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Old 31-05-2014, 02:42 PM
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John K
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Is this flaring and what could be causing it?

Hi guys,

I am getting some type of very slight flaring in some of my images and wanting to try and isolate what it may be.

I am using a 12.5" f/5 scope with a Canon 400D.

The scope and secondary cage are seen here:

https://www.flickr.com/photos/johnka...57594183210993

https://www.flickr.com/photos/johnka...57594183210993


Essentially I am getting a 5th spike in some of my images but only on a couple of stars and not across what appears to be the entire field.

I am wondering if this is simply reflection from stray light?

I have checked the secondary and the only suspect is a twin set of small screws at the bottom end of the secondary.

I anyone has experienced the same can you share your thoughts.

Latest shot taken with this scope that the attached crop was taken from here:

https://www.flickr.com/photos/johnka...pods/lightbox/
Thanks in advance.

John K.
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  #2  
Old 31-05-2014, 02:57 PM
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what happens if the star with the 5th spike is moved to the 4 corners of the chip, does the spike stay in the same spot or rotate around the star? What happens if you change the camera?
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Old 31-05-2014, 03:26 PM
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Quote:
Originally Posted by Joshua Bunn View Post
what happens if the star with the 5th spike is moved to the 4 corners of the chip, does the spike stay in the same spot or rotate around the star? What happens if you change the camera?
Good question but have not tried this - the question for me is what is the typical cause of a fifth spike in a Newtonian with a 4 vane secondary?
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Old 31-05-2014, 04:11 PM
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Not something like a spider web strand inside the tube?
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Old 31-05-2014, 04:18 PM
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Quote:
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Not something like a spider web strand inside the tube?
It's a truss - so nope, it's not that.
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Old 31-05-2014, 05:52 PM
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Even a strand of hair, anywhere along the optical path, focuser etc?
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Old 31-05-2014, 06:10 PM
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John, it isn't just the two stars near the edge. How apparent they are appears to correlate with magnitude.
UCAC4-329-121610 mag 5.45 is the brightest object with the most noticable spike.
UCAC4-329-122821 mag 8.67 (for instance) on the other side has a slight spike pointing in the same direction too.
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Old 02-06-2014, 03:16 PM
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You mentioned 2 screws at the secondary I would blacken those and see if it changes.

I get a little diagonal spike from my FLI Proline camera which is typical of that model. Its not objectionable so the camera is a possibility although unlikely.

I would rotate the camera or do as Josh suggested and see what happens.

You also may need extra baffles. Modern scopes seem to have a lot of baffles these days.

Greg.
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Old 03-06-2014, 09:01 PM
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Hi John

it'll probably be diffraction from something intruding into the light column (screw, spider support, OTA edge etc). Maybe try back- projection if you are having trouble isolating it - you will soon see if there is anything amiss. Put some translucent baking paper over the entrance aperture and shine some light in backwards through the scope from the centre of the focal plane - the shadow pattern on the paper will pick out any intrusions. http://www.iceinspace.com.au/forum/s...ad.php?t=82230

Centering the light column by adjusting the secondary in-out can also help reduce diffraction patterns on the edges of the field - back projection can help you do this, but you need to do the following sequence: collimate scope> check if light column is centred> if not, adjust adjust secondary in or out a couple of mm> collimate scope> check if light column centering is better or worse> adjust secondary more if improving, reverse direction if it got worse> collimate scope> check light column centering.....etc

Last edited by Shiraz; 03-06-2014 at 09:24 PM.
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Old 03-06-2014, 09:59 PM
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Thanks for the advice guys,

and in particular Greg and Ray - really like the idea of backlighting the light path to chase down the culprit.

My rotating truss and tub are way overdue for some new flat black paint as well.

Clear skies.

John K.
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Old 04-06-2014, 08:06 AM
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My CDK17 had a lot of stray light reflections from out of field bright stars. Planewave developed this aspect of the scope for a while with a different secondary shroud (that has a slightly triangular section to it), extra baffles that they supplied including a fairly large one around the primary mirror tube (about 30mm wide).

That's what it seemed to take to stop the problems. It does not seem to be an issue anymore but I was surprised at how many baffles, bits it took to handle it.

I also got improvements by blackening some areas in the light path. Black anodised aluminium is reflective so it should be blackened as well.

Greg.
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Old 04-06-2014, 10:20 AM
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It is consistent with the diffraction artifacts one would expect to see as a result of the bottom of the focuser tube extending in to the light path
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Old 04-06-2014, 10:55 AM
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I'm with Clive,

Is your focusser at 45* to your vanes, just like mine is?

Is the focusser in the same 'quadrant' as the spjke in the shots

I'd suggest that turning the camera will result in the spike remaining where in the same quadrant

VAZ
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Old 05-06-2014, 03:48 PM
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Quote:
Originally Posted by vaztr View Post
I'm with Clive,

Is your focusser at 45* to your vanes, just like mine is?

Is the focusser in the same 'quadrant' as the spjke in the shots

I'd suggest that turning the camera will result in the spike remaining where in the same quadrant

VAZ
Yep = correct. see:

https://www.flickr.com/photos/johnka...7594183210993/

So is one of the solutions to simply turn the camera?

Flat black paint bought yesterday for an overdue coating.
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Old 05-06-2014, 05:33 PM
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Quote:
Originally Posted by John K View Post

So is one of the solutions to simply turn the camera?
No.. turning the camera will not solve the problem.
However, one step towards positively identifying the source of the flaring is this;
Set the telescope up with the camera attached and in focus, point the telescope towards the horizon, walk away from the telescope in the same direction it is pointing, when you get 20 feet away from it, turn around and look at your primary mirror. Move your head side to side, up and down. Is there an orientation where the focuser tube extends in to the light path of the primary mirror?

Last edited by clive milne; 05-06-2014 at 07:49 PM.
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Old 29-06-2015, 11:06 PM
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Well, as is the case with things, I did get to the bottom of things, and it was in fact the shroud of my scope.

So now the idea is I have simply made a large black cardboard tube that surrounds the mirror to shield it from stray light when I do AP. This also means that the exposed mirror gets more cool air and remains closers to ambient then before. I have not however used it in very cold conditions this winter and likely will need a hair dryer close by!

An oversized shroud for my truss tube will do the trick.

John K.
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