ETX125 - photo adapter + Canon EOS 500 problem

Hello,

When I try to use my camera to an ETX with a Meade photo-adapter, I am
loosing the 24x36 format reflex vision.
About 15% of the top part is cut off... as if I had a new 20x36 format (with
the focus controls de-centered towards the bottom)

I didn't take any picture so far. So, I am not sure that this problem shall
be visible on a picture itself.
Maybe is it a mirror problem on my camera....maybe not...

Anyone any thoughts?

Thanks a lot

Erick




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Visit Mike Weasner's Mighty ETX site -- read his section on
Astrophotography, and, send an e-mail to Mike -- may be helpful.

http://www.weasner.com/etx/menu.html

--

----
Joe S.

"Erick" wrote in message
...
Hello,

When I try to use my camera to an ETX with a Meade photo-adapter, I am
loosing the 24x36 format reflex vision.
About 15% of the top part is cut off... as if I had a new 20x36 format
(with
the focus controls de-centered towards the bottom)

I didn't take any picture so far. So, I am not sure that this problem
shall
be visible on a picture itself.
Maybe is it a mirror problem on my camera....maybe not...

Anyone any thoughts?

Thanks a lot

Erick




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Thanks Joe,

I knew about this site. I will send an e-mail to Mike....who knows!

Erick



"Joe S." wrote in message
...
Visit Mike Weasner's Mighty ETX site -- read his section on
Astrophotography, and, send an e-mail to Mike -- may be helpful.

http://www.weasner.com/etx/menu.html

--

----
Joe S.

"Erick" wrote in message
...
Hello,

When I try to use my camera to an ETX with a Meade photo-adapter, I am
loosing the 24x36 format reflex vision.
About 15% of the top part is cut off... as if I had a new 20x36 format
(with
the focus controls de-centered towards the bottom)

I didn't take any picture so far. So, I am not sure that this problem
shall
be visible on a picture itself.
Maybe is it a mirror problem on my camera....maybe not...

Anyone any thoughts?

Thanks a lot

Erick



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I 've got already a reply but I am not satisfy with that answer.
Any other idea's?


Here is his answer:

What you describe is typical, depending upon the camera and telescope.
The focal plane is only so large in diameter and so may or may not all
appear on the film. You can use one or both portions of the T-64 and see
whether you notice much change.

Mike Weasner

----------------------------------------
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| "Best Meade ETX Telescope Web Site" |
| -- according to ETX users worldwide |
----------------------------------------

Here is my reply:


Hi Mike,

Thank you very much for your quick answer. I appreciate.
Nevertherless, after having tried the 2 remaining possibilities with each
part of the adapter, I must admit that the phenomena is less important with
the shortest part. 10% instead of 15% and top part that I said "missing" is
in fact only darkened.(but a lot)

What I don't understand is why this part more than another (left, right or
bottom)
I can correct this if I hold the camera at the rear without adapter and
that
I play with the angle made by the optical axis and the film plane. If I
reduce this angle, the darkened part diminishes and even dissappears. But
in
that case, I don't have a 90° angle anymore.

Attached to a Pronto, I had already noticed this problem in the past but
less important, about 5%.

Do you think that all this is related to the focal lenght of the telescope.
Or related to the camera itself....with a misaligned mirror maybe??? Should
I ask to Canon?





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Hi Pierre,

I red already a lot about vignetting but I don't think that my problem has
anything to do with vignetting or with collimation.
Don't forget that the shape of the darkened upper part of the viewfinder is
a rectangle of about 5mm height equally darkened on the full width.
Furthermore, I have also experienced this problem with a very short
refractor (Pronto without baffles).
How is this possible....I will have to try with another camera anyway.

Erick


"Pierre Vandevenne" wrote in message
. 180...
"Erick" wrote in
:

I 've got already a reply but I am not satisfy with that answer.
Any other idea's?

Well, have a look at this (if you haven't already checked)

http://www.weasner.com/etx/astrophot...ignetting.html

and then try to see if something is moving inside the scope.

from that disassembly

http://people.freenet.de/Astroseiten/kollimaten.html

it seems that the baffle could move a bit if the back camera adapter is
loaded.

--
Pierre Vandevenne - DataRescue - www.datarescue.com
Home of the IDA Pro Disassembler - leader in hostile code analysis
Home of PhotoRescue - risk free data recovery for digital media.


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What you are describing is mirror cutoff and it only appears in the finder,
not on film. The mirror in an SLR is not long enough to intercept rays at
the bottom of the mirror box due to the need for it to fit within the
limited depth of the body when it swings up. This is of no consequence with
camera lenses which are relatively close to the mirror but you may lose part
of the image in the finder when using long focus lenses. Take some pictures
to make sure this is the correct explanation in your case.

Michael

Hi Michael,

Thank you very much for your explanation.
I was in fact expecting this kind of behaviour with the mirror.
I am actually taking some pictures and will report later about this subject.

Erick



"Michael L. Pipkin, M.D." wrote in message
...
What you are describing is mirror cutoff and it only appears in the
finder,
not on film. The mirror in an SLR is not long enough to intercept rays at
the bottom of the mirror box due to the need for it to fit within the
limited depth of the body when it swings up. This is of no consequence
with
camera lenses which are relatively close to the mirror but you may lose
part
of the image in the finder when using long focus lenses. Take some
pictures
to make sure this is the correct explanation in your case.

Michael




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