Cutting the Bird out the Jones

[mark RD]

Bit of a back story before we get to the juicy bits.

I have had a really long break from astronomy and was looking for a reasonable telescope, Ideally a short tube refractor. 

Well anyway I know they are looked upon as been a pretty poor telescope I ended up buying a Seben Big Boss mainly as the

EQ3 style mount looked like it would handle something like a SW 80ED with ease however the mount turned out to be little better then the EQ1

and the so called short tube 6" reflector turned out to be a bird jones at 1400mm.

I finally got a break and got a good deal on a BRESSER Messier AR-127L/1200 and scope and  a EXOS-2/EQ5 mount.

So if you have been paying attention you may have worked out that the mentioned scopes are far from rich field.

So here comes the fun part. The Seben big boss has a lens in the focuser. I discovered this could be unscrewed. This got my hopes up.

I dropped any eyes in it and bang I could not get it to focus so taking the focuser off I lowered the eye piece by hand undil I found a point

where it would focus but sadly not anywhere close to the focuser rang.

So I though there is only one thing for it. I removed the mirrors and focuser and got the table sew out and cut 50mm of the primary mirror end.

reassembled and it was closer to getting into focus but no cigar. A second attempt of another 50mm and boy it was just half a turn on the focuser short.

Today I went for another 50mm cut and we have cloudy skys but I think this time I will have a 6" reflector at F4.  with a mount that can handle it no problem.

Once I get a chance to get this under stars I will update.  Pictures to follow soon.

[Cornelius Varley]

I suspect that you might be disappointed with the outcome. The "Barlow" lens has two functions. The first is to give the telescope it's focal length of 1400mm and the second is to correct the spherical aberrations caused be the spherical primary lens. You now have a 600mm focal length telescope with a spherical primary.

[Mr Spock]

I tried that many years ago with a Helios 6". All I ended up with was a pile of junk... 

[Peter Drew]

Never mind, always good to experiment, no real harm done.  Next project will be to make a truss to restore the original tube length

[mark RD]

3 hours ago, Peter Drew said:

Never mind, always good to experiment, no real harm done.  Next project will be to make a truss to restore the original tube length

Thats a possibility. Or I could just rebuild the original tube with fibreglass resin.

 

As I got a good deal on the mount and scope I can get my money back on the going rate for the mount so its not going to be an expensive experiment.

In daylight the scope preforms very well,

 

Edited February 16, 2023 by mark RD

[benzomobile]

Hi.

Barlowed newtonians unfortunally are not Jones Bird!

A Barlow lens simply increases the focal length, whereas the negative doublet of a Jones Bird is a true corrector that eliminates both spherical aberration and coma at the cost of increased astigmatism and field curvature. 

Sub-aperture lens correctors for single-mirror telescopes (telescope-optics.net)

A true Jones Bird was the Celestron (Vixen) Comet Catcher Jr. but it is no longer in production.

I have one (which I modified to be easily collimated): for planetary imaging it is a very good instrument.

Beppe

Edited February 17, 2023 by benzomobile

[benzomobile]

PS: 

The mirror of the Seben Big Boss - if it is not a real Jones Bird,  should  be parabolic.   In this case, good images at the primary focus require a coma corrector, optimized for f/4 aperture ratio.

If, on the other hand, the mirror is spherical - as I believe, the result will unfortunately be far from good 😞

B

Edited February 16, 2023 by benzomobile

[benzomobile]

9 hours ago, Cornelius Varley said:

I suspect that you might be disappointed with the outcome. The "Barlow" lens has two functions. The first is to give the telescope it's focal length of 1400mm and the second is to correct the spherical aberrations caused be the spherical primary lens. You now have a 600mm focal length telescope with a spherical primary.

Ordinary  barlows  used in low-priced short newts (as Seben ones are),  actually, don't correct SA. It only extends the distance of the focus.

Beppe

Edited February 16, 2023 by benzomobile

[davidc135]

The spherical aberration of a 6'' f4 sphere will be:

Aperture(ins) x 22.55/ f.ratio cubed = 6 x 22.55/ 64 = 2.1 waves under-correction.

Would that be tolerable at x25 magnification?

Both BJ corrector or common Barlow will try to correct the SA if it can be screwed into the nose of the ep but the Barlow only very slightly. With a small increase in f ratio.

David

Edited February 16, 2023 by davidc135

[mark RD]

1 hour ago, benzomobile said:

Ordinary  barlows  used in low-priced short newts (as Seben ones are),  actually, don't correct SA. It only extends the distance of the focus.

Beppe

Thanks for all your info.  There seems to be a lot of miss gudence on the internet about bird-jones and I have most likely fallen victim .

That was what I felt regarding the Barlow lens. And this was most likely done to keep the size of the scope down and to play the number games on people who think that high magnification makes a good scope.

 

 

[mark RD]

OK an update,

I had clear skys here and the chance to test out this little project. I found no major issues with CA and the stars looked consistent across the field of view. 

The only issue I have found is that the secondary mirror been closer to the primary mirror is noticeable and really needs to be reduced in size. 

The result is that its a little fussy getting your eye in just the right spot.  Overall I am really happy with the result, I was not expecting it to work as well as it did.

 

 

 

[benzomobile]

On 21/02/2023 at 12:52, mark RD said:

OK an update,

I had clear skys here and the chance to test out this little project. I found no major issues with CA and the stars looked consistent across the field of view. 

The only issue I have found is that the secondary mirror been closer to the primary mirror is noticeable and really needs to be reduced in size. 

The result is that its a little fussy getting your eye in just the right spot.  Overall I am really happy with the result, I was not expecting it to work as well as it did.

 

 

 

Your scope has  got  a  parabolic mirror  🙂

B

Edited February 27, 2023 by benzomobile

[mark RD]

On 22/02/2023 at 17:08, benzomobile said:

Your scope have  got  a  parabolic mirror  🙂

B

Yes it  looks that way, Its now a fairly decent scope.  I will be looking into a smaller secondary mirror possibly upgrading the focuser and a paint job.  

[Peter Drew]

The closer the secondary mirror is to the primary mirror, the larger it needs to be to benefit from the full aperture, the further away from the primary the smaller the secondary can be provided that you can still access the focal point.  One of the reasons a Bird-Jones design incorporates a Barlow/corrector is to achieve this.     🙂