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Reducing a Tele Vue 2x barlow

JamesF

By JamesF
in Discussions - Eyepieces

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JamesF

JamesF

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I've been looking for a 1.6x-ish barlow for some time for planetary imaging.  My current setup is a C9.25 with a secondary focuser, filter wheel, 2x TV barlow and ASI120MM, but that gives me a focal ratio of around f/27 (from memory) which is a bit over the top.  What I'd really like is f/20 to f/22-ish, hence the idea to use a 1.6x barlow instead.

I've not found anything I particularly like the look of though and today it struck me that as the TV barlow has a detachable lens unit perhaps I could machine up a shorter body to which I could attach the lens unit to get the lower power.  I know that some barlow lens units are designed with this in mind, but the TV barlow doesn't use a standard filter thread so I assume that's not the intention in having the lens unit removable.  I guess it's just so it can be cleaned or something.

In fact, I could probably make up a two-piece unit that would screw into the M42 thread on the camera, hold the lens unit at the right distance from the camera sensor and screw into the M42 thread on the filter wheel at the same time, reducing the number of potentially wobbly parts in the entire imaging train.

Does this sound like a reasonable thing to do or have I missed something obvious?

If I know the overall focal ratio of the optical train and the distance from the lens unit to the camera sensor, is it possible to calculate what distance I need for the reduced power?

James

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JamesF

JamesF

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I know for the Tal barlows you can unscrew the lens element and put it on the EP for x1.4, not sure if that would work for imaging but I can't see why not. Should be able to do the same for the TV one.

That's pretty much my reasoning, but it seems odd that TV should use a different thread unless they specifically wanted to prevent it in which case you have to wonder why.  It's not like they sell a lower power barlow.  The 3x barlow is very long and also has a screw-off lens unit.  I wonder if the lens unit is the same for both and they just control the power with the length of the barrel?  That hadn't occurred to me before, but I guess it would make a certain amount of sense.  Perhaps it was just an oversight on their part when they chose the thread size that it wouldn't fit a standard filter thread.

James

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SkySurfer

SkySurfer

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That's pretty much my reasoning, but it seems odd that TV should use a different thread unless they specifically wanted to prevent it in which case you have to wonder why.  It's not like they sell a lower power barlow.  The 3x barlow is very long and also has a screw-off lens unit.  I wonder if the lens unit is the same for both and they just control the power with the length of the barrel?  That hadn't occurred to me before, but I guess it would make a certain amount of sense.  Perhaps it was just an oversight on their part when they chose the thread size that it wouldn't fit a standard filter thread.

James

Hi,

Have you seen the 1.6x Barlow lens on astroboot, they are for screwing onto a camera nosepiece, German made, and only £8 each, cheap I know but for that price maybe worth a try.

This is from the site

New item. Paralux (German brand) 1.5x screw-on Barlow lens. This Barlow 'lens head' fits the filter thread of 1.25" eyepieces and camera nose-pieces. A useful accessory.

£8.00

:) :)

SS

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nightfisher

nightfisher

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Posted

Hi,

Have you seen the 1.6x Barlow lens on astroboot, they are for screwing onto a camera nosepiece, German made, and only £8 each, cheap I know but for that price maybe worth a try.

This is from the site

New item. Paralux (German brand) 1.5x screw-on Barlow lens. This Barlow 'lens head' fits the filter thread of 1.25" eyepieces and camera nose-pieces. A useful accessory.

£8.00

:) :)

SS

for that price worth a try

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YKSE

YKSE

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How about Baader VIP barlow?  You can have 1.5x, 1.75x and 2x depend on if none/one/two 15mm extension tube is used. It has male T2(M42x0.75) thread on eyepiece side too. And if you get more 15mm or 7.5mm extention tube, you get many more combinations of barlow effect you want.

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JamesF

JamesF

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IIRC, the three element 2.5x APO Barlow from Revelation gives 1.6x as a nose piece. less than £40 and lot's of feedback on it on here from planetary imagers.

Russell

That's very interesting.  I have been a happy user of that barlow in the past -- right up until I changed cameras and optical train and it became too powerful.  I didn't realise it was usable if the chrome barrel was split from the rest of the body though.  I shall have to look into that.

James

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nightfisher

nightfisher

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for that price worth a try

I have just got one of these, i will let you know if its any good when i get to try it, my plan is to screw it into the DSLR 1.25 nosepiece and use it with the 150p ds to get 1200mm focal length

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