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Newbie with a 6" reflector that may get a new primary mirror.

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#1 jje1701

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Posted 05 December 2021 - 08:00 PM

Hello everyone.

 

I am in need of a bit of advice.

 

I have a 6" (153mm), focal length 750mm,  f/4.9 Orion telescope that is ~25 years old.

 

I would like to either re-figure the primary mirror or possibly purchase a new mirror that is of better quality.

 

I can find a 6" f/5 mirror rather easily, but my question is, will that mirror work in this scope without other modifications?

 

F/5 and f/4.9 are close, but the focal length difference is ~15mm.

 

 



#2 vtornado

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Posted 05 December 2021 - 09:07 PM

If as you say the new mirror will be f/5 and your old mirror is f/4.9 then you need 15mm of out focuser travel more.

Put in your eyepieces focus the scope and determine if you have 15 more mm of travel. 

If you don't you can use a short extension tube.

 

You may also be able to pull the mirror back another 15mm with the collimating screws.

 

That being said I'm not sure when these big scope sellers celestron/orion/meade .. sell a scope at f/5

is that nominial or is that the exact focal ratio????


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#3 jje1701

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Posted 06 December 2021 - 09:29 PM

Thanks for the confirmation.  That is what I was thinking, but wanted to be sure.  I do have enough travel in my current focuser to accommodate an additional 15mm travel.  The new focuser I am looking at will have more available travel.

 

The Orion scope I have from the mid '90s seems to be fairly accurate with it's specs, however it would be nominal.  Each part of the scope has tolerances.  Main mirrors seem to be ±1.5mm.  Seems they made the mirror slightly smaller at some point, which made it an f/5 and probably saved them a few $ per mirror.

 

I have sent the primary off to OWL for evaluation.  I will see what they come back with and decide if it is better to keep the original or go with a new mirror.

 

OWL produces mirrors, but only lists a 6" f/6.  Not sure what the cost will be for an f/5.

 

Also looked at GSO and Edmunds Optics.  One seems too good to be true, the other a bit on the expensive side.


Edited by jje1701, 06 December 2021 - 09:34 PM.

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#4 DAG792

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Posted 07 December 2021 - 07:10 AM

Have you done any testing at all on your original mirror to deem it bad? The GSO mirror might be an improvement, but its a shot in the dark. 

The Edmund mirror will definitely be better, but like you sad, its expensive.


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#5 SteveG

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Posted 07 December 2021 - 08:18 PM

I would bother sending it off, it will be too costly to refigure.

 

Just get this and test it after installing it - return if not at least 1/4 wave.

https://agenaastro.c...-set-6-f-5.html




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