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?
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?
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.
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.
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
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.
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.
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.
