Celestron Ultima 2x Barlow: Can you explain this?

My wife gave me as a present a "do not ever by this!" department store
70/700 scope wich came with 2 eyepieces, a Kellner 4mm an a SR 20mm.

The 4mm gave 175x which was over the recommended (140x) magnification
for the scope.

It realy hooked me into astronomy so I decided to buy a 11mm Televue
Plossl eypiece witch was a big improvement and today I bougth a
Celestron Ultima Barlow 2x.

The 11mm TV gave me 64x so with the barlow I shoud get 128x which
should be just fine for the apperture.

BUT when I try the 11mm TV with the Barlow on Mars I get a BIGGER
planetary disk than with the 4mm eyepiece. I can tell this because
with my other eye I can compare the planet to the holes on the tripod.
And yes I put the barlow after the diagonal.

What happens here? I suppose 175x is bigger than 128x. Is this right?
What am I missing?

The 4mm would have very little eye relief as opposed to the 11mm TV.

It could just be a matter of perception caused by the greater amount of eye
relief.

Just a thought...
Todd

http://www.backyardastronomy.com
http://www.skynewsmagazine.com
http://www.simpleastrophotography.com

(Nikos Aslanakis) wrote in sci.astro.amateur:

My wife gave me as a present a "do not ever by this!" department store
70/700 scope wich came with 2 eyepieces, a Kellner 4mm an a SR 20mm.

The 4mm gave 175x which was over the recommended (140x) magnification
for the scope.

It realy hooked me into astronomy so I decided to buy a 11mm Televue
Plossl eypiece witch was a big improvement and today I bougth a
Celestron Ultima Barlow 2x.

The 11mm TV gave me 64x so with the barlow I shoud get 128x which
should be just fine for the apperture.

BUT when I try the 11mm TV with the Barlow on Mars I get a BIGGER
planetary disk than with the 4mm eyepiece. I can tell this because
with my other eye I can compare the planet to the holes on the tripod.
And yes I put the barlow after the diagonal.

What happens here? I suppose 175x is bigger than 128x. Is this right?
What am I missing?

If the description on the eyepieces and on the barlow as well is
correct, and your scope is f 700mm the magnification with a 2x barlow
indeed is 700/5.5 as opposed to 700/4.

If you are absolutely certain that this is no visual effect, and not due
to a smaller FOV of the Plossl (maybe 60 deg versus 50 degrees), there's
only one conclusion left: the labels on the eyepieces are wrong. Maybe
the 4mm Kellner is a low end "department store accessory" and in the
Chinese factury they switched the plastic label - after all, probably no
one there could reach that latin-symbol gibberish..... :P

--
CeeBee


Uxbridge: "By God, sir, I've lost my leg!"
Wellington: "By God, sir, so you have!"


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"Nikos Aslanakis" wrote in message
m...
My wife gave me as a present a "do not ever by this!" department store
70/700 scope wich came with 2 eyepieces, a Kellner 4mm an a SR 20mm.

The 4mm gave 175x which was over the recommended (140x) magnification
for the scope.

It realy hooked me into astronomy so I decided to buy a 11mm Televue
Plossl eypiece witch was a big improvement and today I bougth a
Celestron Ultima Barlow 2x.

The 11mm TV gave me 64x so with the barlow I shoud get 128x which
should be just fine for the apperture.

BUT when I try the 11mm TV with the Barlow on Mars I get a BIGGER
planetary disk than with the 4mm eyepiece. I can tell this because
with my other eye I can compare the planet to the holes on the tripod.
And yes I put the barlow after the diagonal.

What happens here? I suppose 175x is bigger than 128x. Is this right?
What am I missing?

Hi Nikos,

Are you sure it isn't the other way around.....4mm SR and 20mm Kellner?

Best regards,
Bill

"Nikos Aslanakis" wrote in message
m...
My wife gave me as a present a "do not ever by this!" department store
70/700 scope wich came with 2 eyepieces, a Kellner 4mm an a SR 20mm.

The 4mm gave 175x which was over the recommended (140x) magnification
for the scope.

It realy hooked me into astronomy so I decided to buy a 11mm Televue
Plossl eypiece witch was a big improvement and today I bougth a
Celestron Ultima Barlow 2x.

The 11mm TV gave me 64x so with the barlow I shoud get 128x which
should be just fine for the apperture.

BUT when I try the 11mm TV with the Barlow on Mars I get a BIGGER
planetary disk than with the 4mm eyepiece. I can tell this because
with my other eye I can compare the planet to the holes on the tripod.
And yes I put the barlow after the diagonal.

What happens here? I suppose 175x is bigger than 128x. Is this right?
What am I missing?

I wonder if your 4mm eyepiece is really 4mm. Maybe you're lucky and it's
really 6mm

More than one person has claimed that the Ultima 2X barlow is in fact 2.5x.


I get larger images with the Ultima than I do with a 2.5x Powermate.

More than one person has claimed that the Ultima 2X barlow is in fact 2.5x.


I get larger images with the Ultima than I do with a 2.5x Powermate.

I have seen this suggested other times.

One thing to consider is that the magnification of a barlow is not fixed,
rather it depends on some geometric factors. The fact that putting the barlow
in front of the diagonal (nice trick on a Newt) is said to give 3X
magnification is only an approximation.

I think Stephen Paul is on the right track here.

jon

(Nikos Aslanakis) wrote in message om...

BUT when I try the 11mm TV with the Barlow on Mars I get a BIGGER
planetary disk than with the 4mm eyepiece. I can tell this because
with my other eye I can compare the planet to the holes on the tripod.

I have carefully measured the magnification of my "2X" Ultima
Barlow at anywhere from 2.4X to 2.6X, depending on which eyepiece
it is coupled with. Note that the magnification is extremely
sensitive to the distance between the Barlow and the field lens
of the EP; you can boost it quite perceptibly by pulling the
eyepiece 1/8 inch out of the Barlow's barrel.

Also, I wouldn't trust the 4mm stamped on a cheap eyepiece farther
than I can throw it.

- Tony Flanders

Well, almost all of you are right in one way or another.. ;-)

I have made some test today and here are the results:

I put in the 20m eyepiece looking on a nice external air conditioning
unit some hundreds of meters away. Looking down with one eye on the
eyepiece and the other to the floor, I marked where the projection of
the air condition on the floor started and where it ended. Now I had
35x.

Then I put the barlow with the same eyepiece and noted the projection
which was now almost double. Good thing. At least my Barlow was a 2x
one ;-) I marked the projection limits now at 70x.

Then I put the 11mm Televue eyepiece. The projection was somewhat
smaller which was ok at 64x. I put the barlow on and it normally
doubled. That was 128x.

At last I put the "4"mm eyepiece and guess what!!: the projection was
smaller at about 80% of the 128x one!!

So the "4mm" eyepiece is not a 4mm but something from 6mm to 7.5mm.
They have a wrong label on it saying "K 4mm".

Tomorrow I will do the same tests with greater accuracy to calculate
the exact mm of the eyepiece.

Thank you all for your help anyway!

Well, after careful calculations here are the results:

The Celestron Ultima Barlow magnifies by a factor of 2.2x with all
eyepieces.

And, of course, the "4mm" eyepiece is not a 4mm but a 6.5mm one,
giving 108x on my scope.

I guess it is still a kellner. It wouldn't bother me if it was a
Radian though...