Weird tails on stars

[Liam Bremner]

Hi all,

Excuse this if it's a daft question but I'm new to this.

I've got the Skywatcher Explorer 130p with a standard eq2 mount. Supplied 10mm and 15mm eyepieces plus a Skywatcher UWA 4mm EP too.

I've managed to view Jupiter and the Orion Nebula recently and was really impressed with what I was seeing. Then last night, after a lot of reading on starhopping and practicing it in Stellarium (due to constant cloudy weather outdoors) I went out and decided I was gonna try to see M51. I managed to get to the right area of the sky after comparing it with my sketch maps I'd planned my hops with but couldn't see it. Probably due to the moon being a bit bright last night.

The thing is, when I was focussing in on stars to follow for my hops I kept noticing something that seemed a bit, not out of focus but weird. I've tried to recreate what I saw in this terrible doodle attached.

Just to add, it happened in both the 10 and 25mm eyepieces (didn't try the 4mm). The telescope had been outside for around 45 minutes to cool down before using.

Can anyone advise if this is a problem with collimation or something else?

Cheers in advance

Liam

[Stargazer_00]

Most likely a fast scope coupled with not the best eyepieces so its probably coma.  I assume the tails were pointing towards the edge of the field of view? 

Another possible explanation is diffraction spikes caused by the spider veins holding the secondary mirror, although those are usually cross shaped.

[ollypenrice]

That's miscollimation, pure and simple. Absolutely classic. The spikes off to one side will turn into concentric rings once you have recollimated the scope.

Astro Baby tells you how to do it in this thorough guide. http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm

Olly

[Liam Bremner]

Brilliant.

Cheers for the quick answers guys.

Stargazer - yes they were pointing to the edge of view. All star 'tails' were pointing the same direction.

Ollypenrice - collimation time it is then then. Is this do-able without a cheshire/laser collimator as mentioned on that astrobaby link? This will be my first attempt at colliamtion so don't want to get it wrong

Cheers

Liam

[Stargazer_00]

It's best to use at least a cheshire collimator to align the mirrors really.

With patience it can be done just pointing at a fixed star (polaris).    This is quite a challenge though, particularily if you haven't put thumbscrews on your secondary mirror tilt adjustment.

[Liam Bremner]

Nice one.

I'll have a spy about for a Cheshire today then

Thanks again

Liam

[nicks90]

alternatively you can get it more or less spot on by punching a 2mm hole in to the dead centre of an old 35mm film canister (if you have one) and using that until you get a proper sighting tool.

[Liam Bremner]

Darn it. Just missed your reply Nicks90. I do have a spare film cannister as well. I've ordered a cheshire collimator now.

From reading the astrobaby article on collimation, step 2 mentions a 'collimation cap' on the focusser and step 4 mentions the cheshire collimator.

My question is, are these 2 different tools or does the cheshire include a 'collimation cap'?

Sorry for the noob questions!

Cheers

Liam

[Moonshane]

if they all point the same way, I agree with Olly. if they all radiate from the centre then I agree with Graham. from what you say it seems like collimation.

collimation is easy and I would get the Cheshire.

this http://stargazerslounge.com/topic/197640-collimation-and-star-hopping/ might help.

[Liam Bremner]

Cool,

Yep, they all radiate from the centre so I'm going down the collimation route.

Cheers for the input Moonshane

Liam

[Moonshane]

if they all radiate from the centre then this sounds more like coma. this is something that's inherent in a fast Newtonian and can only be corrected with a coma corrector. that said, many people learn to live with it and after a while it's something you can ignore. collimation is something you'll need to learn anyway so have a go at that too and see if it improves things.