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Real numbers for GSO RC-8?

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#1 TinySpeck

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Posted 23 April 2020 - 08:56 PM

I'm collimating my TPO-branded GSO 8" Ritchey-Chretien for the first time, and the first thing I want to do is get the focal length correct by adjusting the spacing between the mirrors and using plate solving to tell me the image scale / focal length.  But there doesn't seem to be a good answer to what the focal length should be.  There is about an even split between 1600 mm and 1625 mm out there, which seems to hinge on whether the aperture is 200 mm or 8" (203.2 mm).  If you assume that the f/8 spec is exact that will give you the two focal lengths of 1600 and 1625 mm, respectively.

 

So I went and measured my primary mirror, to try to tip the scale one way or another.  I used a U-shaped piece of cardboard with two marks exactly 200 mm apart, reached in and held this up against the mirror.  I could get a pretty accurate reading this way.  And guess what?  The active, reflective portion of my primary mirror is about 3 mm LESS than 200.

 

I now have three different mirror apertures and no consensus on what the focal length should be.  I don't even know if f/8 is exact or rounded.  I've inquired to Guan Sheng Optical but I don't hold much hope for a response.  The focal length of an R-C scope is supposed to be set pretty carefully to avoid aberrations, so what's a poor astronomer supposed to do?



#2 TinySpeck

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Posted 24 April 2020 - 04:39 PM

Anyone? 

 

In the absence of any other input I'm likely to go with Craig's post here, since he's contacted GSO and is pretty adamant that all these scopes are 1600 mm.  The aperture is supposed to be 200 mm, too, though, and mine is more like 197.  Maybe GSO shaves off a couple mm for their re-branded scopes.  grin.gif  There's such a mess of misinformation out there I wish I could be more confident on this.



#3 werper

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Posted 29 September 2020 - 02:34 PM

I have the same scope.  Did you ever get a definitive answer on this?



#4 Bomber Bob

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Posted 29 September 2020 - 03:18 PM

203? 200? 197?  Ya gotta be kidding me!

 

Is there no way to safely remove the primary & measure it directly?  Old / vintage / antique Casses like mine are kinda clunky, but at least the design & hardware is owner serviceable, and easy to break apart & reassemble...



#5 TinySpeck

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Posted 29 September 2020 - 03:25 PM

I have the same scope.  Did you ever get a definitive answer on this?

I'm sure that it's 1600.  The back focus spec is 250 mm and you don't get that unless the focal length is 1600.  Some sites (like OPT) are now starting to spec this scope as 1600 mm also.

 

203? 200? 197?  Ya gotta be kidding me!

 

Is there no way to safely remove the primary & measure it directly?  Old / vintage / antique Casses like mine are kinda clunky, but at least the design & hardware is owner serviceable, and easy to break apart & reassemble...

Sure, you can pull the mirror relatively easily, but then you have to recollimate.  I didn't think to measure my diameter when I had the mirror out for cleaning.  It would be nice to know, but I'm sure the focal length is 1600.


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#6 werper

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Posted 01 October 2020 - 01:44 PM

Thank you for the reply.  I'm going with 1600, 203 dia.



#7 andysea

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Posted 02 October 2020 - 09:18 PM

Even better, you may be able to determine the exact spacing with a Ronchi eyepiece. I’ve never been able to use it precisely tho.
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