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Refractor star test aberration~ cause?


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Guess part of me is still the 4 year old asking "why?"

mercury 707 frac, cheap as chips, slightly floppy focusser, can't expect too much @ end of day.

Don't anyone get too hung up about it, just trying to understand the star test.

Works fine for what it is (cheap small low power scope), but a highish power star test shows a diffraction pattern like an out of collimation newt~ the rings are not concentric but drift off to one side. (old hands will know what i mean)

Spent some time adjusting focusser & moving cell from one extreme to other, plus trying to get best allignment with a cheshire.

Nothing seems to change the diffraction pattern.

My two theories~

1) lens wedge? but i'm thinking this would give spurious chromatic effects?

2) lens elements tilted in respect to one another? Guess this could be rectified playing with the thickness of the lens spacers~ maybe one is different?

 

Can anyone shed any light?m Even if it's uncorectable i'd like to understand the cause. Thankyou.

 

diffraction pattern drifts off to north of view regardless of what i do to focusser or lens cell.

 

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I have a similar issue with a 90mm F/11.1 achromat that I purchased last year. It was very cheap and, having tried adjusting the tension and tilt of the focuser, the tilt of the objective, the positioning of the lens spacers and the seating of the lens elements I've had to accept that it was a low cost scope and I've probably got it as good as it is going to be. The star test is a bit out with the diffraction rings skewed a little and brighter on one side than the other. Using the cheshire to test the objective tilt (after getting the optical axis of the focuser properly in line with that of the objective with a laser) reveals two slightly overlapping disks but not that nice fully overlapping image that I would like to see.

I've concluded that either the figure of the objective (or one of it's elements) is not that great (maybe some wedge ?) or that there is some inaccuracy in the build and fit of the (plastic) objective cell that I can't get to the bottom of.

Every now and then over the past months I've got the thing out and had a play, re-seated the focuser and the objective, even had the objective out of the cell and checked the spacers, etc, etc but I just can't get any further with the thing :rolleyes2:

I suspect my scope is from the same factory that made yours except mine has Celestron branding.

The actual performance under the stars is not bad to be fair. It splits quite tight double stars, showed some nice detail on Mars during the opposition, a sharp lunar surface, good white light solar views etc. Not much CA at F/11.1 either. But that star test and the cheshire test niggles me.

The pics below are not from my scope but are close to what I see. My cheshire images look like the right hand one and the star test of the scope at high power (200x) is somewhere between the two center ones below.

I've kind of given up trying to get things better to be honest :dontknow:

It's been quite fun playing around with it and it was cheap !

fraccolli01.jpg.da03157db6be3c13c24218235ded3549.jpg

 

fraccolli02.jpg.ef802fb647e3dd20f6069c04b8dbf26d.jpg

 

So I will be interested in any suggestions as well :smiley:

 

 

Edited by John
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The images in the bottom row are indication of tilt, usually.  Not sure of the Cheshire image to the right, looks like gross decentring of the individual lens components which is unlikely.  If the cell does not have a facility for collimating it I would suggest first rotating the lens pair to see if the deformation turns with it.  If it does, remove the lens, mark the original orientation of the two components relative to each other then slightly separate them and turn the front one relative to the rear.  Replace the lens complete in its cell and retest.    🙂

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Thanks Peter. These cells have no facility for adjusting the tilt or centering of the elements so it's a matter of using tape or similar as a shim to apply tilt. My issues rotate with the objective. Next time that I feel like playing with it I can get the objective out and try rotating one element relative to the other and see what happens.

 

 

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Thankyou Peter, I'll try the rotation method.

When you say "....are an indication of tilt,usually" do you mean of the objective as a whole, or of the elements relative to each other? Thanks.

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2 hours ago, SiriusB said:

Thankyou Peter, I'll try the rotation method.

When you say "....are an indication of tilt,usually" do you mean of the objective as a whole, or of the elements relative to each other? Thanks.

Tilt of the complete objective.    🙂

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On 23/02/2021 at 22:02, John said:

 

The actual performance under the stars is not bad to be fair. It splits quite tight double stars, showed some nice detail on Mars during the opposition, a sharp lunar surface, good white light solar views etc. Not much CA at F/11.1 either. But that star test and the cheshire test niggles me.

 


Hi John.   In the past I’ve also been very unhappy with refractor star tests, but the scope did produce great views. So isn’t that the important thing ?   I recall Al Nagler saying something like - don’t worry about the views when the scope is out of focus, just be happy if the in-focus views are good 👍

On 23/02/2021 at 22:02, John said:

fraccolli01.jpg.da03157db6be3c13c24218235ded3549.jpg

 

fraccolli02.jpg.ef802fb647e3dd20f6069c04b8dbf26d.jpg

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