.

 

Thanks

Paul

[KevinS]

There may be a formula out there but I do this:

 

Replace the "point" in your scenario with a "disc" of sufficient width to match the eyepiece field stop or sensor you plan to use. (Around 30mm should cover most amateur endeavors) If the light cone edges fall inside that disc width at the focal length, you will not have full illumination of it and have vignetting.

 

There are some good guides online including in this forum (somewhere) but I find it most useful to draw a full scale cross section on paper.

Edited by KevinS, 06 March 2015 - 08:18 AM.

[Mike I. Jones]

 

Here's the formula for calculating the inside diameter of an internal baffle at any point down the light cone to the image plane.  It doesn't say where to put the baffles along the tube, though.  There is a wide variety of positions that will work well.  One at the BFL midpoint is a good start.  Three equally spaced baffles along the BFL is better.  They should be the flattest black possible, and the ID's should ideally be tapered to sharp points, rather than just drilled perpendicularly to the baffle plate.

Mike

 

[Nils Olof Carlin]

The idea behind baffling a refractor is to prevent strong, grazing reflections in the tube wall from reaching the focal plane. The top half shows the height and placing of baffles that will completely hide the wall from all points in the focal plane. The lower half shows how to place baffles so that each baffle will hide the part of the tube wall not in complete shadow from the baffle before it - you can save a baffle that way, perhaps. What the picture does not show is baffling of the focuser drawtube. You may consider flocking its inside.

Nils Olof

PS baffling the tube of a Newtonian is something totally different!

[GShaffer]

I used a combination of the following.....

 

1. I used the exact formula Mike Jones posted above for determining the size of the baffle opening at the point where I placed them.  Because he posted the same thing to my build thread quite some time ago 

 

2. I used the process at the following link to determine placement. http://www.berfield.com/baffles.html Except I modified the process by skipping the green line process and using the redline process only which depending on the FL of the scope could cause you to use more baffles.  I then modified it even further by moving the red line/tube intersect point to exactly half the distance from the opening in the baffle to the tube wall. Resulting in a total of 5 baffles in the 8" F16 build. I did this since I am leaving a small gap between the outside of the tube and the baffle (3/16") in hopes of mitigating tube currents having to spill over the baffle into the light path. I also didn't want any chance of light that passes thru that gap to not be stopped by the next baffle and within reason of course you cant have too many baffles

 

While I have not as yet tried it there seems to be a consensus in both professional and home theater crowds that this paint is the bomb when it comes to flat black "Roscoe Supersaturated Velour Black"...... I will be trying it shortly though......

[Sean Cunneen]

The light cone should not come to a point, you have to decide what your illuminated field will be. Normally I set the illuminated field to the field stop of the eyepiece I expect to use most or if you use cameras for AP, you set the diameter to cover the image sensor. You can the work out your baffle spacing from there. Remember to consider the focuser tube as often we are limited by the focuser barrel acting as a baffle.

[Paul C-I]

Kevin, Mike, Nils, Greg & Sean,

 

Thank you all for responding so quick. Overwhelmed buy your Responses. Will be very valuable for the last stages of putting my version of an open truss tube design to completion. (full build thread coming soon)

 

I seem to remember now that there may have been a formula to work out the image plane based on the focal length and objective size?

 

Thanks

Paul

[mikey cee]

Yeah I used about 1". A little too big never hurts. The baffles can do their job just as effectively whether your on the money or "way" too big. But too small and the light cone suffers.  Sally forth!     Mike

[Sean Cunneen]

1" is a good number to start with. If you are a Nagler fan or an Ethos fan, go with 1-1/2". 

[catboat]

If you could baffle only the forward half of the OTA or the rear half, which would you choose?  

 

I plan to build the OTA for an old 6” f/15 doublet.  For practical reasons, I’ll use two phenolic tubes, with one sliding into the other for compact storage and transport.  This means that only half of the extended OTA can be baffled.  The other half will have only flat paint inside.   
 

Would it make a difference whether I baffle the forward half or the rear half of the OTA?

Edited by catboat, 08 March 2015 - 05:28 AM.

[m. allan noah]

Many of the really cheap 60mm size scopes only have a single baffle right at the back, basically touching the inside of the focuser when it is fully racked in. They seem to work well enough.

 

allan

[dan_h]

If you could baffle only the forward half of the OTA or the rear half, which would you choose?  
 
I plan to build the OTA for an old 6” f/15 doublet.  For practical reasons, I’ll use two phenolic tubes, with one sliding into the other for compact storage and transport.  This means that only half of the extended OTA can be baffled.  The other half will have only flat paint inside.   
 
Would it make a difference whether I baffle the forward half or the rear half of the OTA?

I would baffle the front half. This is where off axis light gets in. If you can control it there, it won't get to the second half.

dan

[Bob4BVM]

baff.jpgThe idea behind baffling a refractor is to prevent strong, grazing reflections in the tube wall from reaching the focal plane. The top half shows the height and placing of baffles that will completely hide the wall from all points in the focal plane. The lower half shows how to place baffles so that each baffle will hide the part of the tube wall not in complete shadow from the baffle before it - you can save a baffle that way, perhaps. What the picture does not show is baffling of the focuser drawtube. You may consider flocking its inside.

Nils Olof

PS baffling the tube of a Newtonian is something totally different!

Thanks Nils for simplifying this with a diagram. The last thing I have to do I have to do on my 6"f5 RFT is the baffles. I just found this post, so I am on my way to the shop, thank you !

Bob

[ntd252]

baff.jpgThe idea behind baffling a refractor is to prevent strong, grazing reflections in the tube wall from reaching the focal plane. The top half shows the height and placing of baffles that will completely hide the wall from all points in the focal plane. The lower half shows how to place baffles so that each baffle will hide the part of the tube wall not in complete shadow from the baffle before it - you can save a baffle that way, perhaps. What the picture does not show is baffling of the focuser drawtube. You may consider flocking its inside.

Nils Olof

PS baffling the tube of a Newtonian is something totally different!

Hello! Your explanation just opens my mind!

I just have a small problem. If my telescope has objective put like this (photo), how do I identify the placement of the first baffle?

[Nils Olof Carlin]

I just have a small problem. If my telescope has objective put like this (photo), how do I identify the placement of the first baffle?

If the tube diameter and aperture are the same, this design of baffles breaks down. If you can't increase the tube diameter, you can apply flocking material to the first part of the tube. I imagine your best strategy would to design the baffles "backward" starting in front of the focuser, proceeding forward, and flocking only the foremost part. The forward end of the focuser drawtube will work as a baffle itself, and you could design forward from here.

Hope this helps you!

[ntd252]

To Nils: I dont really understand. Why does it break down in my situation?I just think the first baffle can be made randomly somewhere in the first part of the tube.
And I also dont recognize which line is the first in your suggestion. Can you talk about it a little more?

[MitchAlsup]

To Nils: I dont really understand. Why does it break down in my situation?I just think the first baffle can be made randomly somewhere in the first part of the tube.
And I also dont recognize which line is the first in your suggestion. Can you talk about it a little more?

 

If the tube is no larger than the aperture there is always a piece of the tube wall that is visible from the focalplane. You flock this part of the tube. Once the light cone has converged a bit, you can revert to the standard baffle placement method; You do not want to be able to see any part of the tube wall (Nils top 4-baffle) from the focal plane or you do not want to see any part of the tube wall which is illuminated by stray light (Nils bottom 3 baffle) from the focal plane.

[GlennLeDrew]

Note that if you intend to always use a light shield/dew cap, you should be able to reduce the baffle count--by at least one in any event.

 

Effectively shadowing/blocking from view the very front of the main tube just behind the objective is the hardest bit, especially if the tube I.D. is hardly larger than the objective clear aperture. And the faster the scope, and the larger the field stop/sensor, the more crowded together must be the first couple or few baffles behind the objective.

 

Remember, what you want is this. Sighting from the focus, and from the very outer edge of the largest TFoV you will cover, you must see *only* the shadowed back side of any baffle and, if any part of the tube wall can be seen, *only* that portion which is fully in the shadow cast by the baffle just ahead.

 

You must never be able to see 'over' a baffle and thence just beyond it some part of the tube wall illuminated by any light which can enter the objective from the steepest angle permitted by whatever opening lies ahead of the objective. This is why a light shield/dew cap is handy; the more forward opening restricts conical angle within which light can enter the objective.

 

Draw to scale the layout with the aim always in mind to never permit a sightline from your largest field edge which allows to directly see any part of the tube wall which might be illuminated by the farthest off-axis light that can light up that part of the tube. (Crikey, but I do put together rather long sentences!)

[ntd252]

Aha I got it!

I mostly understand the idea of how baffle works and where it should be put, just a little confusion in designing in real scale and calculating.
So if my forward end of the focuser drawtube can't be used as a baffle, that means I can choose the placement of the first baffle anywhere of the tube part near the objective and draw lines and flock?

[GlennLeDrew]

Yes, I start with the first baffle behind the objective. A ray drawn from the outer edge of the dewcap to the opposite side of the rear opening of the objective cell, thence to the inside wall of the main tube, locates the edge of the shadow formed immediately behind the objective cell. The longer the dewcap, and the larger the tube I.D. relative to the objective aperture, the farther back this shadow extends.

 

Your first baffle is placed so that as seen from the focus and from the opposite side of the FOV it just safely enough 'hides' the illuminated tube wall and thereby presents to view only the shadowed back side of the baffle and no more than some shadowed part of the tube wall.

[ckh]

Aren't baffles also required in the dew cap?

[GlennLeDrew]

Baffles in the dewcap are beneficial if some part of its inner surface can be seen from the focus. Otherwise, if no part of same can be seen from the focus, and if it's already deadened with flat black paint, any scatter from that surface will only very dimly illuminate the interior of the scope's forward tube, even if rather strong light falls upon the cap's interior.