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Anyone tried looking at your binocular FOV as if it were an AP "flat"?

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#1 revans

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Posted 19 April 2024 - 01:07 PM

There is a common technique in astrophotography where a stretched pillowcase is tied over a refractor objective and a photograph taken which is known as a "flat."  This is a well known calibration frame type used to remove vignetting, gradients, dust motes and other optical issues from astrophotos taken with the refractor.

 

Out of pure curiosity I tied stretched pillowcase linen over both objectives of my Oberwerk 15x70 Deluxe and then looked through the eyepieces pointing in the direction of the dimly lit horizon.

 

I paid attention to any shadows or variations in brightness that were observed.  What I saw was a dim slightly shadowed area extending from the extreme periphery of the FOV, inwards for a distance of about 10% of the radius circumferentially.  Otherwise the FOV was uniformly bright.

 

Interesting...

 

Rick


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#2 sevenofnine

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Posted 19 April 2024 - 01:53 PM

That is interesting hmm.gif  Have you tried the test with your other more expensive binoculars like the Zeiss 20x60? borg.gif



#3 revans

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Posted 19 April 2024 - 01:59 PM

That is interesting hmm.gif  Have you tried the test with your other more expensive binoculars like the Zeiss 20x60? borg.gif

Not yet.  My Orion BT-100 90 just arrived.  I was chagrinned after all my pre-planning, to discover that it simply will not attach to my Oberwerk 280 fork mount.  The 3/8 thumbscrew isn't accepted by the Orion.  Oh well.... on the bright side I was able to mount on my HAZ31 alt/az GOTO mount. 

 

It seems to hold securely, but the footplate on the Orion lacks the bevel that the HAZ31 mounting plate has.  That makes me worry a bit but I'm not sure what if anything I should do about it.

 

Update: OK, I just did the pillowcase test on my Zeiss 20x60 IS.  There is uniform brightness across the field with no shadows seen.

 

Rick


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#4 sevenofnine

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Posted 19 April 2024 - 02:20 PM

Check that the Orion doesn't have a 1/4" adapter screwed into the 3/8" base. That's what it looked like to me from the picture you posted. If it is, you can simply unthread it using a flat head screw driver...Anyway, I just tried your new test on my Oberwerk 20x70EDU only I used a folded sheet of white printer paper. The FOV looked evenly lit to me. The only "defect" I noticed is the unsharp field stop that bothers Fiske but not me borg.gif


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#5 revans

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Posted 19 April 2024 - 02:41 PM

Check that the Orion doesn't have a 1/4" adapter screwed into the 3/8" base. That's what it looked like to me from the picture you posted. If it is, you can simply unthread it using a flat head screw driver...Anyway, I just tried your new test on my Oberwerk 20x70EDU only I used a folded sheet of white printer paper. The FOV looked evenly lit to me. The only "defect" I noticed is the unsharp field stop that bothers Fiske but not me borg.gif

You were right.  I found the world's largest flat head screwdriver and was able to remove the adapter, revealing the 3/8" thread. Here it is on the fork mount now.  I've put it on backwards, but it doesn't matter for the mounting test.

 

Orion BT 100 90 On Oberwerk280forkmount
 
Rick

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#6 TOMDEY

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Posted 19 April 2024 - 02:51 PM

Yep --- that's a good and interesting casual quick-test/check for vignetting and more. At work, I was responsible for readiometric ceretification of huge telescopes. As long as the ~pillowcase~ is both uniformly illuminated and Lambertian in transmission... then the test is both qualitatively valid and quantitatively accurate. I found that some (few/select) cleanroom garb materials are nice and Lambertian with near zero specular component... rather surprising and welcome. The logistics of everything gets challenging as the size goes up. --- and FUN! I have some patents on these kinda things.

 

PS: I checked my Zeiss 20x60 and (as one would expect) --- nice over the entire field.

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  • 311 Tom's Jumbo Lambertian Calibrtion Screen.jpg
  • 312 Ken Mason Tom Dey Patent Satellite Radiometric Calibration p1 1200.jpg

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#7 steveincolo

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Posted 19 April 2024 - 03:56 PM

Check that the Orion doesn't have a 1/4" adapter screwed into the 3/8" base. That's what it looked like to me from the picture you posted. If it is, you can simply unthread it using a flat head screw driver...Anyway, I just tried your new test on my Oberwerk 20x70EDU only I used a folded sheet of white printer paper. The FOV looked evenly lit to me. The only "defect" I noticed is the unsharp field stop that bothers Fiske but not me borg.gif

Same result here on the Oberwerk 20x70 EDUs.  I don't notice it during night time use.   By the way I used a white t-shirt like many imagers do!


Edited by steveincolo, 19 April 2024 - 03:56 PM.


#8 sevenofnine

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Posted 19 April 2024 - 07:12 PM

Rick...I'm glad my hunch was right and you got the Orion's mounted waytogo.gif I just had to do the "White Tee Shirt Challenge" on my Obie 20x70EDU. Maybe my 74 year old eyes just aren't good enough to see subtle shading anymore but the FOV looked evenly lit to me. I'm looking forward to your evaluation of these BT's as they are the ones that interest me the most  borg.gif



#9 revans

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Posted 20 April 2024 - 06:13 AM

Rick...I'm glad my hunch was right and you got the Orion's mounted waytogo.gif I just had to do the "White Tee Shirt Challenge" on my Obie 20x70EDU. Maybe my 74 year old eyes just aren't good enough to see subtle shading anymore but the FOV looked evenly lit to me. I'm looking forward to your evaluation of these BT's as they are the ones that interest me the most  borg.gif

Thanks again for telling me how to get the Orion mounted.  I was on the verge of buying the Orion fork mount but don't have any place to store it, but beyond that, it doesn't seem to have an elevator which, to me, seems essential.  My HAZ31 is fine for moving quickly from target to target, but it isn't made for sweeping an area of the sky and star-hopping. 

 

I should have clear skies Sunday, Monday and Tuesday.  Before and after that looks like clouds and rain. I'll probably give some first impressions with the Orion sometime mid next week.  It certainly did a good job in daylight on a distant hill.  It gave really crisp and clear views with excellent color saturation.  The FOV looked uniform and bright.  And I think I'm going to like the 90 degree eyepiece orientation more than the 45 degree (for astronomy at least).

 

The best part of the Orion was the price.  I see why the Oberwerk BT-100 ED costs more.... it has a more elegant case and the objective covers are metal rather than plastic... and little things like that. Plus the fact that it is 4 lbs lighter.  But the Orion does have that 90 degree eyepiece orientation which really should help with zenith viewing.  And the optics look good so far. 

 

Thanks again !  

 

Rick


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#10 GlennLeDrew

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Posted 22 April 2024 - 12:00 AM

Illumination fall-off occurring over a suitably small range in visual angle is much easier to see than is a more gradual fall-off.

 

This edge darkening suggests some relatively near-to-focus obstructor. Such as the combo of a small rearmost prism aperture and a large eyepiece field stop. Certain eyepieces can exhibit edge darkening all by themselves, too.

 

Most binos will suffer some amount of a gradual fall-off, which can commence immediately from the field center or not far from the field center. A frontmost prism aperture is the usual cause of this.

 

if the fall-off is gradual, and commences from the field center or nearly so, a most surprising amount of diminution is required in order to be sensible. Of this form, I would challenge anyone to reliably detect a fall-off to 50% brightness. Indeed, a nicely uniform dimming from field center to field stop that has illumination at the field edge of just 25-30% can be hard to discern.




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