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Size of light baffle for a dob

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#1 avarakin

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Posted 21 January 2021 - 10:04 PM

I finally bought a 48x96 roll of kydex and getting ready to make a light baffle for my 14.7" dob.

Question is - how large it needs to be. 

After some thinking, I decided to go with the following approach for figuring this out:

Take a laser and shine it into focuser from different points and observe whether the straight beam of light passes through the eyepiece.

If it passes from some point, then that point needs to be protected, if not, then it can stay unprotected.  

Does this approach make sense?

Another approach is to remove eyepiece, do similar test and make sure that the beam does not hit the eyepiece at all, but I think this is overkill.

Any other suggestions?

 

Alex


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#2 Augustus

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Posted 21 January 2021 - 10:06 PM

I would just make it go around the entire UTA, no harm in doing so


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#3 avarakin

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Posted 21 January 2021 - 10:09 PM

Agreed, but I am also getting light from above and below UTA, so I need baffle to stick above and below of UTA



#4 Oberon

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Posted 22 January 2021 - 07:39 AM

I finally bought a 48x96 roll of kydex and getting ready to make a light baffle for my 14.7" dob.

Question is - how large it needs to be. 

After some thinking, I decided to go with the following approach for figuring this out:

Take a laser and shine it into focuser from different points and observe whether the straight beam of light passes through the eyepiece.

If it passes from some point, then that point needs to be protected, if not, then it can stay unprotected.  

Does this approach make sense?

Another approach is to remove eyepiece, do similar test and make sure that the beam does not hit the eyepiece at all, but I think this is overkill.

Any other suggestions?

 

Alex

Put your eyepiece or CC in and build a big enough baffle so that you cannot see glass from behind the focuser no matter from what angle you look. 
 

Conveniently for Merope I found that the right sized baffle was the same diameter as the UTA, meaning I could make the baffle do double service as a protective cover when not in use. But I have a deep focuser, so maybe that helps.
 

gallery_217007_4913_71994.jpg

 


Edited by Oberon, 22 January 2021 - 07:40 AM.


#5 Dick Jacobson

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Posted 22 January 2021 - 07:51 AM

I just used an ordinary flashlight, moved it to the side until the light no longer flooded the eyepiece.



#6 starcanoe

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Posted 22 January 2021 - 08:28 AM

Not trying to get the focuser as low as possible helps a lot in this regard...



#7 TOMDEY

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Posted 22 January 2021 - 11:13 AM

Just putting your eye at the center of the draw-tube, at the focal plane, and looking in will indicate where stray specular light can come from. Anything bright outside of the image of the primary mirror is a "sneak path" that will need to be baffled out.    Tom



#8 starcanoe

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Posted 23 January 2021 - 09:44 AM

Tom...

 

Doesn't he need to place his eye EVERYWHERE in the focal plane check for stray light?....and for that matter I get a bit further INWARDS of the focal plane to do the test as well as make whatever baffle size was indicated a bit bigger still just to play it safe.



#9 MitchAlsup

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Posted 23 January 2021 - 06:30 PM

Consider the following drawing::

 

Paracorr-top.jpg

 

a) There is a line drawn from the edge of the light baffle at the righ side,

b) that just passes through the entrance aperture of the Paracorr 2

c) that converges at the orignnal prime focal plane opposite that of the baffle edge.

 

These lines are drawn from both the top (to the bottom of the focal plane) and from the bottom (to the top of the focal plane)

 

Do the same drawing looking done the optical path instead of across the optical path and this will give you the elipse you need to cut the baffle to. In practice the elipse is nearly circular and can be cut so.

 

{WHile the drawing was originally drawn to illustrate the vignetting of the focal plane by the (minimal) secondary--the baffling characteristics are well shown.}


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#10 avarakin

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Posted 24 January 2021 - 11:34 PM

I tried several methods, described in the thread:

1. Laser, as I described in the beginning

2. Observing flashlight from eyepiece

3. putting your eye at the center of the draw-tube, at the focal plane, and looking in will indicate where stray specular light can come from
4. build a big enough baffle so that you cannot see glass exit pupil of eyepiece from behind the focuser no matter from what angle you look.

 

 

I don't think method 3 is accurate as it does not account for optical properties of eyepiece. There is also a lot of variability in terms for positioning the eye in at the center of draw tube

Methods 1,2,4 produced about the same result.

Method 2 was a little difficult to do during daylight due to scattered light.

Method 4 turned out to be the easiest, followed by method 1.

 

Alex


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#11 starcanoe

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Posted 25 January 2021 - 08:51 AM

One thing to consider....there is the minimum sized baffle that just doesn't allow any direct stray light to hit anywhere on the focal plane (and AT the focal plane)...now since most eyepieces project inside the focal plane...if you test at the closest any eyepiece part gets inward of the focal plane that is a more conservative sized baffle (and it just plain lets less total stray light into the focuser tube).



#12 MitchAlsup

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Posted 25 January 2021 - 09:37 PM

One thing to consider....there is the minimum sized baffle that just doesn't allow any direct stray light to hit anywhere on the focal plane (and AT the focal plane)...now since most eyepieces project inside the focal plane..

And this was illustrated above in my figure !!

 

Now, it occurs to me that once one defines the maximal stray-light-free-field and uses a Paracorr 2, the only other dimension needed is the distance from the light-entering-end of the Paracorr to the shield itself ! Everything else is simply linear math.

 

For example::

 

Let us postulate that we want a 46mm focal filed free from stray light (after Paracorr). Before the Paracorr magnification and change in focal plane position, the size of this field is 46/1.15 = 40mm which is 20mm (0.787") off axis.

 

Using the same figure as above:: the position of this uncorrected focal plane is 3" + 9mm = 3.354" from the light-entering-end of the Paracorr 2. The distance from the Paracorr to the light shield is 21.2" for a total of 24.554"

 

The aperture of the light entering end of the Paracorr 2 is 46mm (= 1.811"); so now we have a complete piece of geometry.

 

The "run" is 24,554 and the "rise" is 0.787 + 0.9055 across the Paracorr distance of 3.354 distance

and we are looking for the rise if the distance were 24.554 !

 

1.6925 = 0.787 + 0.9055

12.390 = 1.6925/3.354*24.554

and all we have to do is subtrace the originating offset (0.787) and we have the half diamete of the light shield.

11.60 = 12.390 - 0.787

so the light shield needs to be 23.20" in diameter.

 

You may not need a 46mm illuminated FoV (only the 41 PanOptic has a field stop this large (reduce as desired) and in my case since this is an F/3 the field stop of the 21E is only 36.6mm allowing the slight shield diameter to shrink to 21.1"


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#13 starcanoe

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Posted 26 January 2021 - 12:52 PM

And this was illustrated above in my figure !!

 

 

 

 

Sorry...did not mean to imply that is wasn't (though that diagram is a bit visually busy).

 

I was pointing out what was in your graphic as the first point so that I could then move onto the second point....that after making sure you don't allow any direct stray light to reach any point in the focal plane the next step better is to make sure no direct stray light can hit the interior sides of ANY part of the eyepiece barrel (or in other words the interior lower part of the eyepiece barrel that lies between the last optical component the secondary. And if you can well flock any part of the focuser tube that is not occupied by the eyepiece barrel even better.

 

IMO the best is when no direct stray light can reach ANY part of the interior focuser tube....but thats almost impossible when trying for the lowest possible focuser/focal plane. I even suspect that most larger scopes would give better images in general if "you" went up to next sized diagonal and had a taller focuser that you baffle in such a manner.

 

Oh...and very nice example and calculation you gave there ! Very helpful for somebody try to solve this with pencil and paper rather than eyeball.


Edited by starcanoe, 26 January 2021 - 12:55 PM.

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#14 Mirzam

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Posted 26 January 2021 - 01:15 PM

If you have clearance (i. e., without causing light path intrusion), you can add a snout on the end of the Paracorr.  This will help limit off-axis light and reduce the size of the requisite baffle opposite the focuser. 

JimC


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#15 MitchAlsup

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Posted 26 January 2021 - 03:59 PM

LightShield.jpg

 

The above is a de-cluttered version of the previous drawing--sized to the dimensions on my 13" F/3, illustrating how to setup the calculation(s).

 

One takes the field stop diameter of the EP and reduces this by the magnification of the Paracorr (1.15×) and moves it inwards by 46mm, leaving it 9mm above the exit lens of the paracorr 2.

 

The rest is just straight lines, and simple algebraic "math"

 

Enjoy.


Edited by MitchAlsup, 26 January 2021 - 04:01 PM.

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#16 MitchAlsup

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Posted 26 January 2021 - 05:09 PM

The problem with such baffles is 1. They are not perfectly black, and with no shadow on them still reflect lots of light, and 2. They catch wind like a sail more efficiently than a short tube does.

On a non windy night at a dark site, I agree they are better than nothing. And even a short tube without a shroud does not caste a shadow much, with dew shields bring recommended.

The thread is about how big the baffles need to be, not about all their deficiencies.


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#17 Oberon

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Posted 27 January 2021 - 02:27 AM

Just a note for Mitch that your drawing permits unbaffled light into the Paracorr (although not directly into the pupil). My preference is to prevent any light entering the first objective in the eyepiece train, to avoid scattering within the eyepiece or CC, which requires a slightly larger baffle, and hence my method described as...

”...build a big enough baffle so that you cannot see glass from behind the focuser no matter from what angle you look.”



#18 starcanoe

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Posted 27 January 2021 - 08:30 AM

Thanks for the more obvious drawing there Mitch...will be very helpful for others in the future.

 

Yes, Oberon...you are more inline with my school of thought. Though personally I would more aim for no direct rays hitting anywhere inside the focuser tube (pretty much impossible with a low profile focuser). 

 

I have a 12 inch F5 mirror...it most very likely ain't no Zambutto....I could push for as small a secondary mirror as possible...but this is a deep sky scope...so I'll go up a size or maybe even two from smallest possible secondary so I can do some really good baffling.

 

I just thought of an easy experiment to try. Take your focuser (or even just a tube with the same dimensions of the focuser. Put your eyepiece/paracorr/barlow in there. Make your baffle.  Aim "focuser" towards said baffle at the...same distance as it would be in scope...look through eyepiece at baffle at your typical observing site....what you see is how much light that will be added to your view in the telescope.

 

People often talk about how great the views are in unobstructed designs or refractors. Besides planetary/moon views....I suspect much of the improvement is how well and easily you can baffle those designs compared to a newt and especially a newt with minimal baffling.




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