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MCT Secondary Baffle Mod

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#26 Asbytec

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Posted 02 August 2013 - 01:45 AM

That dark "edge" is not the primary's edge...

Or maybe it is... So hard to tell, hard to illuminate that area to get a good look. Here's an image from the visual back (no diagonal.)

On further inspection, I believe that "hard edge" is the primary. The foam ring is just outside the primary's edge, it appears, anyway. Okay...thinking...if so, then from the visual back the entire primary can be seen with room to spare.

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  • 6002540-Oblige look at primary edge.jpg


#27 GlennLeDrew

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Posted 02 August 2013 - 02:00 AM

Now, from that well off axis viewpoint just shown, as your sight line gradually moves back toward the optical axis, does the primary edge still remain visible when the edge of the foam lines up with the edge of the primary baffle opening? If so, that's encouraging.

Does the primary edge become hidden before your sight line lies on-axis? If so, not good.

Most importantly, such observations must be made from the focal surface, which is farther back when a diagonal is used.

#28 Asbytec

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Posted 02 August 2013 - 02:25 AM

as your sight line gradually moves back toward the optical axis, does the primary edge still remain visible when the edge of the foam lines up with the edge of the primary baffle opening?


I believe it does, on closer inspection the full primary can be seen from the visual back with room to spare. And from the diagonal, the primary's edge appears to very closely coincide with the primary baffle opening (image below.) The secondary baffle 'base' (the foam ring) lies just outside the primary's upper opening and the primary's edge just inside. And I do mean, "just." LOL

----------------------

Okay, it looks as though the primary's edge does J U S T fit inside the primary baffle at the diagonal back focus. The edge can be best seen about 8 o'clock. The 'fuzz' is the flocking paper. It seems consistent with the star test, too, as I can see that "intrusion" near 11 o'clock position.

Man, that was difficult to discern. A game of millimeters, or fractions of them...

But, apparently the primary's edge and the foam baffle ring are very close together with the primary's edge just inside it. So much so, I thought the foam ring obscured the primary's edge...all this time. But, such a design makes sense allowing the secondary to catch off axis light.

So, at diagonal back focus, there is so little room for flocking the end of the baffle. I might just have to live with it or risk reduced aperture.

And the tight fit is consistent with a very small unvignetted TFOV...that I want to observe again during collimation. I never noticed anything in the 11' arc FOV at high power, previously. However, vignetting became apparent nearer to the edge of 20' arc FOV. So, I suspect the FOV is admittedly small, unvignetted, and just fine for planetary observing (apparently what it's designed for, or advertised as such. Acclaimed to do, maybe, lunar and planetary.)

So, in the end, the secondary baffle seems almost useless except for low power viewing directly in the visual back (which I rarely do...never done, in fact.) Some efficient flocking darkened up the mechanical innards to prevent off axis reflections back at the diagonal. I might just make the flocking permanent and be done with it...leave the secondary baffle off and run with a 30.7% co and full aperture.

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  • 6002554-Diagonal View (Mirror Edge).gif


#29 Asbytec

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Posted 02 August 2013 - 02:42 AM

Glenn, I really appreciate you nursing me along. Thank you.

#30 GlennLeDrew

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Posted 02 August 2013 - 02:56 AM

You're most welcome, Norme! There's nothing like going hands-on to really understand the workings of a complex optic. Well, not so complex when the proverbial light bulb flicks on. :grin: Your well-documented probings and ruminations have forced me to delve more deeply into a detail or two, and so I've come away with something, too.

#31 azure1961p

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Posted 02 August 2013 - 03:54 AM

I have to say I truly appreciate how you guys have ferreted out the details of what's happening here. It almost seems a little maddening but you fellas have navigated your way through it all. I'm impressed out how little a secondary baffle can benefit viewing at medium to high power. I'm not going to mod my 6" sct as far as you have Norme but I have a better understanding about what does what where and when. This is a most educational thread. I'm a little regretful it isnt in the cass forum but I understand the point of it being here instead.

I'm truly looking forward to your astronomical views !!

Thanks guys.

Pete

#32 Asbytec

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Posted 02 August 2013 - 04:45 AM

Guys, they don't make 'smileys' big enough, so this will have to do: :grin:

#33 Jmel

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Posted 05 August 2013 - 09:04 PM

I have a c5, and I always suspected that reflections on these surfaces (due to a lack of flocking) were causing problems in my images...

#34 Asbytec

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Posted 05 August 2013 - 09:15 PM

Try observing the low power exit pupil and see if you can isolate the cause. If the reflection is in the camera, though, that might prove tricky to lock down. And if you stumble onto something, post it. Occasionally people ask about those rings or arcs, but far as I know no one has located the source.

For the sake of completeness in my own investigation, there does appear to be sufficient fully illuminated FOV for planetary and high power lunar work. Star testing, I could detect a very slight "squeezing" of the outer most de-focused rings near the edge of 11' arc FOV. The affect is very small suggesting an entirely very minimal light or resolution loss on this scale.

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  • 6008504-Vignetting.jpg


#35 Ed Holland

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Posted 07 August 2013 - 01:10 AM

What an intersting thread. I well remember the beginnings of this subject :)

It also got me thinking again about the 127Mak, but from a different perspective - its use as a camera lens. While doing some tests recently it was obvious that stray light could be a considerable problem under certain daylight conditions. If this could be tamed, it offers some cracking photographic possibilities, image quality is otherwise excellent.

Ed
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#36 Asbytec

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Posted 07 August 2013 - 06:01 AM

Hi Ed, you DO and should remember well the origins of this thread. As do I, what a great learning experience. Yea, you started this over a year ago. :lol:

#37 Ed Holland

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Posted 08 August 2013 - 02:02 PM

Yes, but I feel responsible for the trouble I caused ;)

:tonofbricks:

Looking more at my 127mm scope, there seems to be a lot of scattering from the baffle tube, so perhaps some flocking there would help. The problem, for daylight photography seems to be most prevalent where one is photographing a dark(er) scene in a situation where there is lots of light outside the FOV e.g. trees at a distance, but lots of sky out of the frame. This is easy to encounter, since the field of view is very narrow, but the acceptance angle for light which can scatter to the camera seems to be large. One answer might be a hood, but I suspect it would have to be absurdly long... and then baffled/flocked.

The motivation is there, though, because the image quality is excellent under favourable conditions. I might invest in some flocking material for experimentation.

Ed

#38 GlennLeDrew

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Posted 08 August 2013 - 03:29 PM

Ed,
I gather you have some direct light passing theough the meniscus and making its way out the back end of the scope? You could determine the required length of a hood as follows.

Place a long-ish stick against the side of the scope, extending a good couple of feet beyond the front end.

Position your eye near the position of the focus, far enough off-axis to see the crescent of direct skylight, and on the opposite side of the stick. Hopefully you will see the stick as fully crossing the crescent; if not, push it farther forward until it *just* fully crosses the crescent. Or if the stick is initially fully crossing the crescent, pull it rearward until its end just becomes visible, then push forward until just fully crossing the crescent.

Hold the stick in place and measure the length protruding beyond the scope's front edge; this is the required hood length.

Note that the hood opening diameter impacts the needed length. In the test above, it's assumed the hood will be a cylinder snugly fitting the OTA. If a wider hood is envisaged (with a 'spacer' type ring providing stand-off), it will need to be longer, or a ring aperture should be added to its front end. This latter approach is actually a good way to baffle the hood itself, with additional rings added along its length. And it stiffens the thing too.

If you do this test on required hood length, please let us know the result!

#39 Ed Holland

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Posted 08 August 2013 - 07:13 PM

Thanks Glenn, (And sorry Norme if I'm going off topic)

I'll experiment with the stick method, and try to make some more detailed general observations to determine what is happening.

Ed

#40 Asbytec

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Posted 08 August 2013 - 09:21 PM

Ed, not at all. It's on topic as far as I am concerned. If it deals with blocking stray light, have at it. Hey, post away, study, learn and share you're results.

The stick method sounds appropriate. In fact, I kind of liked the circular aperture inside the hood idea. I don't want to use a dew shield, but maybe some short hood and baffle combination, if needed, could be the ticket. I just hope it doesn't lead, necessarily, to a reduced effective aperture, again.

:gotpopcorn:

#41 GlennLeDrew

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Posted 08 August 2013 - 10:45 PM

Norme,
A hood will reduce the on-axis aperture only if you make the opening smaller than the working aperture. A same-as-aperture hood opening will clip off-axis light, but for a small-field instrument like this, the impact will be miniscule.

Let's work out an example. Let's say the aperture is 127mm, the hood opening is 127mm, the field semi-angle is 0.7 degree, and the hood extends 450mm beyond the meniscus edge. Light entering the system from 0.7 degree off axis has its entrant bundle offset by

TAN(0.7) * 450 = 0.012 * 450 = 5.5mm

The effect is like offsetting a 127mm aperture mask laterally 5.5mm, or 4.3% of the aperture's diameter, which reduces the illumination by no more than a measely 4%. Negligible!

And besides, there is already worse vignetting being introduced by the primary baffle.

Trying to eliminate that wee bit of off-axis vignetting introduced by the hood only results in having to make it a bit longer still.

#42 Asbytec

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Posted 08 August 2013 - 11:31 PM

Glenn, bear with me while I consider your comments. That 5.5mm mask sounds like reduced effective aperture in addition to minuscule drop off in field illumination. I don't mind the minuscule field illumination, but prefer every millimeter of full aperture (and every millimeter of CO) due to it's affect on effective CO ratio.

A hood of 450mm is nearly twice the length of the OTA making the OTA 3x it's length. It might be effective, but other than being gaudy, it also affects balance and set up time. It has to fit pretty well and not be skewed. I've noticed such skewed affects in star tests with my own ill fitted dew shield (which I promptly aligned.) However, the longer the hood, it might mean the more accurately it need to be aligned.

Something much shorter with an aperture ring (preferably at least the working aperture) seems preferable. Not really sure if that's possible. But, such a hood does almost seem to act like a secondary baffle outside the telescope. That's interesting.

#43 GlennLeDrew

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Posted 09 August 2013 - 01:10 AM

Norme,
The hood could be made so as to seat itself nicely aligned. An inner ring some distance from the lower end could rest against the front cell, guaranteeing proper alignment. This will ensure no larger than 1mm of offset, if just some reasonable care is taken during construction.

So. With a properly aligned hood, there is no worry about on-axis aperture reduction, even when the hood's opening is exactly equal to the aperture. As the earlier calculation showed, for each one millimeter of offset, the resulting aperture clipping amounts to roughly one per cent of light loss. And to stress again, the induced contribution to off-axis vignetting is of no concern, it being much less than that imposed by the primary baffle.

The figure of 450mm for the hood length I threw out there was merely a quick guess for illustration of the calculation. Hopefully a shorter one will do.

#44 Asbytec

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Posted 09 August 2013 - 01:27 AM

...there is no worry about on-axis aperture reduction, even when the hood's opening is exactly equal to the aperture.

You're correct, I missed that part getting confused with the 5.5mm offset aperture mask mentioned above. Could not follow exactly what that meant.

It would be interesting to see if a same aperture hood could be made short while still providing sufficient off axis shielding (relative to the secondary baffle) even at the expense of negligible illumination off axis. But, thinking about your math above, the same aperture hood would have to be pretty long by design, especially if the secondary baffle is small. Yes?

Sure, I understood you to throw some numbers out there, they don't seem unreasonable. Maybe a reduced semi aperture FOV, provided someone like me might accept that reduction to a few arc seconds, could figure into a (edit: longer) hood.

#45 GlennLeDrew

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Posted 09 August 2013 - 02:00 AM

Norme,
Yes, the larger the scondary baffle, the shorter can be the hood.

I believe you've established already for your scope that the secondary is large enough block any direct light through the meniscus from reaching that part of the focal surface sampled by any of your eyepieces. If this is so, then a hood need not be so long, for your only concern now is to block direct illumination of the stuff outside the field stop which can scatter light into the field.

#46 Asbytec

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Posted 09 August 2013 - 02:21 AM

Yes, direct blockage does seem sufficient at the diagonal. In that case I am wondering if a hood is useful at all. A shorter hood would block off axis rays that are already blocked or flocked. Since my secondary is on the small side, and to get any additional - even perfect - protection, it looks like a longer hood would be called for.

...its use as a camera lens. While doing some tests recently it was obvious that stray light could be a considerable problem under certain daylight conditions.


Not sure what Ed might be thinking about attempting, but there's enough good info (thank you, again) for him to get started. Lets see what he runs with, I'd be interested in seeing his stray light analysis.

#47 Jmel

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Posted 09 August 2013 - 05:34 PM

You can see what I mean about the reflections here:

Posted Image

It only happens with really bright stars that are not dead center of the image... or slightly less bring (but still bright) stars near the edge of the fov, near the vignetted part of the scope edges...

I've just taken ownership of an edgehd 800 though, so I plan to see if it continues with the new scope and old camera or not.

#48 GlennLeDrew

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Posted 09 August 2013 - 06:41 PM

The large, dim, asymmetric 'donut' exactly opposite Alnitak is a ghost formed by the Schmidt corrector (a classic problem with Schmidt systems.) That is, it does not result from scatter off non-optical surfaces. And one or two of the others are likely to be purely optical as well.

#49 Ed Holland

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Posted 09 August 2013 - 08:07 PM

OK, I had a moment to look at the 127mm Mak. The crescent of light able to enter the primary baffle tube directly from the corrector was observed. Armed with my trusty stick, I was able to determine the length of a hood that would be required to block this stray path. Since the crescent could be observed from a number of positions across the visual back, I made the following series of measurements. My viewpoint was approximately at the distance behind the rear port corresponding to that of my camera's sensor plane:

Eye at nearest point to center of field where crescent is just visible, stick just intrudes: 355mm


Eye at edge of 2" visual back, stick all the way into the visible crescent. 260mm

Eye at edge of 2" visual back, stick just intrudes into crescent. 190mm

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.

Interestingly the "complete hood" length suggested by the 355mm measurement is longer than the OTA which is ~320mm. It would double up as an epic dewshield!

Next: to find some appropriate materials from which to fashion an experimental hood :)

#50 GlennLeDrew

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Posted 09 August 2013 - 08:29 PM

Ed,
Your sense of variation in required length vs distance off axis 'protected' is in accord with expectation. I'm glad your result is rather shorter than my earlier 'wave-arm' guess of 450mm! :grin:


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