Do i have bad collimation?

[Thell]

Hey all, so i was out observing tonight, having not been out for2-3months. I noticed that when viewing my first target Jupiter, i seem to have some sort of glare.

I would decribe it as points of light shooting out from jupiter making an X with jupiter being in the middle. I could see the GRS tonight so am getting fairly good detail, despite bad light pollution here!

I also get this on brighter stars and am wondering whether its due to collimation or is this normal? I would expect to just have a very sharp bright image surrounded with only darkeness.

I have a cheshire eyepiece but the process doesnt seem that accurate to me!

I have a Skywatcher 150p if that helps!

Thankyou for reading

[mdstuart]

You should just get a spot for a star with no rays of light. It does not sound like collimation...as that tends to give irregular shapes..

Maybe something to do with the spider holding the secondry...

Sorry not much help?

Perhaps you coudl take a pic with a digital camera somehow?

Mark

[Ramjo]

I get the same thing sometimes, not sure what causes it

it's not culmination mines spot on, to check go out of focus on a star you should see a disc of fuzzy light with a dark disc in the middle that's you secondary mirror you should also see the four lines of it's supports, if that's all central then your culminations fine

[umadog]

The cross that you see is due to diffraction of light around the secondary support vanes. This is normal: http://en.wikipedia.org/wiki/Diffraction_spike and you just hadn't noticed it before. Seeing detail on Jupiter is not related to how light polluted your skies. Jupiter is so bright that light pollution is irrelevant. Instead, seeing detail depends upon how steady the atmosphere is. If it's very steady you can use higher powers and will see more detail. If it's not steady, these high powers will make the views mushy. The steadiness of the atmosphere is known as the "seeing", and is independent of light pollution and, generally speaking, of sky transparency.

[Parus major]

Sounds like normal Newtonian behaviour to me.

[Thell]

Thankyou for replying , seems like it is normal for this scope then. I do get the dark disc in the middle if i use the focuser to zoom out so thats good.

I just went out to play around with it a little and not suprisingly its clouded over nicely

At least i got to see the GRS for the first time!

Heres hoping for clear skies soon

[umadog]

Just so you know, the focuser is used to focus, not to "zoom." You will always see the shadow of the secondary when you stray far from focus: its presence doesn't indicate anything in particular.

[Thell]

Doesnt the shadow of the secondary show good collimation? Provided it is viewed as being central against the primary?

[John]

If the shadow is central that is a sign of good collimation. Just out of interest, how did the Great Red Spot appear to you - was it as clear as you expected or did you find it a challenge ?.

[umadog]

Yes, a scope that is well-collimated should have a central shadow. The problem, however, is that's quite hard to judge if the shadow is central. Defocussing just a little, so you see the first few diffraction rings, allows you to conduct a "star test". This is more accurate than looking at the shadow because it's easier to see by eye whether the rings are concentric. However, poor seeing often makes this hard to do well. It also won't help to diagnose adjustment of the secondary. It is for these reasons that we rely on collimation tools. It's good to double-check your collimation with a star test once or twice, but once you know that your tools are doing the job then you don't need to bother with star testing.

[Moonshane]

agree with comments above. relates to diffraction spikes from the secondary vanes. you will also get it on bright stars like Betelgeuse etc.

if you were seeing the GRS then your collimation will not be far out but it can always be tweaked a little to get it spot on. I do my newts every time I observe (and sometimes during).

[Thell]

Alot of helpful replies here thanks alot

Jahmanson - the grs was quite apparent tonight, although i have looked at Jupiter many times before and never seen it!

I suspect the reason is due to it not actually being visible at the times i have been observing.

I just wish the clouds would have stayed away longer for me to spend more time viewing it, but hey theres always next time

Thell