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Help with late model C8se

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#1 davelid

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Posted 23 February 2013 - 03:47 PM

Hi.
Anyone have experience with late (circa 2010 or so) model C8se's? The previous owner of my scope removed the corrector and secondary for cleaning and I am not sure if he put them back in the correct orientation. Unlike earlier C8s with an engraved serial number on the front edge? of the corrector, this scope has the letters "ClV" engraved at two locations on the side of the corrector. They are located 180 degrees apart. Any idea how these should be oriented? I tried placing the marks as suggested by Celestron at 180 deg to the focuser (3:00 position), but neither position gave sharp views. The diffraction rings are nice and circular, but the view seems a bit mushy. I suppose I can try the "rotate the corrector 10 deg at a time and star test", but was hoping that there might be some factory orientation to start from. On another note, I removed the secondary mirror from its holder and there are absolutely no orientation marks on the secondary or its housing. This must be the new production model for Celestron (their website says that post 2009 scopes may not be marked/oriented the same as previous scopes).
Thanks!
Dave

#2 Alfredo Beltran

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Posted 23 February 2013 - 08:38 PM

Hi Dave

I own a C6 which came in 2009 brand new to me from the US. As I explain in this thread two years ago I had to take out the corrector plate and thus had to collimate it. These days I had trouble trying to collimate it (completely my fault) and I think you'll find very useful information.

You can also contact Dean at Starizona. He is very helpful and knows a lot about SCT telescopes.

You can see my telescope in the link at my signature.

Let me tell you if I can be of more help

Best regards

Alfredo

#3 davelid

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Posted 25 February 2013 - 01:41 PM

Hi Alfredo,

I have been following your progress with your C6 with interest.

Thanks!
Dave

#4 orion61

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Posted 25 February 2013 - 02:57 PM

Post 2009 Celestron has discontinued the practice
of hand correcting the secondary mirrors..or this is rumored
And remember " They cant put anything on the Internet that isn't true"
I am waiting for my Swedish Model to stop by later for our first date....

#5 Alfredo Beltran

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Posted 25 February 2013 - 05:03 PM

Hi Dave

That's good! You can see I finally collimated my scope.

Best regards

Alfredo

#6 Eddgie

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Posted 26 February 2013 - 09:56 AM

The reason that Celestron used to touch of the seconary mirrors was to null out astigmatism.

I think most likely your scope just needs fine collimation, but lets pretend that this is not the case.

I can't comment on whether they do this any longer and it isn't relevent to your post (whether they do or not), but that is why they used to do it.

If there is a bit of astigmatism in the primary, rather than take it out by polishing, it was much easier for a skilled optical worker to simply introduce a bit of opposite sign astigmatism in the secondary to cancel out the astigmatism in the primary.

What this means is that if the secondary was retouched, then for it to work properly, it has to be in the correct alighment to the primary.

There is such good news for you though. If the secondary was re-touched to balance out astigmatism, the effect of a secondary that is not aligned will be that there will be astigmatism in the image and this is very easy to check for.

I would advise the use of an artificial star for this test because seeing is rarely good enough to test unless the astigmatism is quite pronounced.

You can start with a real star tough.

Rack your mirror out of focus by turning the knob clockwise a few turns.

When you see the Fresnel pattern, start turning the knob slowly in. As the rings collapse, you are watching that they remain perfectly round, especiallly just before the bright ring collapses into the Airy Disk.

Perfectly Round.

As you continue to turn counterclocwise though best focus, you should see the rings emarge perfectly round. You want to see this as close to the point that you emerge from best focus as possible.

If the rings change to either ellipictal or a cross shape, then there is astigmatism.

This is only good for rough testing.

For fine testing, it is best to use an in-focus star (artificial pin-hole would be best for this). What you are looking for is that when a star (or your pinhole source)
is at best focus, the ring is evenly illuminated.

If it has "Nodes" where there are bright and dim parts in the ring, this indicates that there is some kind of astigmations. NOt if the ring is like a "C". That is collimation. More like an even ring with one part bit brighter or dimmer. This may indicate a rotational error.

I think your problem is fine collimation though. Using the defcused image to collimate only gives you a rough collimation. To get really good collimation, you should use an in-focus star, or if seeing is not so good, use an artifical source.

Not saying that you may not have astigmatism, but I think fine collimation may be more the issue.

But when you do fine colliation, you will see if there is any astigmatism there, so it is the best place to start.

#7 davelid

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Posted 27 February 2013 - 12:48 PM

Thanks Eddgie!

Working with artificial star last night I cannot detect any astigmatism per your instructions, so it appears that secondary orientation is not an issue. I might still try rotating corrector to see if that matters. Both corrector and secondary appear to be mechanically and optically centered.

Dave

#8 gfeulner

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Posted 27 February 2013 - 01:52 PM

Aaaah, Bonjour! LOVE that commercial! Gerry






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