From: "Dwight K. Elvey" 
To: atm{at}shore.net
Reply-To: "Dwight K. Elvey" 


>From: "Jeff Anderson-Lee" 
>
>Sure, just put the barlow in backwards. ;-)
>
>Actually you could probably do make a reverse barlow, but it might end up
>with half the fiield of view.  Simpler/better to buy a longer focal-length
>eyepiece.

Hi
 A focal reducer needs to be bigger than the field stop by
the field stops diameter plus the distance between the field stop and the
focal reduce times the f-ratio of the primary to not vignette any.
 Actually, in a Newton or SCT, it can be smaller because the
secondary dominates the off axis vignetting. Still, it should be larger
than the field stop of the eyepiece. In the case of using a 18mm and a
focal reducer, it would most likely need to be larger than the 1-1/4 inch.
This would mean that a 36mm 1-1/4 would be better. If the reducer used was
a 2 inch, and the eyepiece was a 1-1/4 18mm on adapter, that would most
likely work with any practical primary.
 This can all be drawn out on a piece of paper to show what
is happening.
Dwight


>
>Jeff
>
>----- Original Message -----
>From: "Paul Cousoulis" 
>To: 
>Sent: Wednesday, March 19, 2003 12:50 PM
>Subject: ATM Reverse Barlow lens
>
>
>>
>>
>> Is there any such thing as a reverse barlow lens? Something that would
>> make an 18mm eyepiece work like a 36mm eyepiece?
>>
>> Thanks
>> Paul
>>
>
>
>

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