seban fix ??

[steelfixer]

With the discusion over the possible replacement of the correcting lens in a seban still fresh in my mind I had a brief moment of excited thinking followed by the sobering and depressing thought of it being a seban.

Several people have had the idea of replacing this lens with a better quality barlow. The problem is to keep the replacement barlow in the correct alignment.

My solution is this.

1. Wind the focuser fully in.

2. operating through the front of the tube look at the base of the focuser. You will see this lens. Unscrew it from the tube.

3. unscrew the extension tube stroke/eye piece holder from the focus tube.

4. in the kit that came with my scope was a very long 2x barlow lens dig it out from where ever you stored it.

:hello2:A word of warning here you might want to remove the focus tube from its carrier before you do this next bit as you can easily break the teeth of the rack. No prizes for guessing how I found that one out.

If you do break the teeth just turn the tube round in the carrier.

5. Insert this barlow into the focus tube. It is a snug fit but I managed to get mine in about half way. I figure the further the better as it will mean your focus point will be further out.

I found with my scope the focus point on a star was with the focuser nearly all the way in.

This will also act as your new eye piece holder.Refit the focus tube.

6. pop in an eye piece and try it.

I have had a quick try and I found I can even focus my 25mm plossl attached to a 3x barlow.

I recon if you or you know someone who has a damaged or spare 3,4,or 5x barlow you coud transplant the lens to replace the rubbish lens in the Seban barlow you might just get a half decent image.

Alternatively and better still you could use the donated lens to replace the corrective lens you first removed.

Anyway if the clouds lift long enough tonight I will try and see if this make any real difference.

It does proove that this lens can be changed though.

Keep smiling all you seban owners and take it from one who knows, things can only get better.

thanks

graham

[haitch]

So essentially you are using the barlow to replace the built in one?

Have you managed to view with it yet?

[steelfixer]

only day time.toooooo cloudy

[steelfixer]

If this does improve things I intend to see if I can get hold of another decent lens from somewhere and try that.

All good fun.

[haitch]

Something like a Tal barlow would probably be nice but it's whether you would consider it worthwile sacrificing a nice barlow like that.

[John]

From what I've read about the Seben's, I believe they use primary mirrors with a fast focal ratios (ie: around F/3 or F/4) and spherical, rather than parabolic, figuring. That means the corrector lens at the bottom of the drawtube is (supposedly) doing a bit more than just increasing the effective focal length as it would also need to correct the errors inherent to a fast spherical primary mirror.

Personally I'd be wary of sacrificing a decent quality barlow lens to this project but it may be worth playing around with a cheap one to see what the results are.

[haitch]

John, thanks - I was trying to put across the question of whether it is worth it but reading it back I think I may have been a little too subtle.

[Cornelius Varley]

I believe the best "fix" for a 1400/150 Seben weighs about 10lb.

As John said, the primary mirror in this type of telescope is spherical not parabolic and the corrector lens in the focuser is not a barlow lens. The 1400/150 is a Jones-Bird telescope and the lens in the focuser corrects for the spherical abberation. The image towards the centre of the mirror is relatively sharp becoming less so towards the edge. At apertures below 6" a Jones-Bird is adequate but not brilliant. A non destructive modification would be to take a piece of stiff black card or plastic the same diameter as the primary mirror and cut a hole in the card about 4.5" diameter. Attach this to the front of the primary making the primary effectively a 4.5" mirror. This should reduce the amount of spherical abberation whilst keeping the telescope intact.

Peter