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DPAC Test - Mathias Wirth MN86

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#1 Jeff B

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Posted 13 February 2020 - 05:22 PM

Some more DPAC fun, this time with my APM/Mathis Wirth MN86.

 

Now, I carped in my C5/TEC7 DPAC thread here in this forum that getting the aperture to fully show in DPAC just can't be done with my Ronchi screen holder and, especially due to the tiny fully illuminate fields those scopes have.

 

Well, the MW MN86 is exactly the same way!  It has a tiny secondary mirror with a fully illuminate FOV (when properly collimated!) of 3-4mm...maybe.  Just look at the tiny size of the secondary on the meniscus and its shadow in the DPAC images!   As a result this scope is terribly difficult to collimate while getting even illumination over the full aperture as there is little room for error due to the tiny secondary.   However, with care, I can do exactly that and the scope is indeed a planet/double star killer.

 

These images were taken today inside of focus (outside of focus were basically the same), but in order to see both edge of the primary (that's typically where bad stuff hangs out so I want to see it), I have to flip the Ronchi screen/LED holder over.   Note that on the top of the "rightside up" image, the bottom of the focuser draw tube is clipping a small segment of the aperture a bit.  In the "upside down" DPAC image you don't see that as that view falls off of the edge of the secondary.  Also the slight differences you see in the overall shapes of the dark lines between the photos are due to slightly different screen orientations and focus positions of the holder when it is flipped.  The image of the scope in DPAC is with the holder "rightside up".

 

Hover your cursor over the images to read their titles.

 

But note how smoooooth, even and straight the lines are!  The edge is very good.  At focus the surface is very, very smooth (so smooth, I really couldn't get a good shot as there was nothing there to really photograph). This is a very good optic...but I knew that before taking a look in DPAC, which really confirms my visual assessments.

 

Comments appreciated.

 

And I'll being doing the same thing for my Intes MN76.

 

Jeff

 

 

Attached Thumbnails

  • MW MN86 secondary.jpg
  • MW MN86 In DPAC.jpg
  • MW 86, Holder Right Up, Inside Focus.jpg
  • MW 86, Holder Upside down, Inside Focus.jpg

  • eros312, denis0007dl, davidc135 and 3 others like this

#2 Richard O'Neill

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Posted 13 February 2020 - 06:21 PM

 Very interesting but what is the hemispherical object on top of the optical flat and what is it's purpose there, if any? 



#3 Jeff B

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Posted 13 February 2020 - 08:04 PM

 Very interesting but what is the hemispherical object on top of the optical flat and what is it's purpose there, if any?

Its purpose is to generate questions concerning its purpose. grin.gif

 

Actually while the optical flat is really, really, really flat, the front face of the meniscus cell is not and the flat would rock around a bit.  This made it difficult (impossible really) to align the flat with the optical axis of the scope.  I resorted to using very tin paper shims around the circumference of the cell to line things up and used the additional weight of the paper weight (Hah!) to stabilize the set up.

 

Jeff



#4 Richard O'Neill

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Posted 13 February 2020 - 09:08 PM

I understand completely. In other words, what it is and it's actual purpose is a closely guarded proprietary secret. wink.gif 

 

Richard



#5 Jeff B

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Posted 13 February 2020 - 11:31 PM

In reality, it is half of a very large ball eyepiece.




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