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Old 25-11-2018, 07:10 PM
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GSO RC8 native focal length

Hi,

Can someone please confirm the native focal length of GSO RC8 scopes. Both 1625mm and 1600mm are being mentioned on the internet.

Thanks in advance
Luka
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Old 25-11-2018, 08:10 PM
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blink138 (Pat)
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should be 1600 luka .......... 200mm times (F)8?
pat
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Old 25-11-2018, 09:10 PM
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I suppose my question can be rephrased if the scope is a true 8" (203.2mm) or 200mm.
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Old 25-11-2018, 09:35 PM
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multiweb (Marc)
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Quote:
Originally Posted by luka View Post
I suppose my question can be rephrased if the scope is a true 8" (203.2mm) or 200mm.
Just measure the front aperture.
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Old 25-11-2018, 10:22 PM
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Stonius (Markus)
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I also wonder about these scopes as the correct spacing of Primary and Secondary is critical (IIRC, 1mm misalignment moves the focal plane by 10mm).

Given the scope is spec'd to 1600mm I wonder how accurate are they when figuring the mirrors? Is it possible that the manufacturing tolerances for focal length may be +/- 1mm or more?
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Old 25-11-2018, 11:14 PM
DJT (David)
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Mine was 1628. Plate solve is your friend
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Old 26-11-2018, 12:20 AM
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Stonius (Markus)
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Here's where I get confused. It's not like a newtonian in that the secondary isn't flat, so I wonder if the setup of the secondary (spacing and accuracy of it's figure) contributes to the final image as measured by the plate-solver. Kind of like if you tried to measure your focal length with a focal reducer in place.

IOW, Is the focal length of the system for plate solving *only determined by the primary or does the secondary play a part?
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Old 26-11-2018, 01:01 AM
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Thank you for replies.

I should have probably explained that I "played" with the mirrors and changed the spacing so it is not correct now. I needed to know the correct value so I can set it with the help of plate solving.
(I have also emailed the question to GSO but no reply yet...)

Marc, the aperture at the very front is 220mm but I think the size of the primary mirror is the important factor for the focal length (200mm*f8 or 203mm*f8).

Marcus, when you said that the scope is spec'd to 1600mm, have you got a confirmed source that it is 1600mm and not 1623mm?
To answer your other question, the adjustment of the distance between the primary and secondary (which affects the final focal length as measured by plate solving) is achieved by shifting the secondary mirror (there is a screw in the middle of the primary to shift it in/out).

David, did it plate solve to 1628mm when the scope was new? I have found an old image when my RC8 was relatively new and it was 1610mm. Not very conclusive as it is in the middle between 1600 and 1623.

Thank you again for replies.
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Old 26-11-2018, 02:35 AM
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Quote:
Originally Posted by luka View Post

Marcus, when you said that the scope is spec'd to 1600mm, have you got a confirmed source that it is 1600mm and not 1623mm?
To answer your other question, the adjustment of the distance between the primary and secondary (which affects the final focal length as measured by plate solving) is achieved by shifting the secondary mirror (there is a screw in the middle of the primary to shift it in/out).
No Source, just asking the question as to what the tolerances are for the scope to vary from spec. In which case, setting to the published spec could put the scope at less than optimum performance.

Really the question is how hard is it to fashion a mirror where the focal length is accurate down to the millimeter, and do they bother or does it vary?

M
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Old 26-11-2018, 06:10 AM
glend (Glen)
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Just measure the front aperture.
The primary mirror is not equal to the front aperture. I used to own one, I know.

Last edited by glend; 26-11-2018 at 07:22 AM.
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Old 26-11-2018, 09:36 AM
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Quote:
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The primary mirror is not equal to the front aperture. I used to own one, I know.
Is it smaller than the front aperture? All catadioptrics or cassegrains I've played with had a primary larger than light path entry as to clip the spider vanes bolts or other attachments, same for the secondaries. They have a slightly oversize disc that covers any bolts or protruding bits. Never seen a GSO RC though. They might do things differently.
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