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double through a 5 and 10 inch

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#1 quilty

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Posted 22 September 2022 - 10:51 AM

This is armchair theorism, but the nights are getting colder and the armchair is quite teasing.

 

below you see a star pattern through a 5" and a 10" scope at the same power and below the same star with a b star in about 1.2" distance.

(Apologies for my poor sketching) The 10" scope shall be a scaled up version of the 5"

 

Can you imagine that splitting is easier through the 5"?

 

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#2 Old Southern Man

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Posted 22 September 2022 - 10:58 AM

I have heard of secondaries being difficult to detect due to being on the diffraction rings of the primary.  I would suspect, however, that if the 10" is of comparable type and optical quality, that it would usually beat the 5".  It might take different magnifications.  For one thing, the secondary will be brighter in the 10" and, if seeing is good, more magnification can be used effectively.  This might move the secondary off of the ring. 



#3 quilty

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Posted 22 September 2022 - 11:23 AM

changing power might help but it won't shift the b star with respect to the star pattern.


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#4 Ihtegla Sar

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Posted 22 September 2022 - 01:45 PM

It all depends on seeing and cooling. If both scopes are completely at ambient temperature with no tube currents or whatnot and the seeing is good enough than the 10" will split tighter doubles, of course, since the Dawes limit for a 10" is quite a bit smaller than a 5".

But a larger scope is harder to keep at ambient than a smaller scope and any tube currents in the larger scope can reduce its ability to split tight doubles that might still be split in a smaller scope that is at ambient temperature.

I've been able to split tighter doubles in my TEC 140 (5.5 inches) that were easily within the ability of my 10" CC but where I live the temperatures fall all night and the 10" mirror sometimes has trouble keeping up, even with fans.

Then there is seeing. If seeing is good enough that the effects of seeing are hidden in the larger airy disc of a 5" but not good enough to hide the effects of seeing in the smaller airy disc of a 10" then the 5" may give a more aesthetically pleasing view of a double star that can be split by both scopes.

#5 quilty

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Posted 23 September 2022 - 03:26 AM

Yes, seeing is almost everything at starsplitting and smaller apertures are less demanding. But my point was under special circumstances, seeing let alone, at a certain star distance (which equals then the distance to the first side maximum of the a star) and certain magnitude the separation just might be easier through the smaller aperture (as long as it's still capable of a complete split)

Anyway I'll try next opportunity Zeta Her through a 4". For this is an example where I think it might apply.



#6 Ihtegla Sar

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Posted 23 September 2022 - 03:29 AM

Another thing that might work in these situations is an aperture mask, particularly one with a geometric shape like a hexagon that breaks up the diffraction rings and can be rotated into the best position to show the secondary.
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#7 quilty

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Posted 23 September 2022 - 04:04 AM

Oh yes, great idea. This does two things in a time. A pity that it applies to refractors in the first place. Guess it was in order to reduce colour errors first. In a Cass with 30-44 % CO an aperture stop might not be too helpful. But I've seen garlands around the spider vanes in order to subdue the spikes. That's maybe worth a try.




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