300mm Dobsonian...blur

[Loeina]

Hello. I just upgraded from a small 70 mm newtonian to a 300 mm dobsonian telescope (GSO Dobsonian 300C) but i cant se anything exept for blurry images (even of trees).

I have collimated the telescope and I have let the fan run for about an hour in my room with all windoes open so it's freezing in here but when I tried to look at some trees to align my finderscope I just saw blur through my 32mm eyepiece. I removed the eyepies and looked down the focuser expecting to see trees without blur but the blur was still there. I held my hand infront of the opening so I had my hand and the trees in view but the image of my hand was clear but the trees were not. what am I doing wrong?

[Naemeth]

How far away are the trees? They have to be quite a significant distance away to get focus with the focal length of a 12" dob.

[8kids]

You may need a 50mm extension tube. I needed one for my Explorer 300pds. Try holding the eyepiece further out of the draw tube and see if you can reach focus.

And welcome to SGL.

[Loeina]

about 10 meters. but im not looking through an eyepiece at the moment. just down the focuser

[Naemeth]

about 10 meters. but im not looking through an eyepiece at the moment. just down the focuser

Needs to be further away I'd expect, and you'll need to look through an eyepiece .

[Moonshane]

you need an eyepiece to focus and you really need to focus on something a little further away than 10 metres. space is effectively at inifinity of course.

[Loeina]

Alright will try that once these clouds go away... which should be sometime next week since it's a new telescope but abit worried when i saw nothing but blur on the mirror

[AlexB67]

Also make sure you use the right adapter and not screw in the 1.25 inch adapter into the 2 inch adapter, but use the 1.25 inch directly and screw it in the focuser only. That may be your problem ?

edit: of course only applies if the 32mm is 1.25 inch, is it a 2 inch eyepiece ?

[Loeina]

the 32mm eyepiece is a 2 inch  got a 9mm and a 5mm that are 1.25 but wont even begin to use those untill I can get the telescope to get a clear picture of something. But I just thought of another question regarding the collumation screws at the bottom. how far can they generally move and will the screw fall out if turned to much?

[AlexB67]

From your description I doubt collimation would cause such serious out of focus issues.  Perhaps it is what is already said, to do with focus distance and needing some sort of extension tube. I don't know about the model and the design of the screws, Someone with a GSO will know no doubt and comment in due course. 

[themos]

Contrary to what many people think, you do not need an eyepiece to view the telescope image. 

 http://www.telescope-optics.net/system.htm#magnification_factor

[ronin]

You will need to start with something about 1Km away not 10 mtr, 2Km or 3Km would be better.

They are build to look at stars, nebulas and (the nearest option) planets.

Assumoing something like f/5 then you will have to move the eyepiece further out by about an additional 27mm from the normal position, there will not be enough travel on the focuser for this.

You do not need an extension tube unless you intend to view trees 10 meters away and binoculars would be better for that. :grin:

[John]

As ronin says, the closest focus of a newt is a lot further away than 10 metres.

[Loeina]

Alright, I just thought since my hand was clear the trees behind it should also be clear with no eyepice. but will wait for the skies to clear up! So thats why I got the extension tube... I was wondering about that one. Will let you know how it went once i have had a decent go with it!

[Tinker1947]

Nothing wrong with your scope, just need something a long way off a EP 30mm or there about's and turn the focuser really slowly, you may find you need a 2" extension if you can get focus with the focuser right out, slowly remove the EP while still looking through it.....

[brantuk]

You should be able to focus an eyepiece on the clouds - so long as it's not raining of course.

[Loeina]

Aye only reason I tried to focus on the trees was because I was able to do that with my old telescope. there is a uniform cloudcover that coveres the entire sky so no shapes

[astronymonkey]

Hi and welcome to the forum. I stepped up from a 6inch to a 16 inch at the start of this year and had similar issues.

firstly the nearest I could focus was much further away, probably 100m would be the closest I can focus although I can get a recognisable image of some chimney pots about 60m away which I use to set my finder .

secondly the cooldown time was much longer. When stood outside with the fan on it takes about one and a half hours to be able to get a sharp star focus, before this the stars are irregular and spikey especially if usedas soon as it is brought outside.

so im sure the scope is fine, just let it cool before using it and dont expect anything to focus thats closer than 100m.

Cheers

[rwilkey]

You need to use the telescope outside to get it to cool down properly to the ambient temperature.  Using it inside just does not work because of the warmer air currents inside the house.

[laowho]

I'd be surprised if you needed an extension tube, unless you got a mirror whose focal length is that far off...and most EP focal planes are within relative reach of each other. Was at a star party and couldn't get it to focus, even though the fellow had been using it. He said it was collimated, but didn't see that the return of his laser collimator wasn't even hitting his secondary.

But you say it's collimated...and have probably tried other EPs by now, and on a target at least a km away...and if it isn't also fog/dew/condensation on your surfaces, then the need for an extension tube would have me sending it back. You want the EP as close to the secondary as possible, otherwise you'll be losing illumination and probably vignetting too. Remember, your light cone coming off the secondary and through the focuser tube converges on the focal plane of the EP...if you have to pull the EP focal plane further back, that means that the secondary isn't catching the whole cone and you're losing light/reducing the aperture of your primary. If it were a build, you'd have to raise the secondary away from the primary to rein the focal plane in.

[Loeina]

Finally had a night with clear skies! I had the telescope outside for about 2 hours before I used it and after collimation i turned to look at the moon it works wonderful! some warm air currents did a number on the view but apart from that it was amazing! The moon was abit bright though as I have no moonfilter (yet).

[Moonshane]

nice one. at 2 hours outside, more likely to be seeing I'd say? don't forget that the larger the scope, the more effect poor seeing has.

[estwing]

nice one. at 2 hours outside, more likely to be seeing I'd say? don't forget that the larger the scope, the more effect poor seeing has.

now you tell me!

[Loeina]

nice one. at 2 hours outside, more likely to be seeing I'd say? don't forget that the larger the scope, the more effect poor seeing has.

sorry, not following

[AlexB67]

sorry, not following

He means the influence of  the atmospheric effects on the view. Bigger aperture scopes are more sensitive in that regard, often times atmospheric turbulence will be the limiting factor as the quality of the views you get, there may at times planets/moon appear be a bit blurry and unsteady when seeing is not good, this effect it goes up with magnification also.

Estwing now knows that his whopping big moonshane dob is no good for planetary views, He'll be selling it soon