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Fixing an Achromat

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#1 Jeff B

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Posted 05 April 2020 - 12:24 PM

I've written about doing this sort of thing before but here is another example of what turned out to be an easy fix for my Istar 8" f9 achromat.  The build and some of the DPAC results are documented here:


https://www.cloudyni...uild/?hl= istar


During a build I align the focuser with the center of the lens using Glatter (RIP sir) laser.  I do this by leaving the screws holding the tail piece to the tube snug and then tapping on the tail piece's flange to move the laser dot to the center of the objective (see photo), then, carefully tightening the screws down to lock in place the position of the tail piece.  I then use a Cheshire eyepiece to line up the dots that come from the rear surfaces of the objective elements using the objective's push/pull collimation screw sets.  Everything with this build went quickly and turned out well.  The scope was now "collimated".   


Or so I thought.


In DPAC however, I saw "clocking", a small rotation of the grating lines as I would rack between inside and outside of focus.  This suggested astigmatism, which, unfortunately, was confirmed when I took the beast outside for a live star test one night.  Looking at Aberrator and the images of astigmatism at Vlad's site Telescope Optics.net, I estimated it to be ~1/3 wave at full aperture, decreasing to a still very visible 1/5 wave-ish at 7" aperture.  




But I have run into this before with many other achromats, particularly, the Synta sourced jobbies like the standard 6" F8's.  So I've gotten clever in dealing with mild to moderate amounts of astigmatism in achromats.


Specifically, since astigmatism is by far the dominant off axis aberration in achromats (which are coma free), I've learned I can typically dial it out using the objective's collimation screw sets.  I do this outdoors at high power with a 100 micron artificial star placed ~65 yards away from the scope (I have since learned to place a high quality barlow lens over the star's 1.25" nosepiece, which makes the star even smaller).  I then tweak the collimation screw sets and watch what happens to the astigmatism.  I've gotten to the point now where, depending on the orientation of the astigmatism and how it rotates through focus, I can pick out which collimation screw set (s) to start with.


Sure enough, within about 10 minutes, at ~230x, I had dialed it out.  Subsequent live star testing a couple of nights ago confirmed it and the moon was now very, very sharp at the 7" aperture (but of course with a shade of purple as it is, after all, an achromat).   Popping it back into DPAC, confirmed, no clocking of the grating lines through focus (however, now with the Cheshire eyepiece, I see two, rather than one, reflection dots).  This is further proof to me of how effective using both DPAC and star testing can be, especially to diagnose and fix problems. 


So, even though the traditional tools available to me said the the scope was initially "collimated", it was not.  In fact, the axis of best optical correction (least astigmatism in this case) was slightly off the axis indicated by the Cheshire eyepiece.   However, some may, quite correctly,  say what I was actually doing was, via the collimation screws, adding astigmatism of the opposite sign and orientation, cancelling out what was there "on axis".


So any of you achromat owners out there stuck with a scope that shows mild to moderate astigmatism, don't despair, you may have recourse available to you if your scope has objective collimation screws up front...and a clear night....or an artificial star of some kind.  



Attached Thumbnails

  • Laser A.jpg

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#2 Sol Robbins

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Posted 05 April 2020 - 07:47 PM

Interesting. Thanks for this info.

#3 stevew


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Posted 05 April 2020 - 10:45 PM

Jeff, can you show us what clocking looks like...

#4 oldmanastro


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Posted 06 April 2020 - 08:54 AM

On smaller long focal length refractors I have used the Cheshire always with success in getting the correct collimation. On larger apertures, even though the Cheshire says its collimated, small adjustments have been needed at the eyepiece while observing a star. Your adjustment may have corrected a tilt in the lens cell that was there from the beginning but so small that it is not noticeable until you do the star test. I had a telescope with an astigmatism problem that was traced back to the lens cell.  Thank you for your this information.

#5 Jeff B

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Posted 06 April 2020 - 10:14 AM

Jeff, can you show us what clocking looks like...

Sure, here are inside/outside of focus DPAC images through the Istar stopped to 7" aperture with the Chromacor U I in the light path.  You see the "clocking" of the lines as a clockwise rotation of the outside of focus lines relative those inside of focus.  Place the cursor over each image to read their titles.



Attached Thumbnails

  • 8 Istar, Green, 7 Stop, UI, Inside.jpg
  • 8 Istar, Green, 7 Stop, UI, Outside.jpg

Edited by Jeff B, 06 April 2020 - 10:15 AM.

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