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Astigmatism in objective

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#1 Doug Doonan

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Posted 19 April 2024 - 04:19 AM

I have some astigmatism in the air spaced doublet objective in my home built 80 mm x 500 tscope.

Is here a way to make a very thin conic lense that I could add to this arrangement that would cancel out the astigmatism.



#2 RichA

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Posted 19 April 2024 - 04:28 AM

I have some astigmatism in the air spaced doublet objective in my home built 80 mm x 500 tscope.

Is here a way to make a very thin conic lense that I could add to this arrangement that would cancel out the astigmatism.

You could go to a glasses optician, they may be able to make you a sub-diameter lens that would do the job.  They even coat them.  But simpler, get another objective. 3rd option, try rotating one element against the other to see if some of the astigmatism can be partially eliminated.


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#3 Ben the Ignorant

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Posted 19 April 2024 - 05:42 AM

It looks like a lot of hassle for a scope that's only an achromat, am I presuming right? How much did it cost in time and money? How much would an extra lens cost? How much for a trouble-free used 80mm semi-apo, or even another well-made achro?



#4 Darren Drake

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Posted 19 April 2024 - 06:16 AM

Make sure the astigmatism isn't in your eye or in the diagonal before anything.   Then double check the collimation of the objective.  In short, however ,the answer to your question is no.


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#5 peleuba

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Posted 19 April 2024 - 08:35 AM

I have some astigmatism in the air spaced doublet objective in my home built 80 mm x 500 tscope.

Is here a way to make a very thin conic lense that I could add to this arrangement that would cancel out the astigmatism.

 

You may have already tried this...  But, try rotating the elements with respect to each other to see if the aberration in one would cancel out the aberration in the other?  An eighth of a turn then retest.

 

As Darren said, see if its in your eyes, the mirror diagonal etc.


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#6 DAVIDG

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Posted 19 April 2024 - 09:14 AM

 You need to check the complete alignment of system to be sure the astigmatism is not coming from misalignment . To check that the lens is correctly spaced and also aligned, first shiny monochrome or semi monochrome light on it. A common CFL bulb will work well. You should see round and centered interference rings like this. 

 

lens interference pattern CFL bulb.jpg

 

  If the pattern is not round and also centered that means the air gap is not uniform and that will cause lateral color and astigmatism. So you'll need to adjust the spacers to achieve round and centered interference rings. If  the pattern is oval shaped and it rotates then you rotate one element against the other then that means one of  the two surfaces has astigmatism .

 If you achieved a round pattern and centered pattern  place a laser collimator in the focuser and make a paper mask the same diameter as the objective and with the center marked. Place it over the objective and the laser spot from the laser in the focuser should hit the center mark on the mask. If not adjust the tip and tilt of the focuser. Having the focuser misaligned can also cause astigmatism.

  Now remove the paper mask and point the scope at a wall about 6 feet away. You will see on bright laser spot on the wall and if you see other dimmer ones  around the bright one then your lens is not collimated. Adjust the tip and tilt of the lens cell so all the spots merge into one. Now your lens is fully adjusted. 

   If you are using a star diagonal I would observe without to be sure the astigmatism is not from it. You can do this in the daytime by observing a glint of sunlight off a distance object that act as an artificial star.  If  the  astigmatism is still present then you can use a weak cylinder lens over the eyepiece to correct for it. Televue sold them but I would try Surplus Shed to get a few inexpensive one to determine the power that you need.

 

                  - Dave  


Edited by DAVIDG, 19 April 2024 - 09:15 AM.

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#7 Jeff B

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Posted 19 April 2024 - 09:35 AM

I'll bite too.

 

I'm deep into achromat ATM-ing.  Before I do any star testing, I always first collimate my focuser with the center of the objective using a quality laser in it firing down the tube.  I adjust the focuser such that the beam passes through a small hole in the middle of a paper mask placed over the objective.

 

Next, I use a Cheshire eyepiece and adjust the objective cell to make sure the reflection dots from the lens elements line up with each other to form a single reflection dot that is, hopefully, concentric with the Cheshire's peep hole. 

 

Only after I get that stuff "squared away" as best I can, do I do a high power star test to look for coma and astigmatism (also, use a good green filter too).  

 

In my experiences, after all the preliminaries above, I've found it rather common to see a bit of astigmatism at high power, especially with fast scopes like yours.  If it's mild to moderate, I simply further adjust the objective cell to eliminate it or, at least, minimize it, then lock that adjustment in place.  I do this because the dominant off axis aberration in a modern achromat is astigmatism.  I've found that subsequent messing a little with the lens cell tilt often adds astigmatism of opposite sign, cancelling the astigmatism I see.  This has worked very well for me.

 

Now, there is also that coma thing too.  Do you see any of that?  That's a very different animal.

 

Good luck!

 

Jeff

 

Edit:  Hah, Dave beat me to it!


Edited by Jeff B, 19 April 2024 - 09:37 AM.



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