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Fo_Cuss

4mm Omni Plossl - Aperture reduction baffle?

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What is the purpose of the 'aperture reduction baffle', on the lens clamp nut?

The clamp nut has a central 5 mm diameter cylinder.
However, at the top of the cylinder (scope-side), there is an intruding lip of 4.5mm diameter.

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I see no reason for this lip, other than to cast a shadow onto the 5 mm diameter cylinder.
(for all I know, it could even be a repurposed component that comes with the lip)

Does anyone know of any other reason, for the existence of this lip?

If my suspicions are well founded, this lip can be removed, and the cylinder flocked with Black 3.0
In doing so, this would increase the amount of focussed light entering the eyepiece, by over 10%.

For an eyepiece that is already suffering from lack of light .... 10% increase, would be a significant improvement.
The thing is ... the chamfer brilliance will already be defeating the 'shadow creating' lip (if that is what it is).

image.jpeg.be007ddac7c7ba0c0302b9bee7539c7b.jpeg


My guess is that it is this combination that is causing the banding aberration on the image.

To be honest, if I had a 5.2mm drill, or a 5.3mm broach, I'd expand the cylinder slightly, to make way for the Black 3.0.
... then matt the chamfer surface, to complete the clamp nut upgrade.

However, this lip may serve some purpose that I am not aware of.

Does anyone have any knowledge that might inform the next step that I might take ?

🌝

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To stop the lens falling out? The field stop? Often the glass is stopped down as the edge performance is so distorted it’s not worth having. My Tal 25mm plossl is a case in point. It has a larger afov than is standard for plossls but the edge of field is poor compared to the centre 

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It it is scope side and it is first place where light falls - it is probably field stop of the eyepiece, and no, you won't get more light if you remove it (even if it is not field stop).

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As above. Often these serve to provide the field stop for the eyepiece as well as keeping the lens elements in place.

Removing it will not increase the light throughput of the eyepiece. You might get a slightly larger apparent field of view with the eyepiece if it is acting as the field stop but the edges of the field of view are unlikely to be sharply focused.

Next step ? - put it back in the eyepiece and leave it in place I suggest.

 

 

 

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Thanks everyone for the explanation :)
Just by knowing the correct terminology for this baffle, has allowed me to carry out some additional research.

That research threw up a cloudynights thread, where an equation was presented by Starman1 :

Quote

TFOV = (EPFS/TFL) * 57.296

Where TFOV = true field of view, EPFS = eyepiece field stop, and TFL is Telescope Focal Length.

For my scope therefore...
In current spec TFOV = (4.5/700) * 57.296 = 0.368°
In 90/f1000 TFOV = (4.5/1000) * 57.296 = 0.258°

There was some discussion concerning the FS removal ... some did, others didn't.

As suggested here; I'll leave it in place.
My objective is to tidy up the image, not increase the TFOV.

... though I will first flock this component, as it is the most shiny anodising that I have ever seen.

That massive mirror-like chamfer that terminates at the Field Stop, must surely be bouncing light down through the FS aperture.
That (surely) cannot be beneficial.
Or can it ?

🌝

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You could either flock it or paint it with flat black non-reflective paint. Don't make the aperture of the hole any smaller or allow flocking material to project beyond the sharp edge of the hole because that will reduce what is an already small apparent field of view and you will notice that your field edge becomes ragged.

Personally I'd probably go down the flat black paint route.

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4 minutes ago, John said:

go down the flat black paint route

Yes.
My intention is to use a hybrid mix - Black 3.0 and consumer matt black - to gain durability (as used for the EP face and interior).

The Field Stop = 4.5 mm D
The cylinder beneath the FS = 5 mm D

... providing 0.25 mm potential coverage depth

I will not be ladling it on, and it will be baked, so it should be fine  🌝

 

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A quick bit of good news...
The mods to the 4mm Omni plossl appear to have worked  😀

The added chamfer to the EP aperture, has reduced the perimeter shadow ring.

It's a proof of concept.
When the lens is dismantled, to realign the doublet, I will reduce the chamfer angle to make it flatter.

The mid-way banding appears to have gone.
I put this down to the blacking of the EP body and clamp nut.
For good measure, I also flocked the adapter tube internal threads.

There now remains the general lack of contrast; which should be cured by flocking the scope.

I didn't get a benchmark image for the mods (it's rainy), which is a shame.
... but from what I've seen, the EP is definitely better, so the 12mm will undergo the same treatment 👍

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4mm Plössl?

How do you use it?  The eye relief would be about 2.8mm, close to the average distance from the cornea to the pupil in the eye.

I presume you don't see the entire field but move back and forth to see the edges?

As an experiment some time, try a 2X Barlow on an 8mm Plössl.  The 8mm Plössl has about a 5.6mm eye relief, but the Barlow extends that to close to 6mm,

which would be a lot more comfortable.  In addition, the Barlow would provide additional baffling.

There is one good thing about a 4mm Plössl, though--if the lenses are removed it can be used as a collimation cap.😀

Edited by Don Pensack
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Ya I agree I dont think I would get one under 6, now probably nothing under 8mm

Joejaguar 

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I think in this case Fo_Cuss (the original poster) is simply trying to make the best of what he already has :smiley:

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