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can this be corrected ?

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#1 AlRos

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Posted 27 September 2020 - 08:35 AM

Hi Folks

 

I am struggling with trying to identify the cause of coma like aberration on an old C80 ED refractor. aside from the aberration, the focus is not bad across the field.  I posted in the astrophotography forum and got a few answers suggesting tilt was the issue. I was told that in true coma, the tail points away from the centre which is opposite to what is seen here. So I purchased a tilt adjuster, but when I try and correct the coma that is occurring in one corner of the image, as expected the opposite corner gets further out of focus so the end result is worse than the original. I am a complete non expert on optics, but it looks to me when I look at my images from this scope, that the bottom left (as in this example) is the best and If I could move the image so  that section was closer to the middle, I could improve the overall image.

 

this sope is relatively non adjustable. the focus rack and pion type is fixed, but I am wondering if there is a bench test I can use to further diagnose the problem and if for example rotating the objective elements might improve things. I could just give up on this scope, but I am hoping I could learn something about scope optics  adjustments if this can be improved

 

thanks

Al

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#2 AstroBrett

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Posted 27 September 2020 - 09:18 AM

Tilt is usually corrected by placing a tilt adjuster in the optical train. A typical example is:

 

https://agenaastro.c...ds-art8022.html

 

However, you should first check your focuser drawtube and make sure it does not have noticeable play that would allow it to sag under the weight of the camera, and could yield similar results which a tilt adjuster would not be able to compensate. If you want to check that possibility, take an image of a star field, then rotate the OTA 180 degrees and take another (or better take several images at various orientations). If the elongated stars move around the frame, then the likely cause is sag of the focuser tube. If the distortion stays constant, a tilt adjuster should be able to correct it.

 

Brett



#3 davidc135

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Posted 27 September 2020 - 09:22 AM

How does the tilt adjuster work? I would think that tilting/ shimming the entire objective/cell relative to the tube would move the bottom left imagery into the centre.

 

A bit more complicated would be to remove and disassemble cell and objective and check that, if there were spacers between the lenses, that they were even and there was no air wedge.  David

 

PS posts crossed Brett's approach sounds like the first step


Edited by davidc135, 27 September 2020 - 09:26 AM.


#4 AlRos

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Posted 27 September 2020 - 09:50 AM

Hi 

 

Thanks for the responses. So I did do the experiment of rotating the camera and in fact the aberration moves according to the rotation angle as AstroBrett suggested. and I did use a ZWO push pull type tilt adjuster just before the camera. This is the one here that I used which appears similar to the one Brett suggested

 

Looking at the OTA and focuser assembly I can't get it to sag very much with hand pressure and also the tilt correction that partially fixes the issue is a diagonal from top left to bottom right when looking at back of scope using the tilt adjuster. 

 

this is why I am kind of stumped about this issue - I really thought the tilt adjuster would do the trick but it didnt correct. if the objective was slightly tilted would a tilt adjuster just ahead of the camera correct that? or does a tilted objective result in the optical centre being thrown off?

 

Is there a way I can reproduce this aberration on a bench ? I quickly experimented with taking images of a screen door hoping I could see some aberration, but I could not detect any. If there was a bench test I could do,  then I would feel a bit more confident playing with the objective cell 

 

thanks for the assistance

 

Al



#5 AlRos

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Posted 27 September 2020 - 12:22 PM

actually - let me correct something I stated. I rechecked the focuser and there definitely is slop in an up / down direction when the focuser is not locked and when it is locked - the lock screw does push the focuser tube downward slightly. So I will try to see if I can retest the tilt adjuster to see if I can compensate for that when the focuser is locked. but it is odd that the angle of the coma is not in the up down direction but is diagonal.

 

I did look at the objective lenses just now and cleaned them. the spacer between the elements is just three thin small square tape pieces  spaced at 1/3's . There is not much one can do there as the elements just drop in and held by the tightening ring. The only adjustment one could do is create an slightly angled ring that would drop in first then drop the objectives and adjust the rotation of the angle ring.

 

 

Al



#6 MitchAlsup

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Posted 27 September 2020 - 01:08 PM

What you are seeing in negative coma (inward pointing).

 

Certain refractor optical prescriptions can have coma in either a positive or negative sense.

 

The solution is tilt adjustment on the objective--tilt adjustment at the focuser does nothing (very little) to ameliorate this. You have to get the "sweet spot" of the objective at the center of the sensor in order to get the negative coma evenly placed around the image.



#7 AlRos

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Posted 27 September 2020 - 03:26 PM

ok thanks MitchAlsup - this is what I was thinking was going to be the issue. I will start working on a shim for the objective lens.

 

is there any "bench" test I could use - this will be a very slow process if done with just star field testshocked.gif

 

thanks again

 

Al



#8 davidc135

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Posted 27 September 2020 - 05:59 PM

How is the objective cell fixed to the tube? Is there any play that would allow a little tilt? Doesn't sound likely but the other way to properly align the objective's optical axis with that of the tube/focuser would be to tilt the objective within the cell. I suggest three supporting tabs placed under the lens and positioned in line with the spacers could tilt the objective the right amount. The difference in thickness of these tabs could be calculated in proportion to the shift of best image needed at the focal surface. Would that work? 

 

David

 

PS I think three supporting tabs would be as good or better than a ring. If the ring is rigid the objective will only ever be supported at three positions. With three tabs you determine those positions.

 

Just as an obvious check the objective retaining ring isn't too tight?


Edited by davidc135, 27 September 2020 - 06:23 PM.


#9 AlRos

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Posted 28 September 2020 - 01:11 PM

Hi David

 

Yes no way to adjust the objectives. Now at least the cell is not overtightened.

 

I experimented with the shim / tab idea last night. I started with one tab positioned positioned inline with the spacer (I didn't think of doing three tabs last night). I over did it with tab thickness just so i can get a sense of which angle would counter the coma direction I am seeing. I must admit it was super hard as each adjustment meant opening the cell rotating it so the tab was at a different angle and reassemble the cell and tighten and reinstall the scope and take a star shot.

 

The challenge for me is that it was difficult to interpret the result. I could tell when an angle was definitely worse, but I didn't find an angle  that corrected the coma. now that I think of your suggestion of using three tabs. I am thinking that using only one tab like I did probably allowed the objectives to rotate side to side during tightening and maybe that is why I had inconsistent results,

 

I need to find a way to bench test this experiment as it is too challenging to do it with star testing. The other option I was thinking was if one could purchase a push pull adjustable cell for 80mm lenses that I could fit on to the tube. that would allow real time adjustments.

 

Thanks so much

Al



#10 davidc135

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Posted 28 September 2020 - 03:10 PM

Is 3D printing a possibility?  David



#11 davidc135

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Posted 28 September 2020 - 06:18 PM

If the point of best imagery has to move y mm to reach the centre of field then I think 2 tabs will vary from the third by a bit over .1y mm if everything is aligned and the f.l is 600mm. But the adjustable cell would be better.

 

I'm surprised that the doublet has coma and would have thought it could be designed out, as in achromats?   David


Edited by davidc135, 28 September 2020 - 06:19 PM.


#12 AlRos

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Posted 28 September 2020 - 07:09 PM

Thanks for that tip David.

 

I bought this scope over 12 years ago when I was less able to discern a difference in quality. I think it was a bad sample but for the price it did pretty well as mostly a guide scope. You kind of read my mind regrading 3D printing an adjustable cell. I am starting to try and draw something up when I ran across this post. I contacted the auther to see if he still has the CAD files. but in the meantime I will try the tab idea.

 

 

I have a new mount on order (EQ6R) and I think as soon as I get it I will switch my main scope from a older SCT to a refractor - thinking of the skywatcher 120 triplet as I have gotten to the point now where bad star shapes are getting to me. but that scope is pretty pricey, I have heard the 120 doublet is very good but whatever scope I get must be adjustable. even tossing around getting edgeHD for the reach on smaller galaxies etc and with focal reducer otherwise. decisions / decisions ! I am only into astrophotography now.

 

thanks for your help.

Al


Edited by AlRos, 28 September 2020 - 07:10 PM.


#13 AlRos

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Posted 28 September 2020 - 09:55 PM

Well I started fooling around with fusion 360 and came up with this. Not sure 3d printed plastic will be sturdy enough, but credit for the idea goes to David and the original designer Chriske. I used his design to come up with this version

 

Al

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  • Adjustable Cell.JPG

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#14 AlRos

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Posted 04 October 2020 - 09:13 AM

Its gone through a few iterations, but this is what I have so far- certainly not perfect, but at the very least I can rotate the lens elements without disassembly of the scope. Just waiting for a clear night to test it out now. Still wish I could figure out a daytime bench test.

 

Al

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  • 3d Printed Lens adjustable Cell.JPG

Edited by AlRos, 04 October 2020 - 09:14 AM.

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#15 davidc135

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Posted 04 October 2020 - 10:36 AM

That looks great. How is the lens retaining ring held in place? Threaded?  Although it could be that the plastic moves over time, with heating and cooling.

 

At least the three pairs of push/pull screws will make alignment adjustments easy, viewing Polaris at high power. Hopefully that will be enough to resolve the problem.  David



#16 AlRos

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Posted 05 October 2020 - 11:41 AM

Hi David

 

the end of the scope has a 1.5 cm smooth ring, then the thread for the metal dew shield.  I have the lens ring scaled to fit on this 1.5 cm section by friction for now so I can rotate it. If this ends up solving the issue, I will extend it so it threats on to the dew shield threads. but as is it is surprisingly tight fit and solid, but I fear you are correct that it may shrink and crack in the cold.

 

Al


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