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Refractor baffles for several upcoming builds

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#1 PaulEK

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Posted 28 January 2019 - 05:52 PM

It's been a while since I've put together a refractor, and I'm having trouble finding information on baffles. Can someone point me to a description of how to plan them? I had a document saved on an old computer, but even that is gone.

 

Also, one of the four scopes I'm making is a 4-inch f/4.5. Would baffles be worth having in such a short tube? Heck, for all I know, at least one might be more important in such a short tube than in a longer scope.

 

By the way, the other scopes I'm putting together are a 4-inch f/7.8, and two 90mm f/10s.

 

Last night, after spending much of yesterday making the tools needed to make the tube for the 4-inch f/4.5, I was able to cut and mill the tube. The material came from a chunk of 4-inch PVC sewer pipe ($5 at Menard's). I'm getting metal for the others, but this was a good fit for the cell I have.

 

 



#2 plyscope

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Posted 28 January 2019 - 07:42 PM

Here you go.

 

http://www.berfield.com/baffles.html


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#3 PaulEK

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Posted 28 January 2019 - 08:01 PM

Great! That's the one I was looking for.

 

My tube on the 4-inch f/4.5 is only going to be 6 inches long (!); I'll draw a plot of the lines and see where it gets me for baffles. I'm thinking there will only be one.



#4 csrlice12

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Posted 28 January 2019 - 08:59 PM

My 81mm f7.7 has six baffles....maybe a bit overkill.



#5 figurate

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Posted 30 January 2019 - 07:38 AM

I use fewer baffles as a rule and try to make them conical in order to cut down on any reflections back into the system from the baffles themselves (which, although black, are usually smooth, flat surfaces capable of some amount of reflection). I also tried some schemes in the past to create less-reflective tube walls using slits painstakingly cut into black foam sheeting that was then snugly fitted into the tube assembly, similar to what is mentioned in the linked article, and that was a tricky operation. Newtonians, however, can use a similar approach where baffles would be problematic.  

 

For refractors, that means I go straight to the kitchen wares department at Target with a ruler, or I use a funnel from an auto parts store and cut it down to size. Foam sheet tube inserts locate the baffle position on either side and keep it centered.



#6 oldtimer

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Posted 30 January 2019 - 11:35 AM

My 100mm F4.5 (Surplus lens objective) required no baffles as the 2" focuser draw tube was exactly in the right place.



#7 PaulEK

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Posted 30 January 2019 - 09:08 PM

figurate: I like your idea of using a funnel, but unfortunately, I won't be able to use foam in the tube (or at least not right in the front) since the inner diameter of the tube is just barely larger than the diameter of the lens. But I'll mull it over. I had planned to use 1/4 inch Baltic birch, possibly with a beveled opening. It would be held in place with dowels attached to the 3/4 inch Baltic birch ring that the focuser will be centered in.

 

oldtimer: I have the same lens, and have plotted the scope to also have the draw tube in the right place. It might be overkill, but I'd like to put in a baffle, just for the fun of making it. (And I really like your signature!)

 

Here's the PVC tube getting turned to fit into the lens cell. I had to make a crude tailstock for my minimill in order to hold it steady enough, making the mill work like a sort of lathe. It worked very well, and I was as close to being a machinist as I'm likely to get (which is not very close!).

 

lathe setup.jpg


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#8 Mike I. Jones

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Posted 31 January 2019 - 12:00 AM

This spreadsheet does the calculations Scott Berfield gives on his website.  User inputs are in BLUE.

 

I'm not convinced it's an optimum baffle solution, but it has the merit of simplicity.  It would be good to do some Monte Carlo stray light simulations to try different approaches.  Post your known quantities for your refractor lens and I can do that.

 

Mike

 

Attached File  Baffle calculator.xlsx   31.26KB   36 downloads

 


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#9 PaulEK

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Posted 31 January 2019 - 01:11 PM

The known quantities I have are a clear aperture of 100mm, and a focal length of 450mm. The first is very accurate; the second is close. I didn't really measure carefully when I had everything assembled on my makeshift test bed. I just put the lens, focuser, diagonal, and a few different eyepieces together, so I could get a length for the tube. I did then do a full-scale sketch, and the focal length came out to 450mm when I put in the measurements I'd made with the stuff strung together. If it's off, it's only by a few millimeters.

 

Do you need other measurements?

 

Thanks, by the way!



#10 Mike I. Jones

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Posted 31 January 2019 - 04:48 PM

The known quantities I have are a clear aperture of 100mm, and a focal length of 450mm. The first is very accurate; the second is close. I didn't really measure carefully when I had everything assembled on my makeshift test bed. I just put the lens, focuser, diagonal, and a few different eyepieces together, so I could get a length for the tube. I did then do a full-scale sketch, and the focal length came out to 450mm when I put in the measurements I'd made with the stuff strung together. If it's off, it's only by a few millimeters.

 

Do you need other measurements?

 

Thanks, by the way!

There is a difference between clear aperture and back aperture.  For an achromat the clear aperture is usually the ID of the front retainer ring.  The back aperture will be smaller, as light has already begun to converge by the time it reaches the last glass surface.  There could be a 2-3 mm difference in the two.

 

Also, is the focal length you mention the effective focal length (EFL), or the distance between the last glass surface and the image, called the back focal length (BFL)?  There could be 8-10 mm difference between the EFL and BFL, depending on how thick the lens is.  BFL can be directly measured with a tape measure.  EFL can be indirectly calculated from images of stars having known angular distances between them and counting pixels.

 

I can set up the stray light model using what you've already provided, but if you provide more accurate measurements I can refine the model.  I'll use Berfield's baffle plate locations; what will change will be the baffle plate ID's.



#11 PaulEK

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Posted 31 January 2019 - 07:41 PM

To get what you're asking for, I think it would be best to wait until I can set up the test bed and do some more measuring. But it's still mighty cold here (-33 F last night; -15 right now), so it will have to wait. We're supposed to be up into the 40s by Sunday, but of course it will also be clouding up. Sometimes I'm glad I like making these scopes more than looking through them, but sheesh!


Edited by PaulEK, 31 January 2019 - 07:43 PM.


#12 Mike I. Jones

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Posted 01 February 2019 - 07:14 PM

The Berfield baffle positions look OK to me, providing good stray light rejection at focus.  The last baffle might have to be moved forward and the hole in it enlarged a bit to make room for your focuser (drawtube shown in orange).  Three is enough for this small a scope.

 

The tube is 120mm ID and is perfectly absorbing, as are the three baffles.  What I found out doing Greg Shaffer's refractor was that the reflectivity of the inside of the tube doesn't really matter, as long as the baffles are really black.  The "Black 3.0" thread running concurrently with this one is a good discussion for blackening baffles.

 

These plots show the ray paths, with stray light in RED.  Sky background is assumed to be Lambertian (equal scattering in any direction).  You can see that of 5000 launch rays, only a few single rays survive to focus, whereas all the direct rays reach focus at any point in the focal plane.

 

100x450 refractor baffling ray paths.jpg



#13 Mike I. Jones

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Posted 01 February 2019 - 07:18 PM

This shows the stray light level at focus relative to a 6th magnitude star at the center of the FOV.  The upper curve is roughly equal to 17th magnitude/sq. arcsec (4.0E-8 w/cm² irradiance), a very bright sky background.  The lower curve shows the sky background at about 22nd mag/sq. arc-sec (4.0E-10 w/cm² irradiance).  You couldn't see a 6th mag star with your naked eye in 17th mag skies, but the refractor's extra light grasp should show it (if you can find it!).

 

100x450 refractor stray light wrt 6th mag star.jpg



#14 PaulEK

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Posted 01 February 2019 - 09:08 PM

Wow! I am in no way an optics expert, but those illustrations really make clear how much the baffles improve the image.

 

I checked the focal length of the lens tonight on the stars. As you said, Mike, the back focal length was the easiest to measure, and it turned to be 426mm. So, it was off more than I thought.

 

My issue with putting baffles in the tube is that, rather than an inner diameter of 120mm, it has one of 102mm. making one really thin for the front end might not be doable. And at the eyepiece end, I'll be constrained, of course, by the focuser and diagonal. I'm wondering what the benefit of just one baffle would be, placed about where the second one from the front is? This is going to be a scope used primarily for wide-field viewing, so I think I want a wide illuminated field (25mm? 30?)

 

I only paid $50 for the lens, and cannot spend too much on the whole thing I have a router jig that will allow me to cut circles in 1/16th-inch increments, and have very thin plywood that I can cut the apertures in, and then glue to a thicker piece with a bigger hole in it. Is there a way to figure out the placement of one baffle, within a 102mm tube, that has an aperture of, say, 3.375 inches? Or does this not make sense at all for effective blocking of stray light? I know that it won't be nearly as good as what's shown above, but will it be significantly better than just blackening the inside of the tube?

 

I'm grateful for the help -- and the learning! -- so far, and feel like a mooch asking for more. But I figure this information would be helpful to others, too.



#15 sunrag

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Posted 18 January 2020 - 04:58 PM

I made the baffles for my OTA before reading this forum, and realize I calculated the baffle Apertures incorrectly. I calculated them assuming that rays come to a point at the focal plane instead of coming to a ~ 20 mm disc. It is very difficult to undo the mistake now, but visually it looks ok. If I remove the eyepiece and view through the focuser, i see the full aperture of the objective. And I don’t see any of the tube side walls. So is this proof that there is no vignetting?
Thanks for any feedback.

#16 Chuck Hards

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Posted 18 January 2020 - 05:14 PM

I made the baffles for my OTA before reading this forum, and realize I calculated the baffle Apertures incorrectly. I calculated them assuming that rays come to a point at the focal plane instead of coming to a ~ 20 mm disc. It is very difficult to undo the mistake now, but visually it looks ok. If I remove the eyepiece and view through the focuser, i see the full aperture of the objective. And I don’t see any of the tube side walls. So is this proof that there is no vignetting?
Thanks for any feedback.

The primary image is a defineable circle, not a point.  If it's a point, you have essentially nothing that's fully illuminated.

 

Telescopes need to be baffled for a larger circle of 100% illumination, and that depends on personal choice based on use.   Some folks try and fully illuminate a diameter equal to the field lens diameter on their lowest-power eyepiece, for visual use, but you need not get that extreme.



#17 sunrag

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Posted 18 January 2020 - 11:08 PM

Yes, I understand that the baffle aperture calculation assumes a certain non-zero spot size, small spot-size for short FL eye-piece, and big spot-size for long FL eye-piece. But if i can see visually the full aperture of objective from a mid-FL image plane, should that mean there is no vignetting?

It is possible that i mis-calculated the position of the baffles too, and placed them closer to the objective than I intended (So hoping that the two errors cancelled each other). Just looking for a logical explanation why i see full aperture objective, when i am supposed to get some masking by the incorrectly cut baffles.

I will try to position my eye with a little radial offset around the center axis (say 10mm) a few degrees apart and check again. Worst case, i will just have to replace the last baffle (one closest to ep) as this will be contributing most to the ray blocking.

I also did the same visual check on a Meade 395 (90mm/F11) and noticed the same phenomenon.

Edited by sunrag, 18 January 2020 - 11:16 PM.


#18 Jeff B

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Posted 18 January 2020 - 11:44 PM

For decades I've used a simple Y =MX + B formula to calculate my baffle locations as I have fixed sized holes saws of of 6", 5", 4.5", 4.0", 3.75", 3.5", 3.0", 2.5" and smaller.   Depending on the aperture and focal ratios involved, I use 2-4 baffles.  I pay special attention to the area between the back of the lens and the first baffle as that's where most stray light comes from.  I typically use 1/16" or 1/8" modelers plywood.   

 

Here is the calculation from my latest 8" F9 refractor OTA as well as pictures of their construction.

 

I've also included some "before" and "after" shots from other builds using the same calculation method.  It may not be a fancy ray trace method but it's a very simple means which has given excellent results.   I try to shoot for a 1" fully illuminated FOV but I also look to see where the "choke" points are in the light path sans baffles, for example the inlet diameter to the focuser draw tube, the 2" diagonal inlet nose diameter, or the inlet aperture to my bino-viewers.  They can easily restrict the fully illuminated FOV.  

 

Jeff

 

Attached Thumbnails

  • Design Page 1.jpg
  • Design Page 3.jpg
  • Baffless.jpg
  • Baffled.jpg
  • Baffles Rear.jpg

Edited by Jeff B, 18 January 2020 - 11:48 PM.


#19 dogbiscuit

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Posted 19 January 2020 - 12:15 AM

I made the baffles for my OTA before reading this forum, and realize I calculated the baffle Apertures incorrectly. I calculated them assuming that rays come to a point at the focal plane instead of coming to a ~ 20 mm disc. It is very difficult to undo the mistake now, but visually it looks ok. If I remove the eyepiece and view through the focuser, i see the full aperture of the objective. And I don’t see any of the tube side walls. So is this proof that there is no vignetting?
Thanks for any feedback.
 

It is not proof there is no vignetting.

If done as you say, there is vignetting.

You should redo the baffles. 



#20 dogbiscuit

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Posted 19 January 2020 - 01:08 AM

Yes, I understand that the baffle aperture calculation assumes a certain non-zero spot size, small spot-size for short FL eye-piece, and big spot-size for long FL eye-piece. But if i can see visually the full aperture of objective from a mid-FL image plane, should that mean there is no vignetting?

What is mid-FL image plane?  There is only one focal plane, and it's location is fixed. The focal plane diameter of full illumination is a choice.

 

Drawing shows baffle sized for illuminating only a point on axis at the focal plane and rays from the edge of the lens to that point and rays to the edge of (let's say) a 20 mm diameter field.  The baffle vignettes the rays to the edge of the field.  Eyeball sees the edge of the lens.

baffleVignetting.jpg

Two (or more) reasons you can see the edge of the lens with baffles as you describe...

Light from all around the edge of the lens passes through the focal plane on the optical axis. If your eye is located on axis at the focal plane you will see the edge of the lens.  

 

The iris of your eye has size so your eye can be a little off axis and still see the edge of the lens all around.

 

If you only look the edge at one location around the lens at a time, you can be far off axis and by moving your eye around scan around to see all of the edge.  

 

You could misplace your eye inward from the focal plane to see the edge.

 

I recommend you recalculate baffle diameters and positions for your desired fully illuminated field diameter at the focal plane and redo the baffles.


Edited by dogbiscuit, 19 January 2020 - 01:14 AM.


#21 Bob4BVM

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Posted 19 January 2020 - 01:39 AM

All this got the gears turning here too.  I will need to look back & see how I calc'd the baffles for my 6"F5. I think I used a 1" IP, but not sure. Seem to remember using a baffle formula from Mel's website., maybe.

If necessary I will remake them, easy enough to do. Been a while since I've done any mods to that scope, probably it's feeling neglected  :)

CS
Bob



#22 MartinPond

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Posted 19 January 2020 - 02:48 AM

One thing to watch out for in commercial (mainly cheaper) scopes:

There can be light leakage around the outside of the irises

    that they don't plug.

A minimum of 2 irises tends to eliminate any concerns about that though.

 

Most ST80s and clones use steps instead.

That works really well, though there are 3 steps. 



#23 dogbiscuit

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Posted 19 January 2020 - 03:08 AM

 Look at post # 4 from Nils Olof Carlin at this link

 

https://www.cloudyni...le-setting-out/



#24 Jeff B

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Posted 19 January 2020 - 10:23 AM

Yes, I understand that the baffle aperture calculation assumes a certain non-zero spot size, small spot-size for short FL eye-piece, and big spot-size for long FL eye-piece. But if i can see visually the full aperture of objective from a mid-FL image plane, should that mean there is no vignetting?

It is possible that i mis-calculated the position of the baffles too, and placed them closer to the objective than I intended (So hoping that the two errors cancelled each other). Just looking for a logical explanation why i see full aperture objective, when i am supposed to get some masking by the incorrectly cut baffles.

I will try to position my eye with a little radial offset around the center axis (say 10mm) a few degrees apart and check again. Worst case, i will just have to replace the last baffle (one closest to ep) as this will be contributing most to the ray blocking.

I also did the same visual check on a Meade 395 (90mm/F11) and noticed the same phenomenon.

Just move them backwards in the tube if you can.  What scope is it?



#25 sunrag

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Posted 19 January 2020 - 10:52 AM

Yes I understand now. Thank you all for taking the time to explain.

The baffles are made from a thin plastic sheet (from Michael’s for $1.99 for a 22”x28”sheet) and glued to thick cardboard rings cut from a cardboard tube. Then the cardboard rings are held by a few drops of glue inside the OTA tube.

I will try to break the glue bond and push the baffles to the correct position. If that is successful, it will take care of the vignetting.

The refractors i am working on are (1) 80mm/F11 (2) 90mm/F10 (3) 106mm/F8.5 (4) 152mm/F7.8. All except the 106mm/F8.5 have the baffles already glued in position, so will need rework.

My OTA tubes are all made from standard PVC tubes and fittings, as i lack machining tools.

Edited by sunrag, 19 January 2020 - 10:53 AM.



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