Re: true magnification of Televue 3x?


"Jason Watters" <jw@xxxxxxxxxxxxx> wrote in message
news:Jqbsf.10653$nm.8986@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> Hi Roger,
>
> Thanks for the very useful info. The only thing I want to correct is
> that I
> do have the JMI installed between the scope and the Televue, and have
> been
> wondering how much this distance also adds to the magnification with the
> Televue 3x.
It doesn't directly (this helps a _lot_ in the figures... :-)
However it _does_ increase the focal length of the scope itself. The
secondary on the SCT, is also acting as an image multiplier. Increasing
the spacing from this to the image plane, makes it's factor increase, and
makes the SCT, act as a longer focal length scope.

> I've had these vignetting problems for some time, especially since
> switching
> from a webcam sized CCD to a 1/3" format. I did at one point borrow a
> friend's 4x Powermate in the 2" size and no longer had vignetting, but
> the
> image scale was smaller than with the 3x barlow. I'd be tempted to try
> the
> 5x Powermate, but it is only available in 1.25" size so I may still have
> vignetting problems plus the distance above the top of the barlow would
> be
> increased by the filter wheel/ camera. If there was a way to shorten
> the
> Powermate 5x (and thereby get less overall magnification once the filter
> wheel and camera are added), and not have vignetting problems due to the
> 1.25" size, it might be ideal.
>
> Jason
What focal length do you actually 'want'. What is the pixel size of your
CCD?. Whose CCD is it?. What is the filter wheel?.

Best Wishes

> "Roger Hamlett" <rogerspamignored@xxxxxxxxxxxxxxxxxxx> wrote in message
> news:vC9sf.15657$f7.6495@xxxxxxxxxxxxxxxxxxxxxxx
>>
>> "Jason Watters" <jw@xxxxxxxxxxxxx> wrote in message
>> news:05Arf.10191$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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