lluiscl
member
Reged: 09/03/08
Posts: 11
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Hi everyone. New here which a question. Bought my first scope by ebay... a Fastar C-8... but arrived with the corrector lens craked!! Testing the rest of the glasses I discovered that the famous Schmidt corrector plate is a single piece of flat glass!! I can't detect any curb or bend in the glass. Simetrically in all parameters (read with a digital caliper) with not optical effect looking across it. This is what Celestron is selling as a SCT units? or perhaps it's a fake lens? Thanks in advance.
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Lee Jay
sage
Reged: 02/27/08
Posts: 420
Loc: Westminster, CO
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The complex curve in an SCT corrector plate is pretty subtle.
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Clive Gibbons
Mostly Harmless
Reged: 05/26/05
Posts: 10349
Loc: Oort Cloud
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Hi and welcome to CN.
The Schmidt corrector plate has a very, very weak complex curve. It won't be measurable with a caliper.
The best way to see it's effect is to look at a reflection off the corrector plate. Like the edge of a wall.
Stand several feet back from the corrector plate and make sure the object you're looking at it a good distance away.
This is best done with the scope in a darkened area.
Move your head from side to side while looking at the reflection in the corrector plate. You should see some "deformation" of straight lines.
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A few telescopes of dubious value.
Understanding wife and three curious cats.
"Semper ubi sub ubi"
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lluiscl
member
Reged: 09/03/08
Posts: 11
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Thanks a lot for your advice. I think was OK. I can see any subtle kind of deformation. In order to replace it, how to determinate the external side?
(obviously Celestron make a different Schmidt lens than http://www.oldham-optical.co.uk/Manufacture.htm)
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rmollise
Carpal Tunnel
Reged: 07/06/07
Posts: 1662
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Quote:
Hi everyone. New here which a question. Bought my first scope by ebay... a Fastar C-8... but arrived with the corrector lens craked!! Testing the rest of the glasses I discovered that the famous Schmidt corrector plate is a single piece of flat glass!! I can't detect any curb or bend in the glass. Simetrically in all parameters (read with a digital caliper) with not optical effect looking across it. This is what Celestron is selling as a SCT units? or perhaps it's a fake lens? Thanks in advance.
You won't. This is a shallow complex curve. If you want to see it, you will have to lay it on an optical flat and look at the resulting bands.
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Uncle Rod
Watch for Rod's New Book:
Choosing and Using a New CAT--coming in December!
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rmollise
Carpal Tunnel
Reged: 07/06/07
Posts: 1662
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Quote:
Thanks a lot for your advice. I think was OK. I can see any subtle kind of deformation. In order to replace it, how to determinate the external side?
(obviously Celestron make a different Schmidt lens than http://www.oldham-optical.co.uk/Manufacture.htm)
Ouch! You'll need to find the serial number on the edge. It should be mirror correct when the correct side is out. I hope you also marked the position of any shims and the rotational position of the secondary holder!
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Uncle Rod
Watch for Rod's New Book:
Choosing and Using a New CAT--coming in December!
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lluiscl
member
Reged: 09/03/08
Posts: 11
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Really I am not sure now: lay it on an optical flat... look sides with magnifying... Fully flat!! Not serial number on the edge... Both sides not coated!!
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rmollise
Carpal Tunnel
Reged: 07/06/07
Posts: 1662
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Quote:
Really I am not sure now: lay it on an optical flat... look sides with magnifying... Fully flat!! Not serial number on the edge... Both sides not coated!!
If this is a Fastar corrector, it is coated, period. The serial number is very tiny and at the edge. Did you lay it on an actual OPTICAL FLAT and look for fringe patterns?
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Uncle Rod
Watch for Rod's New Book:
Choosing and Using a New CAT--coming in December!
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Chris Rowland
member
Reged: 02/28/05
Posts: 25
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This was bought on eBay, maybe the real corrector has been replaced with flat, uncoated, glass.
Chris
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Luigi
Carpal Tunnel
Reged: 07/03/07
Posts: 2061
Loc: Massachusetts
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I haven't tried this with a corrector but try propping up the a piece so that you can view a distant scene throught it and stand 10-20 feet back from the piece. You might see some optical effect then.
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17.5" f/5 Discovery Truss
IM715 7" f/15 MCT, Eon-120ED
Lunt 60mm single etalon HA
CG5A coffee grinder, Orion Skyview Alt-AZ
35,19,15 Pans.9 Nag. Meade 24.5 4kSWA, 4.7 5kUWA.
BO-TMB 7mm planetary.
Zeiss Diascope 85
Zeiss, Leica, Canon IS, Fujinon, Nikon binos
One each generic rescue Greyhound (pictured)
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rmollise
Carpal Tunnel
Reged: 07/06/07
Posts: 1662
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Quote:
I haven't tried this with a corrector but try propping up the a piece so that you can view a distant scene throught it and stand 10-20 feet back from the piece. You might see some optical effect then.
Very unlikely.
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Uncle Rod
Watch for Rod's New Book:
Choosing and Using a New CAT--coming in December!
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Luigi
Carpal Tunnel
Reged: 07/03/07
Posts: 2061
Loc: Massachusetts
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Oh yeah. I forgot to mention that you can try looking through the piece from 10-20 feet away using bins or a telescope and compare the the distant object through the and around the piece. Still might not be enough tho.
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17.5" f/5 Discovery Truss
IM715 7" f/15 MCT, Eon-120ED
Lunt 60mm single etalon HA
CG5A coffee grinder, Orion Skyview Alt-AZ
35,19,15 Pans.9 Nag. Meade 24.5 4kSWA, 4.7 5kUWA.
BO-TMB 7mm planetary.
Zeiss Diascope 85
Zeiss, Leica, Canon IS, Fujinon, Nikon binos
One each generic rescue Greyhound (pictured)
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lluiscl
member
Reged: 09/03/08
Posts: 11
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Thanks Luigi. You were right. Making this test I have discovered subtle distorsioned lines (haven't seen with a flat glass filter). So...no serial number, no coated, no bends, but apparentely was good.
Now I am thinking of replacing it.
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rmollise
Carpal Tunnel
Reged: 07/06/07
Posts: 1662
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Quote:
Thanks Luigi. You were right. Making this test I have discovered subtle distorsioned lines (haven't seen with a flat glass filter). So...no serial number, no coated, no bends, but apparentely was good.
Now I am thinking of replacing it.
How do you know it's not coated? StarBright coatings are not easy to see.
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Uncle Rod
Watch for Rod's New Book:
Choosing and Using a New CAT--coming in December!
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lluiscl
member
Reged: 09/03/08
Posts: 11
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Probably it was also very subtle (not the different colors reflections in MC optics).
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HughB
journeyman
Reged: 12/28/07
Posts: 55
Loc: UK
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Quote:
the famous Schmidt corrector plate is a single piece of flat glass!!
This is EXACTLY what I thought when I saw a SCT in the flesh for the first time!
Out of curiosity, did you have any comeback on the delivery company?
I've saved for years for my new scope. It's arrived but I'm working away from home; I haven't examined it yet.
Apparently, there are lots of dents and holes in the box and the couriers delivered it to 3 wrong addresses before it arrived at my house. Aaarrrgggghhhhhh
HUGE sympathy for your cracked scope blues, 
and curse the eejit who packed it! 
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Naked Eyes
Enhanced Optics OEW (Open Eyes Wider)
4.5" Newt CheapScope (The Last Scope I Will Ever Buy)
Orion Optics UK OMC140 (The Very Last Scope I Will Ever Buy And This Time I Mean It)
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lluiscl
member
Reged: 09/03/08
Posts: 11
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Shipment was unsured... seller are looking for indemnity (was international Priority Mail). Comes with 2 boxes (one inside other) fully muffled!! Incredibly the external one was OK... May be played to (soft) football with it?
Looking for a used plate corrector... anyone?
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Kolenka
professor emeritus
Reged: 06/01/08
Posts: 600
Loc: Seattle Area, WA, USA
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Ouch, that was shipped pretty poorly using bubble-wrap like that. I am not surprised it didn't make it. 
As for replacing the corrector, you would need a new one made specifically for the scope, if I recall correctly. You might get extremely lucky and find a match from someone else who busted some other part of a Fastar C8. Usually the corrector, primary, and secondary have to be matched and aligned to provide proper views.
Sorry the news isn't better.
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Meade 10" LX200R
Orion 80ED
Nagler 7T6, 9T6, 13T6, 17T4, 26T5
Canon XSi (Modded), TIS DMK 31AF03
Northwest Astro Photoblog
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sang33ta
professor emeritus
Reged: 07/08/08
Posts: 539
Loc: UK
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Bah! Another reason I like Maks, the corrector is built like a tank. I always thought the SCT glass was flat too but everyone here insists it's got a slight curve. The glass in my Mak has an almost impossible to detect hint of blue in it rather than the obvious purple UHTC coatings you get on Meades.
Couriers don't have cover for shipping glass normally it's excluded in their small print so I'd probably look to split the loss with the seller (I'm a nice person).
Sorry for your loss, wish you luck in finding a replacement corrector.
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Hioptic 152mm f12.5 Maksutov
Celestron Advanced CG5-GT Mount (Mr Noisy!)
Meade 4000 Super Plossl Set
Casio QV-2900UX
Got fed up of waiting for Meade ETX-150 so put this together for £500/$1000 
Edited by sang33ta (09/05/08 12:55 PM)
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rmollise
Carpal Tunnel
Reged: 07/06/07
Posts: 1662
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Quote:
Bah! Another reason I like Maks, the corrector is built like a tank. I always thought the SCT glass was flat too but everyone here insists it's got a slight curve.
It can't be flat. If it is flat, what do you have? Something like the Hubble Space Telescope before the first "fixit" mission. SCTs use a spherical primary mirror, and without the corrector to CORRECT for--to counteract--the inherent spherical aberration of the mirror you would have--a MESS.
Let me add that a replacement corrector can work acceptably, especially if some time is spent finding the optimum rotational orientation. Do not expect the performance of the original _matched_ corrector, however. 
--------------------
Uncle Rod
Watch for Rod's New Book:
Choosing and Using a New CAT--coming in December!
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