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MCT Secondary Baffle Mod

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#51 Ed Holland

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Posted 09 August 2013 - 08:52 PM

Thanks Glenn,

Well you weren't too far off the mark :) - I'd expected this to require somewhat large measures, if not extreme. Next up will be an experimental hood, and some with/without photographic comparisons. I have an excellent and consistent subject for this - a house in the trees on a hill ~ 1 mile away. The "sheer curtain" effect produced by the stray light reaching the camera sensor is very pronounced in this situation.

I may find time for some tinkeing this weekend. If so, I'll pop back with any intersting findings, pictures etc.

Ed

#52 Asbytec

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Posted 09 August 2013 - 10:28 PM

Jmel, yes, that seems to be the artifact others have mentioned. I'm sure Glenn is correct, but it's curious the corrector causes that. I'm not sure how it does, though. It would appear to be an internal reflection, but others have had difficulty locking it down. Interesting.

Interestingly and informatively, not only could the stick be observed directly, but, in wiggling it slightly, I could observe its diffuse shadow in the light scattering down the baffle tube when looking on axis.

Ed, that's a fascinating observation. Is your baffle interior flocked? I have to chuckle at your measurements being close to Glenn's number tossing math approximation. I guess that shows how good he is without much effort. :lol:

Glenn, was thinking about the hood concept, even if its useless info. On axis light waves are parallel and normal to the axis, but the hood presents a real perspective aspect as viewed from the visual back. At same aperture, the distant hood opening "appears' smaller thereby narrowing the angle of direct lighting.

When viewed from a distance looking down the hood back toward the objective, the outer hood opening would appear to be larger than the corrector - even if they are the same diameter. If you could shift the view slightly off axis where the objective's edge just touched the hood's outer diameter, that angle would define (half) of the fully illuminated FOV.

But, both the hood and objective are the same diameter, so the affect is a very real perspective not unlike looking down a set of railroad tracks. I thought that was interesting.

#53 GlennLeDrew

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Posted 09 August 2013 - 11:37 PM

Norme,
To best grasp the hood's action on blocking non image-forming light, the perspective considered should be that from a *very great* distance out in front of the scope. Because our targets lie at optical infinity, where incoming light rays are parallel, we must picture the hood's aperture as it would be seen from the target's perspective. A hood opening equal to the working aperture would subtend the very same angular diameter.

And so only on axis would the full light bundle pass through the system; at any angle off axis the hood opening becomes displaced with respect to the working aperture, resulting in a small amount of aperture clipping on one side of the incoming light bundle.

With hood in place, if one looks through the front opening and cannot see *any* part of the focal surface within the eyepiece field stop or camera frame, it's doing its primary job of blocking direct sky light making its way to the image field. The real test comes by trying to sight *just* outside the edge of the secondary baffle (or the secondary itself, if the baffle is removed.) This is the path of the unwanted light which enters the system at the shallowest off-axis angle, and which comes closest to the optical axis at the focal surface. This determines the larger value for the hood length.

#54 GlennLeDrew

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Posted 09 August 2013 - 11:57 PM

I neglected to stress the following...

With the basic 'stick test', as formerly outlined, the result applies to a hood whose front opening equals the OTA's outside diameter. This results in a longer hood, which is generally undesirable. How to shorten it? Do this.

Measure the OTA O.D. Subtract the actual working aperture (as determined by the flashlight test for aperture), and divide by 2. This is the distance between the actual aperture's edge and the outside edge of the OTA. Attach a piece of paper or cardboard to the end of the stick so that it extends inward by an amount equal to this distance.

For example. Suppose the working aperture is 127mm and the OTA O.D. is 155mm. The length by which the inward extension protrudes is (155 - 127) / 2 = 14mm. A slip of paper taped to the font end of the stick should extend inward by 14mm. The result will be a useful reduction in hood length.

#55 Asbytec

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Posted 10 August 2013 - 01:13 AM

...we must picture the hood's aperture as it would be seen from the target's perspective. A hood opening equal to the working aperture would subtend the very same angular diameter.


Ah, okay, that makes sense. I was 'viewing' the hood from much closer allowing perspective to enter the equation. It does make sense from the objects 'optically infinite' distance both same apertures would subtend the same angle. However, at some point an on axis wave will be positioned just outside the hood prior to being truncated by the aperture (the viewpoint I took.) Off axis waves similarly so.

A greater angle would be subtended viewing closer to the hood and the same angle at infinity? But, if you work the geometry, there is an angle involved at all distances. That angle being constant subtends a greater width dimension at very large optically infinite distances. This is where you derived the trigonometric tangent function above to calculate the hood's length.

Still working through visualizing this...a same aperture hood 'closes in' on the secondary baffle 'apparent' diameter at pretty large distances (say, 400mm or so) to achieve 'perfect' FOV protection. A point where the apparent aperture of the hood does not allow one to see any sky between the secondary outer edge and the hood's inner diameter. At any angle...LOL (I chuckle because I'm confusing myself thinking if you cannot see the sky, how can any light enter the scope?)

Gaaaa! I'll stick to the 'stick' method. Translating between parallel waves, normal rays and perspective (angles) is confusing. Shorted a synapse or two...Lemme think on it.

For example. Suppose the working aperture is 127mm and the OTA O.D. is 155mm. The length by which the inward extension protrudes is (155 - 127) / 2 = 14mm. A slip of paper taped to the font end of the stick should extend inward by 14mm. The result will be a useful reduction in hood length.


Excellent point to fine tune the parameters.

#56 Asbytec

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Posted 10 August 2013 - 06:52 AM

Pictures are worth a 1000 words...if it's correct.

Attached Thumbnails

  • 6017126-hood.jpg


#57 GlennLeDrew

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Posted 10 August 2013 - 09:23 AM

Norme,
Your most recent diagram gets to the crux of the matter, whereby light 'sneaks' between the secondary baffle and the front aperture of the primary baffle, directly reaching the focal surface. This is the most egregious source of veiling glare to be found in any optical system, and must be fought tooth and nail!

#58 Asbytec

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Posted 10 August 2013 - 12:01 PM

Sure. Reflections probably run a close second.

The orange triangle in the diagram, I believe, is the trig you used above to determine hood length. From the diagram, it's clear only on axis is un-vignetted. The hood, in the process of restricting direct off axis glare, vignettes off axis at an angle that would normally have access to the corrector (though still not fully illuminated.)

#59 Jmel

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Posted 10 August 2013 - 01:26 PM

Jmel, yes, that seems to be the artifact others have mentioned. I'm sure Glenn is correct, but it's curious the corrector causes that. I'm not sure how it does, though. It would appear to be an internal reflection, but others have had difficulty locking it down. Interesting.


Funny thing is that it does it without the focal reducer in place, and I can see it ever so slightly even with an eyepiece (without a camera of course). I always wondered about it, and it really frustrates me. I read some about flocking the primary and secondary baffles, but opening the scope scared me. Then most people go, oh, it's a c5, it sucks, blah blah... but surely this is something that can be corrected.

#60 Asbytec

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Posted 10 August 2013 - 01:40 PM

Opening my scope was scary, that's understandable. I have a MCT, it's apparently a piece of junk with a small FOV and thermal issues, too. :lol: Then why do I love it? Cuz it does what I want it to do.

You can flock without tearing your scope apart. Just insert some flocking paper into the baffle from the visual back. You don't even need to glue it, just press it in nicely with a snug fit. Do the same with all the attachments, if needed.

I trust Glenn's diagnosis on the corrector causing it. He's probably right. In that case, there's not much you can do about it. But, remember, scopes are never perfect. They all are design trade offs and even a lowly C5 has advantages preferred by the market. If not, they wouldn't be on sale.

Next time someone blah blah's our C5, point out the design faults of theirs. Na...don't do that. Be nice and get some support from other SCT affectionados. And stay out of the refractor forum. :grin:

Sorry, in a giddy mood...

#61 GlennLeDrew

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Posted 10 August 2013 - 04:26 PM

Norme,
The orange zone in your previous sketch is essentially where direct sky light is visible in the absence of the hood. This is *best* (and so easily!) determined visually with the 'stick test.' Absolutely no need for number crunching here.

The trig I posted earlier had to do with the small angle corresponding to the edge of the FOV, to get a sense of the effect on edge-of-field illumination when the hood aperture equals the working aperture (which is minimal for narrow-field instruments like yours.) In your sketch this angle, being hardly larger than a degree at most, would be shallow indeed (which should reinforce the very inconsequential illumination fall-off at the field edge.)

If I come across as repetitive about the inconsequentiality of off-axis fall-off in illumination when the hood aperture equals the working aperture, that's because I get the sense that you still needlessly worry about it. :grin:


Jmel,
The Schmidt ghost, typically manifesting as a much defocused (and sometimes distorted) star exactly opposite the field center from its source, is caused as follows. The image of a bright star is focused somewhere on the sensor. This intense point becomes a source of light which is reflected and fairly well collimated by the mirrors and front corrector, forming an image back on the detector, opposite and equidistant from the field center.

If you placed a tiny penlight at the focus and shone it roughly up into the scope, you would form an image of the light beside itself, the position of the image always being opposite the field center from the light.

The presence of auxiliary optics, such as a reducer, does modify the ghost image, but the system in focus will always work to collimate and place a ghost image opposite the source.

#62 Asbytec

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Posted 10 August 2013 - 09:51 PM

Glenn, you don't come across as repetitive. I am just a perfectionist. :grin: No, I'm not worried about low single digit fall off, but would like to get as much full illumination as easily possible.

For my own project, I think it's there and the secondary plus flocking will be sufficient and may show some improvement. Much of this hood study is out of interest and laying some foundation for Ed.

Glenn, is the ghost image, then, an artifact of the 'relatively' flat glass corrector surface? The corrector, of course, does have a curve, but it's not nearly as curved as a MCT meniscus. I don't see ghost images.

Anyway, those ghosts seem to have been a mystery in some recent posts. Solved? Interesting. Are they limited to largely off axis or just more noticeable off axis?

In any case, it's an internal reflection and not off topic, in my view. I was hoping we might address and solve it. Seems it has been solved.

#63 Ed Holland

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Posted 11 August 2013 - 06:22 PM

OK, I finally had time for some fun today, after starting the promised cabinetry work for Mrs H, and completing repair of one of our magneplanar loudspeakers....

What really is obvious on this scope is that, regardless of stray light admission, the internal surface of the baffle tube really does its best to ensure it reaches the focal plane. So my first stop was to experiment with flocking. With no "professional" materials to hand, I was forced to improvise. Thinking about rough surface texture and dark colouring I made a couple of quick experiments, and settled on 100 grit sandpaper, sprayed with flat black Rustoleum. This was allowed to dry thoroughly in the sun, then cut, rolled and fitted inside the tube. I also already had a long dew shield, constructed from card.

The home made flocking definitely makes a visible reduction in scattered stray light - the baffle tube looks much darker in an A - B comparison. Adding the hood, which is not quite the required length calculated in my earlier post, does seem to offer further improvements.

A quick photo test confirms the (hoped for) boost in contrast. I chose two subjects and photographed them with the unmodified scope, with flocking, and hood plus flocking. I'll try and post some images later

#64 Jmel

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Posted 11 August 2013 - 08:20 PM

Awesome, I eagerly anticipate your results.

#65 Asbytec

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Posted 11 August 2013 - 11:00 PM

:goodjob:

Yes, I find flocking the baffle to cut out a bunch of light bouncing around into the visual back. Looking up the visual back toward the moon, that is apparent. In dark skies, though, it's really hard to tell if there's much visual improvement. But, there has to be improvement. I'd think observing the moon or bright planets benefits most, as it seems observing bright landscapes. Observe and eliminate all those reflective surfaces from the baffle back to the eyepiece.

#66 Ed Holland

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Posted 12 August 2013 - 02:07 PM

Thanks Norme,

I'd never noticed any difficulty with stray light during night time use, but will be interested to try it now. However, in daytime conditions, reflections from the unflocked baffle tube are very strong - it is very poor at trapping stray light and is was this that really captured my attention, since the scope works beautifully with my camera. I need to get my test shots up onto Flikr so they can be seen here.

Also I want to experiment with a "full size hood" but am short of appropriate materials. A big roll of black foam would be ideal, with velcro to join the edges - it could roll up in the scope bag and double as protection for storage & transport, and dewshield :)

#67 Asbytec

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Posted 12 August 2013 - 10:05 PM

Ed, I noticed the same glare affect observing the moon directly up the visual back which motivated me to flock the baffle. I made a dew shield from similar material and velcro. (Added padding...LOL, hey!)

Keep us posted, I'm interested in your study.

#68 Ed Holland

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Posted 14 August 2013 - 01:31 PM

Ok, here are some images, taken today with different combinations of defence against stray light. Harry Potter might consider this "Defence against the Dark Arts" :)

All the pictures were taken at 1/500 sec, within a few minutes of each other. In this fascinating scene, the full sun was above the features by aproxiately 20°

Firstly, the unmodified OTA

Posted Image

#69 Ed Holland

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Posted 14 August 2013 - 01:33 PM

Now, the OTA with home-made painted 100 grit flocking:

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#70 Ed Holland

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Posted 14 August 2013 - 01:35 PM

Lastly, flocking and hood in combination. The hood is still somewhat short of the requirement estimated by the stick test, as posted earlier.

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#71 Ed Holland

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Posted 14 August 2013 - 01:57 PM

No post processing of the images was done, they are as delivered by the camera, save for the jpg conversion.

The contrast improvement brought about by reducing the baffle tube scattering is particularly significant. In fact, the difference was easily noticable in the viewfinder. Addition of the hood seems to bring about a further improvement, though it is of a much smaller magnitude. Of course, there is far less to be gained once the light trapping efficiency of the baffle tube is improved. However, I do want to experiment with a hood of sufficient length that the direct light path from corrector to baffle tube is blocked.

I think these tests are a good endorsement of the methods we have discussed, particularly when daytime photography is a consideration. I had in mind using this photgraphic setup for bird photography, particularly at coastal/baylands environments. In these locations, there is typically wide open sky, bright backgrounds, the angle of the Sun cannot be predicted - all factors which could exacerbate the stray light problem and destroy contrast. Now a reasonable solution is at hand to improve performance.

The final question is: do I buy proper flocking material, or stick with the Heath Robinson approach (Substitute Rube Goldberg for American readers)

Thanks for the inspiration, Norme :)

#72 GlennLeDrew

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Posted 14 August 2013 - 05:46 PM

A most illuminating series! Or should I say darkening?

The painted sandpaper seems to be very effective.

I wonder how much of a crescent of direct sky light is slipping past your not-quite-long-enough hood?

It's worth pointing out this source of unwanted light illuminates only outside a certain radius from the optical axis. Installing first a short hood, and then ever longer, the outer part of illumination is trimmed back, resulting in an ever narrowing annulus on the focal surface. The inner edge of the annulus, which does not move, is the last to go.

The current question is this. Does this annulus lie outside your sensor's FOV, or somewhere within?

If within, its effect will be apparent only when the annulus falls on a darker part of the scene. Spilled skylight falling on sky will hardly impair contrast. The ideal test has a dark target which *just* fully fills the field and which is surrounded by a large area of rather brighter light (sky), extensive enough to ensure direct illumination sneaking past the edge of the secondary baffle.

#73 Ed Holland

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Posted 14 August 2013 - 06:26 PM

Thanks Glenn, and apologies, since I intended to credit you with inspiration along with Norme, but got carried away & hit "submit"

I was musing along similar lines regarding an ideal dark target as a test subject - as you wrote, to fill the FOV, but allow for some "worst case" illumination outside it, e.g. daytime sky. I'd considered a can, or tube blocked at one end, blackened inside, and with the open end facing the imaging system - should be a pretty good light trap. My chief difficulty was in arranging such a target in a manner to facilitate tests, especially given the long-ish minimum focus distance presented by the scope/camera combination. The dark glazed insulators of our pole transformer made an acceptable substitute, at least for simple verification.

Cheers

Ed

#74 Asbytec

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Posted 14 August 2013 - 09:44 PM

Ed, I agree, those images seem to be telling and very interesting. Amazing improvement with flocking. Makes me feel even better.

Glenn, interesting test set up, maybe simulating observing near the moon. That does seem like a good, real world test. On dark sky background, it's always a question I had considering what amount of light coming from an already dark sky to spoil image contrast. Still, it's doesn't hurt to flock that away, anyway, even if it's just a feel good improvement. It feels good to have the optical path dark, and it might even help DSO observing as Dick Parker noted in his Cass baffle design.

He commented, "The problem was that extended objects, like galaxies and nebulae, got dimmer with increasing distance from the center toward the edge of the FOV, and the sky brightened also with increasing distance toward the edge of the FOV. Not a lot. You would not notice this looking at planets or even bright globular clusters, but with low surface brightness objects, it was a noticeable issue. Stephen’s Quintet, for example disappeared when not in the center of the field."

http://mirrorworksho...affleStory.html

I see Glenn has been around this block before...LOL
http://www.cloudynig...5800932/Main...

Rock on, Ed. :)

#75 Ed Holland

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Posted 14 August 2013 - 11:03 PM

That article by Dick Parker is excellent. It really explains how the baffles are intended to work. Thank you for bringing it to my attention :)


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