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Barlows and focal length and f/


dawson
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Two questions, sorry.


 


1. How much does a Barlow increase the focal length by?


 


2. I've just watched Damian Peach's Planetary Imaging DVD again (thank you Kim) and he is saying you want an f/ of 20-40 for planetary stuff, nearer f/40 for Mars, nearer f/20 for Jupiter. So how does one go about altering this?


 


Thanks for any replies.


 


James


 


 


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Guest Kheldar

1. By the factor of the Barlow. Taking an f5 1M scope for example, a 2x Barlow will make it an f10 2M scope


 


2. Start with a nice long focal length / ratio scope ... and add Barlow to suit :)


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OK. Thank you. I didn't think it was that simple. Now I see why I need a good quality Barlow!


 


Right, any suggestions? I need one before Wednesday night :)


 


James

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Don't get a barlow, get a Tele Vue powermate, 2xpowermate is approx £265.


 


A powermate will blow the socks off a barlow any day!


Edited by Daz Type-R
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Blimey, hadn't planned on £265. Also, it's a 2" and my scope takes 1.25" EPs.


 


I think this will have to be another idea to put on hold for the time being until I can get to a Star Party and see what other peoples kit will do on my scope.


 


I'll just suffer with the bog standard Barlow supplied with the scope for now :)


 


James

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Guest Kheldar

Blimey, hadn't planned on £265. Also, it's a 2" and my scope takes 1.25" EPs.

 

I think this will have to be another idea to put on hold for the time being until I can get to a Star Party and see what other peoples kit will do on my scope.

 

I'll just suffer with the bog standard Barlow supplied with the scope for now :)

 

James

 

Powermate is of course the top end, but you did as for quality :) Try a Meade Telextender instead, what are they priced at?

 

I thought a Powermate was a Barlow, just a branded type of Barlow!

 

Nope, Powermate's are built optically differently - they are an amplifier I think not a Barlow ... Kim can probably advise better

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Guest Ely Ellis

Does that mean that you can buy other branded Powermates, other than TV?


Or maybe that type of 'amplifier' but with different name?


 


I know Powermates are the bees knees, would like to have one, but cannot justify the expense.


I ended up with a 2 x Tal Barlow.


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Hi Martin.


 


Powermates give you all the benefits of a barlow, without the downsides to a barlow.  They are 2 completely different things.


 


You can get into some serious tech talk if you want but the crux of it is, a power mate is the best.


 


One of my future purchases is going to be a 2" 2x powermate.

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Guest Ely Ellis

James, Yes they are, I was going for the 1.25" one, but they are still expensive.


 


Daz, are there other companies that make equivalent to Powermates? Possibly cheaper alternatives but made in the same lens arrangements.


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Like most things James, people's opinions differ.


 


Rather than trusting the word of one persons views, I think the only thing you can do is either get to a meet and ask to borrow one for an hour or so, then you can make your own mind up, that way, the only person you can blame if you don't like it, is yourself.


 


I have looked through TV powermates and EP's and have been blown away, where as Pat seems to think they are not worth the money.


 


Both valid points, but both our own views, the only view that matters, is your own!


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Do these powermate dealies increase the focal length of the scope in the same manner as barlows? Also does the 2 x actually give 2x mag, as the 5x apparently gives 7.7x mag

(I know barlows dont physically increase focal length, just has that effect)

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To add a further piece into the mix James if you put a plain short extension tube between the Barlow and the EP you can increase the effect of the Barlow lens by almost double. :)


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Daz, yes I meant a 2" powermate wouldn't fit into the 1.25" focuser of my scope; would it?


 


I'll wait until I can compare them both on my scope and see what effects they yield.


 


Certainly given me more food for thought.


 


James

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Yup - Powermates are amplifiers - and the brilliant thing about them is they just do the magnification and you don't even know they are there. It's hard to see the glass when looking through one in daylight - they just get right out of the way by night.


 


A good value for money barlow is the Celestron Ultima 2x (around £85 new or £60'ish s/h). They've been around for a long time and very popular in the astro community. Also the Orion Shorty Plus - essentially the same eyepiece as the Ultima. At the budget end a great barlow is the TAL 2x at around £40 new or s/h - they're rare but hold value cos they're so good for the price. :)


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