Dobsonian + Barlow lens?

[j1mb093]

Only just bought my first decent telescope in the past 2 weeks and just need some guidance

It's probable something small that i'm doing wrong but I cannot for the life of me get my barlow lens to focus.

My scope is a Saxon 8" Dobsonian, with a Focal length of 1200mm.

I use either my 25mm Plossl eyepiece or 10mm eyepiece. On their own it's amazing, currently I like to sit and have a good long look at Saturn. But while wanting to get a closer look, the barlow I have (from an old cheap telescope) just will not focus.

It's like when I goto focus inward I run out of space where I can keep focusing down.

The barlow I have now is quite a long tube, I was wondering would it make any difference if I buy a barlow lens with a much shorter tube? Or should I buy a better eyepiece like a 4mm? I would like to be able to get an even closer view of Saturn and eventually Jupiter when it's visible at a more friendlier time of night.

Any help would be AWESOME

[Charic]

On some Barlow pieces the Barlow lens cell can be unscrewed from the tube, and can therefore be screwed directly to the eyepiece.

Try this if the option is provided, confirming any need for a shorter tube?

On my 8" F-1200 f/6 scope a 6mm provides an exceptable 200x power. Although I have a 3.2 and a 5mm a 4mm eyepiece would not get used so much unless seeing conditions are good, or when viewing the bright Moon.

Of note, the Barlow increases the focal length of the scope, so if using a 2x Barlow, your scope produces a new focal length of 2400. Used on say a 12mm EP you achieve the same power as I could achieve using the 6 mm alone, and possible you could maintain the eye relief. This also doubles the range of your eyepieces. If you buy just a 4mm its pretty limited at 2mm? But an 8mm can be Barlowed to obtain the same power.

[Dave In Vermont]

Charic has given a very good description of the Barlow-lens. But there remains one other very important piece to using a Barlow to the best purpose - invisibility. I will explain:

Armed with the information that a Barlow magically doubles (and triples and more) the number of eyepieces us astronomy lovers have at our disposal, you'd expect to see most telescope-owners with a Barlow in the star-diagonal, or whatever, of their scopes. But they do not. So why is this? Because there is a down-side to Barlows and one of their side-effects. This being that you are putting more glass in the pathway that the image you are seeing has to travel through. More glass = dimmer image. Usually. The Barlow will also double any imperfections in the variety of EP's (eyepiece = EP) it's used with. What this boils down to is that the best Barlow is one which is invisible in terms of dimming the image and any imperfections (such as make for a blurry view). And this will cost you.

Take a look in the used equipment ads for astronomers. Invariably you will spot a bunch of Barlows in "hardly ever used" condition. People buy a Barlow, use it, and then it sits in their EP-case. This is because it did dim the image, and/or make it blurry and unstable. So they sell that Barlow, and buy a new one of a different brand. This can go on until the person gives up on Barlows altogether. Unless they manage to find one that IS invisible. But these generally don't come cheap.

I'm sure that others will wish to add other brands here, but I'll use the one's I use as an example: TeleVue Barlows. I don't generally like to plug a TeleVue on people as it's products are among the most expensive on the market - new AND used. But I'll make an exception here. The reason being that TV's are the last Barlow's you would ever buy. They work better than most - effectively being invisible in the optical-pathway. So if you want to use a Barlow, you won't go wrong buying a TV or two. Their PowerMates are also top-notch, but aren't Barlows - they work on a different principle that can wait for another thread.

So here's addition to Charic's post. Between them, you now have a good overview in all-things Barlow!

Have fun -

Dave

[BritAngler]

IMO a barlow is okay as a stop-gap until you buy an eyepiece to suit; I have 2x and 3x barlows but never use them. What would be useful is an anti-barlow, one that decreases the magnification of an EP. Not done any serious looking but I imagine they exist already....

[Dave In Vermont]

IMO a barlow is okay as a stop-gap until you buy an eyepiece to suit; I have 2x and 3x barlows but never use them. What would be useful is an anti-barlow, one that decreases the magnification of an EP. Not done any serious looking but I imagine they exist already....

Try a Focal-Reducer if you want less magnification. There are many types to consider. I'll start you off:

http://www.firstlightoptics.com/reducersflatteners/celestron-f63-focal-reducer.html

Have fun -

Dave

[BritAngler]

Try a Focal-Reducer if you want less magnification. There are many types to consider. I'll start you off:

http://www.firstlightoptics.com/reducersflatteners/celestron-f63-focal-reducer.html

Have fun -

Dave

Ah, seen those things but didn't take much notice of them, until now

Thank you very much Dave

[beamer3.6m]

I think the oeion shorty barlow is good.

Not expensive either.