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Could use some help figuring out cause of extra flare

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#26 azure1961p

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Posted 13 August 2014 - 08:43 PM

This is an interesting thread. My first thought was "oh brother, astigmatism" but it'd be a fine thing if this just adjusted out with some tweaking.

 

Pete



#27 gnowellsct

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Posted 18 August 2014 - 11:59 AM

No, astigmatism shows as an out of focus star which is oval.  When you rotate the mirror in its cell, the axis of rotation will change with it.

 

Someone ask you to mask off the outer edge (of the tube, or the mirror).  That's a good suggestion.  *If* the spikes were evenly distributed all the way around, it would be a fairly simple diagnosis, which you confirm by masking the edge, and seeing if the spikes go away.  But that's the way modern machines would create TDE (turned down edge).    But you have an f/8.3 which is an unusual focal length for mass produced Newts and suggests that your mirror was made by an amateur telescope maker (atm).  A hand ground mirror by an atm who was just learning might have something odd like TDE in some parts of the mirrors and not others.  The bigger your mirror the more likely you will have TDE.

 

It is also possible that you have TDE *and* a mispositioned secondary or primary misalignment.  

 

Bad TDE shows itself as long tendrils that come out of a star that is in focus.  There will be the classic four spikes from the secondary and then the tendrils will be long and thin and come out way past the vane spikes.  But where TDE is less severe you get shorter spikes, and what you have in your pic is not particularly bad if that is all there is.  Still, if they go away when you mask the outer edge of the mirror, that is a hint.

 

The bad news is that I had a mirror which showed asymmetric spiking like that, it was almost as if the spike was a stick and the star a popsicle.  This effect showed even on planets such as Jupiter.  The mirror in question was badly deformed by incomplete annealing and now serves as a doorstop.   A foucault test might show a grand canyon type valley in the mirror.  In my case the mirror was so bad you could see three ghost images of Jupiter in addtion to Jupiter when in focus.  There is a currently active thread about replacing a GSO mirror that has the pics.  I'll try to find the link.   We cannot rule out (yet) that you have a similar defect, but perhaps not as severe.  In any event even a less severe fracture than what I had would prove fatal to the mirror's prospects.  

 

Greg N

 

http://www.cloudynig...mary/?p=6158215


Edited by gnowellsct, 18 August 2014 - 12:14 PM.


#28 gnowellsct

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Posted 18 August 2014 - 12:39 PM

No, astigmatism shows as an out of focus star which is oval.  When you rotate the mirror in its cell, the axis of rotation will change with it.

 

Someone ask you to mask off the outer edge (of the tube, or the mirror).  That's a good suggestion.  *If* the spikes were evenly distributed all the way around, it would be a fairly simple diagnosis, which you confirm by masking the edge, and seeing if the spikes go away.  But that's the way modern machines would create TDE (turned down edge).    But you have an f/8.3 which is an unusual focal length for mass produced Newts and suggests that your mirror was made by an amateur telescope maker (atm).  A hand ground mirror by an atm who was just learning might have something odd like TDE in some parts of the mirrors and not others.  The bigger your mirror the more likely you will have TDE.

 

It is also possible that you have TDE *and* a mispositioned secondary or primary misalignment.  

 

Bad TDE shows itself as long tendrils that come out of a star that is in focus.  There will be the classic four spikes from the secondary and then the tendrils will be long and thin and come out way past the vane spikes.  But where TDE is less severe you get shorter spikes, and what you have in your pic is not particularly bad if that is all there is.  Still, if they go away when you mask the outer edge of the mirror, that is a hint.

 

The bad news is that I had a mirror which showed asymmetric spiking like that, it was almost as if the spike was a stick and the star a popsicle.  This effect showed even on planets such as Jupiter.  The mirror in question was badly deformed by incomplete annealing and now serves as a doorstop.   A foucault test might show a grand canyon type valley in the mirror.  In my case the mirror was so bad you could see three ghost images of Jupiter in addtion to Jupiter when in focus.  There is a currently active thread about replacing a GSO mirror that has the pics.  I'll try to find the link.   We cannot rule out (yet) that you have a similar defect, but perhaps not as severe.  In any event even a less severe fracture than what I had would prove fatal to the mirror's prospects.  

 

Greg N

 

http://www.cloudynig...mary/?p=6158215

 

p.s. in addition to masking the mirror another test--for the kind of fracture in the link--is to rotate the mirror in the cell and see if the defect rotates with it.   A defect in your *alignment* ought not to rotate with the mirror.  



#29 Whichwayisnorth

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Posted 18 August 2014 - 07:55 PM

Just a quick update, I am actually testing tonight and I'll try to do a few of the suggestions here.  I'll mask the outer edge to see if I can block whatever is causing the spikes.  Also I will, if I can, try to rotate the mirror.

 

One thing I have done in the interim is I followed the procedure in another thread for using a webcam, properly centered in the focuser, to align the optics.  I found a few things that were not right.  Mainly the secondary mirror was not aligned correctly.   I have been testing another OTA but I'll have time tonight for this one and I'll see what I can find out.

 

Stay tuned..



#30 Whichwayisnorth

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Posted 29 August 2014 - 06:34 PM

Ok hopefully both of these videos work unlike one of the videos I tried to post previously.

 

This is me trying to work out the collimation process.  If you have 15 minutes to spare to watch these videos I would appreciate some feedback on anything you notice that I am doing wrong.

 

http://youtu.be/sraXh5W74kg  - Part 1

 

http://youtu.be/wVK7bAtMHE0 - Part 2



#31 Starman1

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Posted 29 August 2014 - 08:01 PM

Oh boy.  You made it a LOT more difficult than it should have been.

Collimation is 3 steps, and your 3rd step isn't the one you need to follow.

1) Centering the secondary mirror under the focuser.  You can use the sight tube/cheshire combination tool for that.

2) Aligning the secondary with the focuser axis.  You can use the sight tube part of the combo tool, or a simple beam laser--your choice.

But you want to use as few adapters as possible to avoid registration issues.

3) Aligning the primary with the pre-determined focuser-to-mirror axis.  This requires the cheshire part of the combo tool, or a barlowed laser.

Unfortunately, the accuracy of the return beam of the simple laser on the 45 degree screen is not accurate enough for f/3.8.

You needed to barlow the laser or use the cheshire part of the combo tool.

 

A 4th step would be desirable at f/3.8 because of the tight tolerances in that f/ratio and because of the use of a coma corrector

(virtually mandatory at that f/ratio) which tightens collimation tolerances even more.  That is the use of an autocollimator like the Catseye XLKP.

It will help eliminate the residual errors that will keep you awake wondering why the camera images aren't right.

 

Rather than go into all the description necessary, here is a tutorial to read, with illustrations:

http://www.cloudynig...escope-v4-r2599

and here is the math part of it, left out of the article:

http://www.catseyeco... Tolerances.pdf








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