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2x BST and 3x Xcel-lx focal point


sorrimen

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Hi all

Very niche question here. I’ve been imaging with a 2x BST shorty barlow but have the option to buy a used 3x Xcel-lx to get to optimal f-ratio. Currently my 2x + camera focuses very very close to the bottom of the focuser, so I’m wondering whether the 3x will push the focal point backwards or forward. I understand that this may depend on the barlows in question so have included them in the title, but if it’s just a matter between 2x and 3x even better! 

TIA

Ross

p.s. I’ve put this in eyepieces rather than imaging because I would imagine more people browsing the eyepiece and barlow topic would have experience with the two focal points than in the imaging topics.

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Hi Ross, I know your predicament from your other thread about the ADC, I think something you could do is to sell the 2x barlow you’ve got now and get another one with a screw off lens cell, then you could screw it onto the nosepiece of the ADC and you can gain a lot of inwards focuser travel that way.

For example, the Baader Q 2.25x barlow is 2.25x natively and has a stated focal length of 44.5mm. 

The distance from the nose of the ADC to the top is about 84mm so putting the barlow cell on the nose should give you a barlow power of (44.5+84)/44.5 = 2.9x (roughly, this assumes your sensor will be level with the top of the ADC).

Just avoid Televue Barlows for this as although their lens cells screw off they do not have a standard m28 filter thread and so they won’t screw into anything else apart from their barlow body. 
 

 

 

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5 hours ago, CraigT82 said:

Hi Ross, I know your predicament from your other thread about the ADC, I think something you could do is to sell the 2x barlow you’ve got now and get another one with a screw off lens cell, then you could screw it onto the nosepiece of the ADC and you can gain a lot of inwards focuser travel that way.

For example, the Baader Q 2.25x barlow is 2.25x natively and has a stated focal length of 44.5mm. 

The distance from the nose of the ADC to the top is about 84mm so putting the barlow cell on the nose should give you a barlow power of (44.5+84)/44.5 = 2.9x (roughly, this assumes your sensor will be level with the top of the ADC).

Just avoid Televue Barlows for this as although their lens cells screw off they do not have a standard m28 filter thread and so they won’t screw into anything else apart from their barlow body. 
 

 

 

Cheer, Craig. Having a real headache over this one 😆  That seems like a good idea, though I’ve seen a couple of posts saying the ADC body itself is 30mm which would change the calculation a little. Either way, my best bet is probably to leave you folks alone and utilise FLO 30 day returns to see what works best! Thanks again.

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