Focus issues: Weather or Scope?

[TF Ratio]

Hello all,

Apologies for the poor weather recently , my Telescope (skywatcher 200p - cheers Steve) was delivered 10 days ago and I have been chomping at the bit to get out there.

Now before I ask my question, please let me say I do live in London and the light pollution is a complete..... ..... its never really dark, even at 1 am!

I went out tonight and the conditions are quite poor, but its the first dry night so I had to at least try, I managed to focus on a few stars in between the clouds, not for long, but enough to calibrate the spotting scope etc...

My question is this, given the terrible conditions, i can understand that an image would not be pin sharp etc, however, the few stars i was able to look at seemed to be flared, i dont know how to describe it better than that, the stars were clear but with a bit of a flare in two directions to the north east and south east of my view..? The flare was the same colour as the star, limited to only the main star in the field of view, its almosts like i was unable to resolve the focus on the object at all..? would this be cause by meterological conditions or do i need to collimate my scope or something else...?

cheers in advance for help

[GazOC]

Hi TF,

You should n't get the flaring no matter what the weather conditions. If you havn't collimated your scope then that will probably be the problem.

Gaz

[Kaptain Klevtsov]

The flaring is the diffraction spikes, unavoidable with the Newtonian design with spider supports for the secondary mirror. Quite normal.

The not getting dark is a seasonal thing, it'll pass to some degree as the year gets older.

Welcome to SGL BTW.

Kaptain Klevtsov

[TF Ratio]

diffraction spikes, interesting.

have looked it up on the net and i think you are spot on, cheers.

[Ant]

Mmmm, not sure about deffraction spikes...

There would be two or four of them (depending on the design of the spider). To have a "flare" North East and South East. They should be opposite each other.

Question. Were the stars in the centre of the Field of view or to the edge.

Question. Have you collimated your scope - this really is VITAL on a F5 Reflector.

Ant

Note: I have been known to be wrong - quite often actually.

[TF Ratio]

Question. Were the stars in the centre of the Field of view or to the edge.

Well they were in the main field of view but the spikes were more prominant when the star was off centre..

Question. Have you collimated your scope - this really is VITAL on a F5 Reflector.

Um.... no. I was being lazy and thought i may get away wothiout it..... guess i better go learn how to do it.

[GazOC]

I agree with Ant, there would be 4 "flares" not 2 if it were diffraction spikes.

You really MUST collimate a f5 Newt, the fact the problem gets worse towards the edge also supports thats its a collimation issue rather than diffraction spikes.

[Ajohn]

If the vanes are only causing 2 spikes like that it probably means that the collimation is slightly out. Not the word spikes. If it's broad flare it could be a number of things but likely to be collimation. As you said that the focus is a bit iffy what magnification were you using?

John

[TF Ratio]

I tried all my both my supplied eyepieces with and without the barlow....

I have looked on the web and downloaded about 10 how to guides for collimation....

Looking through the scope with no eyepeice I can see a nice little circle in grey on the primary mirror, and its not dead centre. So I think we have our answer.

I have scoured the house for a 35m film case or actual film but cannot find one. So I will wait till tomorrow and go and buy a collimation tool in London.

I have soooooo much to learn, Its great :hello1:

i love it.

[GazOC]

Nice one! Collimation is one of those things thats a pain the backside to do the first time but after that its very easy.

[Ajohn]

Try reading this before you do anything. I don't think that spot should be central on your scope.

http://stargazerslounge.com/index.php/topic,26383.0.html

My post is some way down and includes a link to another web page.

Hope I'm not tooooooooooo late.

John

[TF Ratio]

Ajohn, that is one hell of a good video.

I went to town today and bought a lazer collimator, in for a penny in for a pound.

And we haver the answer folks, my collimation can be defined as major ugly!

Watching the video in the link on AJohns post I laughed... I now need to get some mini Allen keys 'spelling'... typical

Cheers for all the help peeps it should mean that I am ready to go on the next clear night.

[GazOC]

I know it may sound obvious but if you are going to make adjustments to the secondary mirror make sure the scope is slightly tilted mirror up or at least level. You wouldn't believe the number of people who have dropped allen keys or screwdrivers onto their main mirror......not good!! :crybaby:

[Ajohn]

Just watch it with the offset 2ndry mirror that skywatcher scopes seem to use. Best to use the visual methods 1st and maybe use the laser to tweak last.

The 2ndry mirror on skywatchers seems to be move off center away from the focuser. They do it because the 2ndry sits in a 45 degree cut through the cone of light of the mirror. The cone of light is wider where the bottom of the 2ndry is than at the top. You could work out what the offset should be if desperate. I don't think it's worth doing but if you have a telescope that's made like that there can be other reasons why it should always be offset.

( You wouldn't believe how many people have had the 2nry drop off too ! It does sometimes happen )

One of the reasons I like a central 2ndry is that I can then ensure that the 2ndry is central and square to the axis of the focuser and that I can also see the entire bottom of the tube central in that. Getting that part right helps with the rest and tends to ensure that the focal plain is square to the eyepiece or what ever. That's important at higher magnifications. It doesn't matter nearly as much with the usual dobsonian low power viewing. The same thought processes could be used with an offset 2ndry but not so easily.

John

[lunator]

TFR

Welcome to SGL

The only piece of advice I would give is take your time when collimating. Do it bit by bit and it should be ok.

Cheers

Ian

[Astro_Baby]

Very slow and methodical is the way to collimation, dont rush it because it will be frustrating, do it when you have ample time and turn those screws SLOWLY - little by little does the trick.

A few weeks ago I was clueless at it - my little 130 gave me the basic practice at it - its frustrating at first but then you kind of get to 'feel' what you should be turning and all of a sudden voila its 2nd nature ( I think ) and yes - keep that scope tube level cos dropping things down there is definitely not good news.