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Brandt telescopes

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#26 Jeff B

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Posted 28 June 2020 - 10:29 AM

 A lens has two major types of optical aberrations, spherical and chromatic. As you know spherical aberration is when  the figure on the glass causes light not to come to the same focus position.  A  fully reflective system has no color error just errors caused by the figures. 

   In an achromat even if the lens is perfectly figured as per the design to have no spherical aberration, that only happens at one wavelength which is usually in the green. That is why  you test with a green filter to isolate that  wavelength. With the  blue and red  image, they  that don't come to the same focal position and two these wavelengths will show spherical aberration even thou the lens is perfectly figured in the green. This is called spherochromatism ie spherical aberration as a  function of wavelength. 

    So even if you have a perfectly figured achromat in the green you still have the problem that red and blue don't come to same focus. So you have a wavefront error. You have a sharp image formed from the wavelengths that come to focus well enough to form a diffraction limited image swimming inside a blurry image caused by the other wavelengths that don't come to focus. That is what causes the purple halo around bright  objects.

    Now think about what happens  when the lens is not figured correctly ie it shows spherical aberration in the green. Now the image formed in green light has errors but this also translates to  the red and blue image which already have spherical aberration. So these wavelengths  have increased  in spherical  plus it causes more  defocus across all wavelengths.  You already have  chromatic aberration to start and now it  has also increased.

   So to summarize in a lens, spherical and chromatic go hand  in hand so if one changes so does the other. Hopefully this makes sense and explains my above statement.

 

             - Dave 

Ok, now I understand your comment.  You were speaking more broadly to include other wavelengths other than green.  I thought you were referring to longitudinal chromatic aberration in just green , which didn't make sense to me since the image was taken in a very narrow band around green light.  

 

Regarding spherochromatism.  It's been my experience in DPAC that achromats, which can have boat loads of longitudinal CA, especially with a fast achromat, have comparatively little spherochromatism relative to the longitudinal errors but also when compared to some faster APO's.  I just posted in the refractor forum some DPAC shots of two 6" F10 achromats. While the longitudinal CA is readily seen, especially in blue, both samples show very little spherochromatism.   I suspect that the under/over correction seen in Roberts scope in green, may very well carry over into both the red and blue.  But that's a guess.

 

Jeff   


Edited by Jeff B, 28 June 2020 - 10:30 AM.


#27 clamchip

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Posted 28 June 2020 - 11:14 AM

Here's the lens at focus.

It has a diamond shape ring? in the center affecting the lines.

The lines bow OUT inside focus, and bow IN outside focus, so maybe the lens spacing in not quite there, but very close.

The black mark at 2 o'clock is a clamchip.

I check with red and blue filters and the correction looks the same both sides with a slight possibility the lines are not as

sharp in blue, and maybe just maybe slightly straighter in red.

Robert

 

IMG_9860.JPG

IMG_9861.JPG

IMG_9857.jpg


Edited by clamchip, 28 June 2020 - 11:55 AM.

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#28 starman876

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Posted 28 June 2020 - 12:11 PM

It is so great to be able to check in both the red and blue ranges of the spectrum.   If you check a apochromat in all three colors you will see rather straight lines for all three colors.   That is the great benifit on an apochromat.   Also, makes it great for imaging with CCD imagers. 


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#29 clamchip

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Posted 28 June 2020 - 12:11 PM

I went over my notes from about 10 years ago about the respacing of this Brandt 6 inch f/13.3 lens.

10 years ago! it seems like last year.

The lens came from ebay with .040" spacers. At focus it had color fringing, slight de-focus color free.

So I knew something was wrong.

I used the color fringing to adjust the spacing. I just kept playing with the thickness of the spacers

until color fringe was equal distance ether side of focus, purple one side, yellow/green the other.

I settled on .005" spacers.

 

Robert 



#30 Jeff B

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Posted 28 June 2020 - 01:01 PM

Robert, your lens is slightly over-corrected in green.  That zone seems to cover maybe ~30% of the aperture.  However, that's only 11-12% of the glass area and it's at the center where the glass does minimal work.  You should be able to clearly see it though in the star test, as a bright spot in the center of the diffraction pattern close to focus (5-6 rings) on one side of focus and a dark spot on the other side.  It should disappear into the airy disk at focus, with a trivial amount of light from the zone spilling into the first diffraction ring.

 

Being a little undercorrected  may not be such a bad thing when the scope is trying to cool down as the cooling process tends to add undercorrection to the figure in green.  However, taking it from a cool house to a warmer outdoors will do the opposite. 

 

But what's so nice about DPAC is that it's such a great, quick and easy tool to see what's going on.  It would be a relatively easy way to tweak the spacers some more to maybe get straighter lines in green (except for that little kink in the middle).  I've done this before with a couple of achromats with good results.   However, if you get great views with the lens as is, why bother.

 

I will have to dig around for some photos of the OTA I wrapped around a Brandt 8" F13.3 objective I had in the 1980's.  A wicked good lens.  However, Brandt was very upset with me when I told him I was having a local coating shop apply a Mgfl coating to the lens.  He said I was taking a terrible chance on messing up the lens.  It came out fine...though, I have to say I saw no real differences in image brightness and contrast after coating.

 

Jeff


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#31 DAVIDG

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Posted 28 June 2020 - 05:26 PM

"However, if you get great views with the lens as is, why bother." That is problem you can not be getting a great image with the errors showing in double pass autocollimation of this lens. The test is showing problems that prevent a great image. This what I keep seeing over and over,  which is that the image produced by optics like this are believed to be producing a great image. It is only when you have actually looked through a telescope that bench tests  to  show to have a total polychromatic  wavefront of 1/8 wave or better will you see what you have been missing. This would be especially evident if you have this scope and another of the same aperture with true 1/8 wave or better optics  and operating at the same magnification to view an image of Jupiter with. It would be like seeing an image produced by the old analog TV next to modern HD one. Until then one is easily fooled that the image they are view is "great".

 

          - Dave 


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#32 tim53

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Posted 28 June 2020 - 05:41 PM

Is there a quick way to determine which surface has the greatest error, and perhaps refigure it?



#33 starman876

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Posted 28 June 2020 - 06:39 PM

on the other hand I have one lens that puts up nice straight lines and will not provide a decent image when looking throught it.  It is an old Brandon birder.  I suspect it was optimized for low power viewing.   It looks awesome viewing birds lol.gif



#34 DAVIDG

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Posted 28 June 2020 - 07:20 PM

on the other hand I have one lens that puts up nice straight lines and will not provide a decent image when looking throught it.  It is an old Brandon birder.  I suspect it was optimized for low power viewing.   It looks awesome viewing birds lol.gif

 I would be very interested in seeing that lens and the DPAC results.  One way that  the lens could show excellent results in DPAC when tested with monochrome light and poor on the stars is that it has ton of chromatic aberration.  If you tested in Red, Green and Blue,  and you find that they  come to different focal points were red and blue should focus at the same point behind green then that is your answer. The odds of that are low because that requires  some major design flaw and I can't think of any reason why a scope designed  for bird watching would be designed that way. 

    The other simple answer is that you  tested the lens in  the correct orientation on the test stand but it is installed backwards in the telescope.  

   Here is  a picture  I have shown many times of a Tinsley 3" lens that was installed backwards in a scope that my clubs has. When tested correctly it shows straight lines but a large amount of spherical when backwards BUT many club members said the image was great when backwards.

   DPAC doesn't lie and there is most likely a simple  answer to this issue.

 

                     - Dave 

tinsleydoublepass.jpg


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#35 starman876

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Posted 28 June 2020 - 07:34 PM

 I would be very interested in seeing that lens and the DPAC results.  One way that  the lens could show excellent results in DPAC when tested with monochrome light and poor on the stars is that it has ton of chromatic aberration.  If you tested in Red, Green and Blue,  and you find that they  come to different focal points were red and blue should focus at the same point behind green then that is your answer. The odds of that are low because that requires  some major design flaw and I can't think of any reason why a scope designed  for bird watching would be designed that way. 

    The other simple answer is that you  tested the lens in  the correct orientation on the test stand but it is installed backwards in the telescope.  

   Here is  a picture  I have shown many times of a Tinsley 3" lens that was installed backwards in a scope that my clubs has. When tested correctly it shows straight lines but a large amount of spherical when backwards BUT many club members said the image was great when backwards.

   DPAC doesn't lie and there is most likely a simple  answer to this issue.

 

                     - Dave 

attachicon.giftinsleydoublepass.jpg

Been there and done that Dave smirk.gif  It is just a strange lens.   I have not tested it in red and blue.   Other lenses I have that are not worth the effort to look through also look bad in DPAC.  I tested the Brandon lens mounted on the tube so no  chance of reversing the elements. 



#36 Jeff B

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Posted 28 June 2020 - 07:42 PM

"However, if you get great views with the lens as is, why bother." That is problem you can not be getting a great image with the errors showing in double pass autocollimation of this lens. The test is showing problems that prevent a great image. This what I keep seeing over and over,  which is that the image produced by optics like this are believed to be producing a great image. It is only when you have actually looked through a telescope that bench tests  to  show to have a total polychromatic  wavefront of 1/8 wave or better will you see what you have been missing. This would be especially evident if you have this scope and another of the same aperture with true 1/8 wave or better optics  and operating at the same magnification to view an image of Jupiter with. It would be like seeing an image produced by the old analog TV next to modern HD one. Until then one is easily fooled that the image they are view is "great".

 

          - Dave 

Dave, I'm usually with you but I have to respectfully agree and disagree.  I've had, and do have, and use optics that fit that criteria, or close to it, from C to F.  They are truly great scopes and give great, sharp images on Jupiter and Saturn as well as the moon.  I also have some achromats that give great images on those objects, even those that have errors similar to those of Robert's lens and even in comparison with well documented APOs.  But they are achromats. 

 

I recently did extensive comparisons of an Istar 6" F10 achromat with and without a matched Chromacor O1.  In DPAC, the base lens was very similar to Robert's lens but undercorrected ~ 1/5 to 1/6 wave in green with a similar central zone.  Red and green are very close to each other in focus, blue, not so much.  There is scant, if any astigmatism and no coma seen in high power star testing.   Taken on its own the lens produced some great viewing with sharp images of doubles, the moon, and, yes, Jupiter (though low in the sky, the seeing was quite steady).  However, the addition of the Chromacor did clean up the images nicely giving  slightly sharper, and basically color free views.   Those views were great too.  On the bench, with the Chromacor, the optical system was very neutral spherically from blue to red with just a tiny bit of residual blue focus error and the center zone was still there but seemed a little reduced in intensity.  But, for me, I still found the base lens providing me with "great', sharp, views. 

 

I'm a very skilled and experienced observer, as are you, so, I suspect we differ on what we consider to be a "great" view.  

 

Jeff



#37 DAVIDG

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Posted 28 June 2020 - 08:46 PM

Been there and done that Dave smirk.gif  It is just a strange lens.   I have not tested it in red and blue.   Other lenses I have that are not worth the effort to look through also look bad in DPAC.  I tested the Brandon lens mounted on the tube so no  chance of reversing the elements. 

 What model Brandon scope is it ? There must be other optics between the lens and the eyepiece like set of prisms to erect the image. There is a simple answer to this, like I said DPAC doesn't lie.

 

             - Dave 




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