What difference will adding a barlow x2 to a 150p dob do?

[foxesjon]

Hi guys, me again (apologies for all the endless questioning!).

Had my first real go with my 150p last night and saw some beautiful constellations. Just wondering what the addition of a x2 barlow lens will do and what would I need to add to my 150 dob to see saturn, jupiter etc.?

Thanks in advace,

FJ

[Peter Drew]

The addition of a 2X Barlow lens will increase the effective focal length by 2X, this results in doubling the magnification of each eyepiece you use.

[Scott]

Hi FJ,

your 150 dob with the supplied ep's will give you views of Saturn and Jupiter, no problems. this of course can be improved with better ep's etc but for starters, stick with what you've got.....unless you really want to spend your money then go for it

[ronin]

I would go with the idea of slowing acquiring better eyepieces. A 2x barlow will in effect half the focal length of the eyepieces but there is the unmentioned assumption that both the eyepieces and the barlow are all good.

The supplied eyepieces are not good, I don't think that Skywatcher provide even the basic plossl any more with their scopes. Then add it that at f/5 you will need some reasonable eyepieces when trying for magnification and I think that you need to consider lowering your bank balance and look at a few better eyepieces.

[Scott]

I would go with the idea of slowing acquiring better eyepieces. A 2x barlow will in effect half the focal length of the eyepieces but there is the unmentioned assumption that both the eyepieces and the barlow are all good.

The supplied eyepieces are not good, I don't think that Skywatcher provide even the basic plossl any more with their scopes. Then add it that at f/5 you will need some reasonable eyepieces when trying for magnification and I think that you need to consider lowering your bank balance and look at a few better eyepieces.

I agree, better e.p.'s are certainly one of the first "upgrade" options to look at. however, If I could see the cassini division in my 8" f5 sw with the standard 10mm ep, I'm sure the OP will have no troubles with his/her f8 albeit 6". Having said that, I'm told the sw ep's can be a bit hit and miss in the quality department

And yes, I'd definately agree that investing in ep's rather than barlows is the way to go. I only ever use barlows when planetary imaging (not often these days).

[ronl]

Have a look at this great site it is a guide about eyepieces for beginners,

http://www.swindonstargazers.com/beginners/eyepieces.htm it tells you

all about the different types, and gives you the advantages and disadvantages

it really helped me.

Good luck and Clear sky's

[russ.will]

I'm not a particular Barlow fan, but that's easy to say with a box full of decent EPs, that can cope with F5 or lower scopes and have decent eye relief. If I were sitting in the position of having a decent scope with the starter EPs, I think I'd actually recommend getting a decent Barlow first for several reasons -with caveats.

1. A Barlow doubles the focal length (and therefore focal ratio) of the scope - It does not half the focal length of the EP. Although a 20mm EP plus Barlow will give exactly the same magnification result as a 10mm EP alone, it's a distinctly different proposition. At double the scope focal ratio, cheaper EPs will have a much easier time in terms of astigmatism at the edge of field - They will be sharper to edges.

2. Cheaper EPs tend to have tighter eye relief (and manufacturers figures can be a touch optimistic) so a Barlowed budget EP can be easier to look through. A 20mm Plossl isn't too much of an issue, but a 10mm can start to feel a bit tight depending on how luscious your eye lashes are.

3. With a decent Barlow in the box, your EP selection is automatically doubled, with all of the advantages above. A decent Barlow like the GSO 3-element 1.25" 2.5x isn't exactly expensive and will work well with decent Plossls like those available from Skywatcher, Celestron and particularly Vixen, which can easily be found for £15-20 a pop second hand. I've messed with a few Barlows and whilst none would sway me from my ES Focal Extender, the GSO is the best I've personally played with and that includes the 'legendary' TAL 2x Barlow.

A 32mm Plossl shows the maximum FOV you can extract from a 1.25" EP and as you want a 32mm EP, lets assume you bought one. These are typical results culled from the SW/Celstron Plossl range:

32mm plus the GSO 2.5x Barlow gives 13mm approx. That's 37.5x and 92x covered.

25mm plus the GSO gives 10mm. That's 50x and 120x covered. (included because you already have it)

20mm plus the GSO gives 8mm. That's 60x and 150x covered.

17mm plus the GSO gives 6.8mm. That's 70x and 176x covered.

15mm plus the GSO gives 6mm. That's 80x and 200x covered.

12mm plus the GSO gives 4.8mm. That's 100x and 250x covered.

I'd (personally) pick the 25mm, 17mm and 15mm to give a 37.5, 50, 70, 80, 92, 120, 176 & 200x magnifications with the barlow from three EPs.

The caveat is that if you're particularly into planetary, you may find it beneficial to dump one of the longer focal length EPs (32mm?) and get the 12mm to give you a denser coveage of magnification at the top end. When observing planets, moving from an 6mm to 4.8mm focal length EP, can have seriously differing results on different nights, despite the 1.2mm difference. By comparison, 25 to 20mm can be basically interchangeable regardless of atmospheric conditions.

Just a few thoughts to chew on, but don't dismiss a decent Plossl, because they remain handy even once you start upgrading EPs and trust me, you will!

Russell