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DIY Coma corrector "Tunable top"

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#1 Marcopolo_M

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Posted 07 August 2020 - 07:03 AM

Hello all,

 

a few months back a few of you may have noted my 3D printed ultra-low profile 2in Crayford focuser for use in my 12in F3.3 scope.

 

I wanted to make another addition to the system along the lines of what the more expensive coma corrects have for example The Paracorr visual model tunable top

because as far as I can tell the distance that the eyepiece sits behind the coma corrector lens can vary greatly from eyepiece to eyepiece.

 

So what my question is, can this simple be a tertiary focuser mechanism that drops into my 2in coma-corrector? (in my case a low cost Revelation coma corrector).

Initially I suppose I could just use different spacers to find the range I need.

 

This does not effect or change the crayford focuser that sits at the bottom of it all.

 

I suppose another question i've got is when you are using a tunable top style corrector does changing the position of the eyepiece also effect the focus whereby you have to refocus the main focuser drawtube?

So say you drop an eyepiece in, find focus, decide the coma correction is not optimal, change the tunable top position, do you then have to refocus?

 

thanks in advance,

 

Marco.

 

 



#2 TOMDEY

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Posted 07 August 2020 - 07:26 AM

The purpose of the tunable top is specifically so that you don't change the relationship of the CC to the telescope because there is one unique spacing to scope that nulls the field coma of the paraboloidal Newtonian Primary Mirror. The range that you build into your concoction will have to accommodate that for all of your eyepieces.    Tom



#3 Marcopolo_M

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Posted 07 August 2020 - 07:40 AM

That's interesting Tom, so by that logic is it fair to say that when using  coma corrector the purpose of a focuser really becomes defunct EXCEPT for to compensate between altering the focus when changing from say to looking at the moon to say looking a a galaxy. In both instances I would have assumed they are close enough to infinty anyway to consider different focus position minute.?

 

But by your logic does it not also mean that your focuser really only has one optimal position it will sit in for it's entire life (if or course you assume you're always using that particular coma corrector)? and that the only thing you'll ever change when switching eyepieces is the tunable top?

 

regards,

 

Marco.

 

PS post edit. I do realise there are other reason eg mirror cooling/heating, altitude the scope is pointing depending on cell design etc... that may effect focus so it's really just a theoretical postulation and that you'll always need a focuser of sorts!!! But all that accounted for on an evening viewing when everything is settled down technically you could be at one position with your focuser and the only adjustment you make when switching eyepieces is the tunable top.


Edited by Marcopolo_M, 07 August 2020 - 07:48 AM.



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