Re: true magnification of Televue 3x?
- From: "Roger Hamlett" <rogerspamignored@xxxxxxxxxxxxxxxxxxx>
- Date: Tue, 27 Dec 2005 11:31:39 GMT
"Jason Watters" <jw@xxxxxxxxxxxxx> wrote in message
news:05Arf.10191$nm.14@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>I have been trying to figure out the true magnification of my Televue 3x.
>I
> use it with my C 9.25 SCT and it is inserted into a JMI motofocuser
> which
> adds some length between the barlow and visual back, plus I have some
> length
> between the barlow and CCD camera due to a filter wheel in the path. I
> realize that the magnification is way beyond 3x- is there any way to
> figure
> out the true magnification?
>
> Thanks,
> Jason
The Barlow lens, is simple to calculate the 'magnification' fator
produced. It is:
Mf = (f-s)/f
Where 'Mf' is the effective factor applied to the scopes focal length, 'f'
is the focal length of the Barlow assembly, and 's' is the seperation
between the optical centre of the assembly, and the focal point.
Now, if you have a filter wheel, adding perhaps 1.2" to the length,
together with a CCD, perhaps 1" inside it's own camera body. The JMI, will
add perhaps another 2", making 's', perhaps 7.2". It is designed to be
about 3.2", and the focal length if I remember correctly, is about -1.65".
With your spacing, this will give:
(-1.65-7.2)/-1.65 = 5.3*!...
You are also talking about vignetting. This will be being made worse by
the long distance between the lens assembly, and the CCD (you don't say
how large the CCD is, which is critical data).
For Powermates, these are effectively complete 'scope' assemblies used to
alter the light paths in a much more complex way.
If you look at:
http://www.televue.com/engine/page.asp?ID=42
This gives graphs for the magnification of the various PowerMates, with
different spacings.
First general comment, put the JMI, in front of the lens assembly. You do
not want variable lengths in the light path after the Barlow. Now try to
shorten the light path as much as possible. You may even be able to get a
threaded adapter to replace the top tube of the Barlow, and screw directly
into the filter wheel. Provided your CCD is not too large, this should
solve the magnification/vignetting problems. However if the CCD is big (a
unit like the ST2000 or larger), then you should be looking at using 2"
accessories, rather than 1.5", to keep the light path as large as
possible. You may well also find that results would be better without the
JMI, but using a RoboFocus on the main scope focusser instead (this keeps
the length/weight at the back of the scope down, brings the optics closer
to the focal point they are designed for, and reduces potential flexure
points behind the scope).
Best Wishes
.
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