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Silly question about baffles inside refractor tubes.

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#1 emflocater

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Posted 30 July 2019 - 10:25 AM

I understand the concept of baffles inside refractor tubes to ward off stray lights. However does not the diameter opening of the baffle which I assume is smaller than the objective lens diameter, reduce the lens aperture thus causing images to be dimmer and the focal ratio to change. In other words if you read the specs on a refractor you want to purchase and that scope tube has baffles, should not the specs be changed in conjunction to the use of baffles in that scope?

 

Cheers

Don 


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#2 JGass

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Posted 30 July 2019 - 10:32 AM

The objective of a refractor brings the light to a focus, thus the converging light narrows like a cone, as you progress down the tube.  The baffle sizes should be such that the entire cone passes through their openings at the position of each baffle.

 

A flashlight placed at the scope's focus should fully illuminate the objective, if there is no vignetting occurring.  A piece of paper in front of the objective can confirm this, while the light is on at the focus.

 

 


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#3 emflocater

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Posted 30 July 2019 - 10:43 AM

So a baffle inside the tube works different than say a cap with a smaller diameter hole on the end of a dew shield just before the objective lens?

 

Cheers

Don 


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#4 RAKing

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Posted 30 July 2019 - 10:50 AM

Yes, baffles are different.  The small cap on the front reduces the aperture and increases the focal ratio - the focal length stays constant.

 

If our scopes were built perfectly, they would be cone-shaped going back toward the focuser.  But economics and common sense suggest that we build the scope with a constant diameter tube and add baffles to control stray light.

 

Cheers,

 

Ron


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#5 Sketcher

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Posted 30 July 2019 - 11:13 AM

Baffles can, if their diameters and placement are not proper, effectively reduce a telescope's aperture; but the only time I've seen this (and it's something I always check when getting a new refractor) has been with very cheap OTAs.  The finder of this scope:

 

Buttercup (CNa)

 

has a pseudo-baffle, the purpose of which is (most likely) to reduce the objective's aberrations by intentionally stopping down the aperture.  The 60mm primary telescope has one or more (I don't remember the number) true baffles.  It utilizes its full 60mm aperture; and has great optics!

 

The last telescope I saw where a pseudo-baffle was used to stop down the aperture was a 42mm f/14.3 singlet refractor.  That's right, the objective wasn't even an achromat!  But even there, I strongly suspect that the aperture was intentionally stopped down -- for obvious reasons!  I rebuilt that scope and have used it as a 1/2-inch f/47.2 singlet refractor -- to see what some people had to deal with back in the good old days.   Even at f/47.2 the chromatic aberrations from the singlet objective were quite obvious.  Yet, the telescope was "usable".  I've observed M33, among other "targets", with it. smile.gif


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#6 sg6

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Posted 30 July 2019 - 11:16 AM

Yes, baffles should be different.

As the incoming light is focused down as in cone shaped then the baffles get smaller the further down the tube. Their purpose is to reduce stray light and so reduce reflections.

 

Now the unscrupulous can actually do what your question is: Chop out some of the incoming light, well some of the edge stuff.

 

Take a poor 100mm lens, add 2 or 3 baffles and have to first one cut into the light cone and so block the light from the edges of the poor lens and so reduce chromatic and spherical aberrations. Person buys therefore a 100mm aperture scope, thinks it is fairly good as CA and SA are small and never realise thay really have an 80mm scope. Of course the cost is as per a 100mm scope.

 

You are safe with recognised named scopes. Thet cannot afford to do such and no point in them even considering such. But bet you can find on the internet some large aperture unknown name scope at silly prices and they may even boast/highlight the baffling they impliment.

 

In a way if you suspect and realise such then, even if reduced down in effective aperture but if the cost is still worth it then the choice is yours.


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#7 TOMDEY

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Posted 30 July 2019 - 11:25 AM

OK... I just made this cartoon that shows what to be aware of... pretty self-explanatory.    Tom

Attached Thumbnails

  • 04.2 vignetting jpg.jpg

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#8 Nippon

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Posted 30 July 2019 - 11:33 AM

Yes, baffles are different.  The small cap on the front reduces the aperture and increases the focal ratio - the focal length stays constant.

 

If our scopes were built perfectly, they would be cone-shaped going back toward the focuser.  But economics and common sense suggest that we build the scope with a constant diameter tube and add baffles to control stray light.

 

Cheers,

 

Ron

So true. Just look at the Canon and Nikon manual focus super telephotos before they had to have room in the tube for focus motors and anti vibration stuff. They were indeed kind of funnel shaped. The converging cone is why Tak FC100s work with a 95mm tube and why the TV85 works with a 76mm tube.


Edited by Nippon, 30 July 2019 - 11:36 AM.

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#9 RAKing

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Posted 30 July 2019 - 03:01 PM

So true. Just look at the Canon and Nikon manual focus super telephotos before they had to have room in the tube for focus motors and anti vibration stuff. They were indeed kind of funnel shaped. The converging cone is why Tak FC100s work with a 95mm tube and why the TV85 works with a 76mm tube.

I was thinking about my Tak FC-100 as I wrote that!

 

Cheers,

 

Ron



#10 emflocater

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Posted 30 July 2019 - 04:10 PM

Are Baffles tack welded to the inside of the tube or what other methods do manufactures use to secure Baffles so they stay put and don't tilt or move due to a scope being jolted or banged against or God forbid dropped or even from expansion and contraction due to temperature differences?

Cheers

Don



#11 howardcano

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Posted 30 July 2019 - 04:13 PM

Are Baffles tack welded to the inside of the tube or what other methods do manufactures use to secure Baffles so they stay put and don't tilt or move due to a scope being jolted or banged against or God forbid dropped or even from expansion and contraction due to temperature differences?

Cheers

Don

All the refractors I've owned simply used a press-fit of the baffles into the tube.  They have several radial "fingers" that keep a spring force against the tube to absorb tolerances and temperature changes.  I've never had one move (unless I did so intentionally).


Edited by howardcano, 30 July 2019 - 04:14 PM.

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#12 Jeff B

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Posted 30 July 2019 - 04:44 PM

Some good explanations here about how hey work and how they control stray light.  However, some lower-end refractors have baffling that does indeed clip the aperture.  I'm not sure why they are designed that way, other than to mask the outer edge of the lens, where the majority of optical issues occur.  

 

The baffles also set what's called the fully, 100%, illuminated field at focus.

 

A neat and informative thing to do is to note the focuser position when looking at astro-stuff.  During the day, run the focuser to that position and remove the eyepiece, then, simply put your eye right up to the opening and look inside the tube.  You should be able to see the edges of the objective. Now pan right to left a bit  You will more than likely see the edges of at least one baffles start to intrude on the edge of the objective.  That "diameter" before that happens is your fully illuminated field.  In modern, fast  APOs, especially those for imaging, that diameter can be very large.    For slower, more visual scopes, that diameter may be rather small but you will also notice that, even when you position your eye all the way over to one side, not that much of the objective is masked.  The other thing you will see is just how deep black the blackness surround the objective is.  Even simple baffling is very, effective at killing off stray light.

 

Jeff

 

 

Attached Thumbnails

  • Baffless.jpg
  • Baffled.jpg
  • Baffles Installed.jpg
  • Baffles Front.jpg

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#13 daquad

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Posted 30 July 2019 - 07:06 PM

Some good explanations here about how hey work and how they control stray light.  However, some lower-end refractors have baffling that does indeed clip the aperture.  I'm not sure why they are designed that way, other than to mask the outer edge of the lens, where the majority of optical issues occur.  

 

The baffles also set what's called the fully, 100%, illuminated field at focus.

 

A neat and informative thing to do is to note the focuser position when looking at astro-stuff.  During the day, run the focuser to that position and remove the eyepiece, then, simply put your eye right up to the opening and look inside the tube.  You should be able to see the edges of the objective. Now pan right to left a bit  You will more than likely see the edges of at least one baffles start to intrude on the edge of the objective.  That "diameter" before that happens is your fully illuminated field.  In modern, fast  APOs, especially those for imaging, that diameter can be very large.    For slower, more visual scopes, that diameter may be rather small but you will also notice that, even when you position your eye all the way over to one side, not that much of the objective is masked.  The other thing you will see is just how deep black the blackness surround the objective is.  Even simple baffling is very, effective at killing off stray light.

 

Jeff

I suggest doing it from the objective end.  Sight down the tube while shining a flashlight into the tube (to see the baffles) and place your line of sight so that the edges of all baffles (on that side) are coincident.  With an eyepiece inserted you can see whether or not the entire field stop is visible or if it is cut off to some degree.  Just make sure the field stop of the eyepiece is at the focal plane of the objective.

 

With my 4" f/15 refractor, I can see the entire field stop of most eyepieces.  However the field stop of my 32 mm UO MK 80 Konig (Masayuma) is cut off by about 25%.  So there is some vignetting with this eyepiece, but hardly noticeable in actual practice.

 

I once owned a Unitron 60 mm where more than half the field stop of a 25 mm eyepiece was cut off, thus reducing the effective aperture.  The only eyepiece that showed the entire field stop was a 4 mm ortho.  Repositioning the baffles was not on my agenda so I sold the Unitron.

 

Dom Q. 


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#14 emflocater

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Posted 30 July 2019 - 08:41 PM

So is it safe to say (based on some reply posts here...if I'm reading correctly) that some refractors with baffles can cause vignetting? If so, is this because of miss-positioned baffles in the tube? 

 

Cheers

Don


Edited by emflocater, 30 July 2019 - 08:43 PM.

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#15 GShaffer

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Posted 30 July 2019 - 10:48 PM

So is it safe to say (based on some reply posts here...if I'm reading correctly) that some refractors with baffles can cause vignetting? If so, is this because of miss-positioned baffles in the tube? 

 

Cheers

Don

 

Yes.....

 

Another example of baffle positioning.....used CAD software to setup distances and inner diameter on the plates when building my 8" F/16. Those are carbon fiber arrow shafts as spacers and 1/16" aluminum knife edge baffles that are mounted to the focuser plate. They slide in and out as an assembly and are actually spaced out using spring clips from the tube wall to allow tube currents to flow along the wall of the tube so that they dont pile up at each baffle and spill into the light path. They are spaced so that each baffle from back to front does not see the outer edge of the one in front of it thus no light can spill thru around the edges when looking from the rear of the scope yet still allow currents to flow along the tube wall. No vignetting and no stray light.

 

build78.jpg build77.jpg

 

Cutting the inner sizes and knife edges of the 5 baffles as one assembly on the mill.

 

build65.jpg


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#16 AndresEsteban

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Posted 31 July 2019 - 06:51 PM

I understand the concept of baffles inside refractor tubes to ward off stray lights. However does not the diameter opening of the baffle which I assume is smaller than the objective lens diameter, reduce the lens aperture thus causing images to be dimmer and the focal ratio to change. In other words if you read the specs on a refractor you want to purchase and that scope tube has baffles, should not the specs be changed in conjunction to the use of baffles in that scope?

 

Cheers

Don 

Don, 
the light cone from the objective to the focal plane, ends in an illuminated circle called focal area. This area has a diameter and it's usually given by the maximum internal diameter of the eyepiece barrel. Common practice is to consider:
EP size / Focal area diameter
0.965 in / 17 mm
1.25 in   / 27 mm
2 in       / 46 mm

With this in mind the ray tracing starts on the inner diameter at the objective side and ends at the focal plane with a circle of certain diameter, but NOT as a focal point as usually depicted (but wrong). In fact, if we cut the OTA in its middle section, longitudinally, what you'll have is a truncate cone, represented in the cut as an isoceles trapezoid. It's height is the Back Focal Length or BFL, which is the focal distance between the inner face of the objective and the focal plane.

Better than all these explanations, read my paper as "How to Calculate Baffles", both graphically (following Scott Berfield graphical methods) and mathematically (my work). The link to download it is:

https://mega.nz/#!xZ...JQoXEPOoJ-sHNFU

 

Hope you find it useful!

Clear skies for us all!
Andy

 


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#17 AndresEsteban

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Posted 31 July 2019 - 07:16 PM

So is it safe to say (based on some reply posts here...if I'm reading correctly) that some refractors with baffles can cause vignetting? If so, is this because of miss-positioned baffles in the tube? 

 

Cheers

Don

Yes, you're right, Don. Many excellent vintage japanese refractors have their baffle system (position and ''hole' diameter) calculated for 0.965 in eyepieces, which means in theory a maximum 17 mm  focal area diameter. When you switch to 1.25" eyepieces (which I mostly recommend, specially to use wide field 1.25" eyepieces), is not a matter of simple using a 0.965" to 1.25" adapter, but use a new drawtube terminal with already 1.25" output in it.

 

1.25" eyepiece - focuser adapter for vintage refractors with 0.965" original ep
 
1.25" eyepiece - focuser adapter for vintage refractors
 
1.25" adapter for vintage refractors
 
76.2mm (3-in) f/16 Dean Beam (OKKK), vintage refractor
 

But even doing so, you'll find in many cases that baffles somehow limit the "view" from the eyepiece side, because now, with a 1.25" ep. you have a 27 mm diameter focal area. Therefore you'll need to reposition all the baffles for this new ligth cone.

Doing so has several vantages: Full use of objective aperture (no vignetting), full illuminated focal area (excellent for prime focus imaging) and better contrast.
 

Example of a 76 f/16 vintage refractor: rebaffling and flocking (flocking = recover inner tube walls with an antirreflective material)

 

Vintage refractors rebaffling - 27mm focal area diameter
 
Clear skies for us all!
Andy

 



#18 emflocater

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Posted 31 July 2019 - 08:32 PM

 

Yes, you're right, Don. Many excellent vintage japanese refractors have their baffle system (position and ''hole' diameter) calculated for 0.965 in eyepieces, which means in theory a maximum 17 mm  focal area diameter. When you switch to 1.25" eyepieces (which I mostly recommend, specially to use wide field 1.25" eyepieces), is not a matter of simple using a 0.965" to 1.25" adapter, but use a new drawtube terminal with already 1.25" output in it.

 

 
 
 
 
 
 
 
 

But even doing so, you'll find in many cases that baffles somehow limit the "view" from the eyepiece side, because now, with a 1.25" ep. you have a 27 mm diameter focal area. Therefore you'll need to reposition all the baffles for this new ligth cone.

Doing so has several vantages: Full use of objective aperture (no vignetting), full illuminated focal area (excellent for prime focus imaging) and better contrast.
 

Example of a 76 f/16 vintage refractor: rebaffling and flocking (flocking = recover inner tube walls with an antirreflective material)

 

 
 
Clear skies for us all!
Andy

 

 

So if what you wrote and me reading correctly. if for example say someone has a refractor that it's focuser is only made for .965 eyepieces and someone uses a .965 to 1.25" adapter, then there could be possible vignetting when using a 1.25" eyepiece? Can this vignetting be from the adapter or the baffles (if the scope has them) or both, since the original focuser provides a max. 17 mm  focal area diameter but with the adapter your now using a 1.25" eyepiece with a 27 mm  focal area diameter, but the focuser will only allow 17 mm  focal area diameter of light in.

 

I have an older 60 mm Jason 311 Towa made refractor that has several baffles internally. I replaced the .965 focuser with a custom made Crawmack 1.25" focuser so I can use 1.25" eyepieces. I have not noticed any vignetting using 1.25" eyepieces up to 32 mm. The baffles appear to have large diameter center holes which I guess are allowing a larger focal area diameter for light to enter including the help of the larger replaced focuser that has helped prevent vignetting...so it seems!

Cheers

Don


Edited by emflocater, 31 July 2019 - 08:37 PM.

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#19 TOMDEY

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Posted 01 August 2019 - 10:26 AM

The nice thing about baffle design and build --- there are many perfectly good ways to do it.

 

Functionally desirable (stray light analysis and execution), is to make the telescope tube itself generous on the diameter. That is, "deep baffles" trap stray light a lot better than ones where the tube walls are barely covered. I was involved in coronagraph/planet-finder design/build for satellites... where stray light must be controlled a million/billion to one.

 

And here's one that many folks are unaware of: >>> In the most critical applications, glossy black paint provides better stray light control than flat/matte black. I know that is counterintuitive, but the specular reflections can be ray-traced and sent to benign traps... whereas the Lambertians blow all over the place!  The glossy paints also do not attract moisture and dirt... can be cleaned. The near last thing done before a satellite is shipped to integration for launch... is cleaning inside and out!    Tom


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#20 punk35

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Posted 01 August 2019 - 03:06 PM

Very interesting about the glossy paint! 



#21 AndresEsteban

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Posted 01 August 2019 - 11:22 PM

So if what you wrote and me reading correctly. if for example say someone has a refractor that it's focuser is only made for .965 eyepieces and someone uses a .965 to 1.25" adapter, then there could be possible vignetting when using a 1.25" eyepiece? Can this vignetting be from the adapter or the baffles (if the scope has them) or both, since the original focuser provides a max. 17 mm  focal area diameter but with the adapter your now using a 1.25" eyepiece with a 27 mm  focal area diameter, but the focuser will only allow 17 mm  focal area diameter of light in.

 

I have an older 60 mm Jason 311 Towa made refractor that has several baffles internally. I replaced the .965 focuser with a custom made Crawmack 1.25" focuser so I can use 1.25" eyepieces. I have not noticed any vignetting using 1.25" eyepieces up to 32 mm. The baffles appear to have large diameter center holes which I guess are allowing a larger focal area diameter for light to enter including the help of the larger replaced focuser that has helped prevent vignetting...so it seems!

Cheers

Don

You got it 100% right Don! The best way to notice vignetting by the baffles inside teh OTA is to use a "collimation cap" like the ones we use for newtonian telescope collimation. You'll find in it a central hole. Just do two more holes at (27/2 = 13. 5) aprox 14 mm from the center hole (this is for 1.25in eyepieces). Put the cap and look through the lateral and opposite new holes: you should see the complete objective inner area without anything obstructing it. Do this with the focuser at its normal working position. That's the best way to check if no vignetting is happening. it's very simple and intuitive!

Many vintage 60 mm anf 76-80 mm vintage refractors will have the baffles calculated for a central focal point. So even with the original 0.965" eyepieces there was some vignetting.

All my vintage refractors are used with 1.25" eyepeices, and all (except my 60 f/15, no need to) have been modified to prevent vignetting and make use of the full objective aperture.

Hope this helps!

Clear skies for us all!

Andy


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#22 emflocater

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Posted 01 August 2019 - 11:50 PM

 

All my vintage refractors are used with 1.25" eyepeices, and all (except my 60 f/15, no need to) have been modified to prevent vignetting and make use of the full objective aperture.

Andy can you share what you modified to prevent vignetting? Was the modification to the baffles (if so how?) or as I did, did you replace the original focuser with a 1.25" in focuser?

Cheers

Don


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#23 peleuba

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Posted 02 August 2019 - 11:26 AM

Some good explanations here about how hey work and how they control stray light.  However, some lower-end refractors have baffling that does indeed clip the aperture.  I'm not sure why they are designed that way, other than to mask the outer edge of the lens, where the majority of optical issues occur. 

 

 

 

Yes, to mask the extreme edge and all the issues contained therein as you've pointed out and to offer better color correction.  A stopped down aperture will have better color correction then the published focal ratio will lead you to believe.

 

Years ago, Burgess marketed an achromat that was claimed to have  very good color correction.  A friend looked at the telescope and did a back-of-the-napkin measurement at NEAF and it turned out to be stopped down.  IIRC, it was an 80mm telescope stopped to something like 70mm (or less).  It was years ago, I am at the office, am fuzzy on the details, will check my notes this weekend.  But it happened.

 

Burgess was incredulous when we brought it up to him...  He (Bill B.) was very proud of the telescope and its color correction.


Edited by peleuba, 02 August 2019 - 11:37 AM.


#24 Scott in NC

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Posted 02 August 2019 - 11:30 AM

Yes, to mask the extreme edge and all the issues contained therein as you've pointed out and to offer better color correction.  A stopped down aperture will have better color correction then the published focal ratio will lead you to believe.

 

Years ago, Burgess marketed an achromat that was claimed to have  very good color correction.  A friend looked at the telescope and did a back-of-the-napkin measured it at NEAF and it turned out to be stopped down.  IIRC, it was an 80mm telescope stopped to something like 70mm (or less).  It was years ago, I am at the office, am fuzzy on the details, will check my notes this weekend.  But it happened.

 

Burgess was incredulous when we brought it up to him...  He (Bill B.) was very proud of the telescope and its color correction.

Something very similar reportedly happened with the earliest iteration of what later became known as the Stellarvue Nighthawk, although IIRC this wasn't a baffling issue, but related to the inner end of the focuser protruding into the light cone.



#25 peleuba

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Posted 02 August 2019 - 11:47 AM

Something very similar reportedly happened with the earliest iteration of what later became known as the Stellarvue Nighthawk, although IIRC this wasn't a baffling issue, but related to the inner end of the focuser protruding into the light cone.

 

Yes, I often get accused of SV bashing so did not want to bring it up.  

 

I think when these imported 80mm telescopes first hit the market and offered from several different sellers, a lot of them were stopped down unintentionally do to production nuances.  Focuser drawtubes to long, baffles too wide, beauty ring to thick etc.  The end result looks great to the user - nice color correction and no edge issues.  It gives the impression that these early 80mm achros were terrific telescopes defying long standing optical theories.  When, in reality, they were longer focal length ~70mm telescopes.  


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