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Flares on bright stars.

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#1 stevehiebert

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Posted 06 April 2018 - 01:41 PM

Hi Everyone, I recently purchased a PN208 (208/812mm) scope from Explore Scientific.  I've seen some really excellent images coming from this scope.  I have attached a couple of images (Don't judge them on image quality as these are just to show the issue) I am getting some serious flares happening on brighter stars.  At first I thought it was collimation however I have run through laser collimation several times and the results remain the same.  I'm hoping someone would have an answer as to why is causing these flares(I am NOT referring to the diffraction spikes, I would expect those) .  As you can see on the M45 image the flares seem to go in different directions depending on the location in the frame.

 

Any help would be appreciated.  Thank you.

Attached Thumbnails

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#2 theastroimager

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Posted 06 April 2018 - 02:11 PM

Have you tried collimating without the laser device? 

 

I would examine both mirror surfaces carefully. It's almost like there is something transparent in front of one/both of the optical surfaces.

 

I would certainly send these images to ES and suggest that if they don't have a quick fix, that they need to replace the 'scope.



#3 WadeH237

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Posted 06 April 2018 - 02:39 PM

I have a refractor that displayed very similar behavior when I bought it.  The problem turned out to be pinched optics.  There were 4 screws that went radially into the primary cell and they were too tight.  I backed them off just a bit and the problem went away.

 

Given that this is a Newtonian, I doubt that it's exactly the same issue.  But I would start by looking for any optical element held by 4 points (which I've never seen on a Newt).  Or by looking at intrusions into the light path 90 degrees apart (or thereabouts - your artifacts don't look exactly 90 degrees apart...)

 

Actually, the aberration looks different to me on different stars.  Looking at just the last image above, the star in the lower left looks like there are 4 artifacts around the star.  The star in the upper right might be an interaction with 3 aberrations at 120 degrees apart combined with diffraction spikes at 90 degrees.

 

It might be worth checking the clips that hold the primary mirror in the cell to make sure that they are not too tight.  Also (depending on how the secondary mirror is mounted to the holder), make sure that the secondary collimation screws are not too tight.



#4 leveye

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Posted 06 April 2018 - 03:15 PM

Could be pinched optics. The primary mirror clips are putting pressure on the primary mirror surface and distorting it.They should not touch the surface at all.They are there just to hold the mirror in the cell it it's tilted too much.


Edited by leveye, 06 April 2018 - 03:16 PM.


#5 nemo129

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Posted 11 April 2018 - 07:57 AM

What coma corrector is being used and is it possible that it is in the light path? I had an issue a Baader RCCI 2" Coma Corrector where it extended slightly out of the focuser tube into the light path and my stars looked a lot like the ones in the OPs pictures. If not that then is is possible that there is something else in the light path ...well except the spider vanes and the secondary?



#6 Cotts

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Posted 11 April 2018 - 09:49 AM

I don't buy the 'pinched optics' theory.     This has an effect on the shape of the diffraction pattern as seen at high power. (At 208mm aperture and 812mm focal length the Airy pattern will be so small it may fall entirely on only one pixel and be completely unresolvable in your images. For 812mm f.l. and a typical 5-micron pixel the image scale will be very close to 1 arc second per pixel - the Airy disc of a 208mm scope is only half that size - you'll never image it)  

 

Stick the highest power  eyepiece you have in the scope and examine in focus the diffraction pattern of a 3rd magnitude (or so) star nearly overhead and exactly in the centre of the field to see if it is a tight central spot with a circular ring or two concentrically around it.  If the clips aren't touching and the scope is collimated well you will get a good diffraction pattern.  Once you have a good diffraction pattern (which your images can't resolve anyway) you need to move on to what is really causing the problem.

 

Look for anything that is even slightly in the light path.  Any irregularity at all, a rough edge somewhere, a screw or knob, anything can cause diffraction spikes and 'fans'. Also look for shiny surfaces that are parallel to the light path - the edge of the secondayr mirror, for instance....  The mirror clips.....Does the focuser tube intrude into the light path? (Test: focus your camera.  Remove the camera and insert an eyepiece.  Don't move the focuser! Examine the out-of-focus image by sliding the eyepiece in and out.  Anything large, like your drawtube, will be clearly visible)   Also are there reflections inside the telescope tube?  Are there baffles in the telescope tube?  If so do they have rough edges?  Could there be reflections inside the focuser draw tube?  The camera draw tube, too...

 

Test images can narrow down the problem.   If you are using a field flattener, a coma corrector or a focal reducer lens (I don't think your scope has a built-in flattener/coma corrector) and it is removeable, remove it and take a test image.  If the garbage disappears, then that lens is responsible....  Also, take an image like the Pleiades and then take another with the camera rotated 90 degrees.  If the extra spikes and fans also rotate the blame lies in your camera.....You'll be able to clearly notice the rotation because the four spikes from the spider will not rotate making any change in the orientation of the garbage quite obvious.... 

 

Be a detective!!

 

Dave


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#7 Cotts

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Posted 11 April 2018 - 09:58 AM

Another thought:  The big, V-shaped 'shadow' in the 'garbage' in your images is oriented between the spider vane diffraction spikes at almost exactly at the same angle as the focuser is in its website image here:  https://www.astrosho...ab_bar_1_select.   I'd venture that the focuser draw tube is the primary culprit.  It only needs to intrude a millimetre into the light path to cause great havoc!  If it is shiny/silver coloured, even worse!!!

 

Do let us know what you find.

 

Dave



#8 petercoxphoto

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Posted 29 August 2020 - 04:54 AM

I know this is an old topic, but I found it while searching for a solution to the exact same problem. In my case (and the flares looked exactly the same as the ones in the OP's image) the problem was caused by a combination of diffraction patterns from the primary mirror retaining clips and a badly centered secondary.

 

My secondary was not centered directly under the focuser on delivery and as I only got a laser collimeter I never thought to check it, nor had I the tools to. I got a sight tube and discovered the problem. After centering the secondary properly the asymmetric flare patterns resolved into three evenly-spaced wedges which matched the pattern of the mirror clips. I then made a cardboard mask the same outside diameter as the inside of the OTA and the inner diameter is enough to cover the mirror clips.

 

Here's the before and after:

 

Unaltered OTA - see the mirror clips holding the primary in place:

IMG_20200603_112313 (2).jpg

 

With my cardboard mask:

IMG_20200829_020517.jpg

 

I plan to get a mask 3D printed to replace this rough one. 

 

Hope that helps others with the same problem. A basic takeaway is don't trust your telescope is properly set up when you get it!

 

Also, the mirror clip problem and the mask idea are described in this video: https://www.youtube....5B0B3dwA&t=179s

 

Cheers,
Peter

 




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