Collimation or conditions?

[gooseholla]

Hi, these past couple of nights I have noticed when observing Mars and Saturn that there are points of light coming off of them. Now, I simply thought it was the conditions, light being affected by thin cloud. But... then when I moved the image around in the eyepiece I started to notice that 2 points could become 8, or 4, etc, or a ring. So does this suggest that actually it is a collimation issue?

Or is it perhaps a combination of both, or something else completely?

Thanks

John

[acey]

If "points of light" means spikes then it could be diffraction spikes. These are mainly caused by the arms that hold the secondary mirror in a Newtonian, but can be caused by any obstruction in the light path. They always come in pairs, which accords with your description, though I don't see how you'd be able to vary the number (or get a ring). Poor collimation would most likely manifest itself in coma, meaning that stars would tend to look like streaks or elliptical blobs. Another possibility is stray light creating artefacts in the eyepiece. Diffraction and light scattering are most visible on bright targets such as planets. If you feel it's spoiling the view then do some more investigation in order to identify (and maybe solve) the issue. If it isn't spoiling the view then there's no problem. Newtonian users have been living with diffraction spikes for centuries (though some opt for a curved secondary in order to replace spikes with a less obtrusive diffraction pattern). Light scattering can be reduced through baffling. Some eyepieces are more prone than others to internal reflection, in which case the solution is to get a better eyepiece.

[gooseholla]

Hi,

Cheers for the reply. Got me thinking. Wasn't there before, but this past week I turned my secondary mirror whilst re-collimating and measuring bits. One side had an accident with some glue and well, yeah the edge is ruined... It hasn't caused a problem before. I am wondering if, by turning it, the light is now scattering a bit - would explain perhaps why at certain points of the image it was making all kinds of weird shapes then disappearing in other parts?

Thanks

John

[acey]

An "accident with some glue" certainly sounds like the sort of thing that could affect your views. If the edge of the secondary is scattering light then the best thing might be to cover it (with something black and non-reflective, e.g. flocking material such as Protostar). You would experience a slight loss of light at the eyepiece, dependent on how much you cover. There is of course the possibility that by trying to fix the problem you only make matters worse, so as a first step I might put an obstruction in front of the secondary (e.g. piece of card fixed to the spider arm rather than secondary mirror itself) to see what effect it has. Or you could look out for a replacement secondary mirror.

But if glue is on the side of the mirror rather than on its reflective face then this should have little or no effect. The first thing to do, I think, would be to look through the focuser (without eyepiece) in daylight. Look at whatever light is coming to the eye other than what is being reflected from the secondary mirror. If, say, you can see a big shining blob of glue then that could indeed be your problem. But the part of the telescope tube visible around the secondary (and anything else visible) is also a possible source of scattered light. And that's before you consider possible internal reflections in your eyepiece - try other eyepieces at night and see if they produce the same effect.

[gooseholla]

Yeah it happened about two years ago, the holder holding the secondary split in two and I panicked, poured some glue into the gap to seal it up, next thing I know glue is drip down onto the edge of the secondary. It covers a patch of it on the flat face. You can definitely see it looking down the tube. However, I suspect the old angle it used to sit at meant only the extremities saw any problems.

I haven't tried to remove the glue under recommendation from many on here. I will, however, cover it if it proves to be the problem.

Not sure it is just one particular eyepiece, all of them were doing it, from a T5 20 mm Nagler, through a 6mm TMB Planetary II.

[gooseholla]

UPDATE: You are going to love this. Cos it is clear I could follow Mars longer tonight... right round over our house. The light scattering disappeared... as I turned away from my neighbours hugely bright security light! Guess it is scattering and reflecting in the tube!

[acey]

Problem solved - but  I suggest you have a word with those neighbours.

[gooseholla]

Problem solved - but  I suggest you have a word with those neighbours.

Yeah... not gonna do anything. I think they are the paranoid type. Moment they get in shut all the blinds so no one can see them. Leave a light on all night, and part of day too... etc. As my next door neighbour said, best just to let them be. They're not harming anyone really, can see Mars from my back garden, just like to share scope with my neighbours and nice to be out front on dry parts when it is wet. Oh well, as you say, problem solved!!