Not enough inward travel on heritage 130p for 2.5x barlow to focus

[willinliv]

Hi, 

I'm new to using telescopes to look at the stars, decided to buy a Skywatcher 130p dobsonian reflector, also a 10mm Vixen NPL and a 32mm Super Plossl eyepieces, and a barlow lens. The guy in the shop advised that the Revelation Astro 2.5x barlow lens he had in stock was better than the other 2x one he had in stock (not sure what brand), so I went with that.

Trying things out for the first time last night, everything's working well, and already found experience breathtaking. The scope looks to be really good quality and pleased I went with this one rather than one which potentially has more features but less pleasing in quality. No problem focusing the 10mm, the 32mm or the eyepieces that came with the telescope if looking at a star/moon etc.. But if I put in the barlow lens and then an eyepiece I just can't get them to focus (with the 32mm or the 10mm lens). 

By this I mean that it is apparent that turning right down the focuser (this telescope has an eyepiece that you screw in and out 'helical focuser') so that it goes into the body of the telescope you can start to see it getting into focus but the focuser gets to the end of it's permissible travel before actually getting into focus. Looking through the eyepiece it looks like a round doughnut shape with the secondary mirror as a cutout in the middle. With photography I think this is called bokeh, whereby when unfocused the blur takes the form of the aperture. So anyway focusing in (turning it inwards) this doughnut gets smaller and smaller, but you can't turn the focuser enough so that it comes to focus. Have tried turning it the otherway and it just gets bigger and more out of focus, so I think definitely inwards travel is needed.

I have experimented by unscrewing the focuser completely off the scope and in it's hole lowering the barlow and lens slowly into it's hole, and eventually the picture comes to focus. I estimate it needs about another 8mm-1cm of travel inwards than the focuser currently allows.

I did try not putting the barlow lens all the way into the focuser as another thread suggested (http://stargazerslounge.com/topic/95687-barlow-3x-focus-problem/) but it did not fix and instead again suggests that the barlow lens needs to go further inwards not outwards. One thing I might try tonight is not to put the eyepiece all the way into the barlow lens - I need to take a look at a diagram of how these two work, but maybe it might change things.

I am going to speak to the telescope shop tomorrow to see what they suggest, and I am inclined to see if a 2x barlow lens would be better suited to this scope. My main question (beyond the ramblings below) is: Could anybody suggest if this in theory would be more suited to my situation i.e. is a 2x barlow in some ways requiring less inwards travel. Or would a so-called 'shorty' be a good option? Any recommendations?

Thanks for reading, Will

A few shops actually bundle a x2 barlow in with the scope.

http://stargazerslounge.com/topic/172667-cant-focus-25-barlow-on-130mm-scope/ this guy has a similar problem with the same scope and barlow, again suggestion of an extension tube - but would that help with an inward travel problem?

http://stargazerslounge.com/topic/187402-a-problem-trying-to-focus-using-a-barlow/ another one. Here like the one above he finds if he doesn't extend fully the scopes flextube he can fix the problem (shortening focal length). Is this a good thing to do?

http://www.youtube.com/watch?v=ZTqYQrdR65M This guy got it working with a different barnd of barlow (which actually looks like same build as astro revolution). 

some info:

Skywatcher Heritage 130p

focal length 650mm, primary mirror diameter 130mm (f/5)

10mm Vixen NPL

32mm Super Plossl 

Astro Revolution x2.5 barlow lens

[willinliv]

Sorry that's Revelation Astro x2.5 barlow not Revolution

[Schorhr]

Hello,

too bad you are having trouble to get into focus with the barlow!

The good thing about the Heritage: You can slide the tube in just a bit to achieve focus.

As for beeing good or bad to do: It's a bit of a hassle and the secondary mirror moves further in, so

if it has to be done very far ilummination may be an issue, but honestly, visual and with higher magnification it's not really that much of an problem. I also think remembering that the secondary is rather large even for the f/5 scope.

For using my camera I have to slide the tube in as well.

But if it is possible, return the barlow, and either get another one, or simply invest the money into aditional eyepieces.

You got a 32mm eyepiece? Usualy it comes with a 25mm, but the 32mm (Plössl?) would be a good choice as it shows the maximum field on the 1.25" focuser.

I do not have that barlow, I have Orbinar 2x and Meade 3x, but both are achromatic, not apochromats.

I hope you can get it sorted out :-)

[knobby]

As Shorhr says, try not extending the truss tubes fully...

[willinliv]

Hi Both, thanks for this. Just been out and indeed got it into focus by not fully extending the telescope. I'd prefer it if it was a bit easier so think I will return it for the cheaper 2x model and give that a try, and if that doesn't work maybe get a smaller eyepiece equivalent to what I would have had. Thanks again, Will

[Reeny]

Hi Both, thanks for this. Just been out and indeed got it into focus by not fully extending the telescope. I'd prefer it if it was a bit easier so think I will return it for the cheaper 2x model and give that a try, and if that doesn't work maybe get a smaller eyepiece equivalent to what I would have had. Thanks again, Will

Try not to return the 2.5x Barlow if you can help it.

It is a quality bit of kit

Most of the 2x Barlows have the all but useless one element achromatic lens.

They aren't good.

To use my 2.5x revelation, I needed to unscrew and modify the plastic holder on my focus tube.

Removing about 5-8mm from the thickness of the holder meant that I could extend the focus travel.

I now have a few mm to play with.

A radical solution - but worth it.

[John]

A few really cheap barlows are singlet lenses. Most are achromatic doublets and a few (like the Revelation 2.5x) are triplets. The Revelation is good, for it's price, though so worth hanging onto if you can get it to focus. Barlows usually do move the focal point inwards so can cause focusing problems in scopes with limited focuser travel. 

I assume the scope is not at focus Reeny - thats a lot of focuser tube in the light path if it is !

[Reeny]

A few really cheap barlows are singlet lenses. Most are achromatic doublets and a few (like the Revelation 2.5x) are triplets. The Revelation is good, for it's price, though so worth hanging onto if you can get it to focus. Barlows usually do move the focal point inwards so can cause focusing problems in scopes with limited focuser travel. 

I assume the scope is not at focus Reeny - thats a lot of focuser tube in the light path if it is !

It wasn't far away from being focused John.

It normally sits 2 or 3mm further out from there during use.

I measured 40mm internal obstruction into the 5" diameter tube (across the path of the 76mm mirror)

On the plus side - the 2.5x has been used to upgrade the 200P dob with no problems.

And the cheap 2x barlow (unbranded) has had the single element lens unscrewed to use as a 1.5x magnifier on the old scope pictured.

The 1.5x magnifier actually pulls the focus point away from the tube, removing the internal intrusion into the tube.

I couldn't use the 2x as a Barlow, and assumed all of this type were of similar poor quality.

[John]

Sometimes the barlows supplied free with scopes are poor quality singlets. Thats why they are free I guess