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Converting C8 with broken corrector plate to Cassegrain/Ritchey-Chretien

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#1 mhammady

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Posted 20 June 2021 - 06:32 PM

Hello,

 

I have a C8 that I broke its corrector plate by accident. Since then, I moved forward and got C9.25, but still, this C8 sitting there hurts me in a way. It was a good one. Of course, I tried to replace the corrector plate, but as you may know, it cost as equal or even more than a new C8, and definitely more than a nice used one.

 

So, I've been thinking is it possible that I eliminate the corrector plate, and change its design to be used by only 2 mirrors? a design similar to basic Cassegrain or Ritchey–Chrétien telescope. I designed a secondary mirror holder like the one I saw in GSO RC model, and it achieved its purpose of handling the mirror.

 

Before proceeding further in this direction, does someone has an idea if this can work?

 

Thank you

 


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#2 Garyth64

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Posted 20 June 2021 - 07:28 PM

I believe that the C8 has about an f/2 spherical primary.

If you want a Classical Cass, the primary would have to be a parabola, and the secondary a hyperbola.

For a RC, the primary and secondary would both have to be hyperbolas.

 

Either way, it would be a lot of work.  Refiguring the primary, and making and figuring the secondary.


Edited by Garyth64, 20 June 2021 - 07:28 PM.


#3 benklerk

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Posted 20 June 2021 - 07:37 PM

I was about to say the same thing.

 

Its going to be a lot of work to change a c8 to either a RC or classical cassegrain. Both have completely different optic setups as stated above.

I don't have enough experience in this field to say more.



#4 Robindonne

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Posted 20 June 2021 - 08:24 PM

I assume he just want to know if the c8’s secondary mirror can be used in a spider instead of corrector plate. I dont know but i’ve read some posts about this issue and the outcome was negative. Not sure why exactly but i assume you miss the correction. Cant see where you are from but Italy is supposed to have a company that makes these corrector plates

Edited by Robindonne, 20 June 2021 - 08:25 PM.


#5 *skyguy*

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Posted 20 June 2021 - 08:31 PM

 Cant see where you are from but Italy is supposed to have a company that makes these corrector plates

That would be Toscan Optics:

 

https://www.toscanop...corrector-plate


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#6 davidc135

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Posted 21 June 2021 - 02:32 AM

The corrector has to do a massive amount of work. The 8'' f/2 primary produces 22.55 waves of spherical aberration of which 6.5 waves is corrected by the secondary which leaves around 16 waves for the corrector plate to deal with. Below is one surface of a 4'' corrector which corrects about three waves.

 

P5141546a.jpg

 

The photo shows a neutral zone at around .7r separating the inner hill from the outer concave zone. 

 

In the 8'' scope 16 waves can be put on the Schmidt plate, on the primary or secondary or both, or even perhaps on a sub diameter corrector attached to the secondary. All the options are tough.

 

David


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#7 maroubra_boy

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Posted 21 June 2021 - 02:57 AM

The other option is to place a wanted ad for an 8" corrector plate. I did this a few years back & found one surprisingly quickly. You never know your luck and all manner of misfortunes happen to people & scopes. For a corrector to come up most often it is a cleaning mishap that sees the primary or secondary damaged beyond salvation. This is what happened to the corrector I picked up, a wayward screwdriver fell on to the primary...

Alex.

Edited by maroubra_boy, 21 June 2021 - 02:58 AM.


#8 luxo II

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Posted 21 June 2021 - 07:03 AM

Before proceeding further in this direction, does someone has an idea if this can work?

Well, if (a) you are a master optician with thousands of hours prior experience (b) you have unlimited time, and © value your time at zero $ per hour, you could find-grind, repolish and parabolise the primary mirror, then redo the secondary mirror as a hyperbolic to make it a classical Cassegrain. But an f/2.2 parabola and a convex hyperbola are a pretty tough order for very experienced opticians, never mind a noob with no prior experience, so quite frankly while yes its POSSIBLE it is well out of your abilities and will cost some serious $$$$$ for anyone to do that, as a one-off. 

 

So, no, this is not a viable option.

 

Either:

 

(1) you acquire a new corrector, secondhand as above, or from Celestron or Toscan Optics; or

(2) replace the scope with with another good scope, secondhand, offer the scope you have for parts on AM, and whatever is left over goes in the trash.

 

NB correctors are not matched to the mirrors, as some would have you believe; that the ones from Toscan work better than the Celestron ones says it all.


Edited by luxo II, 21 June 2021 - 07:26 AM.


#9 rmollise

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Posted 21 June 2021 - 10:56 AM

As you've been told, the mirror is an f/2 sphere. Your choices?

 

  • See if Celestron can sell you a new corrector.
  • Look for a surplus one. 

 

Anything else will be beyond the realm of what is practical...like refiguring the primary and secondary and building a classical or other Cassegrain scope out of it. Or having someone produce a new corrector.

 

A new or surplus corrector should work acceptably well with your optics. 



#10 Stopforths

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Posted 21 June 2021 - 12:39 PM

what about a dall kirkham ??  be easier than the other 2 one surface the primary needs to be an ellipse.   Wouldn't be easy to figure an f2 primary mind you :(



#11 Don W

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Posted 21 June 2021 - 12:46 PM

It would be like transforming a VW Beetle into a Lamborgini. Is it possible? I guess so. Is it cost effective. Not in the least.


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#12 carolinaskies

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Posted 21 June 2021 - 12:51 PM

There might be some ATMer willing to spend the time to self-grind a corrector.  If YOU aren't that guy, list it in the Classifieds and let someone else take up that headache. 

BTW, Celestron WILL outfit your telescope with a whole new optical set for something like $800. 


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#13 Paul Hyndman

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Posted 21 June 2021 - 01:31 PM

There might be some ATMer willing to spend the time to self-grind a corrector.  If YOU aren't that guy, list it in the Classifieds and let someone else take up that headache.

Absolutely the way I would go.

 

It would be like transforming a VW Beetle into a Lamborgini. Is it possible? I guess so. Is it cost effective. Not in the least.

Couldn't resist grin.gif   While not the "real-deal", sometimes pretend can be fun (from my gear-head days):

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#14 rmollise

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Posted 22 June 2021 - 08:04 AM

Absolutely the way I would go.

 

Couldn't resist grin.gif   While not the "real-deal", sometimes pretend can be fun (from my gear-head days):

Alas, methinks few with the talent to make a new SCT corrector would care to spend the time on the project to revive a garden-variety C8. cool.gif


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#15 carolinaskies

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Posted 22 June 2021 - 09:20 AM

Alas, methinks few with the talent to make a new SCT corrector would care to spend the time on the project to revive a garden-variety C8. cool.gif

LOL, though some in the ATMer lot think they'd love such a challenge just for kicks.  



#16 RichA

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Posted 23 June 2021 - 12:14 AM

Hello,

 

I have a C8 that I broke its corrector plate by accident. Since then, I moved forward and got C9.25, but still, this C8 sitting there hurts me in a way. It was a good one. Of course, I tried to replace the corrector plate, but as you may know, it cost as equal or even more than a new C8, and definitely more than a nice used one.

 

So, I've been thinking is it possible that I eliminate the corrector plate, and change its design to be used by only 2 mirrors? a design similar to basic Cassegrain or Ritchey–Chrétien telescope. I designed a secondary mirror holder like the one I saw in GSO RC model, and it achieved its purpose of handling the mirror.

 

Before proceeding further in this direction, does someone has an idea if this can work?

 

Thank you

Get creative.  Build a sub-diameter corrector mounted on the visual back that can correct for SA of the mirrors!!



#17 luxo II

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Posted 23 June 2021 - 01:23 AM

If YOU aren't that guy, list it in the Classifieds and let someone else take up that headache. 

IMHO the OP was suckered into thinking he could fix a busted scope on the cheap.

 

I know its disgraceful to suggest this as IMHO either you pony up the $$$ to Celestron, or bin it and write that loss down to "lack of knowledge", but I reckon he'll cut his loss by selling it to another (sucker, that is).


Edited by luxo II, 23 June 2021 - 01:27 AM.

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#18 Old Speckled Hen

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Posted 24 June 2021 - 10:34 AM

That would be Toscan Optics:

 

https://www.toscanop...corrector-plate

"Copyright 2015 Toscanoptics Italy"

 

And off we go again. The owner has a nice Mercedes and lots of electric guitars on FB but "the LAST time" I looked on google Earth the address was either a disused plant, a grassy lot or apartments depending on what timestamp you were handed from zeus atop mount google....

 

So despite the really handy option of having a European producer of sct corrector plates after zero replies to the business E- mail  I gave up the trail as cold as of approx a year ago.

 

Now, if anyone HAS found them back in production, that would be FANTASTIC. Please let us all know.



#19 gfstallin

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Posted 24 June 2021 - 12:37 PM

"Copyright 2015 Toscanoptics Italy"

 

And off we go again. The owner has a nice Mercedes and lots of electric guitars on FB but "the LAST time" I looked on google Earth the address was either a disused plant, a grassy lot or apartments depending on what timestamp you were handed from zeus atop mount google....

 

So despite the really handy option of having a European producer of sct corrector plates after zero replies to the business E- mail  I gave up the trail as cold as of approx a year ago.

 

Now, if anyone HAS found them back in production, that would be FANTASTIC. Please let us all know.

I would not take business addresses in some parts of Italy too literally. Additionally, Google Maps has quite a difficult time with places like Matera, where Toscano Optics is located. The address you seek is down the street from the apartment building retaining wall that comes up on Google Maps. It is a storefront in a different apartment building (both of which are 23, Via Collodi, Matera). This is also the same business address for Real Music. Given the music-centric Facebook posts of the apparent owner of Toscano Optics, I'd argue Real Music and Toscano Optics are using the same address and are possibly owned, at least in part, by the same person. I did reach out to them in Italian about 16 months regarding a 6" SCT and received a nearly immediate reply via email. 

 

Regarding Facebook, the apparent owner of the page has also published photos of newly-produced/sold OTAs and optical blanks as of Nov. 2020. Granted, his latest post, from May of this year, was an homage in Italian to the unparalleled natural beauty, style and class of actress Charlize Theron. As you pointed out, that is not exactly what many would expect from a master optician hard at work producing and selling world-class OTAs. It reminds me of some classic posts from otherwise trusted and adored family members that convinced me to mothball my own Facebook account. Perhaps you can reply to his Charlize Theron post with an homage in English to the unparalleled natural beauty, style, and class of responding to emails about SCT corrector plates and related optics. "Unlike Charlize Theron, I'm actually interested in talking to you." grin.gif

 

None of this is to badmouth the business, ongoing concern or not. It is a boutique company in a niche market - something we know pretty well in this endeavor of ours. There are a handful of small, well-regarded (by their few customers) producers of astronomy equipment in Italy, most of which have the saddest of social media presences and tiny internet footprints. A well-branded social media presence wouldn't even be necessary if they were a little more active in maintaining their webpages with updated pricing and product availability. It is as if they depend solely on word of mouth advertising and personal contacts for sales, and part of me suspects this is the actual case. 

 

George


Edited by gfstallin, 24 June 2021 - 12:43 PM.

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#20 KLWalsh

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Posted 26 June 2021 - 12:08 PM

It may be that you can get a sharp image on-axis and within a very small zone around the optical axis. If so, this would give you a scope that’s ok for planets and other small objects. (Jupiter might be ok, but it’s moons well out from Jupiter may show a lot of coma.)

Maybe rewrite and repost your question in the ATM forum. Someone there might be curious and run the optical analysis for you.

#21 luxo II

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Posted 26 June 2021 - 03:22 PM

That isn’t the case - the spherical aberration resulting from no corrector will be terrible and applies everywhere across the focal plane including the centre. SA does not vary by the distance off-axis.


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#22 mhammady

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Posted 04 July 2021 - 06:01 PM

Thank you, everyone, for your postings, opinions, and ideas. I already moved on and got a great C9.25, but I feel bad seeing what was a good piece of equipment is gathering dust. I think I will post an ad asking for a used corrector plate, or I will order the one from toscanoptics.com

 

PS: I already contacted Celestron, but I was quoted about $800 excluding shipping and taxes which may end up close to the cost of a new C8 tube, and most probably less than the cost of a used one.


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#23 RalphMeisterTigerMan

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Posted 05 July 2021 - 10:18 AM

Has anyone ever thoiught of trying what Viven does with their SCT designs. They have a subdiameter corrector in FRONT of the secondary mirror. I have no idea what type of lenses (2) would be required, not being an optician myself, but perhaps it might be something that someone could look into. Couldn't a Ray-Tracing program tackle a problem such as this if one could figure out the type of glass for the lenses and the curves needed for all sides? (I didn't say it would be easy, maybe just something to play around with).

 

Clear skies and keep looking up!

RalphMeisterTigerMan



#24 rmollise

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Posted 05 July 2021 - 12:06 PM

Has anyone ever thoiught of trying what Viven does with their SCT designs. They have a subdiameter corrector in FRONT of the secondary mirror. I have no idea what type of lenses (2) would be required, not being an optician myself, but perhaps it might be something that someone could look into. Couldn't a Ray-Tracing program tackle a problem such as this if one could figure out the type of glass for the lenses and the curves needed for all sides? (I didn't say it would be easy, maybe just something to play around with).

 

Clear skies and keep looking up!

RalphMeisterTigerMan

 

 

The problem, as with every other solution other than getting another SCT corrector? It would cost far more than the scope is worth to have someone do that. And anyone with the skill to do so on their own probably wouldn't want to waste their time with this telescope. ;)


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#25 luxo II

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Posted 05 July 2021 - 09:03 PM

If you asked me, I'd happily charge you a hefty fee to make you a new corrector, but what I'd actually do is go find an old secondhand C8, and give you the corrector from that.

There are many if you look and they're cheaper than a new corrector.

 

Which does rather highlight that its cheaper to just replace the scope with a good secondhand one, and as for the one you have, break that up for parts. Someone else recently sold a pair of C8 mirrors for a nominal $80, the fork might interest someone, likewise the handset and cables.

 

Secondhand C8's are not rare.


Edited by luxo II, 05 July 2021 - 09:09 PM.

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