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Uses for 6" f/8 GSO primary?

ATM Astrophotography Beginner Mirror Making
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#1 Keudn

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Posted 06 March 2021 - 10:51 AM

Hi guys,

 

I have recently come into possession (for free) of a 6" f/8 GSO primary mirror, and I have been thinking about what I could use it for. I'm not really interested in using it for visual as I have an 8" dob, so I'm leaning towards having it refigured and ground down to f/5 or so for astrophotography use. I've currently got an ED80 on my LXD75, and having a short newt might be useful for planetary and planetary nebulae. My concern, and the reason I am making this post, is that I have read some threads here about issues with stress in these cheap mirrors, and I'm wondering if people think its worth it to put some money into refiguring and buying a secondary for it. I have access to lots of spare 80/20 and a 3D printer, so it won't cost me much to build an OTA for it. Do you think its worth utilizing? Would I be better off just selling it, if its even worth selling either?


Edited by Keudn, 06 March 2021 - 10:53 AM.


#2 DAG792

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Posted 06 March 2021 - 11:13 AM

It would be a lot of work to refigure the F/8 mirror to an F/5. I reckon you could buy two GSO F/5 mirrors for the same price. You could sell it but personally, if I had it... and if I had the time, I'd buy another 6 inch F/8 GSO mirror and make a binoscope. It is a lot of work though, and while the binoscope may not show as much as your 8 inch dob, it would be a really nice visual experience.

However, enough fantasizing, I really think its not a good idea to refigure an already fine optical surface. If you need an F/5 mirror, you're better off selling this one and buying a new GSO F/5 mirror. 


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#3 starcanoe

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Posted 06 March 2021 - 11:54 AM

Easy project. Make a 6 inch dob...BUT just slap a 2 INCH diagonal secondary mirror (maybe even a bit bigger) in there.. Use that set up with a 2 inch long focal length eyepiece like one of the more modestly priced 50mm ish plossls or 40mm ish wider field eyepieces.

 

The poor man's NP101 bigger cousin.



#4 TOMDEY

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Posted 06 March 2021 - 12:12 PM

At that point it's no more than just a cheap, questionable piece of glass. If you want a custom 6" F/5, the shop would want to provide a reliable substrate, rather than start whittling on something of meh pedigree. Taking a mirror from F/8 to F/5 is not refiguring --- it's starting from scratch.    Tom


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#5 starcanoe

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Posted 06 March 2021 - 12:48 PM

. Taking a mirror from F/8 to F/5 is not refiguring --- it's starting from scratch.    Tom

 

Naw...you just got polish longer and harder than usual...



#6 TOMDEY

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Posted 06 March 2021 - 02:58 PM

Naw...you just got polish longer and harder than usual...

But the sun might only last another four billion years.    Tom


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#7 starcanoe

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Posted 06 March 2021 - 03:01 PM

Dang Tom...I was hoping you'd take the bait and do the calculations....



#8 davidc135

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Posted 06 March 2021 - 03:44 PM

If, as is likely, it's a sphere, I'd see if I could go the extra step and figure it to a smooth paraboloid with a 1'' diagonal. Or make it available for someone else to turn it from good to excellent. If the coating was stripped it could easily be checked for strain.  David


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#9 MKV

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Posted 06 March 2021 - 04:25 PM

My advice is -- keep the f/8. The 6-inch f/8 is my favorite all around telescope.


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#10 Steve Dodds

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Posted 06 March 2021 - 04:55 PM

The cheapest route is to sell the f/8 and buy the f/5 you want.


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#11 LU1AR

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Posted 11 March 2021 - 04:09 PM

My advice is -- keep the f/8. The 6-inch f/8 is my favorite all around telescope.

I own a TAL 6" F/8 and enjoy it, from a quarter of century now.
With a mask, become a beautifull planetary device.
Is tolerant to my microscope eyepiece adaptations.
Regards.
Edgardo


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