Re: [ATM] Tough questions about Aperature Stops

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

Richard O wrote:

> The main one is to compensate for atmospheric seeing.

Another is to compensate for a mediocre mirror figure - in general, the 
surface defined by a sub-aperture mask will be a better approximation to a 
perfect paraboloid than is the full mirror - e.g. a turned-down edge will be 
almost completely eliminated from active service.

> I can't think of why you would need to adjust the secondary.  An aperture
> mask does not turn your Newtonian into a Herschelian.  A Herschelian
> involves tilting the primary and simply adding an aperture mask does not.

It does define a piece of mirror "tilted" wrt the optical axis, just like an 
ideal Herschelian with an off-axis paraboloid. But there is of course no 
need to adjust the secondary. It is in principle possible that you might 
improve things (lowering coma) by tilting the focuser/EP some relative to 
the usual position where the focuser/EP axis is coincident with the 
*optical* axis of the (unrestricted) paraboloid, but I have no good answer 
to this. Somebody with a running ray-trace program may check where the 
tightest focus lies as you go off-axis - it would be interesting to see if 
this focal plane is tilted. If nothing else, aiming the EP at the center of 
the masked area would minimize the aberrations of the EP itself.

Nils Olof

>
>
>> Playing around with an aperature stop for my Orion
>> XT10 when viewing Mars. IT REALLY WORKS WELL! Doubling
>> and blurryness are gone and I'm getting very, very
>> nice edges and actually quite alot of surface detail
>> (dark and light areas).
>>
>> Only problem is, I want to understand more about some
>> of the aspects of using an aperature stop. I can't
>> quite wrap my brain around the optics. Can anyone
>> help?
>>
>> I understand that by using an aperature stop on a
>> newtonian telescope, you are in effect creating a
>> folded herschelian (because of the secondary mirror)
>> with no diffracation spikes because the secondary and
>> spider is no longer in the way, and less aperature
>> gathering light pollution with). But here's my sticky
>> questions:
>>
>> 1. Why don't you need to alter the angle of the
>> secondary mirror (and the angle and position of the
>> focuser in tandem) be turned to face the new angle of
>> the path of the reflected light caused by offset
>> aperature opening? Real Herschelian scopes do this.
>> They have the secondary (or just the eyepiece with no
>> secondary) turned along the axis of the returned
>> light, not the axis of the incoming column of light.
>> Without doing this, your secondary mirror is still
>> aligned as before along the axis of the incoming
>> column of light, thus the light entering the eyepiece
>> probably isn't centered. Why doesn't this seem to
>> matter? Or does it?
>>
>> 2. Does the shape of the opening in the aperature stop
>> matter? Could you use a square for instance? Why not a
>> half circle? Could you use more than one opening in
>> another place? Why do you use a circle, and why that
>> specific size?
>>
>> I've looked all over the internet for an answer to
>> these two questions for two days and have come up with
>> nothing. Please help me! I'm dying to know...
>>
>> Thanks for any advice
>>
>> -Mike Carambat
>>
>>
>>
>>
>>
>> Mike Carambat
>> Design Wizardry - http://www.designwizardry.com
>>
>> PLEASE IGNORE THE BIT OF SPAM BELOW (free email requires it):
>>
>>
>>
>>
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