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Reeny

£20 Seben Zoom 7.5mm-22.5mm - first light

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This is a decent 10mm to 22mm zoom.

But not so good at the higher magnifications between 10mm and the 7.5mm maximum Zoom.

I checked it out this morning on the moon in semi daylight, 

and compared it with 12mm / 10mm / 9mm Plossls on the 700/76 3"beginners scope - then on the 200p dob.

You need to back the zoom off to gain focus, only using the bottom two thirds of the Zoom available.

The story is the same if you use a Barlow.

I used a 1.5x magnifier = bottom one third of the zoom was available = 10mm equivalent

And a 2x Barlow = no zoom is available = 10mm equivalent

It matches the Plossls for moon detail up to it's maximum limit of magnification.

So it will make a very nice 22mm / 10mm dual role eyepiece.

But it is never going to work at the 7.5mm quoted in the specification.

And when you swap onto the next eyepiece - the target has moved due to a slack fit and misalignment in the focus tube. (maybe I got a bad-un)

Advantages: Price / wide views / one size fits all / good construction  - with black coated metal threads for the filter (not cheap painted threads)

Disadvantages; You need to use the rubber eye cup for internal reflections / usable range = 10mm equivalent / there is misalignment when swapping onto the next eyepiece.

So in summary - just keep the 25mm / 10mm EP's which come with the Skywatcher gear.

Then get yourself a Barlow, or buy a decent Plossl like the 6mm Revelation to achieve your higher magnifications.

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When I tried mine out it zoomed perfectly through all mags, it did stuggle a bit with the barlow at 7.5mm to around 10mm though but seeing wasn't great and there is no major slack on the heritage focuser that I noticed. Because they are only £20 there may be some quality control issues with some but mine seems ok and i'm happy with it especially seeing I didn't pay for it either (xmas prezzie).

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Nice review. I'm always surprised how well these zooms seem to fair from peoples reports on them.

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When I tried mine out it zoomed perfectly through all mags, it did stuggle a bit with the barlow at 7.5mm to around 10mm though but seeing wasn't great and there is no major slack on the heritage focuser that I noticed. Because they are only £20 there may be some quality control issues with some but mine seems ok and i'm happy with it especially seeing I didn't pay for it either (xmas prezzie).

I may have needed to collimate the scopes - but I doubt that would help very much.

The barlow lens is a definite no-go area with my example.

It won't go past 10mm zoom, so starting from a barlowed 11mm leaves me no-where to go with the additional zoom magnification.

So no way in a million years will it reach down to 4mm.

It isn't worth sending back - so it will come to bits and be re-assembled to see if the 3-piece lens system will square up a little bit better.

- The rubber eyecup unscrews

- The top lens come out with watch case back remover

- Plastic spacer and zoom assembly needs to be encouraged out.

- Then the 2-piece zoom lens sliders are held in by brass screws

It will make a cracking EP for the cheapy if I can get it to work properly.

So all is not lost yet - watch this space.

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I had no problems focusing over a whole range of zoom.

I refrained from sharing my experiences so far, because weather has prevented me from doing everything I wanted before reporting back, but so far (and for the price of £20) it is definitely useful piece.

I didn't bother with Barlow (can't see a point of doing it).

I had no problems when swapping to another piece apart from having to re-focus but that happens with other eyepieces when I swap the as well.

BTW - it is amusing to note that someone is trying to flog them for £29 from Spain on AB&S ("...never used 29 shipped...)

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Wow - there is an easy fix for this assembly problem.

Assuming the design is good - the issue must have been with spacers and/or alignment.

I tried to shuffle a few things about to realise that it is better with the eyepiece lens screwed out, further away from the zoom assembly.

A spacer was required.

The quick fix is to remove the top lens assembly, reverse the glass in the holder, put the holder back in upside down.

two negatives make a positive.

Result = the lens is the correct way up / the lens is spaced correctly.

I'm not sure what happened - but it's working great now.

BigDaz & Bamuko - Thanks for the encouragement to get it fixed.

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Wow - there is an easy fix for this assembly problem.

Assuming the design is good - the issue must have been with spacers and/or alignment.

I tried to shuffle a few things about to realise that it is better with the eyepiece lens screwed out, further away from the zoom assembly.

A spacer was required.

The quick fix is to remove the top lens assembly, reverse the glass in the holder, put the holder back in upside down.

two negatives make a positive.

Result = the lens is the correct way up / the lens is spaced correctly.

I'm not sure what happened - but it's working great now.

BigDaz & Bamuko - Thanks for the encouragement to get it fixed.

Great news, glad you got it sorted. We will await another review from you with the corrected lens. They must have had a trainee make the one you purchased. :grin:

I had no problems focusing over a whole range of zoom.

I refrained from sharing my experiences so far, because weather has prevented me from doing everything I wanted before reporting back, but so far (and for the price of £20) it is definitely useful piece.

I didn't bother with Barlow (can't see a point of doing it).

I had no problems when swapping to another piece apart from having to re-focus but that happens with other eyepieces when I swap the as well.

BTW - it is amusing to note that someone is trying to flog them for £29 from Spain on AB&S ("...never used 29 shipped...)

I have seen two zooms, the same as we have, for sale and both wanted around £30 for them, one also wanted £10 postage from Germany :confused:

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Great news, glad you got it sorted. We will await another review from you with the corrected lens. They must have had a trainee make the one you purchased. :grin:

I have also tried it with the 1.5 barlow magnifier, and had no problems.

I was worried I might have stopped down the zoom when respacing the top lens.

No worries though.

It barlows down with excellent results - exactly what I bought it for.

The image is good until it goes grainy at 140x because of the light capture limitations of the 3-inch scope.

I won't be using it on the 8" dob normally - but that will be the next and final test.

The 3" travel scope will be a nice bit of kit when everything is working properly.

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All sounds very promising. I can see me using my sons Seben zoom on occasion.

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