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Orange C90

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#351 kansas skies

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Posted 22 July 2018 - 09:08 PM

Absolutely! It is a pretty weak spring though, at least in my version. It also favors one side.

That would have been my next thought. In my mind, it goes along with taking time to let the entire assembly settle in before making adjustments to any component in the system, then trying not to adjust more than one component at a time (with a lengthy amount of settle-in time between adjustments) until the best level of collimation is attained.

 

Bill



#352 Der_Pit

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Posted 23 July 2018 - 07:45 AM

Hello C90 fans :)

 

Found this thread during the weekend, and enjoyed reading through it.  I'm also one of the owners of those old beauties, it had actually been my very first scope, back in the mid /end 80s (I don't remember exactly - got it as Xmas present).  I've just entered the details in the Registry

 

One thing I wonder about:  Mine had two T2 extenders that connect to the SC thread on the back of the scope.  One is 21mm, the other 121mm.  With a MAK it's easy to get to focus at either location, but the focal length varies a lot.  Has anyone yet checked/compared the optical quality for such different configurations?  I'm not familiar with MAK optics, but I'd assume the correction to have an optimum/nominal value only for one focal length/backfocus...



#353 wfj

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Posted 23 July 2018 - 01:28 PM

To determine focal length, take an large focal length EP and measure exit pupil, one way is to trace on a piece of paper it carefully and measure with a caliper. The f/ is the EP focal length divided by exit pupil. That f/ times the effective aperture gives you focal length.

 

Be advised that you're measuring focal length at that point of focus, when you adjust mirror spacing between secondary and primary to change focus, you are also changing (slightly) magnification of a Cassegrain (also, roughly the effective f/ of the scope is the distance to the focus from the secondary divided by the secondary's diameter). (You also should use the flashlight test to insure what the functioning aperture is at that focus.)

 

For a 40mm 0.965 EP and a prism 0.965 diagonal I have, I get a 5.1mm exit pupil, thus f/7.84 and 89mm aperture thus 698mm (shortest optical path diagonal with EP  that puts the focus point almost entirely through the diagonal and in the front.

 

Now, with the same diagonal and an EP that focuses near the top  of the diagonal, the f/ is closer to 10. If I put a 1.25" adapter and standard 1.25" prism with a EP that focuses just above the prism inside the EP barrel, I get f/11.

 

Note also that when you DPAC test, you'll get under corrected for small f/ , corrected best at close to 11, and over  corrected for more than 11. Interestingly the barlow lens that these were sold with not only magnifies at a highly reduced f/, it also appears to have the proper correction at the 2.5 meter effective focal length.


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#354 Der_Pit

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Posted 23 July 2018 - 02:19 PM

To determine focal length, take an large focal length EP and measure exit pupil, one way is to trace on a piece of paper it carefully and measure with a caliper. The f/ is the EP focal length divided by exit pupil. That f/ times the effective aperture gives you focal length.

ISTR that I had done it (some 20+ years ago) by looking at image scales, and came to a focal length difference of 200mm between the two extremes.  I should probably do it again, some images with a camera should do, no?

But for sure I'll also try the exit pupil method - I definitely like that kind of 'simple' approaches :)

 

For a 40mm 0.965 EP and a prism 0.965 diagonal I have, I get a 5.1mm exit pupil, thus f/7.84 and 89mm aperture thus 698mm (shortest optical path diagonal with EP  that puts the focus point almost entirely through the diagonal and in the front.

 

So indeed a huge difference.  I don't have an issue with that by itself (rather the contrary).  The main question is, is it still a good scope at f/7.8, when it was designed as f/11?

Ah wait - sorry.  I first missread your last paragraph.  You did DPAC it, yes?  And it indeed is best at nominal.  So I'd have to find out where that is, and use it there :D

 

Thanks so much for sharing your knowledge!


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#355 wfj

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Posted 23 July 2018 - 03:27 PM

Yes I did DPAC mine. Best correction is about 930-950mm. When I draw a line across each Ronchi line (after correcting for "keystoning" due to slightly off axis - widens/narrows top bottom of lines), I measured an average that was less than 1/8 wave, accounting also for "dual pass" accumulation of error. And the nature of the residual error looks like a spherical harmonic, thus it's likely HSA. Think mine was "overdone" on the aspherization a bit, so it handles stuff better shorter, but then more HSA too.

 

Once I get the baffling better I hope to measure this deviation carefully. It might be the case I can place an element (backwards meniscus, or two - asymmetical achromat or DCX /PCV pair) in the baffle tube to "floor" this. And then see what's left. Again - hobby.

 

As to the lack of correction at shorter FL, since you can only get this at large EP/low mag, it's a nuance that mostly affects resolution. The large exit pupil are useful with filters like Hydrogen Beta and nebula like the Horsehead and California, and that's annoying but an advantage over the f/3 Mak's who can't make it to 5mm exit pupil easily, so "lose edge definition" vs. "soda straw FOV" is the trade. It's not like I expect crystal clarity on these with 90mm of aperture.



#356 wfj

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Posted 23 July 2018 - 04:46 PM

Well, I decided to go all in.  Even with the screws I still can't get a decent alignment.  I can tell that the factory used screws around the mirror to get the primary centered.  But I still wonder about the baffle tube center vs primary center?  My laser alignment tool does put a nice center spot on the secondary, so pretty sure that is aligned.  Maybe I'll work on that baffle next, it was melted anyhow so I won't feel too bad about replacing it.

 

attachicon.gif FullSizeRender.jpg

You are definitely "all in". Now let's see if we can get you "out" again.

 

Could you take pictures so we can see the side and top views WRT where the tripod plate attaches please (want to examine spring, inside of rear casting, and side of center tube from two different positions at 90 radial to each other). Looking for oblong things, tool marks, casting defects, etc.

 

Laser can't help you much for this.  What might help is to use a caliper to measure multiple different radial measurements for the outside of the center tube to the inside of the casting. You'd be attempting to tell if the tube is centered, tilted, out of round, bent, or  irregular. You might also inspect the mirror hole to see if it also has an irregularity (chip, some of the "goo" on the side, wedge in the hole, glue line,  ...). Do not attempt to clean it up. The idea is to solve the puzzle of how it might be failing. The clues might tell us how to "fix it".

 

As to the baffle, pics of the base where it joins metal might help. I once had one where it was put on at a tilted angle. Is it seperable from the metal?

 

add:

 

As Bill outlined above, think these things start with a casting that is machined/turned on a metal lathe. Bored out and threaded at a minimum. They also likely do the inside bore and surface the bottom as plane/true.


Edited by wfj, 23 July 2018 - 04:53 PM.


#357 PeriodicTrends

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Posted 23 July 2018 - 06:57 PM

I'll do my best.  As a general observation everything looks really well done.  The casting/machining appears to be on center and whatnot.  If there is anything I detect (which is really hard to show in pictures) is that the center primary hole might be about 0.5mm off center with the outside edge, and the baffle cone on the secondary spot might not be glued exactly centered...but the spot itself seems ok.  With the center spot I am mostly making a visual observation but also observing my laser align-er (if I turn off the lights) I can see just barely leaking thru the coatings dead center of the secondary spot.

 

Anyhow, lots o fun.  There are many screw holes in the cast base that could be used to center the primary.  It did have some goop inside the center hole, and some also fell out on removal.  The spring seems to be off center, but not sure because it sorta levels out when you compress it.  Springs are funny like that.

 

I'm currently trying to think about how to make an upside down rig to center everything with the primary loose and resting on the three screws.  I'm also wondering if some leveling screws the right threading glued to the primary might be handy?

 

image1.jpeg image2.jpeg image3.jpeg image4.jpeg


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#358 PeriodicTrends

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Posted 23 July 2018 - 06:59 PM

More photos...

 

image5.jpeg image6.jpeg image7.jpeg IMG_6424.JPG


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#359 wfj

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Posted 23 July 2018 - 09:10 PM

Thank you, more helpful.

 

Looks like in disassembly you tore the fabric of glue on the inside of the mirror hole. The pattern looks like it might be used as an "index" to match against one on the central tube it is mounted against, so you can restore the original orientation (which may or may not be "good enough", but it's a start). Don't see any chips, cracks or bore issues on the mirror.

 

I'd take pictures closer in and with more illumination of where the clip holds the mirror. Looking for details/marks/glue/glue lines. Since it is black, try to capture the "sheen" so you can see details, by carefully illuminating.

 

If you can match up it's original position, make a pencil mark (not on the mirror surface) on the mirror (underside/edge) and casting so that if you lose the glue/marks (try to the glue residue for the moment) you can return to the original state as much as possible.

 

Can you also capture a photo of the edge of the mirror only, looking for marks like those used by a fixture? There may be matching marks on the inside of the casting (or none - perhaps no fixture was used and it was just "slapped together?).

 

Hope this helps you.



#360 PeriodicTrends

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Posted 23 July 2018 - 11:37 PM

Nothing that I could see. Doesn't mean there aren't any, but nothing that appears to my eye and various flashlights. The glass still seems to have a lot of rough casting marks on it, but nothing I would assume was from a fixture. Aside from the numbers on the back, there are two blue sharpie marks on the back rim. They don't line up with anything, although I was hoping they would. Using the glue residue I made a mark for an alignment feature on both the casting and mirror. The clip area is pretty marred up from taking it on and off with all of my experiments from before. I plan to sand the washer nice and smooth again too, although in my scope the mirror didn't press against it too hard, perhaps because of the weak spring. I guess next up will be to sand and fix the baffle tube, then maybe try some general alignments and see if I can position the primary on a more favorable axis. I think I'll get some nylon screws and try to see how much centering play I can get out of it.

#361 wfj

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Posted 26 July 2018 - 04:26 PM

Thinking about this a bit more. The problem with the spring and potting goo (sometimes RTV, sometimes other stuff) is that you're not assured a bearing surface to the mirror to communicate load with.

 

If those pressed on a plate that had a bearing surface with the mirror, then the plate might be stressed but the mirror wouldn't be. If the plate had an edge support that contacted at the center of mass plane (three spots), it would allow fixation of the mirror w/o stress, independent of orientation.

 

If this plate could be fixed to a nylon sleeve that could ride the baffle tube, then all you'd need would be to preserve the enough adjustment compliance radially to centrate the mirror to the mechanical/optical axis of the threads/corrector/baffle tube.

 

That's an outline of how to improve the design.



#362 wfj

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Posted 28 November 2018 - 05:46 PM

Thanks to inspiration of recent back/forth over classic C90's, I went back and swabbed the inside of the baffle near the inside glue line.

 

The baffle popped off with my finger.

 

I can now make custom 3D printed baffles and experiment with them!

 

c90-primary-baffle-remove.jpg


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#363 wfj

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Posted 30 November 2018 - 06:07 PM

Meet my nemesis - primary baffle. Polished, glossy polystyrene cylinder 27.7mm outside (25.4mm inside) diameter, 27.1mm long, with a 15mm hole.

 

Top scatters a bright target's light, diffusely illuminating secondary. And there is sky flooding unless you either use a 1.25" eyepiece holder and mirror diagonal with the focus above the shoulder, or a long 0.965 2.5x barlow with a diagonal off the end.

 

c90-primary-baffle.png

 

Ideally it should be a cone with a 14 degree slope with textured microbaffle sides and as thin as possible. About twice as long.

 

Make a spool of the appropriate dimension of Teflon, spray with silicone grease, coat with viscous metal filled epoxy. Tightly wound with the thinnest carbon fiber thread from top to bottom. Autoclave. Trim, and insert.

 

 



#364 wfj

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Posted 30 November 2018 - 09:02 PM

Here's a prototype of a conical baffle cut out of scrap roof flashing and finger formed. When I find some glue I'll cement the seam and finally form the cone, then spray it flat black.

 

This is what it looks like fitted from the front. You can see the seam.

prototype-primary-baffle.png

 

Using a 500 watt light from the front, can't see any leakage. Already this seems and improvement, as I can see the absence of a bright ring on the secondary from the former flat baffle, as all the secondary sees is the edge of the baffle, which is still well within the shadow of the secondary since it's closer. The longer baffle also allows better use of the hybrid and 0.965 diagonals. Need to check for vignetting with the 1.25" eyepiece adapter and 1.25" diagonal case.

 

If the moon beats the clouds tonite, I'll give it a peek just at this raw stage. It would be ironic if such a trivial thing were to show promise.


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#365 wfj

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Posted 01 December 2018 - 01:16 PM

Can you measure the hole inside diameter of the mirror? Thanks.



#366 PeriodicTrends

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Posted 01 December 2018 - 03:36 PM

Can you measure the hole inside diameter of the mirror? Thanks.

Gimme just a sec, I’ll be right back with that.

Getting about 29mm.

 

 

Attached Thumbnails

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Edited by PeriodicTrends, 01 December 2018 - 03:42 PM.


#367 wfj

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Posted 01 December 2018 - 04:33 PM

Thank you. Considering an primary baffle replacement of a grander scale.



#368 Moffett

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Posted 01 December 2018 - 05:00 PM

Thank you. Considering an primary baffle replacement of a grander scale.

Considering buying one from you when you finalize it. 



#369 wfj

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Posted 01 December 2018 - 07:18 PM

The weather is rainy and cold. Glues and paint don't setup well. Still at the test prototype stage. (Last night the moon rose into clouds.)

 

Considering buying one from you when you finalize it. 

Hold your horses. We don't even know if it'll improve things. Just at the experimental stage here.

 

(Kids, don't try this at home ...)

prototype-baffle-too.png

 

I think why they didn't do this is that it sticks out. And, you need to examine lots of cases to make sure they all work right.


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#370 wfj

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Posted 02 December 2018 - 04:21 PM

Some experience with the prototype on daylight targets, the waning crescent moon, Orion Nebula, dissimilar doubles, Pleiades, and Venus. Conditions were poor.

 

Two most striking differences were on the Moon and Venus. Neither appeared as "washed out" as before in looking for detail, but in the case of Venus the residual color stood out more - I wondered if in my next tests if I "tune" BFL to eyepiece I'll find a minimum point. That's supposed to be at best correction, around 1000mm EFL, but it certainly isn't what you get with a ~920mm FL you get with a stock 0.965 diagonal or a hybrid.

 

At the same 230x didn't notice color on the Moon's limb(!). What I did notice was that I could make out more craters near the limb than ever before, as well as the subtle hues that I usually see with a good refractor. In fact the view reminded of that more so than with the stock baffle. But what you don't see in a long refractor is HSA, and that was in evidence in looking at details inside of craters. Conditions were far from perfect to judge this matter.

 

The short answer is that a better primary baffle lets you see existing optical defects better, and more range of contrast/color.

 

Taking out the eyepiece and using the Moon to judge scattering inside the OTA, the baffle only shows the secondary mirror and the reflection of the primary in it. Correctly sized, it doesn't even show the secondary baffle, which is now superfluous. One cannot see the thin edge of the primary baffle, so the flat area of the original primary baffle is entirely removed - which was the objective. However in it's place is the diffuse illumination on the inside of the baffle, which wasn't apparent before with the original's "hole" - there was nothing to directly illuminate. Even sprayed flat black, it's apparent although the net trade is better. Microbaffles on the inside could reduce this further.

 

Also with this design is a larger opening at the top of the cone. That's because the closer to the secondary, the larger the opening has to be for the f/11 of this optical design to be not vignetted (it isn't).  This also has increased the illuminated field size usefully (one of the main reason astrophotographers in the 1970's did this, to illuminate better a 35mm film frame. A 32mm eyepiece shows this readily.

 

For the first sample of this, I chose the smallest conical baffle design that would work with a 0.965 diagonal with a BFL to the base of the diagonal for a lowest power eyepiece with a adjacent field lens, as well as allowing the removal of the secondary baffle. This is the most convenient configuration to pack in my backpack, and it's also the most rugged - even a thin baffle has a moment arm, and dropping the scope would put force on the base where it attaches. This one is attached friction fit on the outside, so that no reduction in baffle base would occur.

 

Downsides of this mod were that it was very difficult to both form a proper cone, easy to knock it out of round, and hard to get a good mechanical connection at the base (it is only a prototype). And you have to be very precise in fabrication, because you have to exactly "outline" the secondary for this to work. If you are out of round, or off in dimensions, it's very apparent (you can tell lopsided or intrusion into optical path.

 

There appear to be less prominent benefits too. You can make out dissimilar doubles a bit better that were contrast reduced already by the large CO.

 

No negatives on DSO's like M1/M42. In the Pleiades could see some of the fainter orange stars. near the base of the "handle".

 

All of this leaves the question open of what happens with the removal of the secondary baffle as well - does the CO reduction help much? Also, does it change the star test then, because the large CO hides HSA as well as the HSA is compromised by some intentionally left LSA (which may not be present on this sample - part of the reason I got interested in playing around with it).

 

I did do a star test with just the A|B test of the factory/prototype baffle, and unsurprisingly nothing has changed with that (meaning the outside/inside additional area seen doesn't have zones to worsen things with the baffle being changed.

 

All of this has been a "control" for the greater experiment - to rework all of the baffles of the scope. I needed to determine that there wasn't other surprises present (did deal with little light leaks in the seam as I was fitting things) that could throw off the results. And before I go further, I need to a)improve the outside friction fit of the baffle, b) reduce the inside baffle tube glare, and c) stiffen the top so it is less out of round (glue didn't set up completely).

 

Am pleased that this optical sample might allow me to explore a larger scope of an experiment. If It had a turned edge , zone, or "hole", I'd worry that I'd be not measuring one change (baffles) but that and unmasking other flaws besides known HSA.

 

Other things I've been thinking about - making the baffle longer with larger diameter at the top, making the baffle even thinner and under tension (springs back?).

 

Considering a revised prototype and a consolidated change in baffles along with flocking to see if a maximal change provokes a substantially different result.

 

None of what I've done do I suggest you yet try. The sample here appears uniquely ideal to test this out on due to its relatively fewer flaws. Most/all other C90's I've encountered didn't have good collimation, DPAC, star tests, and planetary detail. It's likely if you do this you'll amplify the effect of an aberration. Indeed, I've already found that to be true on this sample as well. You may not like the result.

 

Leads me to believe that Celestron made this scope for high manufacturing yield, with the baffles a worthy compromise on top of many others.

 

I'm interested in eliminating artifacts here at the cost of raising a single one - residual HSA - to see if I can cancel (for this sample only) it, to see if it is possible to remove compromises and see what remains. Perhaps a form of "tilting at windmills".

 

And I'm still assessing if there is a useful project here to do. Or not. Let me know what you think.


Edited by wfj, 03 December 2018 - 02:52 PM.

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#371 ftwskies

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Posted 03 December 2018 - 04:35 PM

Could the same improvement be made by simply flocking the face (the donut shape that faces the secondary spot) and the interior of the stock baffle tube?



#372 wfj

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Posted 03 December 2018 - 06:23 PM

Could the same improvement be made by simply flocking the face (the donut shape that faces the secondary spot) and the interior of the stock baffle tube?

You could decrease the effect somewhat that way. But the limits of it are a) flocking works best at a grazing angle, the the front of the baffle isn't grazing, b)  for 0.965 diagonals, the baffle is too short and some "leaks" through on the edge, and c) the cylinder outside itself is glossy and conveys light from a grazing angle into the secondary/baffle.

 

Have been examining  the proper correction distance for both residual color and spherical aberration near 1000mm (use a prism 1.25" Celestron diagonal and a 1.25" visual back. My prototype baffle obscures the  too much of the edge in such. If I remove the secondary baffle, I cannot have both 0.965 and 1.25 cases (different EFL/BFL's) for both, one will be sacrificed for the sake of the other. The residual color is an artifact of using an f/2 primary, as a fast mak will have to have an achromatic corrector (or a second "counter" corrector in the primary baffle) to accomplish such a wide range of secondary magnifications.

 

One could just trim back the secondary baffle with a cone that would cover both. But the smallest CO, best contrast, would be a 10mm longer conical baffle of 0.05mm thickness and no secondary baffle at all, with a  diagonal immediately attached to the rear. If I remove the mirror center support from the tube, and edge support the mirror, I could use a baffle and add 1.25" support for the highest gain of the design. But I have no idea how the HSA would appear, and the residual color on Venus/Sirus would be the lasting defect that way.

 

FWIW, the measured 1000mm EFL has a  BFL of 5" from the rear shoulder on the eyepiece tube, just ahead of the threaded portion that receives the C90's 1.25" visual back. (Which makes sense when you consider the 4" of optical length for a porro prism erector and another 1" for its eyepiece mounting, or the size of a 35mm camera adapter, T to camera mount , and mount to film length.

 

For the 0.965 prism diagonal the BFL is 2". There's a considerable difference in designing for these.


Edited by wfj, 03 December 2018 - 06:59 PM.

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#373 Moffett

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Posted 03 December 2018 - 10:52 PM

Are you using any sort of dew shield to help with stray light too? I have the original rubber celestron one and have noticed it makes a pretty big difference if there are any offending light sources that can shine on the corrector. 


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#374 wfj

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Posted 04 December 2018 - 06:55 PM

I typically use a sheet if plastic and rubber bands under dark sky. In the tests of the baffles, no, didn’t, because it doesn’t effect sky flooding or lunar/planetary scatter inside the instrument.

 

Baffles depend on each other as a consolidated whole. While I’ve started out piecemeal, it’s become clear that complete solutions need to be built and somehow quantitatively measured. Because otherwise too much quickly becomes subjective.


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#375 Bomber Bob

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Posted 04 December 2018 - 07:36 PM

All of my refractors came with baffles & blackened interiors -- except The Mogey.  It's a case study for me, because I improved it in stages.  First, I re-blackened the interior.  Next, I cannibalized baffles from a Celestron frac, made a light path diagram, and installed 3 baffles at the correct distances from the lens.  Finally, I flocked from the lens cell to the first baffle, and the inside of the focus tube.  At each stage, I saw improvements in contrast (more) & resolution (less significant).  Yes, that's subjective, but I'm not sure how I could measure those attributes objectively at home as a hobbyist.




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