Re: [ATM] Schief secondary

[Guntram Lampert <glampert@aon.at>]

Hello Gary,

yes, it is an advantage to interference test them against each other.
Until one is faced with the task of cutting out the secondary. I had to
give up the optics of a 6" anastigmatic Schief because of this. Could
not cut it out. And I  did not want to mount a grotesquely oversized 6"
secondary into that tube.
However, if you can do the trepanning, this is the way to go.

There is nothing special about making these "small" secondaries. They
are quickly made since the curvature is shallow. One can make them a
little oversize anyway, and so does not need to worry about edge defects
anymore.
Just use reasonable care. The secondary is interference tested against
the spherical tool, just as one would do in a Dall-Kirkham.
Using a different curvature opens up another degree of freedom for the
designer.

Both will provide very satisfying views.

Best regards,

Guntram



GARY FUCHS schrieb:
> Hello Guntram,
>
> My understanding of the Schief design with equal but opposite curvature of primary and secondary is that a big advantage of this is that you can easily interference test the secondary against the primary. With different curves how would you test your secondary?
>
> Also, if you were using that design for a 4.25" or even 6"  Schief would you do anything different when working with such small blanks?
>
> Thanks,
>
> Gary Fuchs
>
>
>
>   
>> Hi William,
>>
>> you can use a full diameter secondary since it is a sphere. Just mount 
>> it like you would the nominal size secondary. You only use the small 
>> part of its area that is specified by the design, typically about  55mm 
>> in diameter.
>>
>> Having made a 4.25 inch Kutter Schiefspiegler myself, I would not make 
>> the design that calls for equal but opposite curvature of primary and 
>> secondary design anymore.
>>
>> There are designs in existence which call for two different radii, and 
>> offer even somewhat better correction. I would be happy to share design 
>> data.
>> This way, one can avoid cutting the secondary from the tool. You simply 
>> make a separate secondary, and enjoy fantastic images.
>> I was successful making a 85mm diameter convex secondary for another 
>> telescope design from ordinary 1cm thick window glass.
>>
>> Guntram Lampert
>> _______________________________________________
>> ATM mailing list http://www.atmlist.net/
>>     
>
>
>   

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