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Questar out of focus star images

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#1 oldmanastro

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Posted 27 February 2020 - 10:29 PM

I took this tonight under excellent skies and after a two hour cooling period the "new" Questar was at work undergoing a star test . The star is Rigel and the image was taken using a firmly held iPhone. Just as the telescope is coming into focus there is a slight decentering of the bright central spot. Thus it seems just a tad out of collimation. The defocused images seem to show it. In focus (last image) the first diffraction ring seems brighter on one side. Opinions are welcome.

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  • Rigel1.jpg
  • Rigel2.JPG
  • Rigel3.JPG


#2 Mike Allen

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Posted 28 February 2020 - 10:57 AM

I looked very close at the images.  The in-focus image appears to be just slightly out of focus.  I see the first diffraction ring to be the same in brightness and thickness around its complete circumference.  I also see the spurious disk of Rigel’s companion star near the 7 o’clock position and touching the first diffraction ring.  Please note that when Rigel is observed at perfect focus with the Questar 3.5 visually, the spurious disk of Rigel’s companion star is found just outside of the first ring, not touching it.  The disk of the companion in the image looks perfectly round, as does the disk if the primary, except for some minor bumps which I might attribute to unsteady air.

 

The out of focus images show a slight de-centering which could easily be caused by not having the star not perfectly centered in the field of view.  I’ve noticed visually that even a slight de-centering in the field of view will cause issues with the out of focus image.  This issue is resolved across the field of view when the image is brought to best focus.

 

I would do the following.  Observe a second or third magnitude star near the zenith, on a night with steady seeing and after the telescope has cooled.  Importantly, choose one that does not have a companion star, and make sure the star is centered in the field of view.  Use a magnification that will readily show the spurious disk and the first diffraction ring.  Do not use the internal barlow.

 

Its difficult for me to say this, because it’s obvious you put some effort in posting your images, but there is nothing I see in the data that would cause me to believe your optics are out of collimation.  If you follow the steps I suggest in the paragraph above, please let us know what the results are.  I wish I could be there to see for myself.


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#3 oldmanastro

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Posted 28 February 2020 - 01:13 PM

The slight decentering can be observed visually right at the center of the field of view just like it looks in the defocused imaged. You are right. Rigel is not well focused. At focus the secondary is well out of the first diffraction ring. The cooling down period for this test was more than two hours before I took those images. It is really a very slight deviation. I also observed Procyon. It was right overhead for me and the deviation was there. At very high powers using my UO Orthos I can see it even more clearly. I will test again on the Zenith and try to get some images with my planetary camera. They should come out much better. One thing is that the mark on the corrector is not at 6 o'clock as it should. It is at 5 o clock. Maybe that is the slight adjustment that is needed.

 

Thank you for the recommendations. I will report on my next test.

 

Clear Skies!



#4 Les Aperture

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Posted 28 February 2020 - 08:47 PM

Did you notice if the brightness shifted to the other side of the diffraction ring as the star drifted from one side to the other in the field of view? 

 

"Les"


Edited by Les Aperture, 28 February 2020 - 08:50 PM.


#5 oldmanastro

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Posted 28 February 2020 - 09:20 PM

Did you notice if the brightness shifted to the other side of the diffraction ring as the star drifted from one side to the other in the field of view? 

 

"Les"

It seems that the problem has been resolved. I found that the corrector can move quite easily in the cell. Using a pair of nitrile gloves and being very careful, I moved it so that the mark on the side of the corrector fell at the 6:00 o'clock position. Then I tapped the cell lightly to make sure that the corrector was evenly seated. After a 60 minute cooling period outside, the in and out of focus images are now concentric and as one approaches focus everything remains centered. At the end a star image has an even first diffraction ring all around. Apparently the corrector moved out of place during the three day trip across the pond. I do find that it may be too loose in the cell. When my spanner wrench arrives I will make sure that the retainer ring is holding the corrector just snugly as it should.

 

One object that I observed for the first time today with the Questar was the moon now past crescent. It was awesome to see the level of magnification that this instrument can handle without losing definition. I had never seen something like this before. Can't wait to take some images with my planetary cam.

 

Thanks for your replies and have Clear Skies!

 

Guido


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#6 Mike Allen

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Posted 29 February 2020 - 11:40 AM

Glad to hear that you resolved the collimation issue.  The last time my Q returned from New Hope from being serviced, there was a spot of clear cement on the retaining ring that appeared to have been placed there to prevent the retaining ring from coming loose.  As an additional precaution, I’ve noted the position on the spanner wrench holes in the ring so I can detect any future movement of the ring that might occur.  Best wishes.


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#7 oldmanastro

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Posted 29 February 2020 - 03:32 PM

Glad to hear that you resolved the collimation issue.  The last time my Q returned from New Hope from being serviced, there was a spot of clear cement on the retaining ring that appeared to have been placed there to prevent the retaining ring from coming loose.  As an additional precaution, I’ve noted the position on the spanner wrench holes in the ring so I can detect any future movement of the ring that might occur.  Best wishes.

I was surprised when the corrector moved so easily and thought that the retaining ring could be loose. Then I tried to move the retaining ring using two small allen wrenches that fit the side holes. It is tight and probably has some adhesive to hold it. If everything remains in place I probably won't touch it.




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