Used Skywatcher 250 won’t focus

[Daznok55]

Hi

Im at a loss as how to go forward, wife bought me a used 250 dobsonion with a 2” focuser and a badder 8x24 click eyepiece on eBay but I cannot get it to focus. After reading here on other people having the same trouble, I purchased a 1.5 inch extender from FLO. It didn’t work came close but no cigar. Do I continue extending it out or do I have some other problem?

Getting very frustrated can anyone shed more light on this.

 

[Bruce Leeroy]

You should be able to draw the eyepiece out by hand to see if you need a further extension, if you have a barlow lens you can normaly remove the lens element and use the body to provide a further extension.

If you can gain focus on some roof/tree close by then your close to focusing to infinity.

Edited July 6, 2020 by Bruce Leeroy

[CraigT82]

Keep extending! They do focus quite far out. Are you using the stock 2" to 1.25" adapter? Some pics could help

[Victor Boesen]

I use a 50mm extension tube and then the 2" to 1.25" adapter which is also quite long in itself. Sometimes you'll be surprised with the distance required to reach focus and it also varies from eyepiece to eyepiece.

[John]

If you can post a photo of the focuser as you have it setup for observing that will help diagonse the issue

[Daznok55]

Here she is.

1. Stock 2 inch foucuser (isn’t it?) 2. The 1.25 extender 3. The eyepiece 

you guys were right I slowly moved the extender about an inch Out and blimey it popped into focus. (Happy days)

However this only leaves half of the extender inside the focuser and the eyepiece is quite heavy so I think getting a bigger extender would be wise. 
I thank you all for such a rapid response.

 

Getting closer to a working telescope. Skywatcher 250px

[Carbon Brush]

To help get focus back into the tube, rather than mid air, It might be worth looking at the main mirror fixing.
On a secondhand scope, someone might have deliberately moved the mirror up the tube a few millimetres to focus (for example) a DSLR where the sensor is a long way back in the body.

The main mirror fixings might have been extended, or they might just be near the end of travel.

HTH and keep us updated.

David.

[Daznok55]

Hi Carbon

here are the screws t the back, not sure if this helps. How far out or in is good?

[JamesF]

I have an after-market focuser, but that focus position is much further out than my flextube 250P, I'm sure.  It may be a silly question, but just to make sure, have you definitely got the truss tubes extended all the way out?

James

[Corncrake]

James beat me to it !

Another possibility : perhaps the previous owner modified the truss tubes (with some packing rings or something) to make for a (shortened) photographic position ?

 

 

Edited July 6, 2020 by Corncrake

[Daznok55]

Tubes are fully extended and locked.

[Daznok55]

Tried the Barlow idea, took the lenses out and it works great!

Another question for you guys.

No 1.25 eyepieces came with the scope only the adapter should I invest in a couple and if so what do you recommend?

[Daznok55]

Oh yes I forgot to mention upside down view so a diagonal will be needed also??

 

[Peter Drew]

6 minutes ago, Daznok55 said:

Oh yes I forgot to mention upside down view so a diagonal will be needed also??

 

Diagonals are not used with Newtonian reflecting telescopes.  The view orientation is normally upside down and back to front astronomically.  I would also stick with the extension solution to your focus problem, fiddling with the primary mirror adjustment, at this stage would most likely result in much more complex issues.      🙂

[bomberbaz]

when I had the 12" version I didn't have any issues focusing even albeit a different focal length ota. Strange.

[Stub Mandrel]

19 minutes ago, Daznok55 said:

Oh yes I forgot to mention upside down view so a diagonal will be needed also??

 

People don't normally use a diagonal with a Newt, it's function is to make the viewing position more convenient.

A Newt's view is rotated 180 degrees, not flipped, so just turn your star atlas upside down.

[Ricochet]

15 minutes ago, Daznok55 said:

Oh yes I forgot to mention upside down view so a diagonal will be needed also??

 

The view being reversed does not matter for astronomical targets. If you are noticing this because you are trying to use the telescope for terrestrial observations then it would explain your focusing issues. Terrestrial targets will require the eyepiece to be further out than the focuser is designed for. If this is the case then try the telescope on astronomical targets and see if the problem with focusing persists. 

[Geoff Barnes]

Regarding the truss tubes, they have two positions. When you extend the telescope from its compact state the tubes will click and seem to lock into position. This is not the full extent, you need to pull the scope past that first click lock another few inches until it will go no further, then it is in the correct extended position and any eyepiece will come to focus.

I have had my truss Dobsonian for three years now and still make the mistake of not extending the trusses fully on the odd occasion and wonder why I can't focus on anything.

[bomberbaz]

25 minutes ago, Geoff Barnes said:

Regarding the truss tubes, they have two positions. When you extend the telescope from its compact state the tubes will click and seem to lock into position. This is not the full extent, you need to pull the scope past that first click lock another few inches until it will go no further, then it is in the correct extended position and any eyepiece will come to focus.

I have had my truss Dobsonian for three years now and still make the mistake of not extending the trusses fully on the odd occasion and wonder why I can't focus on anything.

Forgot about this, make sure it clicks twice

[Daznok55]

Just went and checked the tubes, only get one click! Pulled hard as I dare but won’t move past first click? 

[Geoff Barnes]

2 minutes ago, Daznok55 said:

Just went and checked the tubes, only get one click! Pulled hard as I dare but won’t move past first click? 

Then you need to be brave and pull harder, you must reach the second click. Try a bit of lubrication on the poles if it helps, my poles get a bit stiff after a while and I have to wipe them with some silicone spray to loosen them.

[bomberbaz]

Is there any of the truss pole still sticking out of the base section when you get the first click?

 

[fozzybear]

7 hours ago, Daznok55 said:

Here she is.

1. Stock 2 inch foucuser (isn’t it?) 2. The 1.25 extender 3. The eyepiece 

you guys were right I slowly moved the extender about an inch Out and blimey it popped into focus. (Happy days)

However this only leaves half of the extender inside the focuser and the eyepiece is quite heavy so I think getting a bigger extender would be wise. 
I thank you all for such a rapid response.

 

Getting closer to a working telescope. Skywatcher 250px

I see on your photo off the focuser on the wheels they are missing  the rubber covering. if needed have a look at

https://www.astroboot.co.uk/AstroBoot/telescope-and-astronomy-stuff.html

they are only £2-90 plus shipping plus you can get a host of bit's n pieces from them

[Daznok55]

White dwarf and proto star

Guys took a few pictures of tubes Entended (as far as I can) and stowed.

[Geoff Barnes]

That looks fully extended, strange that it doesn't click before you get there though.

Oh well, we can rule out the poles as the problem I think.