Is my focal length too long?

[LuckyPhill]

Hello everyone. I'm brand new to star gazing and I'm having some troubles. I was hoping someone might be able to help me work out where the issue is.

I recently bought a Sky-Watcher 200p Dobsonian and for the life of me I can't get anything to come into focus when I use the eye pieces. Last night I was looking at the moon as my first ever observation, and I could line it all up no troubles, but when I brought my eye up to the eyepiece it was all out of focus, no matter how much I adjusted it. Eventually I found that if I took out the eyepiece and held my eye roughly 40-50cm away from the secondary mirror then I could bring it into perfect focus.

Before I buying it I read about how reflector telescopes work to make sure I had a good idea of what I would be doing. I'm a far cry from an expert, but the only way I can explain this behaviour is if the focal length of the mirror is longer than it should be. I can't see how that's likely though given this telescope is professionally manufactured - I'd hope they have all sorts of checks and balances for quality control.

Can someone help me diagnose the problem here? Is there something I'm completely missing? I was really excited last night to start star gazing, but I've had nothing but frustration so far.

 

[Dark Vader]

Welcome to SGL, you're in the right place. I'm quite new to this as well and went for the same scope. 

Could you post a picture of your focusser? A frequent error when putting the scope together is to use both eyepiece adapters. Assuming you have the eyepieces that came with it, you just need the 1.25" one.

Hope that helps 

Andy

[Waldemar]

Hi and welcome, LuckyPhil,

You do not mention collimation anywhere, while this type of telescope really needs to be collimated every time you set up, because of the open construction, which leaves room for flaws in collimation. So I kind of expect that that will be your issue. Read about collimation with this particular scope and try to get it right.
You should be able to get real nice views with this great telescope.

[LuckyPhill]

Hi Andy, thanks for helping out! I was about to post the picture, but I worked out what you were referring to. I did in fact have both adapters attached, and as soon as I removed the extra piece, everything worked! I guess I had it around the wrong way, the focuser was too long!

Waldemar -  thanks for the tip regarding collimation. I checked it out just to be sure, and if I'm following the process correctly, then I think it's all lined up properly.

I'm really excited to try it out tonight now, fingers crossed the skies are clear!

Phill

[Dark Vader]

No problem at all Phill.

I managed to get my 200p out last night for an hour for only the 2nd time and got my first proper views of Jupiter and Saturn. Cloud belts visible on both and and the Cassini division in Saturn's rings nice and clear. Blown away, went to bed grinning like an idiot 😀. 

Clear skies

Andy

[Waldemar]

1 hour ago, LuckyPhill said:

Hi Andy, thanks for helping out! I was about to post the picture, but I worked out what you were referring to. I did in fact have both adapters attached, and as soon as I removed the extra piece, everything worked! I guess I had it around the wrong way, the focuser was too long!

Waldemar -  thanks for the tip regarding collimation. I checked it out just to be sure, and if I'm following the process correctly, then I think it's all lined up properly.

I'm really excited to try it out tonight now, fingers crossed the skies are clear!

Phill

Good for you, Phill !  Enjoy your new hobby and be prepared to be hooked for life 😎 I will keep my fingers crossed for you (and myself)

[jambouk]

Lots of potential causes. I wonder if the most likely cause is that the Moon wasn’t in the fielD of view of the eyepiece when focusing. Use the lowest power eyepiece first. Test it out in the day time on a distant terrestrial object to ensure you can actually achieve focus.